The molds used to shape the silk-protein solution into optical devices are patterned with nanoscale features. Such fine detailing is important in optics, since light interacts best with features at a scale no bigger than its own wavelength–about 400 to 700 nanometers in the case of visible light. In the ambient light of the lab, the plastic molds’ nanopatterned regions shine softly, like the inside of an abalone shell.
One device the researchers have made is a hologram, demonstrating that silk has the same versatility as other optical materials. At the lab bench, postdoc Jason Amsden uses a pipette to deposit silk solution onto a mold etched with the Tufts logo. He leaves the mold on the counter at room temperature for about eight hours–long enough for the proteins to set into a flexible, irregular oval displaying the logo in a three-dimensional pattern of iridescent pinks and blues.
In other molds around the lab, different types of optical devices have already finished drying. Amsden selects one and gently peels it from the mold using tweezers. The device is a translucent red card impregnated with hemoglobin and patterned with several optical elements, including a diffraction grating that splits white light into its component colors.
The card acts as a simple oxygen sensor: light passing through it changes wavelength slightly, depending on how much oxygen has bound to the embedded hemoglobin. These changes can’t be seen with the naked eye but can be detected by a photodiode, a chip that turns light into electrical current. When a drop of oxygen-rich blood is placed on the sensor, for example, the hemoglobin draws in oxygen from it, and the wavelength of light registered by the photodiode shifts.
Oxygen is just one possible target for Omenetto’s devices. Gratings with antibodies and enzymes embedded in them could sense just about any medically interesting molecule, be it glucose or a tumor marker. And the Tufts researchers envision not just lab sensors but implantable ones. One application Omenetto has developed will be particularly important: silk optical fibers for carrying light from the surface of the skin to the implanted sensors and back, so that it can be read by a photodetector. The sensors could be implanted during surgeries such as tumor resections and then used to monitor patients for signs of infection or recurring cancer. Omenetto and Kaplan also hope to integrate the sensors into future tissue-engineering structures that would help doctors track how well a new tissue is being incorporated into the body. The devices would dissolve harmlessly with the rest of the tissue’s supportive structures.
Future sensors, Omenetto says, will have designs that lead to more dramatic color changes when the sensors bind to their targets. To create sensors that can be read with the naked eye, he drew inspiration from another insect, the morpho butterfly. Its shimmering blue color is due not to pigments but to the way light interacts with nanoscale protein pillars on its wings. Changing the pillars’ structure eliminates the color. Omenetto imagines a silk-based sensor patterned with nanoscale structures that make it appear blue; a target molecule binding to proteins in the sensor would subtly change the nanostructures, making the color change or disappear. Omenetto says that the basic technologies for doing this are in place; it’s simply a matter of designing the right molds.