Wearable and Implantable Sensors Use Less Power

R&D Digest: The monthly review of new technologies and medical device innovations.

A smaller version of a motion sensor used for navigation control is being tailored for medical applications. The novelty of the sensor isn't its size, but its low power draw and low manufacturing cost. These characteristics, along with a wireless capability, could make the sensors a more suitable and prevalent component in medical devices.

Developed at the University of Florida (Gainesville), the sensors detect motion or vibration of a monitored object. They can tell in what direction an object is moving and can measure acceleration, which can be converted to precision or velocity. Measuring 3 mm², the unit consists of a wireless chip packed with a battery. It integrates signal processing and wireless transmission.

“Our goal is to have a wearable sensor,” says Huikai Xie, assistant professor in the department of electrical and computer engineering at the university. Xie and the Florida researchers designed the sensor to be manufactured with complementary metal oxide semiconductor technology, which uses less power. The designers estimate that if the component were mass-produced, it could cost less than $10 to manufacture.

Xie envisions two types of sensors—one worn externally, the other implantable. He wants to be able to embed the chip in clothes or wrap it around a particular body part. In this case, the sensors could be used for tracking a person or for rehabilitation purposes. For example, a stroke patient might not remember how to move. The motion sensors could help remind the patient how to move or indicate how well the person is progressing. Athletes could use the sensor to optimize their energy consumption.

Tracking a person involves putting wireless radio-frequency circuits on the chip, an important feature for medical purposes. Instead of having a nurse on constant watch, the sensor could help the nurse monitor the patient from a distance.

Implantable sensors would allow doctors to monitor recovery progress or potential problems inside the body. “Because the whole unit is small, we can implant the sensors into the body, in bones or an organ to study motion,” says Xie. “In that case, it's even more important to have wireless [capability], because you don't want to have a wire coming out of the body.”

Xie says the option of implanting the sensor is imperative to better understand body processes. “If you want to know exactly how the bone moves, it's better to implant the sensor into the bone,” says Xie. To make it implantable, the sensor unit must be encased in a biocompatible material. Putting the chip inside the body also raises issues with power consumption.

“You may want the unit to stay [implanted] a few weeks, months, or a year,” says Xie. “With power management, the lifetime of the battery can be easily extended up to one year.”

It can continuously run for about a month, but it doesn't usually need to be turned on all the time. Data sampling may be a more effective way to save power. To run for a year, the unit would use about 0.001 W of power.

More information about the technology can be found in the December issue of the journal Institute of Electrical and Electronics Engineers Transactions on Circuits and Systems.

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