The brain is made of pliant soft tissue that flexes and moves within the cranium as we go about our days. Studying the organ and being able to interface with it on the level of individual neurons is exceedingly difficult when the electrodes used are rigid and so can’t maintain a solid contact with the neurons they’re interfacing with. As the brain shifts, traditional electrodes lose the signal they’re homing in on, and end up only useful when covering a large number of neurons at once. Now scientists from Arizona State University and Sandia National Labs, a subsidiary of Lockheed Martin, are working on neural interfaces that can communicate with individual neurons while the brain moves around naturally.
The team is focusing on building a device that features electrodes that can move around and maintain contact with neurons. Since the research is aimed at being able to study animals while they’re awake and moving, the new brain-computer interface also has to be miniaturized to be small enough for implantation. The probes the investigators have built so far, about the size of a dime, have microscale actuators and electrodes that allow the probe to follow single cells as they shift under the device. Each probe has three tiny electrodes and a set of actuators that work with them. Running a current through a thermal actuator makes the electrodes move out of the probe and toward the brain tissue. These actuators are so sensitive that they can be heated to hundreds of degrees Celsius and cooled back down a thousand times a second. Each cycle can move the electrode a tiny distance toward or away from neurons and 540 cycles are needed to extend the probe all the way. The team tested the device on rodents in a variety of situations, including long-term testing, and have found that the new technology lets the researchers gather much higher quality signals than with previous devices. The team also developed a closed-loop control system that moves the probes autonomously, leading to long term stability of the neural recordings.
Thermal actuators have been used for years at Sandia and elsewhere, but the scale of this system is unique. “The idea that we could build this system to achieve multiple millimeters of total displacement out of a micron-scaled device was a significant milestone,” said Sandia engineer Michael Baker, who designed the actuator. “We used electrostatic actuators in the past, but the thermal actuator provides much higher force, which is needed to move the probe in tissue.”
The microelectrodes are made of highly conductive polysilicon, which the team discovered has a number of advantages. It is almost metal-like in its conductivity, but durable enough for millions of cycles. It provides a signal-to-noise ratio much greater than previous wire probes and provides high-quality measurement signals.
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