University of California, Berkeley engineers have built the first dust-sized, wireless sensors that can be implanted in the body, bringing closer the day when a Fitbit-like device could monitor internal nerves, muscles, or organs in real time. These batteryless sensors, called neural dust, can also be used to stimulate nerves and muscles, opening the door to “electroceuticals” to treat disorders such as epilepsy, stimulate the immune system, or tamp down inflammation.

The sensor mote contains a piezoelectric crystal (silver cube) and a simple electronic circuit that responds to the voltage across two electrodes to alter the backscatter from ultrasound pulses produced by a transducer outside the body. The voltage across the electrodes can be determined by analyzing the ultrasound backscatter. (Credit: Ryan Neely, UC Berkeley)

The neural dust, which the team implanted in the muscles and peripheral nerves of rats, is unique in that ultrasound is used both to power and to read out the measurements. Ultra sound technology is already well developed for hospital use, and ultrasound vibrations can penetrate nearly anywhere in the body, unlike radio waves, the researchers say.

“I think the long-term prospects for neural dust are not only within nerves and the brain, but much broader,” said Michel Maharbiz, an associate professor of electrical engineering and computer sciences. “Having access to in-body telemetry has never been possible because there has been no way to put something super tiny super deep. But now I can take a speck of nothing and park it next to a nerve or organ, GI tract, or a muscle, and read out the data.”

How It Works

The sensors, which the researchers have already shrunk to a 1-mm cube (about the size of a large grain of sand), contain a piezoelectric crystal that converts ultrasound vibrations from outside the body into electricity to power a tiny, onboard 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, which changes the echo detected by the ultrasound receiver, typically the same device that generates the vibrations. The slight change, called backscatter, allows them to determine the voltage.

In their experiment, the UC Berkeley team powered up the passive sensors every 100 microseconds with six 540-nanosecond ultrasound pulses, which gave them a continual, real-time readout. They coated the first-generation motes, which were 3 mm long, 1 mm high, and 4⁄5 mm thick, with surgical-grade epoxy. However, they are currently building motes from biocompatible thin films that would potentially last in the body without degradation for a decade or more.

While the experiments so far have involved the peripheral nervous system and muscles, the neural dust motes could work equally well in the central nervous system and brain to control prosthetics, the researchers say. Today’s implantable electrodes degrade within 1 to 2 years, and all connect to wires that pass through holes in the skull. Wireless sensors could be sealed in, preventing infection and unwanted movement of the electrodes.

Researchers had previously estimated that they could eventually shrink the sensors down to a cube 50 μm on a side, about half the width of a human hair. At that size, the motes could be used in the brain and central nervous system, replacing wire electrodes.

The team is working now to miniaturize the device further, find more biocompatible materials, and improve the surface transceiver that sends and receives the ultrasounds, ideally using beam-steering technology to focus the sounds waves on individual motes. They’re also working to expand the motes’ ability to detect nonelectrical signals, such as oxygen or hormone levels.

“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,” said 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.”

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