Spinning Silk into Sensors

Silkworm cocoons shipped by the boxful from Japan to an optics lab at Tufts University will meet a different fate from those headed to textile factories around the world. Rather than being woven into curtains or clothing, the strong protein fibers that caterpillars once spun around themselves will be used to build optical materials that can serve as the basis for sensors and other devices. Bioengineer Fiorenzo Omenetto, who creates the devices, ultimately hopes to build implantable, biodegradable sensors that could help monitor patients’ progress after surgery or track chronic diseases such as diabetes.

Omenetto realized that silk was good for more than shirts and ties, he says, when he got to talking with David Kaplan, the head of Tufts’s biomedical-engineering department, with whom he shares a hallway. Kaplan turns silk proteins into cell-friendly scaffolds for engineering biological tissues, including corneal implants. The strongest natural fiber known, silk is favored by tissue engineers because it’s mechanically tough but degrades harmlessly inside the body.

Trained as a physicist, Omenetto figured that if silk made good artificial corneas, it might also make good optical devices. As it turns out, he says, the silk devices he’s making work as well as those made from traditional optical materials like glass and plastic–in some cases, even better. And unlike those materials, silk doesn’t need to be processed at high temperatures or with harsh chemicals.

That’s one reason that silk is so well suited for use in biosensors: because silk devices can be manufactured in a gentle environment, it’s possible to incorporate additional biological molecules (such as proteins) into them as they are being built. These molecules serve as sensors that, once integrated into the silk devices, can remain active for years. In the devices that ­Omenetto and Kaplan are developing, proteins embedded in the optical material efficiently bind to a target such as oxygen or a bacterial protein; when they do, the light transmitted by the sensor changes color.

Optical Recipe
Omenetto’s recipe begins with cocoons spun by the silkworm Bombyx mori. First, he says, “you cut the cocoon and remove the worm–much to the chagrin of vegans.” Senior research technician Carmen Preda then boils the cocoons in a solution containing the salt sodium carbonate. This helps dissolve sericin, a gluey glycoprotein that holds the cocoons together but causes immune reactions in humans. After the silk fibers dry, they’re dissolved in a solution of lithium bromide. When it cools, Preda uses a syringe to load it into a dialysis cartridge. She sets this inside a beaker of water, which draws out the salt.

What’s left in the cartridge is a clear, viscous solution of the purified protein silk fibroin. Preda removes this silk “syrup” from the cartridge with a syringe and loads it into a row of test tubes; this is the starting material for Omenetto’s optical components. If he wants to use the components in a biosensor, he can add a protein targeting a particular molecule–say, oxygen-binding hemoglobin–at this stage. “You have this nice water-based solution that you can mix anything into,” Omenetto says.

Hemoglobin is a relatively stable ­protein, but the silk materials can also preserve the activity of less resilient proteins, such as enzymes. As a test case, the Tufts researchers have made silk structures containing a volatile horseradish enzyme called peroxidase; glucose sensors might incorporate hexokinase, an enzyme that binds to the sugar.

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.

Silk Sensors
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.