A Plastic as Strong as Kevlar, and Environmentally Friendly

1-11-15 Squid thermoplastic
The researchers used recombinant techniques to manufacture the small, synthetic SRT molecule used to produce eco-friendly thermoplastics. Adriás Bago/Pennsylvania State University

Melik Demirel has long been interested in creating materials that mimic nature. With his colleagues, he's fabricated materials based on butterfly wings and gecco footpads. Most recently, the engineering science and mechanics professor and his team at Pennsylvania State University have produced a thermoplastic that replicates squid ring teeth (SRT), a protein complex extracted from the squid tentacles' suction cups.

They've made "an eco-friendly material with remarkable mechanical properties," Demirel says, one that "provides unique opportunities for a range of applications including drug delivery, materials coatings, tissue engineering, and wet-adhesives." Their study was published as the cover of the December 17 issue of the Advanced Functional Materials journal.

The problem with previous materials he's studied in the past, Demirel says, is that they could not be engineered. In other words, they could not be easily reshaped or remolded into different forms, limiting their real-world applications. He turned to squid ring teeth, a structural protein (like hair or nails) similar to silk in its semi-crystalline structure, which could be shaped into any geometry using polymer processes that have been around for a century, as well as newer 3D printing methods.

The amount of SRT protein that can be extracted from squid is severely limited, "even if you caught all the squid" in the world, says Demirel, with a touch of hyperbole to drive home the point. "The natural abundance will be the limiting factor." To produce the material in larger quantities, Demirel and his fellow researchers used a common bacteria-based fermentation technique that originated during the 1980s biotech boom.

The researchers took genes from a squid and put it into E. coli bacteria. "You can insert genes into this organism and while it produces its own genes, [it] produces this extra protein," Demirel explains. He compares the process to making wine or beer, except that instead of the fermentation process producing alcohol, it produces more of the synthesized squid protein.

They began producing the material in a 1-liter tank, but by now have started using a 300-liter tank and can make 30-40 grams a day. In addition, they've made several changes to make the production process cheaper, whittling the cost down from $50 per gram to $100 per kilogram. Demirel says they are looking at using algae instead of bacteria to cut down costs further.

The squid-inspired thermoplastic has several advantages over traditional plastics, Demirel says. It's lighter than carbon-based materials and its versatility allows it to be made into fibers and thin films. Because it's a protein, it's also biodegradable and tuneable. In other words, you can modify the properties of the gene at the start of the production process to adapt the material to different needs.

Finally, the production process requires less energy and is more environmentally friendly than that of traditional plastics. Unlike the "high temperature refinery processing of plastics," which are manufactured from fossil fuel sources like crude oil or from synthetic oils, the creation of the SRT thermoplastics can take place at room temperature

Demirel and his team are not alone in touting the potential of such proteins. In July, the American Chemical Society's journal ACS Nano published research on a few dozen proteins from a number of species of squid and one cuttlefish.

"We envision SRT-based materials as artificial ligaments, scaffolds to grow bone and as sustainable materials for packaging, substituting for today's products made with fossil fuels," Ali Miserez from Nanyang Technological University in Singapore told the ACS. "There is no shortage of ideas, though we are just beginning to work on these proteins."

Demirel and his team have already started working on a number of applications. They've received funding from the army to work on developing new textile materials for soldiers, and they are currently working on making backpacks. They've begun collaborating with colleagues in medicine as they think about using colloids—a kind of substance they've already succeeded in making from the synthesized SRT protein—for drug delivery that targets certain cells, such as tumor tissue in cancer patients, as well as 3D printing devices like prosthetic arms.

The researchers are also looking toward the adhesive market. Since they're working with a protein-based material that evolved in a marine environment, it can resist water and stick to other surfaces even when wet, a huge advantage compared to other adhesives, Demirel says.

In the (far) future, Demirel can also see applications in space. He imagines, for example, using the material to produce equipment up on Mars—rather than lugging stuff along on a mission. "One of key advantages," he says, is that "you can take bacteria and grow it anywhere you like."

"It's completely futuristic, Demirel admits, but it seems he can't help but "start imagining what could be done."