New ‘Droplet Battery’ Could Power Next-Generation Wearable Devices and Implants

Small bio-integrated devices with the capability to interact with and stimulate cells have potential therapeutic applications, including targeted drug delivery and faster wound healing. However, these devices require a power source to function. Until now, finding an efficient method for providing power at the microscale has been a challenge. Scientists have now developed a miniature power source that can alter the activity of cultured human nerve cells. Drawing inspiration from how electric eels generate electricity, this device employs internal ion gradients to generate energy.

Researchers at University of Oxford (Oxford, UK) have created a miniaturized soft power source that works by depositing a sequence of five nanoliter-sized droplets of a conductive hydrogel (a 3D network of polymer chains infused with a significant amount of water). Each droplet possesses a distinct composition, generating a salt concentration gradient across the chain. These droplets are separated by lipid bilayers, which provide structural support while preventing ions from moving between the droplets. The power source becomes active when the structure is cooled to 4°C and the surrounding medium is changed. This disrupts the lipid bilayers and causes the droplets to merge into a continuous hydrogel. This allows ions to travel through the conductive hydrogel, from the high-salt droplets at the ends to the low-salt droplet in the center. Connecting the end droplets to electrodes converts the energy released from the ion gradients into electricity, enabling the hydrogel structure to serve as a power source for external components.

In the study, the activated droplet power source produced a sustained current for over 30 minutes. A unit comprising 50 nanoliter droplets yielded a maximum output power of around 65 nanowatts (nW). The devices maintained similar current levels even after 36 hours of storage. The research team then demonstrated how living cells could be attached to the device, allowing their activity to be directly regulated by the ionic current. Human neural progenitor cells stained with a fluorescent dye were connected to the device. When the power source was activated, time-lapse recording exhibited waves of intercellular calcium signaling in the neurons, induced by the local ionic current. According to the researchers, the device's modular design could facilitate the combination of multiple units to enhance generated voltage and/or current. This potential advancement could pave the way for powering next-generation wearable devices, bio-hybrid interfaces, implants, synthetic tissues, and microrobots. By linking 20 sets of five-droplet units in series, the researchers were able to illuminate a light-emitting diode requiring approximately 2 Volts. They envision that automating device production, such as using a droplet printer, could result in droplet networks comprised of thousands of power units.

“This work addresses the important question of how stimulation produced by soft, biocompatible devices can be coupled with living cells. The potential impact on devices including bio-hybrid interfaces, implants, and microrobots is substantial,” said Professor Hagan Bayley from the Department of Chemistry at University of Oxford, who was the research group leader for the study.

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