via 3D Print.com : Sometimes, if the weather is nice and I’m feeling motivated enough, I’ll go for a brisk walk around the neighborhood, but I can’t make it very far if I don’t have my phone with me…sassy dance workout tunes keep me going. Wouldn’t it be great if, before you went for a jog or hit the gym, you didn’t have to worry if your iPhone or portable Bluetooth radio was fully charged? A team of engineers from the UC San Diego Jacobs School of Engineering has announced a breakthrough invention that could fix this issue – stretchable biofuel cells which extract energy from human sweat and can power wearable electronics, like Bluetooth radios and LEDs. These cells actually generate 10 times the power per surface area than existing wearable biofuel cells.
Just yesterday, we told you about how NASA is using astronauts’ recycled urine to make omega-3 fatty acids and 3D printable material – so why not use human sweat to power the electronics we wear while exercising? It works because the cells have an enzyme that oxidizes lactic acid in sweat and generates current. The team built a stretchable electric foundation by using screen-printing, lithography and 3D printing to create carbon nanotube-based cathode and anode arrays.
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.
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.
The biofuel cell was connected to this circuit board, and given to four test subjects, who wore the biofuel cell-circuit board combo while riding a stationary exercise bike. With their sweat, the test subjects were able to use the invention to power a blue LED for four minutes.
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.