A ZINC-HISTIDINE, IRON PHOSPHATE, AND ZINC HYDROXIDE BIO-COMPOSITE IN CUTTING AND PUNCTURING TOOLS

Non-technical summary

How do small animals like ants and scorpions so easily puncture human skin, while humans would have a difficult time biting through similar skin, even with their vastly stronger jaw muscles? The answer is that organisms with small muscles focus their tiny forces using very sharp mouth parts, stings and claws. But, sharp tools are easily damaged, and if an organism is not strong enough to cut with blunted tools, damage and wear may be fatal.

Because of this pressure, special materials may have evolved to help small organisms make sharp tools and keep them sharp. Humans may learn from these materials, and use this knowledge to improve human-made materials. A good candidate for such a material was recently discovered in scorpion mouth claws.

This material may minimize and even “heal” damage and wear of the cutting edges. This grant will confirm and further study the composition of this scorpion material by disassembling the material nearly atom-by-atom using Atom Probe Tomography. The material is thought to contain two different minerals, iron phosphate, and zinc hydroxide, as well as a protein component that contains about 10% zinc.

The hardness and other mechanical properties of the new material will be investigated using techniques that can measure these properties for tiny samples. In addition, the behavior of the sharp edges during cutting will be studied using a testing device that is integrated into an electron microscope and cuts and punctures with the claws. The scorpion material is strengthened by zinc-histidine cross links, which act like cross beams strengthening a building, but these cross links are sometimes broken under the pressures of cutting.

One possible reason for the large quantities of extra zinc is to quickly repair broken zinc-histidine cross links. In addition, the zinc hydroxide may fill in and bond opposite sides of developing cracks. The electron microscope will allow visualization of wear, damage and self-healing during cutting, and the testing machine will measure the forces required for repeated puncture.

The scorpion material will be compared to other materials like human-made blades and tips, and the researchers will investigate how the scorpions make the material by examining scorpions at various stages of development. In addition to potential contributions to materials technology and materials science, this project will further the understanding of the importance of materials in biology, bridging the fields of material science and organismal biology.

A graduate student and dozens of undergraduates will also receive research training through this project. Technical summary

Small animals often overcome force limitations using very sharp mouth parts, stings and claws. But, sharp tools are easily damaged, and if an organism relies on sharp tools for defense or food acquisition, damage and wear may be fatal. The project will investigate a recently discovered complex biomaterial in scorpion mouth claws that may have evolved to produce sharp structures while minimizing and possibly “healing” damage and wear.

Atom Probe Tomography will be used to confirm and extend the preliminary findings, which suggested that the material contains two different biominerals, iron phosphate, and zinc hydroxide, as well as a histidine - rich protein that binds Zn in ~10% concentrations. Nanoindentation will be used to measure hardness, modulus of elasticity and damping (loss tangent), and a puncture/cutting tester will be installed inside a Scanning Electron Microscope in order to visualize how the material wears during the puncture/cutting process, and whether the claw tips and edges have self-healing properties.

The self-healing hypothesis is that broken zinc-histidine bonds re-establish in a manner similar to the minute-scale healing of zinc-histidine bonds recently discovered in mussel byssal threads. The rate of repeated punctures in the puncture testing machine, and repeated indentations with the nanoindenter, will be varied in order to investigate self-healing.

One possible explanation for the presence of zinc hydroxide is that it acts as a ready supply of zinc for this self-healing mechanism, helping bonds re-establish quickly. A cyclical breaking and re-establishing of many zinc-histidine bonds could act as a damping mechanism, absorbing energy that might otherwise be available for fracture. Finally, scorpions will be examined at various stages of development in order to begin to understand how they produce this complex material.

In addition to the potential of inspiring new materials technology and materials science, this project will further the understanding of the importance of materials in biology, bridging the fields of materials science and organismal biology by translating material property differences into differences in required force and muscle mass. A graduate student and dozens of undergraduates will also receive research training through this project.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.