Micromechanics of the equine hoof wall: optimizing crack control and material stiffness through modulation of the properties of keratin

ABSTRACT Small-scale components of the equine hoof wall were tested to determine their mechanical roles in the morphological hierarchy. Macroscale tensile tests conducted on samples of the inner wall tubules and intertubular material showed a sixfold difference in mean initial stiffnesses (0.47 and 0.08 GPa, respectively), indicating that the inner wall tubules stiffen the wall along its longitudinal axis. The similarity in material properties of tubule and intertubular samples from the mid-wall suggests that tubules in this region offer only minor reinforcement along the longitudinal axis. Microscale tests conducted on rows of keratin strands from the inner wall tubules and intertubular material, and on intertubular keratin strands of the mid-wall, produced estimates of the stiffnesses of the hydrated matrix (0.03 GPa) and intermediate filament (IF; 3–4 GPa) components of the nanoscale (α-keratin) composite. The results from these tests also suggest that the properties of the keratin composite vary through the wall thickness. Birefringence measurements on inner wall and mid-wall regions agree with these observations and suggest that, although the keratin IF volume fraction is locally constant, the volume fraction changes through the thickness of the wall. These findings imply that modulation of the hoof wall properties has been achieved by varying the IF volume fraction, countering the effects of specific IF alignments which serve another function and would otherwise adversely affect the modulus of a particular region.