Don’t sweat it: Smartwatch monitors stress levels

“I anticipate that the ability to monitor variations in cortisol closely across time will be very instructive for people with psychiatric disorders,” said co-corresponding author Anne Andrews, a UCLA professor of psychiatry and biobehavioural sciences, member of the California NanoSystems Institute at UCLA and member of the Semel Institute for Neuroscience and Human Behaviour. “They may be able to see something coming or monitor changes in their own personal patterns.”

Cortisol is well-suited for measurement through sweat, according to co-corresponding author Sam Emaminejad, an associate professor of electrical and computer engineering at the UCLA Samueli School of Engineering, and a member of CNSI.

“We determined that by tracking cortisol in sweat, we would be able to monitor such changes in a wearable format, as we have shown before for other small molecules such as metabolites and pharmaceuticals,” he said. “Because of its small molecular size, cortisol diffuses in sweat with concentration levels that closely reflect its circulating levels.”

In the new smartwatch, a strip of specialised thin adhesive film collects tiny volumes of sweat, measurable in millionths of a litre. An attached sensor detects cortisol using engineered strands of DNA, called aptamers, which are designed so that a cortisol molecule will fit into each aptamer like a key fits a lock. When cortisol attaches, the aptamer changes shape in a way that alters electric fields at the surface of a transistor.

The invention — along with a 2021 study that demonstrated the ability to measure key chemicals in the brain using probes — is the culmination of a long scientific quest for Andrews. Over more than 20 years, she has spearheaded efforts to monitor molecules such as serotonin, a chemical messenger in the brain tied to mood regulation, in living things, despite transistors’ vulnerability to wet, salty biological environments.

In 1999, she proposed using nucleic acids — rather than proteins, the standard mechanism — to recognise specific molecules.

“That strategy led us to crack a fundamental physics problem: how to make transistors work for electronic measurements in biological fluids,” said Andrews, who is also a professor of chemistry and biochemistry.

Meanwhile, Emaminejad has had a vision of ubiquitous personal health monitoring. His lab is pioneering wearable devices with biosensors that track the levels of certain molecules that are related to specific health measures.

“We’re entering the era of point-of-person monitoring, where instead of going to a doctor to get checked out, the doctor is basically always with us,” he said. “The data is collected, analysed and provided right on the body, giving us real-time feedback to improve our health and well-being.”

Emaminejad’s lab had previously demonstrated that a disposable version of the specialised adhesive film enables smartwatches to analyse chemicals from sweat, as well as a technology that prompts small amounts of sweat even when the wearer is still. Earlier studies showed that sensors developed by Emaminejad’s group could be useful for diagnosing diseases such as cystic fibrosis and for personalising drug dosages.

One challenge in using cortisol levels to diagnose depression and other disorders is that levels of the hormone can vary widely from person to person — so doctors can’t learn very much from any single measurement. But the authors foresee that tracking individual cortisol levels over time using the smartwatch may alert wearers, and their physicians, to changes that could be clinically significant for diagnosis or monitoring the effects of treatment.