Tubular structure in the equine hoof wall. Modulus and hardness mapping via nanoindentation shows a higher stiffness in the tubular areas. Energy absorption by buckling and cracking of the tubules are observed by in-situ synchrotron X-ray computed tomography compression tests.Horses hooves and sheep horns have to withstand large, sustained impacts. When bighorn sheep clash horns, for example, the forces can rival those experienced during a car crash. Horses hooves have to protect the animal’s skeletal structure from repeated collisions with the ground when trotting or galloping.
“There is an urgent need for lightweight energy absorbent and impact resistant materials in automotive and aerospace engineering,” points out researcher Joanna McKittrick. “By looking into nature, bioinspired designs based on the hoof could result in new materials and structures that have superior energy absorption capabilities.”
Horses’ hooves, along with bighorn sheep horns, hair, nails, claws, beaks, wool, and scales, are made from the protein keratin. Keratin comes in two crystalline forms, alpha and beta, which are found in mammals and reptile and avian species, respectively, and an amorphous form. At the nanoscale, the hoof is made up of tiny crystalline filaments of keratin dimers embedded in a sulfur-rich amorphous keratin matrix. These tiny fibers clump together to form larger, disk-shaped keratinized cells that are, in turn, stacked together in a layered structure.
“In our study, we used different characterization techniques such as electron microscopy and synchrotron micro-computed tomography to uncover the hierarchical structure of hoof,” explains McKittrick, who led a team of scientists at the University of California at San Diego, Riverside, Berkeley and Davis.
The amount and orientation of filaments at the nanoscale varies in different keratins, producing different properties at the macroscale. In structural materials like horns and hooves, the lamellar structure also contains micrometer-sized tubules.
“We found that the tubules act as a reinforcing ‘fiber’ in the hoof and are stiffer than the surrounding area,” says McKittrick. “The difference in strength between the tubular and intertubular areas arises from the different ratio of the crystalline to amorphous phase.”
The tubular structure acts as the energy absorber, buckling and rebounding under impact. This crumpling and cracking protects the whole hoof from catastrophic failure. The researchers found that the hoof can absorb over 2.5 times more energy than the horn of bighorn sheep, which is one of the most studied high-strength keratin materials.
The properties of the hoof depend markedly on the level of hydration, the researchers also found. High levels of hydration soften the tubules, reducing the material’s ability to absorb energy as effectively.
“The reinforced tubular structure at the microscale and the materials design at the molecular scale give us inspiration for new designs of energy absorbent synthetic materials,” says McKittrick. “We are currently applying additive manufacturing to mimic the tubular structures and aim to fabricate prototypes that are lightweight and have excellent energy absorption capabilities.”
Huang et al. Acta Biomaterialia (2019), https://doi.org/10.1016/j.actbio.2019.04.003