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 a "shocking" place: electric eels.
The team used a state-of-the-art fabrication method to layer multiple types of hydrogels - a water-rich material capable of conducting electricity - in a specific 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. They published their findings in Advanced Science.
According to Joseph Najem, assistant professor of mechanical engineering and corresponding author on the paper, 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. 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 electrocytes in electric eels are ultra-thin biological cells, capable of generating over 600 volts of electricity in a brief burst," Najem said. "These cells achieve very high-power densities, meaning they can produce a lot of power from small volumes."
The team built their power sources from only hydrogel to ensure the batteries remained non-toxic and flexible, even as they became more powerful.
"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," Najem said. "This motivated us to develop our strong power sources in a hydrogel-based system, which would operate well within biological environments."
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 - a fraction of the width of a human hair. This thin geometry reduces internal resistance, which is essential for producing high power, while preserving mechanical strength and flexibility, Najem explained.
