Abstract
As the only component that contacts the ground and rock, the hooves of blue sheep may play a crucial role in their excellent climbing abilities. In this study, we used a combination of techniques, including scanning electron microscopy, infrared spectroscopy and nanoindentation, to characterize the surface morphology, structure, material composition, and mechanical properties of blue sheep hoof and investigate the potential contributions of these properties to the establishment of passive contact stability. Straight and curled microscopic lamellar morphology were found on the hoof surfaces. The cross section of the hoof revealed four layers, and each layer had a unique structure. Finite element analysis was employed to verify that the surface morphology and microstructure effectively contributed to the slip resistance and impact cushioning, respectively. Analyses of the energy and infrared spectra showed that the organic and inorganic substances in different regions of the hoof had similar components but different contents of those components. The hoof was mainly composed of keratin. From the outside to the inside, gradients in both the modulus and hardness were observed. These factors help the hoof alleviate high impact strengths and increase contact stability. These findings further our understanding of the unique mechanism of blue sheep hoof and may help in the development of novel biomimetic materials and mechanical components with enhanced friction and contact stability properties.
Highlights
Terrestrial animals have developed complex feet to enable dynamic locomotion in different environments (Woodward and Sitti, 2018)
Abad et al (2016) proposed a multibody compliant robotic foot inspired by goat hooves and presented analytical and experimental explanations for how the morphological computations performed by the goat hoof significantly reduce slipping on both smooth and rough surfaces
This study experimentally characterized the microstructure, surface morphology, material composition and mechanical properties of blue sheep hooves to analyze the potential contributions of these properties to establishing passive contact stability
Summary
Terrestrial animals have developed complex feet to enable dynamic locomotion in different environments (Woodward and Sitti, 2018). Few studies on how the hooves of blue sheep achieve their biomechanical functions during locomotion have been reported. To the best of our knowledge, only a few studies have analyzed the biomechanical functions of goat hooves, but goats have weaker climbing abilities than blue sheep. Zhang et al (2017) modeled goat hooves as an equivalent mechanism with decoupled flexionextension and lateral movement. The upper part of the equivalent mechanism can flex and extend, while the lower part performs the lateral movement By combining these two parts, the mechanism can adapt to the longitudinal slope (anterior-posterior) and transverse slope (medial-lateral). Based on this model, they explained that the mechanism ensures terrain adaptability by selecting the appropriate configuration for the terrain conditions (Zhang et al, 2017). A recent experimental study of goat hooves found that pore structures with inclination angles of 55◦ existed in the hoof capsule and contributed by cushioning impacts (Tian et al, 2019)
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