Worldwide, poultry meat processing industry produces several million tons of waste feathers every year [1]–[3]. The feathers, which are natural fibrous material, are considered waste and only small amounts are processed into valuable products such as feather meal for livestock and fertilisers. The remaining waste is disposed of by incineration or by entombing [3].
Keratin, comprising up to 90% of chicken feathers, is a promising protein reservoir. Recently, keratin has gained a lot of attention due to its remarkable biocompatibility and biodegradability characteristics [4]. However, it is still challenging to recycle feathers.
Until now, most technologies for recycling keratin from biomass waste have been harmful, expensive, often toxic and difficult to handle [5]–[10]. Keratin can be extracted using several methods such as chemical hydrolysis, enzymatic and microbial treatment, dissolution in ionic liquids, microwave technique, steam explosion technique and thermal hydrolysis or superheated process [4]. However, these conventional techniques have several weaknesses. High temperatures used in chemical hydrolysis tend to destroy the amino acids [11]. Enzymatic hydrolysis, which is considered to be environmentally safe and cost-effective, is carried out under the influence of certain reducing agents which can destroy keratin’s disulphide bonds [12]. Dissolution in ionic liquids and microwave irradiation give low yield [12]. These procedures are primarily time-consuming and have hardly changed over the decades despite considerable technological progress. In addition, the conventional technologies currently used for keratin extraction generally have limited solubility and result in a lower molecular weight.
Due to the various weaknesses of existing technologies, it is crucial to develop new and more acceptable keratin extraction technologies that preserve the molecular weight of keratin and improve its functionalities. In this way, subcritical water (SubCW) has been gaining increasing attention as both an environmentally friendly solvent and attractive reaction medium for a variety of applications. It is cheap, non-toxic, non-flammable, non-explosive, and offers essential advantages compared to other chemicals, particularly in the field of “green chemistry” [13]. In our previous work, SubCW was successfully applied for the extraction of keratin from keratin-rich substrates such as waste wool and poultry feathers, demonstrating its versatility [14], [15]. However, systematic research on the applicability of keratin isolated by hydrothermal processing has not yet been extensively investigated. The present study therefore extends this research beyond the current state of the art, particularly with regard to the subsequent formation of keratin- based bioparticles while maintaining high yields and preserved functional properties.
Importantly, keratin is a multipurpose biopolymer that has been widely used in the production of fibrous composites, and with necessary modifications, it can be developed into gels, films, nanoparticles, and microparticles [4]. The global market for keratin materials and products (hair beauty products, cosmetics, personal care products, medical products, food and beverages and other consumer goods) is booming in terms of share, size, and growth. Consequently, the keratin market is expected to grow at a growth rate of 7.30% during the forecast period from 2022 to 2029 and is expected to reach USD 1.96 billion by 2029 [16].
The complexation capability of keratin with various compounds has been studied in the context of biomedical and cosmetic applications, introducing several new functionalities. In recent years, the synthesis of keratin in combination with other compatible materials has been the subject of intensive research. The existing literature mainly deals with the blending or grafting of keratin with non-biodegradable synthetic polymers, including polyethylene (PE), polypropylene (PP), polystyrene and poly(vinyl chloride) [17]. Conversely, increasing emphasis is being placed on the use of natural polymers, particularly in response to the environmental problems associated with plastics. Consequently, the combination of keratin with other natural polymers, such as polysaccharides, is becoming increasingly important as it has the potential to improve properties without compromising the desired biocompatibility [12], [17], [18]. Complexation is emerging as a key mechanism in the modification of polymers, helping to improve or introduce new properties, such as higher molecular weight, thermal properties, surface properties, biocompatibility and biodegradability—properties that individual homogeneous materials cannot confer independently [19]. Several researchers have shown that keratin can form complexes with chitosan, alginate, and sodium tripolyphosphate (TPP), each with its own unique characteristics [20]. Zhai et al. demonstrated the use of keratin-chitosan blended nanocomposite scaffolds for tissue engineering applications [21]. Chitosan is a positively charged polymer that can form complexes with keratin via interfacial complexation, resulting in the formation of microcapsules. This interaction is based on opposite charge interactions between keratin and chitosan, leading to the formation of polyelectrolyte complexes [22].
Alginate, a negatively charged natural polymer, can also form complexes with keratin. This has been studied in the context of hydrogel preparation for cosmetic and biomedical applications [23]. The complexation is based on the ionic interaction between the amide networks of keratin and alginate, leading to the formation of dual crosslinked fibres [24].
A number of studies conducted in the last few years have concentrated on creating biocompatible and biodegradable nanocarriers through complexes among biopolymers and could be useful for a variety of biomedical applications, such as imaging and diagnosis as well as medication delivery systems [25]–[30]. Due to their abundance, biodegradability, low immunogenicity, non-toxicity and biocompatibility, keratin has been used to create nanoparticles that encapsulate and sequester drugs before releasing it [31]. These nanoparticles interact with drugs via electrostatic interactions, hydrogen bonding, disulfide bond formation, or chemical conjugation providing long-term, sustained release of the drug from its carrier [32].
In drug delivery systems, keratin is often blended with other biopolymers to enhance its properties and performance. Alginate-keratin blend systems have triggered great interest in recent advances in the field of biomedical applications, including drug delivery systems, due to their favourable properties and potential for various medical applications [25]. The use of alginate in blends with keratin for drug delivery systems can enhance the properties and performance of the resulting materials, making them suitable for various drug delivery applications. The biocompatibility, mucoadhesiveness, swelling capacity, sol/gel transition and improved mechanical properties of alginate make it a promising biopolymer for drug delivery applications [33], [34].
Also, chitosan has been used in blending with keratin for drug delivery systems. According to a study by Wilson et al. [22], keratin and chitosan electrostatic interactions result in the production of stable microcapsules, which suggests potential uses in biomedicine. Keratin-based nanoparticles, including those blended with chitosan, have demonstrated their potential for targeted drug delivery and other biomedical applications [32].
To our knowledge, direct research on the complexation of keratin with tripolyphosphate (TPP) remains scarce, with existing literature primarily focusing on TPP's utilization in complexes with chitosan and alginate [16]. In our study, we delve into uncharted territory, investigating the potential of recycled keratin extracted with subcritical water to form complexes with TPP that potentially results in nano/microparticles. Additionally, we aim to conduct comparative analyses with keratin-alginate and keratin-chitosan particles, formed directly without additional crosslinking agents. Notably, while TPP has already been successfully employed in nanoparticles alongside chitosan for antibiotic drug delivery [22], the specific examination of keratin-tripolyphosphate particles for drug delivery systems has not yet been explored, which highlights the novelty of the present study. This endeavour promises to shed light on innovative avenues for utilizing keratin in biomedical applications.
Overall, the utilization of recycled keratin for formulating bioactive nano/micro-particles offers a sustainable, cost-effective, biocompatible, and versatile approach for drug delivery systems with potential benefits for both patients and the environment.