Touch Sensors Power Themselves

The touch sensors on smartphones and tablet computers allow people to sling angry birds across the screen or scroll through pages of cat pictures with a gentle finger swipe. To detect a user’s touch, sensors like those require an external power supply. Now researchers have used a sheet of plastic nanowires to build a sensor that powers itself (Nano Lett. 2014, DOI: 10.1021/nl5005652). The easy-to-assemble device could be used in security systems or even as flexible touch-sensitive skin for robotics, the researchers say.

Conventional touch sensors for mobile devices draw power constantly because they work by generating an electrical field. A user’s finger distorts the field, and the sensor detects these distortions. In recent years, researchers have been working on flexible plastic touch sensors that also run on external power. These sensors would serve as skins for robotics or haptic surgical devices.

Zhong Lin Wang, a materials scientist at Georgia Institute of Technology, wanted to build a device that is powered by the very mechanical input it senses. His group has spent many years developing nanowire devices that can harvest mechanical energy and convert it into electrical power. In particular, they have exploited the triboelectric effect, the phenomenon in which some materials pull electrons from others through friction. It’s the mechanism behind static electricity.

To build their self-powered touch sensor, Wang’s group stacks thin films of different materials. They start with a piece of nylon that serves as structural support. Then the team adds two films of indium tin oxide (ITO), a transparent conducting material, with a sheet of polyethylene terephthalate sandwiched in-between. The top layer of the device is fluorinated ethylene propylene (FEP), a flexible polymer known to excel at sucking up charges transferred through friction.

When someone steps on the sensor or brushes a fingertip across it, the FEP gathers charges on its surface and creates a net electric field, or voltage, across the two ITO electrodes. This voltage is strong enough to power an external circuit attached to the sensor. Wang’s group showed that the sensor was most sensitive when the FEP sheet was coated with 1.5-µm-long, 150-nm-diameter nanowires of the fluorinated polymer. The device can detect pressures as small as 0.03 kPa, which is less than the pressure typically produced when swiping a touchscreen, Wang says.

The researchers tested the sensor by attaching it to a doorknob or hiding it under carpet, and then connecting the device to an external control circuit for an alarm system. They showed that the alarm went off when someone touched the knob or stepped on the mat. Wang thinks the devices could be used in energy-efficient security systems that wouldn’t draw power until they were triggered.

Zhenan Bao, a materials scientist at Stanford University who has been developing flexible touch sensors, says the work also shows promise for use in electronic skin. The new sensors produced different electrical responses when touched with different types of glove materials, she says. So unlike many current tactile sensors, these devices may be able to detect subtle differences in texture, similar to what our skin can do.

Ali Javey, an engineer at the University of California, Berkeley, is also impressed by the ability of these sensors to power themselves. “This work represents an important advance in the field,” he says.