Organisms possess unparalleled control over the structure and properties of mineralized tissues such as teeth and bones, creating curved single crystals as well as tough and lightweight self-repairing skeletal structures. Organic matrices play an integral role in the selective formation of metastable mineral precursors and their transformation into the final biomineral. The chiton is a model system for the extracellular, matrix-mediated iron oxide. Its radula is a ribbon-like rasping tongue with many rows of extremely hard, wear-resistant, and self-sharpening teeth designed to withstand the stresses of grazing algae on rocks. So far, there have been numerous studies on the cuspid heads of the tooth, which consists of one of the hardest and most wear-resistant biogenic materials, a magnetite/chitin composite. The underlying base, also called stylus, has not been investigated.
In this study we aim to establish a better understanding of the biominerals in the radula tooth stylus. Modern material fabrication requires large amount of energy to synthesize materials for semiconductors, medicine, or construction, often employing extensive top-down fabrication steps at high temperatures and pressures. Biominerals can serve as inspiration for finding innovative methods and new materials and may also allow to decrease the carbon footprint of these processes. Such knowledge could inform the synthesis of new nanocomposite materials with outstanding properties.
A comprehensive characterization of the stylus was performed with a suite of techniques that provide insights into structure and compositions at multiple length scales, including synchrotron Mößbauer spectroscopy (SMS), solid-state nuclear magnetic resonance (ss-NMR), X-ray spectroscopy and electron imaging. Mechanical properties such as hardness and reduced modulus were tested by nanoindentation.
We find evidence that the cusp-adjacent stylus is mineralized by santabarbaraite, an amorphous iron hydroxy phosphate that was not known to occur as a biomineral. The mineral occurs in very small nanoparticles adjacent to α-chitin nanofibers. The stylus possesses outstanding mechanical properties that are comparable to steels. Based on our insights, we formulated bio-inspired inks for additive manufacturing, demonstrating that the mechanical properties can be tuned over a wide range also in in vitro. The fabricated composite is biodegradable, non-toxic, and processed at room temperature.