Regulation of hard a-keratin mechanics via control of intermediate filament hydration: matrix squeeze revisited

Mammalian hard a-keratins are fibre-reinforced biomaterials that consist of 10 nm intermediate filaments (IFs) embedded in an elastomeric protein matrix. Recent work suggests that the mechanical properties of IFs are highly sensitive to hydration, whereas hard a-keratins such as wool, hair and nail are relatively hydration insensitive. This raises the question of how mammalian keratins remain stiff in water. The matrix squeeze hypothesis states that the IFs in hard a-keratins are stiffened during an air-drying step during keratinization, and subsequently locked into a dehydrated state via the oxidation and cross-linking of the keratin matrix around them. The result is that even when hard a-keratins are immersed in water, their constituent IFs remain essentially ‘dry’ and therefore stiff. This hypothesis makes several predictions about the effects of matrix abundance and function on hard a-keratin mechanics and swelling behaviour. Specifically, it predicts that high matrix keratins in water will swell less, and have a higher tensile modulus, a higher yield stress and a lower dry-to-wet modulus ratio. It also predicts that disruption of the keratin matrix in water should lead to additional swelling, and a drop in modulus and yield stress. Our results are consistent with these predictions and suggest that the keratin matrix plays a critical role in governing the mechanical properties of mammalian keratins via control of IF hydration.