Just 3 miles from the summit of L’Alpe d’Huez in the French Alps, Chris Froome felt as if he were about to hit “the wall”. Taking a rule-breaking sugar-gel cost him a 20-second penalty but he finished the stage and went on to win the Tour de France. Perhaps he won’t need last minute snacks if he gets a tattoo before next year’s race.
A new biosensor, worn like a temporary tattoo on the skin, tells athletes when lactate levels in their body are rising, offering an early warning that the energy stores in their muscles are running low, which can lead to the physical symptoms associated with hitting the wall, such as sudden loss of energy and extreme fatigue.
When physical activity is particularly intense, aerobic metabolism may be unable to provide the body with all of its energy needs. To make up the deficit, muscles consume their stores of glycogen and release energy through an anaerobic process.
A by-product of this process is lactic acid. As this builds up in the body, it increases the acidity in muscle cells, which is sometimes felt as a burning sensation. This slows or stops the person who is exercising, allowing aerobic metabolism to kick back in.
Athletes can monitor lactate levels in their blood, but this would involve regular blood tests – not practical during prolonged exercise.
Joseph Wang at the University of California in San Diego has a solution. Because lactate levels rise in sweat as well as in blood, his team realised they could develop a skin sensor that monitors lactate levels without disturbing the athlete.
Their sensor, which looks similar to a temporary tattoo, contains an enzyme that oxidises lactate to release electrons. These electrons then generate a current through the sensor’s thin and flexible electronics: the more lactate is oxidised, the larger the current. A small device attached to the tattoo measures the current to provide real-time information on lactate levels.
In future, those readings could be transmitted by bluetooth to a cellphone, says Wang.
The prototype device performed well in tests involving 10 volunteers using an exercise bike. The sensors, which were stuck to each volunteer’s right shoulder, remained attached and functioning throughout a 33-minute-long cycling session. What’s more, the readings they gave were as accurate as those obtained by scraping sweat off the volunteers and analysing the samples in the lab.
“I like the approach – their demonstration experiments are quite convincing,” says John Rogers at the University of Illinois at Urbana–Champaign.
Roisin Owens at the National Graduate School of Engineering in Saint Étienne, France, agrees. “It’s a really nice piece of work. It was one of those ‘I wish I had done that’ moments when I read it,” she says.
“The highpoint of the study is that the authors show real-time monitoring of live subjects during exercise and show that the measurement of the concentration of lactate matches very well the effort of the subjects,” says Owens. “They are also able to distinguish between a fit individual and an unfit one.”
Journal reference: Analytical Chemistry, DOI: 10.1021/ac401573r
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