Power sources used in devices found in or around biological tissue must be flexible and non-toxic, while still powerful enough to support demanding technologies such as medical devices or soft robotics. To achieve this balance, researchers at Penn State are taking inspiration from electric eels.
The team used a state-of-the-art fabrication method to layer multiple types of hydrogels in a pattern that mimics the ionic processes electric eels use to generate electrical bursts. Their approach produces power sources with higher power densities than other hydrogel-based designs, while remaining flexible, support-free, environmentally stable and biologically compatible.
Researchers have looked to the biology of electric fish, such as eels, as inspiration to develop soft power sources previously. However, most existing eel-inspired devices produce limited power and require mechanical support to function. The electrocytes in electric eels are ultra-thin biological cells, capable of generating over 600V of electricity in a brief burst.
To address these problems, the team adjusted the material chemistry to fabricate very thin hydrogels, which can produce more power without the need of mechanical supports. The team built their power sources from only hydrogel to ensure the batteries remained non-toxic and flexible, even as they became more powerful.
Using spin coating, a technique that deposits ultra-thin layers of material on a rotating surface, the team layered four different hydrogel mixtures, each only 20 micrometers thick. This thin geometry reduces internal resistance, which is essential for producing high power, while preserving mechanical strength and flexibility.
“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,” said first co-author Wonbae Lee. “Conventional formulations would simply fly off the spinning surface during spin coating. Optimizing the viscosity and mechanical strength of our hydrogel was essential to making this approach work.”
Their new sources had power densities of about 44 kW/m3, which is enough to efficiently power complex devices like implanted medical sensors, soft robotics controllers, and wearable electronics.
By incorporating the chemical glycerol, the power sources remain functional at temperatures as low as minus 80 °C without freezing. The material also retains water longer than conventional hydrogels. While standard hydrogels can dehydrate within a few minutes and lose conductivity, the new formulation can remain hydrated for days in air.
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