Date: 25th March 2021
Thermoregulation is a form of homeostasis, and in humans is in partly controlled by sweat. Sweat is continuously secreted during sedentary and routine activities and changes in sweat rates and composition can reflect underlying health conditions such as nerve damage, chronic stress, and autonomic and metabolic disorders. However, collecting resting thermoregulatory sweat for non-invasive analysis of body physiology is challenging, as secretion rates are low and it can quickly evaporate. Now, researchers use microfluidics for wearable patches that combat evaporation, sweat analysis was compatible with sedentary, routine, and daily activities, facilitating autonomous monitoring of body physiology at rest.
The rate of resting sweat secretions can reflect sympathetic nervous system activity stemming from underlying health conditions. Whilst, changes in sweat rates can be indicative of diseases such as Parkinson’s, diabetes and cerebrovascular diseases, it can also be affected by chronic psychological stress, anxiety and pain, including post-traumatic stress disorder. Resting sweat is uniquely poised to give us insights into these conditions, without the complication of vastly higher but variable rates that are induced by exercise or other external triggers.
Now, researchers at Lawrence Berkeley National Laboratory and the University of California, US, led by Ali Javey, present a wearable patch for the continuous measurement of resting thermoregulatory sweat composition and rate, overcoming evaporation by entrapment of sweat within a microfluidic sensing channel.
The device consisted of three major components: a microfluidic layer, electrochemical and electrical sweat sensing electrodes, and a laminated hydrophilic filler. The microfluidic layer contained a collection well and a microfluidic channel, and the collection well’s area could be modulated such that it could acquire varying amounts of sweat. The device was then attached to regions of the body, via a skin adhesive.
To start the team performed on-body sweat collection at rest on various body sites, including shoulder, chest, bicep, wrist, abdomen, finger, thigh, and leg. Sweat collection took between 2 and 60 min depending on the targeted locations, with the finger patch showing the highest secretion rate, ranging between the order of 0.1 and 1 μL min−1 cm−2 .
Having determined the patch could detect low levels of sweat from numerous locations, the team then integrated electrochemical sensors for pH, Cl−, and levodopa monitoring into the design. Sweat pH could potential reflect acid-base disorders, chloride levels can be used to monitor cystic fibrosis, whilst levodopa testing could be used towards precision medicine for Parkinson’s disease – levodopa is the most effective medication for treating Parkinson’s disease.
The patch was then used to explore dynamic sweat behaviours of healthy volunteers during light physical activity, such as walking or doing lab work. It could also detect stressful events over a 24 h period, such as public speaking, thereby identifying when the body moved into physiologically deviating states from the baseline measurements.
A diabetic subject also wore the patch to investigate hypoglycemia-induced sweat secretion. After insulin was injected, glucose started to decrease rapidly, whilst an increase in sweat rate was observed.
Parkinson’s patients usually suffer from abnormal sweating, and hyperhidrosis occurs when the blood levodopa concentration is low. As such sweat could be a promising non-invasive way to continuously monitor levodopa levels in the body, but currently little is known about these dynamics. To start to address this, a healthy patient took a natural source of levodopa (broad beans) at an equivalent dose to that of levodopa medication consumed by Parkinson’s patients in a day. They saw that levodopa concentrations in the sweat generally increased with increasing doses, with the highest peak occurring ~30 min after initial intake and then slowly decreasing, suggesting sweat monitoring could be an effective way to determine levodopa levels in the body.
The team here presented a wearable device for rapid uptake of low levels of thermoregulatory sweat at rest. This allowed near-real-time sweat analysis, enabling rates and composition to be analysed.
The researchers will now expand on these preliminary data to decode how sweating patterns relate to broader physiology. By building personalised and universal correlations between sweat rates and composition, for example during hypoglycaemia or for monitoring drug levels, the device can facilitate optimal dosage and intervals for drugs such as insulin or levodopa in diabetic or Parkinson’s patients. Of course, other analytes can be used as biomarkers for various diseases giving the device flexibility for a broad range of applications.
There is currently much interest and investment into the development of biosensors, and wearables that can monitor health. Recent developments mean that wearable health sensors can even be printed directly on the skin. Moreover, this is not the first device created to monitor sweat biomarkers, and we have seen motion-charged biosensors able to do this. However, that device required 30 mins of exercise, limiting the use to those patients that can exercise and currently eliminates the possibility of continuous monitoring over long periods of time. The revolution here, is the ability of the device to monitor during rest, which will promote a fundamental understanding of at-rest sweat secretion and its relation to diverse health conditions.
Nyein, H. Y. Y., M. Bariya, B. Tran, C. H. Ahn, B. J. Brown, W. Ji, N. Davis and A. Javey (2021). “A wearable patch for continuous analysis of thermoregulatory sweat at rest.” Nature Communications 12(1): 1823.