Engineers from the University of California – San Diego have developed stretchable fuel cells that extract energy from an often-unpleasant source: sweat. The flexible UCSD-developed devices are capable of powering wearables and electronics such as LEDs and Bluetooth radios.

A researcher demonstrates the biofuel cell's flexibility and stretchability. (Credit: UCSD)

To turn sweat from an annoyance into power, the biofuel cells are designed to chemically react with lactate, a component of sweat. An enzyme, lactate oxidase, removes electrons from the lactic acid present in human perspiration to generate current.

"Sweat is a neglected, unappreciated biofluid," Amay Bandodkar, a former graduate research scholar at UCSD and first author of the biofuel cell study, told Tech Briefs. "However, from an energy point of view, it contains certain chemicals that can be exploited to produce usable energy."

The researchers demonstrated that the biofuel cells, when connected to a custom-made circuit board, could power an LED while a wearer exercised on a stationary bike.

Remember to Stretch

Many wearable-device batteries are large and make the entire system bulky. To address the growing challenge, the UCSD engineers sought a more flexible solution.

Because the aim of the project was to develop a biofuel cell to power wearable devices, the technology was designed and fabricated in a soft, stretchable format – one that can easily mate with the soft, curvilinear nature of the human skin.

A “bridge-and-island” design gave the device a layout of isolated “islands” interconnected with thin, spring-like structures. The fuel cell’s anodes and cathodes were fabricated on top of the standalone enclaves.

When the island-bridge structure is stretched, most of the strain is accommodated by the serpentine interconnects, while leaving the islands unharmed. The spring-like gold structures, manufactured via lithography, stretch and bend – making the cell flexible without deforming the anode and cathode.

"Since island-bridge architecture permits negligible strain on the islands, we had the freedom to deposit active anode and cathode materials in a dense fashion without the fear of them experiencing mechanical stress and the subsequent degradation when the device stretched during routine use," said Bandodkar.

Powering Up

Carbon nanotube-based cathode and anode arrays, developed through lithography and screenprinting processes, formed an essential piece of the technology. To increase power density, engineers screen-printed a 3D carbon nanotube structure atop the anodes and cathodes.

The 3D nature of the carbon-nanotube pellet system allowed higher loadings of the lactate oxidase enzyme – and higher electron transfer. The arrangement led to power density levels higher than previously reported devices: approximately 1mW/cm2 of power from human sweat.

To test the technology, the UCSD researchers equipped four subjects with a biofuel-cell-and-board combination. While exercising on a stationary bike, the participants were able to power a blue LED for about four minutes.

A circuit board, connected to the fuel cell, evened out any fluctuating power generated by the fuel cells. A DC/DC converter turned the flow into constant power with a constant voltage.

"The very fact that we were able to increase the power density to almost 10 times as compared to previous works, make the device soft and stretchable, and power an energy-hungry device, such as a Bluetooth radio, is most exciting to me," said Bandodkar, who is currently a Postdoctoral Fellow at Northwestern University.

The team still has several challenges they hope to address, including the stabilization of the lactate oxidase enzyme, which degrades over time, and increase of the system's power density.

"It would also be great if we can combine the biofuel cell with other forms of wearable energy harvesting systems, such as wearable solar cells and thermoelectrics, so that such an integrated energy harvesting system can produce energy from multiple sources," said Bandodkar.

Professor Joseph Wang, who directs the Center for Wearable Sensors at UC San Diego, led the biofuel cell research, in collaboration with electrical engineering professor and center co-director Patrick Mercier and nanoegnineering professor Sheng Xu, both also at the Jacobs School of Engineering at UC San Diego. The engineers reported their results in the June issue of Energy & Environmental Science.