A hologram created out of optical silk

Got Silk?

The exotic material is even cooler than you thought By michael blanding

Ever since Byzantine merchants smuggled silkworm cocoons out of China around 500 AD, silk has had a special status among fabrics—at once ostentatious and intimate, sumptuous and secretive. Ancient kings forbade nonroyals from wearing it, closely controlling its manufacture. In the Middle Ages, Italian merchants carried worms up the Silk Road to establish their own factories in northern Italy, eventually making Milan a fashion capital of the world.

As a child, Fiorenzo Omenetto grew up just a few miles from the factories of Armani, Versace, and Gucci. He remembers passing groves of mulberry trees—the only food silkworms will eat. Several decades and thousands of miles later, Omenetto is an associate professor of biomedical engineering at Tufts and one half of a collaboration that is truly uncovering the secrets of silk. He and David Kaplan, chair of biomedical engineering, are turning the stuff of fancy clothing into the stuff of science fiction.

“The only reason I do anything with silk is because I share a hallway with Dave,” says Omenetto—who goes by Fio, specializes in optics, and talks an Italian-accented mile a minute. “I never would have thought in a million years to use this.” Kaplan had been working with silk proteins for years—boiling down silkworm cocoons and using the biocompatible goo to form artificial organs. Having trouble making a cornea one day, he asked Omenetto if he could shoot some holes in it with lasers to make it more permeable. The closer Omenetto examined the material, however, the more he found it mirrored the properties of high-quality optics, the kind used in making holograms and DVDs—with one crucial difference: it is completely bio¬degradable.

Pulling out a transparent card a few inches across, Omenetto shows off a hologram that looks like the ones on a credit card. In fact, it was created by pouring liquid silk protein over a microscopic mold. “It’s like a very fancy Jell-O mold, with features on the nanoscale,” he says. Those features are just 1/200 the width of a hair, he says, or about equal to the wavelength of visible light. The result is a surface that scatters light waves into their component colors, just like a butterfly wing.

By changing the surface patterns—say, by binding a bit of bacteria or a chemical to them—you can change the color. That makes silk an intriguing substance for forming sensors to detect hazardous materials. Silk sensors dropped into a bag of spinach could turn from blue to green if E. coli bacteria were present, or sprinkled into a river could turn a coastline red if it was contaminated with PCBs. Here’s the cool part: Unlike plastic, the silk sensors would be completely safe for the environment. “You could use this anywhere, and after you are done, it would degrade away,” says Kaplan, excitedly. Or you could just eat it along with the spinach. “It tastes like nothing,” says Omenetto, adding that “it needs BBQ sauce.”

Silk devices have another unique property: they can support living cells. Omenetto holds up a silk card that was mixed with hemoglobin from blood before it dried on the mold. Even though hemoglobin rapidly becomes inactive in open air, the slide still has a reddish tinge. That means some of the blood cells are still capable of reacting with oxygen, even though the slide has sat on a shelf for more than a year. Omenetto and Kaplan theorize that silk contains little hydrated chambers that help sabilize the protein.

The two scientists foresee dramatic new medical applications. “Right now, to distribute most drugs and vaccines through the world, you have to freeze-dry them or put them in a refrigerator,” says Kaplan. “Here you could package a vaccine, ship it to South Africa or South America or wherever, and it could sit there for months without refrigeration. Then when you are ready, you could literally just pop it in your mouth and eat it, or adhere it to your skin and let it be absorbed.”And because of silk’s optical properties, a sensor could be included in the vaccine to show doctors whether it was still fresh.

Kaplan and Omenetto have filed 10 patents for devices utilizing the silk protein materials, but they are just getting started. They dream of a Silk Institute that would be dedicated to studying how the material might be applicable to different fields—not just replacing plastics but also silicon or other materials to create biodegradable electronics. The two run wild with thoughts of programmable drug capsules that could gauge exactly how much medicine to release, or stay in the system for months or even years before releasing their charge (the more crystalline the material’s structure, says Kaplan, the longer it takes to biodegrade). “I could implant a small chip in my forearm, and it could tell me every day how I am doing,” says Omenetto, “or it could be like that movie Fantastic Voyage, where these people shrink down and go inside someone’s body to fight disease.”

“See what I have to deal with?” says Kaplan, the straight man to Omenetto’s humorous spectulation. In truth, however, they are equally passionate, each interrupting our interview several times to share an idea of a new application. “I don’t sleep much anymore, and neither does Dave,” says Omenetto. “Anywhere between two a.m. and six a.m. is fair game to email each other.”

MICHAEL BLANDING is an award-winning magazine writer whose work has appeared in The Nation, The New Republic, Boston Magazine, and the Boston Globe Magazine.