Cornell researchers have developed a neural implant so small that it can rest on a grain of salt, which can wirelessly transmit brain activity data in a living animal for more than a year, opening up new possibilities for neural monitoring and bio-integrated sensing.
The device, called a microscale optoelectronic tetherless electrode (MOTE), is approximately 300 microns long and 70 microns wide, making it the smallest neural implant capable of wirelessly transmitting brain activity data. The breakthrough demonstrates that microelectronic systems can function at an unprecedentedly small scale.
Powered by red and infrared laser beams that pass harmlessly through brain tissue, the MOTE transmits data back using tiny pulses of infrared light, which encode the brain’s electrical signals. A semiconductor diode made of aluminium gallium arsenide captures light energy to power the circuit and emits light to communicate the data.
“As far as we know, this is the smallest neural implant that will measure electrical activity in the brain and then report it out wirelessly,” said Alyosha Molnar, the Ilda and Charles Lee Professor in the School of Electrical and Computer Engineering, who co-led development of the device. “By using pulse position modulation for the code — the same code used in optical communications for satellites, for example — we can use very, very little power to communicate and still successfully get the data back out optically.”
Sensory information from whiskers
The researchers tested the MOTE first in cell cultures and then implanted it into mice’s barrel cortex, the brain region that processes sensory information from whiskers. Over the course of one year, the implant successfully recorded spikes of electrical activity from neurons, as well as broader patterns of synaptic activity, all while the mice remained healthy and active.
“One of the motivations for doing this is that traditional electrodes and optical fibres can irritate the brain,” Molnar said. “The tissue moves around the implant and can trigger an immune response. Our goal was to make the device small enough to minimise that disruption while still capturing brain activity faster than imaging systems, and without the need to genetically modify the neurons for imaging.”
Molnar said the MOTE’s material composition could make it possible to collect electrical recordings from the brain during MRI scans, which is largely not feasible with current implants. The technology could also be adapted for use in other tissues, such as the spinal cord, and even paired with future innovations like opto-electronics embedded in artificial skull plates.
Development was co-led by Molnar and Sunwoo Lee, an assistant professor at Nanyang Technological University who first began working on the technology as a postdoctoral associate in Molnar’s lab. Molnar first conceived of the MOTE in 2001, but the research did not gain momentum until he began discussing the idea about 10 years ago with members of Cornell Neurotech. The breakthrough was detailed in Nature Electronics.
