Salty science
Materials science and engineering professor Junqiao Wu has spent over a decade working with vanadium dioxide, a compound known for its ability to shift between insulating and metallic states. By harnessing this unusual property, his lab has developed innovative technologies ranging from “smart” roofing materials to ultra-precise thermography techniques.
His team recently discovered a new application for this versatile material: helping electronic sensors more efficiently interface with wet, salty systems. Using this material, researchers created a new in-memory sensor, or memsensor, that can both detect and remember its chemical environment, enabling it to adapt to challenging aqueous conditions.
“This breakthrough could pave the way for simpler, more energy-efficient sensors and adaptive robots capable of operating in complex environments,” said Wu. “It may also open exciting possibilities for next-generation computing systems that integrate memory and sensing in liquid settings, much like how biological neurons function in the brain’s wet, ionic environment.”
To create the memsensor, researchers attached a thin layer of vanadium dioxide to a small piece of indium, an extremely soft, silvery metal. When the device is placed in salt water, indium releases ions where the solution touches the vanadium dioxide. With the built-in electric fields at the solid-liquid interface, the ions are drawn to vanadium dioxide’s surface, causing part of the material to turn metallic — a resistance change that persists over time.
This shift in conductivity effectively “records” the history of salt exposure, and the rate of conductivity change correlates with the salt concentration. More importantly, the memsensor can sense and store this information without the need for any external voltage.
While engineering their memsensor, the researchers took inspiration from a surprising source: the tiny nematode C. elegans. “This worm uses specialized neurons to remember salt exposure and guide its movement toward or away from certain environments when searching for food,” said Ph.D. student Ruihan Guo. “Mimicking this behavior, we used our sensor to direct a small robotic boat to navigate salt gradients — avoiding undesirable zones and seeking favorable ones — based on its prior salt exposure history.”
Learn more: Researchers develop energy-efficient memory sensor for wet, salty environments; Memsensing by surface ion migration within Debye length (Nature Materials)