Paper thin transistor measures heart health
28 May 2013
A postage-stamp-sized heart monitor developed at Stanford
University is thin enough to be worn under a plaster on the wrist
and is sensitive enough to help diagnose cardiovascular problems.
Called a flexible pressure-sensitive organic thin film
transistor, the sensor can be used for non-invasive, high fidelity,
continuous radial artery pulse wave monitoring. It has the potential
to continuously track heart health and provide doctors a safer
method of measuring a key vital sign for newborn and other high-risk
The sensor has a thin middle layer of rubber covered with tiny
pyramids. Each pyramid is only a few microns across — smaller than a
human red blood cell.
The pressure-sensitive organic thin film
that can monitor heart health.
Linda Cicero / Stanford News Service.
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. 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 can measure the pulse wave as it passes through the
"The pulse is related to the condition of the artery and the
condition of the heart," said Zhenan Bao, professor of chemical
engineering at Stanford, who has developed the sensor. "The better
the sensor, the better doctors can catch problems before they
A heart beat is made up of two distinct peaks, the first, larger
peak is from the heart pumping out blood. Shortly after a heartbeat,
the lower body sends a reflecting wave back to your artery system,
creating a smaller second peak.
"You can use the ratio of the two peaks to determine the
stiffness of the artery, for example," said Gregor Schwartz, a
postdoctoral fellow and a physicist for the project. "If there is a
change in the heart's condition, the wave pattern will change."
"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 believe the sensor could replace devices inserted
directly into an artery, called intravascular catheters. These
catheters create a high risk of infection, making them impractical
for newborns and high-risk patients. Thus, an external monitor like
this sensor could provide doctors a safer way to gather information
about the heart.
Bao's team is working with other Stanford researchers to make the
device completely wireless. Using wireless communication, doctors
could receive a patient's minute-by-minute heart status via cell
phone, all thanks to a device as thick as a human hair.
"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
Schwartz G, et al. Flexible polymer transistors with
high pressure sensitivity for application in electronic skin and
health monitoring. Nature Communications, 4, Article number: 1859.