Crustacean shells contain the useful compound calcium carbonate.
Seafood lovers around the world savour the tender meat of crustaceans but want nothing to do with their exoskeleton packaging, which is whisked from the table in short order. All those shells amount to a lot of waste; diners might consume three quarters of the caught weight of a typical fish such as salmon or tuna, while eating much less than half of the weight of a crab or lobster. A 2014 estimate by the Food and Agriculture Organization calculated some six to eight million tonnes of shellfish parts are annually discarded worldwide.
Waste, of course, is in the eye of the beholder; these many shells are actually a repository of potentially valuable agents, including protein and calcium carbonate. Perhaps the most compelling chemical constituent is chitin (C8H13O5N), a long-chain polymer that strongly resembles the polysaccharide cellulose, complete with its nanofibre structure.
“If you take a crab shell and etch away the minerals and protein, you are left with this chitin film,” says Mark MacLachlan, a chemist at the University of British Columbia. “Because of its structure, it’s actually iridescent. It has a helical pitch and the pitch is similar to the wavelength of light.”
MacLachlan and his students have been using the shells to make composites from chitin that swell when immersed in liquids to create a variety of colours. It is just one of many chitin-based materials his lab has been investigating. The cellulose-like structure could lend itself to thin containers, such as biodegradable coffee cups. It might also be wound into a strong fibre for surgical thread, which would likewise break down on its own after the job is done. However, among the most commercially appealing prospects of chitin may lie in the field of battery design.
“If you heat up the shell in an inert atmosphere, the chitin is converted into nitrogen-doped carbon,” says MacLachlan. “That leaves a very porous, conductive material with a high surface area, which is good for supercapacitor electrodes and possibly battery electrodes.”
Such components are typically made out of carbon black derived from fossil fuels such as natural gas. In addition to offering a biological, renewable replacement for this source, chitin has additional virtues for manufacturing. “Its natural organization allows more control over the structure that you get in the material,” says MacLachlan. However, he concedes that before chitin enters the industrial mainstream, the highly dispersed seafood waste stream will have to be turned into a separate enterprise that can provide a reliable supply. That much is obvious from his own supply lines. One student simply approached the owner of a local seafood restaurant, who agreed to provide her with their leftover shells as they became available. Another, apparently a big fan of crab, buys a number of them at a local grocery store and brings the shells in to work. “I told him we would not be able to expense his dinner from our research grant,” MacLachlan quips.