Recent Trends and Future Directions in 3D Printing of Biocompatible Polymers

Abstract

Three-dimensional (3D) bioprinting using biocompatible polymers has emerged as a revolutionary technique in tissue engineering and regenerative medicine. These biopolymers mimic the extracellular matrix (ECM) and enhance cellular behavior. The current review presents recent advancements in additive manufacturing processes including Stereolithography (SLA), Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS), and inkjet printing. It also explores the fundamentals of 3D printing and the properties of biocompatible polymers for 3D bioprinting. By mixing biopolymers, enhancing rheological characteristics, and adding bioactive components, further advancements have been made for organ transplantation, drug development, and tissue engineering. As research progresses, the potential for 3D bioprinting to fundamentally transform the healthcare system is becoming obvious and clear. However, the therapeutic potential of printed structures is hindered by issues such as material anisotropy, poor mechanical properties, and the need for more biocompatible and biodegradable architectures. Future research should concentrate on optimizing the 3D bioprinting process using sophisticated computational techniques, systematically examining the characteristics of biopolymers, customizing bioinks for different cell types, and exploring sustainable materials.

1. Introduction

Three-dimensional (3D) bioprinting has garnered significant interest in the past 10 years as a transformative tool in tissue engineering and regenerative medicine [1,2]. This innovative technique enables the creation of complex biological constructs that precisely mimic natural tissues in both structure and function by carefully depositing living cells and biomaterials [3]. Additive manufacturing scaffolds can be used to mechanically support cells; they can also be biocompatible, biodegradable, and employed for controlled medicinal release [4].

As the building blocks for 3D bioprinting, bioinks are biocompatible polymers that encapsulate cells and provide them with biochemical signals that direct tissue construction and cell development [5]. Recent developments have made a greater range of biopolymers available, including natural ones like chitosan and alginate, as well as synthetic ones like polycaprolactone (PCL) and polyvinyl alcohol (PVA) [6,7]. The distinct characteristics of each biopolymer, which affect cell activity, scaffold architecture, and general functioning, can direct customized constructs for various medicinal applications [8].

Recently, one of the main objectives of 3D bioprinting has been the integration of new materials and technologies to enhance tissue compatibility and functionality [9,10]. The utilization of ethically sourced bioinks is becoming more popular, which may allay concerns about the immunogenicity of animal-derived materials [11]. Using cutting-edge biocompatible polymers from marine sources is one example. The hybrid systems that blend multiple biopolymers to enhance scaffold performance have gained popularity in the bioprinting industry [12]. The right combination of biopolymers is essential for tissue integration and regeneration because it results in improved bioactivity, mechanical stability, and tailored degradation rates [13,14]. Using smart materials that react dynamically to environmental stimuli is another innovative approach to developing multifunctional scaffolds for tissue engineering and drug delivery applications.

In the field of bioprinting, a variety of printing techniques have developed, each with unique advantages that are ideally suited to particular applications. Stereolithography (SLA), which uses laser-induced polymerization to create intricate structures with a high degree of precision, is the method of choice for applications requiring exact geometries, such as organ-on-a-chip models that accurately replicate physiological environments [15]. By extruding thermoplastic filaments, Fused Filament Fabrication (FFF) enables the quick and affordable production of larger structures at a lower resolution than SLA [16]. Selective Laser Sintering (SLS) creates intricate scaffolds with complex shapes without the requirement for support structures by fusing powdered materials with a strong laser [17]. Inkjet printing and direct ink writing have also gained popularity in the bioprinting industry. High-throughput production and a plethora of options for material selection are made possible by the rapid deposition of bioinks enabled by inkjet printing [18]. Direct ink writing excels when working with extremely thick materials, such as bioinks that contain live cells, which require precise control over flow patterns and rates [19,20]. These techniques increase the likelihood that multiple types of cells and materials can be combined into a single build and enable the fabrication of intricate structures.

In contrast to earlier works that are largely centered on singular aspects of biocompatible polymers or singular applications of 3D bioprinting, our review takes a broad and integrative perspective that includes the basic principles, challenges, recent developments, and future directions of this technology, thereby offering a more general understanding of the field. In addition, although previous studies might have considered some of these innovations in bioprinting methods or material science, our paper uniquely integrates such innovations into the overall context of their meaning for medicine, highlighting the promise of 3D bioprinting to revolutionize medical therapy and devices. In addition, this review calls out areas where the current literature is lacking and delineates research directions that explicitly address these deficiencies, with a view to guiding scholarly research toward new methodologies and applications that further improve the effectiveness and biocompatibility of 3D-printed constructs. As such, this paper not only functions as a storehouse of existing knowledge but also as a visionary guide for researchers and practitioners who want to navigate and contribute to the developing field of bioprinting technologies.

This review aims to provide a comprehensive overview of biocompatible polymers and 3D bioprinting, including the fundamentals, present problems, new developments, and possible future directions that could enhance technology’s impact on healthcare. By dissecting these elements, we aim to demonstrate how 3D bioprinting could revolutionize the development of therapeutic approaches and medical equipment that address the intricate needs of modern medicine.

2. Advantages and Limitations of Biocompatible Polymers

Biocompatible polymers are vital for 3D bioprinting because they enable the creation of scaffold structures used in tissue engineering and regenerative medicine [21,22]. There are both synthetic and natural polymer types, and each has advantages and disadvantages that dictate which is most appropriate for specific purpose. Because of their inherent biocompatibility and ability to promote cellular interactions, cellulose, dextran, alginate, gelatin, and chitosan are among the most widely utilized natural biopolymers for soft tissue applications and cell encapsulation [23]. These materials possess well-known ability to mimic the ECM that is crucial for promoting tissue regeneration and cell adhesion. However, due to their long-term mechanical strength and stability, natural polymers are not always able to perform well in load-bearing applications [24].

Conversely, synthetic polymers with superior mechanical properties, such as increased tensile strength and variable degradation rates, include polylactic acid (PLA), polycaprolactone (PCL), polyvinyl alcohol (PVA), poly β-amino ester (PBAE), polyethylene glycol (PEG), and polyvinyl pyrrolidine (PVP) [25]. This renders it attainable to be utilized in challenging applications such as customized implants and bone scaffolding. By creating synthetic polymers to enhance specific properties, scientists can alter their functionality to correspond with the mechanical and biological needs of various tissues. When selecting materials, it is crucial to remember that using synthetic materials may result in chemical changes that compromise biocompatibility [26].

The conflicts between mechanical performance and biological compatibility must be considered when selecting a biopolymer for tissue engineering [27]. This rigorous procedure ensures that the materials used not only provide the necessary structural support but also promote cellular functions essential to successful tissue integration and regeneration. One of the main objectives of current 3D bioprinting research is to optimize both natural and synthetic biopolymers in order to improve the efficacy and therapeutic outcomes of patient-specific treatments. By facilitating the ability to create complex tissue structures, research into biocompatible polymers for 3D bioprinting has the potential to significantly impact healthcare.

Young’s modulus states the stiffness of a material representing how much material will deform when stress is given to it [28]. This parameter is especially important when choosing biopolymers for use in applications where mechanical stability of a specific kind is required, for example, load-bearing tissues (bone and cartilage). For instance, Young’s modulus values for typical biopolymers can be highly variable: in alginate, values are generally in the range of 0.5–2.5 MPa, whereas PCL has much greater stiffness, generally around 200–500 MPa [29,30]. These differences are significant to certify that the published constructions sustain physiological loads and remain intact over time.

Tensile strength measures peak stress that can be abided by a material that when stretched or pulled until it fails. This is of specific importance in situations where the integrity of the structure is of highest concern; for example, chitosan displays a tensile strength of about 35–50 MPa and can find application in scaffolding for soft tissue purposes, whereas gelatin has relatively lower tensile strengths, of about 1–2 MPa, and can potentially be restricted to use in mechanically challenging situations [31,32].

UV curing is a method used in 3D bioprinting that uses ultraviolet light to induce polymerization of photoresponsive bioinks. This mechanism is useful to print hydrogels like Gelatin Methacryloyl (GelMA) because it enables rapid setting times and high resolution in printed constructs that employ reactive species development through UV-induced photoinitiator absorption resulting in cross-linking and rapid solidification of bioink [33,34]. This degree of rapidity would be able to increase bioprinting throughput, enabling rapid build-up of highly intricate tissue morphologies. There are numerous benefits of UV curing utilization in improved print speed and spatial resolution with a cost incurred for biocompatibility. Prolonged exposure to UV radiation can lead to cytotoxic effects, damaging the implanted cells and barring tissue incorporation and function. The intensity and duration of UV irradiation should be formulated to maintain cellular viability since over-irradiation can lead to unreacted species that will also survive in polymer matrix and can be inflammatory hazards [35].

Chemical cross-linking, on the other hand, depends on the action of chemical reagents to form covalent bonds between polymer chains, thereby strengthening the mechanical integrity and stability of the resulting hydrogels. This is best done by the addition of cross-linking agents like glutaraldehyde or genipin that cross-react with biopolymer functional groups. This approach has greater manipulation of the properties of printed hydrogels so that mechanical properties such as Young’s modulus can be finely tuned and swelling behaviors that would be suitable for specific applications in tissue repair. While benefits of chemical cross-linking include tunable properties and greater stability, its limitation is that it is difficult to be biocompatible [34]. The choice of chemical cross-linkers may be critical; there are agents that can produce cytotoxic effects or cause inflammation in the biological environment, and their application might necessitate stringent evaluations to determine safety. For example, it has been observed through research that while genipin has emerged as a less toxic cross-linking agent, there is a requirement for its concentration to be carefully controlled so that undesirable effects on cell viability are avoided. Furthermore, post-crosslinking can result in low degradation rates, and this will affect long-term integration of the bioprinted construction into biological systems.

3. Fundamentals of 3D Printing and Biocompatible Polymers

Based on the biopolymers utilized to create 3D scaffolds, they can be divided into two categories: synthetic and natural [36].

Table 1. Advantages and disadvantages of natural and synthetic polymers employed for 3D bioprinting.

Types	Polymers	Advantages	Disadvantages	References
Natural	Chitosan	Non-toxicity; biocompatibility; biodegradability	Poor mechanical properties	[37,38]
	Cellulose	Adhesive and bioactive; abundant and biodegradable	Mechanical stability lost during processing	[39]
	Alginate	Ease of use for 3D printing; rapid gelation with divalent cations	Poorly adhesive; may damage cells during printing	[40]
	Collagen	Adhesive and bioactive; abundant and biodegradable; tolerant of functionalization	Mechanically weak; contamination can lead to immunogenicity	[41,42]
	Dextran	Cost-effective; biocompatibility	Low reproducibility due to variations in composition	[43,44]
Synthetic	Polylactic acid (PLA)	Degradable by hydrolysis; properties dependent on monomer feedstock	Hydrolysis products may cause inflammation; physically cross-linked gels are weak	[45,46]
	Polycaprolactone (PCL)	Degradable by hydrolysis; stable hydrogels over wide concentration range	Insufficient mechanical strength: crystallinity may slow hydrolysis beyond relevant timeframe	[47,48]
	Polyvinyl alcohol (PVA)	High elasticity; high biocompatibility and hydrophilicity	Non-degradable; non-adhesive	[49]
	Poly β-amino ester (PBAE) Polyethylene glycol (PEG) Polyvinyl pyrrolidine (PVP)	Biocompatibility; biodegrability; tunable chemistry; ease of synthesis Hydrophilicity; biocompatibility; controlled drug release Biocompatibility; non-toxicity	Limited structural variability; degradation affected by pH High concentrations required for drug delivery Viscosity adjustment required; thermally unstable at high temperatures	[50,51,52,53]

lists the advantages and disadvantages of these polymers.

3.1. Natural Polymers

Natural polymers are essential in the field of 3D bioprinting due to their inherent biocompatibility, hydrophilicity, and structural resemblance to the ECM [54]. The characteristics of natural polymers, including chitosan, cellulose, alginate, collagen, and dextran, which encourage cell division and proliferation, can be advantageous for a variety of tissue engineering applications [55,56,57].

3.1.1. Chitosan

Chitosan, a biopolymer made from chitin by alkaline deacetylation, is a promising material for 3D bioprinting and has garnered attention due to its biocompatibility, biodegradability, non-toxicity, and ability to form hydrogels [58]. The unique rheological characteristics of chitosan make it ideal for use in 3D bioprinting for tissue engineering and regenerative medicine, enabling the production of complex and accurately defined tissue scaffolds [59,60]. Zhang et al. combined silk microfibers, nanoparticles, and nanofibers to generate chitosan/silk composite scaffolds (Figure 1) using an extrusion-based 3D printing technique. The hydrophilic surface of the produced scaffolds promotes stable cell growth. The geometry of the silk nanoparticles had a significant impact on the mechanical capabilities, and the scaffolds that were 3D-printed and packed with silk nanofibers showed the longest shape and the fastest rate of fibroblast growth [61].

3.1.2. Cellulose

Cellulose is a β-1,4-linked glucopyranoside polymer that is renewable, biodegradable, safe for the environment, and compatible with living things [62]. It can also be covalently connected to a variety of bioactive substances. Cellulose can be used in 3D bioprinting to create cell-rich structures that mimic the ECM found in nature. The study found that cellulose nanofibers (CNFs) are perfect for bioprinting because of their shear-thinning properties [63]. This guarantees that the printed item maintains its structural integrity and facilitates extrusion processes (Figure 2). Nanocellulose increases the rheological properties of bioinks, improving both printability and cellular interactions [64]. Furthermore, studies have shown that cellulose-based bioinks may support many cell types, ensuring sufficient cell activity and multiplication even after printing [64]. Additionally, by altering the physical and chemical characteristics of cellulose through various formulations, researchers have been able to change how the material behaves for specific bioprinting applications [65,66]. This enables them to modify materials to achieve the required bioactivity and mechanical properties.

3.1.3. Alginate

Alginate, a naturally occurring polysaccharide derived from brown algae, is one of the most crucial components in 3D bioprinting [67]. It is hydrophilic, biocompatible, biodegradable, and non-toxic. Alginate bioinks may be made more printable by varying their alginate content or mixing them with other polymers like gelatin or nanocellulose [68]. For instance, methylcellulose and alginate together improve the shape fidelity of bioprinted structures, allowing for the progressive dynamic release of components and mechanical stability. Shang et al. proposed a calcium alginate hydrogel fabrication method that combines electrodeposition and 3D printing [69]. Sodium alginate and CaCO₃ nanoparticles were sprayed onto a conductive substrate through the injector nozzle as fillers, and the calcium alginate hydrogel was created by the release of Ca²⁺ ions from the CaCO₃ particles under electric pressure, which caused the alginate to cross-link (Figure 3) [70]. The injection syringe was connected to a conventional 3D printer.

3.1.4. Collagen

Collagen is regarded as an essential biomaterial in the field of 3D bioprinting because of its abundance in nature and proximity to the ECM found in live organisms [71]. Surprisingly, collagen is easily adaptable to many bioprinting methods. Because the bioink gels at body temperature, it may be utilized to produce intricate 3D structures that resemble genuine tissues. Jeong et al. showed that 3D-printed collagen scaffolds could effectively maintain the physiological parameters necessary for cryopreserved melanoma explants, emphasizing the significance of collagen in mimicking in vivo conditions in vitro [72]. A recent study by Lee et al. used a dual-material printing technique to create an elliptical left ventricular model. Collagen bioink was used to reinforce the outside and interior walls to maintain their intended geometric shape and provide adequate structural support (Figure 4) [73]. Directional action potential transmission and coordinated contraction were made possible by the central core area in conjunction with high-density cell bioink (such as cardiac muscle cells derived from human stem cells).

3.1.5. Dextran

Dextran, a naturally occurring polymer created when bacteria ferment sucrose, is a crucial part of 3D bioprinting due to its biodegradability, versatility in physicochemical properties, and favorable biocompatibility [74]. When added to cross-linked hydrogel formulations, dextran enhances the bioink’s printability, swelling behavior, and overall mechanical strength, particularly when paired with other polysaccharides like hyaluronic acid. Pescosolido et al. revealed the idea of mixing dextran and hyaluronic acid to increase the structural stability of semi-interpenetrating networks [75]. Tao et al. combined GelMA solution with β-lactoglobulin (β-LG) nanoparticles/dextran solution mixture to prepare bioinks and print tissue structures using 3D bioprinting technology based on digital light processing (DLP) (Figure 5) [76]. After solidifying, the ink was submerged in the culture media to form in situ pores that allow nutrients and oxygen to enter the 3D-printed hydrogel structure.

3.2. Synthetic Polymers

3.2.1. Polylactic Acid (PLA)

PLA, a thermoplastic aliphatic polyester, is biodegradable and bioabsorbable [77]. It features repeating lactic acid units, contains both D- and L-stereoisomers, and can be enantiomerically pure (PLLA only contains L stereocenters) or derived from renewable resources. PLA possesses low viscosity, superior thermoplastic, and heat stability, making it an ideal material for FFF and related techniques [47]. PLA exhibits a moderate glass transition temperature of roughly 65 °C, enabling successful extrusion at relatively low temperatures, which aids in maintaining cell viability during the bioprinting process. Several nano-additives have been added to PLA to increase its mechanical strength. FFF technology was used in K. Dave’s study to produce porous scaffolds that were combined with raw materials [78]. To create filaments that could be 3D-printed, they first melted and mixed PLA with amphiphilic nanomaterial carbon dots (CDs) [24,79]. On the contrary, the therapies enhanced cell adhesion, motility, and proliferation in living systems, creating opportunities for in situ scaffold and cellular environment monitoring and scanning [80].

3.2.2. Polycaprolactone (PCL)

PCL is a biodegradable aliphatic polyester with insufficient mechanical strength [81]. In terms of bioprinting, the ability to mechanically reinforce PCL is a significant advantage. PCL-based composites with bioinks that are adaptable to cells enable tissue ingrowth, cell survival, and structural support while preserving the mechanical integrity of the composites [82]. According to Rathan et al., the addition of PCL to cartilage ECM bioinks facilitated the creation of mechanically superior structures that support essential cellular functions for cartilage regeneration [83]. Fang et al. emphasized the significance of PCL as a robust framework to enhance the mechanical stability of bioprinted osteochondral structures and injected biocompatible hydrogels into PCL scaffolds. In order to support cell development and nutrient transfer, this combined approach leverages the benefits of hydrogels and PCL’s resilience [84]. Furthermore, the application of PCL combined with β-tricalcium phosphate to produce tailored bone grafts has shown promising results in correcting bone defects, as illustrated in the case study by Javkhlan et al. [85]. This represented a prime example of PCL’s exceptional capacity to adjust to the specific mechanical and biological needs of various tissues.

3.2.3. Polyvinyl Alcohol (PVA)

PVA has become a popular biopolymer for 3D bioprinting due to its exceptional mechanical characteristics, biocompatibility, and biodegradability [86]. The viscoelastic properties of PVA are comparable to those of articular cartilage. PVA’s hydrophilicity and hydrogel-forming properties allow it to adapt to biological behavior in bioprinted structures [87]. Zeng et al. highlighted the significance of PVA in developing granular gel baths for embedded extrusion printing, discovering that it enhanced the stability and functionality of the bioprinted structures [88,89]. By mimicking the structure of real tissue, this property of PVA facilitates the creation of 3D structures that activate cellular processes required for tissue regeneration [90].

3.2.4. Poly β-Amino Ester (PBAE)

PBAE are a class of biodegradable and biocompatible polymers that have gained significant attention in 3D bioprinting applications because of their tunable mechanical properties and ability to aid cell adhesion and proliferation [91]. These polymers are synthesized by step growth polymerization typically involving the reaction of diacrylates with primary or secondary amines. Their hydrolytic degradation results in nontoxic byproducts, making them highly suitable for biomedical applications. PBAEs are widely explored as bioinks for 3D printing particularly in tissue engineering and regenerative medicine because of their ability to support the formation of intricate cellular structures [92]. Their cationic nature enables effective interaction with negatively charged cell membranes and ECM components, promoting cell attachment and growth. However, by modifying the polymer backbone, researchers can fine-tune properties such as degradation rate, mechanical strength, and hydrophilicity, making PBAE a versatile candidate for printing scaffolds aimed at controlled drug release and tissue regeneration [93].

3.2.5. Polyethylene Glycol (PEG)

PEG is a hydrophilic, biocompatible polymer extensively used in 3D bioprinting due to its excellent water solubility, nontoxicity, and capacity to form hydrogels suitable for cell encapsulation [94]. PEG-based hydrogels are widely used as bioinks because they provide a hydrated environment that mimics the native ECM, supporting cell viability and differentiation. One of the key advantages of PEG is its chemical versatility. It can be functionalized with various bioactive molecules, peptides, or cross-linkers to tailor its mechanical and biological properties [95]. PEG hydrogels can be cross-linked by photopolymerization or enzymatic reactions, allowing precise control over the printing process and scaffold architecture. In 3D bioprinting, PEG is frequently used to engineer tissues such as cartilage, bone, and soft tissue constructs due to its ability to provide a mechanically stable yet biologically permissive environment. However, PEG is used as a combination with other natural or synthetic polymers to enhance its bioactivity and promotes cell attachment, making it a fundamental component in scaffold fabrication for regenerative medicine and drug delivery applications.

3.2.6. Polyvinyl Pyrrolidine (PVP)

PVP is a water-soluble synthetic polymer widely utilized in 3D bioprinting because of its superior biocompatibility, nontoxicity, and capacity for stable hydrogel formation. PVP possesses a special combination of film forming, adhesive and stabilizing properties making it useful bioink additive. Its high water-retaining capability allows the generation of hydrated environments that promote cell viability and proliferation in printed structures. Additionally, PVP is frequently utilized as a bioink viscosity modifier to improve printability and preserve structure integrity during and after printing [95,96]. It can be also mixed with other biomaterials such as alginate or PEG to enhance mechanical strength and bioactivity. PVP-based hydrogels in tissue engineering applications are used for wound healing, cartilage regeneration, and controlled drug release systems due to their capability to encapsulate and slowly deliver bioactive molecules. However, non-immunogenic nature of PVP guarantees a very low inflammatory response, making it a suitable candidate for biomedical uses in 3D bioprinting.

4. Recent Trends in 3D Printing of Biocompatible Polymers

Recently, 3D printing technology has advanced significantly, enabling the production of complex tissue structures. The use of biocompatible polymers holds potential for transforming tissue engineering and regenerative medicine [91,97]. The 3D bioprinting process consists of several crucial components, which can be broadly divided into three main phases: preprocessing, which involves creating the digital 3D model to be printed; processing, which includes preparing the bioink and printing the model; postprocessing, which focuses on stabilizing and maturing the bioprinted structure (Figure 6) [98].

During the preprocessing phase, a variety of software programs are used to support computer-aided design (CAD) [99]. The next step after creating a design in CAD is the slicing stage, which entails translating the CAD design to a standard tessellation language (STL) file by creating G-code. Computers and printers can communicate by using G-code language. When creating a bioprinted model, it is essential to consider the arrangement and proportions of the various tissues that will be replicated. Both the STL slicing parameters and the mechanical properties of the finished structure are impacted by this ratio [100,101]. Comparing and analyzing the data from the initial design with the outcomes after printing would be quite beneficial. Selecting the appropriate biomaterials and bioprinters is particularly essential due to their impact on the rheological, biological, and degrading properties of the output. Biocompatibility is an important factor to consider when choosing a biomaterial as it interacts with living tissues or bodily fluids and because the development of a toxic or immunological reaction might be lethal [102].

4.1. Additive Manufacturing Technologies

4.1.1. Stereolithography (SLA)

SLA, a popular additive manufacturing technique, uses light to polymerize and solidify photopolymer resins layer by layer, making it perfect for creating precise 3D objects [103]. This method has advanced to the forefront of 3D bioprinting due to its ability to print with high-resolution features and a range of biomaterials appropriate for biological applications. The fundamental process involves immersing a construction platform in a photopolymer resin solution. The building of the object is then completed by exposing particular areas to light, usually ultraviolet (UV) radiation, which initiates the polymerization process [104].

SLA is an additive manufacturing technique that enables the flexible creation of rapid prototypes [105]. This process creates an object by layering photosensitive polymers and subjecting them to UV light. The shrinkage and consequent distortion of the photosensitive materials change their dimensions, resulting in reduced precision in printed goods. The majority of the materials utilized in SLA are either traditional epoxy, acrylate resins, or thermoplastic elastomers [106]. The roughness of the surface can change when exposed to UV light. In SLA, the effect of process parameters on part quality (surface integrity and dimensional accuracy) has already been investigated [107]. The dimensional accuracy of 3D SLA prints is influenced by a number of parameters, such as component orientation, printing technology, post-processing techniques, and support material utilization as listed in Table 2.

The primary components included in all photocurable SLA resins are as follows: 1. Precursors: SLA molecules are liquid at first, but when they come into contact with one another, they can solidify into a 3D network (a process called polymerization); 2. Photo initiators (PIs): These are parts of the resin that react to light [108]. They are activated by applying radiation with the proper wavelength, preparing them to begin the curing process. The choice of a suitable PI is determined by the type of precursor. The type and amount of PI used in the process have a major impact on the kinetics, light intensity, conversion, cross-linking density, and, to a lesser extent, the mechanical properties of printed components; 3. Absorbers: Most SLA methods use light absorbers, which reduce the quantity of light that can pass through the resin and hence the depth to which it may cure [109]. The Beer–Lambert law states that as the concentration of light absorbers rises, light is absorbed more powerfully, and less light is permitted to flow through a substance. This attenuation limits the effective cure depth, which is crucial for accurate layer development and structural integrity in bioprinted structures; 4. Filled resins: Filled resins with glass or carbon fiber fillers reinforce the polymer network without sintering or debinding. For instance, in some formulations, powdered resins are combined with metal fillers that are typically sintered to achieve the proper mechanical properties. These fillers enhance performance without the need for heat treatments [110]. This versatility provides the way for the development of composites with unique properties that are ideal for a variety of applications, including those that call for increased structural stability or thermal conductivity. The filled resins are not limited to conventional sintering techniques; they also create new opportunities in additive manufacturing that capitalize on the unique properties of various fillers to improve the overall properties of the material [111]. A high filler percentage helps to reduce shrinkage and cracking; 5. Additives: To keep slurries stable over longer printing runs and to increase their shelf life, rheological stabilizers and additives are required [112].

Another well-known feature of SLA is its rapid prototyping capabilities, which enable the production of complex anatomical structures with micro- and nanoscale properties, as well as tissue scaffolds [113]. This level of precision is necessary for tissues that are designed to mimic the intricate structure of living things. SLA’s layer-by-layer technique allows for the precise placement of cells and biomaterials, and it also uses biological molecules to enhance tissue integration and functionality, as well as cell connections [114].

Table 2. Comparison of 3D bioprinting techniques.

Bioprinting Technique	Description	Advantages	Applications	Limitations	References
Stereolithography (SLA)	Uses UV light to polymerize resin in a layer-by-layer fashion.	- High resolution and detail; fast print times for small objects	Tissue engineering; organ models	- Limited material options; resin toxicity during printing	[115]
Fused Filament Fabrication (FFF)	Extrudes thermoplastic filament through a nozzle to create layers.	- Cost-effective; widely available technology; can use various materials	Drug delivery systems; scaffolds for tissue repair	- Lower resolution compared to SLA; limitations in material strength	[116]
Selective Laser Sintering (SLS)	Utilizes a laser to fuse powdered materials based on a digital model.	- Can utilize various materials; no support structures needed due to powder bed	Bone grafts; complex structures requiring precision	- Expensive equipment; post-processing needed for powder removal	[117]
Inkjet Bioprinting	Droplets of bioink are deposited to form 2D and 3D structures.	- High throughput; suitable for living cells; allows for precise patterns	Printing cell arrays; skin substitutes; vascular models	- Limited viscosity range; cell damage from heat during droplet formation	[118]
Direct Ink Writing (DIW)	Involves extruding a gel-like bioink through a nozzle to create 3D structures.	- High versatility in material use; good control over structure	Soft tissue engineering; cell-laden constructs	- Requires precise control of bioink viscosity; limited to certain material types	[119]

Table 2. Comparison of 3D bioprinting techniques.

Bioprinting Technique	Description	Advantages	Applications	Limitations	References
Stereolithography (SLA)	Uses UV light to polymerize resin in a layer-by-layer fashion.	- High resolution and detail; fast print times for small objects	Tissue engineering; organ models	- Limited material options; resin toxicity during printing	[115]
Fused Filament Fabrication (FFF)	Extrudes thermoplastic filament through a nozzle to create layers.	- Cost-effective; widely available technology; can use various materials	Drug delivery systems; scaffolds for tissue repair	- Lower resolution compared to SLA; limitations in material strength	[116]
Selective Laser Sintering (SLS)	Utilizes a laser to fuse powdered materials based on a digital model.	- Can utilize various materials; no support structures needed due to powder bed	Bone grafts; complex structures requiring precision	- Expensive equipment; post-processing needed for powder removal	[117]
Inkjet Bioprinting	Droplets of bioink are deposited to form 2D and 3D structures.	- High throughput; suitable for living cells; allows for precise patterns	Printing cell arrays; skin substitutes; vascular models	- Limited viscosity range; cell damage from heat during droplet formation	[118]
Direct Ink Writing (DIW)	Involves extruding a gel-like bioink through a nozzle to create 3D structures.	- High versatility in material use; good control over structure	Soft tissue engineering; cell-laden constructs	- Requires precise control of bioink viscosity; limited to certain material types	[119]

4.1.2. Fused Filament Fabrication (FFF)

FFF has gained popularity in 3D bioprinting because of its versatility in material selection, affordability, and accessibility. FFF involves the successive layering of molten thermoplastic materials; it is a great method for creating the complex scaffolds and biological structures required in tissue engineering. The basic idea behind FFF is the melting and molding of raw materials to produce new shapes. After being heated to a semiliquid condition and pulled by a driving wheel, the roll-shaped filament enters a temperature-controlled nozzle head. By precisely extruding and guiding materials in an ultrathin layer, the nozzle builds structural components layer by layer [120].

One significant advantage of FFF in 3D bioprinting is its adaptability to a wide range of biocompatible polymers, including PCL, and PLA [91,121]. These materials’ favorable mechanical properties and safe biodegradation make them suitable for use in regenerative medicine applications. Additionally, due to advancements in composite materials that combine thermoplastics and bioactive chemicals, FFF-printed structures can now more accurately mimic the ECM seen in real tissues. The mechanical characteristics of FFF-printed bioprinted structures can be improved by adjusting printing parameters like extrusion rate, nozzle design, and printing speed [66,122] (Table 2). There are several phases involved in making filaments with an extrusion machine. Selecting the desired filament diameter at the outset is followed by configuring the extrusion parameters, inserting the material as a pellet, and extruding it from the nozzle die hole to the roller machine [123]. The steps involved in making filaments are depicted in Figure 7.

The mechanical strength and biological effectiveness of the finished scaffolds are impacted by these changes. For instance, structures with the ideal porosities and mechanical strengths for use with particular tissues, such as bone or cartilage, can be designed [124]. FFF’s adaptability further demonstrates its versatility as a customizable approach to solving a range of tissue engineering challenges.

4.1.3. Selective Laser Sintering (SLS)

SLS is categorized in the Powder Bed Fusion category by the American Society for Testing and Materials [125]. SLS is a sophisticated additive manufacturing technique that has gained attention lately because of its possible applications in 3D bioprinting. It involves using laser radiation to “neck” powder particles in order to form objects [126,127]. Features of this additive manufacturing method include high resolution, powder recycling, and no pre-processing. Stricter quality and safety regulations also apply to the manufacturing of medications. This emphasizes how crucial it is to use printing ingredients that are stable during the printing process and printable, including pharmaceutical-grade powders that are recognized to be safe for ingestion.

SLS is ideal for creating intricate biological scaffolds and other structures required for tissue engineering due to its accuracy and efficiency in creating complex geometries [128]. SLS is highly adaptable and, as a result, provides a huge advantage in the bioprinting industry since it can work with a variety of biomaterials, such as thermoplastics, ceramics, and composites. Because of its ability to create complex and practical structures with enhanced mechanical and biological properties, SLS has found a place in the bioprinting industry [129]. For instance, SLS can produce scaffolds that mimic the ECM found in nature, which offers an environment that is conducive to cell adhesion, proliferation, and differentiation. Furthermore, the intrinsic porosity and mechanically adjustable properties of SLS-printed scaffolds make them perfect for tissue engineering of bone and cartilage (Table 2).

Figure 8 illustrates the sintering process. The powder is first fed into the reservoir platform and then sled-distributed around the construction site to create a level, consistent layer [130]. Before sintering starts, the powder needs to be preheated in the printer. The initial stage in the printing process is turning on the blue diode laser, which scans the powder bed in a predetermined pattern along the X and Y axes in accordance with the object’s predetermined design [131]. The degree to which powder particles fuse together depends on the amount of energy that is delivered. By lowering the printing bed and applying another layer of powder on top of the sintered layer, the item can then be constructed along the Z axis. Until the task is finished, the process is repeated [132]. Lastly, any remaining powder is swept off once the printed dosage forms are removed from the construction platform. Unlike other 3D printing techniques, this one does not require the construction of extra support structures because the unconsolidated powder remains in place and supports the object as it is being built.

4.1.4. Inkjet Printing and Direct Ink Writing

Inkjet printing and direct ink writing have become more popular in the 3D bioprinting sector because of their accuracy, versatility, and ability to work well with biological materials [133]. Inkjet printing is a non-contact method of applying tiny bioink droplets to a surface. Among the numerous reasons that this method is useful for bioprinting are its ability to provide high spatial resolution and the accurate positioning of biomaterials, cells, and biological components (Table 2) [134]. For inkjet printing to be successful, appropriate bioink composition is essential. In order to maintain tissue construct integrity and cell viability, these bioinks ought to be able to transition from a liquid state during printing to a solid or gel state after deposition (Figure 9).

Recent studies have emphasized the significance of bioink surface tension and viscosity in droplet formation and precise deposition. For instance, by decreasing the distribution of droplets and increasing their size, bioinks with appropriate rheological properties enhance print quality [135]. Additionally, inkjet printing enables the design of multiple cell types and biomaterials in a single structure, which expedites the creation of complex tissues that resemble actual organs anatomically. Many research labs can use inkjet printing settings since they are typically simple and affordable to implement.

The rheology of bioinks, particularly their shear-thinning properties and yield stress, is an important factor that determines the fidelity of 3D printing processes [136]. Shear-thinning properties allow bioinks to decrease their viscosity when subjected to shear stress, allowing them to extrude smoothly through small nozzles and minimize the likelihood of clogging, which is required for uniform deposition during printing. Such properties not only create precise layer-by-layer build-up but also allow the printed structure to maintain structural integrity upon the stress removal from shearing. Moreover, yield stress of a bioink becomes appropriate; it gives the threshold value of stress that initiates flow, hence precluding early deformation of bioinks during printing. When the yield stress is excessively low, the bioink will not hold its shape when it is extruded, and structures printed with it will exhibit poor dimensional fidelity [137]. Conversely, an optimum yield stress offers the desired mechanical stability to intricate geometries without compromising cell viability entrapped within bioinks. Moreover, rheological property optimization of bioinks is crucial for enhanced cell survival during printing; excessive shear stress can destroy sensitive encapsulated cells; therefore, there needs to be a fine balance between viscosity and printability. Therefore, the understanding and regulation of the rheological properties of bioinks are the focal points for the establishment of the efficacy and credibility of 3D bioprinting technologies in tissue engineering [138].

However, direct ink writing is a technique that creates 3D structures by extruding a paste-like bioink through a nozzle for materials that require a thicker consistency than what is typically utilized in inkjet printing [139]. By precisely controlling the material deposition process, the approach makes it possible to create scaffold designs that may be tailored to different tissue engineering requirements. One benefit of direct ink writing is its versatility in using hydrogels and conditioning polymers, which are bioinks that can contain living cells and biomolecules [140]. Applications involving tissue regeneration require complex porosity scaffolds, and this method enables the creation of structures with a range of filament widths. Additionally, direct ink writing excels at creating mechanically robust structures that can support cells and provide the environment they require for growth and differentiation [141].

The direct ink writing method creates 3D objects by extruding bioink a blend of hydrogels, polymers, and living cells through a nozzle [138]. It is crucial to maximize the rheology and viscosity of bioinks when printing with them, particularly when they include live cells, in order to minimize shear stress, which could damage the cells. During direct ink writing extrusion, the bioink experiences high shear stresses; nevertheless, its viscoelastic characteristics can be altered to safeguard the living cells that have been implanted. Optimizing these rheological properties is essential for cellular survival because studies have shown that high-viscosity bioinks ensure printability while preserving cell integrity. Live cells can be included into scaffolds printed via direct ink writing, although cell viability is significantly decreased. Direct ink writing’s ability to support living cells is also being enhanced by advancements in bioink formulations and printing parameters [130].

Other 3D printing techniques, including inkjet and laser-assisted bioprinting, have the potential to print living cells; they also provide a distinct set of challenges that must be carefully considered. Inkjet bioprinting uses a process called droplet production in which bioink is applied in tiny droplets to a substrate [142]. Cell viability may be lowered by the introduction of mechanical and thermal stresses during this process. Specifically, localized heating during the printing process may result in cell death if the bioink is not designed with adequate thermal properties. Furthermore, sensitive cells may be harmed by the shear stress brought on by mechanical forces during droplet production and ejection, which could result in decreased post-printing survival rates.

Laser-assisted bioprinting may represent the most effective option for high spatial resolution [143]. This technique precisely patterns bioinks using high-energy lasers. However, the enormous amount of energy needed by this process may be excessive for fragile living cells to handle. However, recent data indicate that laser-assisted bioprinting can be adjusted to lower mortality rates. The high energy laser remains a big concern when it comes to printing cells without causing damage to them. The method’s complexity requiring precise focusing and laser intensity regulation makes it unsuitable for tissue engineering applications where preserving cell viability is crucial [144].

Direct ink writing is capable of creating both macro- and microstructures. Vascular networks in engineered tissues can now be produced more readily using bioprinted materials [145]. For the tissues to remain viable, these networks enhance the flow of waste products and nutrients. Furthermore, the technique makes it possible to print structures with incorporated functional bioactive molecules, improving the structures’ ability to integrate with tissue and their post-implantation utility. New opportunities for the production of advanced bioprinted organs and tissues are presented by the combination of inkjet printing with direct ink writing. With ongoing advancements in bioink compositions, printing technology, and process optimization, these techniques will continue to progress in regenerative medicine and clinical applications, expanding their potential (

Table 3. Bioink formulations and their applications.

Bioink Composition	Cross-Linking Method	Mechanical Properties	Target Tissue	References
Alginate–Gelatin	Ionic cross-linking (calcium ions)	Moderate stiffness; bioactive	Cartilage; bone	[146]
Chitosan–Silk	Chemical cross-linking (glutaraldehyde)	High tensile strength; moderate flexibility	Skin; cartilage	[147]
Gelatin Methacryloyl (GelMA)	Photopolymerization (UV light)	Adjustable stiffness; high biocompatibility	Cartilage; vascular structures	[148]
Decellularized Extracellular Matrix (dECM)—Gelatin	Solvent evaporation (non-crosslinked)	Variable viscosity; low tensile strength	Liver; cardiac tissues	[149]
Alginate Dialdehyde	Chemical cross-linking (transglutaminase)	Moderate, adjustable; adhesion-enhancing	Bone; skin	[150]
Hyaluronic Acid–Fibrin	Ionic cross-linking (calcium ions)	Soft and flexibility; promotes cell adhesion	Cartilage; muscle	[151]
Keratin–Glycol Chitosan	Chemical cross-linking (methacrylation)	Moderate stiffness; good biocompatibility	Skin; connective tissues	[147]
Methacrylated Hyaluronic Acid (MeHA)	Photopolymerization (UV light)	Tunable mechanical properties	Cartilage; vascular tissues	[152]

) [97].

4.2. Advancements in Polymer Modification

The advancement of polymer modification for 3D printing is a rapidly emerging topic that significantly increases the potential of additive manufacturing. Polymers are essential components of this technology and are used in tissue engineering, drug delivery, and personalized medicine [153]. Scientists have employed a variety of techniques, such as cross-linking, composite synthesis, surface functionalization, and nanomaterial integration, to produce polymers that are ideal for 3D bioprinting [154]. The variety of biocompatible materials available for 3D printing has significantly increased. Chitosan, PLA, and PEG are examples of older materials that are supplemented by contemporary hydrogels, elastomers, and composites. Rimington et al. highlighted the feasibility of using a range of polymers to support neuronal, myogenic, and hepatic cell types by showing that distinct photopolymerized and laser-sintered polymers are compatible with specific cell types for distinct purposes [155]. Improvements in composite materials have been crucial in enhancing the mechanical properties of 3D-printed structures. Fillers that have been discovered to improve tensile strength and impact resistance include graphene, carbon nanotubes, and natural fibers like kenaf [156]. According to a study by Hamat et al., natural fibers combined with polymers like PLA provided an environmentally friendly alternative to conventional composites while simultaneously enhancing print quality [157]. It is possible to create flexible scaffolds that are resilient to the physiological pressures encountered upon implantation by combining polymers and reinforcements.

Modern methods include dual-extrusion systems, digital light processing (DLP), and multi-material jetting, which enable the precise creation of heterogeneous structures that more closely resemble the architecture of actual tissues. Zhang et al. showed how to create extremely robust hydrogel structures using novel printing methods, highlighting its potential for uses such as artificial meniscus [158]. The ability to print multiple materials simultaneously opens up new possibilities for modifying scaffolding properties. Bioprinting is increasingly being used not only to create structural scaffolds but also to create functioning tissues [10]. This integrates growth factors and living cells into the printing process with the aim of controlled release and optimal cell-nurturing conditions. The development of self-healing hydrogels, which can adapt to various environments and recover from harm while enhancing cell viability, is one example of this trend, according to Advíncula et al. [159]. Potentially, these advancements could lead to functional tissue transplants that are indistinguishable from the host tissue [160].

The development of algorithms and software that enable the production of scaffolds specific to each patient has transformed the use of 3D bioprinting in medicine [161]. When using additive manufacturing techniques to manufacture biocompatible implants, the unique anatomy of each patient can be taken into consideration. Pappas et al. employed reverse engineering techniques to produce one-of-a-kind ocular explants as an illustration of the potential applications of 3D printing in personalized medicine [2,162]. These developments boost the possibility of effective tissue integration and repair by making it simpler to construct customized implants. The development of bioactive scaffolds that enable the consistent release of growth factors or therapeutic agents can lead to better regeneration outcomes. Yu and Dunn’s research on extrusion-printed continuous fiber composites serves as one illustration [163]. By incorporating bioactive compounds into a polymer matrix, these materials enable more accurate delivery of these therapeutic agents. These advancements allow for the fine-tuning of the properties of the immediate environment of the cells and offer a wide range of new applications.

Research into sustainable and biodegradable polymers has increased as the shortcomings of traditional materials become more obvious. Innovations in bio-based polymers derived from sustainable resources are becoming more prevalent. According to research, 3D printing with natural fillers and renewable polymers can significantly reduce environmental effects without compromising functionality [164]. This is essential since biological applications are increasingly requiring more ecologically friendly solutions. Three-dimensional bioprintable polymers that can manage the complexities of tissue engineering have advanced as a result of the interdisciplinary approach that combines computational design, biomedical engineering, and materials science [165]. As the field continues to innovate, new materials and technologies are being used to produce breakthrough solutions in regenerative medicine, which could lead to improvements in patient care and outcomes. Volumetric bioprinting, a new technique that uses light-based tomographic reconstruction to fabricate intricate cell-laden structures in seconds [166]. By projecting multiple light beams into a volumetric resin, volumetric bioprinting enables the fast fabrication of intricate three-dimensional constructs with high resolution and complexity. This technique is particularly valuable for vascularized tissue and organoid generation because more than one type of cell and one component of the extracellular matrix can be deposited at once, providing a more biomimetic environment for growth [167].

Multi-material printing is a sophisticated feature of 3D bioprinting that enables the simultaneous or sequential deposition of dissimilar materials, e.g., various bioinks, to create heterogeneous tissue constructs that more closely resemble the intricacy of natural tissue [168]. This capability is essential to replicate the intricate architecture and function required in biomedical applications such as tissue engineering, drug delivery, and organ regeneration. Multi-material printing allows for precise spatial organization of distinct cell types and materials within one construct to facilitate cellular interaction, which is essential for tissue function. An example is that of Mazzaglia et al., which was able to effectively demonstrate the use of extrusion bioprinting for generating compartmentalized tumoroids that consist of immune cells and cancer-associated fibroblasts, successfully recreating the TME with distinctive 3D architect [169].

4.3. Applications

The advancements in biopolymers that enable 3D bioprinting have made a significant impact on tissue engineering, drug delivery systems, and biosensors [170]. As research continues, these developments have opened the door to improved biocompatibility, new functionalities, and broader applications (

Table 4. Key applications and case studies of PLA, PEEK, PEG, PVP, PCL, and PVA in 3D printing for biomedical applications.

Biomaterials	Key Applications	Explanation	References
Polylactic Acid (PLA)	3D-Printed Face Implantations	PLA applications in developing face implants in reconstructive surgery with emphasis on patient outcomes and surgical accuracy.	[171]
		Use of PLA in customized surgical guides and implants for facial surgery with improved integration with surrounding tissues.	[172]
		PLA scaffolds in face implants aided complex reconstruction with improved outcomes.	[173]
Polyether Ether Ketone (PEEK)	Personalized Cranial Implants	Formation of customized PEEK implants for cranioplasty, illustrating mechanical stability and patient-specific adjustments.	[174]
		PEEK utilization in management of complex acetabular fractures to improve surgical results and accuracy.	[175]
		Assessment of PEEK implants in total talus replacements with a focus on restoration of anatomical naturalness.	[176]
Polyethylene Glycol (PEG)	Drug Delivery Systems	PEG hydrogels as localized drug delivery systems for effective healing in tissue repair processes.	[177]
		PEG formulations in bio-inks for the 3D printing of soft tissue scaffolds to enhance cell viability and function.	[54]
		Regeneration of cartilage and drug delivery systems as an application of PEG hydrogels in clinical research.	[178]
Polyvinyl Pyrrolidone (PVP)	Drug Delivery and Stabilization of Nanoparticles	Nanoparticle stabilizing agent for the use as PVP in formulation of drugs to deliver prolonged release profiles.	[179]
		Case studies clarifying applications of PVP in enlightening delivery effectiveness of therapeutics with the help of nanocarriers.	[180]
Polycaprolactone (PCL)	Tissue Engineering Scaffolds	PCL scaffold formation for engineering of bone tissue with improved cell adhesion and propagation in vitro and in vivo.	[181]
		PCL mixed with biodegradable polymers use to form hybrid scaffolds for soft tissue engineering.	[182]
Polyvinyl Alcohol (PVA)	Bio-Inks and Hydrogel Applications	PVA in 3D printing to be used as bio-inks, improving mechanical properties and cell viability in tissue engineering applications.	[183]
		Examples depicting PVA hydrogels in curing wounds and drug delivery systems.	[184]

).

4.3.1. Tissue Engineering Scaffolds

The creation of scaffolds for tissue engineering using 3D bioprinting biopolymers represents a significant advancement in regenerative medicine [185,186]. This technology allows for the precise construction of a scaffold that mimics the structure and function of genuine tissues, supporting cell proliferation, differentiation, and ultimately tissue regeneration. The advent of 3D bioprinting has given researchers greater flexibility in creating complex scaffolds that can be designed to mimic the target tissue’s ECM [187]. The ability to build intricate, multi-layered structures enhances cell adhesion and proliferation, two processes essential for effective tissue regeneration [89,188].

Polymers such as PLA, gelatin, and polycarbonate are widely used due to their beneficial properties. Polyetheretherketone (PEEK) is notable for its excellent mechanical strength, chemical resistance, and biocompatibility. The ability of 3D-printed PEEK to replicate the properties of human bone makes it advantageous for load-bearing applications in surgical implants [189]. The capacity of the scaffold to integrate with both soft and hard tissues is essential for successful tissue engineering; this is where composite formulations, such as hydroxyapatite reinforcement, are useful. Polyurethane (PU) has a unique chemical structure made up of alternating hard and soft segments [190]. Due to its structure, mechanical properties can be adjusted to meet specific biological requirements. The ability of scaffolds to mimic the flexibility and robustness of genuine tissues is essential for creating scaffolds that encourage cell attachment and proliferation. PU scaffolds have the potential to be particularly helpful for soft tissue repair without damaging surrounding healthy tissues. Furthermore, PU’s high degree of biocompatibility facilitates its simple integration with host tissues, hastening the healing and regeneration processes while providing adequate mechanical support [191].

The water-soluble synthetic polymer known as PVA has garnered a lot of interest in tissue engineering and bio-ink research because of its non-toxic properties and superior biocompatibility. It is hydrophilic, and it can create hydrogels with a high water retention rate, which makes them perfect for encasing cells and promoting their integration with adjacent tissues. It is pliable to chemical changes, and scaffolds formed of it can mimic the ECM and provide a space for cells to join and proliferate [192].

PVA-infused bio-inks are essential for bioprinting because they improve printability, mechanical strength, and adhesive properties. PVA hydrogels may be easily treated using a range of 3D printing techniques, enabling the creation of complex tissue architectures with regulated and reproducible geometries [193]. Furthermore, PVA hydrogel’s biodegradability makes it an excellent material for temporary scaffolds that encourage tissue regeneration; these scaffolds may be readily taken down as new tissue forms. PVA can potentially be utilized as a matrix for drug delivery applications. It can provide localized and prolonged release when combined with medications, significantly increasing therapeutic efficacy and reducing side effects [194].

Moghaddaszadeh et al. emphasized the use of carbonated nanohydroxyapatite insertion into PCL scaffolds for bone tissue engineering in order to demonstrate its effectiveness [195]. The bioactivity and mechanical robustness of the scaffolds were enhanced by the inclusion. Among other qualities, hybrid scaffolds, which are created by integrating multiple biopolymers, can satisfy tissue engineering requirements by having increased mechanical strength and bioactivity. The potential of 3D-bioprinted scaffolds is also being actively explored in the fields of dental and cartilage tissue engineering. Choi et al. demonstrated the impact of a scaffold composed of methacrylated gelatin and calcium silicate cement on human dental pulp stem cells [196]. This demonstrated the scaffold’s capacity to encourage cell division and proliferation. Moreover, Maihemuti et al. developed fish gelatin scaffolds specifically for cartilage tissue engineering by utilizing the accuracy and customization provided by 3D bioprinting [197].

New research focuses on smart scaffolds that can sense and respond to pH and temperature changes. Through dynamic interactions with cells, Wang et al. enhanced the scaffold’s regenerative capability by incorporating self-healing double-network hydrogels. These smart materials provide more flexible tissue engineering solutions that adapt to the body’s evolving physiological needs [198]. Another area of scaffold design innovation is how to incorporate growth hormones, antibiotics, or other bioactive substances into the scaffolding material. According to Fu et al., 3D-bioprinted scaffolds with integrated delivery systems can encourage osteogenic differentiation and tissue growth [199]. This breakthrough allows for the controlled release of medicinal chemicals, which improves healing processes and increases tissue regeneration. Using the freedom of design, the range of materials, and new technologies, researchers are well positioned to develop flexible scaffolds that enhance tissue regeneration and repair.

4.3.2. Drug Delivery Systems

The use of 3D bioprinting technology in drug delivery systems has revolutionized the field by providing novel approaches to the controlled, customized, and targeted distribution of pharmaceuticals [200]. By enabling the building of complex geometries and the use of various biocompatible polymers, 3D bioprinting allows for the creation of customized drug delivery systems that enhance treatment efficacy and patient compliance [201]. PEG, a versatile polymer, is widely utilized in drug delivery systems, particularly in the form of hydrogels. High-water-content hydrogels act as a biomimetic replication of the ECM due to their remarkable hydrophilic properties. PEG’s physical and chemical tunability makes it a good option for pharmaceutical applications since it allows researchers to modify hydrogel characteristics including drug release profiles and degradation rates [202]. PEG-based hydrogels enable the targeted and progressive release of pharmaceuticals. The diffusivity of encapsulated medications is influenced by the cross-link density of PEG hydrogels, enabling fine-grained control of release kinetics.

Recent studies have shown that combining several biopolymers can create composite materials that enhance the functionality of drug delivery systems. Ng et al. developed 3D-printed aerogel scaffolds that are osteo-inductive and antibacterial by incorporating drug-laden hollow mesoporous silica microparticles into silk fibroin biopolymer matrices [203]. According to this study, a single scaffold can be used for a variety of tasks, including drug delivery and tissue regeneration promotion using bioactive compounds.

Through 3D printing technology, it is now possible to envision complex medicinal drug release profiles. For instance, Gioumouxouzis et al. developed pH-responsive solid dosage forms that regulated the release of the chemotherapeutic drug 5-fluorouracil using alginate beads [204]. By optimizing the structure and composition of 3D-printed scaffolds, targeted and extended medication release can be achieved, significantly increasing the effectiveness of therapy. Advanced drug delivery systems that adjust to changes in the body are now possible thanks to the integration of smart technology and 3D bioprinting. Zhang and Wang’s study, for instance, investigated the potential of topical drug administration through the use of 3D-printed hydrogels [205]. They concentrated on the application of stimuli-responsive materials, which change their properties in reaction to pH and temperature changes in the environment. Smart medication delivery systems may be more effective if the release mechanism is customized to meet the specific requirements of each patient. Recent research has also focused on improved drug distribution by 3D bioprinting with materials that are magnetically sensitive or electrostatically charged [206]. To illustrate how outside factors can change the kinetics of medication release, chitosan/bismuth ferrite scaffolds that can be electrically controlled to release medications were developed by Baykara et al. [207]. These innovative techniques improve focused drug delivery while also motivating patients to take their prescriptions on time. The employment of real-time monitoring technologies to track the dynamics of drug distribution is an exciting step in enhancing the security and efficiency of these systems [208].

4.3.3. Biosensors and Medical Devices

One of the most exciting applications of 3D bioprinting technology is the creation of biosensors that can identify specific biological signals. In their description of a fully sterile 3D printer, Hart et al. describe how bioprinting electroactive cell structures is necessary to create sensitive cell-based biosensors [209]. Microelectrode arrays and potentiodynamic techniques can be used to detect physiological changes that signify disease conditions. Creating layered structures enhances sensors’ usefulness and sensitivity. Three-dimensional bioprinting is where contemporary tissue engineering technologies got their start. Ke et al. successfully bioprinted trachea constructions, demonstrating the ability to employ biopolymers to create tissues that closely resemble native architecture. According to Ke et al., these structures were constructed with patient-specific designs and suitable mechanical qualities [210]. The customized approach raises the prospect of designing scaffolds or implants that fit each patient well, perhaps improving surgical outcomes.

The integration of vascular networks into printed scaffolds remains a major challenge for tissue engineering. Yeo et al. emphasized the significance of integrating vascularization strategies with 3D bioprinting [211]. This method makes it possible to create perfusable tissue constructions that can hold larger amounts of tissue. These structures significantly increase the transplant success rate in living patients by utilizing biopolymers that stimulate angiogenesis and endothelial cell adhesion. In addition to biosensors, 3D bioprinting has significantly advanced the field of drug delivery systems. Wu and Hsu developed hydrogels that can self-heal and adjust to different body states [212]. By encapsulating therapeutic molecules in these hydrogels, targeted release is made possible, increasing the efficacy of drug therapies while lowering side effects. The degree of control one can have over the physical and chemical properties of smart materials is one benefit of 3D printing them.

Using extracellular vehicles (EVs) in 3D-printed objects is a novel approach to enhance drug delivery. Han and Ivanovski showed how 3D bioprinted devices might deliver medicines for tissue engineering without the need for cells by utilizing the regenerative capabilities of EVs [14,213]. The intricate regulatory framework that has long surrounded live cell therapies may be simplified by this strategy. Fetah et al. noted that traditional manufacturing techniques are often insufficient to create 3D tissues with complicated topologies, which are necessary for realistic biological modeling [214]. Three-dimensional bioprinting makes it possible to construct organ models layer by layer, improving the capacity to replicate pharmacological reactions and tissue interactions. The development of biosensors and medical devices using 3D bioprinting biopolymers is ushering in a new era of personalized medicine. The capacity to develop custom, biocompatible structures that are suited to individual patient needs both improves therapeutic efficacy and advances precision health.

4.4. Food and Drug Administration (FDA)/European Medicines Agency (EMA) Guidelines in 3D Printing of Biomedical Devices

FDA is involved in the regulation of 3D-printed biomedical devices. The FDA guidance file, “Technical Considerations for Additive Manufactured Medical Devices”, sets ethics for builders with reverence to design, manufacturing events, and safety and efficacy testing [215]. The guideline puts the focus on 1. Material Evaluation: Manufacturers need to demonstrate that resources employed in 3D printing (e.g., PEEK and PLA) are biocompatible for their purposes. 2. Process Validation: The development of construction must be validated so that there is uniformity, dependability, and traceability. 3. Post-Market Investigation: The strategies must be supervised in the post-market for functioning and any negative effects. As expertise in 3D printing progresses, the FDA has been involved in evolving new standards and adapting existing ones to include special attentions for additive manufacturing, such as material variation, quality, and dimensional conformance [216].

EMA’s Medical Device Regulation (MDR) outlines the process of evaluating and approving 3D-printed devices within Europe to ensure they have met high safety, efficacy, and performance standards prior to reaching the market [217]. These include procedures for design, risk management, and clinical evaluation. Like FDA guidelines, the EMA also demands an in-depth risk management plan specifically developed to reflect the unique properties of 3D-printed devices, such as thorough examination of material characteristics and possible risks inherent in their application.

The use of materials in the production of 3D-printed biomedical devices has a few ethical implications [218]. One significant ethical implication is the choice of using animal-derived materials versus synthetic materials. While animal-derived materials may provide natural properties that are useful for tissue integration, synthetic materials typically bring biocompatibility and ethical advantages through reduced use of animals. The ethical implications are related to sourcing practices, animal welfare, and environmental impacts of production [219].

The environmental impact of the materials used in bioprinting is a growing concern. Biodegradable artificial materials like PLA are welcome in certain uses. However, the environmental impact associated with their manufacture is also subject to controversy. Debates regarding sustainability should consider life cycle assessments of materials employed in biomedical devices to have minimal ecological footprint. Customization of 3D-printed devices raises issues of patient autonomy and informed consent. It is important to make sure that patients are properly informed regarding the materials employed, possible risks, and advantages of such devices—such as ethical issues regarding the use of genetically modified material or composite materials whose long-term effects are unknown [220].

The incorporation of FDA and EMA guidelines into the context of ethical considerations enhances the knowledge of 3D-printed biomedical devices. Compliance with regulations not only protect patient health but also promote technological advancement, while the resolution of ethical issues guarantees that the application of these innovative materials is responsible and watchful.

4.5. Commercially Accessible 3D-Printed Biomedical Products

Commercialization of 3D bioprinting from laboratories to clinics has gained traction, with a few companies spearheading commercialization of bioprinted tissues and medical devices [221]. Organovo is pioneering the development of 3D-printed human liver tissues. Its technology focuses on bioprinting functional liver tissues that will be employed in drug testing and regenerative medicine. Organovo’s liver tissues are designed to recapitulate the structure and function of native liver tissues and will provide a more accurate model for preclinical drug screening than traditional two-dimensional cell cultures. Organovo’s liver tissues have been successfully integrated into pharmaceutical research, decreasing efficiencies in preclinical testing and decreasing dependence on animal models [222]. By their capability to provide a realistic model of human tissue, for instance, liver constructs, the improved drug development processes were augmented by an enhanced understanding of drug metabolism and toxicity.

Three-dimensional systems can design state-of-the-art bioprinters for medical applications, e.g., the fabrication of complex tissue architecture and individual patient-specific anatomy models [223]. They offer high-resolution accuracy and capacity for producing extremely detailed structures with use in surgery planning and enhancement of patient results. In hospitals and clinics, 3D systems’ bioprinters are utilized for manufacturing customized surgical guides and implants. One such application was in the development of patient-specific models for surgery, which have been shown to increase surgical accuracy and reduce operation time. The ability to print replicas of anatomy has made possible better pre-surgical assessment, allowing surgeons to rehearse complex procedures prior to the actual operation.

Stratasys is a prominent 3D printing firm that provides solutions to produce customized orthopedic implants and instruments [224]. Their technology facilitates the creation of patient-specific implants that are designed to suit patients’ unique anatomy and anatomy modifications. Clinical applications of Stratasys technology involve orthopedic surgery with implants 3D-printed, which have shown significant improvements in surgery planning and safety. For instance, customized orthopedic models have been utilized to depict intricate fractures, leading to enhanced precision during surgery and, therefore, improved patient outcomes.

NuraLogix leads the pack in additive manufacturing of 3D craniofacial implants in an effort to enhance surgical planning for facial reconstructions [225]. The company’s revolutionary use of additive manufacturing allows them to customize implants with the intended aesthetic and functional specifications of patients. NuraLogix’s craniofacial implants have proved their worth through the production of patient-specific implants that restore function and aesthetics, significantly enhancing the quality of life in craniofacial surgery recipients [226].

5. Challenges in 3D Printing of Biocompatible Polymers

The development of 3D printing technology has opened up new avenues to produce biocompatible polymers with applications in medicine, specifically in tissue engineering and regenerative medicine. Despite the enormous potential of biocompatible polymers, a number of barriers currently stand in the way of their widespread and efficient usage in 3D printing. The choice of appropriate biocompatible polymers is a critical step that affects the mechanical properties and biological performance of printed objects. According to a study by Arefin et al., anisotropic behavior, where mechanical strength varies with direction, is a typical outcome of material property variability brought on by 3D printing parameters. When targeting certain tissue types, it can be difficult to guarantee that the created structures have constant mechanical qualities suitable for load-bearing applications [227]. This explains why balancing mechanical strength and biocompatibility is so challenging.

Three 3D printing techniques, bioplotting, FFF, and SLA, have varying needs for printability and bioink rheology. For instance, bioinks with high viscosities may hinder the extrusion process, hence reducing the fidelity and resolution of the printed structures. Inadequate cross-linking or fluctuations in heat during the printing process may also result in scaffold architectural defects that reduce overall functionality. To address these challenges, print speeds and layer deposition must be carefully optimized for each material and application.

Although cross-linking is required for bioinks to solidify into 3D structures, the cross-linking technique used has a significant impact on the mechanical and biological properties of biopolymers. The concentration and timing of cross-linking agents, such as calcium ions, are critical for alginate-based scaffolds in order to achieve the required mechanical properties and maintain cell viability. However, varying cross-linking rates can produce heterogeneous structures, which may impair the processes of tissue integration and repair. Therefore, research into the most effective strategies for cross-linking optimization is essential to enhance scaffold performance in bioprinting applications.

Biocompatibility is the highest priority when developing 3D-printed medical devices. According to Agueda et al., 3D printing has the potential to alter the characteristics of polymers, resulting in the release of hazardous substances or the creation of non-biocompatible byproducts [228]. The challenge of developing novel materials for clinical use is already significant, even before considering the need to meet stringent safety and efficacy regulations. Since materials must comply with FDA requirements and elicit appropriate biological responses, bringing new ideas to market requires substantial additional effort and investment.

3D-printed structures can operate better if bioactive substances or nanoparticles are included. For example, Chen et al. presented a technique that combines mechanical strength with biological activity in their discussion of how to enhance cellular development utilizing graphene oxide in polymer composites [229]. However, including functional materials presents a number of challenges, including ensuring the stability of bioactive molecules during printing and attaining even dispersion across the scaffold. Targeted bioactive responses and careful mechanical property optimization are necessary for tissue engineering (Table 5).

Post-processing is often necessary to enhance the structural and functional performance of 3D-printed constructions. Mechanical properties are improved by processes like thermal treatment and photopolymerization, although biocompatibility may be impacted. Optimizing these post-processing procedures while maintaining the intended biological utility of the printed objects is a difficult task that requires more study. The study by Rimington et al. focuses on understanding the connection between cell activity and processing factors, which is necessary for creating effective tissue models [230,231]. Continued research and development are required in a number of areas, including material selection, printing procedures, cross-linking dynamics, biocompatibility, functionality integration, and post-processing, in order to fully exploit 3D bioprinting technologies for customized medical applications [210,232].

Table 5. Clinical applications, challenges, and case studies of 3D bioprinting.

Tissue/Organ	Biopolymer Used	Outcomes	Challenges	References
Skin	Collagen and Gelatin	Successful integration with surrounding tissues; improved functionality in wound healing	Limited durability; long-term effectiveness needs further study	[233]
Heart Valve	Alginate and Gelatin	Improved compliance and structural integrity; potential for transplantation	Need for precise mechanical properties to imitate natural heart valve function	[234]
Cartilage	Alginate and Chitosan	Enhanced chondrogenesis with promising tissue regeneration outcomes	Limited mechanical strength; heterogeneity in cellular distribution	[235]
Bone	Hydroxyapatite and Polycaprolactone (PCL)	Demonstrated osteoconductivity; integration into host bone with favorable healing	Ensuring adequate vascularization; long-term integration and biomechanical properties	[236]
Vascular Structures	Gelatin, PEG, and Fibrin	Formation of functional vascular networks within engineered tissues	Minimizing thrombosis; optimizing cell-laden delivery systems	[237]
Nerve Regeneration	Polycaprolactone (PCL) and Gelatin	Preliminary indications of successful neuroregeneration	Ensuring accurate alignment of nerve fibers; biocompatibility	[238]
Liver	Decellularized ECM and Gelatin	Enhanced hepatocyte function; improved model for drug testing	Recreating multi-cell interactions; maintaining liver-specific functions in vitro	[239]
Craniofacial Implants	PLA and PEEK	Customized fitness leading to improved clinical outcomes and patient satisfaction	Ensuring proper mechanical properties for longevity; challenges in integrating with existing bone	[240]
Tendon and Ligament	Gelatin, Fibrin	Improved cell survival and healing outcomes in volumetric structures	Limited understanding of the mechanical cue for differentiation; collagen organization	[241]

Table 5. Clinical applications, challenges, and case studies of 3D bioprinting.

Tissue/Organ	Biopolymer Used	Outcomes	Challenges	References
Skin	Collagen and Gelatin	Successful integration with surrounding tissues; improved functionality in wound healing	Limited durability; long-term effectiveness needs further study	[233]
Heart Valve	Alginate and Gelatin	Improved compliance and structural integrity; potential for transplantation	Need for precise mechanical properties to imitate natural heart valve function	[234]
Cartilage	Alginate and Chitosan	Enhanced chondrogenesis with promising tissue regeneration outcomes	Limited mechanical strength; heterogeneity in cellular distribution	[235]
Bone	Hydroxyapatite and Polycaprolactone (PCL)	Demonstrated osteoconductivity; integration into host bone with favorable healing	Ensuring adequate vascularization; long-term integration and biomechanical properties	[236]
Vascular Structures	Gelatin, PEG, and Fibrin	Formation of functional vascular networks within engineered tissues	Minimizing thrombosis; optimizing cell-laden delivery systems	[237]
Nerve Regeneration	Polycaprolactone (PCL) and Gelatin	Preliminary indications of successful neuroregeneration	Ensuring accurate alignment of nerve fibers; biocompatibility	[238]
Liver	Decellularized ECM and Gelatin	Enhanced hepatocyte function; improved model for drug testing	Recreating multi-cell interactions; maintaining liver-specific functions in vitro	[239]
Craniofacial Implants	PLA and PEEK	Customized fitness leading to improved clinical outcomes and patient satisfaction	Ensuring proper mechanical properties for longevity; challenges in integrating with existing bone	[240]
Tendon and Ligament	Gelatin, Fibrin	Improved cell survival and healing outcomes in volumetric structures	Limited understanding of the mechanical cue for differentiation; collagen organization	[241]

6. Future Directions and Emerging Trends

Biocompatible polymer 3D printing is constantly evolving due to technological advancements and a better understanding of how materials interact with living systems. The main objective for the near future is the development of more sophisticated and useful biomaterials for application in medicine, namely, in the areas of tissue engineering, drug delivery, and regenerative medicine. One area of increasing interest is the creation of hybrid bioinks, which combine several biocompatible polymers. Magli et al. found that 3D-printed chitosan–gelatin hybrid hydrogels had improved mechanical properties and improved cell interaction [242]. As stated by Magli and colleagues, this technique allows for the fine-tuning of material properties to better support cellular processes and replicate complex tissue characteristics.

Caurasia et al. devised a 3D printing process called Freeform Reversible Embedding of Suspended Hydrogels (FRESH), which uses chitosan bioink to create complex biological structures [243]. These techniques enable the use of extremely precise forms to support intricate tissue architecture. The creation of vascularized tissue models is a critical first step toward effective organ regeneration; the FRESH technique and sacrificial support materials hold considerable promise in this respect.

Smart bioinks that respond to stimuli are another domain for the development of 3D printing technology. Researchers are now working on creating bioinks that can mend themselves when they are injured. This is particularly crucial for tissues that are under mechanical stress and undergo continuous change. Shehzad et al. noted that the mechanical performance and resilience of printed structures are enhanced when dual-crosslinking techniques are applied to gelatin hydrogels [244]. The exceptional wound-healing properties of these materials represent a significant advancement in the durability and adaptability of bioprinted tissues. “In situ 3D bioprinting”, the process of printing biological structures inside an organism or during surgery, is a rapidly developing field of research. This technique might fundamentally change how we repair and regenerate injured tissues. As demonstrated by Fisch et al. with enzymatically cross-linkable bioinks for cartilage applications, biocompatible hydrogels that can undergo in situ polymerization may be used to develop better tissue engineering procedures tailored to the anatomy of each patient [245].

Customizing bioinks for each patient based on their medical records is one trend in the field. According to Faramarzi et al., it is essential to consider the distinct biological and biochemical traits of every patient while creating bioinks for tissue and organ regeneration [246]. By reducing rejection rates and accelerating the rate at which the surrounding biological environment accepts and integrates implanted tissues, this customized approach may greatly enhance healing outcomes. The need for using sustainable materials is becoming increasingly apparent. Researchers are looking into hydrogels made of polysaccharides and biodegradable materials like lignin as potential environmentally acceptable substitutes for conventional 3D printing materials. As a way to stop environmental damage and emphasize the value of materials that coexist peacefully with living things, renewable energy is gaining popularity.

As the industry develops, resolving regulatory issues will become increasingly important. Standardized procedures and thorough testing for new biocompatible materials are required to ensure the efficacy and safety of 3D-printed products in healthcare settings. Early involvement of regulatory agencies in the development phase may facilitate smoother translations of research into clinical applications. Innovative hybrid bioinks, advanced printing methods, personalized therapeutics, and smart materials with self-healing properties all suggest promising futures for biocompatible polymer 3D printing. By researching eco-friendly materials and developing robust regulatory frameworks, the viability of bioprinting technologies will be significantly enhanced. As researchers strive to overcome present challenges and capitalize on emerging trends, regenerative medicine has the potential to experience a significant revolution.

7. Conclusions

Three-dimensional bioprinting has made significant advancements in tissue engineering and regenerative medicine by mimicking the ECM and preserving biological activity through biocompatible polymers. As additive manufacturing processes such as SLA, FFF, SLS, and inkjet printing have advanced, the advantages of this technology have advanced as well. The promise of 3D bioprinting for tissue engineering, drug discovery, and organ transplantation has grown due to improved rheological properties, polymer blending, and the integration of bioactive components. However, key challenges persist.

Key Takeaways:

• Advances in bioprinting methods have improved the capacity to print intricate tissue architectures.
• Enhanced rheological properties of bioinks have been essential for successful extrusion and print quality.
• 3D bioprinting has tremendous potential for applications in personalized medicine, drug discovery, and organ transplantation.
• The main obstacles to overcome are the mechanical instability of constructions, material anisotropy, and the necessity for improved biodegradability.

To realize the full therapeutic potential of 3D bioprinting, it is necessary to overcome the following hurdles:

• Maintaining the structural stability of printed constructs under physiological loads is critical for clinical use.
• The creation of isotropic materials that behave consistently under different mechanical loads.
• The necessity for bioinks that not only support structure but also degrade in a predictable manner after serving their purpose in the body.

By addressing these challenges, future research should focus on developing sustainable materials to enhance the structural and functional characteristics of printed constructions, refining bioink formulations, and integrating computer modeling for precision bioprinting. Three-dimensional bioprinting has the potential to significantly advance personalized medicine and improve healthcare outcomes.