UNIVERSITY PARK, Pa. – Researchers at Penn State have developed a fresh, flexible battery technology inspired by the electric eel, potentially revolutionizing power sources for implanted medical devices, soft robotics, and wearable electronics. The battery, constructed entirely from hydrogels – water-rich, conductive materials – achieves higher power densities than previously reported for similar designs whereas remaining non-toxic and environmentally stable.
The research, published in Advanced Science, addresses a critical need for biocompatible power sources that can operate effectively within biological environments. Traditional batteries often contain toxic materials and rigid structures, making them unsuitable for employ inside the human body or in close proximity to living tissue.
“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,” said Joseph Najem, assistant professor of mechanical engineering and the corresponding author on the paper. “This motivated us to develop our strong power sources in a hydrogel-based system, which would operate well within biological environments.”
The team mimicked the ionic processes used by electric eels to generate electrical bursts. Electric eels utilize specialized cells called electrocytes, which are ultra-thin and capable of producing over 600 volts of electricity. “The electrocytes in electric eels are ultra-thin biological cells, capable of generating over 600 volts of electricity in a brief burst,” Najem explained. “These cells achieve very high-power densities, meaning they can produce a lot of power from small volumes.”
Previous attempts to create eel-inspired power sources have been hampered by limited power output and the need for mechanical support. The Penn State team overcame these challenges by fabricating extremely thin hydrogels, only 20 micrometers per layer, using a technique called spin coating. This process deposits uniform layers of material onto a rotating surface.
“We found that using thin hydrogel naturally reduced the internal resistance of the material, which increased the power densities we could output,” said Dor Tillinger, a doctoral candidate in mechanical engineering and co-first author of the study. Adjusting the chemical composition of the hydrogel was also crucial to maintaining its structural integrity and conductivity at such a thin scale.
“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,” explained Wonbae Lee, a doctoral candidate in materials science and engineering and also a co-first author. “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.”
The resulting power sources demonstrated power densities around 44 kW/m3, surpassing those of other hydrogel-based batteries. The incorporation of glycerol into the hydrogel formulation allows the battery to function at temperatures as low as -112 degrees Fahrenheit (-80 degrees Celsius) without freezing. The material also retains water for extended periods, maintaining conductivity for days in open air, unlike conventional hydrogels which dehydrate rapidly.
“To our knowledge, Here’s the first power source entirely contained within a hydrogel solution that requires no external support,” Najem stated. “We are not aware of any other hydrogel technology that can achieve these power densities while remaining flexible and environmentally stable.”
The researchers are now focused on increasing the power density and recharge efficiency of the batteries, as well as exploring the possibility of self-charging capabilities. Additional co-authors on the study include Derek Hall, assistant professor of mechanical engineering, and Haley Tholen, who recently completed her doctorate in mechanical engineering at Penn State. The work was supported by the Air Force Office of Scientific Research.