Wearable sensors decode dynamic sweat composition

By Dr. Liji Thomas, MDAug 19 2019

A new wearable skin sensor developed at the University of California in Berkeley may be very useful to help us understand how the body’s chemical composition changes in a host of situations. For instance, these small patch-like devices could detect the presence of issues like fatigue or dehydration.

Scientists have sometimes looked at sweat as mirroring the body chemicals. However, this remains to be proved. The current sensors are designed to be mass-produced at high speed, enabling large studies to be carried out on sweat, which can help define the future of this field.

In the present study, the innovative sensors were used to look at the rate of sweat production and its composition in terms of salts and glucose. Both exercise-induced and iontophore-induced sweat was examined in volunteers. Iontophores are substances that stimulate sweat production.

New wearable sensors developed by scientists at UC Berkeley can provide real-time measurements of sweat rate and electrolytes and metabolites in sweat. Image Credit: Bizen Maskey, Sunchon National University

Researcher Ali Javey says, “The goal of the project is not just to make the sensors but start to do many subject studies and see what sweat tells us -- I always say 'decoding' sweat composition.”

The sensors were positioned at strategic spots like the forehead, forearm, underarm and upper back, on users who then rode an exercise bicycle. Their measurements showed that the rate of sweat production/ sweat electrolyte levels correlated with exercise-induced fluid/ electrolyte depletion. Worn as wristbands or headbands, this might offer an easy way to predict dehydration and electrolyte imbalance real-time during strenuous exercise. In future, researchers hope to include a resistive temperature sensor as well to compensate for effect of temperature on the electrochemical sensors.

Secondly, iontophore-induced sweat was examined. This method showed significant differences with the type of sweat, whether exercise-induced or iontophoretic, as well as day-to-day differences.

Thirdly, the researchers performed a comparison of blood and sweat fasting glucose levels in healthy and diabetic participants. Previous methods could not achieve an instantaneous comparison due to the inevitable delays in collecting enough sweat for measurement, as well as the necessity of using an averaged level. The current sensors allowed immediate and sensitive measurements of both blood and sweat glucose at the same time. This showed that single tests of sweat glucose were not a very accurate indicator of the blood glucose concentration.

The current study is remarkable for its use of a roll-to-roll rotary screen-printing procedure, in which sensors are printed on to a plastic roll like so many words on a roll of newsprint. This is a great advance over older lithography and etching processes which require a number of steps, in its automated mass production design built for high-speed uniform large-substrate printing. This allows real-time in situ sensors to be produced by a high-throughput process. The practical benefit is the abundant availability of sensors that are reliable, accurate and reproducible for academic and practical applications.

The manufacturing process was worked out in collaboration with the VTT Technical Research Center of Finland. Says VTT’s Jussi Hiltunen, “Roll-to-roll processing enables high-volume production of disposable patches at low cost.”

Sweat sensing is a potentially easier way to assess the levels of various body substances at the point-of-care and without having to draw blood or stick in needles. It contains a host of biomolecules, including the smaller salt and glucose molecules, and larger peptide or protein particles coming from the interior of the body. Thus it could be useful to understand the chemical status of the body at any moment.

Some small studies have already pointed to the possibility of using sweat measurements for certain biomarkers to predict health status. In fact, sweat testing has been an important means of diagnosis in cystic fibrosis, autonomic neuropathy, and dehydration. However, the conventional methods of sweat testing require dedicated facilities, expert training, and specialized collection of sweat. This hinders real-time self-monitoring of sweat on a routine basis, and limits the size of studies.

The new sensors have three layers: a printed layer containing the sensor, a microfluidic double-sided adhesive spacer, which contains the spiral microfluidic tubes and a collection reservoir of 2.5 µL capacity, and a flexible sheet covering the functional components. The cover sheet and microfluidic adhesive layer are both designed to contain encapsulated channels for the sweat. Once the microfluidic adhesive layer is laminated between the other two layers, the microfluidic channels are tightly sealed.

The sweat passes through the collection reservoir at the point of entry where analytes are measured continuously by electrochemical sensors. Once it fills, older sweat is pushed out into the channel by the newly arriving sweat, where it comes into contact with immersive silver electrodes. These measure the sweat rate continuously in real-time by impedance measurements. This prevents evaporation and contamination of the sweat, avoiding the need for mixing and eliminating the risk of carryover effects. The layered pattern allows high printing speeds and flexibility of design.  

With earlier static methods, the sweat would be gathered for a predefined interval and then measured. Now, however, says researcher Hnin Yin Yin Nyein, “Using these wearable devices we can now continuously collect data from different parts of the body, for example to understand how the local sweat loss can estimate whole-body fluid loss.”

Lead author Mallika Bariya says, “There's been a lot of hope that non-invasive sweat tests could replace blood-based measurements for diagnosing and monitoring diabetes, but we've shown that there isn't a simple, universal correlation between sweat and blood glucose levels.  This is important for the community to know, so that going forward we focus on investigating individualized or multi-parameter correlations.”

More work will be needed to understand sweat composition in relation to body weight, age, diet, and hydration status, as well as changes over time. Production techniques will also be refined to avoid fluctuations in performance due to the drop-casting method used currently. By accurately mining the wealth of information contained in sweat, the researchers hope to unveil the true utility and limitations of sweat sensors in medicine and science.

The study was published on August 16, 2019, in the journal Science Advances.