Intracranial nanowires: The basis of implanted brain-computer interfaces

A knockout punch rotates the head so quickly that the brain, inside its cushioned fluid sanctum cannot keep up. The brain is compressed against the skull, shears away small blood vessels, and the boxer’s world collapses around him. The promise of brain-computer interfaces, or BCIs, depends upon intracranial electrodes which penetrate through the cortex or into deeper structures. If electrodes placed in the brain cannot bend with it as it moves, they would eventually scramble it like a fork taken to an egg yolk. Even in the absence of a knockout blow, electrodes would wreck considerable havoc as your brain shrinks and expands normally according to your body’s nutritive or hydrative state. One component of the hangover’s headache results from the brain being, in effect, shrunken with dehydration.

Researchers at the University of Michigan have now created a flexible microthread electrode (MTE) that is only 7 micrometers in diameter, and can be bent into a full circle with a diameter of just a few hundred microns. It consists of an electrospun carbon fiber core with a thin film dielectric coating that is nonreactive inside the body. Electrospinning is increasingly being used to create finely controlled structures on microscopic scales. It can be likened to a form of 3D printing wherein the extruded material is given a charge and “shot” at a moving, oppositely charged target to build up the resulting structure layer by layer.

MTEs were compared with traditional silicon electrodes and were found to be much less attractive to the immune system sentinels (microglial cells) which patrol the brain looking for intruders. Samples taken from tests done with silicon electrodes showed the presence of these microglial cells, and a sparsity of neurons near the electrode, a poor prognosis for humans desiring implants to last perhaps 70 years. The MTEs did not show this pattern, and stable recordings were achieved throughout the duration of the study.

This study dovetails nicely with other studies also using polymer electrospun microfibers to interface with the brain. While electrodes can be simply inserted into existing brain tissue, in many cases it would be desirable to be able to coax neurons into growing into the implants themselves. Neurons that extend for any significant distance in the brain are generally insulated with myelin. Myelin is comprised of cells which wrap fatty lipid membranes up to 50 layers thick around the neuron’s axon.

Researchers at UCSF in California have built scaffolds thinner than human hairs and have induced the formation of myelin around these scaffolds in culture. They have also determined the optimal-si e scaffold for doing this. The researchers initially used polystyrene, however other studies have done the same with the degradable polymer, poly-L-lactide. This polymer is used to fabricate other kinds of biodegradable stents and sutures, and has a long history of success as an implant material. If neurons can later be guided into these custom-built myelin guide tubes, larger-scale brain-computer interfaces may become more feasible. As implant technologies become more user friendly we will begin to see BCIs and BMIs (brain-machine interfaces) migrate from external toy-like curiosities to essential internal components of the post-human machine.

Intracranial nanowires: The basis of implanted brain-computer interfaces