Stretchable fuel cells power devices by extracting energy from sweat
Stretchable fuel cells that extract energy from sweat in order to power electronics, such as LEDs and Bluetooth radios, have been developed by a team of engineers from the University of California San Diego.
The biofuel cells generate 10 times more power per surface area than any existing wearable biofuel cells and could be used to power a range of wearable devices.
The fuel cells are constructed by using lithography and using screen-printing to make 3D carbon nanotube-based cathode and anode arrays.
The biofuel cells are equipped with an enzyme that oxidizes the lactic acid present in human sweat to generate current.
The engineers have tested the device by connecting it to a custom-made circuit board and demonstrating its ability to power an LED while a person wearing it exercised on a stationary bike.
To be compatible with wearable devices, the biofuel cell needs to be flexible and stretchable, so a “bridge and island” structure was used.
Essentially, the cell is made up of rows of dots that are each connected by spring-shaped structures. Half of the dots make up the cell’s anode; the other half are the cathode. The spring-like structures can stretch and bend, making the cell flexible without deforming the anode and cathode.
The basis for the islands and bridges structure was manufactured via lithography and is made of gold. As a second step, researchers used screen printing to deposit layers of biofuel materials on top of the anode and cathode dots.
Increasing the biofuel cell’s energy density or the amount of energy it can generate per surface area was one of the biggest challenges.
“We needed to figure out the best combination of materials to use and in what ratio to use them,” said Amay Bandodkar who worked on the project.
To increase power density, engineers screen printed a 3D carbon nanotube structure on top the anodes and cathodes.
The structure allows engineers to load each anodic dot with more of the enzyme that reacts to lactic acid and silver oxide at the cathode dots. In addition, the tubes allow easier electron transfer, which improves biofuel cell performance.
The biofuel cell was connected to a custom-made circuit board that includes a DC/DC converter that evens out the power generated by the fuel cells, which fluctuates with the amount of sweat produced by a user, and turns it into constant power with a constant voltage.
Researchers equipped four subjects with the biofuel cell-board combination and had them exercise on a stationary bike. The subjects were able to power a blue LED for about four minutes.
The team is now focusing on improving its design in two areas. First, the silver oxide used at the cathode is light sensitive and degrades over time. In the long run, researchers will need to find a more stable material.
Also, the concentration of lactic acid in a person’s sweat gets diluted over time. That is why subjects were able to light up an LED for only four minutes while biking. The team is exploring a way to store the energy produced while the concentration of lactate is high enough and then release it gradually.