This summer, the team published a paper about their work, titled “Soft, stretchable, high power density electronic skin-based biofuel cells for scavenging energy from human sweat,” in the journal Energy & Environmental Science, in which they explained how they connected the cells to a custom circuit board and showed that it was able to power an LED.
The research was led by Professor Joseph Wang, who directs the UC San Diego Center for Wearable Sensors and has worked with 3D printing technology before. Wang collaborated with electrical engineering professor and center co-director Patrick Mercier and nanoegnineering professor Sheng Xu; co-authors include Amay J. Bandodkar, Jung-Min You, Nam-Heon Kim, Yue Gu, Rajan Kumar, A.M. Vinu Mohan, Jonas Kurniawan, Somayeh Imani, Tatsuo Nakagawa, Brianna Parish, and Mukunth Parthasarathy.
In order to work well with wearable devices, the team’s biofuel cell had to be stretchable, so they used a structure called a bridge and island – rows of dots make up the cell, all connected by spring-shaped structures that can bend and stretch. Half of these dots make up the cathode of the cell, while the other half make up the anode, and due to the flexibility that comes with this type of structure, neither is deformed when the biofuel cell bends. The basis for the structure is gold and was manufactured using lithography; the team then deposited layers of biofuel materials on the dots using screen printing.
Then the researchers had to turn their attention to the biggest challenge of the project – increasing the energy density of the biofuel cell.
Bandodkar, one of the first authors and a former PhD student of Wang’s, who is now a postdoctoral researcher at Northwestern University, said, “We needed to figure out the best combination of materials to use and in what ratio to use them.”
The team screen-printed a 3D carbon nanotube structure on top of the biofuel cell’s anodes and cathodes, which let them add more of the sweat-reactive enzyme to each anode dot and silver oxide to the cathode dots. 3D printed carbon nanotubes also improve the biofuel cell’s performance by making electron transfers easier. Mercier’s research group manufactured a custom circuit board – a DC/DC converter that’s able to take the cell-generated power and make it more even (as it changes with the amount of sweat a person produces), then transform it into constant power with constant voltage.
According to the research paper, “This is the first example of powering a BLE radio by a wearable biofuel cell. Successful generation of high power density under practical conditions and powering of conventional energy-intense electronic devices represents a major step forward in the field of soft, stretchable, wearable energy harvesting devices.”
The research team still has work to do, starting with finding a cathode material that’s more stable than silver oxide, which is light sensitive and will degrade over time. In addition, the test subjects could only light up the LED for four minutes, because the concentration of lactic acid in sweat dilutes as time goes on. The researchers are currently investigating ways to store the produced energy at times when the lactate concentration is higher, and then have it gradually released. Discuss in the UC San Diego forum at 3DPB.com.
[Source: Engineering.com, UC San Diego Jacobs School of Engineering / Images: UC San Diego Jacobs School of Engineering]