Got silk? Material no longer just for nice dresses

Because of its smoothness, strength and luster, silk has long been used in clothing. In recent years, however, researchers like Biomedical Engineering Professor David Kaplan have been experimenting with new applications -- particularly in biological integration -- of this unique material.

"Silk is an amazing biomaterial. It's mechanically strong, it's versatile, you can process it in water and it's compatible with the body," Kaplan said.

Fibroin, a protein in silk, can be isolated in a solution. The process begins when silk cocoons are cut, cleaned, boiled and filtered to create a crystal-clear fibroin solution that can then be used in a variety of applications.

In the summer of 2006, Kaplan, while working on engineering an artificial cornea out of silk, went to Associate Biomedical Engineering Professor Fiorenzo Omenetto, a specialist in femtosecond lasers and ultra-fast nonlinear optics.

"David asked me to poke holes on some silk," Omenetto said. "They were making a scaffold to make a cornea out of silk. I looked at it and saw that it looked like it would be a good material for optics. No one had done anything like this, so David and I started talking."

Kaplan and Omenetto immediately began researching silk's optical properties, and over the past two years, silk has proven itself an ideal material in many respects.

Pure silk fibroin is incredibly transparent, and its surface, even on the nanoscale, is smooth. "You can get features down to 10 to 20 nanometers," Omenetto said. For comparison, this is less than one one-thousandth the thickness of a strand of hair.

These qualities make silk ideal for making optical devices such as lenses and diffraction gratings that focus or spread light.

The quality of these devices is excellent, and although similar ones can be made from plastics or glasses, silk is unique in the easy and environmentally friendly way in which it is processed.

"The cool thing about doing things with silk as opposed to glass and plastics is that silk is all processed in water, it's all at neutral pH and it's all at room temperature; so instead of glass at 2,000 degrees and plastics with acids and other toxic mixtures, it's all green. You can put this all back into the environment," Omenetto said.

Silk optical devices are proving most innovative because of their biological functionality. In a recently published article in the journal Nature, Omenetto and Kaplan wrote, "Although the variety and quality of silk optical elements is impressive, the key advantage of using silk essentially lies with its biocompatibility."

One interesting discovery Kaplan and Omenetto made was in preserving certain enzymes in the silk films.

"We did some experiments with some enzymes that if you put them at room temperature, they lose their activity. If you mix them in the silk solution, they stay active for a year, whereas they lose activity outside in two days," Omenetto said.

Enzymes contained in the film, because they keep their functionality, create encouraging possibilities for silk.

"You can start thinking of a pair of glasses with enzymes in them that change the optical properties," Omenetto said.

The fact that silk can be safely implanted into the body and dissolve after a certain period of time means that enzyme-carrying silk devices can serve as biosensors.

"We're dreaming that one day we can make a little optical device that can tell you if you're doing okay or if you should take some medicine," Omenetto explained. Another similar idea is an edible bacteria sensor that could be put in with packaged food and let consumers know if it is spoiled or not.

There still remains a lot of work to be done before such devices can be created, but the possibilities are exciting. The interdisciplinary nature of this research has brought students and faculty from all fields together in collaboration.

"I didn't do any silk [research] before I came here," Omenetto said. "It's a weird thing that happens by sharing the hallway. You know how a lot of people talk about getting people from all disciplines -- well it happens like this."

The duo's success is largely due to their compatible personalities and complementary expertise.

"It's a great team," Kaplan said, "both on the science side and the personality side."

"I'm absolutely excited. This is about as close as I will ever get to having a good idea," Omenetto said. "There are so many things to do. Yeah, I don't sleep much anymore, but neither does David."