Sweat is less invasive to collect than blood, and can tell a lot about a person's health. This is the premise behind the wearable sweat sensors developed by Wei Gao, assistant professor of medical engineering at the California Institute of Technology (Caltech). Over the past five years, Gao has steadily added features to his wearables, making them capable of reading out levels of salts, sugars, uric acid, amino acids, and vitamins as well as more complex molecules like C-reactive protein that can provide timely assessment of certain health risks. Most recently, in collaboration with Martin Kaltenbrunner's group at Johannes Kepler University Linz in Austria, Gao has powered these wearable biosensors with a flexible perovskite solar cell (FPSC).
Perovskite is as much as 1,000 times thinner than silicon solar cell layers, making them "quasi-2D" in Gao's terms. Perovskites can also be tuned to the spectra of different lighting, from outdoor sunlight to various forms of indoor lighting. Importantly, perovskite solar cells can 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. The flexible perovskite solar cell (FPSC) on Gao's wearable sweat sensor has a record-breaking PCE exceeding 31 percent under indoor light illumination.