Attached temporarily to a tooth, the sensor can measure bacteria levels in the mouth and could one day help dentists fine-tune treatments for patients with chronic periodontitis, for example, or even provide a window on a patient's overall health.
The sensor is relatively simple in its construction, says McAlpine. It's made up of just three layers: a sheet of thin gold foil electrodes, an atom-thick layer of graphite known as graphene and a layer of specially engineered peptides, chemical structures that "sense" bacteria by binding to parts of their cell membranes.
"We created a new type of peptide that can serve as an intermediary between bacteria and the sensor," says McAlpine. "At one end is a molecule that can bond with the graphene, and at the other is a molecule that bonds with bacteria," allowing the sensor to register the presence of bacteria, he says. Because the layers of the device are so thin and fragile, they need to be mounted atop a tough but flexible backing in order to transfer them to a tooth. The ideal foundation, McAlpine says, turns out to be silk—a substance with which Kaplan and Omenetto have been working for years.
By manipulating the proteins that make up a single strand of silk, it's possible to create silk structures in just about any shape, says Omenetto, a professor of biomedical engineering at Tufts. Since 2005, he's created dozens of different structures out of silk, from optical lenses to orthopedic implants. Silk is "kind of like plastic, in that we can make (it) do almost anything," he says. "We have a lot of control over the material. It can be rigid. It can be flexible. We can make it dissolve in water, stay solid, become a gel—whatever we need."
Omenetto, Kaplan and Tao created a thin, water-soluble silk backing for McAlpine's bacterial sensor—a film that's strong enough to hold the sensor components in place, but soft