Miniaturized neural-electronic implants

The term neural dust is a fanciful name for a class of very small devices that are capable of interfacing with the brain. They can best be thought of as tiny brain prostheses that can act as both monitor and effector in the brain by recording and stimulating specific neurons that are in close proximity. First demonstrated in 2011, neural dust has spawned a vibrant new field (Wikipedia):

"Neural dust is a term used to refer to millimeter-sized devices operated as wirelessly powered nerve sensors; it is a type of brain–computer interface. The sensors may be used to study, monitor, or control the nerves and muscles and to remotely monitor neural activity. In practice, a medical treatment could introduce thousands of neural dust devices into human brains."

The predecessors of neural dust are microelectrode arrays which are "devices that contain multiple (tens to thousands) microelectrodes through which neural signals are obtained or delivered, essentially serving as neural interfaces that connect neurons to electronic circuitry." These arrays are also small and can be implanted in the brain but require connection via wires to equipment outside of the body for power and to receive and send signals.

The key innovation of neural dust was removing the wires so that the implants were less cumbersome. Instead they employ a novel technology that leverages ultrasound to replace the functionality of the wires (Figure 1). For example, power can be supplied by ultrasonic waves in the following manner:

"Briefly, pulses of ultrasonic energy emitted by the external transducer impinge on the piezocrystal and are, in part, reflected back toward the external transducer. In addition, some of the ultrasonic energy causes the piezocrystal to vibrate; as this occurs, the piezocrystal converts the mechanical power of the ultrasound wave into electrical power, which is supplied to the transistor."

Thus, piezoelectric crystals are used to transduce the sound energy into electrical energy that can power the device. In addition these crystals serve a second function which is getting signals into and out of the device without wires. For the latter, "recorded signals are reported back to the transmitter/interrogator by reflecting and modulating the amplitude, frequency and/or phase of the impinging ultrasound wave." The vibrations of the crystal itself modulated by the neural signals from the recording electrodes alter the wave pattern of the ultrasound waves that are reflected from the device (backscatter communication). By working in reverse, the ultrasound waves can send signals into the device (Figure 1).

The device is stripped down to be as small as possible; it consists of "piezoelectric transducer, surface electrodes for electrophysiological signal acquisition, and a silicon CMOS (complementary metal oxide semiconductor) chip containing electronics for signal amplification and conversion." In addition, a small patch the size of a large Band-Aid worn by the subject could contain the auxiliary circuitry including "a tiny ultrasound emitter to power the devices (neural dust ‘motes’) and transmitters to receive information from the chips inside the patient."

Although much of the research on neural dust has occurred in the academic community, some of the new technology is being commercialized. In 2021, a startup founded by the key inventors of neural dust called Iota was acquired for $429 million by the large pharmaceutical company Astellas based in Japan:

"The nascent brain prostheses sector received commercial validation with the acquisition of University of California, Berkeley spinout Iota Biosciences by Tokyo-based Astellas late last year. The sale of the startup, which is developing ‘neural dust’—minute devices combining electrodes with a piezoelectric crystal that can be implanted in the brain or onto peripheral nerves—signals that bioelectronic medicines are receiving increasing recognition from traditional biopharma companies."

Two broad commercial areas for neural dust are disease surveillance and therapeutic intervention where nerves are involved. For example, clinicians can temporarily place these devices on the surface of the brain to pinpoint the origin of neural signals that may cause seizures. A more ambitious application would implanting the tiny neural dust in muscles or along the spinal cord to help control paralyzed limbs. 

We shall see where this technology leads; it is an example of bioelectric medicine which uses patterns of electrical impulses for therapeutic purposes, instead of a chemical drug or physical surgery. 

Figure 1. Schematic illustration of a neural dust device. The electrodes can record or stimulate neurons in the brain or peripheral nervous system. The piezocrystal serves multiple functions collecting energy from ultrasound waves as well as sending and receiving data/instructions via backscatter communication with the ultrasound signal. The transistor represents a chip capable of signal amplification and conversion.