Researchers at the Viterbi School of Engineering at the University of Southern California, US, have developed thin, flexible polymer-based materials for use in microelectrode arrays that can record brain activity more deeply and with more specific placement than ever before.

Each microelectrode array has eight prongs, each with eight microelectrodes attached, which can record from a total of 64 subregions in the brain all at once.

The newly developed polymer-based material is called Parylene C and is less invasive and damaging to surrounding cells and tissue than previous microelectrode arrays, which are made of silicon or micro-wires.

However, the long thin probes can easily buckle during insertion. This makes it necessary to add a self-dissolving brace made up of polyethylene glycol (PEG) that shortens the array and prevents it from bending. Despite this, the performance of the polymer-based material is on par with micro-wires in terms of recording fidelity and sensitivity.

USC Viterbi Department of Biomedical Engineering professor of biomedical and electrical engineering Ellis Meng said: “The information that we can get out is equivalent, but the damage is much less. Polymers are gentler on the brain, and because of that, these devices get recordings of neuronal communication over long periods of time.”

Insertion of the microelectrode arrays still poses the same problems as with any other prosthetic implant because of the body’s natural immune response to a foreign object. Previous microelectrode brain implants made of silicon or micro-wire have caused neuronal death and the scarring of glial cells. Parylene C provides a solution to this as the material is biocompatible and can be microfabricated in an extremely thin form that moulds well to specific sub-regions of the brain, allowing for exploration with minimal damage.

So far, these arrays have recorded electrophysiological responses of individual neurons within the hippocampus. If this area of the brain is injured it can result in a patient’s inability to form new memories.

Meng claimed the polymer-based material can conform to a specific location in the hippocampus and ‘listen in on a conversation’ between neurons. The large number of microelectrodes also means that much more information about neural interconnectivity can be gathered.

He said: “I can pick where I want my electrodes to be, so I can match up to the anatomy of the brain. Along the length of a tine, I can put a group of electrodes here and a group of electrodes there, so if we plant to a certain depth, it’s going to be near the neurons I want to record from.”

Future research aims to determine the recording lifetime of polymer-based arrays and their long-term signal-to-noise stability. The team plans to create devices with even higher density, including a double-sided microelectrode array with 64 electrodes per prong instead of eight, which would see a total of around 4,000 electrodes placed in the brain at once.