Silk and gold, usually a pairing for the runways of Milan, are now the main ingredients for a new kind of implantable biosensor. Researchers at Tufts University have crafted a small antenna from liquid silk and micropatterned gold. The antenna is designed to spot specific proteins and chemicals in the body, and alert doctors wirelessly to signs of disease. Scientists say the implant could someday help patients with diabetes track their glucose levels without having to test themselves daily.
According to Fiorenzo Omenetto, professor of biomedical engineering at Tufts University, silk is a natural platform for medical implants–it’s biocompatible, and while it’s delicate and pliable, it’s also tougher than Kevlar. Implanted in the body, silk can conform to any tissue surface, and, unlike conventional polymer-based implants, it could stay in place over a long period of time without adverse effects. Omenetto has previously taken advantage of these properties to mold silk into tiny chips and flexible meshes, pairing the material with transistors to track molecules, and with electrodes to monitor brain activity.
Now Omenetto is exploring the combination of silk and metamaterials–metals like gold, copper, and silver manipulated at the micro- and nanoscale to exhibit electromagnetic characteristics not normally found in nature. For example, scientists have created metamaterials that act as “invisibility cloaks” by manipulating metals to bend light all the way around an object, rendering it invisible.
Omenetto and his colleague Richard Averitt, associate professor of physics at Boston University, used similar principles to create a metamaterial that’s responsive not to visible light, but rather to frequencies further down the electromagnetic spectrum, within the terahertz range. Not coincidentally, proteins, enzymes, and chemicals in the body are naturally resonant at terahertz frequencies, and, according to Averitt, each biological agent has its own terahertz “signature.”
Terahertz science is a new and growing field, and several research groups are investigating specific protein “T-ray” signatures. A silk metamaterial antenna could someday pick up these specific signals and then send a wireless signal to a computer, to report on chemical levels and monitor disease.
To engineer the responsive end of such an antenna, the team first created a biocompatible base by boiling down silk and pouring the liquid solution into a centimeter-square film. The researchers then sprayed gold onto the silk film, using tiny stencils to create different patterns all along the film. Each area of the film responds to a different terahertz frequency depending on the shape of the gold pattern. The team then wrapped the patterned film around a capsule to form an antenna.
To test its performance, Omenetto and Averitt subjected the antenna to terahertz radiation and found that the antenna was resonant at specific frequencies. Going a step further, the researchers implanted the antenna in several layers of muscle tissue from a pig, and still detected a terahertz signal.
“We’ll try to sense something next and maybe put the antenna in contact with something we’d like to detect, like glucose,” says Omenetto. “We’ll see if we can replicate a proof of principle, and try to add some meaning to the resonance.”
Rajesh Naik, a materials science expert at the Air Force Research Laboratory at Wright-Patterson Air Force Base, says the research has great practical potential.
“Proteins and other molecules can be entrapped within silk films, allowing one to monitor in-vivo chemical reactions,” says Naik. “Similar resonating structures can be patterned onto other polymeric materials, but silk has an added advantage of being biocompatible.”
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