Eel-inspired 'droplet' battery could power tiny bio-devices

What is the droplet battery?

The team turned to eels for inspiration since the biological animal is capable of generating electricity with the help of internal ion gradients.

In their own battery design, the team used a chain of five nanoliter-sized droplets of a conducive hydrogel. A hydrogel is a 3D network of polymer chains that can absorb and retain large quantities of water. Each of these droplets contained a different composition so that a salt gradient could be created along the chain.

The droplets were separated from each other using lipid bilayers which not only prevented the flow of ions between them but also provided mechanical support.

To start the battery, the structure was cooled to a temperature as low as 39 Fahrenheit (four degrees Celsius), and the surrounding medium was changed. This leads to the disruption of the lipid bilayers and the formation of a continuous hydrogel.

The high salt droplets are at either end, while the low salt droplet is located in the middle. If electrodes are connected to either end of the droplets, the researchers succeeded in using the ionic gradient to generate electricity.

When using 50 nanoliter droplets, the researchers were able to generate a maximum power output of 65 nanowatts (nW), which lasted a little over 30 minutes. Interestingly, the device could also be stored for a period of 36 hours and still generate a similar amount of current.

To demonstrate the droplet battery could be used with human cells, the researchers attached them to human progenitor cells that were stained with a dye. When the battery was turned on, the cells displayed intercellular signaling.

"The miniaturized soft power source represents a breakthrough in bio-integrated devices," said Yujia Zhang, the lead researcher of the work, in a press release. "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 team added that their battery design was modular and multiple units created using a droplet printer could be combined together to increase the voltage or current output. 20 units of five droplet battery systems were sufficient to generate an output of two volts, which could power a light-emitting diode. Even a system containing thousands of power units was not far away.

The research was published today in the journal Nature on August 30.

Study abstract:

Bio-integrated devices need power sources to operate. Despite widely used technologies that can provide power to large-scale targets, such as wired energy supplies from batteries or wireless energy transduction3, a need to efficiently stimulate cells and tissues on the microscale is still pressing. The ideal miniaturized power source should be biocompatible, mechanically flexible, and able to generate an ionic current for biological stimulation, instead of using electron flow as in conventional electronic devices4–6. One approach is to use soft power sources inspired by the electrical eel7,8; however, power sources that combine the required capabilities have not yet been produced, because it is challenging to obtain miniaturized units that both conserve contained energy before usage and are easily triggered to produce an energy output. Here we develop a miniaturized soft power source by depositing lipid-supported networks of nanolitre hydrogel droplets that use internal ion gradients to generate energy. Compared to the original eel-inspired design7, our approach can shrink the volume of a power unit by more than 105-fold and it can store energy for longer than 24 h, enabling operation on-demand with a 680-fold greater power density of about 1,300 W m−3. Our droplet device can serve as a biocompatible and biological ionic current source to modulate neuronal network activity in three-dimensional neural microtissues and in ex vivo mouse brain slices. Ultimately, our soft microscale ionotropic device might be integrated into living organisms