If you’ve ever seen commercials for sports drinks, you’ve probably heard the mention of electrolytes. Mostly known as agents to maintain fluid balance in the human body, electrolyte intake is promoted as a key part of what keeps athletes going.

Scientists, however, believe electrolytes can also replicate physiological processes, potentially improving the energy efficiency and computing speed of future iontronic devices — soft, flexible electronics that use ions in liquids to transmit signals like the human body does. 

Northwestern Engineering’s Monica Olvera de la Cruz, Eleftherios Kirkinis, and Ahis Shrestha in the Center for Computation and Theory of Soft Materials found that placing specific ions and electric charge along the wall of a narrow channel creates a one-way flow of liquid inside it, without needing any external force. While mimicking how the body moves signals, this new method works differently from current approaches that rely on complex shapes or applied electric fields to move the liquid.

Unlike previous methods that depend on specially designed channel shapes or large external inputs, this approach uses a simple combination of wall charge and controlled ion release to drive flow. That shift simplifies the design and avoids common issues in existing systems, such as wear and tear from electrolysis or complications from managing multiple external forces.

“This approach could improve how new iontronic devices carry information by ion transport in an aqueous environment rather than electrons and holes of current solid-state concepts. These devices will be manipulated by both chemical and electrical signals,” Olvera de la Cruz said.

“They may be employed in the design and operation of flexible and wearable electronic devices for medical sensing and monitoring. They can also be used as energy storage devices,” Shrestha said.

Olvera de la Cruz is the Lawyer Taylor Professor of Materials Science and Engineering at the McCormick School of Engineering. Shrestha, the paper’s first author, is a postdoctoral scholar at the Center for Computation and Theory of Soft Materials. The researchers presented their findings in the paper “Self-Generated Electrokinetic Flows from Active-Charged Boundary Patterns,” published June 3 in Physical Review Research.

The new study expands on earlier research by Olvera de la Cruz and Shrestha which showed that a nanoparticle with two differently coated sides — known as a Janus particle — could move on its own when combining ion movement with surface charge. That work demonstrated the potential for self-propulsion at the nanoscale.

In their latest study, the researchers scaled up the concept to fluid motion in narrow channels, showing that a similar mechanism, combining surface charge and controlled ion release, can create a continuous, directional flow of liquid without external input. This advancement not only simplifies the physical setup but also opens the door to applying ion-based signalling in larger, more versatile systems, paving the way for practical iontronic devices that mimic biological communication. 

The researchers’ process could lead to more energy-efficient and powerful alternatives to today’s solid-state electronics.

“The present mechanism avoids known challenges and provides a self-generated propulsion mechanism that could be employed in future iontronic concepts,” Kirkinis said.

One key feature the researchers aim to build into iontronic devices is memory — the ability of a system to retain information about past signals or states, much like how biological synapses strengthen connections based on experience. In current research, this is often measured by the size of the current-voltage hysteresis loop, which reflects how the system responds differently to the same input depending on its history. This kind of built-in memory is essential for developing iontronic systems that can learn, adapt, and perform computations over time, bringing them closer to mimicking the behavior of neurons in the human brain.

In future work, the researchers plan to show that similar memory effects can come from ion flow driven by moving charges or electric potentials along the walls of simple straight channels. This means that these boundary-driven systems could be strong candidates for iontronic devices.

Current methods of measuring memory don’t consider important material properties like frequency or viscosity. To improve this, the team is looking to introduce a new, universal way to define memory time based on how long it takes the system to settle back to normal after the external input is turned off. This new memory measure keeps track of all the system’s past states and doesn’t depend on the specific boundary conditions used. This approach will help guide future iontronic experiments.