Strength, elasticity and the limits of energy dissipation in two related sea urchin spines with biomimetic potential

Abstract

The calcitic spines of the sea urchins Heterocentrotus mamillatus and H. trigonarius are promising role models for lightweight applications, bone tissue scaffolds and energy dissipating processes due to their highly porous and organized structure. Therefore, mechanical properties including Young's Modulus, strength, failure behaviour and energy dissipation efficiency have been investigated in depth with uniaxial compression experiments, 3-point bending tests and resonance frequency damping analysis. It was found that despite a very similar structure, H. trigonarius has a significantly lower porosity than H. mamillatus leading to a higher strength and Young's Moduli, but limited ability to dissipate energy. In order to show reliable energy dissipation during failure in uniaxial compression, a transition porosity of 0.55-0.6 needs to be exceeded. The most effective structure for this purpose is a homogeneous, foam-like structure confined by a thin and dense shell that increases initial strength and was found in numerous spines of H. mamillatus. Sharp porosity changes induced by dense growth layers or prominent wedges of the spines' radiating building principle act as structural weaknesses, along which large flakes can be spalled, reducing the energy dissipation efficiency considerably.The high strength and Young's Modulus at the biologically necessary high porosity levels of the spines is useful for Heterocentrotus and their construction therefore remains to be a good example of biomimetics. However, the energy dissipative failure behaviour may be regarded as a mere side effect of the structure.

High strength and damage-tolerance in echinoderm stereom as a natural bicontinuous ceramic cellular solid

“…2e ). A slightly lower energy absorption has been observed for spine samples with growth rings, which resulted from a greater reduction in plateau stress due to early spallation from growth rings 53 , 54 (Supplementary Fig. 15 ).…”

Hexagonal Voronoi pattern detected in the microstructural design of the echinoid skeleton

“…Porosity is usually highly variable in the skeletal components of echinoids, e.g. ossicles of the buccal apparatus and spines [ 48 , 54 , 55 , 106 ]. Variation in porosity creates distinct structural units contributing to the mechanical strength of the skeleton.…”

“…Regularly ordered pore structures reported here represent structural solutions known to be mechanically efficient when loads are applied from predictable directions [ 107 ]. Recently, these structures have received increasing attention due to their potential application in the fields of electronics, photonics, and material sciences [ 106 – 109 ]. In the case of the sea urchin, TB region, loads and resultant stresses are typically predictable since they are correlated with regularly organized collagenous fibers that penetrate the pore space and spine locking mechanism [ 46 , 47 , 56 , 74 – 76 , 97 , 98 ].…”

Mechanical defensive adaptations of three Mediterranean sea urchin species

“…Variations of the porosity of the material also seem to play a key role in its mechanical properties (elasticity, hardness, etc. ; other factors like the stereom architecture may further affect these properties; Lauer et al, 2018 ).…”