A biomimetic approach to materials would regard the possibility of promoting their self-assembly in a way that allows the growth of autonomous structures to be used for functional purposes [1,2]: this procedure is particularly sought in the case of peptides and proteins [3]. More specifically, the typical structures of proteins, such as collagen, elastin, and keratin, all include some degree of α-helices arrangements [4]. The most recent novelty, however, is constituted by the possibility of obtaining these structures out of the material extracted from animal waste, which enables the establishment of correlations between their aspect and the route by which self-assembly has been realized [5]. As far as protein extraction is concerned, extracted material takes the form of hydrolysates and small peptides [6]. These nanomaterials can have a role in acquiring a biomimetic function towards appropriate training exerted on non-natural protein backbones [7]. In other words, some extraction methods confer to the material a higher capability to resist proteolysis than others and can be regarded as adapted to its biomimicking [8].

In practice, keratin-based waste from different origins/biological structures can be used for the purpose: these include, among others, sheep wool in the form of hydrolyzed peptides [9], or in more structured form for direct blending with biopolymers [10], where a biomimetic approach might reduce the final risk of generating other hazardous material difficult to dispose of [11]. Other keratin residues in search of sustainable routes for disposal are human hair [12], epidermal waste [13], nails [14] or claws [15], and poultry feathers [16], on which this review specifically focuses.

Chicken feathers can be considered dense shells with a porous core, which provides lightness and is able to be compressed or even locally torqued without being buckled [17]. Their application as fibers (chicken feather fibers, CFF) in composites, purposely separated from the rachis [18], though proposed in many cases, does reduce this structural complexity to the bare tensile support of a polymer resin [19]. On the other hand, keratin extraction may result in the destruction of the composite structure, which is the scope of the biomimetic process to avoid. The main issue is unlocking the protein by extracting it without damaging its secondary structure and, therefore, promoting the self-assembly of the keratin structure in another geometry from the original one [20]. The secondary structure is mainly influenced by the position of the side chains and hence the degree of close packing in the protein, which for keratin is controlled by the presence of a number of different motifs [21], namely α-helical, β sheets, β-turn, and random coil structures [22].

In practice, the organization of chicken feathers includes, as the effect of the judicious combination of α and β forms, the combined presence of crossed-lamellar structure (300–600 nm thick) in lateral walls of rachis and barbs, arranged into alternate layers of crossed-fibers [23]. A significant grade of preservation of this structure out of keratin extraction is particularly beneficial in terms of toughness, which also allows its blending with other proteins isolated, e.g., from soy, for the formation of composite films [24].

The isolation of keratin from poultry feathers has been demonstrated of interest in the last few decades, especially, but not exclusively, in the food-related and the cosmetics sector [25]. Here, the preservation of secondary structure is not always considered essential [26], while rather preventive purification processes, based, e.g., on ethanol, ozone, and sodium chlorite, are given larger significance [27]. Another possibility, which is specific to poultry-originated keratin is the capability to absorb some metals, e.g., in soil treatment, such as cadmium, nickel, chromium, and zinc [28]. In some cases, nonetheless, the use of keratin from poultry feathers can be considered biomimetic, more explicitly whenever the architecture of the folding structure is considered and used to achieve specific functionalities. This has been, e.g., recently performed in the case of carbonized feathers therefore used as biochar [29], where the preservation of keratin structure did result in the possible application to the removal of the residues of drugs, such as amoxicillin, from aqueous solutions [30]. Keratin biochar is also particularly adapted to be possibly blended with other similarly abundant biomass waste, such as is the case for sugarcane bagasse, which suggests synergistic effects to be obtained from adapted doping strategies between the two feedstocks [31]. However, the energy-intensive character of these processes based on carbonization has also suggested that to use chicken feathers with a biorefinery approach, biodegradation/metabolism by the action of bacteria would represent a more suitable route towards the extraction of free amino-acids and soluble proteins [32].

To allow the potential use of keratin extracted, hence solubilized, from chicken feathers, a possible approach is based on the fabrication of autogenous cross-linked gels, e.g., serving as plant growth media [33]. On the one hand, this route can provide some mechanical performance that might ease application and compete, at least as blends, with other categories of gel structures, such as those obtained from polysaccharides, e.g., starch [34], alginates [35], or guar gum [36]. On the other hand, restoring secondary structure, hence protein cross-linking, starting from disulfide links, but not limiting to them, would represent a more natural application of keratin, therefore providing also other characteristics, such as controlled water retention, elongation, and strength [37]. These properties were exploited for some uses, such as wound healing, using a blended film with polysaccharides [38], or in a more general sense, in the biomedical engineering sector, including, e.g., also applications for drug delivery [39].

This review concentrates on chicken feathers since they constitute a very large waste of the food-related production sector, and therefore, their functional use would possibly result in a circular economy approach, offering a larger value to waste. Using poultry feathers in a biomimetic way for the production of engineered structures does involve preserving as much as possible their features during extraction, in the understanding that keratin-based structures do present a number of fundamental biomimetic properties in a thermal insulation and low-density context, which include reversible adhesion, structural coloration and the possibility to offer super-hydrophobic surfaces [40]. To achieve this potential, it is important, though, to preserve as much as possible the secondary structure of feather keratin, which will be described in Section 2. A number of methods exist for the extraction of keratin from feathers, which are reported in Section 3.

Following this, the discussion does particularly concentrate on those works where keratin is extracted with the preservation of its secondary structure, which can be suggested to represent a biomimetic application of poultry feathers’ keratin. With this aim, bio-inspired materials based on poultry feather keratin are discussed, after general considerations of the keratin role in these materials (Section 4.1), being either exclusively based on keratin or as a significant component of a blend with another biopolymer (Section 4.2). Finally, applications with particular reference to the bio-inspired potential, therefore with evidence of self-assembly, are discussed (Section 4.3). Conclusions and potential for future research are offered in Section 5.

The design of a chicken feather is constituted on hierarchical levels, depicted in Fig. 1 [41], namely the basal calamus, the main structure of the rachis, from which barbs, and then barbules irradiate. Recent studies also emphasized the variable characteristics of feather design between broiler chicken, reared for meat production, and layer chicken, which are intended at egg production instead [42]. This occurs since the feathers have specialized functions, in particular, contour feathers are dedicated to flight, down feathers serve for insulation, and small ornamental ones are aimed at signaling for social activity. The different dimensional levels represented are deemed to constitute the base for the engineering of feathers’ functionality, comprising flight ability and thermal insulation properties [43]. In other words, they are able to transform through a controlled and tailored spiraliform arrangement, a unidimensional appendage into a volumetric body capable of actively sustaining the flight action [44]. In particular, the section of the rachis comprises epicortex, with crossed-fiber architecture, made of β keratin, which offers a trabeculae-like support [45], then cortex and the medullary pith with its foam-like structure [46]. The latter has recently been proposed as to offer some bio-inspired action of thermal insulation due to its cellular geometry [47]. Keratin fibers are arranged in a cross-like architecture being coated with amorphous protein: this disposition is connected to the need to withstand specific forces during flight, mainly by flexural and shear loads, maintaining a sufficient flexibility notwithstanding the required stiffness offered by the protein structure [48]. In particular, an uninterrupted structural connection appears to be formed between the cortex of the rachis and the barbs [49]. The idea is that the rooting of barbs within the rachis is helpful in withstanding the aerodynamic forces during flight [50]. As a matter of fact, the whole structure of the feather optimizes bending stiffness to sustain loading in flight: in that respect, the heavily deformed structure is then recovered by immersion in water, aimed at simulating air moisture effect [51].

At a microscopical level, though, the structure of feather keratin takes the form of microfibrils, less ordered with respect to alpha-keratin, yet still following a composite arrangement such as fiber + matrix [52]. It has been also suggested that the natural model followed during feather development, and that has therefore to be accounted for in the self-assembly process, is that of a simple twisted β-sheet [53], as indicated in Fig. 2, which details the different dimensional levels. Investigations carried out by small angle X-ray scattering (SAXS) have demonstrated the more compact short-range organization of feather keratins with respect to wool keratin [54].

Going more into detail, into the molecular level, keratin is generally rich in cysteine, which creates a strong covalent disulfide bond, which results in crosslinking, promoting therefore an increased hardness of the structures [55]. This ultimately contributes to cornification, hence conversion into a horn structure, of epithelial cells of feathers [56].

Different methods are available to extract keratin from feathers (a summary of the various techniques adopted is offered in Fig. 3), among which this review gives preference to those that are able to preserve its secondary structure, which results also in maintaining its antioxidant potency [57]. A number of chemical methods like reduction [58], ionic liquid [59] and alkaline hydrolysis [60], are used for keratin extraction, but not all of them preserve the secondary structure. The completeness of the secondary structure preservation is difficult to assess, even for recent studies that concentrate on the morphological characteristics of the keratin extracted from feathers the amount of β sheets crosslinked by disulfide bonds was not easily measurable from microscopical observation [61]. More reliable measurements can be offered by quantitative Raman spectroscopy, especially by comparing the intensity of the peaks representing S-S disulfide bridges and β sheets, typically around 521 and 1662 cm⁻¹, respectively [62]. Apart from the extraction process, it is also worth noting that the application of tensile stress on the keratin structure might lead to a more reduced preservation of disulfide bonds, due to their stretching, which might also be ascribed to the agitation process during chemical action [63].

In particular, alkaline hydrolysis, also facilitated by cetrimonium bromide, resulted in a significant disruption of the hydrogen links and in the virtual absence of peptides at 200°C [64]. Often alkaline methods are assisted by microbial and enzymatic ones [65] using keratinases [66]. These can operate autonomously in a biorefinery concept, involving diversification of products and full use of waste resources [67]. This contributes to the complete use of the whole of feather residues, in a circular economy approach, as reported in Fig. 4. Further assistance can be provided by ultrasound irradiation [68], or by microwave treatment [69], where the texture and morphology of extracted material can be controlled by the time and energy of irradiation [70]. In all these cases, the final product yielded are functional protein + hydrolysates of interest for the food industry [71]. Another important factor, which in particular influenced the preservation of the secondary structure, while not directly related with the adopted method, is the retention time in the alkaline solution, since keratin is typically soluble in variable amounts depending on the pH of the relevant environment [72].

Ionic liquids (ILs) method for the extraction preserves the structure of the keratin, at least partially, with yields as high as 45% in turkey feathers [73]. In particular, on chicken feathers, the use of ILs [Amim]Cl, [Bmim]Cl and [Bmim]Br have been studied by Ji et al. [74], demonstrating that the imidazole liquid [Bmim]Cl can extract keratin from feathers, the solvent of IL is non-volatile and easy recyclable. Other hydrophobic IL have been used, such as [HOEMIm] [NTf₂], which offers molecular weights for extracted keratin in the order of 10,000 [75]. Moreover, with extraction using ionic liquids, there is no discharge of pollutants, which goes in agreement with circular economy and eco-friendliness, since the ionic liquid is easily separated from keratin, and it benefits the recycling of the salt, being not detrimental to the quality of extracted keratin [76].

The reduction method that uses mercaptoethanol has a high yield, and preserves the secondary structure of the keratin, yet being harmful for the possible non-preservation of mercaptan in the final product, hence its possible discharge [77]. The reduction with sulfitolysis method, using sodium sulfate or sodium metabisulfite, is the most extensively used for the extraction of keratin due the good yield, low toxicity, and the likely preservation of the secondary structure of the protein [78]. Treatment with thiourea leads also to sufficient self-assembly properties, which are deemed to depend on the preservation of cysteine residues, with possible re-oxidation of free cysteine thiols, specifically responsible for the possible recoiling of the protein: this method has been widely applied on wool keratin [79]. A symptom of the degree of potential self-assembly of extracted keratin is described by the formation of homogeneous gels, normally at rather neutral pH. As a consequence, methods based on sulfitolysis treatment offer larger availability due to the larger presence of sulfato-kerateines, for biomedical applications [80].

Thermal treatments for the extraction of keratin appear to be quicker and are therefore prevalently used in the industrial sector. The steam flash explosion is a process based on a hydrolysis in high temperature and pressure, the exposure of keratin to these conditions allows obtaining the disintegration of the proteins [81], and a significant modification of the secondary structure with an increase of the disorder domains [82]. Furthermore, it can even assist alkaline hydrolysis into increasing the extraction rate of keratin from feathers up to over 65% [83]. However, although β sheets are comparatively more diffuse than α-helical, β-turn, and random coil structures, steam explosion does maintain on the other side a tertiary structure of partially coiled proteins in a generally more polar environment [84]. In this context, the use of supercritical water does also induce an extensive depolymerization of keratin into aliphatic chains [85]: in other contexts, this process appears particularly suitable for the production of bio-hydrogen [86].

Self-assembly of chicken feather extracted keratin, therefore intended as use in biomedical devices, such as for scaffolds and wound dressing, received some degree of attention in recent studies. On the other hand, not many works that resulted in regenerating the disulfide bonds explicitly declared the potential for prospective self-assembly of obtained biomaterials, despite the fact that the attention towards the synthesis of nanoparticles does appear to be gradually increasing. The outcome of this review indicated the reduction extraction processes as the most suitable for the purpose, including those with use of sodium sulfite or mercaptoethanol, or also those with ionic liquids, and even more surprisingly, the extraction of keratin through steam explosion.

A final comment would concern the fact that future developments in this field, also given the very large availability of poultry feather would also be likely to invest in larger value applications for extracted keratin, which will necessarily involve the preservation of secondary structure to enable its self-assembly process in the form of nanoparticles, gels, and blends with biopolymers.