The bridge and island structure of the biofuel cell allows it to be flexible and stretchable. Photo: UC San Diego

A new, wearable device developed by a team of California researchers shows perspiration can truly be powerful.

A stretchable biofuel cell developed by a team of engineers at the University of California San Diego extracted enough energy from human sweat to operate a bluetooth low-energy radio and an LED light—a development the researchers call “a major step forward in the field of soft, stretchable, wearable energy-harvesting devices.” The device, which reacts with the lactic acid in sweat to produce electrical energy, was able to generate far more power than any existing wearable biofuel cells.

While new manufacturing techniques, innovations in material design and advancements in nanotechnology have allowed researchers to create thin, soft and stretchable wearable electronics, the UC San Diego engineers note in a recent paper that “a key challenge to enable adoption in practical use cases is the lack of correspondingly thin wearable energy sources.”

“Most of the wearable devices mandate batteries that are quite large and make the entire system bulky. We wanted to find a suitable solution to this issue,” Amay Bandodkar told Laboratory Equipment. Bandodkar is the first author of a paper on the biofuel cell that was published in a recent issue of Energy & Environmental Science.

Previous efforts to develop thin, stretchable batteries have faced issues with limited energy storage capacity, and therefore required frequent recharging. Attempts at wirelessly powering wearable electronic devices have also faced obstacles, such as equipment limitations that require users to stay in a particular geographical area.

The paper identifies a wearable energy harvester that scavenges energy from body motion, body heat and body fluids as promising alternative approaches; but notes that of these options “utilizing biofluids, especially human sweat, for generation of electricity by wearable biofuel cells is an appealing avenue since these systems rely on innocuous biomolecules for energy conversion.”

“Sweat is a neglected, unappreciated biofluid. However, from an energy point of view, it contains certain chemicals that can be exploited to produce usable energy,” said Bandodkar, a postdoctoral fellow at Northwestern University who was a Ph.D. student at UC San Diego when the device was being developed.

So, with a focus on finding a way to transform sweat into power, the research team set out to create a biofuel cell that was soft and flexible but still capable of producing enough energy to operate electronic devices.

Skin meets electronics
“There is a distinct mismatch between the mechanical properties of human skin and conventional electronics (including energy source devices). The human skin is soft, stretchable and has complex three-dimensional features at [the] microscopic level. On the other hand, conventional devices are planar and rigid,” Bandodkar said.

In order to overcome that challenge, the UC San Diego team developed a biofuel cell using an “island-bridge structure.”

The cell is comprised of rows of isolated dots, or islands, half of which are the cell’s anode and half of which are the cell’s cathode. Those islands are “interconnected with thin serpentine, spring-like structures,” Bandodkar said, a design that allows for flexibility while also maintaining the integrity of the anode and cathode dots. The foundation for the island and bridge structure was built using lithography and is made of gold.

“When such an island-bridge structure is stretched most of the strain is accommodated by the serpentine interconnects while leaving the islands unharmed,” Bandodkar explained. That structure allowed the research team to focus on the next challenge: improving the device’s power density.

“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 subsequent degradation when the device stretched during routine use,” Bandodkar said.

The team designed the cell’s anodes and cathodes as densely packed 3-D pellets made of carbon nanotube, which improved the cell’s performance by increasing storage capacity for the enzyme that reacts with lactic acid to generate electrons and facilitate the transfer of electrons from the enzyme to the underlying electrode.

“These features helped us ramp up the power produced by the biofuel cell,” said Bandodkar.

The device is capable of generating about 1 milliwatt per square centimeter. That’s nearly 10 times higher than any other previously reported wearable biofuel cell.

This increased power density is particularly important considering the fact that the concentration of lactic acid in human sweat diminishes over time due to the increased sweat rate.

While the paper notes that some biofuel cells have been shown to power custom or commercially available radios, those cells were “optimized to operate either as an implant within the human body, or in fruit, both of which offer higher fuel concentrations that are not available for a wearable device.”

“Thus, increasing power density in wearable [biofuel cells] is essential,” the authors write, “especially when considering the transient nature of lactate concentration in human sweat.”

The team reported in their paper that when the cell was affixed to a person riding a stationary bicycle, the device was able to harvest enough energy to power a blinking LED light for about four minutes before tapering off. A custom-made DC-DC converter circuit board was attached to the cell to produce consistent voltage, since the power generated by the biofuel cell fluctuates depending on the amount of sweat being produced.

After the LED stopped blinking, the researchers disconnected the device as the subject continued cycling to allow for the generation of fresh lactic acid. After a couple of minutes, the device was reconnected and the LED started blinking again.

“Such a demonstration reveals the ability of the [epidermal biofuel cell] as a potential power source for wearable applications,” the authors wrote.

Future challenges
The research team was led by Joseph Wang, the director of the Center for Wearable Sensors at UC San Diego, and also included the center’s co-director, Patrick Mercier, and Sheng Xu, a nanoengineering professor at UC San Diego.

While the team reported that their research demonstrates the practicality of using biofuel cells as a wearable power source, they also acknowledge that “additional efforts must be undertaken to harness the true potential of such systems.”

Among the main challenges researchers face in further developing this wearable technology is finding a way to stabilize the enzyme used at the cathode, since silver oxide is light-sensitive and degrades over time.

“One could look into other active materials, such as manganese oxide, to address this issue,” the researchers note in their paper. The authors also suggest integrating micro-fluidics with the biofuel cell to supply fresh sweat to the fuel cell while directing away used, old sweat to help improve its performance.

“At the same time, efforts must be made to develop soft, stretchable architecture for the DC–DC converter electronics based on recent advances in wearable electronics,” the authors wrote.

In addition to further increasing the power density of the system, Bandodkar said “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, thermoelectrics, etc., so that such an integrated energy-harvesting system can produce energy from multiple sources.”

But, Bandodkar said, “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 able to power an energy-hungry device like a Bluetooth radio is most exciting to me.”

Researchers demonstrated that their flexible biofuel cell can power LEDs and a Bluetooth radio, using perspiration as its energy source.