Monitoring electrical signals from muscles and nerves may become easier with a 1 mm³ sensor that can be implanted in the body, and powered and read out by ultrasound.

These dust-sized, wireless sensors developed by University of California, Berkeley engineers might one day allow for real-time monitoring of nerves, muscles, or organs by Fitbit-like devices. The so-called "neural dust" technology also may broaden prospects for “electroceuticals” to treat disorders such as epilepsy or to stimulate the immune system or tamp down inflammation.

The sensor attached to a nerve fiber in a rat. Image source: Ryan Neely.The sensors – about the size of a large grain of sand – contain a piezoelectric crystal. The crystal converts ultrasound vibrations from outside the body into electricity to power a tiny, on-board transistor that is in contact with a nerve or muscle fiber. A voltage spike in the fiber alters the circuit and the vibration of the crystal. This, in turn, changes the echo detected by the ultrasound receiver, typically the same device that generates the vibrations. The slight change, called backscatter, allows the voltage to be determined.

In experiments with rats, researchers powered up the passive sensors every 100 microseconds with six 540-nanosecond ultrasound pulses. This provided them with a continual, real-time readout. The researchers coated the first-generation motes – 3 millimeters long, 1 millimeter high and 4/5 millimeter thick – with surgical-grade epoxy, but they are currently building motes from biocompatible thin films which would potentially last in the body without degradation for a decade or more (see video).

Tests with the neural dust motes have focused on the peripheral nervous system and muscles. Researchers say the devices could work equally well in the central nervous system and brain to control prosthetics. Today’s implantable electrodes degrade within 1 to 2 years, and virtually all connect to wires that pass through holes in the skull. Wireless sensors – numbering from the dozens to 100 – could be sealed in, avoiding infection and unwanted movement of the electrodes.

Efforts are now devoted to further miniaturizing the device, finding more biocompatible materials, and improving the surface transceiver that sends and receives the ultrasounds, ideally using beam-steering technology to focus the sound waves on individual motes. Tiny backpacks for rats are being designed to hold the ultrasound transceiver that will record data from implanted motes. The capacity of motes to detect non-electrical signals, such as oxygen or hormone levels, is also being expanded.

“The vision is to implant these neural dust motes anywhere in the body, and have a patch over the implanted site send ultrasonic waves to wake up and receive necessary information from the motes for the desired therapy you want,” says Dongjin Seo, a graduate student in electrical engineering and computer sciences. “Eventually you would use multiple implants and one patch that would ping each implant individually, or all simultaneously.”