Silk hydrogel battery could power pacemakers in mice

The smallest soft lithium-ion battery ever made consists of just three tiny droplets formed from a silk-based hydrogel. Droplet batteries can provide pacemaker-style control and deliver defibrillator shocks to beating mouse hearts, but could eventually be used in biomedical implants and wearable electronics for humans. It may supply electricity.

“Potentially, our small battery could be used as an implantable microrobotic battery, which could be moved to a target location by a magnetic field and release its energy for treatment.” . Yuka Zhang at Oxford University.

Zhang and colleagues designed the small battery as three connected droplets that can self-assemble in solution after injecting various components into the liquid with a microsyringe. One droplet contains lithium manganese oxide particles and serves as the battery's negative electrode. The second droplet contains lithium titanate particles and serves as the positive electrode. A central droplet filled with lithium chloride separates these electrodes. UV light energizes the battery by breaking down the layers that separate each droplet, allowing lithium ions to flow freely between the droplets.

Droplet batteries are one-tenth the length of traditional soft lithium-ion batteries. At just 600 micrometers, it is about six times as wide as a human hair. The battery is also 1000 times smaller in volume than similar flexible lithium-ion batteries. The central droplet can also incorporate magnetic nickel particles, allowing the battery to be controlled remotely via an external magnetic field.

Such a small battery also provides an unprecedented amount of energy considering its small size. Wei Gao at the California Institute of Technology. “This energy density is significantly higher than what has been achieved with other similarly sized batteries,” he says.

The droplet battery was tested on a mouse heart removed from the animal's body. These successfully functioned as defibrillators to restore normal heartbeats and as pacemakers to regulate heartbeats. Additional testing showed that the battery retained 77% of its original capacity after 10 charge/discharge cycles.

The simplicity and scalability of such droplet batteries could be a potential advantage over traditional battery manufacturing in the future, Gao said. He suggested that such batteries could power minimally invasive biomedical implants and biodegradable medical devices.

“What impressed me most is how this soft battery mirrors the aqueous environment of human tissue by being hydrogel-based,” Gao says. “However, we still need to consider the safety and biocompatibility of the materials used in this battery, especially as we move towards commercialization and further research applications.”