Wearable wireless polygraph tracks hidden stress signals

Northwestern University engineers have developed a small, wireless polygraph system you can wear.

Unlike polygraphs used in television crime dramas, this wearable version isn't optimized to detect lies. Instead, engineers and physicians designed it to sense underlying stress hidden deep within the body - no interrogation room required.

The lightweight, bandage-like device gently adheres to the chest, where it simultaneously measures heart activity, breathing patterns, sweat response, blood flow and temperature. Together, these signals capture a real-time, whole-body view of stress.

By continuously tracking multiple physiological signals at once, the device could help clinicians detect stress and potential discomfort in patients - including infants or the elderly - who may be unable to communicate, diagnose sleep disorders without cumbersome in-laboratory equipment, monitor mental health over time and even sense early warning signs of medical complications.

The study will be published on Wednesday (May 13) in the journal Science Advances.

A voice for the vulnerable

Currently, detecting babies' stress often depends on what caregivers can see and hear - crying, facial expressions and movement - along with basic vital signs. But these signals can be subtle, inconsistent or even entirely absent.

"Stress is often scored using survey sheets and nursing assessments," Rogers said. "The entries include things like tonality and volume of crying. Infants obviously cannot describe their own pain levels. So, unlike with adults, determining stress in babies can be incredibly challenging. We wanted to take subjectivity out of these assessments."

"This new device tracks the body's stress signals around the clock, helping quantify how long someone is stressed each day and how intense that stress is," said Weese-Mayer, Rogers' long-time collaborator. "The beauty of the device is that both individuals and healthcare providers can now identify stress and objectively monitor the effectiveness of interventions to decrease stress and restore a healthy balance, in a completely non-invasive manner."

All-in-one stress sensing

To do that, Rogers and his team found inspiration in a surprising place: polygraphs. Although they are colloquially called "lie detectors," polygraphs actually don't detect lies. They measure the body's response to stress, which can be triggered by many factors besides deception. Rogers saw an opportunity to build on that core idea. But, while traditional polygraph machines rely on a patchwork of bulky, wired sensors, Rogers aimed to capture those same physiological signals - and then some - in a more comprehensive, fully integrated, accurate and wearable form.

The resulting technology combines several tiny sensors into a single, soft device. Together, these sensors continuously track multiple physiological signals, all of which respond when the body senses stress. A built-in motion sensor and miniature microphone capture subtle mechanical and acoustic signals from the heart and lungs. Other sensors detect skin temperature and heat flow associated with near-surface blood circulation. And another sensor measures changes in the skin's electrical conductivity caused by sweat gland activity - a well-known marker of stress.

"Measuring stress is a complex task because it's multi-dimensional," Rogers said. "It's not possible to reliably determine stress by measuring just one or two, or even three or four, parameters. A broad collection of factors is necessary. So, we crammed as many sensors of physiological processes into this device platform as we could, while maintaining a compact size and lightweight construction and avoiding the need to access biofluids."

The system wirelessly transmits these synchronized data streams to a smartphone, smart watch or tablet, where machine learning algorithms analyze patterns associated with stress in real time. Weighing less than 8 grams (equivalent to eight paperclips) and designed to move naturally with the skin, the device can operate continuously for more than 24 hours.

Proven across realistic scenarios

And, finally, during emergency room training sessions with medical students, the device revealed a striking pattern. Participants with stronger stress responses tended to perform worse, suggesting stress may impair decision-making in high-pressure situations.

"Ultimately, the device could send an alert to a user or caregiver when stress levels hit a certain limit," Rogers said. "Many people might not fully appreciate the level of stress they are under and might not realize it's affecting their performance."

What's next

Rogers also is exploring opportunities for incorporating even more sensors into the device, including the ability to measure brain activity. Adding electroencephalogram (EEG) capabilities would allow the device to move beyond measuring the body's stress response to capturing how the brain perceives that stress. That could bring scientists closer to distinguishing stress from pain - even in the home setting - and understanding how it is experienced in the context of the simultaneously recorded stress biomarkers.

"We are living in stressful times, without sufficient measures to proactively detect stress," Weese-Mayer said. "By identifying stress - whether environmental or disease-induced - earlier, we can introduce intervention before stress' effects become irreversible."

The study, "Wireless, skin-interfaced multimodal sensing system for continuous psychophysiological monitoring - a wearable polygraph device," was supported by the Querrey Simpson Institute for Bioelectronics.

Source:

Northwestern University

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