September 15, 2014
NeuroNexus has developed a true 3-dimensional probe, the Matrix Array. The Matrix Array is a silicon-based probe assembly that is comprised of an array of shanks aligned along a single lateral plane, with each shank containing multiple electrodes vertically positioned. The Matrix Array, then, allows for recording volumes that span both cortical layers and cortical columns. These blog posts will describe some of the developmental process of the Matrix Array, and the testing that we have done to ensure that it is a high quality product. Today, we’ll focus on the methods for implanting the Matrix Array.
Traditional NeuroNexus probes have either a single shank or multiple shanks that are all positioned in the same plane. Thus, insertion of those probes is fairly straight forward. However, probes with a 2-dimensional array of shanks come with special insertion challenges. Depending on how close the shanks are to each other, it is possible to get a “pincushion effect,” in which the penetration of one shank is hindered by tissue dimpling caused by a different shank. (This happened to me with a probe that I was attempting to use during my first postdoc to record in the dorsal root ganglia, and the shanks of the probe bent instead of inserting, ruining the probe.) One way to overcome this effect is to insert the probe with a lot of force, and this is a method that is often used to insert probes with this shape profile. However, high-force insertion can cause tissue damage that manifests in longer healing times, longer periods before recordings can be taken, or possible neural damage. Thus, we spent some time developing and testing alternate insertion methods for the Matrix Array.
The shanks of the Matrix array are thicker (50 µm) than the standard NeuroNexus probe (15 µm). This increased thickness makes the shanks stronger and able to cleanly penetrate tissue. The shanks are still thin enough, however, that the ratio of space between shanks to shank thickness still prevents the pincushion effect. Thus, we are able to do controlled insertion of the Matrix Array without requiring a great deal of force. Finally, we chose a computer-controlled insertion motor with extremely fine step resolution (0.05 µm/step) and speed resolution (0.22 µm/sec).
We first tested our insertion method on models, such as plastic wrap over agar. Once thoroughly tested in that way, we moved onto in vivo testing and eventually implantations with our beta testers.
One such test was done in the lab of researchers in Texas in October of 2013 (pictured above). The surgery was performed by our collaborators, implanting the Matrix Array into the motor cortex of a rhesus macaque monkey. The Matrix Array is held onto the tip of the IST-Motor insertion tool by vacuum. Using a craniotomy 1.6 cm in diameter, we fully inserted the 0.75 mm probe, in increments of 0.2 mm every 30 s. Once satisfied that the probe was completely inserted we turned off the vacuum and withdrew the insertion tool, leaving the probe in the brain. During this experiment we held forceps on the back of the inserted Matrix Array to make sure that it remained in place, but in prior and subsequent tests the forceps were not necessary. At this point, the probe was successfully inserted.
The insertion process is but one step on the path to having successful 3-dimensional recordings, and it perhaps is not even the first step that one might think of. But it is an important step nonetheless, and we made sure to develop a good approach and fully test it to ensure that the Matrix Array provides maximum benefit with minimal energy.
Close-up detail, Insertion methods, and more
A true 3D neural interface, for both acute and chronic experiments.