Structure-Property Relationships in Sea Urchin Spines and Implications for Technical Materials

Abstract

Sea urchin spines have been studied for numerous reasons including their crystallographic and chemical composition, their aesthetic appearance and their enigmatic growth at ambient conditions. Depending on the species, sea urchins use their spines for protection against predators, for burial in the substrate, for locomotion and for withstanding wave energy by wedging into reef cervices. Hence, sea urchin spines are in most cases optimized for bearing load. This study deals with the mechanical properties of the unique spines of Heterocentrotus mamillatus, a large Indo-Pacific Echinoid. They consist as all skeletal elements of Echinoids of Mg-calcite arranged in a porous meshwork (stereom) with very little organic material incorporated (<0.5 wt%). By the overall porosity of 0.6-0.7 their density is similar to sea water and the large and thick spines are not a burden to carry. These properties make the spines of H. mamillatus a promising biomimetic role model for high performance, intelligently structured, lightweight ceramics. Since biological role models are usually a lot smaller than the technical application they inspire, the question of how properties change with an increase in size, is intimately linked to biomimetic research. In contrast to man-made materials, biological materials gain much of their mechanical performance from the elaborate structuring on many hierarchical levels. Therefore, the relation between structure and property was analysed in depth before addressing the question of scaling. Mechanical properties were tested with uniaxial compression, 3-point bending and resonance frequency damping analysis. The structure was visualized by optical microscopy, secondary scanning microscopy and computer tomography. X-ray diffraction, infrared spectroscopy, thermogravimetry and dilatometry gave insight into the crystallography and chemical composition. For scaling analyses theories of Weibull and Bažant were applied. The spines generally derive their high strength, high stiffness and exceptional damage tolerance from their construction out of >107 struts/cm3. The µm sized struts can be bent elastically, demonstrating that they are practically free of surface flaws. The struts are separated by pores which restrict crack growth and keep damage localised. The porous meshwork is covered irregularly by dense layers, the “growth layers” marking earlier growth stages. They provide the spines with additional stiffness and strength. Spines with many growth layers have a significantly higher strength and stiffness. The strength of the spines seems not to decrease significantly with increasing size, contradicting scaling theories. To test this unexpected finding, compression tests on samples with and without growth layers were conducted. A novel micro-compression test, the pin indentation was also applied. Despite the uncertainties induced by natural heterogeneities, it seems that spines of H. mamillatus counteract the size effect by adding more and denser growth layers to larger (older) spines. By this they work against the decrease in strength with increasing size. This hypothesis was confirmed by segments lacking growth layers that show a size effect.