Scientists at University of California San Diego have developed biofuel cells capable of extracting energy from sweat of the person wearing the fuel cell paving way for wearable devices that need almost no external power or battery to function.
Measurements have shown that the biofuel cells generate 10 times more power per surface area than any existing wearable biofuel cells. The biofuel cells were developed after extensive research in chemistry, advanced materials and electronic interfaces.
The cells have been developed using lithography and by 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 thereby turning sweat into a source of power.
In the paper published in Energy & Environmental Science, the creators of the biofuel cell describe how they 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.
The key behind usability of the biofuel cell is to ensure that it is not only flexible but also stretchable and is able to connect with wearable devices to provide power. For this UC San Diego scientists used “bridge and island” structure wherein the biofuel 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.
As with any power generating and storage device, the biggest hurdle for biofuel cells was energy density. 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.
Scientists have already demonstrated the biofuel cell usability by connecting it with a custom-made circuit board that is effectively 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.