On a dry winter day, shuffling across a carpeted floor often can end in a painful shock thanks to static electricity. A team of scientists would like to exploit the phenomenon behind that annoying zap to build useful devices. By harnessing the electron exchange created when certain materials rub together, the researchers developed a simple and inexpensive pressure sensor that doesn’t need an external power source (ACS Nano 2013, DOI: 10.1021/nn4037514). The devices someday could be incorporated into artificial skin to sense contact or used in computer touch screens.
Zhong Lin Wang of the Georgia Institute of Technology has worked for years to develop devices based on the triboelectric effect—the phenomenon behind static electricity. The effect happens when one material, like a person’s socks, rubs against another, like a carpet. The rubbing transfers electrons from one material to the other, making one positively charged and the other negatively charged. Wang and his group have built devices in which a polymer layer rubs against a metal layer to charge batteries and to detect chemicals.
He thought similar devices also could detect applied pressure. When the two oppositely charged layers in the devices move apart, a voltage develops between them that depends on the distance between the layers. So Wang envisioned a sensor that measures pressure based on changes in voltage caused by the two layers moving toward and away from each other.
To build the sensor, Wang and his colleagues first etched a mold out of a silicon wafer and coated it with polydimethylsiloxane (PDMS) to create a uniform grid of pyramids, each 10 by 10 μm at the base. The researchers then peeled the polymer grid off the mold and deposited a gold electrode on the back. Next, the researchers dipped an aluminum film into a solution containing silver nanowires and nanoparticles. The nanomaterials on the film increase the surface area of the metal layer, which provides more contact between it and the PDMS pyramids. More contact between the layers leads to greater electron transfer. Finally, the researchers bonded the gold and aluminum electrodes together so that each layer arched away from the other with a gap between the two. A person can press on the 2-cm-wide oval device and push the layers together.
The team tested the sensor by applying known pressures and then measuring the voltage and current between the two electrodes. As they applied more force, the gap between the layers shrunk and the voltage increased. The current depended on “how fast you close the gap,” which allowed the scientists to gauge the speed of the pressure stimulus, Wang says. After calibrating the sensor, they found that the device could detect pressures as low as 2.1 pascals, which is equivalent to the pressure produced by two $1.00 bills stacked on top of each other and lying flat. The device also was robust: It could handle 30,000 presses.
Wang’s team also built a six-by-six grid of the pressure sensors. They pressed plastic letters—T, E, N, and G—onto the grid and detected the electrical output. A two-dimensional plot of the signal intensities looked like the letters, suggesting such an array of sensors could map patterns of pressure on a surface.
Xudong Wang of the University of Wisconsin, Madison, says the simple design is impressive. “They can achieve very high voltage output that’s almost linear with pressure,” he says, meaning the sensor can provide information about how much pressure is applied, not just whether it is applied. He thinks the researchers next should make the device’s fabrication method suitable for commercialization.