High Performance Impact-Tolerant and Abrasion-Resistant Materials: Lessons From Nature

Current methods for synthesizing impact-tolerant and abrasion-resistant materials are traditionally inefficient and costly and often require the use of environmentally hazardous components and processes. In stark contrast to their industrial counterparts, however, biological systems are well known for their ability to synthesize a wide range of high performance composites at ambient temperatures and pressures, and near neutral pH without the use of caustic precursors of byproducts. One such example is found in the mineralized teeth of the chitons, a group of benthic marine invertebrates common along the North American Pacific Coast. The teeth are anchored to a flexible belt like structure, the radula that is used for scraping algae from rocks, on which the chitons feed. Because of their constant rasping motion, the teeth must be specifically adapted to persist under such harsh conditions.

Elemental mapping via Energy Dispersive Spectroscopy (EDS) in conjunction with electron and X-ray diffraction have revealed that each tooth is composed of two dominant biominerals, (an amorphous ferrous phosphate core and a thick magnetite veneer) that are intimately associated with the tooth organic matrix. Backscattered electron microscopy has also been used to investigate the interfaces between these two mineral phases and their roles in ultimately affecting the mechanical properties of the teeth. High-resolution imaging of this interface reveals that the transition between the two mineral phases is not abrupt as one would typically encounter in synthetic multi-material composites. Following mechanical loading of the teeth, cracks propagating through the core phase are deflected laterally as they encounter the harder magnetite phase, revealing the fact that not only is this architecture specifically adapted for abrasion resistance, but is also very effective in preventing the propagation of large cracks originating form contact-induced surface defects. Nanoindentation of the two mineral phases reveals that there is a gradual 4-fold increase in modulus from the core to the magnetite periphery of each tooth, a design strategy that has been shown through both experimental and modeling approaches to be very effective in increasing fracture toughness of related composite materials via crack deflection, without the problems associated with delamination of the two phases via complications arising from modulus mismatch. The radular teeth thus represent an excellent model system for investigating the properties of mechanically graded materials and future investigation will be aimed at elucidating the various stages of tooth maturation and mineralization. It is hoped that in the not too distant future, this and related research into the structural complexities of biological systems may ultimately guide the fabrication of a new generation of high performance synthetic materials for a wide range of technologically relevant applications.