Researchers develop 'ion switch' that turns current on and off using salt crystals

When saltwater dries, crystals form. Add water again, and they dissolve. Researchers have now harnessed this simple, everyday phenomenon to build an "ion switch" that turns current on and off much like a semiconductor transistor.

The Ulsan National Institute of Science and Technology (UNIST) announced Monday that a research team led by Professor Kim Tae-sung of the Department of Mechanical Engineering had developed an ion switch that controls current by forming and dissolving salt crystals inside channels just nanometers — one-billionth of a meter — wide.

Transistors, the core components of semiconductor chips, act as switches that turn current on and off. A state in which current flows is read as 1; a state in which it does not is read as 0. By combining the states of countless switches, chips store and process information.

The ion switch the team developed performs a similar function, but with a key difference: it controls the movement of ions rather than electrons.

The heart of the device is the salt crystal. The team filled a microchannel about 120 nanometers tall with a solution of potassium chloride or sodium chloride. When ions move freely through the solution, current flows. When the ions cluster together and form a salt crystal, the channel becomes blocked and current is cut off.

For the switch to operate repeatedly, the salt crystals must be able to form and dissolve on demand. The team built a separate control channel above the ion channel, through which either nitrogen gas or water can be passed. A thin membrane permeable only to water molecules separates the two channels.

When dry nitrogen flows through the upper channel, water evaporates from the solution below, raising its concentration until salt crystals form. When water is supplied instead, the crystals dissolve and ions can move again, restoring current flow. The current is switched off and on simply by controlling the evaporation and resupply of water.

In tests using a potassium chloride solution, the ion switch was cycled 32 consecutive times. The on-state current averaged 156 times higher than the off-state current.

In terms of switching speed, turning the current off took an average of 8.6 seconds, while restoring it by supplying water took less than 3 seconds.

The team also connected two ion switches on a single chip and successfully performed logic operations. Both an OR circuit, in which the two switches were connected in parallel, and an AND circuit, in which they were connected in series, operated correctly across all input combinations — demonstrating that basic computing functions can be carried out using ion flow alone.

Existing nanopore-based ion gates are highly sensitive to the chemical environment of the surrounding solution and to electrochemical reactions. The new technology, by contrast, adjusts ion conductance simply by controlling the amount of water in a separate control channel. Because the control channel and the ion-conducting channel are physically isolated from each other, the design also lends itself to integrating multiple ion switches on a single chip and controlling each independently.

The approach carries particular promise for biological applications, given that nerve signals in the human body are transmitted through the opening and closing of ion channels in cell membranes. The technology could eventually be extended to neuromorphic ion devices that mimic the brain's information-processing methods, as well as to biosensors, bio-machine interfaces and molecular valves.

"We started from the simple observation that saltwater forms crystals when it dries and dissolves when water is added, and from that built an ion circuit capable of performing logic operations," Kim said. "Just as transistors play a central role in semiconductors, we expect ion switches to serve as fundamental components of ion circuits that mimic the brain's information processing or handle biological signals."

The findings were published in Nature Communications on Sept. 25.