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The sweat sensor as it would be worn on the body, capable of being powered by both sunlight and ambient indoor light Credit: Jihong Min
Solar-powered wearable sweat sensors developed at Caltech use perovskite solar cells for energy, enabling continuous health monitoring under indoor conditions.
These sensors can track multiple biomarkers, offering a non-invasive tool for managing various health conditions cost-effectively.
Wearable Sweat Sensors
Sweat, like blood, contains valuable health information—but collecting it is far less invasive. This concept drives the development of wearable sweat sensors created by Wei Gao, Caltech assistant professor of medical engineering, Heritage Medical Research Institute Investigator, and Ronald and JoAnne Willens Scholar.
Over the past five years, Gao has enhanced these wearables to detect a wide range of health indicators, including salts, sugars, uric acid, amino acids, vitamins, and even complex molecules like C-reactive protein, which signals certain health risks. In his latest innovation, developed in collaboration with Martin Kaltenbrunner’s team at Johannes Kepler University Linz in Austria, Gao has equipped these sensors with a flexible solar cell for continuous, battery-free operation.
The flexible perovskite solar cell that powers Wei Gao’s wearable sweat sensor. Credit: Stepan Demchyshyn
Advancements in Solar-Powered Wearable Technology
The solar cell used by Gao’s lab is made of perovskite crystal, a material that shares the chemical structure first found in the mineral calcium titanium oxide. Perovskite has captured the attention of solar cell developers for several reasons: First, it is cheaper to manufacture than silicon (the primary material used in solar cells since the 1950s), which must be highly purified through multiple processes. Second, perovskite is as much as 1,000 times thinner than silicon solar cell layers, making them “quasi-2D” in Gao’s terms. Third, perovskite can be tuned to the spectra of different lighting, from outdoor sunlight to various forms of indoor lighting. Finally, and most enticingly for pioneers of solar energy, perovskite solar cells achieve a higher power conversion efficiency (PCE) than silicon, which means they can convert a greater proportion of the light they receive into usable electricity.
Enhanced Efficiency of Perovskite Solar Cells
Silicon solar cells have reached PCE levels that range from 26–27 percent, though in regular use the range hovers between 18 and 22 percent. By contrast, the flexible perovskite solar cell (FPSC) on Gao’s wearable sweat sensor has a record-breaking PCE exceeding 31 percent under indoor light illumination. “We don’t want to only use strong sunlight to power our wearables,” Gao explains. “We care about more real-life conditions, including normal office and home lighting. Many solar cells have high efficiency in strong sunlight but not in weak indoor lighting conditions.” The FPSC on the sweat sensor is particularly well suited to indoor lighting, says Gao, as “the spectral response of the FPSC matches well with the common indoor lighting emission spectrum.”
Wearable sweat sensor showing layer next to the skin (left) and flexible solar cell (right). Intermediate layers include electronics and biomarker sensors. Credit: Jihong Min
Transition to Solar Power in Wearables
Previous iterations of Gao’s wearable sweat sensors were powered by lithium-ion batteries that were bulky and had to be recharged with an external source of electricity. The search for a lighter, more renewable source of electricity to power these high-demand devices led Gao’s lab to try using silicon solar cells, which they found to be too rigid, inefficient, and reliant on strong lighting conditions. They also experimented with harvesting energy from the chemicals in human sweat (a readily available biofuel) and body motion but found these were too unstable or required too much effort on the part of the wearer.
Continuous Health Monitoring With New Sensors
Use of a FPSC has allowed Gao to create sweat sensors that can be worn for 12 hours a day, providing continuous monitoring of pH, salt, glucose, and temperature, and periodic monitoring (every five to 10 minutes) of sweat rate. All of this is accomplished without batteries or a specially dedicated light source. Furthermore, as the power source has become lighter and less cumbersome, the wearable has room for additional detectors to monitor a greater number of biomarkers simultaneously.
Mobile phone application that pairs with wearable sweat sensor via Bluetooth. Credit: Jihong Min
Design and Functionality of the Latest Wearable Sensor
This new wearable sweat sensor, like its predecessors, is assembled in an origami-like fashion, with individual layers dedicated to different processes. The sensor has four principal interacting components. The first is dedicated to power management—disbursing the electricity harvested by the solar cell. The second enables iontophoresis, the induction of sweating without any exercise or exposure to high heat required on the part of its wearer. In Gao’s study, iontophoresis was performed every three hours to ensure that enough sweat was available to continuously monitor the biomarkers under observation. The third enables the electrochemical measurement of various substances in the sweat. The fourth manages data processing and wireless communication, which allows the sensor to interface with a cellphone app to display the ongoing results of the sensor’s monitoring.
Economic and Clinical Potential of Solar-Powered Sweat Sensors
Fully assembled, the sensor measures 20 x 27 x 4 millimeters, and it can handle the mechanical stress associated with being worn on the body. “Most of the elements of the sweat sensor, such as the electronics and the FPSC, are reusable,” Gao adds. “The only exception is the sensor patch, which is disposable, and it can be mass produced at a low cost using inkjet printing.” These sensor patches can also be customized according to what substances the user wishes to measure in their body.
Broader Implications for Health Monitoring
As these solar-powered sweat sensors are put to use, they will be able to measure far more than any fitness or health tracker currently does. For example, they can be used for diabetes management (studies have shown that glucose in sweat is closely matched to glucose in the blood) and for the detection of a range of conditions such as heart disease, cystic fibrosis, and gout. Because they are noninvasive and can perform multiple measurements over short periods of time, these sensors can discern an individual’s baseline for substances such as cortisol, hormones, or the metabolites of various nutrients and medicines. Once the baseline levels for such substances are known, future deviations from these will provide a more effective means of diagnosis than a single blood draw ever could. And because the sensors are relatively inexpensive, the hope is that they can be an excellent diagnostic tool across the globe, including in developing countries.
Reference: “An autonomous wearable biosensor powered by a perovskite solar cell” by Jihong Min, Stepan Demchyshyn, Juliane R. Sempionatto, Yu Song, Bekele Hailegnaw, Changhao Xu, Yiran Yang, Samuel Solomon, Christoph Putz, Lukas E. Lehner, Julia Felicitas Schwarz, Clemens Schwarzinger, Markus Clark Scharber, Ehsan Shirzaei Sani, Martin Kaltenbrunner and Wei Gao, 20 July 2023, Nature Electronics.
DOI: 10.1038/s41928-023-00996-y
The research was supported by the National Institutes of Health, the Office of Naval Research for Space Health, the National Science Foundation, and a European Research Council Starting Grant. Co-authors include Jihong Min, Juliane R. Sempionatto, Yu Song, Changhao Xu, Yiran Yang, Samuel Solomon, and Ehsan Shirzaei Sani of Caltech, and Stepan Demchyshyn, Bekele Hailegnaw, Christoph Putz, Lukas Lehner, Julia Felicitas Schwarz, Clemens Schwarzinger, Markus Scharber, and Martin Kaltenbrunner of Johannes Kepler University Linz.
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