Elegance is as important in scientific design as it is in art and architecture, chemical engineer Nicholas Kotov believes. Sitting in his austere office at the University of Michigan, in Ann Arbor, he shows off a swatch of black cotton; in heft and feel it’s similar to a soft, lightweight dress shirt. But Kotov has transformed the fabric into a biosensor and an electrical conductor simply by dipping it into a solution of carbon nanotubes, antibodies, and a polymer.
Individual, well-formed carbon nanotubes are highly conductive, which makes them promising for applications such as battery electrodes and microprocessors. If molecules such as antibodies are anchored to their surface, they can also serve as very sensitive chemical detectors: when an antibody binds to its target, the nanutobe’s electrical properties are measurably altered. But nanotubes tend to clump together, which prevents them from functioning individually. That seriously degrades their electronic properties, says Kotov.
There are ways around this problem: nanotubes can be painstakingly laid down, one by one, using methods that involve days of solution processing followed by photolithography, or the tubes can be sprayed onto a flat surface in alternate layers with a conductive polymer, which prevents clumping. But Kotov found that this type of layer-by-layer assembly can be further simplified for a complex three-dimensional surface such as a cotton thread: the tangle of fibers provides a structural template that allows him to simply dip the thread into a solution containing both the polymer and the tubes. Glued to the thread by the polymer, the nanotubes form a net with good electrical properties, the tubes overlapping but well spaced.
The method results in a sleek, powerful, and much more wearable alternative to complex intelligent textiles that incorporate heavy, bulky optical fibers or corrosion-prone metal wires. While Kotov is exploring a number of possible applications for these textiles, the most important, he says, would be as biosensors to keep people safe. They could be used to spot blood loss in soldiers on remote patrols or to detect airborne allergens or pathogens such as influenza. And the threads are cheap and sensitive enough for possible use in factories or stores, or even in the home–for example, to test an iffy batch of peanut butter for toxins.
A Quick Dip
In Kotov’s lab, graduate student Jian Zhu mixes commercially available single-walled nanotubes and a polymer called Nafion into ethanol, which prevents the components from sticking together. The Nafion glues the nanotubes to the cotton, but that’s not all it does. Nafion, a long, conductive molecule composed primarily of carbon, acts like a tiny spring, allowing each nanotube some measure of independent movement. This mechanical property, which is critical for biosensing, also allows the cotton to maintain its softness and give: you wouldn’t want to wear a shirt coated in stiff epoxy.
Zhu snips a length of ordinary cotton thread from a spool and uses a pair of tweezers to submerge it in the inky-black solution. After it sits for two minutes, he fishes out the thread and uses a binder clip to hang it up to dry inside a lab hood, a process that can be shortened to only a few minutes with a hair dryer. The electrical resistance of the thread is optimized, Kotov has found, when it has been dipped about 10 times.
In the group’s student office, Zhu demonstrates the electronic properties of a finished nanotube thread, which is indistinguishable from ordinary black cotton. He attaches it to the electrical contacts on a white light-emitting diode using ordinary solder, then draws the ends of the thread through the positive and negative clips on a power source. He turns the power source up to three volts, and the light shines brightly.
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.”