A miniature battery for bio-integrated devices

Inspired by electric eels, researchers from the University of Oxford have developed a miniature “droplet” battery that could, some day, power tiny bio-integrated devices inside animal bodies. The device is made up of a series of droplets of conductive hydrogel, each with a different concentration of salts, creating a gradient that causes the movement of ions to generate current (Nature 2023, DOI: 10.1038/s41586-023-06295-y).

The droplets are separated by a lipid bilayer to prevent the flow of ions until needed. When the structure is cooled to around 4 °C, the lipid layers are disrupted, causing ions to flow from high to low concentrations, and release energy.

“Between the reservoirs of high and low salt concentrations, we put materials that are charge selective,” says first author Yujia Zhang. Therefore, in one-half of the battery, only positively charged ions can go through, and in the other half only negatively charged.

This study builds on earlier work from 2017, which mimicked the cellular structure of the electric eel and how it generates power from ion gradients. The Oxford researchers were able to further miniaturize the original model, shrinking it down 100,000 times and thereby increasing the power density by almost 700 times.

“We use 50 nanoliter droplets,” Zhang says, “and a single discharge will take around 30 minutes.” This setup was enough to modulate neuronal network activity in ex-vivo mouse brain tissue.

For use in an actual bio-integrated device, the researchers will need to develop some kind of encapsulation for the droplets and figure out how to trigger energy discharge. “The ultimate goal is to fully implant this thing in vivo,” Zhang says. Stimulating neurons was a first step, and the researchers envision applications such as stimulating cell growth and wound healing.

Anasua Mukhopadhyay, a biophysicist at the Adolphe Merkle Institute in Fribourg who was not involved in the study says that this work is a step forward for bio-integrated systems. “Implementing these in physiological conditions perhaps needs more understanding,” she adds, but it has “huge potential.”