The Quest for a Neural Probe That Becomes the Brain Itself

​ Brain-wave scans and fMRI snapshots are often pitched as windows into the brain. It's common enough to hear about such and such device "reading" the brain via EEG or otherwise suggesting some profound neural communication via scalp-based electrical activity. In some large part this is hype.

High or even "OK" resolution observations of the brain are in reality still at the edge of science-fiction. To really get the goods, it's necessary to go invasive. That is, to see brain activity at very-high resolutions and absent the damning noise interference found in EEG-based methods, we need to go inside the actual brain—physically.

A new method of neuro-probing, described in ​a recent Nature Biotechnology paper by researchers at MIT, offers a potentially whole new landscape of super-high resolution brain activity observation—and in super-high resolution brain activity manipulation. Using certain fibrous materials, it becomes possible for a probe to essentially become a part of the brain itself, interacting with it not just electrically, but chemically and mechanically as well.

"Since our fiber-based probes are inserted directly into the tissues they provide one with much greater precision than non invasive techniques such as ​ ECoG," Polina Anikeeva, the current paper's lead author, told me. "But as these devices are soft they produce significantly lower damage to the tissue than other 'invasive' technologies.'"

Indeed, the sorts of direct, invasive neural probes currently in use have all sorts of profound limitations. For one thing, they're relatively rigid, and so any movement by the patient or probe is likely to result in tissue damage. This means that eventually the probe will wind up surrounded just by dead neurons and scar tissue, rendering it useless.

Second, probes (and, again, non-invasive methods) have historically been limited to observing only one mode of neural signaling at a time. This is a bit like taking single-dimensional snapshots of a three-dimensional world: useful but also extremely limited.

Neuronal signaling —thinking, if you will—​consists of three interacting channels. One is voltage-sensitive, the electrical impulses most commonly associated with brain activity. The electrical signalling is joined by a "ligand-sensitive" channel (affected by chemical agents) and a "mechano-sensitive" channel (affected by deformations in the neuron's surrounding membrane). They all feedback on each other.

"Our devices can simultaneously interact with the brain electronically, optically, and chemically," Anikeeva said. "This allows us to stimulate and record neural activity, creating a feedback loop."

That is, the probe is able to deliver some or another chemical or electrical impulse to the brain while also immediately observing its effects. Imagine a drug that, once administered, was able to continuously make itself more effective in vivo. The idea makes current psychoactive drugs, and their blunt-force effects, seem like ancient stone tools.

On paper, the new probe design sounds as crude as anything —like tying three hoses together, one for each channel. This hose sandwich, however, is very, very small and flexible enough to match the flexibility of brain tissue itself. This was achieved using a thermal drawing process, in which a much larger-scale version of the probe's hose-sandwich is constructed from one of a variety of different materials—polymer, metallic, composite—and then heated to the point of becoming soft and pliable. Then, it's carefully stretched further and further until instead of a wide stub of the material, we have a long, microscopically thin strand of it.

So: Starting from a section on the macroscale of inches, the probe is drawn out nearly 200-fold in just a single iteration of the process. This can be repeated again and again until the probe has a thickness on the order of a hair's hair. This resulting strand (this fiber, really) is small and flexible enough to effectively blend into existing neurons.

"These authors describe a fascinating, diverse collection of multifunctional fibers, tailored for insertion into the brain where they can stimulate and record neural behaviors through electrical, optical, and fluidic means," John Rogers, a materials science professor unaffiliated with the current study, offered in an MIT statement. "The results significantly expand the toolkit of techniques that will be essential to our development of a basic understanding of brain function."