A Battery-Free Implantable Neural Sensor

A tiny radio chip implanted in a moth harvests power and senses neural activity.

Thanks to the shrinking size of electronics, researchers have been exploring increasingly sophisticated implantable devices, paving the way for new prosthetics and brain-machine interfaces. But a big challenge has been how to deliver power to electronic components embedded within the body.

Pump it up: This low-power neural amplifier collects electrical signals from nerves and minimizes the electrical noise. Credit: Brian Otis, University of Washington

Now electrical engineers at the University of Washington have developed an implantable neural sensing chip that needs less power. Other wireless medical devices, such as cochlea or retinal implants, rely on inductive coupling, which means the power source needs to be centimeters away. The new sensor platform, called NeuralWISP, draws power from a radio source up to a meter away.

The device contains a microprocessor powered by a commercial radio-frequency reader that doubles as a data-collection device. The same equipment is used to power and read information from radio-frequency identification (RFID) tags. In experiments, the researchers used the new device to sense central nervous system activity in a moth in order to study its locomotion.

There have been some advances in reducing the size of neural implants recently, but the majority of implantable devices are still relatively cumbersome. These devices typically require multiple components--such as a clock for timing operations and an antenna for communication and power-harvesting--that are quite large compared to the transistors on the microcontroller, says Brian Otis, professor of electrical engineering at the University of Washington and lead researcher on NeuralWISP.

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"You can have millions of transistors on a chip less that's less than a cubic millimeter in volume, but the problem is with the extra parts," says Otis. "Our goal is to shrink everything onto a single chip and reduce the power consumption of these components so that the chip can be wirelessly powered."

The NeuralWISP is a collection of smaller, more low-power components, such as a specialized signal amplifier, on a circuit board just over two centimeters long. A future version will integrate all components onto a single chip that's one millimeter by two millimeters in size. The circuitry converts usable power from the reader--roughly 430 microwatts--to a voltage that can turn on the microcontroller. This microcontroller, in turn, controls the sensor and its timer, and runs instructions that allow data to be sent back to the reader.