The Quest for Smarter Dust
The 1996 film Twister follows a group of storm chasers who are designing a revolutionary device to understand tornados and create more advanced warning systems. This device would release hundreds of small sensors into the heart of a tornado, collecting structural data from within these natural phenomena. Of course, the movie’s plot is fictitious, but what if such a revolutionary device was actually designed? Researchers at the University of California, Berkeley have designed similar sensors; however, instead of capturing information about natural disasters, these sensors probe the brain. In a recent paper published in the journal Neuron, in-vivo recordings were made using neural dust, marking the first use of the technology in animals1. This device was implanted into the brains of anesthetized rats, wherein it was successfully powered and shown to transmit data on electrical activity from neurons2.
Many tools currently exist to monitor the brain and map its circuitry. One of the most successful invasive brain-computer interfaces, BrainGate, allows a user to control an artificial hand with a small chip implanted in his or her cortex. However, these types of brain-machine interfaces present significant challenges, requiring clunky tethering and having limited life-spans and spatial resolution. BrainGate requires a long cord connecting user to machine, and it is only capable of collecting data from a small, isolated area in the brain. One of the challenges has been to create a more minimalist interface that can function within living tissue long-term.
The Berkeley research team, led by Dongjin Seo and Ryan Neely, devised an ultra-miniature system capable of making more neural recordings over long-term use: neural dust. This technology takes the form of thousands sensor nodes about the width of a strand of hair (10 to 100 microns). The sensors are implanted into the brain to collect electrical data across the entire cortex, and the collected data could be transmitted to a transceiver outside the skull that also powers the sensors wirelessly, opening up the possibility for chronic use3.
The team’s system consists of sensors and complementary metal-oxide-semiconductor circuits, which produce less waste heat than other types of integrated circuits. The system circumvents traditional corded power sources by being powered with ultrasound—vibrating small piezoelectric crystals built into the nodes4. Piezoelectric materials generate electric charge in response to mechanical stress, allowing the cyrstals to transform the mechanical energy of ultrasound into electrical signals that can power a transistor. These electric signals then allow electrodes implanted in the cortex to detect any localized electrophysiological activity. The electrical activity is reported through reflected ultrasound waves, providing an output similar to sonar. While the thousands of neural dusts are placed within the brain, a sub-cranial interrogator, or control terminal, implanted below the skull connects to an external transceiver to provide serve as a power source and transceiver for the system.
However, issues arise when integrating the piezoelectric transducers and the hardware to convert ultrasound to electronic signals; doing so has proven difficult, as the entire system needs to be packaged into a small insulated film to shield from external electric activity5. Another notable problem lies in in building an interrogation system that generates sufficient power without overheating the cortex. For these reasons, ultrasound has been used over electromagnetic waves for the neural dust system; ultrasound facilitates significantly clearer data transmission without the excessive tissue heating from electromagnetic fields. Finally, there is the challenge of actually implanting the neural dust particles, for which the team suggested the use of a wire array to hold the implants in place.
The recently published paper in Neuron presents the successes of Seo’s team in developing neural dust, paving the way for further improvements to be made to the system design. Current implants have limited life span, breaking down in the brain’s native environment. Smaller neural dust implants might be more stable and prevent adverse biological reactions and infections. By successfully testing the device in animals, further research can be done to decrease the size of the neural dust and increase the scale with which this system can be implemented within the brain. Eventually, this device could be used to detect more types of data outside of electrical activity, but this possibility has yet to be fully explored.
4D. Seo, J.M. Carmena, J.M. Rabaey, E. Alon, M.M. Maharbiz (2013) Neural dust: an ultrasonic, low power solution for chronic brain–machine interfaces. arXiv:1307.2196.