Engineers at the University of California San Diego (UC San Diego) have developed a stretchy fuel cell that is powered by sweat. The ‘epidermal biofuel cells’ stick to the wearer’s skin and can power devices like LEDs and Bluetooth radios.

Fuel cells work by turning the chemical energy in hydrogen-based fuels into electrical energy when the fuel is exposed to oxygen. The chemical reaction takes place at the fuel cell’s electrodes and produce electrically charged particles. These are carried from one electrode to the other, completing the circuit and producing a current.

Combining chemistry, advanced materials and electronic interfaces, the team have made an exciting breakthrough. Their new cells can generate 10 times the power per surface area than any existing wearable biofuel cells.

Sweating it out

The novel fuel cells are based on a flexible, stretchable ‘bridge and island’ structure. Each cell is made up of rows of dots or ‘islands’ that are connected by spring-like ‘bridges’. Half of the dots make up the cell’s positive electrode, the anode, and half make the negative cathode.  The spring-like structures allow the cell to bend and flex without deforming the vital electrodes.

When it comes to powering up the cells, the engineers have to work up a sweat. Each of the ‘islands’ are loaded with an enzyme that oxidises the lactic acid present in human sweat. An oxidisation reaction causes the lactic acid to lose electrons, generating a current. This turns the sweat into a source of power.

In order to use the current, the cell connects to a custom-made circuit board. As the amount of sweat we produce varies, the power generated fluctuates. An on-board converter evens out this current, producing constant power with a constant voltage.

Putting the kit to the test, the team found the wearer could power a blue LED for four minutes while exercising on a stationary bike.

Building a biofuel cell

The island and bridge structure is created using a printing technique called lithography and is made of gold. As a second step, a special ‘ink’ is screen printed on top of the dots. The biggest challenge facing the engineers was increasing the cell’s ‘energy density’. This is the amount of energy a cell can create per surface area. The more energy produced, the more powerful the cell can be.

To increase the energy density, the team created a 3D carbon nanotube ‘ink’ that could be printed onto the electrode dots. This meant they could load each anodic dot with more of the enzyme that reacts to the lactic acid in sweat. The cathodic dots were loaded with silver oxide. The nanotube structure also allows for easier flow of electrons, boosting the cell’s performance.

The study was led by Professor Joseph Wang, director of the Centre for Wearable Sensors at UC San Diego. Joining him from the Jacobs School of Engineering at UC San Diego were Patrick Mercier, electrical engineering professor and centre co-director, and nanoengineering professor, Sheng Xu.

Photo credit: University of California, San Diego