Wearable Measures Sweat to Determine Muscle Fatigue

A prototype of the lactate sensor depicting its three electrodes: 1) counter electrode, 2) reference electrode and 3) working electrode. (Image courtesy of ECS Journal of Solid State Science and Technology.)

Researchers have recently developed a prototype of a wearable sensor that can determine lactate levels in human sweat. Lactate levels rise in the body when energy is in high demand, such as during sports and exercise. The ability to monitor lactate levels accurately could potentially offer an indicator of problems such as muscle fatigue, stress and dehydration.

A Sustainable Electrochemical Sensor

While the idea of monitoring human sweat to track body states is not new, the designers took a novel approach in two respects.

First, the enzyme-based sensor is unique in that it detects lactate levels via an enzymatic electrochemical process. The sensor is comprised of three electrodes: a working electrode made of enzyme-based multiwall carbon nanotube paper (called buckypaper, or BP), an Ag/AgCl reference electrode and a BP counter electrode. An electrochemical technique called chronoamperometry is then employed, wherein a potential of 0.3 V is applied to the working electrode and the current is monitored as a function of lactate concentration.

"We're doing it electrochemically, so we're looking at applying a constant load to the sensor and generating a current response," said researcher Jenny Ulyanova, "which is directly proportional to the concentration of our target analyte."

The second novelty of the sensor is that it’s powered by a safe and renewable biofuel cell based on glucose oxidation. This biomimetic approach to the power source means that the sensor can be self-powered by the same enzymatic processes that it uses to measure lactate levels.

Image of the biofuel cell. It measures 1.4 x 1.4 x 0.3 in (3.5 x 3.5 x 0.7 cm) and weighs 8.3 g. (Image courtesy of ECS Journal of Solid State Science and Technology.)

"The biofuel cell works in this particular case because the sensor is a low-power device," said Ulyanova. "They're very good at having high energy densities, but power densities are still a work in progress. But for low-power applications like this particular sensor, it works very well."

From Prototype to Wearable

Since we’re seeing an influx of health-related wearables designed to count steps, measure heart rate and even determine blood oxygen levels, it’s reasonable to expect that a lactate sensor could become popular among athletes looking to refine their techniques. The researchers have already received interest from high school football coaches seeking the sensor for their players.

"The sensor was designed for a soldier in training at boot camp," said researcher Sergio Omar Garcia, "but it could be applied to people that are active and anyone participating in strenuous activity."

Despite the sensor’s success in the lab, it has yet to be developed into a wearable device suitable for on-body tests. The team is currently working to optimize aspects of the sensor’s design, such as making it suitable for adhering to human skin. Once this is achieved, the designers are also looking to implement extra features such as wireless transmission of data as well as more sensors to detect other biomolecules in addition to lactate.

You can read the team’s full paper on the sensor in the ECS Journal of Solid State Science and Technology.

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