Biosensors Comfortable Enough to Wear 24-7

A new kind of sensor could allow for long-term medical monitoring--without direct skin contact.

Many medical sensors, like the sort used for heart-monitoring electrocardiograms (ECGs) or brain-monitoring electroencephalograms (EEGs), require direct skin contact and a sticky layer of gel to help conduct electrical signals. Both technologies can be remarkably precise, but they don't transfer easily from hospital to home. Now researchers at the University of California, San Diego believe they may have solved the sticky situation with a sensor that can read ECG and other data through clothing, without ever touching the skin.

Brain sensor: A new contact-free sensor (bottom) can be embedded into a headband and detect electrical activity through hair and material, without touching the scalp. Credit: Mike Chi, University of California, San Diego

Scientists have had a tough time developing something that can reliably detect the skin's polarity changes without direct contact. ECG electrodes detect the time it takes for waves of changing polarity (caused by heart-muscle contractions) to travel to different sensors, which reveals the electrical activity of different parts of the heart. Currently, these sensors require a gel or an allergy-inducing adhesive. Nonsticky or "dry" sensors are uncomfortable and particularly sensitive to motion, so they can't be used outside the clinic or for long periods of time.

Instead of using electrodes, the UCSD researchers built a capacitive sensor, which conducts much weaker signals but can do so across small distances. While the concept goes back decades, prior attempts at building such sensors have been impractical for mass production--they tended to be either too costly, too sensitive to outside noise, or both. The sensor developed by bioengineer Gert Cauwenberghs and his graduate student, Mike Chi, uses off-the-shelf components and clever circuitry to get around these problems. The resulting sensor can detect faint changes in capacitance, and amplify them, while canceling out the ambient electrical noise that exists all around us. "What's out there today requires several discrete components," Chi says. "Our process makes it reliable and inexpensive, so we have a circuit that can be mass-produced."

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Chi's sensor is barely larger than a quarter, and when multiple sensors are embedded in material and wired together, they create a portable monitor that patients can wear over clothing as they go about their daily routine. This could mean increased monitoring time and better compliance from patients.

Currently, when cardiologists want to know what a patient's heart activity looks like for an extended period of time, they have to send them home with a Holter monitor, a portable ECG device that employs the same wired, sticky electrodes used in the hospital. But this monitor can only be used for up to 48 hours, and abnormal cardiac rhythms don't always occur during such a short window of time. "A lot of these events are transient, and with today's tech you actually miss the events because you can't capture them reliably," Chi says. If a patient could wear a vest over his clothing, such monitoring could go on for as long as a physician required.