Wet Electronics Open Door to New Possibilities

R. Colin Johnson

Gadgets, gizmos and wireless wonders must be fastidiously protected from moisture today, but researchers using circuitry with the consistency of Jell-O claim that the smarter electronics of the future will be all wet.Twice I ran my old Sony-Ericsson cell phone through the washing machine and it miraculously survived, but that is only a testimonial to device's excellent waterproofing technologies. That all may change soon, when ultra-secure moisture-friendly prototypes recently shown by North Carolina State University (NCSU) are commercialized.
Today, electronic devices of all types must be protected from not only submersion in water, but even from humidity in the air. Medical implants, for instance, must be hermetically sealed to secure them from shorting out. By harnessing the synergy between water-compatible hydrogels and liquid metals, NCSU researchers herald a new era of smarter moisture-compatible electronic devices.

  A 2-by-2 array of crossbar switches where memory-resistors at each crossing operate like synapses in the brain. (Source: NCSU)

As you might imagine, materials that can happily be submerged without dissolving or shorting out their circuitry are few and far between. And those that can—such as plastics—have inferior electrical characteristics, making them too slow reacting for medical implants and other mission-critical electronics that must work rain or shine. However, by combining liquid metals with polyelectrolyte hydrogels, which have the consistency of Jell-O, a new class of fast submersible gadgets is on the horizon.
The key to this invention of NCSU professor Michael Dickey, however, is not the water compatibility of the materials themselves, but rather the ability of the metal—an eutectic alloy of gallium and indium—to form a nonconductive oxide skin when current flows through it. The switches can be programmed to act like synapses in the brain. In effect, these crossbar switches remember their "experiences"—an effect called a memory-resistor, or memristor, by their inventor, University of California at Berkeley professor Leon Chua (this technology is currently being commercialized by HP Labs and Hynix).
Consequently, the new liquid-metal/hydrogel combination can be used to create brainlike circuitry that learns from its environment. The first task of these new water-compatible circuits, however, will be much less ambitious, since for one thing they are still being built on the millimeter scale rather than the micron- and nano-scale of circuitry in the brain. However, simple circuitry can be realized with the new approach to create biological sensors that can be directly implanted for medical monitoring.
NCSU doctoral candidates Hyung-Jun Koo and Ju-Hee So also contributed to the work, which was funded by the National Science Foundation and the U.S. Department of Energy.