Extraction of Natural-Based Raw Materials Towards the Production of Sustainable Man-Made Organic Fibres

Abstract

Bioresources have been gaining popularity due to their abundance, renewability, and recyclability. Nevertheless, given their diverse composition and complex hierarchical structures, these bio-based sources must be carefully processed to effectively extract valuable raw polymeric materials suitable for producing man-made organic fibres. This review will first highlight the most relevant bio-based sources, with a particular focus on promising unconventional biomass sources (terrestrial vegetables, aquatic vegetables, fungi, and insects), as well as agroforestry and industrial biowaste (food, paper/wood, and textile). For each source, typical applications and the biopolymers usually extracted will also be outlined. Furthermore, acknowledging the challenging lignocellulosic structure and composition of these sources, an overview of conventional and emerging pre-treatments and extraction methods, namely physical, chemical, physicochemical, and biological methodologies, will also be presented. Additionally, this review aims to explore the applications of the compounds obtained in the production of man-made organic fibres (MMOFs). A brief description of their evolution and their distinct properties will be described, as well as the most prominent commercial MMOFs currently available. Ultimately, this review concludes with future perspectives concerning the pursuit of greener and sustainable polymeric sources, as well as effective extraction processes. The potential and main challenges of implementing these sources in the production of alternative man-made organic fibres for diverse applications will also be highlighted.

1. Introduction

The extraordinary progress in the development of fibrous-based materials, such as textiles, high-performance, or multifunctional products, is irrefutably linked with the extensive exploitation of fossil-based plastics (also designated as synthetic polymers) [1]. Given these polymers’ properties, such as their light weight, durability, and versatility [2], the rate of plastic production has exponentially risen worldwide in recent decades, reaching 400.3 Mt in 2022 [3]. Moreover, due to the absence of efficient plastic waste management infrastructures, only 9% of global plastic production is recycled (mechanical and chemical) [3]. Consequently, the amount of discarded plastic has proportionally increased, either disposed of in landfills or released into the environment, where it ends up in the ocean or waterways, triggering a complex cascade of ecological drawbacks. Recent research has shown the adverse impact of rising plastic deposits in our ecosystems [4,5,6,7,8,9], essentially originating from progressive plastic fragmentation and the subsequent release of particles with different diameters, namely macro- (>20 mm diameter), meso- (5–20 mm), micro- (<5 mm), or nanoplastics (1 µm–1 nm). Additionally, this causes the leaching of additives from plastics, which have known toxic effects on living organisms [1,9]. A similar trend is observed in fibrous materials, where synthetic petroleum-based fibres dominate the market. Subsequently, there is a growing demand for eco-friendly, natural-derived polymers that contribute to carbon-neutral processes, are biodegradable, and can be utilised across various sectors, including the textile, healthcare, and protection industries [10,11,12].

There has been an acceleration of research in this promising area to address the need previously presented and to increase the cost-effectiveness of bioresource biorefineries by coupling biofuel, high-value products, and biopolymer production using waste and wastewater extracts. Such a strategy improves overall sustainability by lowering costs and carbon emissions in biorefineries, eventually contributing towards the much-touted circular, net-zero carbon future economies [13].

This review contributes to the existing literature with a comprehensive overview of alternative sources, particularly involving the exploitation of biomass and biowaste. It focuses on unconventional and more sustainable bio-based sources that could provide valuable polymers (polysaccharides, proteins, phenolic compounds, and lipids) and derived monomers (e.g., glucose, amino acids, and fatty and dicarboxylic acids) suitable for the synthesis of bio-based polymers, envisaging the production of man-made organic fibres. In this sense, this review highlights some abundant sources of biomass and biowaste that could be better explored to supply building blocks for both the production and functionalisation of sustainable man-made organic fibres. Nonetheless, to enable the efficient extraction of biopolymers and their subsequent monomers, the most relevant methodologies for bio-extraction (i.e., the extraction of biopolymers) will be briefly outlined, ranging from conventional approaches to emerging and more environmentally friendly techniques. Moreover, this review provides a discussion of the evolution of man-made fibres and their production, which, in turn, highlights the emergence of new directions to produce more sustainable fibres from greener and renewable sources. New perspectives will be afforded to embrace natural-based polymers and bio-based monomers as sustainable alternatives to fossil-derived sources, to produce valuable and multifunctional man-made organic fibres.

For the above-mentioned topics that this review paper explores, a bibliographic search was performed in the ScienceDirect, ResearchGate, and Google Scholar databases using a combination of the following keywords: biomass; biowaste; sustainable sources; renewable sources; agroforestry biowaste; industrial biowaste; valorisation; biorefinery; biopolymers; green chemistry; chemical pre-treatments; mechanical pre-treatments; physiochemical pre-treatments; biological pre-treatments; lignocellulosic extraction; sustainable fibres; man-made organic fibres; and fibre production. In this regard, it should be mentioned that a remarkable number of documents have been published in the last decade related to these different terms. Only research papers, reviews, and book chapters are considered in the comprehensive analysis presented in the following sections.

2. Bioresources: Biomass and Biowaste

In addressing environmental and sustainability issues, there is an urgent need to explore the potential of abundant biomass and biowaste as privileged polymeric sources available for fibre production. Both biomass and biowaste have been exploited for conversion into electricity, heat, and advanced biofuels, either through thermochemical or biochemical processes [14], but there is another sustainable route to explore for these renewable materials, which is the extraction of valuable biopolymers.

Given that distinct biomass definitions have been provided in different contexts (e.g., ecology and energy) [14,15,16,17], this review proposes the following distinction between biomass and biowaste sources. Biomass or primary biomass comprises any organic material that could be taken directly from nature and, as illustrated in Figure 1, the most abundant resources are vegetable (terrestrial and aquatic), fungal, and Animalia species. On the other hand, biowaste consists of residual biomass obtained from different biomass processing, as shown in Figure 2, particularly from agroforestry and industrial (food, paper/wood, textile) sectors. In fact, contaminated biowaste from municipal waste was excluded from this discussion, as it presents additional costs and challenges for effective biopolymer extraction.

Concerning biomass, the main sources that should be highlighted, due to their abundance and biopolymeric composition, are suggestively classified as terrestrial and aquatic vegetables, fungal, and animal species (see Figure 1).

2.1. Terrestrial Vegetables

Regarding terrestrial vegetables, the relevance of cotton, flax, or ramie as the plant species recurrently used to obtain valuable plant-based natural fibres is well known. However, it is also recognised the negative environmental impact attributed to their cultivation and manufacturing, particularly for cotton [10,11,12]. Hence, searching for alternative sources to obtain valuable biocompounds suitable for fibre production, there are some terrestrial vegetable species that could reduce these environmental concerns, including water use, pesticide and fertiliser application, eutrophication, and greenhouse gas emissions [18]. In this regard, the most promising terrestrial vegetable species that could be considered are hemp (Cannabis sativa Linn.), jute (Corchorus capsularis), common nettle (Urtica dioica L.), and Juncus plant (Juncaceae).

Hemp, one of the world’s oldest bast plants, is known for its exceptionally strong cellulose fibres [19,20]. Beyond its fibres, hemp biomass—including hurds, leaves, and inflorescences—offers great potential for synthesising bioproducts such as high-quality microbial protein [21] and biopolymers like polyhydroxyalkanoates (PHA) and polyhydroxylbutyrate (PHB). These bioproducts can be applied across various industries, including food, textile, and packaging [19,22]. Particularly for the recovery of both cellulosic and non-cellulosic components from hemp, some studies have highlighted extraction methodologies based on deep eutectic solvents (DES) [23,24,25,26] to produce greener functional and bioactive nanofibres with distinct applications [23,27,28]. In addition, jute (Corchorus capsularis L.) is another source of non-wood lignocellulosic content [29,30,31], mostly cultivated in Southeast Asian countries for the extraction of bast fibres with textile and high-quality paper pulp production applications [32]. In fact, jute has been exploited for the extraction of nanocellulose (e.g., cellulose nanofibrils, cellulose nanocrystals), which presents a unique structure and excellent mechanical properties and low thermal expansion coefficient and transparency [33], which are suitable as reinforcing fillers in biodegradable polymeric matrices [34,35,36,37]. Moreover, in comparison with other non-wood bast fibres, a typical jute fibre exhibits high lignin content [38], predominantly composed of syringyl (S) units, that enable a relatively easier delignification [39]. This high lignin content has a negative effect on fibre quality and separation [32], particularly for textile purposes, which is why jute is often subjected to a delignification process to obtain low-lignin fibres. The S-lignin removed during this process is a valuable renewable aromatic biopolymer, with the potential to replace petroleum-based aromatic polymers and to produce biochemicals and bio-based materials [40].

The common nettle (Urtica dioica L.) is a perennial plant with minimal pesticide and fertiliser requirements that is widely distributed all over the world. Attending the high cellulosic content in nettle leaves [41], this renewable source has a high potential for nanocellulose extraction (e.g., nanocrystalline and nanofibrillated cellulose or hairy cellulose nanocrystals) [41,42]. These nanocelluloses are described as lightweight nanofillers with high stable dispersity, offering high strength and a superior strength-to-weight ratio [41], making them suitable for a wide range of nanocomposite materials. Moreover, nettle is one of the most used medicinal plants in traditional medicine and recently has also attracted interest in modern medicine [43]. These attractive pharmacological/medicinal properties of nettle are essentially due to the peculiar phytochemical composition of the nettle plant (root, leaves, stem, flowers, seed), which is rich in terpenoids, carotenoids, fatty acids, polyphenols, essential amino acids, polysaccharides–glucans, lectins, flavonoids, vitamins, and minerals, among others [43,44]. Some studies have reported the high potential of nettle leaf extracts with valuable biological activity, including antioxidant, antibacterial, anti-inflammatory, anti-cancer, anti-diabetic, and hepato-protective properties [45,46]. In this sense, the effective extraction of these biologically active compounds and their further incorporation into fibrous materials could contribute to their application in the biomedical field.

The Juncus plant (Juncus effusus L.) is a perennial plant that can grow in distinct environments, particularly in wet areas. It exhibits stems with a high amount of cellulose and therefore has great potential to serve as an excellent source for producing cellulose derivatives, including micro- and nanocellulose fibres and cellulose nanocrystals [47,48,49,50]. Moreover, some studies have described the effective mechanical and structural reinforcement of these cellulose derivatives when incorporated as a filler in the development of different fibrous and nanocomposite materials [47,48,49,51].

On the other hand, some invasive species could also be considered as an alternative lignocellulosic source due to their high biomass production and growth rate. Some that can be pointed out are as follows: giant reed (Arundo donax L.), a perennial grass recognised by its high productivity and low requirements [52,53,54], and Johnsongrass (Sorghum halepense L.), another perennial plant with high tolerance and the ability to grow in different environmental conditions, as it is constituted by fibres with mechanical properties comparable to some non-wood fibres [55,56,57,58,59]. Additionally, there is an emergent trend to explore abundant cactus and succulent plants as a rich source of polysaccharides, polyphenols, and natural cellulosic fibres for textile, food, paper production, and biocomposites applications [60]. In this regard, Cactaceae species (opuntia species, such as Opuntia maxima, Opuntia ficus-indica L.) [60,61,62,63], Asparagaceae species (agaves plants, such as Agave americana L.) [64,65,66,67] and Asphodelaceae species (Aloe plants, such as Aloe barbadensis) [68,69,70] are good examples of attractive cactus and succulent plants.

2.2. Aquatic Vegetables

Concerning aquatic flora, macroalgal (seaweed) and microalgal species have major significance, not only for biopolymer extraction, but also for providing precursors that could be further converted into bioplastics. This class of biosources has the advantage of presenting a simple cultivation process, which can contribute to the elimination of excess nutrients from the surrounding environment and help reduce greenhouse gas emissions, freshwater consumption, and potential deforestation [71]. Moreover, attending to the extracellular composition of macroalgae, namely the high content of polysaccharides, their extraction has gained particular interest [72]. In this regard, the most valuable polysaccharides extracted are agar–agar and carrageenan, commonly obtained from red algae (Rhodophyta); alginates, fucoidans, and laminarin, derived from brown algae (Phaeophyta); and granular starch and ulvan resulting from green algae (Chlorophyta) [13]. Furthermore, several macroalgal species have been described as having high levels of cellulosic content. These species include the following: green macroalgae such as Cladophora (20–45%), Chaetomorpha (37–41%), Rhizoclonium (39%), and Ulva (2–19%); red macroalgae such as Griffithsia (22%), Ceramium (19%), Chondria (16%), Corallina (15%), Gelidiella (11–14%), Gracilaria (11%) and Hypnea (11%); and also a few brown macroalgae, such as Laminaria (1–20%), Fucus (14%), and Halidrys (14%) [13,73,74].

On the other hand, microalgae are unicellular organisms that grow rapidly even in polluted environments (wastewater) [71,74]. Some species, such as Chlamydomonas [75], Chlorella vulgaris [76], and Chlorella pyrenoidosa [77]), have been mostly exploited for biorefinery production (e.g., biodiesel, biogas, bioethanol), where the resulting by-products could be used to obtain biopolymers (mainly, cellulose and starch) and polyesters (such as polyhydroxyalkanoate, polybutylene succinate, and polylactic acid). In this regard, since the productivity of microalgae is intrinsically dependent on different factors involved in carbon fixation and photosynthesis processes (among others, light, temperature, culture density, species cultivated, and culture medium conditions), microalgae have not been extensively used for biopolymer extraction as macroalgae [13].

Numerous studies have analysed the compositional analysis of both macro- and microalgae, with some examples presented in

Table 1. Lignocellulosic composition (wt%) of different biomass resources and their principal applications. Notation: abs and n.d. mean absence and not determined, respectively.

Biomass Resource	Cellulose (%)	Hemicellulose (%)	Pectin (%)	Lignin (%)	Applications	References
Hemp (stalks)	67–76	12–18	3–18	3–6	Textile, paper, food, cosmetics, packaging, biodiesel	[19]
Jute (stalks)	61–73	14–23	n.d.	12–16	Textile, packaging, biocomposites	[31]
Nettle (leaves, stem, roots)	54–88	4–10	1–4.1	5–9	Textile, food, biocomposites, biomedical	[79]
Juncus (stem)	40–64	20–28	n.d.	6–19	Biocomposites	[51]
Giant reed (leaves, stem, rhizome)	21–38	11–42	n.d.	13–32	Paper, biorefineries, bioplastics, biocomposites	[52]
Johnsongrass (leaves, stem)	40	36	n.d.	10	Bioenergy, biorefineries, biocomposites	[59]
Opuntia ficus-indica L. (stem)	53	11	n.d.	4.8	Paper, food, pharmaceutical biocomposites	[80]
Agave americana L. (leaves)	59.5	17.4	6.80	13.6	Biocomposites	[67]
Aloe vera (leaves)	64.9	25.1	n.d.	4.1	Textile, cosmetic, pharmaceutical, biocomposites	[69]
Chlorella vulgaris (microalgae)	10–47.5	n.d.	n.d.	n.d.	Biorefinery, bioplastics, biocomposites	[78]
Chlorella pyrenoidosa (microalgae)	15.4	31	n.d.	n.d.	Biorefinery, bioplastics, biocomposites	[81]
Cladophora rupestris (macroalgae)	28.5	abs	abs	n.d.	Biorefinery, bioplastics, biocomposites	[82]
Chaetomorpha melagonium (macroalgae)	41	n.d.	n.d.	n.d.	Biorefinery, bioplastics, biocomposites	[82]
Laminaria digitata (macroalgae)	20	n.d	n.d.	n.d.	Biorefinery, bioplastics, biocomposites	[82]
Halidrys siliquosa (macroalgae)	14	n.d.	n.d	n.d.	Biorefinery, bioplastics, biocomposites	[82]
Ulva sp.	40.7	7.1	n.d.	7.9	Paper, biorefinery	[83]
Water hyacinth	12.8–14.9	24–27.5	n.d.	5.9–14.3	Textile, paper, biocomposites	[84,85]
Waterweeds	18.8	5	n.d.	2.3	Biosynthesis of nanoparticles	[86,87,88]

. It is worth noting that the significant variation in polymeric content (e.g., cellulose) can largely be attributed to the influence of cultivation conditions. This was demonstrated by Aguirre and Bassi [78], who emphasised the importance of well-designed cultivation strategies to maximise cellulose production [73,78].

Some invasive aquatic species, such as water hyacinth (Eichhornia crassipes), Canadian waterweed (Elodea canadensis Michx), and yellow waterlily (Nymphaea mexicana Zucc.), could also be considered abundant sources of lignocellulosic content, without negative effects on global diversity, since they need to be regularly removed from waterways and the biomass obtained is generally disposed of without further use.

Water hyacinth (Eichhornia crassipes) is a widely available free-floating aquatic weed, notable for its high content of cellulosic fibres. It has traditionally been used as a textile raw material in the furniture and household handicraft industries, but recent studies have already reported its potential for the synthesis of cellulose nanofibres [85,89,90]. Notably, it has been reported that Eichhornia crassipes fibres have better properties than glass fibres, including higher tensile strength, stiffness, heat resistance, and corrosion resistance properties, and have great potential as a filler in polymer composites [85,90].

Waterweeds or American macrophytes, such as Elodea canadensis and Elodea nuttallii, have also been proliferating at a high-speed rate in waterways over the last few decades. Therefore, this aquatic biomass has been periodically removed and disposed of without further use. To repurpose them, a few works have reported their composition, revealing lignin, cellulose, and hemicellulose within the typical range for fast-growing species, along with a relatively high protein content [87]. In addition, E. canadensis contains bioactive compounds, including chlorophyll, carotenoids, ascorbic acid, flavonoids, and phenolic acid, which have gained interest in the biosynthesis of various kinds of nanoparticles suitable for biomedical applications [88].

Yellow waterlily (Nymphaea mexicana Zucc.) is another aquatic plant with the potential to grow rapidly and spread into waterways. This aquatic plant has not been substantially explored, however, some studies have performed its physicochemical characterisation, which has described different valuable bioactive extracts with anti-inflammatory and anti-microbial properties [91,92].

2.3. Fungi

Fungi, one of the largest groups of eukaryotes, include a diverse variety of organisms ranging from microscopic unicellular yeasts to macroscopic multicellular mushrooms, and are the second-largest community of organisms on the planet [93]. They play a major role in balancing the environment, the carbon cycle, and the mobilisation of biogenic elements, such as nitrogen and phosphorus [94]. Respecting their structural composition, fungi present similar structures to crustaceans, particularly in their polysaccharides, which are among the most significant bioactive components. Fungal polysaccharides have been extensively used in the cosmetic, food, and health product industries [95]. One of those polysaccharides is chitin, the main component of the cell walls of fungi (1–15%) [96], with fungi being the second most common source of chitin after crustaceans [97]. Moreover, fungal chitosan can also be obtained through deacetylation of the chitin [98]. The only fungal source in which chitosan naturally occurs together with chitin is the cell wall of the zygomycetes, a filamentous fungus [99,100]. Fungal chitosan has several advantages over crustacean chitosan, such as better availability and consistent quality. Hence, some research papers have proposed this alternative chitosan source for producing biocompatible materials, such as absorbable sutures [99], and sustainable textiles [100].

Besides chitin, the cell wall of fungi includes many β-glucans, which are linked with different types of β-glycosidic bonds. Since most of them have a highly branched structure and helix conformation, fungal β-glucans have demonstrated various biological functions such as immunomodulatory, antioxidation, anti-inflammatory, and antibacterial [93,101,102]. Moreover, due to the high content, quality, and availability of proteins contained in fungal organisms, attention is being paid to the fungal proteins and their potential application in food and health areas [94]. Among all fungal proteins, lectin is one of the most studied proteins due to its bioactive properties, since it can recognise and interact with a wide range of cell surface glycans/glycoproteins [103]. Several studies have been conducted with lectins derived from mushrooms (82% of fungal lectins), but there is also lectin present in moulds and yeasts, which represents around 15% and 3% of fungal lectins, respectively [94].

2.4. Animalia

From Animalia, insects are one of the most diverse and abundant arthropods. They have been considered a valuable food source since their exoskeleton is constituted of chitin (10–15%), proteins (30–45%), fat (25–40%), and minerals [104]. Recent research has been conducted to explore edible insects as an alternative source of chitin, chitosan [97,105], and proteins [106,107,108]. Some orders have been studied and extensively characterised, namely Blattodea, Coleoptera, Hymenoptera, Lepidoptera, and Orthoptera [109]. Compared to crustaceans, insects typically have a lower inorganic content, allowing extraction methods to be carried out under milder conditions [109], as previously discussed by Philibert et al. [110]. Therefore, insects present the potential to be used as an alternative source of chitin and chitosan [97,111,112,113,114].

Concerning biowaste, two main sectors will be explored within the scope of this review, namely agroforestry and industry (food, wood/paper, and textile). These could provide valuable raw materials to be explored for biopolymer extraction (see Figure 2).

2.5. Agroforestry Residues (Hardwood, Softwood, and Agricultural Residues)

Agroforestry consists of the integration of farming systems with woody perennials in a balanced ecological system that can improve both crop growth and soil fertility [115,116]. Hence, this new concept is rising worldwide to overcome some adverse effects of agricultural practices [117].

Subsequently, from agroforestry practices diverse by-products can be obtained, such as hardwood, softwood, and agricultural residues. These residues can yield various renewable chemicals and materials, with the lignocellulosic fraction (or lignocellulose feedstock)—comprising lignin, cellulose, hemicellulose, and tannin—being the most abundant raw material [118]. This abundance highlights the potential of utilising this biowaste for the production of high-value compounds [119]. Thorenz et al. [120] assessed the bioeconomic potential of agroforestry residues in the European Union, identifying wheat straw, maize stover, barley straw, and rape straw as the most promising agricultural sources. These crops have a high lignocellulosic concentration, exceeding 80% of their dry matter. However, their potential remains largely underutilised, with only 8% of agricultural waste currently being exploited. In forestry, coniferous species such as spruce and pine are the most notable sources, containing approximately 70% lignocellulosic content.

Distinct applications have been attributed to the residual biomass from agroforestry, such as the production of biofuels (biodiesel, bioalcohol), biogas, and bioenergy [121,122]. Along with that, there is also the possibility of using the biomass for the synthesis of chemicals, building block materials, biochar, paper and packaging, oils, bioplastics (e.g., polyhydroxyalkanoates, polylactic acid), and bioadsorbents [117].

Due to their wide availability, wood and its residues have great importance in energy production, the construction sector, and the pulp and paper industry [123,124]. In fact, wood and its inherent main constituents, carbohydrates (cellulose and hemicelluloses) and lignin, are also of great interest as sources of green chemicals, fuels, polymers, and novel nanomaterials. However, the recalcitrant nature of lignocellulose, related to lignin content, could significantly reduce the effective valorisation of this biomass. In this regard, softwood presents a more recalcitrant structure compared with hardwood [125], as presented in

Table 2. Lignocellulosic composition of different biowaste resources and their principal applications.

Biowaste Product		Cellulose (%)	Hemicellulose (%)	Pectin (%)	Lignin (%)	Applications	References
Hardwood	Poplar	50–53	26–29	-	15.5–16	Paper, biorefineries, biocomposites	[126,127]
	Eucalyptus	54	18	-	21.5		[126]
Softwood	Pine	45–50	25–35	-	25–35	Biorefineries, biocomposites	[126,127]
	Spruce	46	23	-	28		[126]
Cereals	Wheat straw	35–39	23–30	-	12–16	Biorefineries, PHA	[120,126,127]
	Corn stalk	35–40	17–35	-	7–18
	Barley straw	36–43	24–33	-	6.3–10
Fruits	Apple pomace	40–44	19–24	9–12	15–20	Biorefineries, bio-fertiliser, biofiller, PHA, xanthan gum	[128]
	Orange pulp	25.3	5.3	15.7–16.3	2.2–3.0		[128]
	Peach pomace	29–30	19–20	21–24	5–6		[128]
	Coconut coir	44	22	-	33		[129]
	Banana waste	13	15	-	14		[130]
	Spent coffee ground	12	39	-	24		[131,132,133]
Vegetables	Soybean straw	25	12	-	18	Biorefineries, bio-fertiliser, biofiller, PHA, PLA, xanthan gum	[120]
	Tomato pomace	9–19	5–12	7.5	3–36		[128,134]
	Potato pulp	17–22	14	2.2	2.6		[128,134]
	Olive pomace	19–37	22–27	16–17	26–40		[134,135]
	Carrot pomace	28–52	7–12	2–4	18–32		[134,136]
Marine animals	Crustacean shells	-	-	-	-	Chitin and chitosan extraction	[137,138,139,140,141]
Abattoir	Meat and poultry waste	-	-	-	-	Collagen, gelatine, keratin, PHA	[142,143,144,145,146,147,148,149,150,151,152,153,154,155]
Dairy	Milk and cheese waste	-	-	-	-	PHA, xanthan gum	[156,157,158,159,160,161,162,163,164]
Eucalyptus black liquor	Paper industry	-	1–2	-	40–42	Biorefineries, biogas, PHA, carboxylic acids	[165,166,167,168]
Discarded textile	Pre-consumer and post-consumer	-	-	-	-	Extraction of cellulose derivatives, polyester monomers, glucose and amino acids, biorefineries, fillers	[169,170,171,172,173,174,175,176,177,178,179,180]

, which could involve more aggressive delignification processes.

Nevertheless, different bio-derived materials can be obtained from soft- and hardwood residues. The most recent advances have highlighted particular interest in emergent materials, such as nanocellulose, hemicellulose, lignin, and activated carbon [123]. For instance, nanocellulose has been applied to design advanced materials with distinct applications in the energy, electronics, and biomedical fields. Moreover, hemicellulose, a heteropolysaccharide that could present distinct compositions, structures, and concentrations [181], can be widely used in the biomedical field and the transportation packaging field. Lignin is a complex and water-insoluble amorphous polymer with phenolic moieties that, in wood, could range from 15 to 30 wt% [182] as the major byproduct from the paper industry and lignocellulosic biorefineries. This macromolecule is suitable to produce bioactive (nano) materials for biomedical purposes (e.g., drug delivery). In the end, all wood residues could be used to produce activated carbon materials, which have potential in energy storage and environmental adsorption areas [123,183].

In addition to wood residues (both soft- and hardwood), which are associated with environmental concerns such as deforestation, abundant agricultural residues—such as stalks, husk, straw, and pomace—serve as sustainable alternatives for lignocellulosic sources. These agricultural residues offer a diverse range of lignocellulosic biomass, with their composition varying significantly depending on factors such as climate, cultivation conditions, crop varieties, extraction methods, and geographical distribution [184].

2.6. Industrial Waste

Concerning industrial waste, the main sectors contributing to massive waste and subsequent environmental concerns are the food, wood/paper, and textile industries.

The growing global population and the corresponding demand for processed food have led to a significant accumulation of by-products from food processing industries, creating serious environmental and economic challenges [185]. Nevertheless, a variety of industrial food waste including fruits and vegetables (e.g., banana, citrus, apple, tomato, and potato skin waste, mango, grape, and pumpkin seed wastes, coffee waste, bagasse, and sugar molasses waste, and peanut husk), grains (e.g., wheat, soybean, and rice husk) or animal-derived waste (e.g., dairy, seafood, meat, and fish processing) could be applied to extraction and synthesis of biopolymers [185]. Some examples of polymers that could be obtained are polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), and polysaccharides, such as xanthan gum, polylactic acid (PLA), and peptides. These present themselves as promising substitutes for conventional petroleum-based polymers.

Generally, industrial fruit, vegetable, and grain waste could include distinct residues, such as skin, seed, fruit, bagasse, molasse, and husk. All of those mentioned are composed of organic matter available for extraction as follows: structural polysaccharides (cellulose, starch, hemicellulose, pectin, glucan); storage proteins (zein, lectin, hordein, gliadin); lignin (and related phenolic compounds); and lipids (and derived fatty acids) [185,186,187]. Concerning the biopolymer extraction from this complex and diverse lignocellulosic biowaste, different works reported in the literature have compiled and discussed distinct strategies for an efficient valorisation of this abundant source in biorefineries and synthesis of bio-derived polymers, for example, PLA and PHA [185,188,189,190,191,192].

Seafood waste consists of crustacean (crabs, shrimp, lobster) shells and other inedible fractions. These present a complex hierarchical organisation consisting of crystalline alpha-chitin nanofibres embedded in a protein matrix with minerals [141,193]. Hence, these exoskeletons have been extensively exploited as the main biosource for chitin and chitosan extraction. Several works have reported different approaches for the extraction and characterisation of these structural polysaccharides from crustacean waste [137,138,139,140,141].

In the meat and fish processing industries, considerable amounts of biowaste are generated and discarded in landfills or incinerated [142,143], causing severe environmental concerns. The meat by-products are mostly from pigs, chickens, and cattle [194] and might comprise hair, wool, feathers, bones, hooves, horns, and beaks. The fish by-products could include carcasses, heads, skin, scales, tails, and viscera. These residues have been considered for their potential to obtain animal-derived proteins and other valuable compounds, such as enzymes, oils, and minerals. Some research has been conducted to develop effective extraction of animal-derived proteins, such as collagen [142,144,145,146], gelatine [147,148,149], and keratin [143,150,151,152]. These biomaterials are suitable for producing multifunctional materials, for example, nanoparticles, nanofibres, scaffolds, and hydrogels with distinct applications (e.g., food, cosmetics, and biomedical fields). Moreover, based on the content of fatty acids obtained from animal fat, meat industry waste has also been considered to produce biodiesel and co-products as PHA [153,154,155].

On the other hand, the dairy industry has also increased worldwide [195], leading to an intensified generation of organic waste from milk and other processed dairy products (e.g., cheese, yoghurt, butter, and ice cream) [156,157]. Dairy waste usually consists of solid and liquid wastes and is rich in lipids and proteins. These molecules can degrade and release toxic substances, which are a danger to biodiversity [196]. Due to the environmental impact of dairy wastewater, recent biotechnological processes have been established for the treatment and appropriate valorisation of dairy by-products [156]. Among a variety of by-products, whey stands out as a notable resource that is rich in lactoferrin and casein. It has been progressively exploited for casein extraction [158,159,160,161] and for biosynthesis of xanthan gum [159,162] and polyhydroxyalkanoates [163,164].

Concerning the pulp and paper industry, the most valuable by-products generated are black liquor and woody residues. Woody residues could be processed, and their valorisation may follow a similar pathway as common lignocellulosic materials, namely bioenergy and bioproducts (composites, biofuels) production [197]. On the other hand, black liquor is the black/brown aqueous phase obtained from the pulping process, generated when the pulp (cellulosic material) is washed after the delignification step. Distinct chemical pulp extraction treatments have been applied for paper production, but the Kraft process is the dominant process. The resulting black liquor mostly constitutes lignin, polysaccharides (as hemicellulose), resinous compounds, and some soluble salt ions [165,167]. Commonly, black liquor is incinerated or gassed as a source of fuel energy. Nonetheless, this waste liquor can be used as a preferable source of lignin and hemicellulose for the synthesis of polyhydroxyalkanoates, biogas, and carboxylic acids (e.g., succinic, glycolic, lactic, formic, and acetic acids), which are essential monomers for polymer synthesis [166,168].

Alongside the industries previously described, the textile industry is undoubtedly responsible for a significant fraction of the amount of waste every year. It has been estimated that around 92 million tonnes of textile waste are produced worldwide, which is one of the biggest contributors of plastic waste [198,199]. Generally, textile waste is defined as any discarded piece of fabric or clothing that is unfit for its original purpose [199]. This can be carried out during several phases of the process as follows: production; manufacturing or processing; defective textiles (pre-consumer waste); and after their end of life, including discarded clothing, bedding, or automotive textiles (post-consumer or post-industrial waste) [169,200].

Involving pre-consumer and post-consumer textile waste, several innovations have been conducted for collecting (deployment of the Internet of Things, IoT, or the use of smart tags and sensors for traceability), sorting (technology for identification and classification of textile waste), and the recycling textiles (innovative mechanical, chemical, biochemical, or thermal recycling). These new systems are implemented to address global demands on the reuse and recycling of waste [199].

Depending on the recycling process conducted with textile waste, different valuable products could be obtained, such as regenerated fibres, fillers, biofuels, bioplastics, biochar, cellulose derivatives, and amino acids. Mechanical recycling regenerates fibres (both natural and synthetic) for applications similar or not to those of the original fabrics without using chemicals. Yet, it is hampered by the availability and quality of waste materials, which generally reduces the quality and strength of the recycled fibres originated [170]. On the other hand, chemical recycling could also be applied to synthetic fibres (such as polyester, polyamide, or acrylic), natural fibres (such as cotton, wool, or silk), or blended fabrics [171]. This process involves depolymerising waste into their oligomers or monomers (e.g., glucose, bis(2-hydroxyethyl) terephthalate, BHET, dimethyl terephthalate, ethylene glycol) through diverse methods, namely hydrolysis, glycolysis, methanolysis, or solvolysis, which frequently requires harsh chemicals. Chemical recycling consists of different steps as follows: separation; purification; and subsequent repolymerisation to produce recycled polymers [172]. In addition, through esterification and etherification processes, glucose derived from cotton waste can be transformed into useful textile additives, for example, cellulose acetate or sodium carboxymethylcellulose [173]. Biological recycling produces similar results to chemical reprocessing, using microorganisms (bacteria, fungi, and algae) and their secreted enzymes for biodegradation [171]. Nevertheless, due to the resistance of synthetic fibres, biological methods are mainly suitable for natural fibres [171]. Particularly for natural fibres, glucose, and amino acids can be recovered to apply to the synthesis of new bio-based polymers and fibres [174,175]. Furthermore, the glucose recovered from cellulosic fibres could be a low-cost raw material for bio-derived monomers manufacture, such as carboxylic acids (e.g., succinic acid) [179], by way of microbial fermentation processes. Based on PET-cotton textile waste, enzymes have been employed in recovering glucose and polyester monomers [176,177,178].

3. Pre-Treatments and Extraction Methods: Insights, Main Advantages, and Disadvantages

Lignocellulosic resources are the point of attention in replacing the dependence on fossil fuels, due to their availability and abundance [201]. Therefore, distinct pre-treatments and extraction methods can be conducted to accurately treat lignocellulosic biomass and biowaste from diverse sources previously described.

For instance, multiple techniques can be followed to extract cellulose and its derivatives, namely physical, chemical, physicochemical, and biological, as schematically illustrated in Figure 3. Essentially, depending on the complexity of the hierarchical structure of the renewable resources schematically illustrated in Figure 4 and Figure 5, appropriate pulping technologies as well as pulping conditions should be applied. These choices significantly influence specific features and the environmental impact of the resulting materials. In this regard, Shukla et al. [201] and Padhi et al. [202] reported an extensive discussion of the main advantages and disadvantages of these different methods commonly used to manage biomasses. Hence, the following sub-sections are intended to highlight the most relevant aspects concerning these different processing and extraction approaches.

3.1. Physical Methods

Generally, physical methods include diverse processing technologies, comprising mechanical, microwave, or gamma irradiation and sonication processes. These methodologies can induce the structural disruption of biomass/waste, leading to size reduction and surface area increase. Physical methods are consistently used as pre-treatments of different biomass/waste since they provide crucial processing of a wide range of resources for further extractive routes. In the case of lignocellulosic resources, sonication [60,129,208,209,210,211] and irradiation [24,54,118,212,213,214,215,216] methods are commonly used to trigger the delignification process, which is essential for the effective extraction of cellulose and other valuable structural polysaccharides and proteins. Besides the simplicity and versatility of these processes, they involve the application of a wide range of temperature and pressure conditions, which implies some costs in energy consumption [201].

3.2. Chemical Methods

Chemical methods are undoubtedly the most commonly employed for extraction, particularly when dealing with challenging resources such as lignocellulosic biomass or waste (Figure 4), as well as animal residues and fungi biomass featuring chitin-protein or a protein-based hierarchical microstructure (Figure 5).

Among others, chemical approaches comprise the following different solvent systems: alkaline (or alkali); acidic; organo-solvent (or organosolv); ionic liquid; and deep eutectic. These can break down the complex structure exhibited by diverse biomass and biowaste, converting their valuable building blocks (e.g., polysaccharides, proteins, phenolic compounds, and lipids) to be more prone to being extracted. Particularly for lignocellulosic resources (Figure 4), these chemical methods have been recurrently used to efficiently disrupt the lignin barrier, transforming the crystalline structure of lignocellulosic mass into an amorphous form and offering an easier extraction of cellulosic content. Among different chemical methods, alkaline and acidic are the most used due to their high extraction yields; however, these conventional methods present a high environmental impact.

Overall, it is undeniable that acidic treatments are the most relevant extraction processes, especially when combined with other physical treatments, such as ultrasound. However, they present several negative aspects, including hemicellulose degradation, corrosiveness, and the formation of inhibitory species that could reduce their efficiency [217].

In this sense, alternative methods have risen. Some examples are organo-solvents [212,213,215,218,219,220,221,222,223,224,225,226], ionic liquids [27,227,228,229,230], and deep eutectic solvents [23,24,26,231], claiming to be greener solvent systems that can be easily reused or recycled, and that generally provide good extraction yields under mild operational conditions [201,232].

3.3. Physicochemical Methods

Based on the combination of physical and chemical methods, physiochemical processes have been developed and explored specifically at an industrial scale for processing some agroforestry and food industrial residues. Among others, the physiochemical methods frequently used are steam explosion, carbon dioxide explosion, ammonium fibre explosion (AFEX), wet air oxidation (WAO), and liquid hot water. It has been reported that these methods have provided an enhanced production of sugar along with a maximum level of delignification [201,233]. Moreover, it should be mentioned that these methodologies allow efficient extraction of hemicellulose and modified cellulose for their further processing and appropriate valorisation in biorefinery and bioenergy purposes [116,234]. Nevertheless, as physical methods, these methods require extreme temperature and pressure conditions, leading to higher energy consumption costs. Consequently, they are considered less environmentally friendly.

3.4. Biological Methods

As an alternative to the previously mentioned extraction processes, greener extractions could be provided by the anaerobic digestion process. These are commonly designed biological or enzymatic methods.

Currently, biological treatments could involve fungi, bacteria, microbial consortiums, and enzymes. Since commercial and pure enzymes derived from bacteria and fungi are too expensive, a cheaper solution could be provided by microbial extracellular enzymes isolated from diverse sources (e.g., plants and animals) [235,236].

Essentially, these biological approaches rely on specific enzymes, mainly hemicellulases and cellulases [237]. Nevertheless, new perspectives have risen to improve the biological extraction of lignocellulose biomass or waste. These include the search for more efficient functional strains or enzymes, additional research into the enzymatic hydrolysis characteristics and mechanisms, improving the strain’s performance through genetic engineering, and immobilising hydrolases on nanomaterials (e.g., nanoparticles, nanotubes, nanosheets), providing better thermostability and reusability of enzymes [235,236].

From this diversity, as highlighted in recent works [122,184,204,209,214,224,236,238,239,240], enzymatic procedures (or bio-pulping) stand out as the most environmentally friendly approach. These are followed by manufacturing processes, including steaming complemented by mechanical processes and soda (or alkaline) chemical procedures.

Generally, on behalf of biological processes, enzymatic processes provide higher selective degradation of lignin at low temperatures and atmospheric pressure. Nonetheless, they require long processing time and tremendous costs involved in enzyme acquisition or microbiological/fungal cultures. Hence, the combination of these biological hydrolyses with other effective methods (chemical and physicochemical) may reduce the time necessary for the whole process, which also helps to strengthen the lignocellulose dissociation efficiency [235].

4. Man-Made Organic Fibres

Until the early years of the previous century, natural fibres were employed as a multifunctional material for several applications; however, they still present significant challenges that must be addressed. Effectively, they have limited availability, and their culture has a high environmental impact, as well as a low production rate insufficient to meet the recent growing consumer demands [12]. To overcome the limitations described, man-made fibres have risen as promising alternatives through two main categories: regenerated fibres with natural-based compositions and synthetic fibres composed of petroleum-based synthetic polymers.

Man-made synthetic fibres are mostly composed of polyesters, polypropylene, and polyamides with high molecular weight, exhibiting high production rate, low cost, durability, and multifunctionality. Nevertheless, it is well known that these materials imply severe environmental and human health impacts since these polymers are mostly petroleum-derived and non-degradable [12].

Alternatively, due to recent technological advances that have potentiated the effective extraction and further processability of natural raw materials into artificial fibres, man-made organic fibres have risen and progressively attracted attention as a greener alternative to synthetic ones. Cellulose, in particular, is the most abundant organic material on earth, and through chemical modifications can be employed for manufacturing cellulose derivatives and regenerated fibres [10]. Besides cellulosic fibres, new chemical routes have been opened to incorporate diverse and multifunctional natural-based raw materials into fibre composition. Moreover, new bio-based monomers as previously described in this article have been recently used to synthesise greener processable polymers. These materials could be derived from polysaccharides, proteins, lignin and other bioactive derivatives, and lipids extracted from renewable resources, as discussed in the following sub-sections.

4.1. Production Methods

This sub-section focuses on the methods usually employed to generate fibrous materials with the goal of valorising previously described biopolymers derived from biomass or biowaste and subsequent monomers.

Essentially, the methodology for producing fibrous materials should be compatible and adaptable with the polymeric composition and its inherent qualities, such as chemical structure, solubility, crystallinity, conductivity, and thermal and mechanical characteristics. It is well known that natural polymers most studied are derived from polysaccharides and proteins, which typically present lower thermal and mechanical properties than synthetic polymers. To address this constraint, biopolymeric compositions are often treated using wet-/dry-spinning and electrospinning. Melt-spinning is only performed on biopolymeric blends that have been functionalised with specified additives or contain graft-functionalised polymers.

Nowadays, diverse fibres or fibrous materials can be spun by drawing a melt or solution of a polymer, polymeric blend, or nanocomposite from a spinneret into a medium (quenching or water, coagulation bath) for solidification. This solidification medium may be air or a specific bath (water, quenching, or coagulation). In addition, the drawing step can be conducted by rollers and winders by high-velocity air stream or by electrostatic force [241]. Attending this recent diversity, this section will focus on a description of the most prevalent fibre fabrication methods, namely melt-spinning, electrospinning, dry-spinning, and wet/dry-jet-spinning.

Melt-spinning, also known as the extrusion method, is the most used method for manufacturing commercial synthetic fibres with multifunctional properties at a large scale. This is due to the simplicity of the production line, high spinning velocities, low production cost, solvent-free process, and environmental friendliness [241,242]. A melt-spinning setup includes a screw extruder, where a thermoplastic polymer or polymeric blend (in the form of pellets or chips) is homogenously melted and pressurised, a melt pump that controls the throughput rate, and a spin pack that ensures polymer filtering and distribution for the spinneret, in order to occur the filament formation. The extruded filaments are cooled (either into a quenching chamber or a water bath) and a spin finish is applied. Afterwards, the filaments move to the filament draw-down unit where they are drawn by several godets and collected on a bobbin through the winder [241]. Since this process involves thermal processing, a basic requirement is that the polymer or polymeric blend becomes fusible below its degradation temperature. Hence, not all monomers/polymers can be processed through this technique. Furthermore, certain additives are frequently combined with polymeric compositions, especially for biopolymers, to improve their thermal processability and provide better mechanical properties and durability of the obtained fibres [241]. Moreover, as presented in

Table 3. Principal fibre fabrication methods and their major solution and processing parameters (adapted from [241,243,244]).

Fabrication Method	Composition	Process
Melt-spinning	Polymer molecular structure Polydispersity Crystallinity Melting temperature Glass transition Decomposition temperature Enhancing and functional additives	Screw extruder (type and velocity) Spin pack design (spinneret) Heating temperature Extrusion speed Draw ratio Coagulation bath (composition and temperature) Take-up speed Environmental temperature and humidity
Electrospinning	Polymer concentration Viscosity Conductivity Solvent evaporation rate Molecular weight	Flow rate Applied voltage Tip to collector distance Collector types Environmental temperature and humidity
Dry-spinning	Polymer concentration Molecular weight Solvent	Spinneret size Drawing ratio Take-up speed Environmental temperature and humidity
Wet/Dry-jet-spinning	Polymer concentration Molecular weight Solvent Viscosity	Spinneret size Air gap Coagulation bath Drawing ratio Take-up speed Environmental temperature and humidity

, several parameters related to polymer composition and processing conditions can critically affect the spinnability and quality of melt-spun fibres. For instance, bio-based polymers with small molecular weights and amorphous chemical structures can greatly influence the processing temperature. On the other hand, the difference between the extrusion speed and collecting speed could increase the degree of molecular orientation and crystallinity. Therefore, it may contribute to the improvement of mechanical performance [243].

Electrospinning is also a low-cost and effective approach to producing high-quality continuous fibres at the micrometre to the nanometre scale. This technique can be applied to a wide range of polymers (synthetic and natural), resulting in high interfibrous pore size and high specific surface area being able to be further functionalised [23,245,246,247,248,249,250,251,252,253]. The principle of this technology is based on the action of an electrical field that is applied to a polymeric solution. Following this, an accelerated liquid jet of polymeric solution is formed and, due to the solvents’ fast evaporation, the polymer’s solidification occurs and is then adequately collected. The spinnability and fibre quality can be affected by several parameters as follows: solution properties, such as concentration, molecular weight, conductivity, and viscosity; processing conditions, such as solvent, feeding rate, spinning distance, voltage, and temperature; and system design, for example, coaxial spinning and rotating collector [245,254,255].

Dry-spinning is a simple solution-spinning method that allows for the production of fibres with distinct compositions. This technology is based on the evaporation of the volatile solvents commonly used to obtain the polymer solution, also known as spinning dope [256]. So, in this method, the dope solution is injected/extruded through a spinneret setup, passing into hot air for quick solvent evaporation before the fibres are collected and submitted to drawing processing [243]. Besides this simplicity, some parameters could affect the structure and performance of dry-spun fibres (see Table 3). In this regard, it should be mentioned that these fibres could present low production efficiency, mostly due to their instant solidification, which may lead to voids and residual solvents within the fibre structure, compromising their mechanical performance [243,256].

On behalf of spinning processes using solidification solution, wet/dry-jet-spinning has risen as a promising low-cost fibre fabrication at the macroscale [257,258,259], since it is a variation in the widespread wet-spinning suitable for highly viscous polymeric solutions with thermal sensitivity. In wet-/dry-jet-spinning, the polymeric solution is injected/extruded into an air gap, which promotes the molecular orientation and an appropriate stretching of the polymeric jet before passing into a coagulation bath. In addition, the coagulated fibres are collected at variable speeds to promote their stretching. As in previous methods, different parameters can affect the quality of wet-/dry-spun fibres, but they generally exhibit better mechanical properties than melt-spun fibres due to their higher draw ratios [243].

4.2. Bio-Based Man-Made Fibres

Fibrous materials produced from natural polymers or biopolymers have received much interest in recent years due to their lower environmental impact in comparison with the dominant petroleum-derived polymers. Subsequently, MMOFs or bio-based man-made have a promising future in different fields since their molecular structure could be fine-tuned for textile, sportswear, healthcare, and protection applications using specific solvents and suitable fibre preparation and processing techniques [228,251].

Hence, the subsequent sub-sections will focus on the main developments in creating polymeric formulations based on promising biocompounds for the manufacturing of fibres employing the previously mentioned spinning processes. To create high-quality, multifunctional man-made fibres suitable for a wide range of applications, methods for tuning the main biopolymer classes—polysaccharides, proteins, lignin, and lipid derivatives (Figure 6)—will be covered.

4.2.1. Polysaccharides

As previously described, different polysaccharides could be obtained from biomass/waste. Among them, cellulose is the most abundant and versatile polymer and is extensively exploited to develop fibres and diverse fibrous materials. In the context of the textile industry, cellulosic fibres derived from cotton or wood pulp prepared by wet-spinning are still playing an important role. Accordingly, different cellulosic solvent systems have been explored to develop regenerated cellulosic fibres (RCF), as it was extensively described by Shen et al. [11] and Sayyed et al. [12]. As illustrated in Figure 7, there are two approaches for obtaining regenerated fibres: through derivatisation processes that modify cellulose before its dissolution or by direct dissolution of cellulose.

From derivatisation processes, the viscose (NaOH/CS₂) [12,258,260,261,262,263,264,265] and its derivative Modal^® [12], obtained from wood pulp, and cellulose acetate [266], obtained from both wood pulp and cotton litters, stand out as the most significant commercially applied methods for textiles and other applications. In this context, recent adjustments have been introduced to make these procedures more sustainable. TENCEL™ Lyocell fibres are viscose fibres produced in a resource-saving closed-loop process that allows for significant solvent recovery. On the other hand, TENCEL™ Modal’s production method is described as resource-efficient, with high rates of recovery and recycling than generic modal. Nevertheless, these two conventional methods still have detrimental environmental effects because they use the volatile and highly toxic carbon disulfide (CS2) in their solvent system; thus, new aqueous solution systems have emerged recently as greener, harmless, and rapid dissolution solvents. Some examples of these aqueous solutions are CarbaCell (known as Carbamate), a sodium hydroxide solution to dissolve cellulose carbamate [216,231,267,268,269,270,271,272,273,274], Alkali/urea [275,276,277,278], and sodium zincate solution (Biocelsol^®) NaOH/ZnO [271,279].

On behalf of direct dissolution processes, cuprammonium (known as cupro or cuprammonium rayon) [12,280,281,282,283] consists of an expensive industrial process to obtain silky man-made fibres. Furthermore, lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) [284,285] represent an organic solvent system recurrently used to dissolve cellulose and its derivatives. Addressing the environmental concerns involved in these direct dissolution processes, Lyocell processes (also known as Tencel^®) [229,286,287,288,289] have recently emerged as advantageous environmentally friendly methods that could use N-Methylmorpholine-N-Oxide (NMMO) or ionic liquids [227,258,264,290].

Moreover, concerning the mechanical performance of these different regenerated cellulosic fibres compiled in

Table 4. Mechanical properties exhibited by common regenerated cellulosic fibres.

Process	Spinning Method	Tenacity (cN/dTex)	Elongation (%)	References
Viscose (NaOH/CS2)	Wet-spinning	1.80 (dry)	18.00 (dry)	[262]
		1.00 (wet)	21.60 (wet)
		2.07 (dry)	19.20 (dry)	[263]
		0.75 (wet)	12.50 (wet)
		2.15	22.60	[264]
		1.77–2.30 (dry)	17.00–25.00 (dry)	[258]
		0.88–1.32 (wet)	21.00–30.00 (wet)	[265]
Cellulose acetate	Air-jet spinning	1.06–1.24 (dry)	25.00–35.00 (dry)	[266]
		0.57–0.66 (wet)	35.00–45.00 (wet)
Alkali/urea	Wet-spinning	3.50 (dry)	8.00 (dry)	[275]
		2.50 (wet)	8.70 (wet)
		3.43	10.20	[276]
		2.20	1.90	[277]
		1.90	2.00	[278]
NaOH/ZnO	Wet-spinning	2.36 (dry)	15.90 (dry)	[271]
		0.73 (wet)	17.80 (wet)
		2.58	12.10	[279]
Cuprammonium	Wet-spinning	2.00 (dry)	10.00 (dry)	[280]
		1.00 (wet)	20.00 (wet)
		1.50–2.39 (dry)	12.00–13.00 (dry)
		1.41–1.50 (wet)	26.00–27.00 (wet)
LiCl/DMAc	Wet-spinning	4.23	7.76	[284]
		1.68–3.27 (dry)	6.00–12.00 (dry)	[285]
		0.53–2.30 (wet)	8.00–18.00 (wet)
Ionic liquids	Dry-jet wet-spinning	3.60 (dry)	10.20 (dry)	[290]
		2.70 (wet)	12.40 (wet)
		3.85	7.90	[258]
		3.09–3.62 (dry)	7.00–8.00 (dry)	[227]
		2.03–2.74 (wet)	9.00–12.00 (wet)	[264]
NMMO	Dry-jet wet-spinning	4.90	4.00	[259]
		4.30	13.10	[264]
		3.97–4.42 (dry)	11.00–16.00 (dry)	[286]
		3.44–3.53 (wet)	16.00–18.00 (wet)	[258]

, it should be mentioned that NMMO-based Lyocell fibres exhibited good fibre strength and other adequate physical properties, and are by far one of the most environment-friendly and non-hazardous cellulose regeneration process. In this sense, Lyocell fibres present clear advantages over conventional viscose or cupro fibres [12,287].

Aligned with new trends in manufacturing regenerated cellulosic fibres, the brand Orange Fiber was a pioneer in producing fabrics from citrus juice waste, owning a patented innovative process to obtain sustainable cellulosic fibres from orange pulp. In fact, Orange Fiber is a viscose fibre made of 100% natural cellulose obtained from citrus fruits waste, such as oranges, lemons, tangerines and grapefruits, through extraction processes that uses only ecological chemicals. This viscose fibre is biodegradable, and it is commonly called vegetable silk due to its silky and airy texture [291]. In addition, the recent partnership between Orange Fiber and the Lenzing group resulted in the first-ever TENCEL™ branded Lyocell fibre made of orange and wood pulp, which claims to provide high-quality comfort and performance, and enhanced sustainability in the textile industry [291].

Besides cellulose and its derivatives, other polysaccharides that play an important role in the structural support of diverse organisms, such as chitosan/chitin [96,99,110,251,257,292,293,294], starch [295,296], and alginate (SeaCell^® fibres) [297,298,299,300], have been increasingly incorporated into the composition of man-made fibres. As shown in Table 5, these biopolymers are recurrently used to produce fibres with advanced (bio)functionalities for textile and biomedical applications through wet-spinning or electrospinning.

Moreover, concerning the synthesis of biopolymers based on polysaccharide-derived monomers, it should be mentioned that polyhydroxyalkanoates (PHAs), unlike other polyesters, can be produced through bacterial fermentation of glucose. These polyesters are environmentally friendly, biodegradable, and biocompatible, and could represent an alternative to petroleum-based plastics, especially in the medical field. However, the main limitation associated with this process is the high production cost (e.g., 5–10 times higher than conventional plastic), which is mainly influenced by the yields of carbon sources, the fermentation process, and productivity [301,302].

Among a range of biodegradable polymers, polylactic acid (PLA) is an aliphatic polyester that presents excellent performance and physicochemical attributes; so, it has become the most used bioplastic in diverse fields, such as biomedical, textile, food packaging, and automotive [303]. This biopolymer is derived from lactic acid, which can be obtained from the fermentation of food crops rich in starch and cellulose (corn, wheat, or rice). After obtaining the lactic acid, PLA synthesis typically involves ring-opening polymerisation. This process necessitates the completion of numerous separation and purification stages, resulting in a high cost of PLA production [303,304].

Focusing on reducing the cost of those bioplastics, different waste materials have been re-purposed as carbon sources, namely whey from the dairy processing industry; wheat, rice bran, starch, sugarcane molasses, and vegetable oils from the food industry; and wastewater from different industries (textile, paper, and abattoir) [301,302].

4.2.2. Proteins

The development of protein-based fibres, RPFs, also known as azlon fibres, occurred during the world wars (throughout the 20th century) when textile manufacturers looked at alternatives for natural fibres and considered food waste a valuable resource. In this sense, scientific and industrial advances resulted in the invention of Ardil, a RPF developed by the British company Imperial Chemical Industries from storage protein extracted from peanut waste (mainly arachin) through a patented process by wet-spinning. However, the discontinuation of regenerated protein fibres was mostly caused by the rise in high-performance petrochemical fibres [305].

Despite the failure of RDF Ardil, the research and development of RDFs remain active, and, in the market, some commercial examples are already available.

QMILK fibres are made using a unique upcycling method that uses a biopolymer derived from non-food milk proteins and 100% renewable raw materials (such as organic oils). It exhibits soft and smooth properties as silk while displaying natural antibacterial and flame-retardant effects and high hydrophilicity. In addition, these MMOFs claim to be completely biodegradable and compatible with natural fibres through thermo-bonding processes [306].

Using supramolecular technology, UMORFIL^® developed an upcycle yarn process that incorporates collagen peptide amino acid—which is extracted from aquaculture fish scale—into viscose, nylon, or polyester fibres to provide a range of bionic functional fibres that offer improved moisture retention, biodegradability and a soft touch [307].

Babysoy company features an innovative collection of sustainable and comfortable clothing for babies made with the soybean protein fibre. This azlon textile fibre is obtained by wet-spinning through a patented process where proteins are extracted from the leftover pulp of tofu or soy milk production. Essentially, this azlon fibre has gained attention because of its silky smooth and cashmere feel, superior moisture transmission for dryness, dimensional stability, natural flame resistance, and better UV protection when compared to cotton or bamboo fibre [308].

In the literature, some works have reported distinct strategies for the functionalisation of man-made fibres with plant-derived and animal-derived proteins. These compounds could provide structural support and enhance physiochemical properties and diverse surface functionalities. In Table 5, some relevant works containing plant-derived proteins are presented as follows: zein and other prolamins (hordein, gliadin) [248,253,300,309,310]; soybean protein [311]; animal-derived proteins, such as keratin [299,312,313,314,315], collagen [316,317,318], and gelatine [298,319], which are largely conducted with versatile electrospinning and few with standard wet-spinning methodology.

4.2.3. Lignin and Other Bioactive Compounds

Lignin and derived phenolic compounds have attracted attention for bio-based polyester synthesis. Among other phenolic compounds, vanillin is a common molecule extracted on an industrial scale from lignin. It could be explored for the formulation of vanillin-based monomers (e.g., diols, diacids/diesters, hydroxyacids/hydroxyesters), as well as for the synthesis of vanillin-based polyesters [320].

Terpenoids are a diverse class of low-molecular-weight organic compounds synthesised as part of plant secondary metabolism. These compounds can be foreseen as a sustainable resource for obtaining terpene-derived monomers used to synthesise bio-based aliphatic polyesters [321,322,323].

Furan derivatives are also valuable compounds that can be extracted from different lignocellulosic biomass, providing key functional groups to synthesise diverse materials [324]. Particularly, bis(hydromethyl) furan or furan dicarboxylic acid (FDCA) has been produced at a large (as the example of Avantium [325]) and further explored for the synthesis of new bio-based polyesters. This could occur through reactions with other dicarboxylic acids or diols, such as furan dicarboxylic acid-based polyesters, which exhibit excellent thermal and mechanical properties [326,327,328].

Besides this biorefinery interest, some works in the literature describe the development of fibres with the incorporation of lignin as a valuable nanofiller. It usually provides structural support, flame retardancy, antibacterial, and anti-inflammatory properties, which is also a greener and cheaper strategy to produce high-performance carbon fibres (see Table 5).

4.2.4. Lipids

In fact, the lipidic fraction of different lignocellulosic wastes and biomasses has been extensively studied as a valuable raw resource to provide bio-based monomers for the synthesis of new bio-based polymers, including polyesters, polyethylene, and polyamides. Similarly to petroleum-derived polymers, these new bio-based polymers might be processed using melt-spinning or electrospinning to produce fibres with a variety of (bio)functionalities for textile and biomedical applications. Regarding bio-based polyesters, recent developments in the preparation of bio-based monomers have provided the fast growth of new bio-based polyesters with tunable properties and a large range of application areas. Zhang et al. [320] complied with the recent advances in the field of bio-based polyesters (including aliphatic and aromatic) and the main aliphatic monomers derived from plant oils. The main compounds highlighted were ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,6-hexanediol, 1,10-hexanediol, azelaic acid, sebacic acid, dodecanedioic acid and glycerol [320].

Moreover, a new generation of polyurethane derived from renewable raw materials has risen to replace petroleum-based ones. Among the most common bio-based polyols applied for bio-polyurethane synthesis, vegetable oils (including castor oil, oleic acid, and linoleic acid) have been increasingly exploited due to their hydroxyl group composition [329,330]. For instance, as shown in Table 5, these bio-polyurethanes can be combined with other functional components to electrospun fibres with advanced functionalities for diverse applications. In the polyamide domain, due to their excellent intrinsic properties and industrial significance in a wide range of applications, recent efforts have been made to develop bio-based polyamides, also known as bionylons. PA6 and PA6.6 are the two most widely used polyamides, as they are responsible for the massive consumption of petroleum-based materials in their synthesis (around 10 million tonnes each year) [331,332]. Since the monomers commonly used in polyamide synthesis (e.g., dicarboxylic acids, diamines, and caprolactam) are highly dependent on petroleum chemical resources, it is urgent to find greener and bio-based monomers to reduce this environmental impact. For instance, some vegetable oils have been extensively studied for this application. Among them, castor oil has gained particular attention due to its high amount of ricinoleic acid, which provides a favourable source of bio-based dicarboxylic acids, such as undecylenic acid and sebacic acid [333].

Some examples of bio-based polyamides already on the market are as follows: PA4.10 and PA6.10, derived from sebacic acid; PA10.12 and PA10T, derived from decanediamine; and two fully bio-derived PA11 and PA10.10, which can be synthesised from ricinoleic acid and the melt polycondensation of sebacic and decanediamine [333,334,335,336,337]. In particular, the Rilsan^® PA11 supplied by Arkema is derived from ricinoleic acid obtained from castor oil (a bio-based content of around 98%) and presents excellent chemical resistance, easy processing, high dimensional stability and long-term durability suitable for different high-performance applications [338]. Furthermore, Radilon^® provides a wide range of polyamides with excellent mechanical and chemical resistance, including long-chain polyamides (Radilon^® D) produced from bio-derived PA6.10 made with 64% renewable source material [339]. Recently, Fulgar^® developed innovative high-performance and extra comfort bio-based yarns, EVO^®, made by bio-polyamide polymerised with monomers partially or totally derived from castor oil [340].

Driven by the sustainability transition, the use of novel high-performance textile fibres has risen. The Bio-based Dyneema^® fibre, for example, uses a renewable bio-based feedstock (a by-product from wood pulp industry) supplied by sustainable forest management to produce green ethylene, making it the first bio-based high molecular polyethylene (HMPE) fibre. This more sustainable Dyneema^® fibre reduces the dependence on fossil fuel-based resources without compromising protection, comfort, or durability [341].

Table 5. Diverse man-made fibres produced at different scales, using natural polymers functionalised or combined with biofillers for distinct applications.

Composition	Spinning Method	Properties	Application	References
Chitosan + Graphene Chitosan + PVA + Graphene Chitosan + Cotton + Polyester + Tencel Chitin + Cellulose	Wet-spinning Electrospinning Wet-spinning Wet-, dry-spinning	Softness, biodegradability, thermal conductivity, antibacterial properties	Antibacterial and technical fibres	[292]
				[293]
				[294]
				[257]
Bacterial cellulose + Silica	Wet-spinning	Thermal insulation	Technical fibres	[342]
Starch + PVA + Glycerol	Wet-spinning	Biodegradability, skin friendly	Functional fibres	[295]
Starch + PLA Starch + PLGA Starch + Nanocellulose	Electrospinning	Hydrophobicity, biodegradability	Functional nanofibres	[296]
Keratin + Cellulose	Dry-jet wet-spinning	Flexibility and mechanical resistance	Precursor for carbon fibres	[312]
Keratin + Cellulose nanocrystals	Wet-spinning	Hierarchically structured fibres with shape-memory features	Functional fibres	[313]
Viscose + Zein	Wet-spinning	High mechanical performance, biodegradability	Technical fibres	[309]
Polyurethane + Keratin + AgNPs	Electrospinning	Biocompatibility and antibacterial properties	Nanofibrous mats for wound dressing	[314]
Poly(hydroxybutylate-co-hydroxyvalerate) + Keratin	Electrospinning	Biocompatibility, bioadhesiveness, biodegradability	Nanofibrous mats for wound dressing	[315]
Collagen + Nanohydroxyapatite	Electrospinning	Biocompatibility, bioadhesiveness, biodegradability	Nanofibrous mats for bone regeneration	[316]
PCL + Collagen	Electrospinning	Biocompatibility, bioadhesiveness, vascularisation	Biofunctionalised nanofibrous mats for tissue regeneration	[317]
PLGA + Collagen	Electrospinning	Biocompatibility, bioadhesiveness	Nanofibrous structures for tissue regeneration	[318]
Gelatin + Tyrosine	Electrospinning	Biocompatibility, bioadhesiveness	Nanofibrous mats for cartilage tissue regeneration	[319]
Zein	Electrospinning	Core–shell structure	Drug-loaded nanofibrous mats	[248]
Soybean protein	Wet-spinning	Controlled drug load and release delivery	Biofunctional fibres for drug delivery	[311]
Zein/Gliadin/Hordein	Electrospinning	Good mechanical properties, biocompatibility	Ultrafine fibres for biomedical applications	[253]
Alginate + Pectin + Gelatin + Glycerol	Wet-spinning	Monofilament is bioabsorbable and capable of drug delivery	Suture for biomedical applications	[298]
Keratin + Alginate	Wet-spinning	Dual crosslinked fibres suitable for complex braid forms	Flexible fibres	[299]
Zein + Alginate + Betanin + TiO2NPs	Electrospinning	Good mechanical performance, hydrophobicity, antibacterial properties	Nanofibres for food packaging	[300]
Regenerated cellulose + Cellulose diacetate/Cellulose acetate propionate/Cellulose acetate butyrate	Wet-spinning	Transparency, thermal and chemical stability	Biopolymeric optical fibres	[343]
Bio-based polyamide 56	Electrospinning	Good mechanical performance, antibacterial properties	Bionylon nanofibres for functional textiles	[336]
Bio-based polyamide 56	Melt-spinning	Good mechanical and thermal performance, flame-retardancy, biodegradability	Bionylon fibres for functional textiles	[337]
Bio-polyurethane + Triclosan + Cyclodextrin	Electrospinning	Good mechanical properties and antibacterial	Antibacterial nanofibrous materials	[329]
Zein + Hordein + Lignin	Electrospinning	Good electrochemical properties, hierarchical porous texture	Supercapacitors precursors for carbon fibres with flame-retardancy	[310]
Cellulose	Electrospinning	Reinforced mechanical and thermal properties	Biodegradable nanofibrous composites	[344]
Cellulose/cellulose acetate Cellulose acetate/polyurethane	Electrospinning	Light transparency and improved mechanical properties	Light transparent nanofibrous composites	[345,346,347,348]

Table 5. Diverse man-made fibres produced at different scales, using natural polymers functionalised or combined with biofillers for distinct applications.

Composition	Spinning Method	Properties	Application	References
Chitosan + Graphene Chitosan + PVA + Graphene Chitosan + Cotton + Polyester + Tencel Chitin + Cellulose	Wet-spinning Electrospinning Wet-spinning Wet-, dry-spinning	Softness, biodegradability, thermal conductivity, antibacterial properties	Antibacterial and technical fibres	[292]
				[293]
				[294]
				[257]
Bacterial cellulose + Silica	Wet-spinning	Thermal insulation	Technical fibres	[342]
Starch + PVA + Glycerol	Wet-spinning	Biodegradability, skin friendly	Functional fibres	[295]
Starch + PLA Starch + PLGA Starch + Nanocellulose	Electrospinning	Hydrophobicity, biodegradability	Functional nanofibres	[296]
Keratin + Cellulose	Dry-jet wet-spinning	Flexibility and mechanical resistance	Precursor for carbon fibres	[312]
Keratin + Cellulose nanocrystals	Wet-spinning	Hierarchically structured fibres with shape-memory features	Functional fibres	[313]
Viscose + Zein	Wet-spinning	High mechanical performance, biodegradability	Technical fibres	[309]
Polyurethane + Keratin + AgNPs	Electrospinning	Biocompatibility and antibacterial properties	Nanofibrous mats for wound dressing	[314]
Poly(hydroxybutylate-co-hydroxyvalerate) + Keratin	Electrospinning	Biocompatibility, bioadhesiveness, biodegradability	Nanofibrous mats for wound dressing	[315]
Collagen + Nanohydroxyapatite	Electrospinning	Biocompatibility, bioadhesiveness, biodegradability	Nanofibrous mats for bone regeneration	[316]
PCL + Collagen	Electrospinning	Biocompatibility, bioadhesiveness, vascularisation	Biofunctionalised nanofibrous mats for tissue regeneration	[317]
PLGA + Collagen	Electrospinning	Biocompatibility, bioadhesiveness	Nanofibrous structures for tissue regeneration	[318]
Gelatin + Tyrosine	Electrospinning	Biocompatibility, bioadhesiveness	Nanofibrous mats for cartilage tissue regeneration	[319]
Zein	Electrospinning	Core–shell structure	Drug-loaded nanofibrous mats	[248]
Soybean protein	Wet-spinning	Controlled drug load and release delivery	Biofunctional fibres for drug delivery	[311]
Zein/Gliadin/Hordein	Electrospinning	Good mechanical properties, biocompatibility	Ultrafine fibres for biomedical applications	[253]
Alginate + Pectin + Gelatin + Glycerol	Wet-spinning	Monofilament is bioabsorbable and capable of drug delivery	Suture for biomedical applications	[298]
Keratin + Alginate	Wet-spinning	Dual crosslinked fibres suitable for complex braid forms	Flexible fibres	[299]
Zein + Alginate + Betanin + TiO2NPs	Electrospinning	Good mechanical performance, hydrophobicity, antibacterial properties	Nanofibres for food packaging	[300]
Regenerated cellulose + Cellulose diacetate/Cellulose acetate propionate/Cellulose acetate butyrate	Wet-spinning	Transparency, thermal and chemical stability	Biopolymeric optical fibres	[343]
Bio-based polyamide 56	Electrospinning	Good mechanical performance, antibacterial properties	Bionylon nanofibres for functional textiles	[336]
Bio-based polyamide 56	Melt-spinning	Good mechanical and thermal performance, flame-retardancy, biodegradability	Bionylon fibres for functional textiles	[337]
Bio-polyurethane + Triclosan + Cyclodextrin	Electrospinning	Good mechanical properties and antibacterial	Antibacterial nanofibrous materials	[329]
Zein + Hordein + Lignin	Electrospinning	Good electrochemical properties, hierarchical porous texture	Supercapacitors precursors for carbon fibres with flame-retardancy	[310]
Cellulose	Electrospinning	Reinforced mechanical and thermal properties	Biodegradable nanofibrous composites	[344]
Cellulose/cellulose acetate Cellulose acetate/polyurethane	Electrospinning	Light transparency and improved mechanical properties	Light transparent nanofibrous composites	[345,346,347,348]

5. Conclusions and Future Perspectives

In recent years, driven by growing concerns over sustainability and environmental challenges, there has been a significant surge in research and development focused on renewable-based polymers.

In this regard, among the abundant biomass/waste sources herein reported, the agroforestry and industrial lignocellulosic residues could be the preferred resources to obtain valuable building blocks—polysaccharides, proteins, lignin, and derived phenolic compounds and lipids—suitable for a wide range of biorefineries.

Attending to the complex hierarchical structure of diverse lignocellulosic resources, the efficient extraction of their major components (e.g., cellulose, hemicellulose, lignin, vegetable oil) could be challenging. Essentially, the processing of lignocellulosic materials typically relies on conventional extraction protocols that combine physical pre-treatments with alkali or acidic chemical methods, which exhibit enormous environmental impact. Consequently, these conventional chemical extractions started to be replaced by greener solvent systems that could also be effective in terms of extraction yields, such as organosolv, deep-eutectic, or ionic liquids extractions. Alternatively, biological methods could be conducted, providing a more selective bio-pulping. However, these last emergent ecofriendly extractions are expensive and excessively time-consuming, requiring further research and optimisation to be further implemented at a large scale.

Respecting the production of man-made fibres, chemical and technological developments observed in the commonly used production methods (e.g., melt-spinning, electrospinning, and wet-/dry-jet spinning) have contributed to a progressive improvement in the fibre quality obtained. Moreover, bio-based polymers such as PLA, PHA, PHB, bio-polyamides, biopolyesters, and bio-polyurethanes are being incorporated into man-made fibres. These can be combined with structural polysaccharides like cellulose, chitosan, starch, and alginate, as well as proteins such as keratin, collagen, and gelatin. This approach creates multiscale fibres with multifunctional properties, which could be applied to textiles, sportswear, biomedical uses, and protective gear. In this way, a new generation of man-made fibres, known as MMOFs, is emerging, which might be built up of a variety of bio-based polymers.

As it was demonstrated by the present review, biomass/waste sources have a high potential for replacing petroleum-based monomers. In this way, based on the biopolymers examined in this research, novel synthesis strategies have been developed that can reach all or almost all current polymers. However, these strategies still present certain limitations, such as the development of new synthesis pathways and the refinement of formulations to achieve the same attributes in biopolymers as we do in petroleum derivatives. Furthermore, if the performance of petroleum goods cannot be met, evaluate product adaptability and expectations for them. On the other hand, it may be intriguing to investigate strategies to lower the cost associated with the production of bio-derived polymers. Otherwise, it can be assumed that the high cost is a reasonable price to pay for sustainable bio-based polymers.