The University of California - San Diego team was able to achieve this breakthrough thanks to a combination of clever chemistry, advanced materials and electronic interfaces. This allowed them to build a stretchable electronic foundation by using lithography and 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, turning sweat into a source of power.
The engineers have reported their results in the journal Energy & Environmental Science. In the paper, they describe how they connected the biofuel cells to a custom-made circuit board and demonstrated that the device was able to power an LED while a person wearing it exercised on a stationary bike.
Stretchable and Flexible Devices
To be compatible with wearable devices, the biofuel cell needs to be flexible and stretchable. So the engineers decided to use what they call a "bridge and island" structure developed in Xu's research group.
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, the researchers used screen printing to deposit layers of biofuel materials on top of the anode and cathode dots.
Increasing Energy Density
The researchers' biggest challenge was increasing the biofuel cell's energy density, which was key to increasing the performance of the biofuel cells.
"We needed to figure out the best combination of materials to use and in what ratio to use them," said Amay Bandodkar, one of the paper's first authors, a former Ph.D. student in Wang's research group.
To increase power density, the team screen printed a 3D carbon nanotube structure on top the anodes and cathodes. The structure allowed the engineers to load each anodic dot with more of the enzyme that reacts to the lactic acid and silver oxide at the cathode dots. In addition, the tubes allow easier electron transfer, further improving biofuel cell performance.
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.
Future work is needed 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, which 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.
For an alternative source of energy for wearable devices, find out how this Thermoelectric Harvester Could Power Wearables.