Ling Li, an assistant professor at Virginia Tech, has made a number of important contributions to our understanding of how nature constructs multifunctional materials. During his Ph.D. studies, Li focused his work on biological materials with simultaneous optical and mechanical properties. A mollusk shell called Placuna placenta comes into view: despite the fact that the shell is made of 99% brittle calcite, it dissipates energy 10 times more efficiently, while, more remarkably, maintaining optical transparency.
It was later found that the shell has a multilayered structure, consisting of roughly a thousand nanoscopic calcite layers. Each mineral layer has a thickness of about 100 nanometers. In contrast, the organic interfaces between adjacent layers are only one or two nanometers. This structural design allows the light to travel through the composite without being significantly scattered by the interfaces, which makes the mollusk shell highly transparent. Moreover, the crystal structure of the calcite layers undergoes a special deformation mechanism at nanoscale, known as twinning, when the structure is exposed to external forces. Such process works synergistically with the thin organic interfaces to constrain crack propagation and hence limit mechanical damages.
During his later research, Li discovered the image forming capability from the unique mineralized eyes of a group of mollusks. These creatures, known as chitons, live in the intertidal waters and have hard mineralized shells over their soft body. Some chitons evolved to develop mineralized eyes with a diameter of less than 0.1 mm embedded in their hard shells. In this work, Li’s team, for the first time, demonstrated that images can be formed through these mineralized eyes, which is remarkable considering most eyes in nature are based on organic materials.
Li said that our in-depth understanding of biological materials will have a great impact on the development of new materials and functional structures. For example, Li recently led his team to systematically investigate a unique class of biological armor that simultaneously provides mechanical protection while being highly flexible. Moreover, the mechanistic understanding of the biological system allows his team to design and develop a new type of bio-inspired flexible armor.
For his future research, Li hopes to focus more on natural porous materials that are both lightweight and strong, such as bones. Meanwhile, how organisms construct their complex microstructures is another focus of Li’s team. He hopes that one day people can make new materials in similar ways to biological systems.