The tiny light is not, at first glance, very impressive. But three volts is enough power for the threads to carry out functions such as biosensing. Kotov can turn the nanotube textiles into sensors simply by including antibodies in the initial ethanol solution. Because antibodies are sensitive to heat, the researchers let the material air dry instead of using a hair dryer; otherwise, the process is the same. The addition of the antibodies causes the fiber’s resistance to vary with the concentration of the antibody’s target molecule. Zhu takes a thread treated with a solution containing the antibody to the human version of the blood protein albumin and hooks it up to a multimeter, which supplies steady voltage to the thread and allows him to watch how its resistance changes. As he dunks the fiber into a dilute solution of blood, the thread’s resistance drops from 60 kilo-ohms to 20.
When the cotton is dipped into a solution of nanotubes, Nafion, and antibodies, the antibodies are physically trapped at intersections in the nanotube nets. When blood molecules adhere to the treated fabric, these antibodies attach to the albumin in plasma. The albumin-antibody complex, which is very soluble in blood, detaches from the nanotubes, allowing them to move closer together. Because current travels between nanotubes by means of “quantum tunneling,” essentially hopping from tube to tube, a small change in the distance between them “can lead to tremendous changes in resistance,” explains Kotov. The decrease in resistance that results when the antibodies detach from the thread is a more reliable measurement of albumin concentration than a decrease in conductivity would be. Reduced conductivity might be caused by dirt or other contaminants, but a decrease in resistance is a sign of just one thing: albumin, and thus spilled blood. Connected to a PDA capable of interpreting and even transmitting the results, clothing made from fabric treated this way could “generate a distress signal if, for instance, you’re unconscious,” says Kotov.
Using antibodies also makes this detection mechanism very specific: when the fabric is exposed to bovine blood, which contains a slightly different form of albumin, its resistance doesn’t change. Treated with antibodies against other proteins, such fabrics might help doctors monitor hospital patients for infections or warn asthmatics of allergens, Kotov says. And the method is so simple, sensitive, and potentially cheap that fiber-based nanotube sensors might even be used in place of emerging chip-based detectors to test blood samples for signs of diseases such as cancer.
Kotov’s sensors, while very reliable, are not reusable: once the antibodies detach from the nanotubes, they’re washed away, so the fabric can’t detect proteins a second time. Kotov says the fabrics should be inexpensive enough for single use. He’s also working on reusable versions, changing the chemistry so that the antibodies release their targets after detection and remain in the fabric.
Kotov is already working with Nico Technologies to develop garments made from the textiles for undisclosed military and civilian applications. However, he notes, future garments might include different types of coated thread, each treated for a different function. “You need just a single [nanotube-treated] thread in a garment,” he says, “and all the fundamental advances of nanotechnology are there.”