Wearable sweat sensors on a bandage

Researchers develop thread-based sensors to detect changes in metabolite levels in sweat.

Metabolism — or the processing and utilisation of energy resources to sustain life — results in a
number of by-product molecules called metabolites. Medical practitioners measure the
concentration of these molecules in body fluids such as blood, urine, sweat and saliva to test
whether the body is functioning normally. While biannual check-ups suffice for the regular
population, people with chronic diseases need to monitor their medical conditions constantly.
Similarly, athletes need to closely monitor all metabolites in real-time to check for hypoxia,
dehydration and muscle fatigue. Such metabolic monitors need to be wearable and minimally
invasive. In a new study, researchers from the Indian Institute of Technology Bombay (IIT
Bombay) and Tufts University, United States, have developed a novel sensor to detect metabolite
levels from sweat. These sensors can be mounted on adhesive bandages and embedded on
garments.

The study was published in NPJ Flexible Electronics and was supported by the Center for
Applied Brain and Cognitive Sciences (CABCS), a U.S. Army Combat Capabilities
Development Command, Office of Naval Research (ONR), Department of Science and

Technology, and the Scheme for the Promotion of Academic and Research Collaboration
(SPARC, Ministry of Human Resource Development), Government of India.
―There is a lot of interest in sweat monitoring as it provides a larger window into your health,
more than any smartwatch,‖ says Prof Sameer Sonkusale. He is a Professor at Tufts University
and a senior researcher in this study. ―Looking for other metabolites in sweat will expand
opportunities in medical diagnostics as well,‖ he adds.

The ions present in sweat present an overall picture of the body’s metabolic state. For instance,
low amounts of sodium ions indicate dehydration, ammonium ions are markers for protein
digestion, liver function and oxygen levels, and increased levels of lactate ions indicate muscle
fatigue. The researchers developed three types of sensors using carbon-coated polyester threads
for sensing electrolytes like sodium and ammonium ions, carbon-coated stainless steel threads to
test the pH (acidity) and polyester threads coated with an enzyme that oxidises and senses
lactate.

The sensors were connected to electronic circuit boards that wirelessly relayed the gathered
information to computer programs that calculated the concentrations of different metabolites.
―As soon as the sensor sees a difference in ion concentration, the data is transmitted wirelessly
almost instantaneously and displayed on the computer (or phone) screen. I would say it takes
around a second for this transmission,‖ says Prof Sonkusale.

The researchers found that the lactate sensor could detect the metabolites within 5–30 seconds.
They propose that this time window is optimal for real-time sensing of metabolite levels. They
also found that the sensor threads were specific to the ions they detected and did not respond to
similar non-target ions in the solution.

Once the sensor threads and the electronic readout system were assessed individually, it was time
to use them on humans. The sensor threads were first activated by placing them in dilute
solutions containing their target ions. This was followed by calibration so that all sensor threads
functioned at standard levels. ―Chemical sensors need pre-calibration since each sensor may be
different due to variations in the fabrication process,‖ explains Prof Sonkusale.

The sensor threads for sodium ions, ammonium ions, and pH were integrated on one adhesive
bandage. The lactate sensors were placed on a separate bandage since their information is
processed using a different measuring device. The change in ionic levels is measured through a
potentiometer that registers changes in voltage while the lactate concentration is measured in the
form of current via an ammeter. The sensor patch was covered with gauze, which provided an
absorbent surface to collect the sweat.

―It is a low-cost band-aid-like patch that can be disposed of, once used. The electronics module
can be reused,‖ says Prof Sonkusale while describing the wearable sensor patch. ―The disposable

textile sensor is biodegradable. For future work, the reusable electronics may also have
integrated energy harvesting modules that can derive energy from the surroundings making the
sensor sustainable,‖ adds Prof Maryam Shojaei from IIT Bombay, another senior researcher
involved in the study.

The complete sensor bandages were placed on the arms, forehead and lower back of the study
participants while performing mild exercises. They took 10–20 minutes to break into a sweat,
which was absorbed by the gauze. The sensor detected different ions and allowed the researchers
to gather data depicting real-time changes in ionic levels. This data can be used to analyse the
fitness of the participants.

―Ionic concentration changes as a function of diet and exercise. These changes happen within
minutes. Different participants will release different levels of these ions based on their overall
physical health and fitness,‖ Prof Sonkusale explains.

The study provides a roadmap towards creating sweat sensors that are easily wearable and quick.
However, making such a sensor platform is not easy. ―Achieving a sensor that seamlessly
integrates with the body without any discomfort and inconvenience to the user is quite a
challenge since every sensor and electronics platform adds to bulk and rigidity making it
incompatible,‖ says Prof Sonkusale. ―Our textile threads serve as ideal substrates since it can be
functionalized to monitor different biomarkers in sweat, and can be integrated on any garment or
even as a standalone patch. The latter was what we did,‖ he concludes.