Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

Abstract

a b s t r a c tA ubiquitous biological material, keratin represents a group of insoluble, usually high-sulfur content and filament-forming proteins, constituting the bulk of epidermal appendages such as hair, nails, claws, turtle scutes, horns, whale baleen, beaks, and feathers. These keratinous materials are formed by cells filled with keratin and are considered 'dead tissues'. Nevertheless, they are among the toughest biological materials, serving as a wide variety of interesting functions, e.g. scales to armor body, horns to combat aggressors, hagfish slime as defense against predators, nails and claws to increase prehension, hair and fur to protect against the environment. The vivid inspiring examples can offer useful solutions to design new structural and functional materials.Keratins can be classified as a-and b-types. Both show a characteristic filament-matrix structure: 7 nm diameter intermediate filaments for a-keratin, and 3 nm diameter filaments for b-keratin.Both are embedded in an amorphous keratin matrix. The molecular unit of intermediate filaments is a coiled-coil heterodimer and that of b-keratin filament is a pleated sheet. The mechanical response of a-keratin has been extensively studied and shows linear Hookean, yield and post-yield regions, and in some cases, a high reversible elastic deformation. Thus, they can be also be considered 'biopolymers'. On the other hand, b-keratin has not been investigated as comprehensively. Keratinous materials are strain-rate sensitive, and the effect of hydration is significant.Keratinous materials exhibit a complex hierarchical structure: polypeptide chains and filament-matrix structures at the nanoscale, Progress in Materials Science 76 (2016) 229-318 Contents lists available at ScienceDirect Progress in Materials Sciencej o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / p m a t s c i organization of keratinized cells into lamellar, tubular-intertubular, fiber or layered structures at the microscale, and solid, compact sheaths over porous core, sandwich or threads at the macroscale. These produce a wide range of mechanical properties: the Young's modulus ranges from 10 MPa in stratum corneum to about 2.5 GPa in feathers, and the tensile strength varies from 2 MPa in stratum corneum to 530 MPa in dry hagfish slime threads. Therefore, they are able to serve various functions including diffusion barrier, buffering external attack, energy-absorption, impact-resistance, piercing opponents, withstanding repeated stress and aerodynamic forces, and resisting buckling and penetration.A fascinating part of the new frontier of materials study is the development of bioinspired materials and designs. A comprehensive understanding of the biochemistry, structure and mechanical properties of keratins and keratinous materials is of great importance for keratin-based bioinspired materials and designs. Current bioinspired efforts including the manufacturing of quill-inspired aluminum composites, animal horn-inspired SiC composites, and feather-i...

Highly polydisperse keratin rich nanofibers: Scaffold design and in vitro characterization

“…In order to overcome these limitations, collagen or its denatured protein-that is, gelatin-have been used in the crosslinking form, 2 or in combination with biodegradable polymers including poly(lactic acid), 3,4 poly(glycolic acid) 5 and their copolymer poly(lactic-co-glycolic acid) (PLGA), 6 and poly(ε-caprolactone) (PCL) 7,8 to improve the in vitro and in vivo biomechanical stability. 20,21 Recent works demonstrated that keratin-based materials may be easily designed in the form of films, 22 hydrogels, 23,24 sponges, 25 fibers, 26 without altering the natural pathway of amino acid sequences, similarly to those required to promote cell adhesion in ECM. For instance, silk-based materials have been largely considered for their capability to improve proliferation also supporting mechanical properties to the scaffold.…”

“…9,10 In this context, alternative proteins have been more recently tested alone or in combination with other polymers to design protein-based fibrous scaffolds with peculiar functionalities for tissue engineering applications. 21,27 In particular, wool keratin is biocompatible, biodegradable, nonimmunogenic, does not induce inflammatory response and promotes wound healing by stimulating human keratinocytes migration. [11][12][13] Moreover, silk fibroin from Bombix mori has been frequently preferred to the collagen for the fabrication of fibrous scaffolds, 14,15 not being extracted from animals.…”

Water adsorption with relative humidity changes for keratin and collagen as studied by infrared (IR) micro‐spectroscopy

“…α keratin is the main constituent of hair, nail, and skin surface (stratum corneum) . Its structure is mainly in the form of α helix by hydrogen bonding among peptide bonds.…”

Study on Obtaining Keratin Extracts from Leather Industry By-products

“…Wool is a keratinous material with specific structure, mechanical behaviour and physical-chemical properties (Wang et al, 2016). A clean wool fibre contains approximately 82% keratinous protein with high concentration of cysteine, approximately 17%, a protein material with a low cysteine content called "nonkeratinous material" mainly localized in the complex of the cell membrane and approximately 1% of the non-protein material is made up of waxy lipids, plus a small amount of polysaccharide material (Lewis and Rippon, 2013).…”