Reverse engineering of a wool fibre to mimic the structural hierarchy of a gecko's foot.

The adhesion generated by a gecko's foot is realised by a structural hierarchy that is also present inside the cortex of a wool fibre. Both structures are based on the same fibril building blocks that belong to the-keratin family. We show here that this hierarchical structure can be released from a Merino wool fibre with a combination of formic acid refluxing with agitation and trypsin digestion with ultrasonication. Thus, the cuticle scales are shown to be removed from wool yarns by mass-loss, FTIR spectroscopy and SEM followed by the breakdown of the cortex to release macrofibrils at the surface of the remaining yarn. SEM and AFM evidence are presented for the exposure of macrofibrils at the surface of cross-sections of descaled, fibrillated wool fibres. Adhesion measurements in the AFM show that regions of the treated wool have high adhesion, up to 58 nN, consistent with exposure of nanoscale macrofibrils. This exposure is not however homogeneous across the entirety of the cross-sectioned surface of a yarn and further digestion is required to optimise the depth profile of the exposure for direct comparison with the macroscale compliance and adhesion of a gecko's foot. Nonetheless, the current work has developed an experimental route to reserve engineer wool back to sub-unit macrofibrils, in order to replicate the format and to some extent the adhesive properties of a gecko's hierarchal foot structure.