A team of researchers from the Departments of Electrical and Biomedical Engineering has developed a new technique to control the flow of ions in fluids — which they predict could push the frontiers of the new field of bioelectronics.

With their new diodes, called “nanoscale fluidic diodes,” scientists can control the flow of ions through membranes better than they can with currently-existing electric diodes, said doctoral student Weihua Guan GRD ’14, who developed the technology. Unlike electric diodes, these diodes rely on the field effect, a principle of electromagnetism used in computers and cell phones, and could be used in desalination techniques to purify water more efficiently.

“You might see the interrogation room in a movie, where the window on the wall is only transparent on one direction,” Guan said, comparing the flow of ions to the light in an interrogation room and the two-way mirror to a membrane. “With our device, we have the ability to switch which side of the room is transparent to the other, as well as how dim or bright it appears, just by a knob on the wall.”

The study’s principal investigator, professor of engineering and applied science Mark Reed ’57, said that this mechanism could serve as a building block for large-scale circuits that manage the flow and concentration of ions and molecules in electrolyte solutions, much as biological systems do. Nevertheless, he added, this method’s way of moving molecules and ions across membranes is very different from that found in nature.

Guan said that their diodes use the field effect, which relies on the idea that opposite charges attract, while like charges repel. Because the ions and molecules in the liquid are all charged, the diode can use electric charges, determined by digital programming, to control the ions’ flow.

“It is through this basic interaction that we achieved the modulation over the diode,” Guan said.

He added that the field effect is the basis of modern electronic components, such as those found in cellphones and computers, but it had not been used in ionic flow before now.

He said that for this project, building the device was more challenging than coming up with the idea for it. Guan said he had to try several methods to create a channel just 8-20 nanometers thick that could be controlled. Insistence and patience helped him past these frustrating moments and ultimately made this project successful, he added.

Other scientists predict that these diodes will alter the way chemical reactions take place in the field of bioelectronics. Li-Jing Cheng, a research assistant professor from the Department of Chemical and Biomolecular Engineering at Notre Dame, said that the ability to control these diodes precisely will advance the study of chemical reactions at the molecular level.

“There is no doubt that [the nanofluidic diodes] will impact the way we do bioelectronics,” Cheng said.

The Howard Hughes Medical Institute International Student Research Fellowships supported the project.