Imagine an edible optical sensor that could be placed in produce bags to detect harmful levels of bacteria and consumed right along with the veggies. Or an implantable device that would monitor glucose in your blood for a year, then dissolve.

Scientists at Tufts University's School of Engineering have demonstrated for the first time that it is possible to design such "living" optical elements that could enable an entirely new class of sensors.

These sensors would combine sophisticated nanoscale optics with biological readout functions, be biocompatible and biodegradable, and be manufactured and stored at room temperatures without use of toxic chemicals.

"For example, at a low cost, we could potentially put a bioactive silk film in every bag of spinach, and it could give the consumer a readout of whether or not E. coli bacteria were in the bag—before the food was consumed," explained David Kaplan, professor and chair of the biomedical engineering department.

Optics – the science of light and its interaction with matter – has fascinated generations of scientists such as Sir Isaac Newton. Current optical device platforms are based primarily on glass, semiconductors, plastics or polymers.

Silk proteins are, literally, a natural for integrating optical and biological functions. They can be processed in water at ordinary temperatures and patterned on the nanoscale to generate a wide range of optical elements, including ultrathin films, thick films, and nanoscale and large-diameter fibers.

Silk proteins also offer excellent surface quality and transparency, which are perquisites for high-quality optics. Equally important, they are mechanically robust.

To form the devices, Tufts scientists boiled cocoons of the Bombyx mori silkworm in a water solution and extracted the glue-like sericin proteins. The purified silk protein solution was ultimately poured onto negative molds of ruled and holographic diffraction gratings with spacing as fine as 3600 grooves/mm. The cast silk solution was air dried to create solid fibroin silk films that were cured in water, dried and optically evaluated. 

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