There’s a thing about the nature that never cease to amaze us. Wings of a butterfly, beauty of a flower, bustling of a bee and calm of the sea, grant us at least a moment of a respite in our busy lifestyles. For me nature looks even more spectacular at the scale it operates: nanoscale. Among the natural structures that I’ve observed in my lab, nothing looked more incredibly complex and amazingly inspiring as the structure of a crab shell.
The natural nanocomposite
Although seems strange, crab shell is one of the strongest biocomposite materials in the natural world. The shell is designed in this way to provide protection to the crab from its predators. Crab shell has elastic modulus values as comparable to nylon; the type of fibers used in parachute cords. Crab shell is a complex hierarchical structure of chitin nanofibers, calcite and proteins. The main constituent that contributes to its strength is the strength of the individual chitin fibers and their special arrangement.
The smallest chitin nanofibers are in the size range of 3-5 nm in size and extremely strong given their size. Scientists have calculated that the strength of the chitin nanofibers is as ten times strong as reinforced concrete. These smaller nanofibers arrange in to fiber bundles in the size range of 60- 70 nm.
The second constituent to the strength is the special arrangement of the nanofiber bundles. These bundles organize themselves in to parallel patterns in horizontal planes. These horizontal planes are stacked on top of each other, rotating slightly each time along the Z axis, thus forming a structure similar to a twisted plywood. The whole chitin structure is immersed in crystalline calcite, cementing each individual fiber in to a very strong shell. Thus formed structure is ideal to absorb and dissipate an applied force along the whole shell making it tough to break.
The Japanese approach
A group of scientists from Kyoto University, Japan has made an ingenious attempt to get this natural nanostructure to good use. They have treated a whole crab shell with hydrochloric acid, sodium hydroxide and ethyl acetate to remove calcite, proteins and pigments respectively. These treatments only leave chitinous structure with its intricate nanostructure. Then the scientists have immersed what left of the shell in an acrylic resin monomer. Following polymerization and drying they obtained an optically transparent crab shell. Amazingly, the shell retained its original shape and detail. Inspired by the idea, the team has expanded the research to make completely transparent large area nanocomposite sheets. The team has also found that the nanocomposite sheet has higher modulus and breaking strength compared to virgin acrylic films. Transparent crab nanocomposite also showed high heat resistance and maintained transmission properties up to 100 degrees.
Transparent nanocomposites will prove to be materials of great practical potential in near future. Scientists are working on nanofiber based transparent nanocomposites that can be pasted inside the windscreen to provide an extra protection in case of a car crash protecting the drivers and passengers from flying shrapnel. They are also tested as smart blinds that can be pasted inside surface of a glass window. The smart blind can detect the changes of light intensity outside and adjust its optical transmission properties to provide the most desired level of light to the inside.