US academics have developed a skin-mounted battery which generates power for a chemical released in human sweat during exercise. If the technology could be improved, it could provide discrete power for disposable biosensors and other wearable devices.
Biosensor
The battery project began with the team from the University of California, San Diego (UCSD) producing a biosensor to monitor lactate mounted on temporary tattoo paper. Lactate is released during exercise as a by-product of the breaking down of glucose in cells to produce energy. Its presence in blood has long been used to gauge how strenuous the exercise an athlete is.
The battery project began with the team from the University of California, San Diego (UCSD) producing a biosensor to monitor lactate mounted on temporary tattoo paper. Lactate is released during exercise as a by-product of the breaking down of glucose in cells to produce energy. Its presence in blood has long been used to gauge how strenuous the exercise an athlete is.
Keen to eliminate the need to invasively take blood samples to measure lactate concentrations, the UCSD researchers printed a biosensor sensitive to the chemical on to the tattoo paper.
The sensor was impregnated with an enzyme which strips electrons from lactate excreted through sweat. As these electrons flow it is possible to equate the size of the current to the level of lactate present.
Sweat power
The next step was to redesign the sensor as a battery by creating a printed power cell with the electron harvesting enzyme on the cathode. As a result of harvesting the current, the sensor became self-sustaining.
The next step was to redesign the sensor as a battery by creating a printed power cell with the electron harvesting enzyme on the cathode. As a result of harvesting the current, the sensor became self-sustaining.
Project leader, Wenzhao Jia explains: 'The current produced is not that high, but we are working on enhancing it so that eventually we could power some small electronic devices. Right now, we can get a maximum of 70 µW per cm2, but our electrodes are only 2mm by 3mm 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. So besides working to get higher power, we also need to leverage electronics to store the generated current and make it sufficient for these requirements.'
Unhealthy power
During research on the prototype battery it was discovered that test subjects who exercised less regularly generated the most power with the enzyme battery. This is thought to be because less fit people move more quickly into a physiological process called glycolysis which produces more lactate.
During research on the prototype battery it was discovered that test subjects who exercised less regularly generated the most power with the enzyme battery. This is thought to be because less fit people move more quickly into a physiological process called glycolysis which produces more lactate.
The printed lactate sensor will be beneficial to athletes interested in their performance and medical patients with conditions like heart or lung disease which increase lactate production. Taking a wider perspective, the enzyme batteries could provide power to a range of other epidermal printed electronic sensors.
The research was presented on 13 August to the American Chemical Society's 248th National Meeting and Exposition in San Francisco.
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