sweat tattoo
August 14, 2014

Sweat Powers Tattoo-Based Biobattery

April Flowers for redOrbit.com - Your Universe Online

Tattoos have been used for many purposes over the years. In ancient times, and in indigenous cultures, they have been representations of status and position. In our more recent past, they were used by sailors, soldiers and bikers to show membership in a "club." Today, they serve cosmetic purposes from covering scars to wearable art. It makes one wonder what the future of tattoos might be.

A new study, presented at the 248th National Meeting & Exposition of the American Chemical Society, reveals that tattoos can be used to monitor your health and possibly power your small electronics. The research team from the University of California, San Diego (UCSD) has developed a small sensor that can be applied as a temporary tattoo. This sensor monitors your progress during exercise and produces power from your sweat.

Lactate, a chemical naturally present in sweat, is what the monitor is designed to detect and respond to. "Lactate is a very important indicator of how you are doing during exercise," said Wenzhao Jia, PhD, a postdoctoral researcher in the lab of Joseph Wang, DSc.

The general rule is simple; more intense exercise equals more lactate production. The body needs more energy during strenuous physical activity, so it activates a process called glycolysis — which produces energy and lactate. Lactate is also measurable in the blood.

During performance testing, professional athletes monitor their lactate levels to evaluate their fitness and training program. Doctors also monitor lactate levels during exercise testing of patients with conditions that normally exhibit extremely high lactate levels — for example, heart or lung disease.

Current methods of lactate testing are inconvenient and intrusive. They involve multiple blood samples being collected at various times throughout the exercise regime and then analyzed.

Jia's team developed a much faster, more comfortable way to measure lactate during exercise by imprinting a flexible lactate sensor onto temporary tattoo paper. An enzyme in the sensor strips electrons from lactate — a process which creates a small electrical current. Ten healthy volunteers wore the temporary tattoos on their upper arms while working out on a stationary bike. The researchers measured the electrical current produced as the resistance levels on the bike were increased over 30 minutes. This allowed them to continuously monitor sweat lactate levels over time and with changes in workout intensity.

Next, the team used their findings to create a sweat-powered biobattery. Energy is produced in a battery when current, in the form of electrons, passes from an anode to a cathode. The new biobattery's anode contained the enzyme that removes lactate from sweat, while the cathode contained a molecule that accepts the electrons.

Differing amounts of power were generated by the 15 volunteers who wore the new tattoo biobatteries. The researchers were surprised to find that people who exercised fewer than once a week produced more power than those who exercised one to three times a week. Extremely fit people, who exercised more than three times a week, produced the least amount of power. The team attributes this to the fact that less-fit people become tired sooner, causing glycolysis to kick in sooner and form more lactate. In the low-fitness group, the maximum amount of energy produced was 70 microWatts per cm2 of skin.

"The current produced is not that high, but we are working on enhancing it so that eventually we could power some small electronic devices," Jia says. "Right now, we can get a maximum of 70 microWatts per cm2, but our electrodes are only 2 by 3 millimeters in size and generate about 4 microWatts — a bit small to generate enough power to run a watch, for example, which requires at least 10 microWatts. 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."

Because they recharge more quickly, use renewable energy sources (in this case, sweat), and do not explode or leak toxic chemicals like traditional batteries, biobatteries are very desirable.

"These represent the first examples of epidermal electrochemical biosensing and biofuel cells that could potentially be used for a wide range of future applications," Wang says.

Image 2 (below): A tattoo biosensor (enlarged above) detects lactate levels during exercise; a biobattery using the technology could power electronics. Credit: Joseph Wang


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