Phase transition-induced elasticity of α-helical bioelastomeric fibres and networks

Abstract

Natural elastomeric fibres play central structural and functional roles in a variety of tissues produced by many organisms from diverse Phyla. Most of these fibres feature amorphous structure and their long-range elastic response is well described within the framework of entropic (rubber-like) elasticity. Recently, it has been recognized that long-range reversible deformation can also occur in biomacromolecular fibres or networks that feature significant secondary structure and long-range order. Their elastomeric response is then associated with conformational changes of the backbone of the constitutive protein-based polymers. Under axially imposed loads, several groups of proteins whose structure is dominated by α-helical coiled-coil structures can undergo unfolding transitions and secondary structure transformations, for example from coiled-coil α-helices to β-sheet strands. In contrast to rubber-like biopolymers, the retractive elastic force in these biomacromolecular materials is not dominated by a return to a maximum entropic state, but is mostly the result of variations in internal energy associated with the conformational changes. Here, a review of α-helix based elastomeric materials is presented that encompasses examples and experimental evidence across multiple length scales, from the molecular to the macroscopic scale. We begin by summarizing the basic thermodynamic formalism of thermoelasticity. While this formalism is well established for amorphous (entropically-dominated) fibres under tensile loading, its extension towards conformational (internal energy-dominated) elasticity is less known. Recent experimental evidence as well as corroborating computer simulations are then reviewed and discussed in the light of secondary structure and nano-scale features of these biopolymers. Comparisons are also drawn with physiologically important structural fibres that share common characteristics at the molecular and the nano-scale, including intermediate filament (IF) proteins from the cell cytoskeleton, myosins from motor proteins, and fibrin from blood clot. We conclude with a discussion on future directions and opportunities for these materials from a biomimetics engineering perspective.

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

“…The whelk egg capsule is a robust proteinaceous biopolymer, and studies have found that it possesses an a-helical coiled-coil structure (wide-angle X-ray scattering, sequencing of proteins) and shows multiple laminate sheets of ordered fibrils with periodicities 50 nm [219,222,223], which is structurally analogous to the a-keratinous materials [18,224]. At the nanoscale, the capsule wall shows distinct structural features: it consists of cross-plied or slightly misoriented sheets of protein fibrils (Fig.…”

“…internal energy driven rather than entropy) are assessed [116]. Other coiled coil proteins, such as myosin II and possibly fibrin, also show this transition [18].…”

“…However, there have only been very few reports on keratin in terms of biological and structural features, e.g. the classic books including Mercer [15] and Fraser et al [16] on keratins and Feughelman [17] on a-keratin, and two review papers covering their structure, mechanical properties [12] and phase transition-induced elasticity of a-helical bioe-lastomers [18]. Here, our aim is to provide a present-day comprehensive review of keratins and keratinous materials, incorporating biological and materials science perspectives to illustrate the structural designs and functional properties in order to stimulate the development of novel bioinspired keratin-based designs.…”

A silk purse from a sow's ear—bioinspired materials based on α-helical coiled coils

“…The importance of the pseudo-hexagonal packing of the a-keratin fibres and how this contributes to the scaling (nanometre to millimetre) and composite potential of keratocyte akeratin materials is now realized [6 ,7 ]. The molecular organization of wool makes it a wonderful textile as it exemplifies phase transition (a-helix to b-sheet [8]) induced elasticity of its component keratin fibres when they are axially stretched [9]. This was first observed and documented by Astbury [4] and subsequently studied in more detail to reveal the importance of coiled-coil sliding and disulphide bonding to the biomechanical properties of hair [10,11].…”

Mechanics of Vascular Smooth Muscle

“…An ideal elastomer is fully entropic (f em = f s ) because f ie = 0, and a predominantly entropic elastomer is defined as f ie /f em < 0.5. This ratio for elastin is 0.26 and for natural rubber it is 0.18 (316,464). Gravity, in the form of a weight, can be used to stretch a tissue in vitro but, in this case, gravity acts as an aid to examine the net electromagnetic force within the tissue.…”