Silk doesn’t absorb or emit light at the visible and infrared frequencies used in conventional telecommunications and optical information technology. So to make it into a fully working laser medium, one needs to add substances that do. Organic dyes (carbon-based molecules) are already widely used, dispersed in a liquid solvent or in some solid matrix, to make dye lasers. The researchers figured they could mix such a dye into silk. They used one called stilbene, which is water-soluble and closely related to chemical compounds found in plants and used as textile brighteners.
In collaboration with Stefano Toffanin and coworkers at the Institute for the Study of Nanostructured Materials in Bologna, Italy, Omenetto and colleagues etched a series of grooves about a quarter of a micrometre wide on the surface of a slice of silicon, which act as a mirror for the bluish-purple light that stilbene emits. They then covered this with a layer of silk spiced with the dye, and found that when they pumped this structure with ultraviolet light, it emitted light with the characteristic signature of lasers: an intense beam with a very narrow spread in frequency.
Making the device on silicon means that it could potentially be controlled electronically and merged with conventional chip circuitry. But that’s not essential – silk-based circuits and devices could be laid down on other materials, perhaps on cheap, flexible and degradable plastics.
This isn’t the first time that biological materials have been used to make lasers. For example, in 2002 a team in Japan made one using films of DNA infused with organic dyes. And last year, two researchers in the US and Korea made lasers from living cells engineered to produce a fluorescent protein found naturally in a species of jellyfish. But the attraction of silk is that it is cheap, easy to process, biodegradable and already used to make a range of other light-based devices.
There might be even more dramatic possibilities. Recently, Omenetto and colleagues showed that silk is a good support for growing neurons, the cells that communicate nerve signals in the nervous system and the brain. This leads them to speculate that silk might mediate between optical and electronic technology and our nervous system, for example by bringing light sources intimately close to nerve cells for imaging them, or perhaps even developing circuitry that can transmit signals across damaged nerves.