A natural energy absorbent polymer composite: The equine hoof wall

Wei Huang, Nicholas A. Yaraghi, Wen Yang, Alexis Velazquez-Olivera, Zezhou Li, Robert O. Ritchie, David Kisailus, Susan M Stover, Joanna McKittrick

Research output: Contribution to journalArticle

Abstract

The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60% compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure. Statement of Significance: The equine hoof wall is found to be an efficient energy absorbent natural polymer composite. Previous studies showed the microstructure and mechanical properties of the hoof wall in some perspective. However, the hierarchical structure of equine hoof wall from nano- to macro-scale as well as the energy absorption mechanisms at different strain rates and loading orientations remains unclear. The current study provides a thorough characterization of the hierarchical structure as well as the correlation between structure and mechanical behaviors. Energy dissipation mechanisms are also identified. The findings in the current research could provide inspirations on the designs of impact resistant and energy absorbent materials.

Original languageEnglish (US)
JournalActa Biomaterialia
DOIs
StatePublished - Jan 1 2019

Fingerprint

Hoof and Claw
Energy absorption
Keratin
Horses
Polymers
Keratins
Composite materials
Strain rate
Natural polymers
Mechanical properties
Water absorption
Buckling
Macros
Intermediate Filaments
Energy dissipation
Reinforcement
Compaction
Hardness
Stiffness
Crystalline materials

Keywords

  • Energy absorption
  • Equine hoof
  • Keratin
  • Mechanical properties
  • Tubular structure

ASJC Scopus subject areas

  • Biotechnology
  • Biomaterials
  • Biochemistry
  • Biomedical Engineering
  • Molecular Biology

Cite this

Huang, W., Yaraghi, N. A., Yang, W., Velazquez-Olivera, A., Li, Z., Ritchie, R. O., ... McKittrick, J. (2019). A natural energy absorbent polymer composite: The equine hoof wall. Acta Biomaterialia. https://doi.org/10.1016/j.actbio.2019.04.003

A natural energy absorbent polymer composite : The equine hoof wall. / Huang, Wei; Yaraghi, Nicholas A.; Yang, Wen; Velazquez-Olivera, Alexis; Li, Zezhou; Ritchie, Robert O.; Kisailus, David; Stover, Susan M; McKittrick, Joanna.

In: Acta Biomaterialia, 01.01.2019.

Research output: Contribution to journalArticle

Huang, W, Yaraghi, NA, Yang, W, Velazquez-Olivera, A, Li, Z, Ritchie, RO, Kisailus, D, Stover, SM & McKittrick, J 2019, 'A natural energy absorbent polymer composite: The equine hoof wall', Acta Biomaterialia. https://doi.org/10.1016/j.actbio.2019.04.003
Huang W, Yaraghi NA, Yang W, Velazquez-Olivera A, Li Z, Ritchie RO et al. A natural energy absorbent polymer composite: The equine hoof wall. Acta Biomaterialia. 2019 Jan 1. https://doi.org/10.1016/j.actbio.2019.04.003
Huang, Wei ; Yaraghi, Nicholas A. ; Yang, Wen ; Velazquez-Olivera, Alexis ; Li, Zezhou ; Ritchie, Robert O. ; Kisailus, David ; Stover, Susan M ; McKittrick, Joanna. / A natural energy absorbent polymer composite : The equine hoof wall. In: Acta Biomaterialia. 2019.
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abstract = "The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60{\%} compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure. Statement of Significance: The equine hoof wall is found to be an efficient energy absorbent natural polymer composite. Previous studies showed the microstructure and mechanical properties of the hoof wall in some perspective. However, the hierarchical structure of equine hoof wall from nano- to macro-scale as well as the energy absorption mechanisms at different strain rates and loading orientations remains unclear. The current study provides a thorough characterization of the hierarchical structure as well as the correlation between structure and mechanical behaviors. Energy dissipation mechanisms are also identified. The findings in the current research could provide inspirations on the designs of impact resistant and energy absorbent materials.",
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