In the moment, stress rarely feels good — but the long-term effects may be even worse.
A wealth of studies find stressful activities are linked to raised levels of the hormone cortisol in the body. That in turn may lead to obesity, depression, or diabetes down the road, the data suggest. Monitoring and managing stress levels may play a crucial role in preventing disease — but recognizing how much stress is too much is difficult (especially when you are stressed).
But cortisol also offers clues that could be harnessed to better manage stress. A team of biomedical engineers and chemists has designed a small, wireless monitor that can be attached to the body to analyze cortisol in sweat. With a smart phone, both scientists and users would be able to check and analyze their cortisol levels in real-time, helping keep stress levels in check.
The device was described in a study published Monday in the journal Proceedings of the National Academy of Sciences.
There is a long history of attempts to disentangle important biomarkers from the salty residue left over when we sweat. A biomarker is a measurable thing in the body that may be indicative of a person's physical — including whether they are at-risk of disease. Other teams have made monitors to measure cortisol, the so-called stress hormone, as well as glucose and vitamin C levels in the body. Together, these offer important insight into one's health. So why aren't we all wearing them?
The problem, these researchers say, is partially down to aesthetics. Older designs have either been inflexible, bulky, or required expensive off-site analysis, meaning any results couldn't be analyzed and acted upon in real-time.
To get over the design hurdle, the researchers turned to 'soft' materials in order to design a "skeletal" monitor that allows for sequential analysis of different biomarkers from small drops of sweat. The information collected can then be wirelessly transmitted to a user's smartphone for real-time analysis in just a matter of seconds after sweat has reached the device.
"The results represent important additions to a portfolio of emerging capabilities in skin interfaced technologies for physiological monitoring, with particular relevance to conditions that follow from unhealthy levels of physical and mental stress," write the authors.
How is it done — The sweat monitor has a flexible, silicone body and small channels for the sweat to run through. The sweat collected in these microfluidic channels is then exposed to a number of different experimental assays, which are essentially chemical tests to detect biomarkers.
Coauthors Roozbeh Ghaffari and John Rogers, both professors of biomedical engineering and physical chemistry respectively at Northwestern University, tell Inverse that sweat droplets travel through these microfluidic channels to be analyzed.
"Once a droplet of sweat is excreted from the skin during exercise, hot bath, or sauna, it travels into an intricate array of microchannels embedded in the microfluidic patch [and] makes contact with assays and electro-chemical sensors inside the microfluidic patch," they say. "At which point in time, reactions occur in real time."
When it detected cortisol, glucose, and vitamin C, the device let off certain levels of fluorescence, and a smartphone camera analyzed the results.
The monitor is also kitted out with a system of electrodes that measure sweat rate and the skin's electrical resistance, both of which can change when experiencing stress.
After testing the device in the lab using artificial sweat, the researchers tested the device on human volunteers to see how it would fare in a less controlled environment.
Hitting the gym — The team recruited four healthy volunteers to test their monitors.
During the trial, researchers mimicked mental stress by disrupting the participants' sleep schedules for seven days, similar to what one might experience when over-stressed from long studying or research sessions. This was then followed by a period of respite, with regular sleep and supplemental vitamin C, that lasted for fourteen days.
Every day, while wearing the monitors, the participants rode exercise bikes and the devices measured each volunteer's sweat and tracked the level of cortisol in their secretions.
Cortisol in the body follows its own circadian rhythm — it peaks in the afternoon and wanes in the evening, but for three of four volunteers, mental stress during the trial led to the disruption of this natural rhythm and caused higher levels of cortisol.
This kind of disruption to someone's natural cortisol levels could affect both their tendency for anxiety and insomnia, write the authors. After two weeks back at their regular routines and taking vitamin C, a vitamin known to reduce cortisol levels, the researchers report that participants recovered their original cortisol rhythms.
Importantly, the results from the sweat monitor were also confirmed by saliva samples from the participants, suggesting it is a reliable measure of cortisol levels.
Chilling out — While the results of this study are fairly encouraging for how a device like this might be used in real-time to address harmful levels of stress, it is important to remember that this was too small of a sample to truly represent how such a device would perform for the average person.
In the future, the researchers tell Inverse that they plan to expand this research by testing the devices in larger clinical groups across a wide range of applications, including workers, the elderly, and even athletes.
Abstract: Soft microfluidic systems that capture, store, and perform biomarker analysis of microliter volumes of sweat, in situ, as it emerges from the surface of the skin, represent an emerging class of wearable technology with powerful capabilities that complement those of traditional biophysical sensing devices. Recent work establishes applications in the real-time characterization of sweat dynamics and sweat chemistry in the context of sports performance and healthcare diagnostics. This paper presents a collection of advances in biochemical sensors and microfluidic designs that support multimodal operation in the monitoring of physiological signatures directly correlated to physical and mental stresses. These wireless, battery-free, skin-interfaced devices combine lateral flow immunoassays for cortisol, fluorometric assays for glucose and ascorbic acid (vitamin C), and digital tracking of skin galvanic responses. Systematic benchtop evaluations and field studies on human subjects highlight the key features of this platform for the continuous, noninvasive monitoring of biochemical and biophysical correlates of the stress state.