Today's neural implants smaller than ever, but often remain bulky and prone to complications. According to researchers at Cornell University, the new iteration is detailed this week in the journal Natural electronics may offer a new way to develop brain implants. Small enough to fit on a grain of rice, a microscale optoelectronic electrode (or MOTE) is significantly smaller than similar implants and its design can be adapted to work on other delicate areas of the body.
“To our knowledge, this is the smallest neural implant that will measure electrical activity in the brain and then report it wirelessly,” electrical engineer and study co-author Alesha Molnar. says the statement.
MOTE is only 300 microns long and 70 microns wide, equivalent to the width of one human hair. It works by encoding nerve signals with small pulses of infrared light and then harmlessly sending the information through brain tissue and bone to a receiver. Although Molnar first conceived of an early version of MOTE in 2001, it took more than two decades before the project actually took off.
He and his colleagues developed an implant based on a semiconductor diode made from aluminum gallium arsenide. This material allows light energy to be collected to produce power, as well as emit light to send data. The diode is supported by a low noise amplifier and optical encoder using the same transmission principles as standard microchips. Data transmission is carried out using pulse position modulation, the same technology used in many satellite optical communications arrays.
“We can use very, very little power to communicate and still successfully return data optically,” Molnar explained.
The team initially tested MOTE in laboratory grown cell cultures before moving on to mice. For testing, they implanted the device into the rodents' barrel cortex, an area of the brain capable of processing sensory information from the whiskers. For over a year, MOTE reliably recorded bursts of neural activity as well as broader synaptic activity in both active and healthy mice.
One of the major drawbacks of most modern brain implants is that they cannot function while the patient is undergoing electrical monitoring, such as during an MRI. However, MOTE is made from materials that completely circumvent this problem. Its wireless capabilities also solve another recurring problem with implants.
“One reason for this is that traditional electrodes and optical fibers can irritate the brain. The tissue moves around the implant and can trigger an immune response,” Molnar said. “Our goal was to make the device small enough to minimize disruption, while capturing brain activity faster than imaging systems and without the need to genetically modify neurons for imaging.”
The implications go beyond brain monitoring. Molnar's team is confident that MOTE's basic design will allow it to be adapted to other tissues, even in sensitive areas such as the spinal cord. It can also be used if embedded in artificial skull plates.
“Our technology provides the basis for accessing a wide range of physiological signals using small and untethered instruments implanted over a chronic time frame,” the study authors concluded.
