According to the engineers, the biofuel cells generate 10 times more power per surface area than any existing wearable biofuel cells.
"We needed to figure out the best combination of materials to use and in what ratio to use them," said PhD student Amay Bandodkar.
To create the epidermal biofuel cells, the researchers used lithography and screen-printing to make 3D carbon nanotube-based cathode and anode arrays. The biofuel cells are equipped with an enzyme that oxidises the lactic acid present in human sweat to generate current.
To be compatible with wearable devices, the biofuel cell needed to be flexible and stretchable. So engineers decided to use what they call a 'bridge and island' structure whereby 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.
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 are said to allow easier electron transfer, which improves biofuel cell performance.
The researchers then connected the biofuel cells to a custom-made circuit board and demonstrated the device was able to power an LED while a person wearing it exercised on a stationary bike.
