Images: Hyung-Jun Koo/Ju-Hee So
7 December 2010— Researchers at North Carolina State University have demonstrated new “soft” electronic components, built from liquid metals and hydrogels. The scientists hope that such components—quasi-liquid diodes and memristors—will work better than traditional electronics to interface with wet squishy things, such as the human brain.
Ju-Hee So, a graduate student in chemistry at NC State, described a quasi-liquid diode at the fall meeting of the Materials Research Society in Boston last week. The device’s electrodes are made of an alloy—75 percent gallium and 25 percent indium—that is highly conductive and liquid at room temperature. The electrode is housed within a plastic casing. Sandwiched between the electrodes are two films made of agarose, a hydrogel commonly used in biochemistry that is more than 90 percent water by weight. Each film is doped with electrolytes; one contains polyacrylic acid (PAA), and the other holds polyethyleneimine (PEI), which is a base.
The resistance of the device can be changed repeatedly by applying voltage. The interface between the electrodes and the agarose naturally develops a thin, resistive coating of gallium oxide. But the high pH level of the basic PEI suppresses the formation of this skin at its electrode. Applying voltage across the diode alters the thickness of the oxide on the PAA electrode; a negative voltage makes the oxide thinner and lowers the device’s resistance. And a positive voltage produces a thicker skin and greater resistance. Varying the voltage allows the researchers to increase or decrease current flow, thereby switching between conducting and nonconducting states.
Because the device retains a memory of its resistance state when the current is turned off, it acts as a memristor. NC State’s So says the group’s memristor held its resistance state steadily for more than 3 hours. A memristor is a basic circuit element—along with the inductor, resistor, and capacitor—that had been only theoretical until 2008, when the first one was built. “You can combine diodes and memristors to make different types of circuits,” So says.
She and fellow graduate student Hyung-Jun Koo built a test version of the device into a crossbar array. The team—at the labs of NC State chemical engineering professors Michael Dickey and Orlin Velev—is also studying the interactions between different electrolytes and metals to find the optimum combinations. One goal will be to increase the speed at which the device can be switched from conducting to nonconducting and back. So believes that they may be able to achieve a speed measured in milliseconds.
So says these quasi-liquid components could one day be used to build bioelectronic circuits to provide connections between living tissue and computers, such as brain-machine interfaces. “People want to put information into the brain and read information out,” she says. Such an interface might, for instance, allow an amputee to control a prosthetic limb the same way he would control his real limb—with just a thought. Similar devices made with conventional technology tend to be rigid and must be encapsulated to protect the electrical circuits from the moisture inherent in biology. So believes the materials her team is working with will be compatible with human tissue. Gallium salts, for instance, are injected into people to improve the contrast in scans of human lungs, and hydrogels have many biological uses. The devices might also be used as components in artificial neural networks, an application to which memristors are already being applied in earnest.
Many of the details of how well such components can work and what devices can really be built from them remain to be seen. The group has been working on them for only about a year and a half. “It’s a very new concept,” So says.