The common limpet (Patella vulgata) can be found primarily in stony coastal regions. With its strong foot muscle it attaches itself to rocks and cliffs, where it feeds from algae rasped from the stone using its radula – a kind of tongue full of miscroscopic teeth. An incredibly high hardness and wear resistance of the teeth is required for such a mechanically demanding task. Indeed, researchers of the University of Portsmouth have reported a strength of up to 6.5 GPa for these teeth in 2015, dethroning spider silk as nature’s strongest material.
Materials scientists from Montanuniversität Leoben have now, in an international collaboration with researchers from South Korea (Sungkyunkwan University), the USA (Brown University) and Germany (University of Wuppertal), investigated the reason for this extraordinarily high strength of limpet teeth.
Prof. Sang Ho Oh (Sungkyunkwan University) contacted Prof. Dr. Daniel Kiener and Dr. Michael Wurmshuber from the Chair of Materials Physics at MU Leoben due to their expertise regarding micromechanical experiments. Through such specialized testing techniques, namely nanohardness measurements and miniaturized compression tests inside an electron microscope, together with microstructural investigations and computer simulations performed by the other cooperation partners, the cause for the unique mechanical properties of limpet teeth was unveiled. The results of this work have now been published in the renowned journal Science Advances.
The microstructure of limpet teeth consists of ceramic nanorods embedded in a matrix of amorphous silica. Especially in the “leading part”, the part of the tooth that leads the rasping motion, one can observe that the nanorods are arranged in bundles, which perform a rotational motion under load. This motion leads to auxetic behavior of the material. Conventional materials usually contract in transverse direction when pulled on. Auxetic materials, however, increase in transverse direction when stretched in longitudinal direction, the material therefore shows a negative Poisson’s ratio. Accordingly, when an auxetic material is compressed, it shrinks in transverse direction. Such an unconventional material behavior commonly also leads to a higher indentation resistance. In the context of the limpet this means that the teeth show a higher hardness and wear resistance when rasping algae from rocks.
In the published work, the rotational movement of the nanorods as well as the negative Poisson’s ratio could now be proven in electron microscopical investigations and microcompression experiments, respectively. Simulation models complemented the experimental results and illustrated the various rotational modes that lead to the auxeticity of the tooth microstructure. These insights are expected to help in the future design of novel materials with increased hardness and wear resistence. Further collaborative research work on this fascinating biomaterial is planned at the Chair of Materials Physics.
Further information
Prof. Dr. Daniel Kiener
Chair of Materials Physics
Montanuniversität Leoben
E-Mail: daniel.kiener@unileoben.ac.at
Tel.: +43 3840 804 412