Microscale Wireless Implant Tracks Brain Activity Over Time

Long-term monitoring of neural activity is constrained by the size and tethering of current implants. Traditional electrodes and optical fibers can irritate brain tissue and provoke immune responses. These limitations impede stable recordings over months, which are important for neural monitoring and bio-integrated sensing. To help address this challenge, researchers have now developed an ultraminiature wireless implant that tracks brain signals in living animals.

Called the microscale optoelectronic tetherless electrode (MOTE), the device was developed at Cornell University (Ithaca, NY, USA) with collaborators at Nanyang Technological University. It is small enough to rest on a grain of salt, measuring about 300 microns long and 70 microns wide. It is the smallest neural implant capable of wirelessly transmitting brain activity data.

The MOTE is powered by red and infrared laser beams that pass through brain tissue without external wiring. A semiconductor diode made of aluminum gallium arsenide harvests the incoming light to power the circuit and emits infrared light to communicate the encoded data. A low-noise amplifier and an optical encoder, built with standard microchip semiconductor technology, condition and encode the brain’s electrical signals for optical telemetry.

The researchers first validated the system in cell cultures. They then implanted the MOTE into the barrel cortex of mice, the brain region that processes sensory input from whiskers. Over the course of a year, the implant recorded neuronal spikes and broader patterns of synaptic activity while the animals remained healthy and active.

The work was detailed in a study published in Nature Electronics. The project developed within Cornell Neurotech, a joint initiative of Cornell Engineering and the College of Arts and Sciences. The team reported that the device’s material composition could enable electrical recordings during magnetic resonance imaging (MRI), could be adapted to other tissues such as the spinal cord, and could be paired with future opto-electronic systems in artificial skull plates.

“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, professor in the School of Electrical and Computer Engineering at Cornell University. “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.”

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