Additive Manufacturing and Chemical Engineering: Looking for Synergies from a Bibliometric Study

Abstract

Additive manufacturing must be highlighted as an innovative technology with the capacity to produce objects with complex and customized geometries using a diverse range of raw materials. Despite its significant potential, research compiling and evaluating the specific contributions of additive manufacturing in the field of chemical engineering was scarce in both quantitative and qualitative terms. Similarly, the application of chemical engineering tools to additive manufacturing has not been specifically reviewed. Therefore, this work conducted a comprehensive review of the scientific literature covering these issues using bibliometric analysis. The search encompassed the entirety of the scientific literature up to the year 2023, yielding 3761 documents in the Scopus database. The principal findings of this bibliometric analysis indicated an exponential growth in the number of publications, which suggests a rising scientific interest in this field. The analysis revealed that English was the dominant language in the documents, and articles constituted the most common document type, indicating the quality and maturity of the research. The thematic distribution proved to be multidisciplinary, with a primary focus on engineering and materials science, as well as basic sciences. The United States was the foremost contributor to scientific production, followed by China and Germany. Keyword analysis and scrutiny of the most cited documents enabled the identification of the main topics, which were found to include biofabrication and biomedical applications. Moreover, bibliometric network analysis using the software SciMAT (v 1.1.06) yielded the corresponding strategic diagrams, evolution maps, and thematic networks, which provided a comprehensive overview of trends and research gaps. The considerable interest in the application of additive manufacturing to biofabrication and other biomedical purposes has overshadowed the specific applications within the chemical engineering field, while the potential contributions that chemical engineering could make to the field of additive manufacturing have been eclipsed too. On the one hand, applications focused on process intensification in chemical engineering could benefit from additive manufacturing to design advanced microreactors and other miniaturized devices or to produce more efficient heat exchangers, catalysts, and adsorbents with complex geometries and separation membranes with innovative materials and structures. On the other hand, life cycle assessment and optimization are established chemical engineering tools that should be more extensively employed in the context of additive manufacturing to ensure a more sustainable outcome.

1. Introduction

Additive manufacturing, also known as 3D printing, has emerged as a revolutionary technology capable of manufacturing parts with complex geometries by adding material layer by layer based on information provided by a 3D model, thus granting extensive design freedom. This unique characteristic allows for the fabrication of complex or customized parts directly from the design without the need for expensive tools or molds, and reduces the necessity of many conventional processing steps. Complex parts, faithful to their design, can be manufactured in a single step without the limitations of conventional processing methods or commercial forms. Additionally, a significant reduction in the number of parts can be achieved by eliminating or reducing the need to assemble multiple components [1]. The origins of additive manufacturing can be traced back to rapid prototyping in the early 1980s, which culminated in the development of stereolithography (SLA) in 1984. The main difference between the two concepts is that rapid prototyping is oriented toward prototype creation, while additive manufacturing primarily focuses on the final parts for the user [2].

A key advantage of additive manufacturing is its ability to decouple design complexity from production cost [3]. This implies a revolution in product design across many sectors, which can leverage topological optimization, generative algorithms, and cellular structures to obtain optimized and high-value products capable of integrating complex assemblies into a single piece [4]. It is also perceived as an environmentally sustainable manufacturing technology, as it can potentially reduce up to 525.5 Mt of total carbon dioxide emissions by 2025 compared to conventional processes [5,6].

The process begins with the digital design of the component in the CAD software, which is subsequently converted into a suitable format, predominantly STL file format, for additive manufacturing. During this conversion, the component surfaces are tessellated into triangles, and a database of the triangle nodes is stored. From these data, two-dimensional layers are generated to determine the deposition of material at each level, while the deposition in the third dimension is determined by the thickness and interlacing of the layers. The geometric data of the component is then encoded in a file format, especially G-code, a programming language that provides the material deposition device the commands it must follow. The component is then constructed in a layer-by-layer manner, with the material of each layer interacting selectively with that of the previous one until the object is fully formed [7]. The precision of this process underpins the ability of additive manufacturing to produce intricate and highly accurate geometries.

The field of additive manufacturing encompasses a wide variety of materials, processes, and devices that enable the creation of three-dimensional objects from digital models. However, this diversity also presents a challenge for the standardization of additive manufacturing technologies. To solve this problem, some categories have been established to group additive manufacturing technologies according to their common characteristics following the criteria of the ISO/ASTM 52900 standard [8]. This document defines seven standard additive manufacturing processes based on the type of material and initial physical form, deposition method, and energy or binding source (Figure 1). An important feature of additive manufacturing technologies is the need for suitable feedstock materials in the required form, such as powders, filaments, pellets, granules, sheets, and resins. The range of raw materials for additive manufacturing technologies is growing exponentially, and new materials are constantly entering the market [9]. The variety of materials that can be printed and the corresponding handling options have led to more than 50 different additive manufacturing technologies.

Table 1. Main feed materials of the different additive manufacture processes.

Additive Manufacture Processes	Feed Material
Material Extrusion (MEX)	Thermoplastic filaments and pellets, paste-like materials, and liquids in syringes
Vat Polymerization (VP)	Liquid photo-curable resins
Material Jetting (MJ)	Polymers and wax-like materials (fillers are optional)
Sheet Lamination (SL)	Paper, polymer, and metal sheets
Powder Bed Fusion (PBF)	Polymer, metal, ceramic, and sand powders
Direct Energy Deposition (DED)	Metal wires and powders and ceramic powders
Binder Jetting (BJ)	Polymer, metal, ceramic, glass, and sand powders

summarizes the major technologies and feedstock materials used in the seven categories of the ISO/ASTM 52900 standard.

Chemicals are the building blocks of today’s technological society, and maintaining this lifestyle would be impossible without them. Chemical engineering is the discipline concerned with the design, manufacture, operation, and maintenance of instruments, equipment, and chemical processing plants. It combines the principles of sciences such as chemistry, physics, mathematics, materials science, and biology with economics to efficiently design, produce, transform, transport, and use chemicals. Chemical engineering faces various challenges that require continuous evolution through rapid and innovative adaptation to shorter life cycles with sustainability in mind [10]. Additive manufacturing is expected to lead to a revolution in reaction and separation processes, drastically impacting chemical engineering as it can facilitate the development of advanced materials and sophisticated devices for various applications [11]. In addition, chemical engineering can provide methods to assess the sustainability of additive manufacturing processes, taking into account environmental, social, and economic aspects [12]. Thus, the integration of additive manufacturing and chemical engineering can provide benefits such as design freedom, time and material savings, product customization, and function integration, but it can also present relevant challenges such as material selection and processing, component quality and reproducibility, device stability and compatibility, and product and process sustainability assessment.

Despite the importance and potential of the synergies between additive manufacturing and chemical engineering, there is a lack of studies that systematically and thoroughly analyze the mutual contributions of chemical engineering and additive manufacturing, both quantitatively and qualitatively [13]. Therefore, it is necessary to conduct a bibliometric review and network analysis of the scientific literature published on the state-of-the-art contributions of chemical engineering and additive manufacturing to obtain a more comprehensive and detailed overview of the research in this area. Bibliometric review is a technique that allows the quantitative analysis of scientific production using objective indicators, while network analysis is a technique that allows the graphical representation and qualitative study of interactions between elements of a system using concepts such as nodes, links, and clusters [14,15]. These two techniques are complementary and allow a broader and deeper understanding of scientific research in a field, as well as the identification of trends, patterns, innovations, leaders, collaborators, competitors, niches, and opportunities. The purpose of this work was to analyze, from a bibliometric perspective, the scientific literature published in Scopus related to the synergies between additive manufacturing and chemical engineering. The documents found as a result of the corresponding bibliographic search were evaluated according to several criteria (such as annual production, main languages, and knowledge categories or most prolific journals, countries, and institutions). In this way, the quantitative characteristics of the research were identified and complemented with the most important hot topics and research gaps in this field through bibliometric network analysis. The findings were applied to provide a concise review of the most relevant trends and the most important research opportunities.

2. Data Sources and Methodology

The Scopus database was used, which is one of the most comprehensive and up-to-date sources of information on scientific publications. It is recognized as the largest abstract and citation database of peer-reviewed literature, covering more than 27,000 scientific journals and 249,000 books [16]. The following search string was used to obtain the bibliography used in this work: “(TITLE-ABS-KEY (“Additive Manufacturing”) AND PUBYEAR < 2023 AND (LIMIT-TO (SUBJAREA, “CENG”)))”. This string retrieves documents containing the exact phrase “Additive Manufacturing” in the title, abstract, or keywords, published no later than 2022 (the last full year when the search was performed in September 2023), and belonging to the Chemical Engineering subject area. The search returned a total of 3761 documents. From Scopus, the filter count option and complete database were exported into CSV and RIS files, which were used to analyze the search results. The files contained fields such as title, author, year, journal, citations, country, affiliation, language, document type, subject area, and keywords for each document. An Excel file was used to create a database with these fields to organize the data according to the criteria of interest.

The developed methodology has two parts: bibliometric performance analysis and bibliometric network analysis or scientific mapping. First, the performance analysis was based on bibliometric indicators that summarize the main characteristics of the scientific documents. This part used the created Excel file. Second, a conceptual bibliometric network analysis was performed based on a co-word network using SciMAT [17]. Although there are several software tools for scientific mapping analysis, SciMAT was chosen because it supports analysis from different perspectives, mainly strategic and dynamic diagrams, and complete cluster networks. The co-word analysis yielded related groups of keywords called clusters. These clusters were characterized by two measures: density, which indicates the internal cohesion of the keywords comprising them, and centrality, which reflects the interaction of a topic with other topics and its relevance in the scientific field studied [18]. A strategic diagram is a two-dimensional space in which clusters are located according to their centrality and density, plotted on the x-axis and y-axis, respectively (Figure 2).

In accordance with their position within the strategic diagram, the clusters can be classified into four distinct categories:

• Motor clusters: Situated in the upper right quadrant and pertain to well-developed and pivotal subjects within the scientific field, exhibiting robust centrality and high density.
• Highly developed and isolated clusters: Located in the upper left quadrant and correspond to topics that have marginal importance in the scientific field. These topics are characterized by being highly specialized and peripheral.
• Emerging or declining clusters: Located in the lower left quadrant and correspond to very undeveloped and marginal topics or to very recent topics having low density and centrality.
• Basic and transversal clusters: Located in the lower right quadrant and correspond to important topics that are not well developed.

In a cluster, the related keywords and their interconnections form thematic networks, each of which is labeled with the most significant keyword of the associated theme, which is represented in the center of the cluster (central node). The volume of the nodes is proportional to the number of documents associated with the keywords of the cluster, while the thickness of the lines between keywords is proportional to their equivalence index (directly related to the co-occurrence of both keywords).

In the dynamic diagram, thematic areas are defined as a group of clusters that have evolved over different successive time periods. Depending on the interconnections between clusters, a cluster may belong to two or more thematic areas or none at all. Solid lines indicate that linked clusters share the same name (meaning they were labeled with the same central keyword) or the name of one of them is a part of the other one. A dashed line indicates that clusters share elements other than the cluster names. The thickness of the links is proportional to the inclusion index of both clusters (maximum value when all the keywords of one cluster are included in the other). The volume of the spheres is proportional to the number of related documents.

The RIS file containing bibliographic data from Scopus was imported into the SciMAT software to construct the database for bibliometric network analysis. In addition, to ensure and improve the quality of the raw data, a keyword normalization process was applied, merging singular and plural forms of keywords, acronyms, and regional spelling variations. Subsequently, periods or groups of years were defined in order to analyze the evolution of scientific production in the field of study. It was decided to divide the data into five periods: 2002–2017, 2018–2019, 2020, 2021, and 2022. This division was chosen to ensure that each period has a similar number of documents, facilitating comparisons between them and preventing some periods from having uncompensated data for co-word analysis. Finally, the configuration of the scientific mapping analysis was set up using the configuration option in the SciMAT program and the parameters used are listed in

Table 2. Parameter configuration used for the analysis by SciMAT.

Parameter	Value
Unit of analysis	Words (author’s words, source’s words, and added words)
Frequency reduction	12
Kind of matrix	Co-occurrence
Edge value reduction	4
Normalization measure	Equivalence index
Clustering algorithm	Simple centers
Maximum network size	11
Minimum network size	7
Evolution map measure	Jaccard’s index

.

3. Results and Discussion

3.1. Bibliometric Performance Analysis

3.1.1. Publication Year, Language of Publications, and Document Type

The graphical representations in Figure 3 illustrate the evolution of the annual scientific output tracked in the Scopus database and the cumulative number of documents from 2002 (the year of the first identified document) to 2022. The pioneering work in the first decade of the 21st century consisted of only six documents published before 2010, but different additive manufacturing technologies were covered, such as stereolithography for manufacturing injection molds [19], laser powder deposition [20], or precision droplet manufacturing [21].

The analysis of the graphs in Figure 3 shows that the scientific production on the topic has experienced accelerated growth in recent years, especially since 2015. The number of publications has increased from 1 in 2002 to 953 in 2022. This increase can be attributed to technological advances and the increased demand for additive manufacturing, which has facilitated the development of new materials, processes, and applications, including contributions from chemical engineering and applications in this field [22]. The most productive years, or those with the highest number of publications on the topic, were the last three: 2020, 2021, and 2022. During these years, 667, 832, and 953 documents were published, respectively, representing 72% of the total. These years coincided with the context of the COVID-19 pandemic, which stimulated research and innovation in materials and processes to address the health and economic crisis [23]. On the other hand, some years in the 2000s appeared without any production, such as 2003, 2004, 2006, and 2009. This decade corresponded to the beginning of the diffusion of additive manufacturing as an emerging and promising technology for chemical engineering and other fields, including its standardization [10].

The observed evolution of scientific production on this topic followed an exponential trend, indicating that the rate of growth has been variable and has progressively increased over time. The exponential regression yielded an equation of the form y = 0.345·e^0.445x, where ”y” represents the accumulated number of publications and “x” represents the year, with the value equal to one for the year 2002. The corresponding R² value was 0.986, indicating a strong correlation between the observed and estimated data by the equation. Consequently, the projected trajectory of scientific production on this topic, based on the exponential regression, suggests that the trend is expected to persist and potentially accelerate in the coming years, presenting both challenges and opportunities for research in this field.

A review of

Table 3. The languages employed by the publications.

Language	Publications	Contribution (%)
English	3691	98.14
Chinese	38	1.01
German	12	0.32
Japanese	8	0.21
Polish	5	0.13
Bosnian	2	0.05
Korean	2	0.05
Turkish	2	0.05
Croatian	1	0.03
Russian	1	0.03

, which outlines the languages utilized in the selected documents, indicates that English is the predominant language, accounting for 98.1% of the total documents (3691). This suggests that English serves as the universal language or international standard for scientific communication, enabling the dissemination of the research findings to a global audience and facilitating collaboration among researchers from different countries and regions [24]. The next most utilized language is Chinese, with 38 documents, which accounts for 1.0% of the total. This suggests that Chinese serves as the national language or local standard for scientific communication in China, which is one of the most active and leading countries in scientific production across a range of research topics, including the topic investigated in this work. Consequently, Chinese facilitates the dissemination of research findings to a national audience, thereby fostering innovation and development in this field. German is the other language with more than 10 documents (just 12 documents), which implied a 0.3% contribution to the total, indicating the relevant role of German research in this topic. In contrast, the contribution of other languages is minimal, with percentages below 0.25%, reflecting a limited presence and dissemination of research in those languages.

The various document types were subjected to analysis, resulting in the identification of 11 distinct categories (

Table 4. The different types of documents published.

Language	Publications	Contribution (%)
Article	2471	65.70
Conference Paper	663	17.63
Review	385	10.24
Book Chapter	134	3.56
Conference Review	41	1.09
Editorial	29	0.77
Note	15	0.40
Book	9	0.24
Erratum	9	0.24
Short Survey	4	0.11
Letter	1	0.03

). The corresponding analysis demonstrated that the most common document type was article, with 2471 documents, representing 65.7% of the total. This indicated that most scientific production on the subject was disseminated through scientific journals, reflecting the level of quality and maturity of the research on this topic. This supremacy of articles over other types of publications has been previously identified by other bibliometric analyses in fields related to engineering [25,26].

The second most prevalent document type was conference paper, with 663 documents, representing 17.6% of the total. This indicates that a considerable proportion of scientific output on the subject is presented at national or international academic events, which demonstrates interest, relevance, interaction, and collaboration among researchers in this field. The third most prevalent document type was review, with 385 documents, representing 10.2% of the total. This highlighted the necessity for a synthesis and evaluation of the current state of the art and advances in additive manufacturing as they relate to chemical engineering, as well as the identification of challenges and opportunities for future research in this field. Consequently, despite the high number of documents available about the topic, conference papers were a more frequent type than reviews. A review of bibliometric studies published previously indicated that with regard to document types, the second position in the ranking is not as definitive as the prominent position held by articles. While contributions presented at congresses and conferences are highly valued in diverse engineering domains, including electronic, civil, and environmental engineering [27,28], there are cases where reviews are given relatively more importance than conference papers [29,30]. Finally, the contribution of book chapters was 3.6%, while the joint contribution of the remaining categories was below 2.9%.

3.1.2. Publication Distribution of Countries and Institutions

As indicated in

Table 5. The top 23 most productive countries (at least 52 documents).

Country	Publications	Contribution (%)
United States	887	23.58
China	569	15.13
Germany	374	9.94
Italy	298	7.92
India	283	7.52
United Kingdom	278	7.39
Australia	144	3.83
Spain	117	3.11
Singapore	109	2.90
France	108	2.87
Canada	89	2.37
South Korea	86	2.29
Japan	83	2.21
Netherlands	81	2.15
Poland	80	2.13
Russian Federation	78	2.07
Malaysia	70	1.86
Austria	67	1.78
Switzerland	67	1.78
Belgium	60	1.60
Portugal	57	1.52
Turkey	53	1.41
Brazil	52	1.38

, which presents the most productive countries regarding the subject under investigation, the United States emerged as the most prolific country, heading scientific research in the areas of additive manufacturing and chemical engineering with a total of 887 documents, representing 23.6% of the global output. The United States has a high degree of scientific development and technological progress, as well as policies and strategies that promote research and innovation. Moreover, the United States can be regarded as a global superpower in scientific production and is at the forefront of global research in numerous fields, including additive manufacturing. The transition from traditional manufacturing to additive manufacturing may offer substantial benefits in terms of supply chain lead time, raw materials and primary energy consumption, emissions, and life cycle costs for certain industrial sectors in the United States. Consequently, key stakeholders are exploring the potential of additive manufacturing to enhance and capture value in various ways within the context of the current global value chains [31,32].

The second most prolific country in terms of scientific output on the subject was China, with 569 documents, representing 15.1% of the total. It is imperative to consider China as a significant player in the field of scientific research, particularly given its status as the only superpower capable of rivaling the United States in terms of scientific output and leadership in specific research areas. China is renowned for its rapidly expanding manufacturing sector, which has given rise to considerable environmental concerns. The transition to a greener industrial manufacturing sector has become a priority and considerable investments have been made in the development of new materials, processes, and applications, including the exploration of the potentiality of additive manufacturing [33,34,35].

The podium was completed by Germany, which ranked third with 374 documents, corresponding to 9.9% of the total. Germany is the leading European country in terms of scientific production on this topic, with other European countries represented in the ranking including Italy, the United Kingdom, Spain, France, the Netherlands, Poland, Austria, Switzerland, Belgium, Portugal, and Turkey. These countries contribute to the advancement of knowledge and innovation from a variety of levels of development, participation, and impact, representing a range of percentages between 7.9% and 1.4%. Indeed, the collective contribution of these 12 countries amounted to 43.6% (exceeding the sum of Chinese and American contributions). In addition to the factors that facilitate the advancement of additive manufacturing in Europe, such as those shared with the United States and China, which are oriented towards more sustainable production processes [36,37], the European continent has also faced significant challenges in ensuring access to essential technological products in recent years. These challenges have been exacerbated by circumstances such as the global COVID-19 pandemic and the ongoing armed conflicts in the region. In this context, one of the most promising solutions is the development of additive manufacturing [38,39]. Consequently, significant research efforts have been dedicated to this field.

In addition to the European countries, other Asian countries have also made significant contributions to scientific production on this topic. These include countries with long-standing research traditions in industrial technologies, such as South Korea (86 documents, 2.3% contribution) and Japan (83 documents, 2.2% contribution), as well as emerging actors in the research landscape, such as Singapore (109 documents, 2.9% contribution) and Malaysia (70 documents, 1.9% contribution). Furthermore, a highly populated country like India (283 documents, 7.5% contribution) has also made notable contributions, ranking fifth in the overall ranking. Indeed, India has been identified as the next knowledge superpower with the potential to attain the levels of the United States and China before 2050 due to its sustained efforts in scientific research and development programs [40]. The research conducted to facilitate a greener transition is based on technological innovation, including additive manufacturing [41].

The analysis in

Table 6. The top 26 most productive institutions (more than 25 documents).

Institution	Country	Publications	Contribution (%)
Nanyang Technological University	Singapore	60	1.60
Ministry of Education China	China	59	1.57
CNRS Centre National de la Recherche Scientifique	France	51	1.36
School of Mechanical and Aerospace Engineering	Singapore	49	1.30
Chinese Academy of Sciences	China	48	1.28
Friedrich-Alexander-Universität Erlangen-Nürnberg	Germany	47	1.25
Politecnico di Torino	Italy	42	1.12
Huazhong University of Science and Technology	China	35	0.93
Virginia Polytechnic Institute and State University	United States	33	0.88
Singapore Centre for 3D Printing	Singapore	33	0.88
Università degli Studi di Padova	Italy	32	0.85
University of Maryland, College Park	United States	31	0.82
Purdue University	United States	30	0.80
Rheinisch-Westfälische Technische Hochschule Aachen	Germany	29	0.77
Beijing Institute of Technology	China	28	0.74
National University of Singapore	Singapore	28	0.74
Monash University	Australia	27	0.72
Georgia Institute of Technology	United States	27	0.72
Tsinghua University	China	27	0.72
Delft University of Technology	The Netherlands	26	0.69
Xi’an Jiaotong University	China	26	0.69
Universiteit Twente	The Netherlands	26	0.69
Politecnico di Milano	Italy	26	0.69
Oak Ridge National Laboratory	United States	26	0.69
ETH Zürich	Switzerland	26	0.69
A. James Clark School of Engineering	United States	26	0.69

, which summarizes the most productive institutions, revealed that the Nanyang Technological University was the most prominent institution, with 60 documents, representing 1.6% of the total. This demonstrated that the scientific production on this topic was highly shared among different institutions. This university, located in Singapore, is one of the most appreciated institutions engaged in research and development in the field of additive manufacturing. It possesses a specialized center in this domain, the Singapore Centre for 3D Printing, which is listed as an independent institution in Table 6 with 33 documents and a 0.88% contribution. The institution engages in research and innovation projects pertaining to a multitude of applications of additive manufacturing, including biofabrication, aerospace equipment, sustainable construction, and innovative electronics. Indeed, the School of Mechanical and Aerospace Engineering from Nanyang Technological University also appeared among the most prolific institutions, with 49 documents (1.3% contribution). Furthermore, another Singaporean university was also present in the ranking: the National University of Singapore, which produced 28 documents (0.74% contribution). These institutions are part of Singapore’s additive manufacturing ecosystem, which is characterized by academic excellence, interdisciplinary collaboration, and strong links with the industrial sector [42].

The second-ranked entity was the Chinese Ministry of Education, which published 59 documents, representing a 1.57% contribution to the total. Another notable Chinese governmental institution is the Chinese Academy of Sciences, which occupies the fifth position in the ranking with 48 documents. These two institutions are critical within the Chinese research framework. On the one hand, the Chinese Ministry of Education is the primary source of funding for national universities and colleges in China, with 75 affiliated colleges and universities. The investment in higher education institutions in China over the past two decades has led to improvements in both human and physical resources. These improvements have been accompanied by new policy factors, resulting in a positive impact on university productivity [43,44]. On the other hand, the Chinese Academy of Sciences represents the largest research organization in the world, comprising 100 research institutes, two universities, and approximately 70,000 full-time employees. Consequently, it has been instrumental in spearheading China’s research endeavors, particularly at the forefront of scientific advancement. Additionally, it serves as the primary catalyst for innovation in science and technology, provides consultative services for technological advancement, and nurtures talent for research applications [45]. Furthermore, four Chinese universities were included in Table 6: Huazhong University of Science and Technology (35 documents), Beijing Institute of Technology (28 documents), Tsinghua University (27 documents), and Xi’an Jiaotong University (26 documents).

The third most prominent institution was the CNRS (Centre National de la Recherche Scientifique), with 51 documents, or 1.36% of the total. This center is France’s primary public research institution, conducting both basic and applied research in various disciplines, including chemical engineering and additive manufacturing. CNRS houses several laboratories and research groups focused on topics such as polymeric materials, catalytic processes, bioengineering, and nanotechnology. In addition to CNRS, other European institutions contributing to the advancement of knowledge and innovation in additive manufacturing include Friedrich-Alexander University of Erlangen-Nuremberg (Germany) with 47 documents; Politecnico di Torino (Italy) with 42 documents; Delft University of Technology (Netherlands) with 26 documents; and ETH Zurich (Switzerland) with 26 documents.

Notwithstanding its status as the foremost nation in scientific output on the investigated topic, the United States presents only just a single institution in the top ten rankings (the most prolific institution from the United States was situated in ninth place). Consequently, production is distributed across a multitude of institutions with comparable levels of involvement and impact. Notable American institutions that have made significant contributions to the advancement of knowledge and innovation in additive manufacturing include Virginia Tech (33 documents), the University of Maryland (31 documents), Purdue University (30 documents), the Georgia Institute of Technology (27 documents), and Oak Ridge National Laboratory and the A. James Clark School of Engineering at the University of Maryland (both with 26 documents).

3.1.3. Distribution of Output in Subject Categories and Journals

Given the scope of this research, which is limited to the field of Chemical Engineering, only documents that are classified into this subject area in Scopus have been considered. As illustrated in

Table 7. The top 10 most popular subject categories (at least 140 documents).

Subject	Publications	Contribution (%)
Chemical Engineering	3761	100.00
Engineering	2119	56.34
Materials Science	1839	48.90
Chemistry	938	24.94
Physics and Astronomy	861	22.89
Computer Science	592	15.74
Biochemistry, Genetics, and Molecular Biology	544	14.46
Energy	240	6.38
Environmental Science	198	5.26
Medicine	145	3.86

, this approach has resulted in the inclusion of all the documents within this area. It should be noted, however, that this category is not exclusive, as a single document can belong to multiple subject areas. This results in the total sum of individual percentages exceeding 100%. The thematic areas most closely related to or intersecting with chemical engineering are engineering, with 2119 documents (56.3% contribution), and materials science, with 1839 documents (48.9% contribution). These areas reflected the interactions and synergies among different branches of engineering oriented towards different applications and the relevance of the materials employed in additive manufacturing. Additionally, two fundamental sciences, chemistry and physics, were represented in fourth and fifth position with 938 documents (24.9%) and 861 documents (22.9%), respectively. These areas highlighted the significance of theoretical and experimental foundations for advancement and innovation in additive manufacturing. The field of chemistry provides the means to create new materials with specific properties, while the discipline of physics offers insights into the process dynamics and material behaviors for optimizing manufacturing techniques. Computer Science constituted the sixth subject area, accounting for over 15% of the total, with 592 documents. The developments in simulation software and computational modeling have facilitated precise control over additive manufacturing processes and material properties. Moreover, the development of new algorithms is necessary to optimize design complexity and enhance the efficient employment of materials. The multiple contributions from different knowledge areas substantiate the multidisciplinary nature of additive manufacturing and the necessity of adapting it to each specific application [46].

The number of documents published in the most frequently utilized sources is presented in

Table 8. The top 14 most popular scientific sources (at least 40 documents).

Source	2023 SJR	2023 IF	2023 JCI	Publications	Contribution (%)
Lecture Notes in Mechanical Engineering	0.167	-	-	447	11.89
Applied Sciences	0.508	2.5	0.56	402	10.69
Ceramics International	0.938	5.1	1.27	178	4.73
Powder Technology	0.970	4.5	0.78	130	3.46
International Journal of Heat and Mass Transfer	1.224	5.0	1.23	114	3.03
Annual Technical Conference ANTEC Conference Proceedings	0.107	-	-	89	2.37
Crystals	0.449	2.4	0.74	70	1.86
Biofabrication	1.769	8.2	1.70	69	1.83
Corrosion Science	1.897	7.4	1.66	58	1.54
Frontiers in Bioengineering and Biotechnology	0.893	4.3	0.83	57	1.52
Macromolecular Materials and Engineering	0.811	4.2	0.71	52	1.38
ACS Applied Polymer Materials	0.982	4.4	0.90	47	1.25
Nanomaterials	0.798	4.4	0.74	44	1.17
Processes	0.525	2.8	0.44	40	1.06

. To assess the relevance of the sources, the corresponding SCImago Journal Ranking indexes (SJR), Impact Factors (IFs), and Journal Citation Indicators (JCIs) of the top 14 sources, which are the only ones that published at least 40 articles, were included. The Journal Citation Indicator (JCI) is defined as the average Category Normalized Citation Impact (CNCI) of citable items (articles and reviews) published by a journal over a recent three-year period [47]. The average JCI in a category is 1.0. Journals with a JCI of 1.35 have a 35% greater citation impact than the mean for that category. This index provides a more precise understanding of a journal’s absolute relevance, whereas the SJR and IF indexes offer insights within a relative framework that facilitates direct comparisons between different journals. The two leading sources, Lecture Notes in Mechanical Engineering and Applied Sciences, accounted for over 10% of the total scientific production, with 447 and 402 documents, respectively, representing 11.89% and 10.69% contributions.

The most productive source was not a scientific journal: Lecture Notes in Mechanical Engineering publishes the latest developments in mechanical engineering reported in proceedings and post-proceedings and it is managed by the publishing company Springer. It is noteworthy that another source included in Table 8 was not a journal. The Annual Technical Conference ANTEC Conference Proceedings was the sixth most productive source, with 89 documents. These proceedings are compiled on an annual basis from a conference organized by the American Society of Plastics Engineers and showcase the latest advances in industrial, laboratory, and academic work focused on plastics and polymer science.

With regard to the second most prolific source in the ranking, Applied Sciences is a multidisciplinary journal that encompasses a range of disciplines within the applied natural sciences, engineering, and technology. Its SJR and JIF values were 0.508 and 2.5, respectively, while its JCI was equal to 0.56, indicating that the journal’s performance was below the average. The next most frequently occurring journal was Ceramics International, with 178 documents, representing 4.73% of the total. This publication is dedicated to the scientific and technological aspects of ceramic materials, encompassing their design, synthesis, characterization, and applications in diverse fields. Its SJR was 0.938, the JIF was 5.1, and the JCI was 1.27, indicating that the journal has an impact above the average performance. The third most prolific journal was Powder Technology, with 130 documents, representing 3.46% of the total production. This publication focuses on the study and application of powders and particles, including their processing, handling, and characterization methods. Its SJR was 0.970, the JIF was 4.5, and the JCI was 0.78, indicating that the performance was below average.

Indeed, most of the journals included in the ranking demonstrated under-average performance, with only four journals exhibiting JCI values above 1.0. These include the previously mentioned Ceramics International, which occupies the third position in the ranking, and the International Journal of Heat and Mass Transfer, which ranks fifth with 114 documents (JCI equal to 1.23). Additionally, Biofabrication holds the eighth position with 69 documents (JCI equal to 1.70), and Corrosion Science is ranked ninth with 58 documents (JCI equal to 1.66).

Two of the journals under consideration provided insights into two pertinent applications of additive manufacturing. On the one hand, the International Journal of Heat and Mass Transfer addresses theoretical, computational, and experimental research related to heat and mass transfer, with a particular focus on developing a fundamental understanding of these transfer processes and their application to engineering problems. It, thus, appears that the implementation of new devices created by additive manufacturing to improve heat and mass transfer represents a significant and relevant topic of investigation. On the other hand, Biofabrication is dedicated to research concerning the utilization of proteins, complete cells, and other biological materials as building blocks for the fabrication of biological systems, including therapeutic products. The advancement of fabrication technologies and the modeling of fabricated constructs and their maturation towards the intended tissue types represent a pivotal aspect of this journal’s coverage, with additive manufacturing playing a crucial role in these innovative fabrication technologies. Furthermore, Corrosion Science is focused on all aspects of both metallic and non-metallic corrosion, which can be relevant to evaluating the performance of components produced by additive manufacturing.

3.1.4. Most Frequently Cited Documents

The list of the top 10 articles, ordered according to the number of citations they have received, can be found in

Table 9. The top 10 most cited papers.

Ranking	Article	Times Cited
1	Title: 3D bioprinting of tissues and organs Author(s): Murphy S.V.; Atala A. Journal: Nature Biotechnology (2023 Impact Factor = 33.1) Year: 2014	4516
2	Title: Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review Author(s): Wang X.; Xu S.; Zhou S.; Xu W.; Leary M.; Choong P.; Qian M.; Brandt M.; Xie Y.M. Journal: Biomaterials (2023 Impact Factor = 12.8) Year: 2016	1344
3	Title: Bioink properties before, during and after 3D bioprinting Author(s): Hölzl K.; Lin S.; Tytgat L.; Van Vlierberghe S.; Gu L.; Ovsianikov A. Journal: Biofabrication (2023 Impact Factor = 8.2) Year:2016	688
4	Title: 3D printed bionic ears Author(s): Mannoor M.S.; Jiang Z.; James T.; Kong Y.L.; Malatesta K.A.; Soboyejo W.O.; Verma N.; Gracias D.H.; McAlpine M.C. Journal: Nano Letters (2023 Impact Factor = 9.6) Year: 2013	664
5	Title: Gelatin-methacrylamide hydrogels as potential biomaterials for fabrication of tissue-engineered cartilage constructs Author(s): Schuurman W.; Levett P.A.; Pot M.W.; van Weeren P.R.; Dhert W.J.A.; Hutmacher D.W.; Melchels F.P.W.; Klein T.J.; Malda J. Journal: Macromolecular Bioscience (2023 Impact Factor = 4.4) Year: 2013	585
6	Title: Biofabrication of bone tissue: Approaches, challenges and translation for bone regeneration Author(s): Tang D.; Tare R.S.; Yang L.-Y.; Williams D.F.; Ou K.-L.; Oreffo R.O.C. Journal: Biomaterials (2023 Impact Factor = 12.8) Year: 2016	442
7	Title: Auxetic mechanical metamaterials Author(s): Kolken H.M.A.; Zadpoor A.A. Journal: RSC Advances (2023 Impact Factor = 3.9) Year: 2017	428
8	Title: Metallic powder-bed based 3D printing of cellular scaffolds for orthopaedic implants: A state-of-the-art review on manufacturing, topological design, mechanical properties Author(s): Tan X.P.; Tan Y.J.; Chow C.S.L.; Tor S.B.; Yeong W.Y. Journal: Materials Science and Engineering C (2023 Impact Factor = 8.1) Year: 2017	361
9	Title: 3D printed quantum dot light-emitting diodes Author(s): Kong Y.L.; Tamargo I.A.; Kim H.; Johnson B.N.; Gupta M.K.; Koh T.-W.; Chin H.-A.; Steingart D.A.; Rand B.P.; McAlpine M.C. Journal: Nano Letters (2023 Impact Factor = 9.6) Year: 2014	349
10	Title: Polymer composites for tribological applications Author(s): Friedrich K. Journal: Advanced Industrial and Engineering Polymer Research (2023 Impact Factor = 9.9) Year: 2018	337

. The number of citations ranged from 332, which corresponded to the least cited article in the ranking, to 1181, which marks the most cited article that ranked first. While a more detailed examination of emerging research areas identified through frequently selected author keywords will be presented in the subsequent section, an analysis of highly cited publications can provide preliminary insights into the most relevant aspects that have captured the attention of researchers engaged in additive manufacturing research.

The analysis of the ranking revealed that the most frequently cited document was “3D bioprinting of tissues and organs”, published in 2014 in the journal Nature Biotechnology, with 4516 citations. This document presents a review of the principles, techniques, and applications of 3D bioprinting, an emerging field that combines tissue engineering and additive manufacturing to create artificial tissues and organs from cells, biomaterials, and growth factors [48]. The second most frequently referenced document was “Topological design and additive manufacturing of porous metals for bone scaffolds and orthopedic implants: A review”, published in 2016 in the journal Biomaterials and cited 1344 times. This document presents the current state of the art and recent advancements in the field of the additive manufacturing of porous metals for use as bone scaffolds and orthopedic implants. The paper presents a discussion of topological design methods, additive manufacturing techniques, mechanical and biological properties, and clinical applications of 3D-printed porous metals [49]. The third most cited document to conclude the top three was “Bioink properties before, during and after 3D bioprinting”, published in 2016 in the journal Biofabrication, with 688 citations. This document presents a review of the physical, chemical, and biological properties of bioinks, which are the materials used to print three-dimensional structures with living cells. The review examines the factors that influence the behavior and functionality of bioinks at each stage of the 3D bioprinting process [50].

These three most cited papers in this field refer to the use of additive manufacturing for biofabrication, with a particular focus on tissue and organ engineering. Indeed, this topic emerged as a prominent trend, as evidenced by the inclusion of other documents in the ranking that address this issue. The fourth item on the list was an article that elucidated the fabrication of bionic organs, specifically bionic ears [51]. The document in the fifth position explained the cultivation of artificial cartilage constructs based on gelatin-methacrylamide hydrogels [52]. Meanwhile, the sixth position of the ranking was occupied by a review of the biofabrication of bone tissue for bone regeneration [53]. Finally, a comprehensive review of the metallic powder-bed based 3D printing of cellular scaffolds for orthopedic implants occupied the eighth position in the ranking [54]. Out of the field of biofabrication, other topics that have been identified, and among the most important ones were the design of auxetic mechanical metamaterials [55], the manufacture of quantum dot-based light-emitting diodes [56], and the development of polymeric compounds for tribological applications [57].

3.1.5. Distribution Analysis of Keywords

The analysis of the most frequent keywords (Figure 4) revealed that the most frequent keyword was Additive Manufacturing, selected as a keyword by 1884 documents. Since “Additive Manufacturing” was selected as the search string to be entered into the search engine in the database, the fact that it occupies the first position in the ranking is not surprising. The second most frequent keyword was 3D Printers, selected as a keyword in 1869 documents. This expression pertains to the concept and general process of additive manufacturing, thereby indicating that it is a central and cross-cutting theme in this field of study. Furthermore, it is notable that numerous other keywords also refer to this same concept, including 3D Printing (605 times as a keyword), 3-D Printing (489 times), Three Dimensional Printing (319 times), 3D-printing (292 times), and Printing, Three-Dimensional (171 times). These keywords are, therefore, to be considered as forming a family with the same meaning. The third position was occupied by Additives, which was referenced 1008 times.

Other significant keywords that appeared among the most common were Tissue Engineering, with 275 occurrences, and Scaffolds (biology), with 231 occurrences. These keywords are directly related to concepts used in biofabrication and bioprinting, specifically in the creation of tissues and artificial organs, which require the use of scaffolds. In addition, keywords such as Sintering, Selective Laser Melting, and Fused Deposition Modeling (with 290, 217, and 200 occurrences, respectively) were also identified, referring to the technologies employed in additive manufacturing. The frequency of occurrence of these keywords suggests that they are significant and emerging topics within this field of study. As there are a plethora of disparate subjects, the keywords were classified into four principal categories based on their correlation with the following thematic areas:

Concept and process: These keywords referred to the general concept of additive manufacturing and the equipment used in this process. The previously mentioned Additive Manufacturing and the 3D Printing family were included in this category, and other examples included Manufacture, Fabrication, and Manufacturing Techniques.

Technologies: This group of keywords corresponded to the different technologies used in additive manufacturing, as well as their principles and characteristics. Some keywords were Deposition, Laser Heating, Stereolithography, Melting, Extrusion, Selective Laser Sintering, and Powder Bed, which were added to the previously commented Sintering, Selective Laser Melting, and Fused Deposition Modeling.

Materials and properties: These keywords represented the various materials used in additive manufacturing, as well as their properties and behaviors. Therefore, Porosity, Mechanical Properties, Particle Size, Microstructure, Polymers, Ceramics, Biomaterial, Metals, Aluminum Alloys, and Titanium Alloys were included in this category in addition to the previously mentioned Additives.

Biofabrication: This category compiled the different applications of biomaterials in the field of biomedicine, specifically bioprinting. Tissue, Tissue Scaffold, Biocompatibility, Bone, Biomechanics, Bioprinting, and Biofabrication were clear examples to accompany the previously mentioned Tissue Engineering and Scaffolds (biology).

3.2. Bibliometric Network Analysis

This section presents the primary findings of the bibliometric network analysis of the scientific literature related to chemical engineering and additive manufacturing. First, the strategic diagrams of each defined subperiod were depicted: 2002–2017 (Figure 5), 2018–2019 (Figure 6), 2020 (Figure 7), 2021 (Figure 8), and 2022 (Figure 9). The strategic diagrams presented 30 clusters in total, of which 12 were identified as motor themes, 5 were classified as basic and transversal, 8 were designated as emerging or declining themes, and 5 were categorized as highly developed and isolated themes. The structure of each cluster can be observed in Figures S1–S30 in the Supplementary Materials. The size of each cluster is an indication of the number of documents included therein. Furthermore, the total number of citations, h-index, absolute centrality, and density values are presented for each cluster in

Table 10. Number of documents, h-indexes, cites, centrality, and density values of the different clusters identified in the bibliometric network analysis.

Cluster	Number of Documents	h-index	Cites	Centrality	Density
Subperiod 1 (2002–2017)
Chemistry	118	54	11,910	386.42	103.97
Cell Culture	76	47	7657	266.77	45.20
3D Printers	369	83	27,622	148.69	38.17
Bone Prosthesis	52	36	5430	183.87	47.02
Scanning Electron Microscopy	55	35	5646	143.35	13.46
Fused Deposition Modeling	57	13	798	38.42	22.15
Melting	37	28	3323	26.58	12.30
Subperiod 2 (2018–2019)
Tissue Engineering	56	33	2866	122.11	81.57
3D Printers	466	62	14,441	124.65	31.47
Powder	44	24	2208	60.33	18.93
Bone	33	21	1723	98.50	16.78
Microstructure	53	27	2145	61.96	13.28
Fused Deposition Modeling	78	20	1341	33.36	14.78
Heat Transfer	28	16	831	13.29	6.84
Subperiod 3 (2020)
Chemistry	53	24	1827	84.35	64.35
3D Printers	377	45	8575	83.04	21.75
Selective Laser Melting	60	20	1291	25.26	32.22
Mechanical Properties	40	15	957	35.57	3.18
Subperiod 4 (2021)
Tissue Scaffold	65	20	1265	95.23	59.05
Particle Size	92	20	1368	61.73	26.67
Additives	331	30	4265	79.65	14.13
Biocompatibility	44	16	649	86.16	12.95
Fused Deposition Modeling	36	12	383	10.04	4.07
Subperiod 5 (2022)
Tissue Scaffold	55	10	380	86.87	61.56
Powder Bed	417	17	1806	88.20	51.91
Biomechanics	55	10	358	78.09	14.33
Austenitic Stainless Steel	44	8	188	40.80	33.66
3D Printing	140	13	743	61.43	8.05
Deposition	53	7	169	25.42	5.91
Selective Laser Melting	34	6	115	27.51	7.95

. Thematic evolution is illustrated in Figure 10, which depicts the progression of the research field across the various subperiods addressed in this study.

The dynamic conceptual evolution of scientific research related to additive manufacturing and chemical engineering was investigated through the analysis of the thematic evolution. Paying attention to Figure 10, three main families of clusters can be highlighted. These families of clusters were clearly identified because they showed continuous evolution from the initial to the last subperiods throughout the complete time interval selected to be studied, meaning there were no lapses in their evolution. This fact revealed that they were attractive thematic areas for researchers throughout all the subperiods considered.

The first cluster family found began with the cluster “Chemistry” in the first subperiod and its structure can be observed in Figure S1. A large number of connections between keywords were observed in this cluster, indicating a high level of cohesion and relevance among them. Keywords with strong connections include Scaffolds, Tissue Scaffold, Tissue Engineering, and Chemistry. In the second subperiod, the cluster was renamed “Tissue Scaffold” (Figure S8). New keywords emerged there such as Bioprinting, Bone Regeneration, and Tissue Regeneration. During the third period, the cluster reverted to the original name “Chemistry” (Figure S15). The connections remained quite similar and the keyword Procedures appeared, which in subsequent subperiods was maintained and presented significant connections. In the fourth period, the cluster changed its name to “Tissue Scaffold” (Figure S19). The keyword Hydrogels was incorporated and it presented a strong connection with Tissue Scaffold. Finally, in the fifth subperiod, the cluster retained its name (Figure S24), and the appearance of the keyword Polyesters must be mentioned. As can be observed in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9, the five clusters that formed this first family were all of motor themes. Moreover, they presented the highest density values and some of them also exhibited the highest centrality values (specifically during the first, third, and fourth subperiods). Consequently, the relevance of this cluster family within the research scenario was beyond any doubt. This cluster family showed a consistent and coherent evolution across subperiods, maintaining connections among the most relevant keywords. The thematic topic represented by this cluster family could be directly correlated to the keyword category Biofabrication identified in the previous section during the keyword analysis. It reflected the growing interest in bioprinting and biomedical applications, especially in relation to the manufacturing of implants, prosthetics, scaffolds, and artificial organs. These applications of additive manufacturing have the potential to improve the quality of life for people suffering from diseases or injuries affecting their tissues or organs. Although these applications fall out of traditional chemical engineering, contributions from the researchers in this field to this topic must be taken into account.

The second cluster family started in the cluster “3D Printers” from the first subperiod (Figure S3). Several connections among quite general keywords, such as 3D Printers, Additive Manufacturing, 3D Printing, Manufacture, or Printing occurred, but the strong link between Sintering and Laser Heating was worthy of mention. In the second subperiod, the cluster retained its name (Figure S9). New keywords appeared, such as Scanning Electron Microscopy, Selective Laser Melting, and Additives. The important connection between Sintering and Laser Heating remained the most relevant within the cluster structure. In the third subperiod, the theme continued with the same name (Figure S16) and connections remained very similar, except for the disappearance of the connection between Sintering and Laser Heating, as the latter was not included as a keyword in the cluster structure. The cluster name changed to “Additives” in the fourth subperiod and connections among keywords were weaker (Figure S21). The appearance of the keywords Aluminum Alloys and Ternary Alloys was noticeable, as their link was strong. Finally, in the fifth subperiod, the cluster was renamed “Powder Bed” (Figure S25). In this subperiod, connections among themes were reinforced and appeared stronger, highlighting the connections among Powder Bed, Powder Bed Fusion, Laser Powder, and Laser Powder Bed Fusion. This second cluster family showed variable and inconsistent evolution across subperiods, reflected in the change in the cluster names, the weakening of connections among keywords, and the movements among different quadrants in the corresponding strategic diagrams (Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9). Although the clusters were motor themes in the second, fourth, and last subperiod, it started as a highly developed and isolated theme from the fourth quadrant in the first subperiod and appeared as a basic and transversal theme from the second quadrant in the third subperiod. Initially, the cluster focused on 3D printing and various general topics of additive manufacturing, which could be easily correlated with the keywords included in the category concept and process during the keyword analysis carried out in the previous section. However, the focus shifted to additives and powder beds (especially during the last two subperiods), which are concepts related to a specific manufacturing technique and the corresponding materials employed. The strong link between Sintering and Laser Heating during the first two subperiods also justified the importance of Selective Laser Sintering (a technology included in Powder Bed Fusion processes), as well as the presence of metallic materials such as aluminum alloys and ternary alloys. Selective Laser Sintering is an additive manufacturing technique where a laser selectively sinters powdered material from a bed, typically metals, layer by layer at specific points in space defined by the corresponding 3D model, without the need for support structures to produce complex geometries [58].

Regarding the third cluster family identified, their clusters were characterized by fewer connections among the corresponding keywords and, in addition, these were weaker than the strong connections found in the two other families. In the first and second periods, the clusters were called “Fused Deposition Modeling” (Figures S7 and S13 represent the cluster structures in the first and second subperiods, respectively), with highlighted connections between the keywords Elastomers and Plastic Products. Additionally, in the second subperiod, keywords directly related to polymeric materials, such as Acrylonitrile–Butadiene–Styrene and Polylactic Acid, appeared. In the third and fourth subperiods, the cluster suffered name modifications and changed from “Mechanical Properties” (Figure S18) to recover once again “Fused Deposition Modeling” (Figure S23), with sparse connections during these periods. Only the connection between Polylactic Acid and Fused Deposition Modeling stood out. Finally, in the fifth subperiod, the cluster name was modified to “Deposition” (Figure S29), with continued weak connections and only the link between Layered Manufacturing and Deposition was remarkable. This third cluster family was more specific and limited, as it exhibited a more discontinuous and irregular evolution across the subperiods, with fewer and weaker connections among keywords. Indeed, all the clusters fell in the third quadrant, so they could be considered emerging themes that have not yet taken off. The clusters were primarily related to Fused Deposition Modeling, one of the most popular technologies for additive manufacturing (a technology included in material extrusion processes). Fused Deposition Modeling uses a continuous filament of a thermoplastic polymeric material to feed a moving heated printer extruder head, which deposits the material on the growing work [59]. Within the structures of these clusters, keywords related to the most used thermoplastic materials can be found, such as acrylonitrile–butadiene–styrene and polylactic acid, although the application of this technique to elastomers has received greater attention in recent years [60].

In contrast to these three fully developed cluster families, the presence of two couples of clusters with strong links but no longer continuity should be commented on. On the one hand, the first couple began with the cluster “Microstructure” in the second subperiod (Figure S12). Its structure revealed the strong connection between the keywords Aluminum Alloys and Titanium Alloys, appearing also important connections among Corrosion, Corrosive Effects, and Corrosion Resistance. In the following subperiod, the cluster changed its name to “Selective Laser Melting” (Figure S17), where similar connections were observed: the relationships between Aluminum Alloys and Titanium Alloys were reinforced and a third very strong connection with Ternary Alloys emerged. While the first cluster was classified as a basic and transversal theme because of its position in Figure 6, the second one fell in the fourth quadrant (Figure 7) and must be considered a highly developed and isolated theme. The thematic area represented by these two clusters reflected an interest in the microstructural aspects of produced items, particularly those made of metallic materials through the Selective Laser Melting technique. The most used and studied materials with this technique are aluminum and titanium alloys, which exhibit superior mechanical, thermal, and chemical properties compared to conventional metals [61,62]. However, these materials also present challenges related to corrosion and corrosive effects, which impact their durability and reliability. On the other hand, the second couple started in the fourth subperiod with the cluster “Biocompatibility” (Figure S22). Several connections were observed when the cluster structure was analyzed, mostly of similar magnitude, with a notable emphasis on the connection between the keywords Biocompatible Materials and Polymers. In the last subperiod, the theme was renamed to “Biomechanics” (Figure S26). During this subperiod, the strongest connections were modified, highlighting two main ones: the one between Polymers and Unclassified Drug, and the one linking Cell Culture and Cell Engineering. The topics derived from this second couple could be directly related to the first cluster family and the keyword category Biofabrication. The biocompatibility and biomechanics of materials produced by additive manufacturing have gained relevance and the interest in the manipulation and cultivation of cells in controlled environments to investigate the interactions among these materials and additional drugs in the framework of cellular processes and tissue engineering is worth mentioning [63,64,65,66,67]. The increasing importance of these research topics was confirmed by the evolution from a basic and transversal theme represented by the cluster in the fourth subperiod (Figure 8) to a motor theme in the last subperiod (Figure 9).

After a review of the clusters representing motor themes not previously mentioned, the presence of “Cell Culture” and “Physiology” in the first subperiod and “Cell Proliferation” in the second subperiod must be highlighted. The cluster “Cell Culture” (Figure S2) included keywords such as Cells, Cell Engineering, Cell Adhesion, Cell Proliferation, and Cell Survival, which clearly invoked once again the topics related to the keyword category Biofabrication, where the interactions between cells and materials produced by additive manufacturing are critical. The cluster “Physiology” (Figure S4) presented a very strong connection between the keywords Bone Prosthesis and Bone Substitutes, appearing additionally with other related keywords such as Bone, Bone Regeneration, and Bone Tissue Engineering. Consequently, again another topic linked to the keyword category Biofabrication was identified, which was focused on the role of additive manufacturing in the synthesis of bone substitutes and bone tissue engineering. Finally, the cluster “Cell Proliferation” (Figure S11) provided a relative continuity to the cluster “Cell Culture” from the first subperiod, with a strong link between the keywords Cell Culture and Cell Proliferation and the appearance of keywords such as Biomaterial, Biocompatibility, and Biocompatible Materials.

4. Review of Research Focused on Additive Manufacturing and Chemical Engineering

While an exhaustively detailed examination of the extensive scientific bibliography related to the interdisciplinary collaboration between chemical engineering and additive manufacturing is beyond the scope of this study, this section aims to provide an overview of the most relevant issues in this field.

4.1. Additive Manufacturing and Biofabrication

Biofabrication, bioprinting, and biomedical applications have been identified as a notable focus in the bibliometric study, with the keyword analysis and the network mapping underscoring its significance. The findings indicated that this topic has become a well-established research area, receiving considerable attention. The impact of this topic is such that even when the search was limited to the chemical engineering subject area, it was identified as a central spotlight. Biofabrication involves the automated generation of biologically functional products with structural organization using bioactive molecules; biomaterials; living cells; cell aggregates such as micro-tissues and micro-organs on chips; and hybrid cell–material constructs. This is achieved through bioprinting or bioassembly, followed by the subsequent tissue maturation processes [68]. Bioprinting specifically refers to the use of additive manufacturing techniques to combine living and non-living materials (including cells, growth factors, bioinks, and biomaterials) to fabricate bioengineered functional structures [48]. These structures can be utilized in a variety of fields, including regenerative medicine, pharmacokinetics, and basic cellular biology studies, although recent efforts have expanded their use to new applications such as biosensing and environmental remediation. Bioassembly refers to the fabrication of hierarchical constructs with a prescribed 2D or 3D organization through the automated assembly of pre-formed cell-containing fabrication units generated via cell-driven self-organization or through the preparation of hybrid cell–material building blocks, typically by applying enabling technologies, including microfabricated molds or microfluidic [68]. Therefore, bioassembly techniques do not require bioprinters for the assembly of larger cellular and material constructions from smaller components. Biomanufacturing in general and bioprinting in particular provide core and vital support for tissue engineering and regenerative medicine. While tissue engineering considers the use of physical, chemical, biological, and engineering processes to control and direct the aggregate behavior of cells, regenerative medicine has been defined as the application of tissue science, tissue engineering, and related biological and engineering principles that restore the structure and function of damaged tissues and organs [68].

The process of bioprinting requires the previous preparation of the model required by the printing device and the materials to be used. The anatomical configuration of the functional biostructure must be delineated through the utilization of appropriate imaging methodologies, including computed tomography and magnetic resonance imaging. Subsequently, specialized software is employed to translate the image into a CAD drawing of cross-sectional layers and the corresponding final format file, thus enabling the printing device to add them in a layer-by-layer process. In addition, the liquid mixtures of cells, matrix, and nutrients known as bioinks must be prepared, although other biological materials can be employed. Bioprinting for fabricating biological constructs typically involves dispensing these bioinks and other additive factors onto a biocompatible scaffold using a successive layer-by-layer approach to generate tissue-like three-dimensional structures. The precision, stability, and viability of these biological constructs are highly dependent on the employed technologies. Subsequently, the constructed biostructure must be subjected to further processing in a bioreactor, which recreates the requisite in vivo environment for maintaining tissue viability during the maturation period. To create a stable structure from this bioprinted construct, both mechanical and chemical stimulations are needed, since these stimulations send signals to the cells to control the remodeling and growth of tissues [69].

4.1.1. Bioprinting Technologies

There are three main technologies for bioprinting: inkjet bioprinting, microextrusion bioprinting, and laser-assisted bioprinting [48]. The selection of these technologies should take into account crucial bioprinting factors such as surface resolution, cell viability, and the choice of biological materials.

Inkjet bioprinting is able to generate microscale organization of deposited cells without compromising cell viability or inducing damage. Additionally, this approach offers the possibility of using multiple printheads to simultaneously print various cell types (high throughput), along with bioprinting biomolecules alongside biomaterials, thereby enabling the fabrication of complex multicellular constructs [53]. The two most commonly used methods for cell printing by inkjet are thermal and piezoelectric printing. Printers have been adapted to precisely print picolitre drops of biological materials at a microscale. On the one hand, in thermal bioprinting, the system produces pulses of pressure by vaporization using a heating element, leading to the expulsion of the droplet from the printhead. Despite the demonstration that this localized heating does not have a substantial impact on the stability of biological molecules, the generated heat and resulting evaporation can cause thermal and mechanical stress on the deposited cells and create transient pores in the cell membrane. Nevertheless, the advantages of thermal inkjet printers include high print speed, low cost, and wide availability. On the other hand, in piezoelectric printing, heating is not used; instead, a direct mechanical impulse is applied to the fluid in the nozzle through a piezoelectric actuator, creating an acoustic shockwave that forces the bioink through the nozzle [70]. The advantages of piezoelectric inkjet printers include the ability to generate and control a uniform droplet size and ejection directionality as well as to avoid exposure of the biological molecules to heat and pressure stressors. However, piezoelectric bioprinters are more restricted to material viscosity (ideally below 10 centipoise) due to the excessive force required to eject drops using solutions at higher viscosities. In both inkjet bioprinting cases, after the material is deposited, gelation of the drops must occur immediately through physical or chemical crosslinking to ensure structure stability and preserve shape fidelity, which slows the bioprinting process and involves modification of the bioprinted materials, changing both their chemical and physical properties [71].

Microextrusion bioprinting is the most prevalent technology in this field due to its relatively low cost, accessibility, and adaptability. These systems typically entail the use of multiple syringe-like disposable containers loaded with desired combinations of biomaterials and/or cells, which yield continuous beads of material rather than liquid droplets. Bioinks, hydrogels, biocompatible copolymers, and cell spheroids can be dispensed through a nozzle by the action of pneumatic or mechanical forces (piston-driven or screw-driven) [53]. Pneumatically driven bioprinters have the advantage of having simpler drive-mechanism components. Generally, piston deposition provides more direct control over the flow of bioinks from the nozzle due to the delay of compressed gas volume in pneumatic systems, whereas screw-based systems can offer greater spatial control and are beneficial for dosing bioinks with higher viscosities [71]. Microextrusion bioprinting technology allows the deposition of higher cell densities than inkjet bioprinting and can manage materials within a very wide range of viscosities. Nevertheless, cell viability after microextrusion bioprinting is lower than that with inkjet-based bioprinting, mainly because of the shear stresses inflicted on cells in more viscous fluids.

In laser-assisted bioprinting, a fast-pulsed laser beam is used to selectively propel bioink from a donor layer onto a receiving substrate. This process utilizes a donor layer made of a laser-energy-absorbing material (such as gold or titanium) supported on glass and coated with the biological material (bioink, hydrogel, or cells). When a high-energy pulsed laser is focused on the absorbing layer, it vaporizes a small amount of the donor layer, transferring energy to the substrate where the bioink is present. This generates a high-pressure bubble and creates a small droplet, which is transferred toward the collector substrate [70]. This technology enables precise deposition of materials and high cell densities in relatively small three-dimensional structures without negatively affecting cell viability or function. It is a nozzle-free method, thus not affected by clogging issues and compatible with a range of viscosities. However, the high resolution of this process complicates the uniform distribution of cells in ejected droplets, requires rapid gelation kinetics for high shape fidelity, and results in relatively low overall throughput. Consequently, generating larger, clinically relevant three-dimensional constructs using this method is time-consuming, limiting its widespread application [71].

In evaluating these technologies, it is essential to consider the relative merits and limitations of each, taking into account the specific material and intended application. Inkjet and microextrusion bioprinters offer distinctive advantages in terms of simplicity, flexibility, and relatively low cost. However, these technologies also have inherent limitations. For instance, inkjet bioprinting is susceptible to cell sedimentation due to the low viscosity of bioinks, while microextrusion bioprinting often results in reduced cell viability, which can be attributed to bioink viscosity and shear stress during deposition [72]. Laser-assisted bioprinting offers a number of advantages, including high spatial resolution and the capacity to fabricate three-dimensional biomimetic structures with intricate architectures. However, it requires the use of bioinks with particular gelation properties and typically entails elevated manufacturing costs [70].

Contributions from the chemical engineering research field have improved all these three main technologies for bioprinting. For instance, an improved inkjet bioprinting approach was developed to create mechanically strong bone and cartilage tissue constructs using poly(ethylene glycol) dimethacrylate, gelatin methacrylate, and human mesenchymal stem cells [73]. The adequate design of a simultaneous instant photopolymerization process during the bioprinting was critical to obtain constructs with improved properties. Moreover, a sonochemical treatment was developed to control the viscoelasticity of a gelatin methacryloyl-based bioink to be applied by inkjet bioprinting by shortening the length of polymer chains without causing chemical destruction of the methacryloyl groups [74]. This approach effectively increased the maximum printable polymer concentration from 3% to 10% and the control of the mechanical properties resulted in fibroblasts with higher spreading on the hydrogels. Microextrusion bioprinting also can be improved by chemical engineering contributions. Scaffolds produced by traditional microextrusion bioprinting lack collagen anisotropy, which provides essential topographical cues to guide tissue-specific cell function. When extrusion-based bioprinting was combined with a magnetic alignment approach in an innovative 4D printing scheme, tridimensional collagen scaffolds with a high degree of collagen anisotropy were obtained, where human mesenchymal stem cells orient uniformly [75]. More importantly, the obtained collagen anisotropy was found to trigger tendon- or ligament-like differentiation. In another example, the production by the microextrusion-based bioprinting of core/shell constructs to develop millimeter-thick scaffolds with embedded microvasculature for potential soft tissue repair was proposed [76]. The photocuring time was critical for these engineered structures, enabling continuous hollow inner cores that could be successfully printed within the scaffolds and allowing the incorporation of endothelial cells during the bioprinting process to form micro-vessels embedded in the constructs. In the case of laser-assisted bioprinting, studies of mass and energy transfer were required to evaluate the potential Laser-Induced Propulsion of Mesoscopic Objects (LIPMO), a method based on the direct transfer of laser optical energy into kinetic energy in the vicinity of the object to be ejected [77]. Using this method, the bioprinting of multicellular cartilaginous spheroids of up to 300 µm was achieved, surpassing the 100 - 150 µm range that limits other laser-assisted bioprinting techniques.

4.1.2. Bioprinting Materials

Regarding the materials used for bioprinting, bioinks and biomaterial inks can be distinguished. A bioink is defined as a formulation of suitable cells for processing using automated biofabrication technology, which may also contain biologically active components and biomaterials [78]. For clarity and differentiation purposes, biomaterials that can be printed and subsequently seeded with cells after printing, but are not directly formulated with cells, are not classified as bioinks and are named biomaterial inks. Such biomaterial inks may be employed in the fabrication of scaffolds for cell seeding, bioreactors, or implants. For bioinks, cells are an indispensable component of the formulation, which may take the form of individual cells, coated cells, cellular aggregates, or a combination of cells with other materials. In contrast, biomaterial inks use a biomaterial for printing, and cell contact occurs after biofabrication. Biomaterial inks are typically used to produce a rigid scaffold for permanent stabilization or slow degradation of the printed structure, whereas bioinks produce a softer scaffold that can be more rapidly replaced by the deposition of a new extracellular matrix by the embedded cell population [71].

Moreover, hydrogels are three-dimensional polymeric networks that are insoluble in water and exhibit the property of swelling to reach an equilibrium volume without losing their shape. Their capacity to swell in biological conditions renders them an optimal category of materials for tissue engineering. Thanks to their elevated water content, they can imitate the extracellular matrix of native tissues, achieving a closer approximation to natural soft tissues compared to other biomaterials (Figure 11) [79]. These hydrogels can be derived from natural or synthetic sources [80] and a multitude of bioinks have been developed from natural hydrogels, which are well established as suitable materials for three-dimensional cell cultures [81]. These natural materials exhibit excellent biocompatibility and are highly tunable in terms of their physical, mechanical, and biological properties [82] and they often exhibit non-Newtonian and shear-thinning behavior, making them suitable for extrusion-based bioprinting. When compared to natural hydrogels, synthetic hydrogels exhibit greater reproducibility with respect to their physical and chemical properties. They have become a significant alternative for scaffold fabrication, as they can be molecularly tailored with respect to block structures, molecular weights, mechanical strength, and biodegradability [79]. Synthetic hydrogels can only provide structural support and do not inherently promote cellular function, as they require chemical functionalization to integrate bioactivity into their structure [80]. Many synthetic hydrogels exhibit non-Newtonian properties like biopolymeric ones, allowing them to be extrusion-printed.

The use of single-component bioinks offers limited value in bioprinting. However, the properties required for optimal printability and print fidelity often conflict with those that enhance cell viability and functions. For instance, natural hydrogels typically exhibit weak mechanical properties, whereas synthetic hydrogels lack bioactivity. Additionally, natural biological tissues are composite materials with anisotropic properties dependent on their components [83]. To address these challenges, various strategies have been developed, including the use of composite hydrogels. By combining favorable biological properties with enhanced structural reinforcement, these composite hydrogels achieve improved printability and precision in bioprinting. Nanocomposites play a significant role in this development. These bioinks aim to mimic native tissues by crosslinking polymers with inorganic or organic nanofillers such as metals, polymers, or ceramics. The presence of these nanoparticles alters the mechanical, physical, and chemical properties of the bioinks, thereby enhancing their structural and functional properties [84]. Commonly used inorganic nanomaterials for developing nanocomposite hydrogels include metal nanoparticles (gold or silver), mineral nanoparticles (natural silicate clay, laponite, or layered double hydroxides as examples), magnetic nanoparticles (typically based on magnetite), carbon nanoparticles (graphene and derivatives like graphene oxide) and polymer nanoparticles such as nanocellulose [85]. Polymeric hydrogels prepared from blends of two natural and synthetic polymers are frequent. Among these, interpenetrating polymer networks are particularly notable, being unique crosslinked polymer blends where at least one network is synthesized/crosslinked in the presence of the other. These can be divided into double networks and ionically–covalently crosslinked networks. While double networks exhibit extremely high mechanical strength and toughness (superior to single-component networks of either constituent polymer and comparable to cartilage and rubbers), ionically–covalently crosslinked networks serve to dissipate energy from applied loads with covalently crosslinked polymer networks that maintain the hydrogel’s elasticity [86].

Researchers with expertise in chemical engineering have been involved in the formulation of more sustainable hydrogels, bioinks, and biomaterial inks. The optimal composition of natural polymers, including silk fibroin, gelatin, and alginate, in a bioink for bioprinting in bone tissue engineering was investigated [87]. Through meticulous parameter optimization, the bioink achieved the highest print accuracy with 4% alginate content, which demonstrated excellent biocompatibility and enhanced alkaline phosphatase activity in osteoblast-like cells when supplemented with alendronate. The use of natural materials derived from plant and animal sources for the production of biomaterial inks has been explored, considering xanthan gum [88], fish scale particles [89], or powdered fenugreek (an annual plant cultivated worldwide as a semiarid crop, especially in the Indian subcontinent) [90]. The addition of xanthan gum enabled further tuning of biodegradability and an improvement in gelatin methacryloyl printability and shape fidelity. Fish scale as a natural collagen and hydroxyapatite source contributed to the biomaterial ink properties for bone engineering applications, providing additional stiffness and osteoinductivity to the hydrogels. Composite scaffolds using polycaprolactone, silk fibroin, bovine bone, and fenugreek powder exhibited enhanced mechanical durability and improved capacity to generate hydroxyapatite in vitro. A highly controllable method to optimize the printability of hydrogels based on internally crosslinked polysaccharides without the need for additives or post-printing treatments has been developed [91]. By introducing pH as a further parameter to be controlled, multiple pH-dependent crosslinking kinetics were possible without varying hydrogel composition. In addition, the results indicated that cells survived and proliferated following bioprinting, which can also interact and reorganize the hydrogel microstructure. In addition to the composition of biomaterials, modification of the bioprinting process itself also influences the properties of the obtained elements. The effects of various printing process parameters (including the air pressure for dispensing, dispensing head movement speed, and crosslinking conditions) on bioprinted Schwann cell-encapsulated scaffolds using composite hydrogels of alginate, fibrin, hyaluronic acid, and/or RGD peptide were investigated [92]. Regulating these parameters, mechanically stable scaffolds with fully interconnected pores were obtained, where the performance of Schwann cells was improved in terms of viability, proliferation, orientation, and ability to produce laminin.

4.1.3. Bioprinting Applications

The field of bioprinting for organ transplantation and regeneration has witnessed considerable advancement in the creation of thick living cellular structures, which are regarded as an intermediate phase towards organ-level complexity. Despite intrinsic biological and technical constraints, additive manufacturing holds considerable promise in printing complete organs, thanks to its ability to arrange cells hierarchically and build tissue blocks within a three-dimensional microenvironment [69]. In recent years, notable progress has been made in constructing various types of thick tissues with diverse shapes, ultimately aiming to print complete organs or body parts for transplant use. Harvesting a patient’s stem cells and printing them into a replacement organ offers the potential to circumvent the complications associated with transplants, such as long donor waiting periods or the risk of immunological rejection of the transplanted organ. Multiple recent advancements have demonstrated the successful 3D bioprinting of tissues, leading to the creation of organ-level structures, including bones [53,93], cartilage [94], heart [95], and skin [96]. While the fabrication of fully developed vascularized organs would facilitate the production of fully functional human organs suitable for surgical implantation, the realization of this objective still confronts a multitude of challenges, particularly with regard to post-processing remodeling associated with tissue fusion, shrinkage, and compaction of the printed soft tissue [69].

The bioprinting of organs lies far beyond the traditional scope of chemical engineering, yet significant contributions have been made in the preliminary stages toward achieving this goal, providing useful scaffolds or improved bioinks. Despite the high demand for hexagonal scaffolds in liver lobule regeneration, traditional fabrication methods are inadequate to produce their intricate three-dimensional design, making additive manufacturing a better choice. Different hexagonal scaffolds were developed for liver lobule regeneration via bioprinting using biodegradable and photocurable polymeric materials: poly(glycerol sebacate) acrylate (PGSA) and poly(ethylene glycol) diacrylate (PEGDA) [97]. The scaffolds were fabricated with diverse structures, varying surface areas, and three-dimensional designs to optimize cell seeding density and improve culture medium diffusivity. A biocompatible and printable instant gelation hydrogel system was developed based on a designed hyperbranched poly(ethylene glycol)-based multihydrazide macro-crosslinker and an aldehyde-functionalized hyaluronic acid [98]. The reversible crosslinking mechanism between the hydrazide and aldehyde groups provided shear-thinning and self-healing properties to the hydrogel. Moreover, the fast sol-to-gel transition of the hydrogel protected the cells during the bioprinting procedure, avoided their damage during extrusion, and improved the transplanted cell survival, demonstrating great potential to be applied to organ bioprinting.

The creation of biomimetic structures within adequate environments for interactions between cells and extracellular matrix can be achieved by bioprinting, thereby mimicking in vivo conditions with high-resolution vascularized tissue. These bioprinted tissues could serve as valuable tools for generating physiologically relevant in vitro models of human organs, useful for drug toxicity testing and disease modeling, and accurately replicating key physiological aspects of the human body. Combining bioprinting with microfluidic technology opens up the possibility of developing miniature in vitro tissue models known as organs-on-chips, which surpass the limitations of traditional models [99,100]. These tissue models circumvent the ethical challenges associated with animal models, making them an appealing option for various industries. However, they have not yet been systematically validated for reliably predicting toxicity. To fully leverage these models in high-throughput drug discovery, systematic validations and standardizations are necessary to ensure their potential effectiveness [69].

A widespread biological component in many organ-on-chip devices is an engineered vascular interface since it is present in almost all organs of the human body. The vasculature and the vascular interface function as the conduits for inter-cellular and inter-organ interactions and regulate drug transport, being particularly susceptible to biomechanical forces. Chemical engineering has contributed with different approaches applied to modeling human vasculature with an emphasis on the engineering of organ-specific vasculatures [101]. Additionally, chemical engineering research has proven its capacity to enhance organs-on-chip and improve the production processes. On the one hand, a conductive ink including poly-(3,4-ethylene-dioxythiophene)-polystyrene sulfonate microparticles doped in polyethylene glycol diacrylate was developed to integrate tunable hydrogel electrodes into bioprinted organs-on-chips [102]. The prepared electrodes successfully induced electrical stimulation to control cell orientations, morphology, and gene expressions when encapsulated in gelatin methacryloyl, resulting in higher cellular activities, cell densities, and spheroid sizes. On the other hand, many current organs-on-a-chip devices are produced via handcrafted processes, which require the expertise and abilities of skilled operators. Automated and scalable fabrication methods are preferred to generate these devices, and recent research efforts are focused on this objective. A high-throughput heart-on-a-chip platform incorporating fluorescent nanocomposite microwires as force sensors produced from quantum dots and thermoplastic elastomer and 3D printed on top of a polystyrene tissue culture base patterned by hot embossing has been developed [103]. The increased fabrication efficiency allowed the availability of numerous units of these ease-of-use systems to gauge drug responses in matured cardiac tissue.

The combination of additive manufacturing, scanning, and imaging techniques offers a new paradigm for medical treatment, providing faster, more economical, and efficient solutions that alter the traditional supply chain of the medical industry. For example, the adoption of additive manufacturing enables the mass production of precise anatomical models that can be used by orthopedic surgeons for surgical planning purposes. Additive manufacturing allows the production of implants with complex internal geometries and structures, as well as custom medical implants that meet specific individual needs based on patients’ computed tomography scans and magnetic resonance images, making them highly promising for clinical applications [104]. Tools, instruments, and parts for medical devices that enhance a clinical operation can also be obtained by additive manufacturing and external medical guide supports, such as long-term postoperative supports, external prostheses, prosthesis sockets, motion guides, fixators, personalized splints, and other orthopedic complements (Figure 12) [105].

Research by chemical engineers has been relevant for the identification of optimal materials and additive manufacturing technologies for the production of anatomical models. For example, different materials across a broad range of 3D printing technologies were evaluated to identify the combination that most precisely represents the parietal region of the skull for burr hole simulation [106]. The study found that 3D-printed polyethylene terephthalate glycol (using fused filament fabrication) and White Resin (using stereolithography) were the best models to replicate the skull. Even the main safety aspects of the drilling of these anatomical models were evaluated by measuring the presence of airborne particles and volatile organic compounds [107]. While the volatile organic compounds measurements for all materials were found to be below safety thresholds, and therefore not harmful, the particulate matter for White Resin and BoneSTN was found to be above the threshold value at PM₁₀, which could be harmful for long periods of exposure without personal protective equipment. Regarding implants, the selection of the most adequate material is also a critical task. Patient-specific composite bioimplants, consisting of crosslinked poly(trimethylene carbonate) (PTMC) networks and β-tricalcium phosphate (TCP), were tested in vivo in twelve Göttingen minipigs [108]. Histopathologic evaluation reveals that neat PTMC bioimplant surfaces were largely covered with fibrous tissue, while in the PTMC+TCP bioimplants, bones attached directly to the implant surface showed good osteoconduction and histological signs of osteoinductivity. However, these PTMC+TCP bioimplants were associated with a higher incidence of necrosis and infection. In the case of external prostheses and other auxiliary medical items, the selection of materials, additive manufacturing technologies, and post-processing alternatives have been investigated too. The cytotoxicity of 3D-printed dental splints made of several materials with different fabrication orientations and post-processing treatments (manual cleaning, automatic washing, or postpolymerization) was evaluated [109]. Whereas the manufacturing orientation did not affect toxicity, both the tested materials (Luxaprint OrthoPlus based on bisphenol A dimethacrylate and V-Print based on various acrylates) provided lower cytotoxicity when standardized washing and adapted post cure were considered.

Lastly, the production of pharmaceutical drugs by additive manufacturing could significantly modify the global healthcare industry, potentially saving costs by minimizing the waste associated with unused or expired medicines. A notable advantage of employing additive manufacturing in the pharmaceutical sector lies in its ability to easily customize medication treatments to meet each patient’s individual needs. Furthermore, there is the possibility of producing medicines on demand, and, this way, the long-term stability of drugs becomes less critical, as these could be printed as needed, enabling the design of more effective drugs with quicker onset by not requiring an extended shelf life [110].

The role of polymeric materials in the additive manufacturing of oral pharmaceutical dosage forms and appropriate characterization methods for understanding and optimizing additive manufacturing processibility and product performance have been reviewed [111]. Several polysaccharides, cellulose derivatives, and polymer-based surfactants were highlighted for their potential in specific additive manufacturing technologies to produce oral drugs. The most important polymer properties for melt-based additive manufacturing are thermal properties and melt rheology, while for solvent-based additive manufacturing, the rheology and surface tension of printing inks are critical. Most of the characterization techniques already used to assess product performance appeared equally suitable for assessing material suitability for drug printability. More specific examples of research have explored the combination of materials and additive manufacturing technologies for drug production. For instance, different ethylene-vinyl acetate copolymers were selected and loaded with 50% metoprolol tartrate to investigate the potential of fused filament fabrication to obtain 3D-printed tablets [112]. The drug-loaded filament with 18% vinyl acetate copolymer was the most promising printable formulation due to its appropriate mechanical and rheological properties. However, poor flowability out of the extrusion nozzle due to the rheological limitation excluded this formulation, so filaments were pelletized to feed a screw-based 3D printer. All drug-loaded pellets were successfully printed, enabling the production of tablets with the highest vinyl acetate content. In addition, the coupling of the final steps of drug production and additive manufacturing has been investigated too [113]. An additive manufacturing system based on Drop-on-Demand printing was integrated with crystallization, which is frequently the final step of active ingredients manufacturing, including a three-phase settling unit operation. Experimental demonstration of the proposed system was carried out for two different active ingredients: celecoxib and lomustine.

4.2. Additive Manufacturing and Chemical Engineering: Future Needs

In this subsection, research topics that have not been prominently highlighted during bibliometric analysis were commented on. This suggests either they have not been fully integrated or have not received adequate attention, especially when compared to topics related to biofabrication, which overshadow other areas. Therefore, the topics discussed here may be considered as future needs or research gaps. Firstly, two fundamental topics where chemical engineering can contribute significantly to additive manufacturing were explored: life cycle assessment and design optimization methods. Next, selected research areas essential to chemical engineering that should not be overlooked are presented. These areas are classified as priority applications of chemical engineering with significant potential for impactful contributions from additive manufacturing to achieve process intensification [114,115,116]. Among the most relevant applications are heat exchangers, catalysts and sorbents, microreactors, and separation membranes.

4.2.1. Additive Manufacturing and Life Cycle Assessment

In its ongoing pursuit of more suitable processes, chemical engineering can utilize life cycle analysis to assess and enhance the sustainability of additive manufacturing. At each stage of the life cycle of a product obtained by additive manufacturing, resources such as raw materials, energy, and other consumables (compressed air, inert gas, refrigerants, etc.) are consumed, leading to waste generation and environmental emissions. Life cycle assessment is a widely recognized and internationally standardized methodology (ISO 14040) used to calculate the environmental impacts of a product or process across its various life cycle stages (Figure 13).

The specific evidence on how the existing applications of different additive manufacturing technologies promote the circular economy transition has been exposed [118,119,120,121,122]. Additive manufacturing has the potential to greatly impact supply chains in a number of positive ways, including increased responsiveness to changing demands, shorter production lead times, lower material usage and waste, customizability, localized production, energy efficiency, and reduced emissions [123]. Some additive manufacturing processes have the potential to close supply chain processes by repairing and remanufacturing in situ defective objects, thereby significantly reducing environmental impacts [124,125]. For instance, an automated laser cladding system was designed to repair worn-out parts assisted by 3D scanning technology [126]. The validation results demonstrated that the streamlined process can reduce the processing time even for high-complexity geometries and at the same time build toolpath strategies taking into consideration process-specific needs such as dimensional accuracy. Furthermore, additive manufacturing is highly resource-efficient, decreasing the required amounts of raw materials when compared to traditional manufacturing [127]. This efficiency in material use is particularly relevant in reducing material waste, a key aspect of sustainability in manufacturing. However, it is important to note that material efficiency does not automatically guarantee a more sustainable process or product. Although additive manufacturing offers significant environmental advantages such as reduced material waste, other aspects like energy consumption and the quality of the final product must be carefully evaluated. For instance, the specific energy consumption of additive manufacturing processes can be 10 to 100 times higher than that of conventional molding and machining processes [128]. Additionally, achieving the desired dimensional accuracy and surface finish of products obtained by additive manufacturing may require post-processing operations such as final machining and heat treatments, which imply additional raw materials and energy consumption [129].

Assessing the energy consumption and environmental impact of the raw materials used in additive manufacturing is essential. A review of the literature reveals that energy consumption is the most studied aspect of the environmental efficiency of additive manufacturing processes, comprising 87% of the articles. However, only 25% of the studies employed life cycle assessment at some level [130]. Furthermore, it has been observed that energy consumption is the primary contributor to the global warming potential of additive manufacturing processes, whereas in conventional manufacturing processes, raw materials are the main source of this type of emissions. Accordingly, this finding highlighted the necessity for enhanced energy efficiency in additive manufacturing processes to boost competitiveness from an environmental perspective [131]. However, other pertinent factors, such as material inputs, inert gases, material waste, and the energy required for post-processing operations, must be taken into account. Despite the growing emphasis on sustainability, there is still a paucity of comprehensive life cycle analysis studies that directly compare the environmental impact of additive manufacturing processes to that of conventional manufacturing processes.

4.2.2. Optimization of Additive Manufacturing

Additive manufacturing is an advanced processing method that enables the creation of customized components with complex geometries and architectural features, taking advantage of its non-linear relationship between complexity and cost [132]. Optimizing the geometry of a structure is crucial to minimizing weight, material consumption, and manufacturing costs, leading to both economic and environmental benefits. The necessity for lightweight structures arises in numerous engineering disciplines, where they serve to enhance the energy efficiency of moving systems and improve physical performance [133]. The current design strategies allow for a multitude of possibilities to identify the optimal solution or a set of optimal solutions for additive manufacturing structure design with functional properties suitable for each application. In this context, topology optimization and generative design are highly effective tools for the creation of optimal designs [134]. These tools diverge from traditional design approaches that employ computer-aided design systems to generate geometry. In contrast to these methods, these tools allow designers to prioritize functionality over form, while the generation of design alternatives is delegated to the optimization tools [135].

On the one hand, topology optimization is a computational design methodology that employs finite element analysis to achieve an optimal distribution of material within a specific design area while adhering to established constraints, loads, and boundary conditions [136]. This process involves discretizing the design area into finite elements and then employing various optimization techniques to determine how to distribute the material most efficiently. Typically, the goal is to minimize mass while avoiding excessive deformation or to maximize stiffness while keeping mass within a predetermined limit. On the other hand, generative design tools utilize artificial intelligence algorithms to generate a range of design options that meet the predetermined criteria based on user input such as materials, design space, functional surfaces, performance requirements, and manufacturing constraints [137,138]. This way, a variety of different design solutions that the designer may not have considered can be generated, often achieving better performance compared to components designed using traditional methods in terms of mass reduction, improved strength, and reduced number of assembly parts.

As depicted in Figure 14, both approaches share the first and last stages of the design process. The first stage involves using computer-aided design systems to define the functional surfaces of the three-dimensional model, while the final stage utilizes computer-aided manufacturing systems to ensure the proper fabrication of the model. In topology optimization, the model is imported into the optimization environment where conditions and objective functions are defined. Simulation is executed, resulting in an optimized geometry. The model is then modified by removing unnecessary material and validated using finite elements analysis tools in an iterative process. Furthermore, generative design starts with functional specifications, manufacturing processes, and desired objectives. The design space is explored, generating optimized and validated conceptual forms and the best alternative can be identified through comparative and trade-off studies [135].

These tools facilitate significant enhancement across a range of application domains, including the application to chemical engineering problems, such as transport through porous media [139], design of hydraulic components [140], enhanced heat transfer [141], or development of structured packings [142] and monolithic solids [143]. It is worth noting that there have been several notable successful cases of topology optimization and generative design in these contexts. For example, in the design of microchannel heat sinks (Figure 15A), topology optimization was employed to optimize the thermal and hydraulic efficiency of the structures, facilitating the creation of designs with sub-lattice features that enhance heat transfer, particularly in applications requiring effective thermal management [144]. Similarly, topology optimization was used to enhance the structural and thermal efficiency of integrated thermal protection systems (ITPSs), with the objective of minimizing thermal conductivity while maintaining structural rigidity (Figure 15B). This resulted in the development of an ITPS with enhanced thermal insulation capabilities and mechanical strength [145]. Moreover, generative design proved advantageous in enhancing heat transfer in internal liquid flows by optimizing the geometry of internal finned channels (Figure 15C). This approach resulted in a notable reduction in total thermal resistance and a marked enhancement in heat transfer compared to conventional designs [146].

4.2.3. Additive Manufacturing and Heat Exchange

Additive manufacturing emerges as a promising alternative in the production of more efficient and compact heat exchangers. The design freedom offered by additive manufacturing allows for process optimization, control of surface roughness, and the use of compatible materials, all of them key aspects for technical viability and economic competitiveness [147]. Additive manufacturing also offers the possibility to create heat exchangers with more complex geometries, maximizing the area–volume ratio to enhance heat transfer efficiency. This capability for advanced design enables adaptation to challenging operating environments, such as high pressures and corrosive media. Additionally, the short lead time for the production and customization of heat exchangers through additive manufacturing brings additional benefits, including weight reduction, consolidation of components, and space optimization, facilitating the implementation of new design concepts in shorter time periods [148].

Specific studies have demonstrated the viability of additive manufacturing in the fabrication of heat exchangers for a range of applications. One study compared the performance of traditionally manufactured heat exchangers (oil coolers for aircraft) to the ones produced using an additive manufacturing technology (Direct Metal Laser Sintering), demonstrating increased heat transfer efficiency for the models obtained by additive manufacturing (Figure 16a,b) [149]. Another study presented a microchannel heat exchanger manufactured via Direct Metal Laser Sintering too (Figure 16c), showing significant improvements in heat transfer density compared to conventional plate heat exchangers [150]. Finally, a study investigated the use of porous lattice structures fabricated through Selective Laser Melting to enhance heat transfer in air-to-air heat exchangers (Figure 16d). The results demonstrated increased thermal conductivity and heat transfer coefficient compared to conventional fin tube heat exchangers [151].

Although additive manufacturing technologies surpass conventional techniques in producing complex geometries, their relatively poor final outcome can be leveraged favorably under specific outcomes. Currently, additive manufacturing does not completely replace conventional methods commercially but progresses alongside them, demonstrating its benefits. Continuous advancements in additive manufacturing will position it as a robust technique for manufacturing efficient heat exchangers in the future. However, challenges remain in heat transfer, such as surface roughness, resolution, equipment costs, and post-processing difficulties [147,148].

4.2.4. Catalysts and Sorbents Produced by Additive Manufacturing

The development of novel catalysts and sorbents is essential for advancing sustainable practices across various industries. From renewable energy production to wastewater treatment, these materials contribute to minimizing environmental impact while optimizing resource efficiency. Innovations in catalyst and sorbent technologies not only improve process economics by reducing energy consumption and waste generation but also pave the way for greener industrial operation conditions [152]. There is particular interest in monolithic structures given their ability to offer significant advantages compared to conventional configurations such as pellets, beads, or granules. Monoliths are structured materials with specifically designed flow channels whose shape and density per cross-sectional area are controllable [153]. Their advantages include low pressure drop, high thermal stability, excellent mechanical integrity, and good mass and heat transfer characteristics [154]. These properties enable overcoming the issues associated with packed beds, such as limitations in heat and mass transfer, high pressure drops, and uneven flows, which can lead to deviations in contact time and decreased selectivity [152].

One particularly challenging aspect in manufacturing monolithic structures via traditional direct extrusion processes is balancing key design parameters such as active load, mass and heat transfer properties, and channel distribution [153]. When additive manufacturing is employed, it becomes possible to fabricate monoliths with a high degree of freedom, varied cross-sectional shapes, channel sizes, and wall thicknesses, and importantly, manufacturing parameters can be adjusted to achieve final products with enhanced mechanical properties. Currently, the most utilized technologies for manufacturing catalysts and adsorbents by additive manufacturing are based on material extrusion, specifically Direct Ink Writing, followed by Fused Deposition Modeling [155].

Applications of catalysts and sorbents produced by additive manufacturing have expanded across various fields in chemical engineering. For example, three-dimensional honeycomb monolithic reactors with interconnected channels were employed in the selective production of dihydroxybenzenes such as catechol and hydroquinone (Figure 17a), demonstrating significant improvements in performance, stability, and sustainability compared to conventional commercial methods [156]. In another study, the manufacture of zeolitic monoliths through 3D printing provided an efficient platform for the synthesis of heterogeneous catalysts applied to the catalytic cracking of n-hexane, resulting in increased selectivity towards light olefins and improved stability compared to powder alternatives [154]. SAPO-34 monoliths produced via Direct Ink Writing showed notable affinity and CO₂ adsorption capacity, suggesting enhancements in gas separation efficiency (Figure 17b). Additionally, these 3D-printed SAPO-34 monoliths enabled selective CO₂ and N₂ separation through adsorption, with adaptable geometry and excellent recoverability [157]. Finally, the 3D printing of activated carbon provided a solution for CO₂ capture in post-combustion scenarios, exhibiting high adsorption capacity and selectivity adjustable based on the selected activation conditions [158].

4.2.5. Additive Manufacturing for Microreactors and Miniaturized Devices

Microreactors, and other microstructured devices, are compact pieces of equipment designed for mass and heat transfer in structures typically smaller than 1 mm. This scale presents numerous advantages over conventional chemical engineering operations at larger scales, particularly in terms of the ease of mixing due to reduced dimensions that facilitate diffusion, as well as high conversion and selectivity. Furthermore, their high area–volume ratio contributes to more efficient heat and mass transfer [159]. Inherent challenges in these devices primarily revolve around the manufacturing process, where the ability to produce three-dimensional structures of extremely small dimensions is critical, and other determining factors that must be taken into account include cost, processing time, and required precision [160]. Additive manufacturing emerges as an alternative technology that mitigates challenges associated with manufacturing costs while offering significant advantages for rapid prototyping. This technique provides remarkable flexibility for the agile manufacturing of virtually any channel geometry alongside the ability to utilize a wide range of materials and customized designs. However, one of the main challenges when printing more complex microreactors via additive manufacturing lies in multi-level connectivity [161].

Some examples of miniaturized devices produced by means of additive manufacturing used in chemical engineering applications can be commented on. Among these found applications, the use of silicone microreactors for photocatalytic hydrogen generation can be highlighted [162], whose microchannels were coated with an Au/TiO₂ photocatalyst (Figure 18A). The successful operation of the microreactor with 500 µm width channels at ambient temperature and pressure was demonstrated under different water–ethanol gaseous mixtures, photocatalyst loadings, and light irradiance conditions. Other research work developed a polyvalent electrochemical microreactor, which was validated in three different electrosynthesis processes: the synthesis of tailor-made mixtures of paraffins by Kolbe electrolysis, the “cation flow” method for generating and accumulating carbocations, and the synthesis of peroxodicarbonate [163], highlighting its precision in process control, uniform mass flow, and short residence time (Figure 18B). High conversion rates, yields, selectivities, and Faraday efficiency levels were reported. Lastly, another example included the study of steam reforming of methanol performance using 3D-printed porous stainless-steel microsupports for microreactors [164], demonstrating their viability for hydrogen production under specific reaction conditions.

4.2.6. Additive Manufacturing and Separation Membranes

The revolutionary approach of additive manufacturing to achieve micro- and nanoscale resolutions has sparked researchers’ interest in its application to separation membrane synthesis, with specific applications ranging from wastewater treatment to water–oil separation and purification of gases [165], representing a significant step towards improved separation processes. The most recent advances in additive manufacturing to achieve greater scalability, material processability, shorter processing times, and higher resolution have promoted the exploration of its potential to produce membranes with specific designs using typical membrane materials. Many applications involve processes based on material jetting, specifically inkjet printing and electrospray printing (also known as electrospinning), due to their ability to produce thin films even when working with mixed matrix membranes. Most powder bed fusion, material extrusion, or vat polymerization processes are not able to achieve the resolutions required for the production of effective membranes [165]. Inkjet printing utilizes a computer printer that ejects ink droplets onto substrates to form a desired pattern, while electrospray printing employs an electrostatic spraying process to deposit material onto a surface in a precise and controlled manner. Electrospinning, in particular, has emerged as a highly valuable technique for the production of thin-film composite membranes, offering precise control over thickness and the ability to deposit polymers for subsequent film formation [166]. This technology has proven highly adaptable to a wide range of separation needs, crucial in a context where membrane customization is essential due to the diversity of fluids to be treated and the different characteristics of the chemicals to be separated.

Examples of thin-film composite membranes produced using inkjet printing and electrospinning are presented in Figure 19 [165]. In Figure 19a, the inkjet printing process is depicted, where patterns of fluorinated amine surface are printed on an m-phenylenediamine-impregnated support, followed by immersion in a trimesoyl chloride solution for interfacial polymerization [167]. The complete set of different patterns generated can be observed. Figure 19b shows an aqueous solution of m-phenylenediamine printed via inkjet printing on the support, followed by immersion in the trimesoyl chloride bath for polymerization [168]. Figure 19c displays a selectively controllable polyamide layer printed via electrospinning [169], while Figure 19d exhibits mixed matrix membranes of carbon nanotubes functionalized with polyamide and carboxylic functional groups, also obtained using electrospinning [170]. Finally, Figure 19e illustrates a selective nanofiltration polyamide layer printed via electrospinning on a Span 80 intermediate layer [171].

The need to address wastewater treatment, especially industrial oily effluents, has driven the search for efficient and environmentally friendly manufacturing methods to produce highly selective and permeable membranes for this specific purpose. Additive manufacturing technologies have proven to be a promising solution, allowing flexible, solvent-free, three-dimensional design with enhanced energy efficiency [172]. Despite the potential benefits, it is important to note that membranes produced using additive manufacturing may face challenges such as higher susceptibility to fouling, particularly in oil–water separation applications. While surface functionalization can mitigate this issue and improve membrane lifespan and economic performance, it remains an area that requires further research and development.

5. Conclusions

The present work constitutes a comprehensive full analysis of the published scientific literature on the state-of-the-art contributions of additive manufacturing to the chemical engineering field. To this end, a detailed bibliometric study of the documents identified was carried out with a view to determining the main quantitative characteristics of the scientific research on this subject. A discernible exponential temporal evolution in scientific production on chemical engineering and additive manufacturing was identified, exhibiting a markedly accelerated growth, particularly from the year 2015 onwards. This increase was attributed to technological advancements and an increasing demand for additive manufacturing. The observed exponential trend indicated that growth in this field would continue at a progressively accelerated rate in the near future, presenting challenges and opportunities for research in this field. Furthermore, the predominance of English as the dominant language in publications underscores its status as the universal language of scientific communication. The most common document type was articles, followed by conference papers and reviews. This reflects the importance of scientific journals as the primary medium for the dissemination of research findings. A thematic analysis revealed an applied and multidisciplinary orientation in additive manufacturing research, with significant intersections with chemical engineering, engineering, materials science, chemistry, and physics. In terms of country participation, the United States is the foremost contributor to scientific production, followed closely by China and Germany. Additionally, notable contributions have been made by European and Asian countries. With regard to the institutions involved, those in Singapore, France, Germany, and the United States were particularly noteworthy, with a focus on biofabrication, geometric design, and materials for additive manufacturing. The major journals covered a wide range of topics, varying in quality and impact. The most highly cited documents focus on themes such as bioprinting, the topological design of implants, and the properties of bioinks.

The most frequently occurring keywords reflected the principal concepts and technologies in the field of additive manufacturing, as well as their biomedical applications. This underscores the significance and diversity of this rapidly evolving field of study. The application of bibliometric network analysis has afforded a comprehensive insight into the evolution and interconnection of themes in additive manufacturing within the field of chemical engineering, tracing the trajectory of these developments from their initial contributions in 2002. This analysis revealed trends and changes in the field of additive manufacturing research within chemical engineering over time, thereby providing valuable insights into its development and future direction.

The global trends followed by researchers represented the evolution over time of the most relevant topics, identifying notable gaps and future needs. Indeed, there has been considerable interest in the potential of additive manufacturing for use in biofabrication and biomedical contexts. However, this has led to a relative neglect of the specific applications of additive manufacturing within the field of chemical engineering. Furthermore, this has resulted in the potential contributions that chemical engineering could make to the advancement of additive manufacturing technologies being overlooked. This analysis intends to provide a comprehensive and detailed perspective on the research on this subject, establishing a foundation for understanding its evolution, scope, and future direction.

The fields of biofabrication, bioprinting, and biomedical applications represent a significant area of research, with a substantial body of work also emerging from the chemical engineering community. Bioprinting employs a range of technologies, including inkjet printing, micro-extrusion, and laser-assisted techniques. Bioinks and biomaterial inks, which may include cells and biologically active components, are employed for the production of scaffolds and other structures. Hydrogels are the most commonly utilized materials in these inks due to their capacity to mimic the extracellular matrix of native tissues. However, composite hydrogels, which combine polymers with inorganic or organic nanofillers, are becoming increasingly popular due to their capacity to enhance the structural and functional characteristics of bioinks. Additive manufacturing has enabled the creation of implants with intricate and bespoke internal geometries and structures. Moreover, bioprinting has demonstrated considerable promise in the context of organ transplantation, with notable advancement in the fabrication of living cellular structures. Additionally, bioprinting has demonstrated utility in drug discovery, enabling the fabrication of in vitro models of physiologically relevant human organs. However, comprehensive validations and standardizations are essential to ascertain the potential efficacy of this approach.

Recent developments in chemical engineering are closely associated with the concept of process intensification, and additive manufacturing has the potential to make significant contributions to this field. For example, additive manufacturing enables the fabrication of heat exchangers with intricate geometries, thereby optimizing the area–volume ratio and enhancing the efficiency of heat transfer. However, the use of these heat exchangers may present certain challenges, including surface roughness, resolution, economic costs, and post-processing difficulties. Additionally, catalysts and adsorbents can benefit from the high area–volume ratio that additive manufacturing provides. The design of monolithic structures offers significant advantages over conventional configurations. Additive manufacturing appears to be a promising method for producing these materials, as it allows for a high degree of design freedom and the ability to adjust manufacturing parameters to obtain elements with high mechanical properties. Nevertheless, several challenges remain, including the need to achieve a balance between key design parameters and the necessity to enhance manufacturing technologies.

The utilization of microreactors and miniaturized devices represents a further significant application of additive manufacturing within the domain of chemical engineering. This compact equipment facilitates efficient mass and heat transfer, offering several advantages over traditional large-scale operations. Additive manufacturing can surmount the challenges associated with economic costs and offers notable flexibility for the agile production of any required geometry. Nevertheless, several challenges remain to be addressed, including the multi-level connection in complex microreactors. The capacity of additive manufacturing to manage micro- and nanoscale resolutions has facilitated the production of separation membranes with applications spanning wastewater treatment and gas purification. Indeed, additive manufacturing has been demonstrated to be an effective solution for the production of membranes with high selectivity and permeability, in a manner that is both efficient and environmentally friendly. However, it should be noted that the membranes produced using additive manufacturing may face the same challenges as traditional membranes, such as fouling.

Additive manufacturing will encounter challenges and opportunities in a number of areas that have yet to be fully integrated and that require further attention. Among these areas, the sustainability of additive manufacturing must be mentioned. Some chemical engineering tools, such as life cycle assessment and process and system optimization, can play a critical role in this regard. Life cycle assessment is a fundamental tool for evaluating the sustainability of a process. While additive manufacturing offers substantial environmental advantages, including reduced material waste, it is essential to conduct a comprehensive assessment of other factors, such as energy consumption and additional treatments, to ensure the optimal use of resources and the production of high-quality final products. Closer attention must be given to aspects such as materials, inert gases, material waste, and the energy demand of post-processing operations. In addition to traditional process and system optimization, design optimization methods such as topology optimization and generative design have the potential to enhance the efficiency and sustainability of additive manufacturing processes. These tools facilitate the fabrication of customized components with complex geometries and architectural characteristics, ensuring optimal material distribution and reducing weight, material usage, and manufacturing costs.