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Engineers develop paper-thin heart health monitors

16 May 2013

Engineers combine layers of flexible materials into pressure sensors to create a wearable heart monitor that is thinner than a sheet of paper.

This flexible skin-like heart monitor is small enough to wear under a bandage (photo: L A Cicero)
This flexible skin-like heart monitor is small enough to wear under a bandage (photo: L A Cicero)

Zhenan Bao, a professor of chemical engineering at Stanford university, has developed a heart monitor thinner than a sheet of paper and no wider than a postage stamp. The flexible skin-like monitor, worn under an adhesive bandage on the wrist, is sensitive enough to help doctors detect stiff arteries and cardiovascular problems.

The devices could one day be used to continuously track heart health and provide doctors a safer method of measuring a key vital sign for newborn and other high-risk surgery patients.

"The pulse is related to the condition of the artery and the condition of the heart," said Bao, whose lab develops artificial skin-like materials. "The better the sensor, the better doctors can catch problems before they develop."

Each pulse beat is actually made up of two distinct peaks. The first, larger peak is the heartbeat, followed by a reflecting wave back to the arterial system, creating a smaller second peak. The relative sizes of these two peaks can be used by medical experts to measure heart health.

"You can use the ratio of the two peaks to determine the stiffness of the artery, for example," said project worker, Gregor Schwartz. "If there is a change in the heart's condition, the wave pattern will change."

To make the heart monitor both sensitive and small, Bao's team uses a thin middle layer of rubber covered with tiny pyramid-like bumps. Each moulded pyramid is only a few microns across – smaller than a human red blood cell.

When pressure is applied to the device, the pyramids deform slightly, changing the size of the gap between the two halves of the device. This change in separation causes a measurable change in the electromagnetic field and the current flow in the device.

The more pressure placed on the monitor, the more the pyramids deform and the larger the change in the electromagnetic field. Using many of these sensors on a prosthetic limb could act like an electronic skin, creating an artificial sense of touch.

When the sensor is placed on someone's wrist using an adhesive bandage, it is capable of measuring that person's pulse wave.

The device is so sensitive that it can detect more than just the two peaks of a pulse wave. When engineers looked at the wave drawn by their device, they noticed small bumps in the tail of the pulse wave invisible to conventional sensors. Bao said she believes these fluctuations could potentially be used for more detailed diagnostics in the future.

Doctors already use similar, albeit much bulkier, sensors to keep track of a patient's heart health during surgery or when taking a new medication. But in the future Bao's device could help keep track of another vital sign.

"In theory, this kind of sensor can be used to measure blood pressure," said Schwartz. "Once you have it calibrated, you can use the signal of your pulse to calculate your blood pressure."

This non-invasive method of monitoring heart health could replace invasive devices such as intravascular catheters, which pose a high risk of infection.

Bao's team is working with other Stanford researchers to make the device wireless enabled, opening up the opportunity of continuous heart health monitoring via a mobile phone.

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