Recent developments in implantable neural probe technologies

This week, I worked hard on writing an application. As I said, I'm very interested in hardware. As I've been working on analyzing data and drawing some clinical conclusion to write papers, I've become interested in how to get more accurate data. So this week, I read an paper titled 'Recent developments in implantable neural probe technologies'. Originally there was more content, but I focused on only high spatial resolution part.

The technologies required to make a neural probe are largely CMOS and MEMS. CMOS stands for complementary metal oxide semiconductor. It is used in most of today’s integrated circuits. Something called chips or microchips is likely a CMOS. It uses MOSFETs which serve as switch or amplifier of current between source and drain according to the applied voltage. MEMS is an abbreviation for ‘micro electro mechanical system’. As its name implies, it refers to a very small machine made through nanotechnology and its manufacturing process. MEMS fabrication process is largely divided into deposition, lithography, and micromachining. For each step, I will study and share step by step soon.

Let’s talk about the paper. Neural probe is consisted of mainly 2 parts. metal microwire and coating with insulation materials. Tetrode array is common stucture. Tetrode is a bundle of 4 closely located microwire and if we assemble multiple tetrodes into array, it can be called tetrode array.

There are 3 major points to be resolved. First is about building an interconnection between electrodes and external connectors. One reference method is the Schaefer and his team’s work. They mechanically pressed the tips of microwire bundle so that they can be connected to the CMOS amplifier array. Second is about the size of the metal wire. To prevent the damage of brain, the diameter of wire should be less than 5-25um. Third point is about random distribution of microwire. microwire continuously move randomly in the tissue. So we need to develop stable position-based spike sorting algorithm that are not affected by shaking of microwire.

There are 2 main types of silicon-based micro electrode array. michigan probes and utah arrays. michigan probes looks like stick. There are few silicon shanks in a planar configuration and each shank has 8-64 micro electrode sites across a depth of several mm. we can get simultaneous recordings from up to a few tens of neuronal cells. Utah array looks like a square plate. there are 25-100 silicon shank and each of them hosts a single microelectrode. it has relatively large spatial scale. Some researchers tries to combine the advantages of both structures by arranging michigan probes in the form of cube. it looks like 3d silicon needle array.

As of may 2023, when this paper was published, the most advanced probe was neuropixel 2.0. It uses more than 5000 recording microelectrodes. One more remarkable thing is the neuro seeker probe. It gets signals from 1344 electrodes simultaneously, and records more than 1000 neurons signal.