Impact resistant and energy absorbent natural keratin materials: horns and hooves
Keratin is one of the most common structural biopolymers with very high strength and toughness but relatively low density, found in various tissues such as hairs, feathers, horns and hooves. The current study focuses on the impact resistant and energy absorbent properties of two representative keratin tissues: horns and hooves. Bighorn sheep (Ovis canadensis) rams hurl themselves at each other at speeds of ~9 m/s (20 mph) to fight for dominance and mating rights. This necessitates impact resistance and energy absorption abilities, which stem from material-structure components in horns. The equine hoof is also considered as an efficient energy absorption layer that protects the bony skeleton from impacts when the horse is galloping. Keratinized tissues are permanent tissues that are not able to remodel or regrow once broken or damaged. This is a severe challenge to horns and hooves that are without self-healing mechanisms but are under high risk from various loadings. Although it is composed of dead cells, the horn keratin is able to recover from severe compression in the radial direction by hydration.
In this thesis work, the hierarchical structure, as well as correlations between the structure and mechanical properties in both horns and hooves were investigated. Structural characterization techniques such as optical and electron microscopy, synchrotron X-ray computed tomography, wide angle X-ray diffraction (WAXD) were applied to reveal the structural features from the molecular scale to macro scale. Compressive properties at quasi- to high- strain rates were characterized to identify the energy absorption performance under different loading orientations and hydration states. In-situ compression tests under synchrotron X-ray computed tomography were performed to further analyze the energy dissipation mechanisms. Tensile tests were conducted to determine the behavior of the crystalline intermediate filaments and amorphous keratin phases. It was found that although both horn and hoof had tubular structures, significant differences were noticed in terms of tubular compositions, shapes, sizes and keratin cell arrangements. The mechanical properties and energy absorption mechanisms were also different due to the structural differences. The microstructure and mechanical properties of horns from four representative ruminant species: the bighorn sheep (Ovis canadensis), domestic sheep (Ovis aries), mountain goat (Oreamnos americanus) and pronghorn (Antilocapra americana), were studied, aiming to understand the relation between evolved microstructures and mechanical properties. Differences were found in the mechanical properties of these four species, which was partially contributed by the structural differences. The differences in mechanical properties among species may relate to their different fighting behaviors. Water-assisted recovery mechanism in bighorn sheep horn keratin was also investigated. It was found that recovery can occur when the amorphous matrix absorbs water and reorganize the amorphous phases. Damages to the cells and fiber breakages cannot be recovered. This recovery and remodeling mechanism is anisotropic and totally different with living tissue such as bones, which could give inspiration for recoverable energy absorbent materials designs. Finally, 3D printing was applied to mimic the tubular and lamellar structure. The lamellar and tubular structures printed out with VeroClear and Tangoblackplus materials showed promising impact resistant and energy absorbent properties.