Flexible, non-toxic hydrogel batteries could power soft, wearable and implantable electronics, according to a study.

Researchers at Pennsylvania State University have taken inspiration from the biology of electric fish, such as eels, to develop a hydrogel-based soft material capable of conducting electricity.

Joseph Najem, assistant professor of mechanical engineering and corresponding author on the paper, said: “The electrocytes in electric eels are ultra-thin biological cells, capable of generating over 600 volts of electricity in a brief burst. These cells achieve very high-power densities, meaning they can produce a lot of power from small volumes.” 

The team’s approach involved layering multiple types of hydrogels – water-rich materials capable of conducting ions – in a specific pattern that mimics the ionic processes electric eels use to generate high-power electrical bursts.

Using a spin coating technique, which deposits ultra-thin layers of material on a rotating surface, they layered four different hydrogel mixtures, each only 20 micrometers thick. This fabrication technique differs from other hydrogel-based designs, which rely on mechanical supports to function, but as a result generates only limited power. 

According to Wonbae Lee, doctoral candidate in materials science and engineering and co-first author, to make their hydrogel thinner they had to adjust the chemistry and then test several approaches before deciding on the optimal mixture. 

Lee said: “We had to carefully tune the chemical mixture so the hydrogel could spread uniformly during spin coating, remain mechanically stable and be thin enough to maintain low electrical resistance. Optimising the viscosity and mechanical strength of our hydrogel was essential to making this approach work.”

The results showed that their solution is capable of producing higher power without the need for mechanical supports, as the thin hydrogel layers naturally reduce the material’s internal resistance while preserving strength and flexibility.

Najem said: “To our knowledge, this is the first power source entirely contained within a hydrogel solution that requires no external support. We are not aware of any other hydrogel technology that can achieve these power densities while remaining flexible and environmentally stable.”

As such, their non-toxic, flexible and support-free hydrogel solution could be used for powering devices in or near biological tissue, such as implanted medical sensors, soft robotics controllers and wearable electronics.

Najem said: “For biomedical and near-biology applications, we have to make sure that batteries are compatible with their surroundings, flexible, safe and ideally capable of using available resources to recharge.

“This motivated us to develop our strong power sources in a hydrogel-based system, which would operate well within biological environments.” 

According to the team, future work will focus on further increasing the power density and recharging efficiency of the power sources, while also exploring self-charging capabilities.

Their study – ‘Electric‐fish‐inspired thin hydrogel electrocytes achieve high power density and environmental robustness’ – has been published in the journal Advanced Science.