Using the skin’s electrical conductance to track sweat loss during both physical and mental activities

Over the past decades, electronics engineers have developed a wide range of wearable devices that can be used to track some physiological processes and collect health or fitness-related data. These devices rely on miniature sensors that can pick up different signals, such as heart rate, blood oxygen levels and body temperature.

Most wearable devices used by individuals worldwide are particularly effective in tracking physiological changes that occur while users are physically active, for instance, while they are walking, running, lifting weights or engaging in other forms of exercise. This is because physical activity typically results in larger physiological changes than mental activities.

A long-standing approach to monitoring mental states, such as stress levels, entails the measurement of electrodermal activity, which is essentially the varying electrical conductance of the skin. This activity is known to be associated with sweating, which tends to increase when people are under stress or mentally exerting themselves.

Up until now, the tracking of skin conductance was not deemed particularly useful for tracking physical activity. This is because while exercising, humans generally sweat a lot and excess sweat tends to accumulate between the skin and sensing devices, which makes detecting any changes in sweat levels difficult.

Researchers at the University of California–Berkeley (UC Berkley) and the Lawrence Berkeley National Laboratory recently demonstrated that electrodermal activity could be used to track sweat loss both when people are mentally and physically active. Their paper, published in Nature Electronics, could inform the development of future fitness trackers, smart watches, bio-medical technologies and other wearable devices.

“Electrodermal activity has long been used for mental activity monitoring by measuring skin conductance at specific locations, such as fingertips, with high sweat gland density,” Seung-Rok Kim, Yifei Zhan and their colleagues wrote in their paper.

“However, electrodermal activity has not been considered useful for physical activity monitoring, where large sweat volumes are generated, resulting in the accumulation of sweat at the skin—electrode interface and, thus, preventing further dynamic response to sweating events. We show that electrodermal activity can be used as a proxy for sweat loss measurement under both low and high physical activity levels.”

As part of their study, Kim, Zhan and their colleagues developed a new sensor, dubbed SkinG, which consists of μ-lace electrodes integrated with a microfluidic channel. Its unique design ensures the rapid removal of sweat from the interface between the skin and the sensor, preventing its accumulation over time and thus also enabling the monitoring of sweat loss while users are engaged in physical exercise.

“We use wearable sweat sensors that consist of water-permeable electrodes and microfluidic-based sweat analyzers and show that skin conductance is proportional to the instantaneous sweat loss,” wrote Kim, Zhan and their colleagues. “We demonstrate that sweat loss during exercise can be estimated by integrating skin conductance over time, which can be applied to assess the body hydration status of exercisers.”

The researchers tested their sensors in a series of experiments, placing them on different locations on the arms of human participants. They showed that some locations were better for detecting sweat produced during physical activity and others for that resulting from mental states.

“From multisite measurements of skin conductance, we show that the wrist, forearm and upper arm are reflective of physical activity levels, whereas the finger is indicative of mental activity,” wrote Kim, Zhan and their colleagues. “Simultaneous measurement of two different sites selectively decouples mental and physical activities.”

This recent study demonstrates that measuring sweat loss during physical activity by monitoring the skin’s electrical conductance is in fact possible. In the future, the sensor they developed and the insight gathered in their experiments could be used to broaden the capabilities of wearable devices, further improving their ability to track users’ physiological changes while they are engaged in a wide range of everyday activities.

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