Over the past decade, researchers have made many skin-like medical and environmental sensors. But adding stretchable wireless communication circuits to these sensors has been difficult. Those radio frequency (RF) circuits rely on diodes, semiconductor devices that help convert AC power to DC power, working at megahertz frequencies.
Researchers have now made the first stretchable high-frequency RF diode (Nature 2021, DOI: 10.1038/s41586-021-04053-6). It could enable conformable sensors and radio frequency identification (RFID) tags—small electronic labels used to track objects—that can wirelessly receive or send signals.
Zhenan Bao, the Stanford University chemical engineer who led the new work, has made other stretchable devices, but stretchy diodes proved to be both a materials and an engineering challenge, she says.
Diodes contain a semiconductor sandwiched between electrodes and current collectors. “Electrodes and semiconductors that can pass a high current and also tolerate high mechanical deformation didn’t exist, so we had to invent those materials,” Bao says. In addition, the team added films of gold and silver at the anode and cathode to ensure charge movement to the current collectors. As a demonstration, the researchers coupled the diode to an antenna printed from gallium-indium ink, a strain sensor printed from carbon nanotube ink, and a stretchable, colored pixel made from a polymer sandwiching a solid electrolyte. The antenna wirelessly receives energy that the diode converts into a DC voltage to power the device. The resistance across the strain sensor changes when the sensor is stretched, increasing the voltage applied to the pixel and causing it to gradually get darker. The concept could be used to make wireless stretchable temperature or pressure sensors or other types of devices, such as stretchable LEDs, solar cells, and transistors.
Kuan Sun, a materials scientist and engineer at Chongqing University, says the stretchable diodes’ long-term stability needs to be investigated further, as silver ions can diffuse under an electric field. But he says the device shows superior performance and “should help bridge wearable electronics with wireless communications.”