At last Friday's Taste of Tufts lecture, Professor FiorenzoOmenetto of the Department of Biomedical Engineering discussed his extensive work with silk. In his research, he collaborates with Department Chair and Professor David Kaplan to apply silk's unique properties to wide-ranging new functions in medical and biological fields.
"It's a topic that has continued to amaze me," he said. "We look at a material that has been around for 5,000 years and we try to reinvent it."
According to Omenetto, silk has huge potential for a large number of new and useful applications. This can be traced back to silk's structure in its original state. In their glands, silkworms produce liquid silk, a combination of water and fibroin protein. From this substance, they create silk and spin it into a tough cocoon, which contains "about a kilometer of uninterrupted fiber," Omenetto said.
Omenetto added that the first step to using silk is breaking it back down into its basic form. Boiling the silk fibers with salt effectively returns silk to its original components of water and protein.
"This is the reverse engineering process, and you literally kind of make pasta. For a good plate of pasta, when you boil your spaghetti, you have to add salt," he said. "That's the first step of getting to that solution. And once you have that solution, you have the starting point. This is where we can go in multiple directions, in multiple material directions."
From here, the silk solution can be turned into a film by pouring the solution on a surface and allowing the water to evaporate. This film has significant technological applications. Silk can conform very precisely to different structures on a given surface, "down to literally 10 nanometers," Omenetto said. It can pick up surface relief patterns that encode data, like those on the surface of a DVD or CD and can also be used to make highly sensitive color shift sensors.
Silk also serves a variety of optical purposes, including "microprism arrays," which are what make running gear reflective, and it can be utilized when building three-dimensional holograms, optical fibers and glowing light tattoos. Silk also has significant medical capabilities; it can be molded into vein replacements and even bones.
"Remember, it starts off as a tissue-engineering scaffold, so you can mold and change it, and you have this large space to turn the material into doing these different things," Omenetto said.
When silk is interfaced with gold or other conducting metals, it can be used as a flexible sensor that transmits information about the conditions of the human body. This is particularly useful in measuring electroencephalographic signals from the brain, Omenetto said, because the flexibility of the film allows it to contact all the folds of the brain. It can also be attached to the skin and monitor basic body functions.
"The idea is, you can mount these things on your skin. You can monitor rates, you can monitor dehydration ... you put on your little gold leaf tattoo, and this becomes a monitor on your skin of what is happening," he said.
Silk is so effective for biomedical applications primarily because of its "biocompatibility" - it is a biodegradable substance that can be integrated easily into human tissue. This quality makes it better both for the body and for the environment, according to Omenetto.
"The fact that it integrates into the body is great," he said. "If we can get it cost competitive ... it's certainly better than having plastics in the landfill. Plastic really doesn't go away, while this will eventually integrate into the environment."
These silk sensors are also edible. For this reason, they can be used as antennas with fruit or dairy products, to transmit information from a distance about their state of maturation.
"The point here is that you have a device that requires no power, that you're interrogating with an external device, and that is giving you a response as a function of the fruit that it's sitting on," Omenetto said.
Another hugely significant medical use of silk is that it can hold biological components such as vaccines, enzymes or antibodies. These substances can be added to the protein and water solution. When the water dissolves, they remain in the protein structure of the film. This renders some of these substances immune to temperature limitations.
"We've found you can store vaccines without need of refrigeration. Penicillin ... is devoted to curing very bad diseases and has to be kept in the fridge," Omenetto said. "We mixed it into silk, we made films and we stored them at 60 degrees Celcius for a couple of months. The penicillin didn't lose any efficacy."
One of silk's unique properties is that its breakdown can be programmed. This gives a high degree of control over how the substance behaves and is especially useful in silk sensors implanted inside the body to monitor infections post-surgery.
"You don't want an antenna inside you for a variety of reasons," Omenetto said. "You can program the degradation of these sensors, make them disappear after, say, 10 days, so that everything gets reintegrated."
According to Omenetto, silk's abilities make it an extraordinary substance that presents many possibilities for advancement in the biomedical field.
"Programmable degradability and the ability to store just takes this material and gives it a kind of a different life," he said. "You have the technological attributes on one side, and then you have the ability to tune the way the material behaves. I think that the unique material outcomes come from exactly this overlap of programmable form and programmable function."
