Electric eels inspire new Oxford ‘droplet battery’

Inspired by the way electric eels generate a charge, the design uses an ionic gradient across a chain of tiny hydrogel droplets to produce electricity. According to the Oxford team, the work is a significant step towards realising miniature bio-integrated devices, capable of directly stimulating human cells. The study is published in Nature.  

“The miniaturised soft power source represents a breakthrough in bio-integrated devices,” said lead author Dr Yujia Zhang, from Oxford’s Department of Chemistry.  

“By harnessing ion gradients, we have developed a miniature, biocompatible system for regulating cells and tissues on the microscale, which opens up a wide range of potential applications in biology and medicine.”

The miniature battery was produced by depositing a chain of five nanolitre-sized droplets of a conductive hydrogel. Each droplet has a different composition, resulting in a salt concentration gradient created across the chain. Droplets are separated from their neighbours by lipid bilayers, which provide mechanical support while also preventing ions from flowing between the droplets.

The battery is activated by cooling the structure to 4°C and changing the surrounding medium, disrupting the lipid bilayers and causing the droplets to form a continuous mass. This allows the ions to move through the conductive hydrogel, from the high-salt droplets at the two ends to the low-salt droplet in the middle. By connecting the end droplets to electrodes, the energy released from the ion gradients is transformed into electricity, enabling the hydrogel structure to act as a power source for external components.

In the study, the battery produced a current which persisted for over 30 minutes. The maximum output power of a unit made of 50 nanolitre droplets was around 65 nanowatts (nW). The devices produced a similar amount of current after being stored for 36 hours.

The Oxford team then demonstrated how living cells could be attached to one end of the device so that their activity could be directly regulated by the ionic current. The team attached the device to droplets containing human neural progenitor cells, which had been stained with a fluorescent dye to indicate their activity. When the power source was turned on, time-lapse recording demonstrated waves of intercellular calcium signalling in the neurons, induced by the current from the droplet battery.

“This work addresses the important question of how stimulation produced by soft, biocompatible devices can be coupled with living cells,” said research group leader Professor Hagan Bayley, also from Oxford’s Department of Chemistry.  

“The potential impact on devices including bio-hybrid interfaces, implants, and microrobots is substantial.”