Runners and cyclists soon may have a new gadget to combine with the pedometers and heart-rate monitors they wear to keep tabs on their performance. Researchers at the University of California, San Diego, have developed a thin decal sensor that’s worn on the skin—much like a temporary transfer tattoo—and continuously monitors lactate levels in sweat (Anal. Chem. 2013, DOI: 10.1021/ac401573r). By keeping track of the metabolite, the sensor can help gauge the wearer’s fitness and physical performance.
In an intense workout, when an athlete is pushing herself as far as she can go, lactate starts to accumulate as a result of glucose metabolism in muscle cells. Trainers, doctors, and exercise scientists look for this rise in lactate levels to gauge when a person’s body has reached its threshold of exertion. Finding this threshold can help determine a person’s overall fitness or whether a metabolic or physiological disorder exists.
Currently, the only way to monitor lactate production during exercise is by taking blood samples at regular intervals with finger pricks and sending the samples to a lab. “The finger pricks are so inconvenient for athletes; you have to stop every one or two minutes,” says Joseph Wang, a researcher in nanoengineering at the University of California, San Diego.
Wang’s group thought their tattoo sensors would be a less invasive way to get the same information. They first developed the tattoo technology when creating sensors for monitoring the pH of skin (Analyst 2013, DOI: 10.1039/c2an36422k). To make the pH or the lactate sensors, the researchers use a screen-printing method to put the electronic components onto commercial tattoo transfer paper. By pressing the transfer paper onto the skin, the sensor sticks to the skin, and the backing paper can be peeled off.
To make the lactate sensor, the researchers printed three electrodes—one out of silver and silver chloride inks and two out of conductive carbon ink—onto the tattoo transfer paper. They covered one of the carbon electrodes with lactate oxidase (LOx), an enzyme that converts lactate into pyruvate and releases two electrons in the process. When lactate in sweat hits the sensor, the released electrons generate a current proportional to the number of lactate molecules present. The researchers measure the currents with a small, cell-phone-sized microampere meter connected by wires to the sensor’s electrodes.
The researchers tested the biosensor on ten volunteers of different fitness levels. The volunteers rode a stationary bicycle at a steady rate for 33 minutes as the researchers increased and decreased the bike’s resistance to change how much effort the subjects needed to exert.
The researchers watched the current generated by the sensor over that period and confirmed that lactate levels in the skin increased with physical effort. The timing of the lactate spike differed for each volunteer, depending on the person’s fitness level, as expected, Wang says. In another experiment, the team correlated data obtained with the tattoo biosensor with lactate measured by finger pricks.
What’s more, the biosensors were robust, Wang says: “We tested for stretching, bending, and twisting of all types, and we found no effect on the measured current.”
Wang and his colleagues have started a company called Electrozyme to commercialize the biosensor. Michael McAlpine, a nanotechnology researcher at Princeton University, thinks the devices hold promise as commercial products. “These sensor systems are generally not too expensive and will come down in cost,” he says. “And the process of putting them on the skin is no different from the temporary tattoos kids do all the time.”