“It’s unexpected and extremely promising!” Luc Thévenaz, the professor in charge of EPFL’s Group for Fibre Optics, recently experienced a eureka moment in his research. Picking up on an idea proposed by a discussion group of the European Space Agency, he shifted his attention away from traditional fibers made of glass and focused on the silk strands that spiders produce for their webs. These strands are perfectly cylindrical, smooth, transparent and extremely solid – some of the same characteristics as glass-based fibers. But there is one major difference: while glass is inert, spider silk is made up of very long proteins rolled into a helix structure whose bonds are sensitive to a number of chemical substances.
Reusable chemical sensors
“The helix in the silk strands unwinds whenever polar molecules like acetic acid and ammonia come into contact with its bonds,” said Dr. Thévenaz. “This measurably modifies the way the strands conduct light, and it gave us the idea of using them to make chemical sensors.” The researchers discovered another remarkable property, that the change in the helix structure was completely reversible. A sensor using spider silk could thus be used several times over. “We are looking at the possibility of creating silks by adding molecules meant to react with the substances to be tested. That is often impossible with glass fibers, which we have to heat to more than 1,000°C in order to stretch,” said the researcher. Also, because silk is biodegradable, it is ideal for sensors that could be implanted in a living body without needing to be removed later on.
Spiders at work
At EPFL, doctoral student Desmond Chow and post-doc Kenny Hey Tow are working on natural silk strands with a diameter of 5 microns. They were produced by Australian Nephila edulis spiders grown at the Department of Zoology of the University of Oxford. Synthetic silks exist, but they are expensive and do not work as well as real ones. The researchers stretch a silk strand taut in a tiny bracket and direct a laser beam at one end of the strand. At the other end, a polarization analyzer is used to measure infinitesimal changes in the light passing through it. If a gas that interacts with the silk strand is present, the device will pick up on this immediately. This research is still in the early stages, but it is already firing the imagination of the fiber optics specialist. “We have presented it at several conferences, where it was met with significant interest,” he said. The study is in the running for research funds. “We are entering a totally new domain that has yet to be explored,” said Dr. Thévenaz. Source: École polytechnique fédérale de Lausanne — EPFL (Author: Emmanuel Barraud)