A Review of Natural Fibers: Classification, Composition, Extraction, Treatments, and Applications

Abstract

This review provides a comprehensive analysis of natural fibers, addressing their classification, chemical composition, extraction methods, treatments, and diverse applications. It categorizes natural fibers into plant-based (cellulose-rich), animal-based (protein-based), and mineral-based types, detailing their unique structural and chemical properties. The paper examines traditional and advanced extraction techniques—including dew, water, enzymatic, chemical retting, and mechanical decortication—highlighting their impact on fiber quality and environmental sustainability. Furthermore, it reviews various chemical and biopolymer treatments designed to enhance fiber performance, reduce hydrophilicity, and improve adhesion in composite materials. The discussion extends to the multifaceted applications of natural fibers across industries such as textiles, automotive, construction, and packaging, underscoring their role in reducing reliance on synthetic materials and promoting eco-friendly innovations. The review synthesizes recent market trends and emerging fiber classifications, emphasizing the potential of natural fibers to drive sustainable development and informing future research in extraction efficiency, treatment optimization, and lifecycle analysis.

1. Introduction

Natural fibers are renewable materials derived from plants, animals, or minerals, and have played a vital role throughout human history. Archaeological findings from Dzudzuana Cave in the Republic of Georgia suggest the use of flax fibers in textiles as early as 34,000 years ago [1]. Later, from around 8000 B.C., records indicate the use of natural fibers for clothing in the Middle East and China, for pottery reinforcement in Central and South America by Inca and Mayan cultures [2], and for the development of construction materials such as nets, ropes, and paper. Civilizations have used natural fibers through time to suit some of their needs, due to the sheer availability of natural fibers around the globe. In the 1930s, the creation of nylon made synthetic fibers an alternative to the natural ones because of their durability, waterproofing, and moisture resistance [3]. During this time, consumer and customer preferences changed from natural fiber products to synthetic fiber products, which led to a rapid global decline in the production of natural fibers [4]. Over the last century, natural fiber quality increased due to continuous research and technological advances, making them more affordable and desirable [5], while synthetic fibers became less desirable due to specific disadvantages such as non-biodegradability, environmental persistence, and toxicity [6].

Today, natural fibers are increasingly considered a sustainable alternative to non-renewable synthetic fibers due to their comparable properties, lower cost, abundance [7], renewability, biodegradability, and low density [8]. They are also generally regarded as safer for both human health and the environment, typically requiring no special handling precautions and having a lower carbon footprint compared to synthetic counterparts [9]. However, while natural fibers are generally associated with low environmental impact, their carbon footprint can vary significantly depending on cultivation practices. Intensive use of agrochemicals, irrigation, and mechanized farming may increase energy consumption and greenhouse gas emissions, partially offsetting their sustainability advantages [10]. Ongoing research continues to explore new sources of plant-based fibers from diverse botanical species and geographic regions to expand the potential of natural fiber applications.

The objective of this scoping review is to compile and synthesize information from key contributions in the field of natural fibers, namely with a primary focus on lignocellulosic fibers, but also addressing animal and mineral fibers. It covers natural fiber classification, chemical composition, extraction methods, surface treatments, and current applications across a range of industries.

2. Methodology

This scoping review followed the guidelines outlined in the PRISMA Extension for Scoping Reviews (PRISMA-ScR) to ensure transparency and rigor in the literature identification and selection process. The aim was to map and synthesize existing knowledge on natural fibers, focusing on their classification, composition, extraction methods, treatments, and applications.

2.1. Search Strategy

A comprehensive literature search was conducted across multiple academic platforms, including Google Scholar and ResearchGate (primary sources), supplemented by Scopus and Web of Science. The search covered the period from 2000 to 2025. The following search terms and their combinations were used: natural fibers, extraction methods, fiber treatment, natural fiber cost, natural fiber crop species, new sources of vegetable fibers, natural fiber applications, natural fiber types, natural fiber reviews, vegetable fiber morphology, vegetable fiber composition, natural fiber footprint, natural fiber and synthetic fiber comparison, natural fiber structure, natural fiber properties, and natural fiber challenges. For more studies on specific natural fibers, we searched their names as terms directly: abaca fiber, alfa grass fiber, bamboo fiber, banana fibers, coir fiber, cotton, curaua fiber, date palm fiber, flax fiber, harakeke fiber, hemp fiber, henequen fiber, isora fiber, jute fiber, loofah fiber, kenaf fiber, milkweed fiber, pineapple fiber, ramie fiber, bagasse fiber, sugarcane fiber, and sisal fiber. Only studies that were accessible in full-text form were considered for inclusion.

2.2. Eligibility Criteria

To be included in this review, studies had to meet the following criteria: (i) peer-reviewed or academically credible publications; (ii) published between 2000 and 2025; (iii) written in English; and (iv) focused on natural fibers in relation to their structure, extraction methods, treatment processes, composition, or practical applications. Studies were excluded if they were written in languages other than English, focused on dietary fibers, lacked full-text availability, or were not relevant to the fields of materials science or sustainability.

2.3. Selection Process

The initial search identified 407 records. After removing duplicates, 382 records remained. These were screened based on title and abstract and 97 articles were excluded due to lack of relevance. The remaining 285 full-text articles were reviewed for eligibility, and 19 were excluded at this stage. Ultimately, 266 studies were included in the final synthesis. The reporting is supported by a completed PRISMA 2020 checklist (Table S1) and a PRISMA 2020 Flow Diagram (Figure S1) detailing records through each screening stage.

3. Classification of Natural Fibers

There are three types of natural fibers, defined by their sources: vegetable (e.g., flax), animal (e.g., silk), and mineral origin (e.g., basalt fibers) [11]. Figure 1 presents a schematic overview of the classification of natural fibers into their respective groups.

While this review focuses on fibers naturally occurring in plants, animals, and mineral sources, there is a growing class of materials derived from natural feedstocks but produced through chemical or biological synthesis. These fibers, such as PHA [12], PLA [13], chitosan [14], viscose [15], and milk-based casein [16], do not originate as anatomical structures in nature but are of increasing interest in sustainable material research. Their development complements the use of natural fibers by expanding the portfolio of biodegradable and renewable alternatives to petroleum-based synthetics.

3.1. Animal Fibers

Animal fibers are protein-based materials, primarily composed of keratin (in wool and hair) and fibroin (in silk), which impart distinctive mechanical and thermal properties. Sourced from mammalian hair, fleece, or secretions, they are classified by species and anatomical origin. Keratin-based fibers like wool (mainly from sheep) are known for their crimp, elasticity, and thermal insulation due to their hygroscopic core and air-trapping structures. Other keratinous fibers include alpaca, cashmere, mohair, and camel hair, each varying in softness and strength [17].

Silk, secreted by Bombyx mori and other wild moths, is a continuous filament of fibroin coated in sericin. Its tensile strength, luster, and biocompatibility make it a valuable resource for both textile and biomedical applications [18].

Lastly, feathers (e.g., chicken feathers), although not classified as fibers in the traditional sense, are increasingly explored as a keratin-based fibrous material that can be used for lightweight and sustainable composites [19].

3.2. Mineral Fibers

Mineral fibers form a distinct class of natural fibers derived from inorganic geological sources rather than biological organisms. Traditionally, this category was dominated by asbestos, a group of silicate minerals known for their high tensile strength, fire resistance, and chemical stability [20]. These properties made asbestos widely used in insulation, construction, and industrial textiles. However, due to its severe health hazards, especially when inhaled as fine dust, its usage is now banned in most countries [21,22].

Beyond asbestos, modern research has focused on safer mineral fibers such as wollastonite and basalt fibers, which are emerging as sustainable alternatives in composites and construction materials. Wollastonite is a calcium silicate mineral with a needle-like morphology. It exhibits low thermal expansion, high dimensional stability, and excellent reinforcing properties when used as a filler in polymer composites. It improves mechanical strength, thermal resistance, and even reduces shrinkage in materials like plastics and ceramics [23].

Basalt fibers, produced by the controlled melting and extrusion of naturally occurring basalt rock, offer high tensile strength, excellent chemical and thermal resistance, and are entirely non-toxic [24]. Their performance is often comparable to, or exceeds, that of glass fibers, while being more environmentally benign in terms of raw material availability and processing [25,26]. Basalt fibers are increasingly used in automotive, aerospace, marine, and construction sectors, particularly in geotextiles and reinforced polymer composites [27]. While basalt fibers are derived from natural volcanic rock, they are processed via high temperature melting and extrusion, like glass fibers. Therefore, some authors consider them closer to synthetic mineral fibers in terms of processing and properties [28].

3.3. Vegetal Fibers

Plant-based natural fibers are known as lignocellulosic fibers due to their high cellulose and lignin content, and they represent the most abundant group of natural fibers [29]. These fibers are divided into two main categories: woody fibers (with a high lignin content) and non-woody fibers (with low lignin content) [30]. The non-woody fibers are further subdivided into six subgroups: (i) bast fibers; (ii) leaf fibers; (iii) stalk fibers; (iv) reed or grass fibers; (v) fruit fibers; and (vi) seed fibers [31]

Table 1. Classification of various plant-based fibers according to anatomical sources and species.

Fiber Type	Species Name	Fiber Origin
Abaca	Musa textilis	Leaf Fiber
Alfa Grass	Stipa tenacissima	Grass Fiber
Bamboo	Bambusoideae spp.	Grass Fiber
Banana	Musa spp.	Leaf Fiber
Barley Straw	Hordeum vulgare	Stalk Fiber
Betel Nut	Areca catechu	Fruit Fiber
Buriti	Mauritia flexuosa	Fruit Fiber
Coir	Cocos nucifera	Fruit Fiber
Cotton	Gossypium spp.	Seed Fiber
Curaua	Ananas erectifolius	Leaf Fiber
Date palm	Phoenix dactylifera	Leaf Fiber
Elephant Grass	Pennisetum purpureum	Grass Fiber
Fig Tree	Ficus religiosa L	Root Fiber
Flax	Linum usitatissimum	Bast Fiber
Harakeke	Phormium tenax	Leaf Fiber
Hemp	Cannabis sativa	Bast Fiber
Henequen	Agave fourcroydes	Leaf Fiber
Isora	Helicteres isora	Bast Fiber
Jute	Corchorus capsularis/C. olitorius	Bast Fiber
Loofah	Luffa cylindrica	Fruit Fiber
Kahili ginger	Hedychium gardnerianum	Leaf Fiber
Kapok	Ceiba pentandra	Seed Fiber
Kenaf	Hibiscus cannabinus	Bast Fiber
Milkweed Fiber	Asclepias spp.	Seed Fiber
Miscanthus	Miscanthus giganteus	Grass Fiber
Nettle	Urtica spp.	Bast Fiber
Oil palm	Elaeis guineensis	Fruit Fiber
Piassava	Attalea funifera	Leaf Fiber
Pineapple	Ananas comosus	Leaf Fiber
Ramie	Boehmeria nivea	Bast Fiber
Reed Canary Grass	Phalaris arundinacea	Grass Fiber
Rice Straw and Husk	Oryza sativa	Stalk Fiber
Roselle	Hibiscus sabdariffa	Bast Fiber
Sisal	Agave sisalana	Leaf Fiber
Sorghum	Sorghum bicolor	Stalk Fiber
Sponge gourd	Luffa aegyptiaca	Fruit Fiber
Sugarcane	Saccharum officinarum	Stalk Fiber
Sunn hemp	Crotalaria juncea	Bast Fiber
Switchgrass	Panicum virgatum	Grass Fiber
Vakka	Calotropis gigantea	Seed Fiber
Wheat Straw	Triticum aestivum	Stalk Fiber
Windmill Palm	Trachycarpus fortunei	Leaf Fiber

.

Bast fibers are extracted from the stems of plant species in the dicotyledons class. This is one of the two class groups into which all the flowering plants are divided, and it is defined by the fact that the seeds have two embryonic leaves, or cotyledons [11]. Bast is a tissue in plants, primarily composed of sieve tubes, that conducts nutrients to where they are needed. From here, we can obtain fiber bundles composed of elongated, thick-walled ultimate cells joined by both ends and sides [32,33]. These fibers are also known as soft fibers due to their soft texture and are generally of greater length than the hard fibers [34]. Soft fibers are generally long, strong, and rich in cellulose, with moderate amounts of hemicellulose and lignin. Their strength and flexibility make them ideal for textiles, ropes, biocomposites, and reinforcement materials [35,36,37]. Examples of bast fibers are flax (Linum usitatissimum), hemp (Cannabis sativa), kenaf (Hibiscus cannabinus), jute (Corchorus capsularis), ramie (Boehmeria nivea), and isora (Helicteres isora).

Leaf fibers, also known as hard fiber, are obtained from leaves or leaf stalks from various monocotyledonous species [38], defined by their lonely embryonic leaf, big parallel veined leave [33,39,40], and it is the other class group of all flowering plants [39,41]. These fibers are part of the sclerenchyma located in the vascular bundles of the plants, which are composed of cortical parenchyma, xylem, phloem, and sclerenchyma [33,42]. Leaf fibers are generally coarser, stiffer, and more durable than bast fibers, and have a higher composition of lignin [43]. Their natural resistance to microbial attack and moisture makes them suitable for cordage, mats, rugs, brushes, and, increasingly, green composites [44].

Examples of leaf fibers are pineapple (Ananas bracteatus), sisal (Agave sisalana), abaca (Musa textilis Nee), curaua (Ananas erectifolius), tampico (Agave lechuguilla), and African palm (Elaeis guineensis).

The classification of natural plant fibers by anatomical origin often presents overlaps and ambiguities, particularly between stalk fibers and grass fibers. While both are technically derived from the stem or stalk regions of plants, they differ in botanical origin and agricultural context. Looking at their botanical origin, both stalk and grass fibers come from the stems of species belonging to the monocotyledon class. These fibers are part of the sclerenchyma, located at the edges of the vascular bundles, which provide structural strength to the stems of these species [33]. Although the species in this group are clearly defined by the previous explanation, many authors divide them into two different groups: grass fibers and straw fibers [8,11]. Grass fibers refer to species from the Poaceae family that are not cultivated agriculturally, while straw fibers are defined as fibers obtained from agricultural residues of grass crops such as rice or wheat [45]. For example, bamboo is classified as a grass fiber because it belongs to the Poaceae family, a group of monocotyledonous grasses. Although its woody stem structure is atypical among grasses, its botanical classification, growth habit, and lignocellulosic composition justify this grouping [46].

Fruit fibers are typically obtained from the mesocarp or pericarp tissues of the fruit located between the exocarp and endocarp, providing structural support and physical protection for the seed [47,48]. These fibers are multicellular fiber bundles with a high concentration of lignin [49,50,51]. Some examples of fruit fibers are the luffa fibers obtained from the species Luffa cylindrica [49] and the coir fiber from Cocos nucifera [51], the latter being a byproduct of the coconut crop.

Lastly, seed fibers are usually developed as the seed protective layer (integument) on their surface, are formed from a single cell, although it is reported that more than one cell contributes to the growth of the fiber [52,53]. These fibers are generally fine, lightweight, with a high cellulose, and low lignin and waxes content [54]. Gossypium hirsutum, or cotton, is the most well-known species that produces seed fibers [54]. Other known seed fibers are kapok (Ceiba pentandra) [55] and poplar (Populus tremula) [56,57]. While grouped together due to their origin in reproductive organs, seed and fruit fibers arise from distinct plant tissues and serve different biological functions, which reflect in their morphology, composition, and industrial applications.

In addition to these six anatomical sources, some studies also reference root-derived fibers such as those from Ficus religiosa L. [58] and Cissus quadrangularis [59] roots, although these are less commonly used in industrial applications and are not typically classified as a major fiber group. Table 1 summarizes the classification of plant-based fibers by anatomical origin and species.

4. Fiber Morphology, Composition, and Physical Properties

4.1. Morphology

Fibers are elongated, cylindrical, and flexible structures with a very small cross-sectional area and a high length-to-diameter ratio (L/D > 100). Lignocellulosic natural fibers exhibit species-specific structures and morphologies [60,61]. Each fiber consists of bundles of elementary fibers held together by pectins located in the outer layer (Figure 2b). These elementary fibers are composed of a primary cell wall and three secondary cell walls [30,62] (Figure 2c), which in turn are made up of crystalline cellulose microfibrils embedded in an amorphous matrix of hemicellulose and lignin [63]. The microfibrils or ultimate fibers are the structural units of the fiber, featuring a predominantly crystalline core, a filamentous shape (10–30 nm in width), and composed of chains containing 2 to 30,000 cellulose molecules [63,64]. While natural fibers are often simplified as cylindrical structures, most plant fibers have irregular cross-sections, which has led to the introduction of correction factors—such as the Fiber Area Correction Factor (FACF)—to more accurately estimate mechanical properties [65].

The cellulose within the cell walls is not entirely crystalline; amorphous regions are present along the filaments [66]. The mechanical strength of the fibers depends on the content of crystalline cellulose and the spiral angle of the microfibrils [11]. The greater the crystalline cellulose content and the smaller the spiral angle relative to the fiber axis, the better the fiber’s mechanical performance [67]. Finally, the central region of the fiber is hollow and known as the lumen.

4.2. Composition

The chemical composition of natural fibers greatly influences their properties. The concentration of the main natural fiber compounds, namely cellulose, hemicellulose, lignin, pectins, and waxes, has a significant impact on all of these properties, from mechanical to thermal to structural to physical. Each compound influences the properties in their own way, and it is very important to know what each compound impacts in the fiber in the development of new products for every industry because, by knowing them, we can better suit the fibers to the types of products, limiting their inherent disadvantages and enabling us to know how we can modify the fibers, providing them with new capacities. Table 2 lists cellulose, hemicellulose, lignin, extractives, and ash contents for various plant-derived fibers.

4.2.1. Cellulose

Cellulose (C₆H₁₀O₅) is a strictly linear polysaccharide composed solely of β-glucose units linked through β-1,4 glycosidic bonds [68,69], and it has a high degree of polymerization [70]. According to Biagiotti et al. [30] (2004), cellulose is a 1,4-β-D-glucan and the primary component of plant fibers, found in the structure of both primary and secondary cell walls, typically ranging from 26% to 92% of fiber composition (Table 2).

The mechanical strength of plant cell walls is largely attributed to cellulose. Within its structure, cellulose includes both crystalline and amorphous regions. The cellulose content and its crystalline structure are two key parameters that directly affect the mechanical properties of plant fibers [71]. This structure facilitates the formation of organized bundles where crystalline regions are joined by hydrogen bonds [62]. Generally, the higher the degree of crystallinity, the greater the tensile strength of the fiber [30].

In addition, the free hydroxyl groups on the surface of cellulose fibers can interact with various functional groups through physical or chemical modifications, allowing the introduction of new properties or the mitigation of undesired ones [72,73].

4.2.2. Hemicellulose

Like cellulose, hemicellulose is also a polysaccharide found in the cell wall of plant cells. However, while cellulose is a homopolymer composed exclusively of β-glucose monomers, hemicellulose is a heteropolymer with side chains composed of various monomers, including: (i) pentoses, (ii) mannoses, and (iii) glucoses. Hemicellulose has a branched structure and a lower degree of polymerization than cellulose [69,74]. Its composition varies among plant species [75,76,77].

Quantitatively, lignocellulosic natural fibers contain between 0.15% and 42% hemicellulose (Table 2), which is located in the amorphous matrix of the cell wall, together with lignin, and is hydrogen-bonded to cellulose microfibrils [30,78,79]. Hemicellulose contributes to the moisture absorption capacity of fibers and is also a key factor in their thermal degradation, due to its lower thermal resistance [38,67,80,81,82].

A higher hemicellulose content reduces the thermal stability of fibers at elevated temperatures. According to Nayak et al. [32], reducing the hemicellulose content through suitable treatments is advantageous, as it enhances thermal stability and lowers hydrophilicity, thereby reducing water and moisture absorption.

4.2.3. Lignin

Lignin is a spherical polyphenolic group [83] highly branched, and composed of complex hydrocarbons with both aliphatic and aromatic constituents. It is essential for the structural integrity of support tissues in all vascular plant species [70]. Quantitatively speaking, the amount of lignin in vegetable fibers is around 0.1 and 45% (Table 2). It has an amorphous and hydrophobic structure [77,84,85], with reticulated (network-like) cells that interact with cellulose microfibrils in the cell wall matrix, where it is found together with hemicellulose [30,84,86]. Lignin imparts properties to the plant fibers such as rigidity, resistance to acids and microorganisms [44,82], and thermal stability since lignin degrades after cellulose and hemicellulose [87]. According to Mohanty et al. [2], the degradation of lignin through heat leads to the formation of ashes, which provides an insulation layer and further stability.

4.2.4. Pectins

Pectins are carbohydrate macromolecules composed mainly of galacturonic acid, a complex polysaccharide present in the middle lamella and primary walls of plant tissue [88]. It constitutes ≈ 30% of the primary walls of the fibers from the gymnosperm and angiosperm plant species [89] Although not as predominant as cellulose, lignin, or hemicellulose, pectins play an important role in the structure of plant fibers. They form massive macromolecules that binds elementary fibers into the final fibers, providing flexibility, cohesiveness, and stability to the structure [90]. Pectins can also bind the cellulose microfibrils with the matrix formed by hemicellulose [88]. Their hydrophilic nature, due to their carboxyl groups [88,91], allows them to retain moisture, influencing fiber softness, swelling behavior, and water absorption [90,92,93]. During fiber extraction processes, pectins are among the first components to be solubilized, facilitating the separation of individual fibers [94].

4.2.5. Waxes

Waxes are hydrophobic substances composed mainly of long-chain aliphatic compounds such as fatty acids, alcohols, and esters [88,95]. They form a thin, protective layer on the outer surface of plant fibers, especially in the cuticle of epidermal cells [88]. This waxy coating plays a crucial role in reducing water loss, preventing microbial attack, and providing mechanical protection against environmental stresses [96]. However, the presence of waxes can hinder the interfacial adhesion between plant fibers and polymer matrices in composite materials [97,98]. Therefore, chemical or physical treatments, such as alkaline washes or plasma treatments, are commonly applied to remove or modify waxes, enhancing fiber surface energy, wettability, and bonding capacity in composite reinforcement applications.

Table 2. Chemical composition of various plant-derived natural fibers, showing typical ranges of cellulose, hemicellulose, lignin, extractives, and ash content. Note: Listed values do not always sum to 100% due to unquantified minor components such as pectins, proteins, waxes, and moisture content, as well as analytical variability between sources.

Fiber Name	Cellulose (wt%)	Hemicellulose (wt%)	Lignin (wt%)	Extractables (wt%)	Ashes (wt%)	References
Abaca	56–64	15–25	7–13	0.8–3	3	[43,69,70,99]
Bagasse	32–55.2	16.8–30	19–25.3	10	-	[30,31,70,99]
Bamboo	26–65	30	1–31	-	-	[31,43,70,99]
Banana	60–83	6–16	5–10	3–5	-	[30,63,70,99]
Coir	32–43	0.15–20	40–45	3–4	-	[43,63,70,99]
Cotton	75–92	2–5.7	0.1–2	0.1–0.6	0.8–2	[43,63,99]
Curaua	70.7–74	9.9–21	7.5–11	0.2–10	-	[31,43,70,99]
Flax	60–81	14–20.6	0.9–2.3	0.9–2.3	-	[43,70,100,101]
Hemp	57–90	14–22.4	3.7–13	0.8–0.9	0.8	[31,63,99,100]
Jute	45–84	12–22	5–26	0.2–5	0.5–2	[43,70,100]
Kenaf	31–72	8–21	8–21.5	0.3–5	2–5	[43,70,100,102]
Mesta Jute	60	15	10	-	-	[88]
Nettle	79–86	6.5–12.5	3.5–4.4	4	-	[43,63,99]
Phormium tenax	45.1–72	30.1	11.2	0.7	-	[43]
Pineapple Leaf	70–83	16–19	5–12.7	2–2.5	-	[43,63,99]
Ramie	68.6–91	5–16.7	0.5–1	0.3–10	-	[30,31,100]
Rice Straw	36–57	33	8–19	-	-	[31,43,70,88]
Rice Husk	35–45	12–25	20	14–17	-	[31,43,70,88]
Roselle	70.2	7.21	14.91	-	-	[88,103]
Softwood	30–60	20–30	21–37	-	-	[88]
Hardwood	31–64	25–42	14–34	-	-	[88]
Sisal	43–78	10–24	4–14	0.4–10	0.6–1	[43,63,100]
Wheat straw	38–51	15–31	12–20	-	-	[31,43,70]
Luffa aegyptiaca	63	19.4	11.2	3	-	[63]
Crotalaria juncea	41–48	8.3–13	22.7	-	-	[63]
Sansevieria cylindrica	79.7	10.13	3.8	0.09	-	[43]
Sansevieria ehrenbergii	80	11.25	7.8	0.45	0.6	[43]
Attalea funifera	28.6	25.8	45	-	-	[43,63,99]
Pueraria spp.	33	11.6	14	-	-	[43,63]
Asclepia syriaca	74.5	-	4.1	-	2.2	[104]
Agave fourcroydes	60–78	4–28	8–13.1	0.5–4	-	[30,63,69]
Helicteres isora	74	-	23	1.09	-	[43,63,99]

Table 2. Chemical composition of various plant-derived natural fibers, showing typical ranges of cellulose, hemicellulose, lignin, extractives, and ash content. Note: Listed values do not always sum to 100% due to unquantified minor components such as pectins, proteins, waxes, and moisture content, as well as analytical variability between sources.

Fiber Name	Cellulose (wt%)	Hemicellulose (wt%)	Lignin (wt%)	Extractables (wt%)	Ashes (wt%)	References
Abaca	56–64	15–25	7–13	0.8–3	3	[43,69,70,99]
Bagasse	32–55.2	16.8–30	19–25.3	10	-	[30,31,70,99]
Bamboo	26–65	30	1–31	-	-	[31,43,70,99]
Banana	60–83	6–16	5–10	3–5	-	[30,63,70,99]
Coir	32–43	0.15–20	40–45	3–4	-	[43,63,70,99]
Cotton	75–92	2–5.7	0.1–2	0.1–0.6	0.8–2	[43,63,99]
Curaua	70.7–74	9.9–21	7.5–11	0.2–10	-	[31,43,70,99]
Flax	60–81	14–20.6	0.9–2.3	0.9–2.3	-	[43,70,100,101]
Hemp	57–90	14–22.4	3.7–13	0.8–0.9	0.8	[31,63,99,100]
Jute	45–84	12–22	5–26	0.2–5	0.5–2	[43,70,100]
Kenaf	31–72	8–21	8–21.5	0.3–5	2–5	[43,70,100,102]
Mesta Jute	60	15	10	-	-	[88]
Nettle	79–86	6.5–12.5	3.5–4.4	4	-	[43,63,99]
Phormium tenax	45.1–72	30.1	11.2	0.7	-	[43]
Pineapple Leaf	70–83	16–19	5–12.7	2–2.5	-	[43,63,99]
Ramie	68.6–91	5–16.7	0.5–1	0.3–10	-	[30,31,100]
Rice Straw	36–57	33	8–19	-	-	[31,43,70,88]
Rice Husk	35–45	12–25	20	14–17	-	[31,43,70,88]
Roselle	70.2	7.21	14.91	-	-	[88,103]
Softwood	30–60	20–30	21–37	-	-	[88]
Hardwood	31–64	25–42	14–34	-	-	[88]
Sisal	43–78	10–24	4–14	0.4–10	0.6–1	[43,63,100]
Wheat straw	38–51	15–31	12–20	-	-	[31,43,70]
Luffa aegyptiaca	63	19.4	11.2	3	-	[63]
Crotalaria juncea	41–48	8.3–13	22.7	-	-	[63]
Sansevieria cylindrica	79.7	10.13	3.8	0.09	-	[43]
Sansevieria ehrenbergii	80	11.25	7.8	0.45	0.6	[43]
Attalea funifera	28.6	25.8	45	-	-	[43,63,99]
Pueraria spp.	33	11.6	14	-	-	[43,63]
Asclepia syriaca	74.5	-	4.1	-	2.2	[104]
Agave fourcroydes	60–78	4–28	8–13.1	0.5–4	-	[30,63,69]
Helicteres isora	74	-	23	1.09	-	[43,63,99]

4.3. Mechanical and Physical Properties

The mechanical and physical properties of plant-based natural fibers play a critical role in determining their suitability for various applications. These properties influence not only the mechanical performance of fiber-reinforced products but also their durability, processability, and compatibility with different matrices. Factors such as botanical origin, fiber maturity, cultivation conditions, and post-harvest treatments can significantly affect the performance characteristics of each fiber type [38,105].

To characterize plant fiber through a set of key parameters that provide insight into their performance in various applications. Table 3 shows the properties of selected plant-based fibers. Density, expressed in grams per cubic centimeter, affects the weight and packing behavior of the fibers within composite matrices [106,107]. The diameter of the fibers, usually measured in micrometers, influences the available surface area for matrix interaction [60]. The crystallinity index reflects the proportion of crystalline cellulose within the fiber structure and is often associated with increased stiffness and mechanical strength [108,109]. The microfibrillar angle, the orientation of cellulose microfibrils relative to the fiber axis, plays a significant role in determining tensile performance, with lower angles generally indicating higher strength and stiffness [110,111]. Tensile strength measures the maximum stress the fiber can endure before failure, while Young’s modulus indicates its stiffness or resistance to elastic deformation [107]. Elongation at break reveals the fiber’s ductility, representing how much it can stretch before breaking [112,113]. Moisture content quantifies the amount of water retained under standard conditions, influencing thermal properties and microbial susceptibility [107,109]. Lastly, water absorption indicates how much water the fiber can absorb over time, which is crucial for evaluating dimensional stability, biodegradation potential, and suitability in humid environments [60]. Table 3 summarizes the mechanical and physical properties of plant-based fibers, including density, diameter, crystallinity index, microfibrillar angle, tensile strength, Young’s modulus, elongation, moisture content, and water absorption.

Table 3. Mechanical and physical properties of selected plant-based natural fibers. Table includes values for key parameters such as density, diameter, crystallinity index, microfibrillar angle, tensile strength, Young’s modulus, elongation at break, moisture content, and water absorption. These properties vary depending on fiber origin, growth conditions, and processing methods, and are critical for determining suitability in industrial applications.

Fiber Type	WA (%)	MC (%)	Den (g/cm3)	Dia (µm)	CI (%)	MFA (º)	Elongation at Break (%)	Tensile Strength (MPa)	Young’s Modulus (GPa)	References
Abaca	-	15	0.83–1.5	17–130	68.2	-	1–10	400–980	6.2–20	[11,31,99,114]
Bagasse	-	8.8	0.55–1.5	10–400	32–96.33	-	0.9–1.1	20–290	2.7–27.1	[31,43,63,102]
Bamboo	-	8.9–10.14	0.6–1.1	25–330	48.0	10–11	1.4–3.7	140–800	11–35.9	[31,43,70,88,115]
Banana	60	10.71	0.8–1.4	12–280	56.2–61.7	11–12	1.5–10	180–914	7.7–32	[31,115,116,117]
Coir	130–180	4.7–11.4	1.0–1.5	10–460	37.28	30.45	2.84–51.4	46.4–500	2.17–26	[8,11,99,115]
Cotton	-	7.8–8.5	1.5–1.6	10–45	65	20–30	2–10	287–800	3.44–12.6	[11,31,43,70]
Date Palm	60–65	9.6–10.7	0.46–1.2	155–250	38.5	-	2–4.5	97–459	1.91–70	[8,43,115,118]
Flax	136	7–12	1.4–1.5	7–600	70–80	5–10	0.2–3.3	343–2000	18.4–103	[31,43,69,101]
Hemp	-	6.2–15	1.4–1.5	16–500	88	2–6.2	1–4.7	58–1100	3–90	[31,67,70,99,117]
Jute	281	12–23	1.3–1.5	20–350	71	8	1–8	187–938	3–78	[11,31,67,70,101]
Kenaf	-	6.2–20	1.22–1.45	40–250	60	2–6.2	1.5–6.9	223–1191	14.5–53	[8,30,101,119]
Pineapple Leaf	-	11.8–13	0.8–1.6	5–194	32–78.7	6–14	0.8–14.5	144–1627	1.44–2.5	[43,88,99]
Ramie	-	8–9	1–1.5	20–80	58	-	1.2–4	56–1000	3.6–128	[11,70,88]
Roselle	286.5	-	0.75–0.8	-	-	-	5–8	147–350	2.76–17	[8,120,121]
Sisal	190–250	11	0.7–1.5	8–300	71	10–22	2–14	268–855	9–38	[8,11,121,122]
Roystonea regia	-	12.09	0.81	175–230	-	-	3.46	549	15.85	[43,115]

Table 3. Mechanical and physical properties of selected plant-based natural fibers. Table includes values for key parameters such as density, diameter, crystallinity index, microfibrillar angle, tensile strength, Young’s modulus, elongation at break, moisture content, and water absorption. These properties vary depending on fiber origin, growth conditions, and processing methods, and are critical for determining suitability in industrial applications.

Fiber Type	WA (%)	MC (%)	Den (g/cm3)	Dia (µm)	CI (%)	MFA (º)	Elongation at Break (%)	Tensile Strength (MPa)	Young’s Modulus (GPa)	References
Abaca	-	15	0.83–1.5	17–130	68.2	-	1–10	400–980	6.2–20	[11,31,99,114]
Bagasse	-	8.8	0.55–1.5	10–400	32–96.33	-	0.9–1.1	20–290	2.7–27.1	[31,43,63,102]
Bamboo	-	8.9–10.14	0.6–1.1	25–330	48.0	10–11	1.4–3.7	140–800	11–35.9	[31,43,70,88,115]
Banana	60	10.71	0.8–1.4	12–280	56.2–61.7	11–12	1.5–10	180–914	7.7–32	[31,115,116,117]
Coir	130–180	4.7–11.4	1.0–1.5	10–460	37.28	30.45	2.84–51.4	46.4–500	2.17–26	[8,11,99,115]
Cotton	-	7.8–8.5	1.5–1.6	10–45	65	20–30	2–10	287–800	3.44–12.6	[11,31,43,70]
Date Palm	60–65	9.6–10.7	0.46–1.2	155–250	38.5	-	2–4.5	97–459	1.91–70	[8,43,115,118]
Flax	136	7–12	1.4–1.5	7–600	70–80	5–10	0.2–3.3	343–2000	18.4–103	[31,43,69,101]
Hemp	-	6.2–15	1.4–1.5	16–500	88	2–6.2	1–4.7	58–1100	3–90	[31,67,70,99,117]
Jute	281	12–23	1.3–1.5	20–350	71	8	1–8	187–938	3–78	[11,31,67,70,101]
Kenaf	-	6.2–20	1.22–1.45	40–250	60	2–6.2	1.5–6.9	223–1191	14.5–53	[8,30,101,119]
Pineapple Leaf	-	11.8–13	0.8–1.6	5–194	32–78.7	6–14	0.8–14.5	144–1627	1.44–2.5	[43,88,99]
Ramie	-	8–9	1–1.5	20–80	58	-	1.2–4	56–1000	3.6–128	[11,70,88]
Roselle	286.5	-	0.75–0.8	-	-	-	5–8	147–350	2.76–17	[8,120,121]
Sisal	190–250	11	0.7–1.5	8–300	71	10–22	2–14	268–855	9–38	[8,11,121,122]
Roystonea regia	-	12.09	0.81	175–230	-	-	3.46	549	15.85	[43,115]

5. Natural and Synthetic Fiber Comparison

The production and research of natural fibers and new green composites have been promoted globally due to new legislation and environmental campaigns [123,124]. This prompted a higher focus on natural fibers to substitute for conventional synthetic fibers like carbon, aramid, and glass. Also, the production of these manmade fibers depends on fossil fuels, needs more energy, and produces pollutant gases released into the environment at a higher rate than natural fibers [125,126]. Still, the production of natural fibers often relies on intensive use of agrochemical fertilizers [127,128], and certain processing and extraction methods, such as stand retting involving glyphosate application, can have notable environmental impacts [129]. Nevertheless, the rate of pollutant gas emissions is significantly higher in the production of artificial fibers, while natural fibers have a high CO₂ assimilation rate, helping reduce the carbon content in the atmosphere [130]. Begum and Islam [6] argue that the energy consumption required to produce polypropylene (PP) fibers is 20 times higher than to produce the same amount of natural fibers. According to Liu et al. [131], the natural fibers produce and absorb the same amount of CO₂, being carbon neutral.

The production of natural fiber is mainly labor-intensive and low-cost, while synthetic fibers are high-cost and require less labor [132]. Natural fibers offer several advantages over synthetic ones: they are mostly non-toxic, have low density, are fully biodegradable, environmentally friendly, renewable, and widely available across the globe [6,7,8,112,123]. However, they also present some drawbacks, such as high moisture absorption, which can degrade their properties, and low interfacial adhesion to polymer matrices [9,11,112]. Also, studies show that the long-term inhalation of cotton dust may cause respiratory diseases during the spinning process [133,134,135]. These limitations, however, can be mitigated through appropriate surface treatments [136]. In contrast, synthetic fibers typically exhibit superior mechanical performance, greater durability, and lower moisture sensitivity [6,137]. Nevertheless, they are often toxic and have been linked to various lung diseases [21,22,123].

Table 4. A comparative overview of properties of natural and synthetic fibers [138].

Properties	Natural Fibers	Synthetic Fibers
Abundance	Infinite	Finite
Recyclability	Good	Moderate
Carbon Footprint	Neutral	High
Environmental Impact	No	Yes
Durability	Moderate	High
Biodegradability	High	Low
Weight	Low	Moderate
Cost	Low	High
Toxicity	Non-toxic	Toxic
Mechanical Properties	Moderate	High
Humidity Sensitivity	High	Low
Thermal Properties	Moderate	High
Acoustic Properties	Moderate	Moderate
Interfacial Adhesion	Low	Moderate

provides a comparative overview of the main physical, mechanical, and environmental properties of natural and synthetic fibers.

6. Natural Fiber Production and Economical Value

6.1. Global Production

The Kyoto Protocols on greenhouse gas reduction and CO₂ neutral production encouraged the transition from fuel-based products to bio-based products [139], which increased the potential for the natural fiber markets. Globally, natural fibers are produced and used on a wide range of products, from traditional uses for rope, net, textiles, and paper [122] to more technological uses for a variety of composites [5,44,88,125,126,130,140], geotextiles for soil conservation [141,142,143], and nanofibers [86,144,145].

The natural fiber production industry is one of the most labor-intensive sectors, and it is used to stimulate industrialization in cheap labor-developing countries like China, India, Indonesia, and Brazil, which increases their socio-economic development. Today, these countries are the largest producers, playing an increasingly greater role in the production, movement, and processing of natural fibers [123,146].

Table 5 represents the most common fibers produced worldwide and their main production countries. Cotton (Gossypium spp.) is the largest fiber crop globally, with 25 million metric tons produced per year, and its demand has grown considerably in recent years [133], while bagasse is the largest crop byproduct fiber globally from commercial species of the genus Saccharum and Sorghum [147], with 75 million metric tons produced. One often-overlooked factor in natural fiber utilization is the seasonality of harvesting, which varies with geographic origin. Some fibers like sisal can be harvested throughout the year due to favorable conditions [148,149], while other fibers, in temperate-zones, are typically harvested only during the summer or autumn, which can limit production, increasing costs of the fibers, which can hinder production systems and supply logistics for fiber-based applications [11,150].

Table 5. Worldwide production of some natural fibers.

Fiber Name	Main Producers	World Production (103 ton)	References
Abaca	Philippines, Ecuador, Costa Rica	70	[31,70]
Bagasse	Brazil, India, China	75,000	[31,70]
Bamboo	India, China, Indonesia, Malaysia, Philippines	30,000	[31,70]
Banana	India	200	[38]
Coir	India, Sri Lanka, Philippines, Malaysia	100	[31,70]
Cotton	China, India, USA, Pakistan	25,000	[70]
Curaua	Brazil, Venezuela	>1	[70]
Flax	Canada, China, Russia, France, Belgium	830	[31,70,130]
Hemp	China, France, Philippines	214	[31,70,130]
Henequen	Mexico	30	[70]
Jute	India, China, Bangladesh,	2300	[31,70,151]
Kapok	Indonesia	123	[38]
Kenaf	India, Bangladesh, USA	970	[31,38,70,130]
Oil Palm	Malaysia, Indonesia	40	[70]
Pineapple	Philippines, Thailand, Indonesia	74	[70]
Ramie	China, Brazil, Philippines, India	100	[31,38]
Roselle	Thailand, China	250	[120]
Sisal	Tanzania, Brazil, Kenya	378	[31,70]
Sunn Hemp	India, Bangladesh, Brazil	70	[120]
Rice Husk	China, India, Indonesia, Malaysia, Bangladesh	160,000	[152]
Rice Straw	China, India, Indonesia, Malaysia, Bangladesh	579	[153]
Wood Fiber	Canada, USA, China	1,750,000	[130]
Palm Date	Iran	4200	[154]

Table 5. Worldwide production of some natural fibers.

Fiber Name	Main Producers	World Production (103 ton)	References
Abaca	Philippines, Ecuador, Costa Rica	70	[31,70]
Bagasse	Brazil, India, China	75,000	[31,70]
Bamboo	India, China, Indonesia, Malaysia, Philippines	30,000	[31,70]
Banana	India	200	[38]
Coir	India, Sri Lanka, Philippines, Malaysia	100	[31,70]
Cotton	China, India, USA, Pakistan	25,000	[70]
Curaua	Brazil, Venezuela	>1	[70]
Flax	Canada, China, Russia, France, Belgium	830	[31,70,130]
Hemp	China, France, Philippines	214	[31,70,130]
Henequen	Mexico	30	[70]
Jute	India, China, Bangladesh,	2300	[31,70,151]
Kapok	Indonesia	123	[38]
Kenaf	India, Bangladesh, USA	970	[31,38,70,130]
Oil Palm	Malaysia, Indonesia	40	[70]
Pineapple	Philippines, Thailand, Indonesia	74	[70]
Ramie	China, Brazil, Philippines, India	100	[31,38]
Roselle	Thailand, China	250	[120]
Sisal	Tanzania, Brazil, Kenya	378	[31,70]
Sunn Hemp	India, Bangladesh, Brazil	70	[120]
Rice Husk	China, India, Indonesia, Malaysia, Bangladesh	160,000	[152]
Rice Straw	China, India, Indonesia, Malaysia, Bangladesh	579	[153]
Wood Fiber	Canada, USA, China	1,750,000	[130]
Palm Date	Iran	4200	[154]

6.2. Natural Fiber Economic Value

Regarding the economic value of natural fibers, various authors present several values [31,155,156], but in general, the value of the fiber crop will depend on its end-use market and cost of production. An example of an end-use market is sisal fiber, which can be used for producing ropes but also for composites used on airplanes [122,140]. Regarding the cost of production, the extraction of fibers plays a major role here, since the extraction will influence the quality of the fibers, which will also affect the end-use market [119]. Another example is the shape and length of the fibers. Fine and long fibers are more desirable and valued because they can be spun into high-quality yarns for textiles [157] or composites [155], while shorter or coarser fibers have a lower value and are used in paper pulp and other materials [155,158]. Table 6 shows the value of some commercial fibers per kilogram.

Table 6. Comparative average commercial prices of plant-based fibers per kilogram (USD/kg) [155,159,160].

Natural Fibers	Cost (USD/kg)	References
Abaca	1.55–2.55	[161,162]
Bamboo	0.25–0.50	[161]
Banana	0.1	[38]
Hemp	0.30–1.60	[38,161]
Coir	0.20–0.84	[38,161]
Cotton	1.71–5.06	[159,162,163]
Curaua	0.44	[155]
Jute	0.25–0.50	[161,162]
Flax	0.30–1.55	[38,161]
Kapok	1.85–7.61	[162]
Kenaf	0.30–0.65	[38,161]
Ramie	1.50–2.40	[161,162]
Piassava	0.37	[155]
Sisal	0.35–1.30	[38,161,162]
Sponge-gourd	0.6	[155]

Table 6. Comparative average commercial prices of plant-based fibers per kilogram (USD/kg) [155,159,160].

Natural Fibers	Cost (USD/kg)	References
Abaca	1.55–2.55	[161,162]
Bamboo	0.25–0.50	[161]
Banana	0.1	[38]
Hemp	0.30–1.60	[38,161]
Coir	0.20–0.84	[38,161]
Cotton	1.71–5.06	[159,162,163]
Curaua	0.44	[155]
Jute	0.25–0.50	[161,162]
Flax	0.30–1.55	[38,161]
Kapok	1.85–7.61	[162]
Kenaf	0.30–0.65	[38,161]
Ramie	1.50–2.40	[161,162]
Piassava	0.37	[155]
Sisal	0.35–1.30	[38,161,162]
Sponge-gourd	0.6	[155]

7. Natural Fiber Extraction Methods

In order to separate the fibers from the plant crops, the fibers must pass through an extraction method to desegregate the fiber from the adjacent plant tissues. Extraction methods generally revolve around a process called retting, which is a process that causes the fiber bundles to be removed from the stem, leaf, or pseudostem due to the degradation of the pectic polysaccharide tissues surrounding the fiber using biological activity from the environment from bacteria and fungi [115]. Other methods do not use the typical retting process, opting for a chemical compound or equipment [44,70,119,164]. The type of method used might depend on the type of fiber, and it has a major impact on the amount of fiber obtained [70]. Also, the method used has an influence on the fiber quality regarding its structure, chemical composition, and other properties of the fibers [70,77,165]. Figure 3 classifies natural fiber extraction methods by process type.

7.1. Biological Extraction Methods

7.1.1. Dew Retting

Dew retting was the first method used for separating fibers, and it is still one of the most used retting processes today [70]. It is a simple method; the fiber-producing crops are harvested and spread uniformly throughout the field. This process takes 3–6 weeks, depending on the weather conditions, like temperature and moisture ranges [166]. These two conditions promote the activity of microorganisms, which enable the separation of the fibers from the cortex and xylem of the plant [44]. Dew retting must be closely monitored to ensure that a homogenous retting is obtained, because if not, the plants might be over- or under-retted. Over-retting means that the fungi and bacteria have degraded the cellulose of the fibers, resulting in fibers with reduced mechanical properties; under-retting, on the other hand, makes the further processing of the fibers harder [101]. Thus, it is important to observe the weather conditions, turn over the plants in the field, and optimize retting time for this method to be successful. Although this method is among the most widely used today, particularly favored by fiber-producing crop farmers for its sustainability and low labor cost, it presents some notable disadvantages. Its heavy dependence on weather conditions makes it unreliable and unsuitable for global application, as temperature and moisture levels cannot be controlled. Additionally, the quality of the resulting fibers is generally lower compared to other methods, as they often acquire a darker color due to prolonged contact with soil [101,167].

A recent study, Orm et al. [168] proposes a microbial enzyme linked monitoring framework for field or dew retting of hemp. They use high throughput sequencing to correlate microbial taxa and enzymatic profiles with retting stages, identifying Pseudomonas, Sphingomonas, and Cladosporium as indicators. This strategy enhances control over retting outcomes and fiber quality in dew retting.

7.1.2. Water Retting

Water retting is another common extraction method used globally, where the fiber-yielding plants are submerged in water for 3 to 35 days [169]. This method can be performed in the running water of rivers and streams, but it can also be conducted in water tanks where the water needs to be replaced every 2 days. The duration of this process depends on the type of water and the temperature. A water temperature of around 28 to 40 °C grants a faster process (3 to 5 days) due to the higher bacterial activity [44]. Anaerobic bacteria are used in this process to break down the pectin that binds the adjacent plant cells to the fibers. This process produces high-quality fibers, but it has some disadvantages, such as the large amount of water consumption, the high cost of the equipment for artificial drying, and environmental problems such as organic wastewater and fermentation gases [167]. Due to environmental concerns, this method has been abolished as an industrial extraction method in most parts of Europe [70].

A recent study showed a new water retting method called micro-pond retting uses 1/6th of the water of the traditional method, with microbial inoculation, and recaptures residues and integrates aquaculture and crops [170]. Hossain et al. [171] explored the use of alternative water sources (tap, pond, and canal water) for bio-retting kenaf fibers. It demonstrates that non-conventional water sources can yield quality fibers with varying lignin and cellulose contents, proposing a more adaptable and resource-efficient retting method. Another study by Harsányi et al. [172] introduces a novel controlled anaerobic water retting method (AWR) for flax fiber extraction. Unlike traditional retting techniques, AWR is performed in closed bioreactors under strict anaerobic conditions, without the use of chemicals or enzymes. This not only produces high-quality cellulose-rich fibers but also generates valuable fermentation byproducts, including organic acids (notably acetic and butyric acid), hydrogen-rich hydrolysis gas, and biomethane.

7.1.3. Enzymatic Retting

An interesting method of fiber extraction is enzymatic retting, where, as the name implies, enzymes are used to promote the degradation of the pectins that bind the fibers to the adjacent tissue. This procedure starts right after the harvest, where the plants are lightly crushed with a crimping machine to enhance enzyme penetration into the tissue and accelerate the process [173]. The fibers are then incubated in a tank with the pectin-degrading enzyme solution at the best temperature range for enzyme activity [44,174] for 2–24 h. This process produces high-quality fibers with good mechanical properties, although it is still a very expensive procedure due to the cost of the enzymes, wastewater treatment, and equipment needed [100].

A study by Ventorino et al. [175] introduces a microbial consortia-based retting approach for hemp fibers using strains of Bacillus, Paenibacillus, and Pseudomonas selected for their enzymatic activity. The consortia exhibited high pectinolytic activity while preserving cellulose integrity, allowing efficient fiber extraction within 5 days. Huang et al. [176], develop a bioaugmentation retting process using a tailored fungal consortium (Aspergillus micronesiensis, Penicillium citrinum, Cladosporium sp.) for sisal fibers. This method significantly improves degumming efficiency and cellulose purity through high pectinase, xylanase, and mannanase activity while avoiding fiber damage from cellulase.

7.2. Chemical Extraction Methods

Surfactant Retting

Chemical maceration, or surfactant retting, is a group of retting processes where a heated chemical solution is used to enable the degradation of the pectins to free the fibers from the plant tissue [77]. The solution can be made with several chemical compounds, such as potassium hydroxide (KOH), sulfuric acid (H₂SO₄), calcium hypochlorite (Ca(ClO)₂), sodium carbonate (Na₂CO₃), or sodium hydroxide (NaOH) [70]. These methods use surface-active agents to help get rid of non-cellulosic parts through emulsion-forming processes [44]. The plants, after harvest, can be prepared before chemical retting; they might be milled, crushed, or decorticated to help with chemical penetration.

Depending on the retting condition, these processes can take between a few minutes and 48 h [45]. After the retting, the fibers are cleaned with running water, dried, and combed. These procedures yield high-quality fibers with good surface adhesion properties [77], although the high cost and water pollution are some of the disadvantages of this method.

A novel chemical extraction developed by Nassar et al. [177] combines three sequential steps, dewaxing, acetylation, and mercerization, with precise time control and the optional use of microwave-assisted heating. This approach enhances fiber purity, crystallinity, and tensile strength. The method efficiently removes surface impurities and amorphous biomass, yielding cellulose-rich fibers ideal for advanced bio-composites. Balasubramanian et al. [107] proposed a modified low-concentration alkali treatment for jute fibers using 2% NaOH followed by ethanol-acetic acid neutralization. This method achieves cleaner fiber surfaces and reduced lignin content while minimizing chemical usage, representing an eco-friendlier and more effective alternative for jute fiber processing.

7.3. Mechanical Extraction Methods

7.3.1. Manual Extraction

Manual extraction is one of the earliest and straightforward fiber extraction processes, involving physical separation of fibers from retted plant material using hand tools such as knives or scrapers. The extracted fiber quality depends on the number of times the scraping of fibers from the leaf sheath, or stems. In some cases, this process is combined with dew or water retting [178].

The main advantages of manual extraction include its low cost, minimal equipment requirements, and suitability for rural and decentralized fiber production. It also enables selective processing of high-quality fiber portions and offers employment opportunities in local communities [179]. However, it is also labor-intensive, time-consuming, and prone to variability in yield and quality [180], making it unsuitable for industrial applications. Despite this, it remains a valuable process for artisanal, research, and niche uses.

7.3.2. Mechanical Extraction

Mechanical extraction, or green retting, is a simple fiber extraction process where the fiber crops are dried in a field for 3–5 days, depending on the weather, or in a drying oven for 24 h at 70 °C [119]. The stem, or leaf, is then lightly crushed and decorticated, mechanically separating the fibers from the adjacent plant cells. After that, the fibers are washed, dried, and combed. This method is cost-effective and environmentally friendly [70], producing good-quality fibers, although they are less fine and have worse mechanical properties compared with fiber from other extractions due to the mechanical damage to the fibers [44,181].

7.4. Hybrid or Combined Extraction Methods

7.4.1. Mechanical and Chemical Extraction

The combined mechanical and chemical extraction method integrates physical disruption with chemical treatments, which improves fiber separation efficiency and quality. In this process, plant materials are first subjected to mechanical actions such as crushing, decortication, or milling to break down the outer layers to expose the fibrous bundles. Then, chemical treatments are used, generally with alkaline solutions that remove non-cellulosic materials like hemicellulose, lignin, and pectin [63,182].

This hybrid approach reduces retting time when compared with biological methods. It also enhances fiber surface reactivity, and uniformity, providing fibers with improved tensile properties and higher adhesion to polymer matrices for composite applications [182]. However, the process involves significant energy and water usage, and the disposal of chemical effluents poses environmental concerns if not properly managed [183]. This method is widely used for fibers like flax, hemp, kenaf, pineapple, and banana, especially in industrial contexts where consistent quality and performance are critical.

7.4.2. Steam Explosion

The steam explosion method (STEX) is another fiber extraction method where the fibers are extracted using temperature, pressure, steam, and additives [77]. In this method, the plant-yielding fibers are first dried, then subjected to high-pressure steaming, which will cause a saturation of the plants with high pressure and temperature steam followed by rapid decompression [164]. This sudden release of pressure causes a thermomechanical force that ruptures the middle lamella, effectively separating the fibers [70,164]. The resulting fibers are well-defined fibers with great fineness and properties, comparable with cotton fibers.

7.4.3. Ultrasound Retting

Ultrasound retting is a process that separates bast fibers from plants and has some similarities with water and chemical retting. In this method, the harvested plants are cleaned, all the non-fiber-yielding parts are removed, leaving only the stems, and then they are slightly crushed and washed. The stems are submerged in a hot water bath that is kept at around 70 °C for 24 h [184] with a small concentration of alkali and surfactants and subjected to a high-intensity (1 kW) ultrasound (40 kHz) [64,185]. As a result, the fibers are separated from adjacent plant cells. This method improves the extraction time and fiber properties, yet it is still not widely available for an industrial scale [186,187].

7.4.4. Stand Retting

The stand retting process is a modified version of the dew retting process that tries to overcome some of its limitations. This method has variations; in one, glyphosate (N-phosphonomethyl glycine) is used before the harvest to help the retting process [70,166], which will enable the production of fibers with better mechanical properties when compared to the dew retting fibers [188]. On the other hand, it is a thermal treatment where the bottom of the plants is heated to 100 °C, then dried for a couple of days [44]. This second treatment is less dependent on the weather. Summing up, the stand retting method has a reduced risk of weather and crop damage compared to the dew retting method, although this method has higher costs of extraction [70,189]. However, glyphosate is increasingly scrutinized for its potential health and environmental risks, including toxicity to humans and impacts on soil microbiota and aquatic systems [129]. As such, this method using glyphosates should be avoided.

7.4.5. Microwave-Assisted Retting

Microwave-assisted retting is an emerging technique that utilizes microwave radiation to enhance and accelerate the fiber extraction process from lignocellulosic plant materials. This method exploits the dielectric heating effect, where polar molecules—mainly water—absorb microwave energy and convert it into heat through molecular friction. The rapid and uniform heating disrupts the middle lamella, weakening the bonds between fiber bundles and non-cellulosic components such as pectin and hemicellulose [190,191,192]. As a result, fiber bundles are more easily separated from the plant matrix, significantly reducing the retting duration compared to traditional methods such as dew or water retting.

Microwave-assisted retting offers several advantages: it provides precise control over process parameters such as temperature and moisture, ensures consistent fiber quality, reduces microbial contamination risks, and lowers overall water consumption [193]. Additionally, it avoids some environmental drawbacks associated with water retting, such as effluent generation and odor problems. However, the technique also presents limitations, including high capital costs for microwave systems and the need for careful process optimization to prevent fiber degradation due to localized overheating [194]. Despite these challenges, microwave-assisted retting holds promise for scalable, eco-friendly, and time-efficient fiber processing.

7.4.6. The Duralin Process

The Duralin process is a method developed to improve the environmental and dimensional stability of natural fibers developed by Ceres B. V. (Wageningen, The Netherlands) [195]. It has been used for flax and hemp fibers, both bast fibers, and it consists of three steps: hydrothermolysis, drying, and curing [196]. The first steps involve a steam bath, where the hemp and flax stems are placed in an autoclave for 30 min at a temperature of 160 °C. Then, the stems are dried in a dry oven. Lastly, the dried stems are heated at 150 °C for 2 h. The lignin and hemicellulose are broken down into lower molecular weight aldehydes and phenolic functionalities. These are then combined by the curing reaction to make a water-resistant resin that holds the cellulose microfibrils together [196,197]. After this process, the fibers are separated from the stems by crushing, decorticating (mechanical separation of fiber bundles from plant stalks), or scutching (manual or mechanical removal of woody tissues from retted fibers) the stems. The fibers obtained by this process are high-quality fiber bundles for use in high-quality composites with great physical properties [195,196]. A side-by-side matrix of extraction methods, their operating principles, benefits, and limitations is presented in

Table 7. Comparative overview of natural fiber extraction methods discussed in this review.

Method	Type	Description	Advantages	Disadvantages
Dew Retting	Biological	Fibers are left in the field to be decomposed by natural microbial activity.	No chemicals or water required; very low cost; low environmental impact	Weather-dependent; long processing time; inconsistent fiber quality, low scalability and efficiency
Water Retting	Biological	Stalks submerged in water to facilitate microbial breakdown of pectins.	Traditional method; low cost; moderate efficiency; medium scalability; good fiber quality	Time-consuming; large water use; moderate–high environmental impact due to wastewater
Enzymatic Retting	Biological	Uses specific enzymes (e.g., pectinases) to break down binding materials.	Controlled process; consistent fiber quality; environmentally friendly	High cost; requires precise conditions
Chemical Retting	Chemical	Use various types of chemicals, like alkalis or acids to dissolve pectin and lignin.	Fast process; high efficiency; high scalability; decent fiber quality	High environmental hazards; moderate cost; fiber damage if not controlled
Steam Explosion	Hybrid	High-pressure steam treatment followed by rapid decompression.	Fast and effective; high efficiency; retains fiber strength; moderate scalability	Requires pressure systems; high cost; moderate environmental impact due to energy use; possible fiber damage
Ultrasound-Assisted Retting	Hybrid	Use ultrasonic waves in liquid medium to accelerate cell wall disruption.	Accelerates retting; high efficiency; preserves fiber quality; low environmental impact	Expensive equipment; high cost; limited scalability
Microwave-Assisted Retting	Hybrid	Applies microwave energy to heat plant material internally and aid pectin breakdown.	Fast, energy-efficient; high efficiency; good fiber preservation; low environmental impact	Expensive equipment; scaling-up challenges
Stand Retting	Hybrid	Field retting with modifications like pre-harvest chemical application or thermal treatments.	Reduces weather dependency; can improve fiber quality	Use of chemicals or energy may raise costs or environmental concerns
Duralin Process	Hybrid	Alkali pretreatment followed by thermal compression and drying for stiffer fibers.	Produces high-stiffness fibers; dimensionally stable; high efficiency; moderate scalability	Involves heat and chemicals; moderate–high environmental impact; medium–high cost
Combined Mechanical and Chemical	Hybrid	Sequential mechanical extraction followed by chemical treatment.	Improved fiber purity and yield; high efficiency; medium scalability	Requires multiple steps; moderate–high environmental impact; moderate cost
Mechanical Extraction	Mechanical	Physical decortication or beating to separate fibers.	No chemicals needed; rapid processing; moderate cost; high scalability	Produces coarse fibers; possible fiber damage; moderate efficiency; moderate environmental impact from energy use
Manual Extraction	Mechanical	Manual physical separation of fibers using scrapers, knives scuffing.	No chemicals needed; straightforward; low cost; low environmental impact	Extremely low efficiency, low scalability; labor-intensive; inconsistent fiber quality

.

8. Fiber Treatments

Natural fibers have some less desirable inherent properties, such as moisture absorption and hydrophilicity, that can reduce the interfacial bonding of the fiber and matrix [183]. To lessen these properties, provide new ones, or simply remove impurities from the fiber surface, several fiber treatments have been developed over the years [69,198]. These treatments increase the suitability of the natural fibers for a given end-use market.

Table 8. Summary of natural fiber treatment methods discussed in this review. Treatments are compared by purpose, effect on fiber quality, and practical advantages and limitations, with embedded considerations of cost, efficiency, scalability, and environmental impact.

Treatment Method	Purpose/Effect	Fiber Quality Impact	Advantages	Limitations
Alkali	Removes lignin, hemicellulose, waxes; increases surface roughness	Enhances adhesion and fibrillation	Widely used; improves mechanical properties; low cost, high efficiency, and high scalability	Excessive exposure may degrade cellulose; moderate–high environmental impact due to chemical effluents
Acrylation	Grafts polymer chains to fiber surface	Increases hydrophobicity and interfacial bonding	Improves compatibility with hydrophobic matrices; high efficiency, moderate cost	Requires initiators and controlled conditions; moderate environmental impact; not highly scalable
Acetylation	Replaces hydroxyl groups with acetyl groups	Reduces hydrophilicity; improves dimensional stability	Enhances moisture resistance and durability; moderate scalability and cost	Uses acetic anhydride; moderate environmental impact; may reduce biodegradability
Silane	Introduces silane coupling agents for bonding	Enhances fiber–matrix bonding	Excellent fiber–matrix adhesion; high efficiency and scalability	Medium–high cost; requires safe handling of chemicals; moderate environmental impact
Peroxide	Initiates free radical grafting or cleaning	Increases surface energy and bonding	Fast surface activation; high efficiency, moderate cost	Risk of fiber degradation; moderate–high environmental impact from oxidizing agents
Sodium chlorite	Delignification through oxidation	Increases cellulose purity	Effective lignin removal; low cost, moderate scalability	Generates chlorine-containing waste; high environmental impact
Ozone	Oxidative surface modification	Enhances bonding; reduces impurities	Dry method; no chemical waste; low environmental impact	Requires specialized equipment; moderate cost; limited industrial use
Plasma	Physical surface activation via ionized gas	Increases surface energy	Clean, dry method; no chemicals; low environmental impact	High cost, low scalability; requires advanced equipment
Corona	Surface oxidation through electrical discharge	Improves wettability and bonding	No chemicals needed; fast; moderate scalability and low environmental impact	Surface effect is shallow; uniformity may vary; moderate cost
Fungal	Biologically removes lignin and hemicellulose	Preserves cellulose integrity	Sustainable and safe; low cost and environmental impact	Long processing time; sensitive to contamination; low–moderate scalability

synthesizes chemical, physical, and biological treatments for natural fibers, presenting each method’s purpose, fiber-quality effects, advantages, and limitations.

8.1. Chemical Treatments

8.1.1. Alkali Treatment

This treatment is one of the most studied and used for natural fibers. It breaks the fiber bundles, removing hemicellulose and impurities, which improves several properties like wettability, fiber–matrix adhesion, thermal stability, heat resistance, mechanical behavior, and moisture absorption [29,182].

8.1.2. Acrylation Treatment

Acrylation treatment, or acrylation grafting, is a method where the acid used reacts with the cellulose hydroxyl groups on the fiber surface, increasing the number of reactive cellulose macro-radicals and providing enhancements to the interfacial bonding between the fiber matrix [69], thermal stability, and crystallinity index of the fibers [199]. This reaction reduces moisture absorption and improves hydrophobicity, tensile strength, and flexural strength [200].

8.1.3. Acetylation Treatment

In the acetylation process the natural fibers are plasticized. The reaction of acetic anhydride at a high temperature causes the substitution of the cellulose hydroxyl groups on the surface of natural fibers with acetyl groups, modifying the surface and making it hydrophobic [182,201]. This reaction improves fiber–matrix adhesion, dimensional stability, and tensile strength while reducing moisture absorption [182].

8.1.4. Silane Treatment

This treatment is known to have one of the most effective surface modification processes for natural fibers, where multifunctional silane chains are deposited on the fiber surface [88]. These molecules improve the interfacial bond between fiber and matrix and reduce moisture affinity [197]. Also, this process removes surface impurities, reduces water absorption, and increases the hydrophilicity of the fibers while improving flexural strength and Young’s modulus [69].

8.1.5. Peroxide Treatment

Peroxide treatment or involves immersing the natural fibers in benzoyl peroxide or dicumyl peroxide in an acetone solution for a set amount of time [201]. This method can help remove impurities from the fiber surface as well as improve interfacial adhesion, thermal stability, and moisture absorption [182,200].

8.1.6. Sodium Chlorite Treatment

Sodium chlorite (NaClO₂) is a treatment that focuses on the bonds between the lignin and carbohydrates, releasing choleric acid (HClO₂), which causes an oxidation reaction and forms chlorine dioxide (ClO₂) that reacts with the lignin present on the fiber surface, degrading it [202]. The bleaching of the fiber surface provides a removal of the surface impurities, an increased adhesion of the fibers, and improves mechanical properties such as the elongation break, tensile strength, and Young’s modulus [202,203].

8.2. Physical Treatment

8.2.1. Ozone Treatment

In this treatment, ozone gas is used as an oxidizing agent for the cellulose, improving the functionality of the fluoromonomers [204]. The ozone degrades lignin and solubilizes a fraction of hemicellulose. It increases the crystallinity index of the treated fibers, improving their mechanical strength [205,206] and the surface contact angle [204]. According to Yang et al. [207], this process is an energy-efficient method with low impact, and the treated fibers can be applied in various fields, such as the medical field and the production of cellulose nanofibrils.

8.2.2. Plasma Treatment

According to Seki et al. [208], oxygen plasma treatment introduces polar or excited groups that can form a strong covalent bond between the fibers and the subsequent polymers used. This method increases the hydrophobicity of the fiber surface, which increases the adhesion of the fiber matrix. This method is regarded as a clean technique that can modify the surface of the fiber without affecting its bulk properties [208,209].

8.2.3. Corona Discharge

This treatment is regarded as a green technique for fiber surface modification [209], where the treated fibers have an increased content of carboxyl and hydroxyl groups on their surface, causing a reduction in hydrophilicity while increasing the mechanical properties of the fibers [31].

8.3. Biological Treatment

Fungal Treatment

The fungal treatment removes lignin and other non-cellulosic components from the fiber surface using enzymes from certain fungi species, such as Ophiostoma ulmi [63,210]. The treatment using this species improves the adhesion of the fiber matrix and the moisture absorption of the fibers [63,183,210].

8.4. Characterization of Treated Fibers

The impact of surface treatments on natural fibers is frequently assessed using a combination of morphological, thermal, and mechanical characterization techniques. Scanning electron microscopy (SEM) reveals improved surface roughness, fibrillation, and the removal of waxes or hemicellulose after alkaline or enzymatic treatment—traits associated with better matrix interlocking in composites [31,38,63].

Thermal behavior is typically evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Alkali or silane-treated fibers often show increased thermal degradation onset temperatures and improved thermal stability due to the removal of low-stability components like hemicellulose [4,31,63].

Regarding thermal conductivity, most lignocellulosic fibers range from 0.03–0.06 W/m·K, making them attractive as thermal insulators in construction and packaging [116,200,211]. Viscoelastic properties, measured through dynamic mechanical analysis (DMA), are also affected by treatments that increase matrix compatibility, resulting in higher storage moduli and improved damping performance [15].

Natural fibers are generally flammable, so their integration into construction and transport sectors often requires additional fire-retardant treatments, such as phosphorus- or boron-based compounds [212,213,214]. Such modifications improve fire resistance but must be evaluated carefully to avoid compromising biodegradability and fiber integrity.

9. Natural Fiber Applications

The range of applications for natural fibers has increased considerably in recent times, not only due to the need for environmental sustainability but also because of the recognition of their superior properties, as demonstrated by research conducted by scientists worldwide [136]. As a result, these materials are increasingly seen as advantageous alternatives to steel, wood, or plastic, offering viable solutions for use in fields such as engineering, sports, construction, transportation, medicine, architecture, design, among others [8,123]. Manufacturers are increasingly turning their attention to natural fibers due to their biodegradability, sustainability, low cost, and more flexible processing possibilities [11,70,99,215]. These materials therefore hold great potential for reducing the consumption of non-renewable materials, which are harmful to the environment and have high energy demands in their production, such as concrete, metals, or synthetic fibers. Below are some examples of applications of natural fibers in various industries.

9.1. Natural Fibers Applied in the Automotive Industry

Recently, automobile manufacturers have shown interest in using biocomposites and hybrid composites (reinforcement: natural fibers; matrix: petrochemical polymer) [67], as the automotive industry faces the need to replace non-renewable materials with renewable ones while maintaining their desirable properties along with the advantages of green materials [123]. Many components for the automotive sector are already made from natural and renewable sources using fiber-reinforced composites [216,217].

One of the main goals of the automotive industry is to reduce the weight of components, which can be achieved with the use of natural fibers, as well as to reduce costs [130]. In Europe, car manufacturers are strongly investing in the use of natural fibers, which are found in various vehicle components such as pineapple fiber mats, thermoplastic panels reinforced with flax and hemp fibers, seat backs and headrests, and engine covers [130,218,219].

Chrysler has officially achieved this goal, using polypropylene composites reinforced with natural fibers in various interior vehicle components, such as instrument panels and door panels. The company also intends to expand these applications to external components using flax and polyester composites [220,221,222]

For consumers, these composites are well-regarded not only for being eco-friendly but also for offering better thermal and acoustic insulation than fiberglass. Additionally, they do not trigger allergic reactions in the respiratory system or cause skin irritation [21,22,223]. The low density of plant fibers also reduces vehicle weight, which means significant fuel savings [132]. Alves et al. [151] used jute fibers in vehicle roof manufacturing as an alternative to fiberglass and found that the material performed better environmentally, socially, technically, and economically.

9.2. Natural Fibers Applied in the Aerospace Industry

In the aerospace industry, natural fiber composites are playing an increasingly important role, mainly due to their low density and weight [11]. They are used in various secondary aircraft structures [123,140], gaining increasing recognition in commercial aircraft. According to Asim et al. [123] the Airbus A380, A350, and Boeing 787 Dreamliner use, respectively, 25%, 53%, and 50% of composites reinforced with natural fibers in their structure. The use of lighter composites in their structure allows for a significant reduction in aircraft weight, which is a crucial factor in improving fuel efficiency [224]. However, there are some disadvantages; Arockiam et al. [225] concluded that some of the limitations of natural fibers for aerospace applications include the poor compatibility of the fiber with various matrices, resulting in a non-uniform dispersion of fibers throughout the matrix. Additionally, the high moisture absorption capacity of these fibers mainly affects delamination and mechanical properties, potentially leading to a loss of microbial resistance [11,225,226,227].

9.3. Natural Fibers Applied in the Construction Industry

Natural fiber composites and fiberboards, similar to plywood, are an alternative to wood in the construction market. These are emerging, low-cost products that can replace wood or other materials in various applications, such as laminates, panels, partitions, door frames, and roofing [29,228], as well as in the manufacture of mobile or prefabricated buildings [118]. Interest in the application of natural fibers in construction has been growing in recent decades. Dweib et al. [229] conducted a study demonstrating the viability of using biopolymer composites reinforced with natural fibers in structural beams for buildings. Lertwattanaruk and Suntijitto [230] revealed that cements with incorporated natural fibers exhibit low thermal conductivity and improve the building’s energy efficiency. Swamy et al. [231] obtained good acoustic results with the application of Dypsis lutescens fibers in structural coating composites. Mechanical tests on sisal fiber-reinforced composites showed promising results in terms of durability [232]. Bavan and Kumar [126] reported the use of straw in polymer composites as a construction material. Ramnath et al. [233] concluded that hybrid composites reinforced with pineapple, jute, and glass fibers are suitable for the housing industry.

9.4. Natural Fibers Applied in Geotextiles

Natural fibers have excellent characteristics for soil conservation and landscaping, as they protect the soil from erosion after application and also serve as fertilizer by providing various nutrients to the soil [141,142,143]. These geotextiles are divided into two subgroups. The first is called erosion control meshes (ECM), consisting of a mesh or fabric of ordered and intertwined fibers, generally made from coir or jute. The second group is known as erosion control blankets (ECB), composed of a layer of disorganized fibers placed over the soil, usually made from wheat fibers, rice husks, or wood chips [142].

The use of eco-friendly geotextiles for soil reinforcement demonstrates a significant improvement compared to artificial geotextiles [143]. For example, in the UK, palm leaf geotextile mats are used for the conservation of sandy and clayey soils, aiming to preserve eroded slopes [142]. Coir fiber is also used to produce geotextiles that are applied in pavements to increase their load-bearing capacity [141].

9.5. Natural Fibers Applied in the Textile Industry

The textile industry was one of the first to use lignocellulosic natural fibers, currently employing cotton, flax, hemp, kenaf, and pineapple fibers. Functionalization of fibers with silver nanoparticles has been used in the textile industry to embed textiles with barrier, repellent, anti-static, and UV protection properties [234]. According to Liu et al. [235], modified bamboo fiber has strong potential for textile applications due to its excellent antiseptic properties, hygroscopicity, electrostatic behavior, and UV resistance. There is also the production of plant-based leather using natural fibers, as in the case of Piñatex^®, which uses pineapple leaf fibers and vegetable tanning methods to develop natural fiber leather [236].

9.6. Natural Fibers Applied in Packaging and Bioplastics

The incorporation of natural fibers in the development of packaging materials and bioplastics has gained significant traction as a response to environmental concerns associated with conventional petroleum-based plastics [237]. These fibers serve either as reinforcement agents in biocomposites or as primary raw material for cellulose extraction, which is then used to create biodegradable films and containers [238]. Fibers such as hemp, flax, kenaf, and jute are commonly used due to their high cellulose content and favorable mechanical properties [43].

One of the most prominent developments in this area is the use of cellulose nanofibers (CNF) and microfibrillated cellulose (MFC) extracted from natural sources to produce packaging films with excellent barrier properties, low density, and biodegradability [144,239]. These materials demonstrate low permeability to oxygen and oils, which is particularly advantageous for food packaging applications. Additionally, their surface can be functionalized to incorporate antimicrobial or antioxidant agents, enhancing food preservation and safety [240].

Another innovation is the development of fiber-reinforced bioplastics, where natural fibers are used to reinforce biodegradable polymer matrices such as polylactic acid (PLA), polyhydroxyalkanoates (PHA), and starch-based polymers. These materials exhibit improved mechanical strength and thermal stability, offering a viable alternative for rigid packaging applications [31,241,242].

In addition to technical benefits, the use of agricultural residues (e.g., wheat straw, rice husks, pineapple leaves) for packaging production adds value to agro-industrial waste streams, aligning with the principles of circular economy and reducing environmental burdens [243]. However, challenges such as fiber–matrix compatibility, moisture sensitivity, and scaling of production processes remain critical issues that require further research and technological development [244].

9.7. Natural Fibers Applied in Biomedical Applications

Natural fibers have emerged as promising materials in the biomedical field due to their biocompatibility, biodegradability, renewability, and the possibility of functionalization [245]. Their unique structural and chemical composition, especially the presence of cellulose, hemicellulose, and lignin, allows for diverse medical uses, ranging from wound dressings to scaffolds in tissue engineering [246].

Cellulose-based fibers, such as those derived from cotton, flax, and ramie, have long been used in wound care for their excellent absorption capacity, porosity, and non-toxicity [247]. These fibers serve as a base for the development of advanced wound dressings that can be embedded with antibacterial agents, anti-inflammatory drugs, or growth factors [248]. Their high surface area and flexibility make them ideal for promoting gas exchange and maintaining moist environments conducive to healing.

In tissue engineering, natural fibers have been explored as reinforcement elements in bio-scaffolds used to regenerate skin, bone, and vascular tissues. Linen and silk fibers, for instance, provide structural integrity while supporting cell adhesion and proliferation [249]. Additionally, composites of natural fibers with biopolymers such as chitosan, gelatin, or collagen are being developed to mimic the extracellular matrix (ECM) and support cellular activities [250].

Nanocellulose, derived from natural fibers, has gained increasing attention for drug delivery systems and biosensing applications due to its nanoscale dimensions, high surface area, and ability to be chemically modified [144]. It can serve as a carrier for therapeutic agents, allowing for controlled release while ensuring biocompatibility and minimal immune response [251].

Despite their potential, the use of natural fibers in biomedical applications still faces challenges related to sterilization, immune response, and reproducibility of fiber properties [252]. Further research is needed to standardize extraction and processing techniques, ensure consistent quality, and fully understand the long-term behavior of these materials in physiological environments.

9.8. Natural Fibers Processed into Nanocellulose

Nanocellulose can be produced from natural fibers, with hemicellulose and lignin being removed through acid hydrolysis to isolate cellulose from the fibers [145]. In addition to natural fibers, this biological material can also be extracted from other cellulose-rich natural resources, such as wood residues, vegetables, or wood pulp [253]. It is currently one of the most promising sustainable materials, with various applications in the pharmaceutical, food, and electronic industries due to its physical, mechanical, and barrier properties [239,253].

Within nanocellulose, there are two main types: cellulose nanocrystals (CNC) and microfibrillated cellulose (MFC). The barrier property of these materials stems from cellulose’s low permeability due to its crystalline nature [144]. This property is of great interest for the food packaging industry, as it enables the replacement of conventional petrochemical-based plastic packaging with biodegradable, lightweight, easy-to-process, and low-cost alternatives [144,239]. According to Khan et al. [239], the use of nanocellulose composites in packaging can extend food shelf life and quality, as nanocellulose can serve as a carrier for antioxidant and antimicrobial agents.

10. Challenges and Future Prospects

Despite the growing interest in natural fibers as sustainable alternatives to synthetic materials, their widespread industrial adoption is still hindered by several technical, economic, and environmental challenges. Natural fibers are highly dependent on growing conditions, harvesting time, and extraction methods, leading to variability in mechanical properties, morphology, and chemical composition [100,101,254,255]. These limitations, stem from intrinsic fiber variability, the impact of extraction processes, environmental constraints, and a lack of standardization. Nonetheless, promising advances are underway, particularly in biomedical and additive manufacturing fields, that may significantly expand the applicability and value of natural fibers in high-performance sectors.

A key limitation of natural fibers is their inherent variability. Fiber morphology, cellulose crystallinity, microfibrillar angle, lumen dimensions, and composition are all influenced by species, growing conditions, maturity, and harvest timing, resulting in considerable discrepancies in physical and mechanical properties [31,45,105,106,107]. This inconsistency complicates design reliability, especially in structural applications. Additionally, their hydrophilic nature leads to high moisture absorption, which impacts dimensional stability and long-term mechanical performance [11,67,112,256]. Although surface treatments such as alkali [29,182], silane [88,197], or acetylation [182,201] can reduce hydrophilicity and improve fiber–matrix bonding, they often increase processing costs or affect the fiber’s biodegradability.

Extraction processes also pose significant challenges. Biological methods like dew and water retting are low-cost and commonly practiced but suffer from long processing times, weather dependency, and inconsistent fiber quality [44,101,167]. Chemical and hybrid methods, while producing higher-purity fibers, raise environmental concerns due to water pollution and chemical residues [70,77,183]. Other extractions, such as enzymatic retting [100,175,176], microwave-assisted extraction [190,191,192,193,194], and the Duralin process [195,196,257], show potential to improve fiber quality and reduce environmental impact. However, these methods still face limitations regarding scalability, capital costs, and standardization.

Seasonality is another logistical concern that affects the continuity of fiber supply. While tropical fibers like sisal may be harvested year-round [148,149], temperate-zone bast fibers are harvested primarily in summer or autumn, disrupting supply chains and increasing costs [11,150].

End-of-life and safety considerations are also relevant. Although natural fibers are generally biodegradable, their integration into polymer composites, particularly thermoset matrices, limits recyclability and circularity [9,183]. Research into reversible adhesives, recyclable thermoplastics, and enzymatic delamination is ongoing to address these gaps [258]. Moreover, while natural fibers are considered non-toxic, potential health risks may arise from pesticide residues or prolonged exposure to airborne fiber dust (e.g., cotton) [133,134,135]. In contrast, synthetic fibers such as asbestos, once widely used, have been banned in most countries due to severe health hazards [21,22].

Looking forward, natural fibers are increasingly being explored for advanced applications that could overcome some of the aforementioned limitations. In the biomedical field, lignocellulosic fibers and their nanostructured derivatives are being studied for applications such as tissue engineering, wound dressings, and drug-delivery systems [240,246,251]. Chitosan–flax composites have demonstrated antimicrobial activity and high fluid absorption for wound care [14].

In additive manufacturing (AM), plant-based fibers have gained attention as reinforcements in bio-based polymer filaments such as PLA and PHB. These natural fiber–reinforced filaments can increase tensile modulus and strength by up to 50% compared to neat polymers while maintaining low density and compostability [31,123,216,259]. Furthermore, direct ink writing using shear-thinning nanocellulose and alginate gels allows for the fabrication of responsive or 4D structures, such as self-shaping medical implants [260]. Continuous fiber additive manufacturing (CFAM) with flax or hemp rovings is also emerging, offering improved mechanical performance and lower carbon footprints compared to glass fiber–reinforced plastics [218,219].

To fully realize the potential of natural fibers, several research and development priorities must be addressed. First, integrating high-throughput characterization (e.g., SEM, µCT, Raman spectroscopy) with machine learning tools may allow the prediction of fiber quality from agronomic data, enabling the selection of optimal cultivars and treatments [65,106,108]. Second, green hybrid extraction techniques combining enzymatic and microwave-assisted steps could reduce environmental impact while improving fiber consistency [193,194]. Third, the development of functional coatings, such as bio-based silanes or tannin-based fire retardants, could enhance fiber durability without sacrificing biodegradability [199,212,256]. Standardization of mechanical testing protocols, chemical assays, and lifecycle analysis (LCA) data will be critical for facilitating certification and industrial uptake. Finally, a circular economy model for natural fiber use will require advances in recyclable matrices, enzymatic fiber recovery, and modular product design.

In conclusion, while natural fibers already provide viable solutions in automotive, construction, and packaging sectors, their greatest potential lies in high-value applications in biomedical devices, additive manufacturing, and smart materials. Interdisciplinary innovation, spanning agronomy, materials science, biotechnology, and manufacturing, is essential to transforming current limitations into competitive advantages, paving the way for a circular, bio-based material future.

11. Conclusions

This review demonstrates that natural fibers represent a promising class of renewable, biodegradable, and low-cost materials capable of addressing environmental challenges associated with synthetic fibers. Their classification into plant-based, animal-based, and mineral-based types allows for a better understanding of their distinct morphological, chemical, and mechanical characteristics. Among these, lignocellulosic plant fibers stand out due to their abundance and broad application potential across industries such as textiles, construction, automotive, aerospace, and packaging.

The morphological and chemical composition of natural fibers, primarily cellulose, hemicellulose, lignin, pectins, and waxes, greatly influence their mechanical strength, thermal stability, moisture sensitivity, and compatibility with matrices in composite applications. These properties vary significantly across species and are further influenced by cultivation conditions, harvesting time, and processing methods. While natural fibers offer several advantages such as low density, biodegradability, and carbon neutrality, they also present limitations such as hydrophilicity and variability in quality that must be addressed through targeted surface treatments.

This review highlights the diversity of extraction methods, biological, chemical, mechanical, and hybrid, that impact fiber quality, yield, environmental footprint, and scalability. Dew retting and water retting remain widely used due to simplicity and tradition, but newer approaches such as enzymatic retting, microwave-assisted retting, steam explosion, and the Duralin process offer improved fiber performance and reduced environmental impact. The choice of method should be guided by fiber type, end-use application, and sustainability considerations.

Surface modification treatments, ranging from alkali, acetylation, and silane chemical treatments to physical methods like plasma or corona discharge, and biological approaches such as fungal treatment, have proven essential to improve fiber–matrix adhesion, reduce moisture uptake, and enhance mechanical performance. These treatments enable the integration of natural fibers into more demanding technical applications, while maintaining their eco-friendly advantages.

Natural fibers are increasingly being adopted in industrial applications, particularly in the automotive and construction sectors, where their low density, sound insulation, and thermal properties provide technical and environmental benefits. In the aerospace sector, they are gaining traction for non-structural components due to weight savings and fuel efficiency, despite existing challenges related to moisture sensitivity and fiber dispersion. Moreover, the economic and social relevance of natural fiber production is significant, particularly in developing countries, where these fibers contribute to local employment and industrial development. However, challenges related to harvesting seasonality, extraction costs, and standardization still limit broader adoption in certain sectors.

In conclusion, natural fibers are essential materials for enabling the transition toward a circular and bio-based economy. Their performance can be tailored through careful selection, extraction, and treatment processes, allowing for broader and higher-value applications. Continued research into optimizing fiber processing, improving environmental performance, and integrating fibers into advanced composite systems will further unlock their potential. Addressing current limitations, particularly those related to moisture sensitivity, environmental costs of extraction, and standardization, remains critical for accelerating their adoption in global sustainable development strategies.