Animal based bio-reinforcements for sustainable green composites advancement: a review on reinforcement classifications, processing and characterization of their bio-composites

The recent shift of attention toward using natural materials for the synthesis and fabrication of advanced materials has led to several areas of research and development where such materials are being used as matrix and reinforcements in composite development. As a result of the increasing demand for environmentally friendly materials and the desire to reduce the cost of artificial reinforcement materials, novel bio-derived reinforcement materials for the fabrication of composites are now being developed (Oladele et al., 2020a, 2020b). Researchers have begun to focus attention on natural and bio-reinforced composites as bio-composites, which can be from either natural or synthetic resins that are reinforced with natural fibers/particles (Olajide et al., 2019). Natural materials are broadly divided into three categories based on their sources, which are plant, mineral and animal (Khatib et al., 2022; Amin et al., 2022; Ramamoorthy et al., 2015). Currently, all these natural resources are receiving more attention from researchers due to the need to protect the environment from various pollutions emanating from synthetic materials. While there are several reports on the use of plants as reinforcement/matrix materials for composites developments, there are limited reports on the use of animal wastes in these regard relatively. Therefore, there is a need to reveal the available promises of animal wastes for composites developments. The enormous potential in animal wastes as reinforcements in composite development is yet to be harnessed optimally, going by their availability and applications in recent times.

The population of all livestock, including poultry and the various bovine, caprine, ovine and swine species, is estimated to be 4.89 billion (Agbeboh et al., 2019). Globally, there are a lot of livestock and poultry, which generate a lot of waste that ends up in the environment and amounts to a few million metric tons without any scientific intervention, causing socioeconomic and environmental problems (Agbeboh et al., 2019). To effectively use these potential waste deposits, suitable waste management procedures are necessary, in which the conversion of these wastes to reinforcement materials is one of them (Mann et al., 2023; Antor et al., 2025). Animal-derived wastes comprise carcass waste/offals, hoofs, skin, tissues and body parts like feathers and hairs. With improvements in animal production techniques, it is now possible to recover these wastes, which will further significantly reduce pollution (Oladele et al., 2018a, 2018b). However, the creation of composite materials reinforced with animal-based wastes may open the door to a more effective technique for handling waste originating from animals, leading to the development of cutting-edge biomedical, pharmaceutical, structural, manufacturing and nanotechnology applications (Oladele et al., 2023). Composting, vermicomposting, the creation of biogas and the creation of value-added products are all conventional ways of waste management that have been thoroughly studied. However, these conventional methods are not yet effective for achieving a circular economy (Reshmy et al., 2021; Sharma et al., 2020). So far, healthful use of these animal wastes has, however, remained the subject of very few studies. Researchers from all over the world are in the quest for fresh, cutting-edge and environmentally responsible methods for the effective management of these animal wastes that will promote the circular economy as well as enhance technological development (Oladele et al., 2022a; Oladele et al., 2022b). Some of the most significant components made from animal-based wastes include hydroxyapatite, calcium phosphate, sponge, hyaluronic acid, keratin, spicules, chitosan and collagen (Zamri et al., 2021; Li et al., 2016; Agbabiaka et al., 2020; Daramola et al., 2021; Adeyanju et al., 2017; Olajide et al., 2017; Oladele and Adewole, 2015; Oladele et al., 2023). Additionally, animal waste management techniques for bio-reinforcement development are more dependable because they recycle different animal waste components into useful end products while offering a sustainable and economical synthesis process (Tarafdar et al., 2021). The creation of animal wastes is a continuous and increasing source of waste and environmental pollution due to the increasing demand for meat globally. Hopefully, this trend is expected to continue as long as human beings are living and feeding. Therefore, using animal wastes to create bio-reinforcement materials is a useful tactic that also contributes to lowering environmental pollution (Oladele et al., 2025a, 2025b).

However, despite these promising availability and potential processing advances, significant research gaps remain. Existing literature has predominantly focused on plant-based natural fibers, with limited comprehensive reviews addressing animal-derived reinforcements. There is no unified classification framework that organizes animal-based reinforcement materials according to their sources, composition and functional performance. Furthermore, documented studies provide fragmented information on extraction and processing techniques, and there is an absence of systematic comparison of the mechanical, thermal and structural properties of composites reinforced with different animal-based materials. The lack of integrated knowledge linking source materials, processing routes, characterization outcomes and sustainability benefits has created a critical gap in understanding their full potential within a circular economy framework. These gaps necessitate a detailed, structured and holistic review of animal-based reinforcements and their derived composites. Hence, this review presents the classification, processing techniques and characterization of animal-based reinforcements and the ensuing composites to encourage further research in the use of animal wastes, which are more easily adaptable to human systems both internally and externally.

As shown in Figure 1, animal-based reinforcements are classified based on two major factors:

1. based on their sources from the environment; and

2. based on forms/shapes.

Bio-reinforcements can be classified based on their sources from the environment. The classification of animal-based reinforcements based on their locations in the environments is broadly grouped into aquatic, aerial and terrestrial. Thus, the location and habitation of the animals within the universe can determine the type of animal and waste that can emanate from them. From these three locations, animals within each of them have common unique features with unique reinforcement sources. Though some animals like chicken and silkworm do exhibit dual habitation potentials.

Aquatic animal-based reinforcements are bio-reinforcement materials derived from animals, either vertebrate or invertebrates, which lives in water bodies (Oladele et al., 2022a). These animals may breathe air or extract its oxygen from chemical elements dissolved in water through specialized organs or through their skin, making them rich in salts and other unique elements like protein, calcium and phosphorous (Adeyanju et al., 2017). The reinforcement materials developed from aquatic sources are rich in protein and calcium, which make them suitable for biomedical applications. Examples are hydroxyapatite particles derived from fish bones and shells from sea animals like snails, fish skin used as a biomaterial for skin grafting and sea sponge spicules used as blueprints for bio-fabrication of biomaterial composites (Oladele et al., 2023; Müller et al., 2009; Oladele et al., 2022a).

Aerial animal-based reinforcements are reinforcement materials derived from animals that live most of their lives or spend most of their time in air. They are also known as avian-based reinforcements (Oladele et al., 2022b). This category consists of birds and other flying creatures. Their reinforcement materials are predominantly feathers, including those of pigeons and other birds (which includes chicken). Chicken is sometimes classified as an aerial-based animal because it can fly at times, but it is predominantly classified as a terrestrial-based animal because it spends most time on land (Agbabiaka et al., 2020). The aerial-based animal reinforcements possess unique properties, which include good mechanical, chemical, thermal, lightweight and acoustic properties (Oladele et al., 2022b; Agbabiaka et al., 2020; Cheng and Ning, 2023). They are predominantly used for the fabrication of biomedical and structural bio-composites because of their biodegradability and unique physio-chemical properties (Oladele et al., 2022b; Agbabiaka et al., 2020; Cheng and Ning, 2023).

Terrestrial animal-based reinforcements are reinforcement materials derived from animals that live predominantly or entirely on land (Tarafdar et al., 2021; Oladele et al., 2020b). This group may include animals that live both in water and on land (e.g. silkworm). Aside from chicken and silkworm, the majority of terrestrial animal-based reinforcements are majorly developed mammals which come in both fiber and particle forms (Oladele et al., 2020a; Olajide et al., 2019). The predominant animals used for developing these reinforcements are man, goat, sheep, cow, pig and chicken. The reinforcement materials gotten from this group are economical, easy to access and have better properties than the aquatic and aerial-based animal reinforcements (Oladele et al., 2022a, 2022b; Zamri et al., 2021; Li et al., 2016). They are also predominantly used in the biomedical, structural and other manufacturing industries because of their excellent combination of physical and chemical properties (Oladele et al., 2022a, 2022b; Zamri et al., 2021; Li et al., 2016). The fiber forms developed from terrestrial animal-based reinforcements have excellent biodegradability, good chemical reactivity, excellent bio-compatibility, light weight, excellent thermal conductivity, good thermal and fluid absorption and unique durability. Examples of these reinforcements are human hair, wool, silk fiber, hydroxyapatite particles and scaffolds (Khatib et al., 2022; Amin et al., 2022; Ramamoorthy et al., 2015; Oladele et al., 2017b; Oladele et al., 2020b).

The frequently used classification of animal reinforcements is based on their forms/shapes. This classification consists of fibers, particles and meshes. These classifications are identified in this section.

Animal-based reinforcements in the form of fibers are in strands with different thickness and length. The animal fiber reinforcements are predominantly used in the structural and manufacturing industries due to their elongation properties, thermal conductivity, good water and fluid absorbent and high tensile strength. This form of animal fibers is majorly sourced from aerial and terrestrial sources. They are found in their raw form in the form of wool, silk, hair and feathers, which are further processed to yield animal fibers (Khatib et al., 2022; Amin et al., 2022; Ramamoorthy et al., 2015; Oladele et al., 2017b; Oladele et al., 2020b). Its application differs in terms of reinforcements and weight fractions, which impact their quality and effectiveness in composite fabrications for the structural and manufacturing industries.

The properties of selected animal fiber reinforcements are described below.

Silk fibers are a vital subdivision of animal fibers. The main component of silk is a natural protein (fibroin) that has excellent mechanical properties with excellent biocompatibility, biodegradability and bioresorbability (Das and Natarajan, 2019). The component of silk is fibroin and sericin. Sericin is the protective protein glue adhered to the fibroin, although sericin is of great importance to the insect (Bombyx mori) producing it, i.e. it creates the perfect conditions for complete metamorphosis (adhesive for fibroins) (Kunz et al., 2016), it has demerits unfavorable for commercial utilization. Therefore, sericin is removed from fibroin by a process called degumming. Sericin has been degummed and discarded in the past to improve the aesthetics, texture and color of the resultant fibroin, but currently, due to its biocompatibility, it has been found to be useful, especially for biomedical applications. The degumming process is done using boiling water (120°C for 105 min) because of the composition of their amino acids. Other standard techniques such as autoclaving, short alkaline (Na₂CO₃) boiling, long alkaline (Na₂CO₃) boiling and ultrasounds can also be used in degumming sericin from fibroin (Carissimi et al., 2019; Wang and Zhang, 2015; Manesa et al., 2022).

Sericin is a hydrophilic polymer in the family of glycoproteins with about 18 amino acids (aspartic acid, serine, glutamic acid, glycine, histidine, arginine, threonine, alanine, proline, cysteine, tyrosine, valine, methionine, lysine, isoleucine, leucine, phenylalanine and tryptophan) which are responsible for properties such as cross-linking and copolymerization, antioxidant, moisturizing, antibacterial, antimicrobial, antitumor and radiation resistance (Elzoghby et al., 2015; Silva, 2017; Li et al., 2022; Rakesh, 2019). Silk consists of 75% fibroin and 23% sericin; the remaining percentage is made of fat, wax and mineral salts (Rakesh, 2019; Song et al., 2025). Fibroin fiber is the most widely utilized component of silk for commercial applications because of its strength and aesthetic properties. There are four main types of silk fibers, which are mulberry, tasar, muga and eri silk.

Mulberry silk is the most abundant silk fiber globally, with the most contributions from China, Japan and Korea (Saha et al., 2024; Mansourieh et al., 2024). The Mulberry silkworm, also known as Bombyx mori silkworm, produces soft and fine silk fibers with lustrous aesthetics (Yang and Fancey, 2025). Due to its composition and qualities, silk has exceptional potential. It has a variety of qualities, including high tensile strength, high elongation and chemical resistance and is made up of highly organized proteins (Das and Natarajan, 2019; Mussig, 2010).

Table 1 compares key properties of different silk fibers, including mulberry, tasar, eri and muga silk fibers, revealing distinct differences relevant to composite reinforcement. Mulberry shows low ash content, higher fatty matter and strong tenacity, making it the most uniform and widely preferred fiber. Tasar has higher ash content and lower strength, with a convoluted cross-section that affects bonding. Eri exhibits the highest moisture regain and elongation, offering good flexibility. Muga combines moderate ash content, golden coloration and strong triangular morphology that enhances mechanical interlocking. Overall, each silk type presents unique structural and mechanical characteristics suitable for tailored composite applications.

Wool is an eco-friendly material (Russell, 2009). The structure and chemical makeup of wool fiber set it apart from other types of fiber; its enormous variety, heterogeneity of properties and benefits are unmatched by any other natural or synthetic fiber (Laitala et al., 2018). Wool has been used in the production of clothing and textiles since antiquity. Wool has not, however, been used to its full capacity. The creation of new composite materials using natural fibers in place of synthetic fibers in technical applications is gaining popularity. Between 2000 and 2050, it is anticipated that the world’s sheep population would grow from 1.7 billion to 2.7 billion (Thornton, 2010). Wool fiber layers are made up of two different types of cells: internal cortex cells and external cuticle cells, which create a sheath around the fiber. It stands out among textile fibers due to its cuticle cells (or scales), which cross over one another like roof tiles. Wool fibers are anchored in sheep skin by cuticle cells, which serve a vital purpose. Each cuticle cells exposed edge points from the fiber root to the tip (Rosegrant et al., 2009). When a fiber is drawn against the scale as opposed to with the scale, this results in a higher surface frictional value. The frictional differential, which also causes wool to feel when stirred in water, aids in the removal of dirt and other impurities from the fleece (Allafi et al., 2022; Pekhtasheva et al., 2012; Huson, 2018). This unique property, which no other textile fiber possesses, makes it possible to create fabrics with extremely dense structures, such as blankets, felts and materials for overcoats. The natural softness of wool, which makes it one of the smoothest textile fibers, is also largely a result of the fiber surface. To optimize thermal processes, reduce the gap between energy demand and supply, use waste heat that would otherwise be lost, implement peak load shifting strategies and improve the integration of renewable energies, thermal energy storage (TES) is crucial (Allafi et al., 2022; Pekhtasheva et al., 2012; Huson, 2018). However, one of the major downsides of the TES system is the expense of implementation; thus, because of its affordability and ease of deployment, wool is a more preferable upgrade to address this issue (Allafi et al., 2022).

Hair, a nondegradable matter, is creating an environmental problem, so its use as a fiber reinforcing material can minimize the problem, and it is available in abundance and also at a very low cost (Oladele et al., 2018a; Oladele et al., 2014). The hair fiber is made of mainly keratin protein with a primarily alpha-helix structure. About 91% of the hair is protein made up of a long chain of amino acids, which are long chains linked by peptide bonds. Human hair is the most abundant hair fiber and majorly used for composite reinforcements. The chemical structure of hair fibers is, like other natural animal fibers like wool, whose main composition is α-keratin (Cavallaro, 2020). The coiling of two α-keratin makes up the α-helix structure (Erik et al., 2008). The constituents of keratin are amino acids such as cytosine, serine, glutamine, threonine, glycine, leucine, valine and arginine (Nanda and Satapathy, 2017). The shaft of hair fibers comprises the medulla, cortex and cuticle (Huang et al., 2018; Chen et al., 2013). The medulla is a noncrystalline region near the cortex center. The cortex contributes the highest mass of human hair and houses the keratin proteins and structural lipids. The cuticle (dead cells) envelops and protects the cortex. The differences in hair structure are easily noticeable between genders and age gaps (Yang et al., 2014). The average composition of hair fibers is composed of 45.68% carbon, 27.9% oxygen, 6.6% hydrogen, 15.72% nitrogen and 5.03% sulphur. Amino acid present in hair are containing cytosine, serine, glutamine, threonine, glycine, leucine, valine and arginine (Yang et al., 2014). Hair fibers, as actual and potential reinforcement composites, offer many advantages: good strength properties, low cost, high toughness and biodegradability, however, in the case of cellulose fiber some disadvantages due to their intrinsic characteristic, incompatibility with hydrophobic polymer matrix, tendency to form aggregates during processing and poor resistance to moisture, finite length and large diameter, pose an important challenge to their use in advanced composite (Oladele et al., 2018a; Oladele et al., 2014; Cavallaro, 2020).

Feathers are by-products of avian species, and the most abundant of these species is chicken.

In the USA alone, the chicken feather industry generates four billion pounds of trash each year (Kock, 2006). These feathers are either sold for a cheap price ($250/ton) as livestock feed or buried in landfills at a cost of $30/ton (Kock, 2006). Due to health concerns, the European Union has prohibited the use of poultry feathers as feed since 2001. Composite industries, which cannot be consumed by many other sectors, might use the large number of feathers. According to reports, chicken feathers have unusual qualities like low density and effective thermal and acoustic insulation (Oladele et al., 2018b). Chicken feather fibers (CFFs) are made up of 88% water, 1% lipids and 91% protein (keratin) (Arunkumar et al., 2013). Arunkumar et al. (2013) investigated the chemical makeup, elemental analysis, morphological structure, aspect ratio, apparent specific gravity, chemical durability and thermal insulation of these feathers. They reported that chicken feather fibers possess excellent physiochemical properties, which give them potential for composite reinforcements for structural and biomedical applications and replace synthetic fibers in composite production (Reddy and Yang, 2007; Kurien et al., 2022a, 2022b). They are found to possess good mechanical, chemical, thermal and acoustic properties. The alignment of avian fibers in a biocomposite provides high stiffness and directional strength. They contain about 90% protein and β-keratin, with about one percent lipids and the rest is water. Its chemical constituents also include 16% serine, 12% proline, 11% glycine, 9% valine, 7% cysteine and numerous amino acids (Subramani et al., 2014). As a biodegradable material, avian fibers possess strong covalent bonds resistant to forming three-dimensional protein structures, especially from the barbed valves (rachis and hollow shaft) (Oladele et al., 2014). They have high thermal insulation properties, tensile strength and are lightweight, which makes them useful raw materials for biomedical industries (Tesfaye et al., 2018). The uniqueness of this type of fiber is in its honeycomb structure, which possesses low density, high flexibility and spinnable length absent in other natural or man-made fibers (Tesfaye et al., 2018). The mechanical properties of chicken feathers are described as having good impact strength, thermal stability and low energy dissipation but poor flexural and tensile strength due to the microvoids present within the biocomposite (Kumar et al., 2021). The advantage of microvoids is their application in acoustic products. Also, chicken fibers are efficient corrosion-resistant agents on steel substrates for epoxy coating. They also absorb sound more effectively than composites made of plant fibers (Salazar and Rios, 2009; Reddy and Yang, 2010).

Animal reinforcements in the form of particles are micro- or nano-sized constituents developed from animal components (Oladele et al., 2022a, 2022b; Agbabiaka et al., 2020; Daramola et al., 2021; Ariwoola et al., 2021). These constituents are majorly developed from drying, crushing, grinding and synthesis of these animal components to yield micro or nano-sized constituents (Adeyanju et al., 2017; Olajide et al., 2017; Müller et al., 2009; Cheng and Ning, 2023). Animal reinforcements in the form of particles have spectacular properties like excellent biodegradability and biodegradability, good thermal conductivity and fluid absorbent properties (Oladele et al., 2022a, 2022b; Agbabiaka et al., 2020; Daramola et al., 2021; Ariwoola et al., 2021). They are majorly used in the biomedical sector and find vast applications in almost all aspects of biomedicine because their chemical properties are similar with that of the human body (Oladele et al., 2022a, 2022b; Agbabiaka et al., 2020; Daramola et al., 2021; Ariwoola et al., 2021). They are mostly sourced from terrestrial sources and are synthesized from solid components.

The properties of selected animal particle reinforcements are described below.

Shell particles are animal products developed majorly from birds and sea animals (Oladele et al., 2022a, 2022b; Agbabiaka et al., 2020; Adeyanju et al., 2017; Guo et al., 2021). Shells provide the embryo with a protective layer against pathogens and give the embryo the necessary nutrients for development (Meng and Deng, 2017; Choi et al., 2018; Ilyas et al., 2022; Chatterjee et al., 2022; Aranaz et al., 2021; Taketa et al., 2018; Piotrowska-Kirschling and Brzeska, 2020; Cheng and Ning, 2023). Shell particles are bio-ceramic material whose structure is columnar calcium carbonate (CaCO₃) and organic proteinaceous matrix (Guo et al., 2021; Sun et al., 2016; Li et al., 2021). This structure protects the embryo from mechanical impact, regulates fluid exchange through the pores and prevents microbial invasion (Samuel et al., 2021). The quality of shell particles can be determined by its breaking strength, deformation, uniformity in thickness and dynamic stiffness of the eggshell (Sun et al., 2012). The calcium carbonate in shells is about 95% and about 3.5% protein organic matrix (Lin, 2004).

The advantage of shell particles over mineral calcium carbonate is due to the lower density and higher crystallinity. These advantages make shell particles an excellent reinforcement even for commercial biocomposite production (Ahmed et al., 2021; Toro et al., 2007). Shell particles can be converted into other commercial products, such as pure calcium carbonate, cosmetic and pharmaceutical products and water purifiers (Ahmed et al., 2021). Shell particles have a cellulosic structure with their constituents as biosorbent amino acids. The chemical constituents of shell particles include CaO, SiO₂, Al₂O₃, Fe₂O₃, MgO, SO₃, K₂O and Na₂O. The physical properties of shell particles include about 1.2% water content, a density of about 0.9 g/cm³ and a surface area of about 18.5 m²/g (Alzerjawi et al., 2018). It is important to note that the carbonization of the shell particles burns off all the impurities and provides better tribo-mechanical properties than uncarbonized shell particles (Zargar et al., 2015; Bain, 2005; Sanni et al., 2018; Guarnieri et al., 2022; Kowshik et al., 2023; Bose et al., 2018).

Bones are among the waste products produced by cattle that have received the greatest attention for their potential use in the creation of various materials with economic and social value (Miculescu et al., 2018; Odusote et al., 2019; Puri et al., 2020). The biocompatible ceramic particles, hydroxyapatite particles, which are extracted from cow teeth and bones are a main and crucial reinforcement component, has found numerous biological applications (Khandan et al., 2014; Nirmala et al., 2011). According to a study by Fratzl (2006), de-fatting cow teeth and bones and then calcining them at 900°C is the best technique for hydroxyapatite production. Another study by Nirmala et al. (2011) used thermal decomposition and silver nitrate reduction with N, N-dimethylformamide to develop hydroxyapatite particles from bovine femur bone conjugated with silver nanoparticles. The synthesized hydroxyapatite-silver nanoparticle complex demonstrated good antibacterial ability and can be used in the biomedical industry for bioimplant fabrication (Nirmala et al., 2011). In recent studies, researchers also developed an interest in developing bioceramic materials generated from fluorinated hydroxyapatite particles. This was motivated by the discovery that the presence of fluorine ions improved hydroxyapatite stability in biological systems and encouraged apatite production. Additionally, particle reinforcements from cow origin are used to develop osteoinductive and osteoconductive scaffolds for bone and tissue engineering (Puri et al., 2020).

Biomaterials reinforcement particles made from pig origin have also been used in the biomedical sector in a number of ways. Bone quality, also known as bone performance, is influenced by the bone’s architecture and material quality in addition to its shape and quantity (Fratzl, 206). Reinforcement particles from pig bones can be used as a biomedical material to replace a broken component. It has been proven by recent studies to be useful in allographing and xenografting (Ratner et al., 2004). By weight, pig bone typically comprises of 60% mineral components, 25% water and 15% organic elements. With a little number of carbonates, magnesium, hydroxyl, fluoride, chloride and citrate ions, calcium and phosphate ions make up the majority of the mineral phase’s ions (Hench and Best, 2004). The reinforcement particles from pig bone biomaterial interface have shown close relativity between the human bone, which exhibits mature bone features with more osteocytes, and porcine bone properties (Stefanini et al., 2021; Orsini et al., 2006).

The main application of the reinforcement particles from bone is the development of hydroxyapatite. Hydroxyapatite particles from sheep/goat sources, which have good biocompatibility, are majorly used for tissue engineering and bone replacement (Balázsi et al., 2007). In comparison to synthetic hydroxyapatites, those generated from natural sources have been proven to be safer in terms of cross-reaction and other immunological reactions (Chattopadhyay et al., 2007). Although hydroxyapatite particle synthesis from sheep/goat bones is frequently challenging and expensive. Cost-effective bioceramics are also manufactured using biological hydroxyapatites that are found in goat bones and teeth. In a study carried out by Akyurt et al. (2012), dentine hydroxyapatite particles from sheep teeth were used as a different source of bioactive biomaterial for grafting. They were extracted, cleaned and calcined in air at 850°C. It was discovered in their study that hydroxyapatite particles from sheep teeth possess exceptional biocompactibility properties. Various techniques, such as calcinations, chemical synthesis with hot-plating, ultrasonication or hydrothermal processes, are potential techniques that can be used to develop particle reinforcements from animal sources.

Mesh is another form/shape of animal-based reinforcements that have not been exploited in the past, but have been gaining a lot of attraction by researchers and scientists in recent times. They are majorly found under the skin and on the tissues of animals, especially mammals. They have found vast application in tissue engineering for tissue and bone replacement. Its major application in tissue engineering is to slow adhesion development and a synthetic parietal side that promotes strong healing. It is also used in improving skin repair procedures. In a study carried out by Stefanini et al. (2021), vertical bone abnormalities were repaired using a modified connective tissue graft wall technique with an enamel matrix derivative. A coronally advanced flap was used, and an acellular dermal matrix made from pig tissue was put beneath it to mimic the buccal soft tissue wall of the bone defect. One year after the operation, coupled with radiographic bone defect fill, the interdental papilla’s position and clinical attachment level gain both increased (Stefanini et al., 2021). Thus, animal biomaterials in the form of mesh possess excellent biocompactibility and biodegradability properties.

Animal fiber/particle reinforcements are located in different and unique parts within the animal body, which shows natural diversity of bio-derived composite materials, as presented in Table 2. Avian species provide feather fibers, bone particles and shell-based particulates sourced primarily from the wings, legs and outer shells. Insects, including silkworms, beetles, honeybees and others, contribute silk fibers typically extracted from the head region, representing one of the most commercially valuable animal reinforcements. Humans provide hair fibers, also obtained from the head, which have been explored for low-cost composite applications. Mammalian livestock such as sheep, goats, pigs and cows supply a wide range of reinforcements, including wool fibers, hydroxyapatite particles, prosthetic mesh materials and hair fibers, obtained from external body parts, bones, teeth, tissues and tails. Fish serve as an additional source of hydroxyapatite particles and graft materials extracted from bones and scales. Collectively, these animals offer abundant and diverse reinforcement materials suitable for sustainable composite development.

There are different techniques for manufacturing polymer matrix bio-composites, which are pultrusion, hand lay-up, vacuum infusion, compression molding, vacuum infusion, resin transfer molding, vacuum bag molding process, spray-up, electrospinning and injection molding (Gómez-Suarez and Córdoba-Tuta, 2022; Rajak et al., 2019). The quality of the final from any chosen technique is bound to be affected by moisture, type of fiber, fiber volume and length of biocomposite, chemical constituent, fiber modification and molding temperature (Gholampour and Ozbakkaloglu, 2020). The schematic representation of selected manufacturing techniques is shown in Figure 2. Each technique has its advantages and disadvantages, which are shown in Table 3.

The hand lay-up technique, also known as the wet lay-up method, is the oldest and one of the simplest fiber-reinforced composite production techniques. The fibers (woven, chopped, knitted or stitched) are placed into a mold and wetted with liquid resin using a hand roller to ensure interaction and adhesion between the fiber and the matrix. The first step is spraying with a mold release agent. Then a thin plastic sheet is placed at the top and bottom of the mold plate to ensure a good surface finish (Raji et al., 2019). The fibers are then cut into required shapes and sizes and distributed uniformly together with the resin using a brush, and then rolled. More layers are created till the desired thickness is achieved and is left to dry at atmospheric conditions. A variation or improvement of this technique is the spray-up technique, where a single-sided tool under atmospheric conditions is used to spray resin and chopped fibers simultaneously (Jamir et al., 2018; Leonelli and Romagnoli, 2015; Middleton, 2016).

This is a process of filling the matrix (mostly thermoset) in a resin bath with continuous fibers. The fibers can be in the form of tape, woven material or mat, passed through a system that’s heated, shaped and formed. The precision is achieved by making cuts of the required size (constant cross-sections with long lengths). Examples of biocomposite parts that can be produced with this include rods, ladder side rails, tool handles and electrical cable tray components. The parameters controlling the final products are the type and speed of fibers and matrix temperature (Joshi, 2012; Verma and Fortunati, 2019; Correia, 2023).

This is also known as liquid molding and is an intermediate molding technique that injects moderately pressurized resin (low-viscosity thermoset) into a closed mold cavity. This technique creates two surface finishes by filling the voids in the mold with resin and wetting all the surfaces of the reinforcing fiber. The animal fiber used in this technique can be in different forms, such as mats, fiber tows and woven structures. This technique is susceptible to microvoids and unwetted dry spots, which limits its utilization for critical applications, especially in the aerospace industry (Erden and Ho, 2017; Gupta et al., 2022; Hamidi and Altan, 2018). The resin transfer molding has different variations based on the modifications in the technique of impregnating the fiber with resins. These modifications include autoclave, vacuum infusion and vacuum-assisted resin transfer attachments (Gupta et al., 2022).

This is a common biocomposite molding technique in the aerospace industry for making prototypes. The main material for the initiation of the molding process is the prepreg, which means the resin (thermosetting polymer) has been preimpregnated with unidirectional or bidirectional continuous fibers and partly cured (120°C and 1 bar). The prepreg is then cut and arranged into the desired shape and size and then placed into a mold, which is the vacuum bag (air-removal), and then sealed. The temperature of the autoclave (180°C and 7–10 bars) is then raised until curing has been achieved. The result from this technique has fewer voids, even with thick laminate structures (Mallick, 2021; Schlimbach and Ogale, 2012).

This uses an open mold rather than a closed mold. Although the tooling costs can be significantly less, the capital costs are high. The reinforcements are placed on the tool with the top layer as a resin distribution fabric. The stack is enclosed and sealed using a flexible plastic bag connected to a liquid resin and vacuum pump, such that upon squeezing through air-removal, the biocomposite takes the shape of the mold. This technique is used in manufacturing boat hulls and wind turbine blades for aerospace applications (Liu et al., 2018; Spasojevic, 2019).

This is a simple technique for manufacturing polymer biocomposites using either thermosets or thermoplastics as a matrix from discontinuous animal fiber reinforcements. The deformable material is placed between two heated halves, resulting in the formation of the intended part. This technique is easily automated to efficiently produce complex geometries with negligible waste (Park and Lee, 2012; Dumont et al., 2023; Greene, 2021a, 2021b).

This is the choice technique for mass production with good surface finishes. The technique of operation is by heating the resin (thermoplastic) to obtain a viscous melt; then, the melt is pressurized through an orifice that has the intended shape of the parts to be produced. The produced parts are then cooled and extracted. The drawback of this technique is the inability to produce complicated shapes through the technique of heat transfer and pressure flow that are required by injection molding. This technique is not limited to polymer matrix composites because it can be used in producing modified ceramic suspensions. The final part produced will be a function of the type of material introduced, the pressure applied to the plunger, plunger speed, temperature of the heating cylinder and temperature of the mold (Ebnesajjad, 2015; Greene, 2021a, 2021b).

This is an electrohydrodynamic technique that produces electrified liquid droplets to stretch and elongate fibers. The technique is simple, with components for a high-voltage power supply, syringe pump, hypodermic needle spinneret and conductive collector. The manufacturing process, upon electrification, is an electrostatic repulsion that deforms the liquid droplet into a straight line, which then turns into a Taylor cone as the projection continues (Xue et al., 2019).

This is a manufacturing process that involves winding a fiber wetted by resin onto a rotating mandrel along a predetermined path. After the wounding process, the mandrel is removed by heating the resin. The process can be automated to produce axisymmetric and nonaxisymmetric biocomposite parts. Parts produced through this technique include pipe bends and aircraft fuselage (Rajak et al., 2019).

Table 3 presents a detailed summary comparison of common biocomposite manufacturing techniques, outlining their respective advantages and limitations in terms of productivity, cost, material quality and process flexibility. Compression molding is highlighted for its fast setup time, low material waste and suitability for producing large, complex composite parts with good surface finish; however, it suffers from low production speed and is restricted to flat or moderately curved shapes. Sheet molding offers excellent reproducibility and high-volume production with minimal labor requirements, yet it is limited to composites with low fiber volume fractions. Extrusion molding is attractive for its low initial costs and continuous processing capability, but the technique is only suitable for uniform cross-sections and provides moderate precision. Injection molding excels in mass production through high throughput and the ability to form intricate geometries with great accuracy, though high initial setup costs remain a major drawback. Hand lay-up is valued for its simplicity, design flexibility and low tooling cost, while its disadvantages include labor intensiveness, dependency on operator skill and emissions from certain resin systems. Resin transfer molding improves product consistency, allows tighter tolerances and reduces maintenance costs, yet wastes more material than compression molding and has slower production speeds than injection molding. Finally, resin infusion molding produces excellent surface finish and uniform laminate quality, but it is characterized by slow cycle times and high consumable costs. Collectively, these techniques illustrate the trade-offs between cost, efficiency, precision and material performance in biocomposite manufacturing.

The morphological properties of animal fibers can be examined using tools like scanning electron microscope (SEM), but due to the nonconductive nature, ultra-high resolution scanning electron microscopy is difficult. The specimens can be sputtered with iridium prior to observation. Transmission electron microscopy (TEM) can be used to study the hair by using osmium staining and lead poststaining (Yu et al., 2017). The SEM and TEM image of human hair is shown in Figure 3.

Another characterization technique is X-ray diffraction analysis; the diffraction data can be obtained by using biological large angle diffraction experiment with 9 kW, i.e. accelerating voltage of 45 kV and current of 200 mA, and wavelength of 1.5418 Å while aligning the axis of the hair parallel to the beam for maximum illumination. The setup for the X-ray analysis is shown in Figure 4.

The functional groups present in biocomposites can be investigated using the fourier infrared technique or attenuated total reflection fourier transform infrared (Gea et al., 2018). In a research carried out by Ali et al. (2021), the FTIR analysis for chicken feather fibers, cow hair fibers and leather fibers as reinforcements in unsaturated polyester resin matrix. The results show peaks that represent the presence of carbonyl (C = O), =C–H, –OH, CH₃ and CH₂ groups as shown in Figure 5.

More importantly, it is noted that there is no chemical reaction between the matrix and the reinforcements. The thermal behavior of the chicken feather fibers, cow hair fibers and leather fibers biocomposite can be obtained using a thermogravimetric analyzer at a predetermined temperature and heating rate (Suarato et al., 2020). The thermogravimetric analysis of chicken feathers, cow hair and leather fiber shows that 5% reinforcement displays good thermal resistance up to a temperature of 315°C, followed by a slow thermal degradation as the temperature increases until it reaches a critical temperature for rapid degradation (Ali et al., 2021) as shown in Figure 6.

Oladele et al. (2018a, 2018b) performed elemental characterization on bird feathers; the result is shown in Table 4. Tesfaye et al. (2018) also made the proximate analysis and ultimate analyses of chicken feathers, and the results are shown in Table 5. XRD analysis and SEM analysis were performed by Awogbemi et al. (2020) on the eggshells, and the results are shown in Figure 7. The constituents of the eggshell, as characterized in XRD, are shown in Table 6. The textural properties of the eggshells are shown in Table 7.

Collectively, the results presented in Tables 4–7 demonstrate that chicken-derived wastes (feathers and eggshells) possess a rich and complementary chemical–mineralogical profile that makes them attractive as bio-reinforcement resources. Elemental analysis of chicken feathers (Table 4) shows the presence of nutritionally and functionally important trace elements such as Mg, Zn, Fe, Cu and K, together with a very high carbon content, reflecting the organic, keratinous nature of the fiber. The proximate and ultimate analyses in Table 5 confirm that chicken feathers are strongly proteinaceous, with crude protein exceeding 80%, low lipid content, modest ash and moisture levels and a dominant elemental composition of C, N, O and S. The relatively high nitrogen and sulfur contents are characteristic of keratin and indicate a high density of peptide and disulfide bonds, which are beneficial for thermal stability, char formation and potential interfacial interactions when used as reinforcement in polymer matrices.

In contrast, eggshells represent an inorganic, calcium-rich by-product. XRD data in Table 6 reveal that raw and boiled shells are dominated by calcite (CaCO₃), with small amounts of magnesium-bearing calcite, graphite and calcium oxalate, whereas calcination transforms the material into more reactive phases such as lime (CaO) and portlandite [Ca (OH)₂]. This phase evolution with processing temperature implies that heat-treated eggshells can act not only as stiff, low-cost fillers but also as chemically active reinforcements capable of contributing to alkalinity, bioactivity or interfacial bonding in cementitious and polymeric composites. The textural properties in Table 7 further show that boiling and calcination progressively increase BET and external surface areas, pore volume and, to some extent, pore radius, indicating the development of a more open and accessible pore structure. Such increases in surface area and porosity generally enhance mechanical interlocking, resin wetting and potential adsorption or catalytic behavior.

Taken together, the organic, nitrogen–sulfur-rich chicken feathers and the inorganic, calcium-dominated eggshells provide a synergistic combination for green composite design: lightweight, protein-based fibers can improve toughness and impact resistance, while mineral-rich eggshell particles contribute stiffness, dimensional stability and possible functional properties (e.g. bioactivity, fire resistance or barrier behavior). The microstructural changes reported by Awogbemi et al. (2020) through XRD and SEM, along with the BET data, also indicate that simple processing steps such as boiling or calcination can be used to tune phase composition and surface characteristics, allowing the tailoring of interfacial bonding and overall performance of poultry-waste-derived biocomposites within a circular-economy framework.

For the FTIR analysis on eggshell particles, the results show CO^2–₃, OH⁻¹ and OAH functional groups in all the eggshells. The thermogravimetric analysis showed that due to the decomposition of calcium carbonate in the calcined eggshells, the decomposition temperature was lower than in the boiled and raw eggshells, as shown in Figure 8. In chitosan, the FTIR analysis showed the characteristic peaks of Amide I and Amide II, while thermogravimetric analysis showed a gradual wavey degradation curve starting at a temperature of 200°C (Guarnieri et al., 2022; Zhu et al., 2019). The FTIR and thermogravimetric plots of Chitosan are shown in Figure 9.

The mechanical behavior of biocomposites is strongly governed by the intrinsic properties of the reinforcement, the matrix and, crucially, the quality of the fiber–matrix interface (Akinwekomi et al., 2024). Natural reinforcements, including animal-derived fibers such as silk, feather, wool, keratin and mineral phases from eggshell or bone, generally exhibit high specific strength and stiffness but also significant variability in diameter, surface chemistry and defect content (Rodopoulos and Karathanasopoulos, 2025; Shelly et al., 2025). When these fibers are combined with thermoplastic or thermoset matrices (e.g. polypropylene, PLA, polyester and epoxy), the resulting mechanical response typically reflects a balance between the load-bearing capacity of the fiber and the deformability of the matrix (Islam et al., 2025; Li and Roh, 2025). Studies on natural-fiber-reinforced polymers consistently report improvements in tensile and flexural properties at low-to-moderate fiber loadings, provided that good interfacial bonding is achieved through appropriate surface treatments or compatibilizers (Ermeydan, 2024; Mylsamy et al., 2024).

Comparative literature on animal fibers shows that silk-reinforced composites often display superior tensile strength and modulus relative to many plant-fiber systems, owing to silk’s highly ordered β-sheet crystalline domains and excellent extensibility (Yang et al., 2025). When embedded in epoxy or polyester matrices, silk fibers can impart high tensile strength and fracture toughness, making such biocomposites suitable for structural or biomedical components (Anulaya and Kandasubramanian, 2025; Faheed, 2024). In contrast, chicken feather and wool fibers, which contain micro‐ and macro-voids in their medulla and a scaly surface morphology, tend to yield composites with lower stiffness but enhanced impact resistance and energy absorption (Gorur Avsaroglu, 2025; Thenmozhi and Thilagavathi, 2024). Several authors have reported that feather-reinforced thermoplastics exhibit reduced density and improved damping behavior, making them attractive for lightweight panels and acoustic insulation, even when tensile properties remain below those of glass-fiber composites (Santhosh et al., 2024). Where keratin fibers are well dispersed and chemically bonded (e.g. via coupling agents or alkaline/oxidative pretreatments), notable gains in tensile strength and flexural modulus relative to neat matrices have been observed; however, poor treatment or high moisture content can promote fiber pull-out and interfacial debonding, leading to premature failure (Raydan, 2024).

Mechanical behavior is also highly sensitive to fiber volume fraction, orientation and aspect ratio (Gonabadi et al., 2025). Many studies report an optimum reinforcement content beyond which tensile and flexural strengths begin to decline because of fiber agglomeration, voids and stress concentrations. For instance, increasing animal-fiber content generally enhances stiffness and strength up to a critical loading, but at higher contents the matrix becomes insufficient to wet and encapsulate each fiber, resulting in brittle behavior and decreased elongation at break. Unidirectionally aligned or woven animal fibers (e.g. silk fabrics) typically deliver higher tensile strength and modulus along the alignment direction compared with randomly oriented short fibers, which provide more isotropic but lower overall properties (Patti and Acierno, 2023; Wang et al., 2025). Similarly, finer fibers with higher aspect ratios improve stress transfer efficiency and crack bridging, while coarser fibers or particles primarily act as fillers that may increase stiffness but offer limited strength enhancement (Arunprasand and Nallasamy, 2025).

The choice of matrix profoundly affects the overall mechanical response. Thermoset matrices such as epoxy and vinylester, with inherently higher stiffness and better wetting characteristics, often produce animal-fiber biocomposites with superior tensile and flexural performance and improved dimensional stability compared with those based on commodity thermoplastics (Thapliyal et al., 2023). Epoxy–silk or epoxy–wool systems, for example, typically exhibit higher tensile strength and flexural modulus than comparable polypropylene-based systems at equivalent fiber loadings, due to stronger interfacial adhesion and lower residual stresses after curing (B H et al., 2024; Onwubu et al., 2025). On the other hand, thermoplastic matrices (e.g. polypropylene, polyethylene, PLA and PHA) offer beneficial ductility and recyclability; feather- or wool-reinforced thermoplastics can show significant improvements in impact strength and toughness even when stiffness increases only modestly (Devarshi, 2023; Tao et al., 2023). Biodegradable matrices such as PLA and PHA, when reinforced with animal fibers, have been reported to display an attractive balance between stiffness, strength and environmental performance, though their brittle nature often requires careful optimization of fiber content and plasticization to avoid catastrophic failure (Radhakrishnan et al., 2024).

Another key finding across the literature is the role of chemical composition and hierarchical structure of animal-derived reinforcements in damage mechanisms. Keratin-rich fibers (feathers, wool and human hair) contain disulfide bonds and heterogeneous microstructures that favor fiber pull-out, fibrillation and microcracking under load, leading to increased toughness and energy absorption (Mishra et al., 2024). This behavior is distinct from more brittle mineral fillers such as eggshell or bone-derived hydroxyapatite, which mainly enhance stiffness and hardness but may induce stress concentrations and microcracks if not finely dispersed (Oladele et al., 2022a, 2022b). Hybrid biocomposites that combine organic animal fibers with mineral phases (e.g. silk or feather plus eggshell particles) have been shown to exhibit synergistic effects: the fibers bridge cracks and provide toughness, while the mineral particles increase rigidity, wear resistance or compressive strength (Oladele et al., 2025a, 2025b). Such hybrids can therefore be tailored to meet specific mechanical performance targets for structural, biomedical or packaging applications.

Fatigue, impact and fracture behavior of animal-fiber biocomposites have also received attention. Under cyclic loading, good adhesion and flexible matrix systems help to delay crack initiation and slow crack growth, resulting in improved fatigue life compared with neat polymers (Malekinejad et al., 2023). Silk-based composites, for instance, often maintain a stable stiffness over repeated loading cycles due to the reversible stretching of the protein chains, whereas feather-reinforced systems may show gradual stiffness reduction but superior damping and vibration attenuation (Lyu et al., 2022). Impact studies generally reveal that the inclusion of ductile animal fibers increases impact strength and reduces notch sensitivity, as fibers can undergo pull-out and plastic deformation, dissipating energy (Oladele et al., 2022a; 2022b; 2025a; 2025b). Conversely, composites filled predominantly with stiff mineral particles from eggshell or bone show higher hardness and compressive strength but lower impact resistance, underscoring the need to balance fiber and particle contents depending on the load case (Oladele et al., 2022a; 2022b; 2025a; 2025b).

To test the mechanical behavior of ceramic matrix biocomposites, Akbar et al. (2022) highlighted the ASTM standards for checking the mechanical properties of human hair fiber concrete. Table 8 shows the technique for measuring the mechanical properties.

There are ASTM standards for determining the mechanical properties of human hair-reinforced polymer biocomposites, described by Rahman et al. (2023a, 2023b), shown in Table 9.

Performing tensile test analysis requires making tensile sample specimens as described by the ASTM standards. The mechanical tests are performed using a universal testing machine. Tensile samples are dog-bone-shaped specimens and are stressed at a predetermined strain rate until failure (Rahman et al., 2023a, 2023b). The flexural strength of the biocomposites can be determined by a three-point bending test or a four-point bending test at a predetermined strain rate until failure. The formula for determining flexural strength and modulus for biocomposites is shown in equations (1) and (2) (Nanda and Satapathy, 2017; Rahman et al., 2023a, 2023b). Compression tests are also important mechanical behavior testing criteria performed on a universal testing machine. To perform the test, there is a need to ensure the stability of the testing conditions to avoid wobbling and misalignments until a strain of 0.5 is achieved (Jerabek et al., 2010). In the shear test, the two edges of the specimen are compressed while examining the force. The relative displacement between the two fixtures (strain gauge elements) placed at the center of the specimen and at 45° to the loading axis gives the estimation of the shear response of the biocomposite.

It is also essential to estimate the voids entrapped in the biocomposite, as it can contribute to lower adhesion between the fibers and matrix. There are different techniques for estimating the voids present in the specimen. These include density measurement, ultrasound testing, X-ray tomography, radiology and other microscopy techniques. Equations (3) and (4) are used in estimating the voids in biocomposites.

Gholampour and Ozbakkaloglu (2020) reviewed the mechanical properties of animal fibers and their composite; the result is shown in Table 10. Animal fibers such as silk, feather and wool generally exhibit high tensile strength relative to their low densities, making them attractive lightweight reinforcement materials. Silk stands out for its exceptional strength and elasticity, while feathers and wool offer moderate stiffness with varying elongation capacities, allowing them to enhance toughness and energy absorption in composites. In contrast, thermoplastic resins such as polypropylene, polyethylene and polystyrene typically possess lower strength and stiffness but offer excellent ductility, processability and cost-effectiveness. Engineering thermoplastics like nylon and biopolymers such as PLA and PHA show improved mechanical performance, making them suitable matrices for structural biocomposites. Thermoset resins, including polyester, vinylester, epoxy and phenolic systems, generally exhibit superior strength, stiffness and thermal stability compared to thermoplastics, making them ideal for high-performance applications. Lightweight animal fibers with durable polymer matrices enable the development of biocomposites with tailored mechanical behavior suitable for diverse engineering applications. In general, the mechanical behavior of biocomposites reinforced with animal-derived materials is characterized by a complex interplay between reinforcement morphology, chemical composition, matrix type and processing conditions. Literature consistently indicates that when fiber–matrix compatibility is optimized – through surface treatment, coupling agents or appropriate processing parameters – animal-fiber biocomposites can achieve tensile and flexural properties competitive with, or complementary to, many plant-fiber systems, while offering unique advantages such as low density, enhanced toughness, acoustic damping and the valorization of waste streams. These findings highlight the technical feasibility and sustainability potential of animal-based biocomposites and motivate further research into microstructural tailoring, hybridization with mineral phases and long-term durability under realistic service conditions:

where E represents the flexural modulus, σ represents the flexural strength, L represents the distance between support spans, w represents the width of the beam specimen, t represents the beam thickness, I represents the beam moment of inertia, δ represents the mid-span deflection and P represents the applied force. (P/δ) represents the slope of the force vs deflection plot. σ for the biocomposites is estimated using the peak value of “P” of the respective plot. In equations (3) and (4), W_f and Wm represent the weight fractions of the fibers and matrix. ρ_f, ρ_m and ρ_c represent the density of the fibers, matrix and composites, respectively. ρ_a and V_v represent the actual density and volume fraction of voids in composites. The actual density, ρ_a, is estimated using the volume and weights of fibers and the matrix used to fabricate the composites (Nanda and Satapathy, 2017). The schematic and equipment setup for testing the mechanical behavior of biocomposites are shown in Figures 10 and 11, respectively.

To perform volume fraction measurement of biocomposites, the density of each of the constituents must be estimated, and volume fraction is calculated using equations (5)–(7):

ρ_Epoxy represents the density of epoxy, ρ_Silk represents the density of silk fiber, ρ_Laminate represents the density of laminate, V_f represents the volume fraction of fiber, V_r represents the volume fraction of resin, M_Laminate represents the measured mass of the laminate, M_Silk represents the measured mass of the silk fibers and V_ν represents the void volume fraction (Hamidi et al., 2018). The percentage of water absorption can be calculated using equation (8). To calculate the thermal stability, thermogravimetric analysis is used. The experiment is performed at temperatures between 25°C and 600°C (heating rate −10°C/min) with nitrogen to prevent oxidation. The temperature at predetermined weight loss is then estimated. Equation (9) is used to obtain the degree of crystallinity of the biocomposite. Where ΔH_m and ΔH_cc represent the enthalpy of fusion and cold crystallization at melting and crystallization temperatures, respectively, w represents the weight fraction of neat polymer in the sample, and ΔH⁰_m represents the melting enthalpy of the 100% crystalline polymer (Aranberri et al., 2017; Jerabek et al., 2010).

The design, processing, manufacturing and characterization of animal-based reinforced biocomposites are accompanied by a set of challenges that are often more complex than those encountered with conventional synthetic or even plant-based systems. At the design stage, one of the most fundamental issues is the inherent variability of animal-derived reinforcements such as feathers, wool, silk, hair, eggshell and bone-derived particles (Roy et al., 2025). Their dimensions, microstructure and chemical composition can vary with species, age, diet and even body location (Oladele et al., 2023). As a result, several studies report large scatter in mechanical properties and difficulties in generating reliable design allowables (Al-Shalawi et al., 2023). Where glass or carbon fibers are sold with tight specifications, keratin fibers or eggshell powders often lack standardized grades, making it challenging to predict stiffness, strength and long-term behavior using conventional micromechanical models. This variability also complicates numerical simulation; models that work reasonably well for plant fibers frequently require re-parameterization or additional assumptions when applied to hollow feather fibers or highly crystalline silk (Kurien et al., 2022a, 2022b).

Processing presents another major bottleneck. Most animal fibers are rich in proteins (keratin, collagen and fibroin) and are thermally less stable than typical polymer matrices (Banasaz and Ferraro, 2024; Timorshina et al., 2022). Many thermoplastics are processed above 170°C–200°C, a regime where feathers, wool or hair begin to degrade, discolor or emit volatiles (Röhl and Müssig, 2022). Several experimental studies on feather- and wool-reinforced polypropylene or PLA highlight a trade-off between achieving good melt flow and avoiding thermal damage to the reinforcement; excessive residence time or shear leads to embrittlement and loss of tensile strength and moisture sensitivity further complicates processing (Kukcu and Dasdemir, 2024). Keratin fibers are hygroscopic and can carry significant bound water; if not carefully dried, this moisture can generate voids, porosity and hydrolytic degradation during extrusion or injection molding (Ramesh et al., 2022). By contrast, mineral reinforcements such as eggshell or bone-derived hydroxyapatite are less moisture-sensitive but can agglomerate easily, requiring high shear to disperse, again at the risk of degrading the organic matrix or fibers in hybrid systems (Oladele et al., 2022a, 2022b).

Interfacial adhesion and surface treatment are recurring themes across the literature (Kabir et al., 2012; Maiti et al., 2022). The outer surfaces of feathers and wool are coated with lipids and possess scale-like or smooth morphologies that do not bond strongly to nonpolar matrices (Ghosh et al., 2015). Many authors therefore use alkaline treatment, oxidation or coupling agents to improve wetting and create chemical bridges (Kabir et al., 2012; Khalid et al., 2021; Mohammadi et al., 2024). While such treatments can significantly improve tensile and flexural properties (Feng et al., 2020; Khan et al., 2018), they introduce new challenges: process complexity, chemical waste, possible fiber damage and difficulties in scaling from laboratory to industrial quantities (Mundhe et al., 2024; Ghosh et al., 2025). Excessively aggressive treatments can erode the cuticle layers of keratin fibers (Onyuka, 2010; Duit, 2016) or partially demineralize or over-react eggshell-based fillers, altering phase composition and degrading mechanical performance (Owuamanam et al., 2021; Sunardi et al., 2023). For mineral fillers, high loadings often increase stiffness but simultaneously reduce impact strength due to brittle behavior and poor stress transfer (Altay et al., 2021), and induce microcracking driven by thermal expansion mismatch between filler and matrix (Awaja et al., 2016). Balancing fiber or particle content, surface modification and processing constraints is therefore nontrivial, as emphasized in filler and natural-fiber composite reviews (Shubhra et al., 2013; Ahmed et al., 2025).

On the manufacturing side, the choice of technique (compression molding, extrusion, injection molding, resin transfer molding and hand lay-up) introduces additional constraints for animal-based biocomposites (Masri and Sam, 2021; Imon, 2024). Injection molding and extrusion are attractive for high-volume production, yet the high shear stresses and repeated passes through the screw tend to shorten fragile fibers such as feathers and hair, reducing aspect ratio and reinforcing efficiency (Rabbi et al., 2021; Elfaleh et al., 2023). Compression molding of preforms or mats can better preserve fiber length but is less suitable for intricate geometries and can suffer from nonuniform fiber distribution (Billah et al., 2021). Hand lay-up and vacuum infusion allow gentle processing of continuous silk or wool fabrics, but they are labor-intensive, sensitive to operator skill and prone to void formation (Imon, 2024). Several studies report that achieving homogeneous dispersion of lightweight hollow fibers (e.g. barbed feathers) is particularly difficult because they float in liquid resins or melt, segregating to the surface; this leads to local stress concentrations and unpredictable mechanical behavior (Murillo et al., 2024). Industrial up-scaling must therefore address not only throughput and cost but also robust handling, dosing and mixing of low-density, irregular reinforcements (Murillo et al., 2024; Olanrewaju et al., 2025).

Characterization of animal-based biocomposites introduces its own set of methodological challenges. Standard test methods for tensile, flexural and impact properties were originally developed for isotropic plastics or synthetic fiber composites, and may not fully capture the anisotropy and heterogeneity of biocomposites (Bharath and others, 2016). Determining the exact fiber volume fraction is nontrivial when dealing with hollow feathers or porous mineral particles; density-based back-calculations can be inaccurate and chemical digestion methods risk altering the microstructure (Patrucco et al., 2024). In mechanical testing, natural scatter arising from biological variability often produces high standard deviations, demanding larger sample sizes and more rigorous statistical treatment (Patrucco et al., 2024). At the microstructural level, distinguishing fiber and matrix contributions in techniques such as DMA, DSC or TGA can be difficult because thermal transitions and degradation peaks overlap (Chang et al., 2020). XRD characterization of keratin and collagen is complicated by their partially crystalline, partially amorphous nature, while SEM sample preparation must avoid artifacts such as charging, shrinkage or pull-out that can misrepresent true interfacial adhesion (Patrucco et al., 2024).

A further challenge emphasized in several works is long-term durability and environmental ageing. Animal-based reinforcements are prone to moisture uptake, microbial attack and UV-induced degradation (Chang et al., 2020). Studies on outdoor exposure of feather- and wool-reinforced composites show gradual reductions in tensile strength and increased brittleness, often attributed to interface degradation, swelling–shrinkage cycles and photo-oxidation of both matrix and fibers (Chang et al., 2020; Patrucco et al., 2024). When biodegradable matrices such as PLA or PHA are used, disentangling the degradation mechanisms of matrix vs reinforcement becomes even more complex (Chang et al., 2020). Accelerated ageing protocols and life-cycle assessments (LCA) are still relatively scarce, and few long-term data sets exist compared with those for glass or carbon fiber composites (Chang et al., 2020).

Finally, there are nontechnical but closely related challenges: sourcing, hygiene and public perception. Animal wastes such as feathers and eggshells are abundant, yet they must be collected, cleaned and sanitized to remove blood, pathogens, fats and odors before use (Murillo et al., 2024). Several studies note that inadequate cleaning can lead to unpleasant smell during processing, corrosion of equipment or even bio-safety issues (Murillo et al., 2024). Standardization of supply chains – from poultry farms or slaughterhouses to compounding facilities – remains underdeveloped in many regions, leading to batch inconsistency and logistic barriers (Olanrewaju et al., 2025). At the same time, regulatory frameworks for using animal by-products in consumer products or biomedical devices can be stringent, requiring traceability and compliance with health regulations (Murillo et al., 2024).

In summary, the literature shows that animal-based reinforced biocomposites hold considerable promise in terms of lightweighting, toughness and waste valorization, but their realization is hindered by multi-scale challenges in design, processing, manufacturing and characterization (Olanrewaju et al., 2025; Chang et al., 2020). Key findings consistently point to the need for better control of raw-material variability, gentler yet efficient processing routes, optimized surface treatments and more sophisticated characterization protocols that capture both the biological complexity and engineering performance of these materials (Masri and Sam, 2021; Patrucco et al., 2024). Addressing these challenges will be essential for transitioning animal-derived reinforcements from niche applications to reliable, industrially adopted components within a circular, bio-based economy.

The applications of animal-based reinforced bio-composites span structural, functional and biomedical domains, reflecting both the versatility of animal-derived materials and the drive toward circular-economy solutions, as presented in Table 11 (Dutta et al., 2024; Donato and Mija, 2019; Khan et al., 2022). Keratinous fibers (feather, wool, human hair and silk) and mineralized wastes (eggshell, bone and fish scales) are increasingly exploited as low-cost reinforcements or functional fillers in polymer matrices (Ali et al., 2024; Rahman et al., 2023a, 2023b; Thimmegowda et al., 2025). Their relatively high specific strength and stiffness, intrinsic porosity and rich surface chemistry allow them to deliver combinations of lightweighting, toughness, flame retardancy or bioactivity that are difficult to obtain with purely synthetic fillers (Kurien et al., 2022a, 2022b; Khan et al., 2022; Mosnáčková et al., 2021). However, as highlighted in many studies, converting heterogeneous animal wastes into reliable, application-grade reinforcements still requires careful control of processing, interfacial chemistry and long-term stability (Dutta et al., 2024; Donato and Mija, 2019; Kaufmann et al., 2025).

In structural and automotive applications, animal-based biocomposites are primarily used where weight reduction, impact absorption and cost savings are critical (Thimmegowda et al., 2025; Kaufmann et al., 2025). CFF–reinforced polymers have been proposed for interior panels, impact-absorbing elements, helmets and car fenders because of their low density, good energy absorption and intrinsic thermal and electrical insulation (Kurien et al., 2022a, 2022b; Khan et al., 2022; Dutta et al., 2024). Kurien et al. (2022a, 2022b) report that CFF composites exhibit favorable impact and energy-absorbing behavior, suggesting their use in protective gear and lightweight automotive parts, while Khan et al. (2022) emphasize their flame-retardant tendencies, making them interesting for fire-safe housings and electronic casings. Waste eggshell particles, rich in CaCO₃, have been used as fillers in polyester, PA12, PMMA and PLA matrices to produce low-cost, stiff, wear-resistant components for machine parts, toys, electrical housings and even aerospace and automotive components (Homavand and Yari, 2024; Mustapha et al., 2020; Alkaron and Alsaadi, 2023; Carranza et al., 2025). These systems typically show improved hardness, stiffness and dimensional stability at relatively low filler contents, although toughness may decrease and careful dispersion is required to avoid embrittlement (Mustapha et al., 2020; Alkaron and Alsaadi, 2023). A key challenge across this literature is optimizing filler loading and surface treatment to exploit stiffness gains without sacrificing impact resistance or processability – particularly important in dynamic, vibration-rich environments like vehicles (Dutta et al., 2024; Mosnáčková et al., 2021; Thimmegowda et al., 2025).

In packaging and insulation, the combination of low density, biodegradability and functional surface chemistry has inspired a range of applications (Chen et al., 2019; Pardo-Ibáñez et al., 2014; Mihalca et al., 2021). CFF and keratin-based biocomposites have been developed as biodegradable cushion packaging, thermally insulating boards and foam-like materials for protective packaging and building insulation, where moderate mechanical strength is acceptable but low weight, thermal insulation and end-of-life degradability are paramount (Kurien et al., 2022a, 2022b; Khan et al., 2022; Dutta et al., 2024; Strnad et al., 2025). Keratin/thermoplastic films and keratin–cellulose nanocrystal biofilms show enhanced tensile strength, barrier properties and controlled biodegradation, making them candidates for sustainable food or cosmetic packaging, although sensitivity to moisture and processing cost remain obstacles (Pardo-Ibáñez et al., 2014; Mosnáčková et al., 2021; Chen et al., 2019). Eggshell-reinforced starch and PLA films, as well as more recent composite films based on eggshell-derived CaCO₃, demonstrate improved tensile strength, elongation and water-barrier performance, as well as UV-blocking capability, suggesting uses in nonfood packaging, flower wraps and UV-sensitive product protection (Homavand and Yari, 2024; Athanasopoulou et al., 2024; Chen et al., 2019). The main challenges here are ensuring hygienic sourcing, overcoming possible odour or contamination concerns and tailoring barrier properties and durability to specific packaging lifetimes (Chen et al., 2019; Khan et al., 2022; Dutta et al., 2024).

Biomedical and health-care applications form one of the most intensively researched areas for animal-based biocomposites, owing to the intrinsic biocompatibility and bioactivity of proteins such as silk fibroin, collagen and keratin (Li et al., 2023; Ma et al., 2024; Donato and Mija, 2019). Silk-fiber-reinforced polymers and silk-based scaffolds have been explored for bone fixation devices, ligament and tendon repair, dental fiber-reinforced composites and load-bearing tissue scaffolds, leveraging silk’s high strength, controlled degradability and favorable cell interactions (Purnomo et al., 2018; Mandal et al., 2012; Sun et al., 2021; Ma et al., 2024). Keratin-based materials – ranging from fibers and films to hydrogels and nanofibers – have been investigated for wound dressings, drug delivery carriers, oral and nerve tissue engineering, and cell-culture scaffolds, often showing good cell adhesion and the ability to incorporate bioactive cues (Donato and Mija, 2019; Mosnáčková et al., 2021; Popescu et al., 2024). Eggshell-derived hydroxyapatite or CaCO₃ has been used as a bone substitute and as a reinforcement in PMMA and other dental resins to improve stiffness and radiopacity, though maintaining toughness and fatigue resistance remains challenging (Kattimani et al., 2014; Opriș et al., 2020; Yerou et al., 2025). Key findings across this field underscore the importance of tuning degradation rate, mechanical strength and bioactivity simultaneously – particularly in load-bearing implants where premature degradation or insufficient stiffness could compromise function (Li et al., 2023; Ma et al., 2024; Popescu et al., 2024). Regulatory approval, sterilization compatibility and reproducibility of animal-derived materials are central challenges limiting rapid clinical translation (Opriș et al., 2020; Li et al., 2023).

Beyond structural and biomedical uses, animal-based biocomposites find applications in filtration, adsorption, agriculture, textiles and specialty functional surfaces (Khan et al., 2022; Solís-Moreno et al., 2021; Zahara et al., 2021). CFF nonwovens and mats have been used as air-filtration media, heavy-metal and phenol sorbents, and oil-absorbent materials due to their high surface area, hydrophobicity and hollow morphology (Solís-Moreno et al., 2021; Zahara et al., 2021; Strnad et al., 2025). Collagen–keratin biocomposites have been developed as pellicle-forming materials for seed coating and controlled release of nitrogen in agriculture, improving germination and early plant nutrition (Niculescu et al., 2019). Feather fibers are increasingly explored as textile fibers in nonwovens and coatings for medical textiles and hygiene products, where their hypoallergenic character and biodegradability may reduce the environmental footprint of the textile industry (Khan et al., 2022; Dutta et al., 2024). Human hair and other keratin fibers have likewise been incorporated into polymer composites for low-cost, lightweight panels in construction and automotive interiors (Rahman et al., 2023a, 2023b; Arinze et al., 2022; Ali et al., 2024). These niche applications often exploit specific properties (e.g. adsorption, acoustic damping and antibacterial activity) more than pure structural performance, but they still face common challenges: variability in waste streams, odour and contamination control and building consistent supply chains (Khan et al., 2022; Dutta et al., 2024; Niculescu et al., 2019).

Current literature shows that animal-based reinforced biocomposites can deliver competitive or unique performance in many sectors, particularly when their natural functionalities – bioactivity, adsorption, flame retardancy and insulation – are harnessed alongside mechanical reinforcement (Khan et al., 2022; Kurien et al., 2022a, 2022b; Dutta et al., 2024). However, the same features that make animal-derived materials attractive (biodegradability, rich chemistry and biological origin) also generate challenges in durability, regulatory acceptance and process control (Li et al., 2023; Opriș et al., 2020; Chen et al., 2019). Continued progress will depend on integrating advanced processing (e.g. electrospinning and additive manufacturing), robust surface-modification strategies and standardized characterization with life-cycle and techno-economic assessment to position these materials realistically in the marketplace (Pardo-Ibáñez et al., 2014; Ma et al., 2024; Kaufmann et al., 2025; Thimmegowda et al., 2025).

The future of animal-based reinforced bio-composites is both promising and multifaceted, offering avenues for material innovation, industrial diversification and meaningful progress toward circular-economy goals. As demonstrated throughout this review, animal-derived reinforcements are abundant, functionally rich and often undervalued. However, their successful integration into mainstream composite technologies will depend on overcoming several scientific and engineering challenges through targeted research. Looking ahead, the next decade of development in this field will likely be shaped by advances in four key domains: standardization and material consistency, process innovation, functional hybrid composites and application-driven performance optimization. A critical future direction lies in establishing standardized characterization protocols and quality benchmarks for animal fibers and mineral particles. Biological variability remains a core limitation that affects mechanical predictability and reliability. The development of material grading systems, similar to those used for plant fibers and synthetic reinforcements, would significantly improve reproducibility. The authors foresee a growing role for machine learning-assisted property prediction, where large data sets of natural fiber morphology, composition and processing history could be used to model mechanical performance and optimize formulation.

Another promising direction is the evolution of processing technologies specifically tailored to animal-based reinforcements. Future work must focus on low-temperature polymer processing, green solvent systems and precision cleaning/sanitization technologies that preserve the structural integrity of keratin, silk, collagen and mineral phases. Emerging techniques – such as reactive extrusion, microwave-assisted compounding, enzymatic surface modification and supercritical CO₂ treatments – hold potential for improving interfacial bonding without damaging the reinforcement. Additive manufacturing (three-dimensional and four-dimensional printing) of silk, keratin hydrogels and eggshell-based bioceramics presents a pathway for fabricating patient-specific biomedical implants and high-value functional components.

The convergence of animal-derived materials with hybrid and multifunctional composite designs represents another frontier. Future research should explore synergistic reinforcement strategies that combine animal fibers with plant fibers, nanocellulose, graphene derivatives or mineral nanoparticles. Such hybrid systems may offer unprecedented combinations of toughness, bioactivity, fire resistance, adsorption capacity and electrical or thermal functionalities. The authors believe that hybridization will be central to designing next-generation bio-composites capable of matching or surpassing the multifunctional performance of synthetic fiber-reinforced polymers. From an application perspective, the future expansion of animal-based bio-composites will rely on application-driven design and regulatory integration. Biomedicine, filtration, environmental remediation, flexible electronics and smart packaging are areas where the intrinsic properties of animal derivatives – biocompatibility, antimicrobial activity and bioactivity – provide clear competitive advantages. However, scaling these applications will require robust sterilization protocols, regulatory compliance and long-term durability studies. Furthermore, LCA and techno-economic analyses must be incorporated into future research to demonstrate commercial viability and environmental superiority over existing solutions. Also, the future of animal-based reinforced bio-composites lies in advancing material standardization, developing specialized processing methods, exploring multifunctional hybrid systems and expanding into high-value application domains. By bridging biological insight with engineering innovation, future research can unlock the full potential of these materials, transforming animal wastes into functional, reliable and commercially competitive composite solutions. Ultimately, the future growth of this field depends on the establishment of sustainable value chains and industrial symbiosis. The authors anticipate that policy incentives, green manufacturing initiatives and global pressure to reduce plastic waste will further accelerate the industrial adoption of more bio-composites.

This review has comprehensively examined the diverse applications of animal-based reinforcement in bio-composites. The collective findings demonstrate that animal-derived reinforcements hold enormous but underexplored potential for sustainable composite development. Although plant-based fibers have dominated natural composite research, the literature clearly shows that animal-based materials possess unique structural and chemical features, including high protein content, hierarchical architectures, exceptional specific strength in the case of silk and mineral-rich phases such as CaCO₃ and hydroxyapatite in eggshells and bone. These attributes enable distinct mechanical, thermal and functional advantages that can be strategically harnessed in composite systems. Besides, animal-based materials are not seasonal like plants but are more readily available throughout the seasons annually. It was reaffirmed from the review that animal fibers can significantly enhance toughness, crack resistance, impact strength, thermal and acoustic insulation, and – in certain systems – flame retardancy. Mineral-based reinforcements derived from eggshells and bone contribute high stiffness, hardness and biocompatibility, particularly in biomedical and structural applications. When carefully processed and surface-modified, these materials can yield performance levels comparable to or exceeding many plant-based systems, highlighting the importance of optimizing interfacial adhesion, dispersion and thermal processing. The authors observe that animal-based materials also provide functional advantages beyond mechanical performance, such as adsorption capacity for filtration, antimicrobial activity in biomedical films, bioactivity in bone scaffolds and UV shielding in packaging films. These functionalities are difficult to replicate using synthetic reinforcements, demonstrating the broader value of animal waste streams. However, the paper highlights critical challenges that must be addressed for industrial adoption which include variability in fiber morphology, protein composition, mineral content and moisture behavior. These still remains major obstacle to consistency in composite design. Processing challenges – including thermal sensitivity, odor, hygiene issues and difficulties with uniform fiber dispersion – must be overcome through standardized pretreatment, improved compatibilization and process optimization tailored to the biological nature of the materials. Characterization methods must be adapted to account for hierarchical structures, overlapping thermal transitions and complex degradation mechanisms. These challenges underscore the need for interdisciplinary research that combines materials engineering, waste management, biotechnology and manufacturing science.

From the authors’ standpoint, the industrial potential of animal-based reinforcements remains largely untapped. The global abundance of these wastes, especially from poultry, livestock and fisheries, presents an opportunity for large-scale waste valorization within a circular economy framework. Their integration into packaging, automotive interiors, building insulation, biomedical devices, agricultural films and filtration technologies promises environmental, economic and societal benefits. Yet, achieving these outcomes will require stronger efforts in developing standardized processing protocols, scalable fiber extraction and cleaning technologies, advanced hybridization strategies and rigorous long-term durability assessment. Thus, animal-based reinforced bio-composites represent a promising frontier in sustainable materials engineering. Their unique combination of lightweight structure, functional versatility and environmental relevance offers a compelling alternative to synthetic and plant-based reinforcements. As authors, we believe that strategically combining these animal-derived materials with innovative processing and hybrid composite designs will accelerate their adoption in high-value applications and contribute significantly to global circular-economy goals.

The authors of this study would like to express their sincere gratitude to the Department of Metallurgical and Materials Engineering, Federal University of Technology Akure; the Faculty of Materials and Chemical Engineering, University of Miskolc; the Department of Materials Science and Engineering, Iowa State University; the Materials Science and Engineering Program, University of Colorado Boulder; and the Circular Economy and Sustainability Program, Universidad Politécnica de Madrid for their valuable collaboration, support, and contributions toward the successful completion of this review.

No funding was received for this work.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.