Keratin: dissolution, extraction and biomedical application

TLDR: This review discusses the various methods available for the dissolution and extraction of keratin with emphasis on their advantages and limitations, and reports the properties of various keratin-based biomaterials and critically examines how these materials are influenced by the keratin extraction procedure.

Abstract: Keratinous materials such as wool, feathers and hooves are tough unique biological co-products that usually have high sulfur and protein contents A high cystine content (7–13%) differentiates keratins from other structural proteins, such as collagen and elastin Dissolution and extraction of keratin is a difficult process compared to other natural polymers, such as chitosan, starch, collagen, and a large-scale use of keratin depends on employing a relatively fast, cost-effective and time efficient extraction method Keratin has some inherent ability to facilitate cell adhesion, proliferation, and regeneration of the tissue, therefore keratin biomaterials can provide a biocompatible matrix for regrowth and regeneration of the defective tissue Additionally, due to its amino acid constituents, keratin can be tailored and finely tuned to meet the exact requirement of degradation, drug release or incorporation of different hydrophobic or hydrophilic tails This review discusses the various methods available for the dissolution and extraction of keratin with emphasis on their advantages and limitations The impacts of various methods and chemicals used on the structure and the properties of keratin are discussed with the aim of highlighting options available toward commercial keratin production This review also reports the properties of various keratin-based biomaterials and critically examines how these materials are influenced by the keratin extraction procedure, discussing the features that make them effective as biomedical applications, as well as some of the mechanisms of action and physiological roles of keratin Particular attention is given to the practical application of keratin biomaterials, namely addressing the advantages and limitations on the use of keratin films, 3D composite scaffolds and keratin hydrogels for tissue engineering, wound healing, hemostatic and controlled drug release

Figures

Table 4 The effect of different temperatures on the physicochemical properties of the keratin fibre

Fig. 4 Schematic diagram of the sulfitolysis reaction that breaks the strong disulfide bonds of the keratin fibre.

Fig. 10 Effect of different plasticizers and crosslinking agents on the mechanical properties of the keratin film.

Fig. 5 Schematic structures of three major ionic liquids that have been widely used for the dissolution of keratin fibres. 1-Butyl-3-methylimidazolium chloride [Bmim]Cl, 1-allyl-3-methylimidazolium chloride [Amim]Cl, and 1-butyl-3-methylimidazolium bromide [Bmim]Br.

Table 9 Application of synthetic polymers with keratin for electrospinning

Table 2 Characteristics of some of the keratin extraction conditions using the sulfitolysis method

Frequently asked questions

Q1. How did Zhang and his team achieve the solubility of keratin samples?

In order to enhance the solubility of the exploded keratin samples, Zhang et al., in a subsequent study, assisted the SFE with an alkaline method, using sodium hydroxide solution to dissociate the hydrogen bonds and to introduce electrostatic repulsion. 

Q2. What is the main aim of many studies investigating the extraction of keratin from wool?

23 Obtaining an undegraded protein and high yield has been the major aim of many studies investigating the extraction of keratin from wool. 

Q3. What is the reason why the yield of keratin from feathers decreased?

Wang and Cao67 observed that when the temperature was over 90 °C, the yield of keratin from feathers decreased markedly which could be due to the scission of the peptide bond at the higher temperature. 

Q4. What is the common method used to break the cystine disulfide bonds?

Isolation of keratin from wool by the reduction method using reducing agents, such as thiols (e.g. mercaptoethanol), has been the most reported technique to break the cystine disulfide bonds (R–S–S–R), and the formation of cysteine (R–S–H). 

Q5. Why are proteins becoming popular for drug delivery?

In addition to their low toxicity, safety, and high abundance, proteins are gaining wide interest for drug delivery due to their technical unique properties. 

Q6. What is the main drawback of performic acid extraction from wool?

80 Despite the water solubility and relatively easier process of keratin extraction from wool using the oxidation method compared to other available methods, the partial oxidation of cystine to cysteic acid by peracetic or performic acid is a major drawback. 

Q7. What is the reason why the regenerated keratin had lower thermal stability than natural?

A thermogravimetric analysis (TGA) showed that the regenerated keratin had slightly lower thermal stability compared to natural wool,61 which could be due to the high crystallinity of natural wool along with its higher molecular weight compared to the regenerated keratin. 

Q8. What did the authors believe was a catalyst for the oxidation of cystine?

The authors believed that the Schweitzer’s reagent acted as a catalyst to facilitate the oxidation of cystine and its subsequent conversion to the cystine residue. 

Q9. What is the effect of using ionic liquids as a co-solvent?

In addition to the ability of using ILs as a pure solvent, these salts can be used as a co-solvent in aqueous systems or in biphasic systems. 

Q10. What are the main methods used to solubilise keratin from wool?

The major methods used to solubilise and isolate keratin from keratin-rich materials are reduction,14 oxidation,15 microwave irradiation,16 alkali extraction,17 steam explosion,18 sulfitolysis19 and ionic liquids20 (Fig. 2). 

Q11. Why was the yield of keratin decreased by increasing the concentration beyond 0.2 M?

the yield and molecular weight of keratin were decreased by increasing the concentration beyond 0.2 M, which can be due to the degradation of keratin and the permeation of the low molecular weight species through dialysis. 

Q12. What is the effect of the keratin solution on the porous structure of the composite?

the pH and concentration of the keratin solution, the presence of a cross-linker, plastisizer, or incorporation of other natural or synthetic polymers into the keratin matrix can also have an impact on the porous structure of the composite, in a way analogous to the described above for films. 

Q13. What is the reason why the keratin product might not be suitable for feed and pharmaceutical?

the final keratin product might not be suitable for feed and pharmaceutical applications due to the presence of copper, which hinders the use of this method commercially. 

Q14. What are the challenges associated with the disposal and management of wool?

With the exception of good quality wool that is used in garments and rugs, there are challenges associated with the disposal and management of these materials. 

Q15. What is the reason why the tensile strength of the composite was not reported?

tensile strength values for non-copolymerized films were not reported and therefore, it is hard to relate the observed improved tensile strength to polymerization, which could be due to the addition of gelatin. 

Q16. How much alkali is needed to extract keratin from wool fibre?

It has been shown that there is a direct relationship between the solubility of keratin and the alkali concentration up to 15% alkali concentration, after which a further increase in the concentration of the alkali will increase the strength of wool fibre, e.g. 38% NaOH increased the strength of the wool fibre by 30% more than the original fibre strength.40