Flexible Skin Technology Enables Thin Non-Invasive Heart Monitor

A new wearable heart monitor is thinner than a dollar bill, possible thanks to combined layers of flexible materials into pressure sensors. The Stanford engineers who developed the skin-like device hope that it could soon provide doctors with a safer and more convenient way to check the condition of a patient's heart.

Zhenan Bao, a professor of chemical engineering at Stanford, developed the flexible heart monitor with the size of a postage stamp that is worn under an adhesive bandage on the wrist and is sensitive enough to help doctors detect stiff arteries and cardiovascular problems.

"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."

The measured pulse of the heart is made up of two distinct peaks. A larger first peak is from the heart pumping out blood. Shortly after a heartbeat, the lower body sends a reflecting wave back to the artery system, creating a smaller second peak. The relative sizes of these two peaks can be used by medical experts to measure the  heart's health.

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

Bao's team made the heart monitor both sensitive and small by using a thin middle layer of rubber covered with tiny pyramid bumps. Each mold-made pyramid is only a few microns across - smaller than a human red blood cell. When pressure is put on 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.

Another very interesting application that could be enabled by this would be using many of these sensors on a prosthetic limb to create an artificial sense of touch, acting like electronic skin.

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.

"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."

The researchers highlight that this non-invasive method of monitoring heart health could replace devices inserted directly into an artery, called intravascular catheters. These catheters naturally create a high risk of infection, making them impractical for newborns and high-risk patients. So an external monitor like Bao's could provide doctors a safer way to gather information about the heart, especially during infant surgeries.

"For some patients with a potential heart disease, wearing a bandage would allow them to constantly measure their heart's condition," Bao said. "This could be done without interfering with their daily life at all, since it really just requires wearing a small bandage."

Paper:

Gregor Schwartz et al., Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring, Nature Communications, 2013, DOI: 10.1038/ncomms2832