BRAIN implants containing microelectrodes are used widely in the laboratory and clinic, both to stimulate nerve cells and to record their activity. Researchers routinely implant electrode arrays into the brains of rodents to investigate the neuronal activity associated with spatial navigation, or into monkeys’ brains to gain a better understanding of the mechanisms of motor control. As a result, we now have brain-computer interfaces that can help paralysed patients to communicate or control a prosthetic limb. Electrode arrays can also be used to assess vegetative patients, and to treat conditions such as Parkinson’s Disease and depression.
In most instances, keeping the electrodes in place for long periods of time is crucial. But this is difficult, for a number of reasons. In experiments involving freely moving rats, for example, the animal’s movements can cause the electrodes to be displaced, and when they do stay in place, the electrodes become gradually become ensheathed with glial cells, causing the signal to deteriorate with time. The devices have to be re-adjusted regularly, and their decoding algorithms recalibrated, to maintain the signal strength. Implants containing movable electrodes can potentially overcome these problems. The latest such device (described in a new paper in the journal Frontiers in Neuroengineering) is the most advanced yet. It uses microelectromechanical systems (MEMS) to move the electrodes up and down, and can record stably for up to 6 months.
Implants with movable microelectrodes have been in development since the early 1980s. The first generation movable implants contained thin wire microelectrodes that were moved manually. More recently, DC motors, piezoelectric motors and hydraulic motors have been used to move the electrodes, but all of the devices developed to date are still large and heavy. The new implant, developed in Jit Musuthwany’s Neural Microsystems Laboratory at Arizona State University, is the first use MEMS technology, and is the smallest and lightest yet.
Thedevice consists of an array of microelectrodes that are fabricated, along with a number of microscopic mechanical components, onto a silicon wafer. Each microelectrode is controlled by four microactuators, one each to deactivate a release-up lock and release-down lock, and one each to move the electrode up and down. The actuators work using electro-thermal strips and are coupled to a ratchet system that drives the centre shuttle of each electrode up or down.
This design allows for each microelectrode to be moved up and down independently of the others. In the resting position, the release-up and release-down locks prevent the microelectrodes from moving. Application of an electrical current to the appropriate electro-thermal strips causes them to expand, pulling the release-up or release-down lock away from the microelectrode to release it for movement, and to move it up or down. The movements occur in incremental 9 micron steps, equivalent to the spacing between the teeth on the side of the electrode. When the applied current is reduced to zero, the strips quickly cool down and return to their normal position, causing the electrode to move accordingly.
The researchers tested the movable microelectrode chip by implanting it into the cerebral cortex of 12 adult rats. After removing a section of the skull, they put the chips in place and anchored them by means of stainless steel screws. The microelectrodes were then inserted into the somatosensory cortex at a depth of about 1mm. Recordings were taken 3-4 times a week for an average period of 2-3 weeks. In 10 of the animals, the microelectrodes were moved an average of 60 microns from their original position, to sample neuronal activity from deeperdown in the cortex. The recordings obtained from them were compared to those from two controls, in which the electrodes were kept stationary throughout.
Overall, the movable implants proved to be highly durable, remaining fully functional for the duration of the experiment. The recordings even got better with time, because moving the microelectrodes improved the signal-to-noise ratio.
The best performance was obtained from the 8 devices that were encapsulated in a nylon mesh to prevent leakage of fluids from the implant site. Of these, 6 gave reliable signals for 3 weeks, and 3 of them lasted for about 11 weeks. One device remained functional for almost 6 months, and only stopped working because of the rat’s wound healing response: tissue regrew and penetrated underneath the dental cement used to anchor the device to the skull, causing it to pop off the top of the skull.
This prototype is still rather cumbersome – although the MEMS chip measures just 3 mm × 600 μm × 6 mm (seen above in black), the packaging, which secures it to the skull and connects it to the computer, is much bigger. Nevertheless, the experiments show that this approach is promising. The device can be scaled up easily without making it any heavier, by incorporating more microelectrodes and reducing their spacing. The researchers suggest that improvements in the way the device is mounted on the skull, and in how the device is packaged, will eventually lead to implants that can give stable recordings for several years.
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Jackson, N. et al. (2010). Long-term neural recordings using MEMS based movablemicroelectrodes in the brain Front. Neuroeng. 3: 1-13. doi: 10.3389/fneng.2010.00010