One day, your morning gym workout or jog could not only be recharging you, but also recharging your smartphone or other small electronic device. Scientists have developed a biosensor mounted inside a temporary tattoo that can monitor the wearer’s progress as they exercise and harness their sweat to produce electrical power.
The innovation is the work of a team led by Joseph Wang, Distinguished Professor and Chair of Nanoengineering at the University of California. They presented their novel biobattery approach at a recent meeting of the American Chemical Society in San Francisco.
Compared with conventional batteries, biobatteries have several advantages: they recharge more quickly, they use renewable energy (in this case body sweat), and they do not explode or leak toxic chemicals.
Prof. Wang says their work shows “the first examples of epidermal electrochemical biosensing and biofuel cells that could potentially be used for a wide range of future applications.”
As we sweat, we produce lactate, “a very important indicator of how you are doing during exercise,” says Dr. Wenzhao Jia, a postdoctoral student in Prof. Wang’s lab.
Generally, the more intensely we exercise the more lactate we produce, as aerobic respiration is not enough to produce the energy we need, and anaerobic respiration kicks in. Anaerobic respiration converts glucose or glycogen to lactic acid, generating energy in the process.
Professional athletes monitor their lactate levels to evaluate their fitness and training performance. Doctors also asses lactate levels during exercise to test patients for heart or lung disease, and other conditions marked by unusually high lactate.
Dr. Jia and her colleagues have developed a faster, easier and non-invasive way to measure lactate during exercise in real time. Before their innovation, the only way to do this was by taking blood samples at regular intervals during exercise and sending them away for analysis.
The new sensor, which can be imprinted onto a temporary tattoo, contains an enzyme that produces a weak electrical current by stripping electrons from lactate molecules.
The scientists tested the new device on 10 healthy volunteers. They applied the temporary tattoos to the upper arms of the volunteers and measured how much electrical current they produced as they exercised.
The volunteers exercised on stationary bikes for 30 minutes, with resistance gradually increasing over the period. The sensors allowed the scientists to monitor sweat lactate levels as they changed with exercise intensity.
The team then developed the technology a stage further and made a sweat-powered biobattery.
They used the enzyme that strips the lactate of electrons to act as the anode, and used a chemical that accepts the electrons to be the cathode. Electrons moving from an anode to a cathode is the basic principle on which a battery works.
To see how the device works, play the video below.
The team tested the biobattery on 15 volunteers exercising on stationary bikes. As before, the device was incorporated within a temporary tattoo applied to their upper arms.
The different volunteers produced varying amounts of power in their tattoo biobatteries. Curiously, the less fit volunteers appeared to produce the most power. Those who exercised only once a week produced more power than those who exercised at least three times a week.
One possible explanation is that less fit people become fatigued more quickly, causing lactate-producing anaerobic respiration to kick in earlier.
The less fit volunteers produced around 70 μW per square cm (cm2) of skin. Dr. Jia says this is not a large amount of current, but they are working on how to to enhance it so it could eventually be enough to power small electronic devices:
“Right now, we can get a maximum of 70 μW per cm2, but our electrodes are only 2 by 3 millimeters in size and generate about 4 μW – a bit small to generate enough power to run a watch, for example, which requires at least 10 μW.”
She says they also need to find a way to store the generated current.
The National Science Foundation and Office of Naval Research are funding the work.
Medical News Today also recently reported how an engineer at Stanford University is working on wireless powering of medical implants deep inside the body.