Silk from the humble silkworm has been used by humans for thousands of years, mostly to create textiles, but it appears that this remarkable material is on the verge of entering a dramatic new role. It may some day save your life.
Researchers at Tufts University and elsewhere are using silk to create sophisticated optics that may eventually warn that the spinach you were about to eat is contaminated with E. coli bacteria. Or the river you were about to plunge into is highly toxic. Or the glucose level in your blood is dangerously high.
There is a wide range of potential applications for this technology, if it can be scaled up to industrial production, and no one is more surprised than the scientists and engineers who are developing it.
"It was serendipitous," said Fiorenzo Omenetto, associate professor of biomedical engineering and physics at Tufts. "We didn't really set out with this in mind, to make optics out of silk."
Omenetto is an expert on more traditional optics, but when he arrived at Tufts he found himself in the company of David Kaplan, chairman of the biomedical engineering department, who is an expert on silk, among other things. The two have already created optical components, like lenses and sensors, out of protein from the silk that can be reformed into incredibly clear films.
The end product has unique properties, lending itself to surface texturing on the nano scale. It is compatible with biological materials and is so benign it can be programmed to self-destruct after its mission is completed, or just tossed on the ground "like giving the silkworm back to the grass," Omenetto said in a telephone interview.
And by the way, if that bag of spinach isn't contaminated, you can eat it, along with the sensor, with no problem. The researchers are calling it "edible optics."
"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," Kaplan said in announcing the research.
Think of it as a litmus test with radical new technology, and many more uses.
Other researchers are pursuing the same goal with different materials, but very few, if any, materials have the basic properties that make silk so attractive. The high temperatures need to form glass or plastic destroy biological agents that could sense the presence of specific chemicals, for example. Some other potential materials are highly toxic, or must be stored at specific temperatures to keep the biological components viable, or produced with harsh solvents.
Films made from silk can be processed at ambient temperatures, and they are biocompatible and biodegradable, and so nontoxic that they can be implanted in humans to monitor such things as glucose, Omenetto said.
"It's hard to find these things co-existing in one material," he added.
Silk used in the textile industry is drawn from the silkworm, which is actually a cocoon, and woven into fabric. But in the Tufts lab, the cocoon is boiled in water, liquifying the silk, and a protein is extracted. Any toxic materials are removed from the protein, and the purified material can be poured into molds that create specific nano-scaled surfaces.
The researchers are developing sensors that can detect the presence of various substances, like E. coli. Light passing through the sensor will appear as a specific color, depending on which substance is detected.
"It's the same thing you see in butterfly wings," Omenetto said. "The color is defined by nano-patterning of the surface. You monitor the color change, and the color change depends on whatever you have put inside the film." If the sensor detects E. coli, for example, the film would change colors to whatever color reflects the presence of the bacteria. If it's red, the film would turn red, as in a litmus test.
Omenetto has turned to a tough audience to test public acceptance of his work.
"I go to my kid's elementary school, and I bring a cocoon and a lens that is made from the cocoon's silk," he said. When they hold what appears to be an ordinary magnifying glass, made out of silk from the cocoon, "it's easy to capture their attention. It's kind of cool."
Progress in the research has come at a fast clip, due partly to funding from the Defense Department's Advanced Research Projects Agency. Films from silk could be used for many things of interest to the Pentagon, like foolproof personal identification. The researchers have fabricated 3-D holograms that could be embedded in an identification card, or for that matter, a personal credit card, revealing whether the person using the card is authorized to do so.
The researchers caution that it's too early in the program to know just where it will lead, but there's little doubt they think they are on to something big.
Food poisoning kills about 5,000 Americans every year, along with 76 million illnesses and 325,000 hospitalizations, according to federal statistics. It's also costly to producers. Spinach growers lost $350 million in the E. coli outbreak in 2006.
So it would be great if an edible sensor in the bag could tell you whether it's safe to eat the veggies. But to work, the researchers said, the sensor has to produce an instantaneous warning that is extremely clear (like turning red) and there is still work to be done there.
"I see a path forward," Omenetto said. "I wouldn't be doing this if there wasn't the hope of doing something useful in the end."
And, he said, it's what makes serendipitous science so rewarding.
"If I hadn't shared the hallway with David, I would never in a million years have thought of making optics out of silk."
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.