Exploring Neural Implant Microtechnology
By: Frederik Bussler
Tags: Neuroscience, Elon Musk, Nanotechnology, Brain, Electronics

Exploring Neural Implant Microtechnology

Image by KENNETH RODRIGUES from Pixabay

Microtechnology is the study of the very small — micro means one-millionth of a meter. Nanotechnology (one-billionth of a meter) and microtechnology are common in the automotive and smartphone industries, as well as the emerging field of neural implants.

This article focuses on the Neuralink’s microfabrication process, or at least what we can glean from its white paper and recent demo.

Overview

The Neuralink is a neural electronic implant composed of up to 96 “threads” with 32 electrodes each, connected to a thin film array with custom chips.

Those threads — a type of microwire — use a polyimide substrate (base polymer) and dielectric (non-conductive material), encapsulating a gold thin film trace (what the micromaterial is carved out of). Conductors carry electric current, while the substrate carries EM waves.

Below, on the left, you can see an example of a silicon wafer, and to the right is a wafer with a metal substrate. On these wafers, microelectronics like the Neuralink can be fabricated.

Image from “Nanotechnology: A Maker’s Course” on Coursera.

Thin-film deposition is done on the wafer, after which stepper lithography and other microfabrication techniques are used to form the metal film. Ten thin film devices are patterned on a wafer, each with 3,072 electrode contacts (for 96 threads * 32 electrodes).

Below, you can see a (highly magnified) rendering of what nanomaterials may look like etched onto a wafer.

Image from “Nanotechnology: A Maker’s Course” on Coursera.

Electron Microscopy

A thread is just 5 to 50 μm in width, while an electrode is 14x24 μm — around the size of an animal cell. On its short side, an electrode is over 1,000x smaller than the diameter of a European penny (similar to an animal cell, and invisible to the naked eye).

Size comparisons: Animal cell (contents not to scale), Neuralink electrode, European penny.

With thread thickness as low as 4 μm, these materials are nearing the limits of light microscopes, and electron microscopes are surely being used.

Having tiny, flexible, and light wires is a must, as otherwise there will be inflammation, neuronal damage, and potential rejection of the implant.

With nearly a meter of vasculature per microliter of the primary cortex, the risk of damage is extremely high, and every precaution must be taken. Therefore, as Musk says, the Neuralink’s threads may get to the sub-micron (nano) level.

Photolithography

As mentioned, one thin-film device has up to 3,072 electrode contacts (96 threads * 32 electrodes). One microfabrication technique that may be used to create these devices is called “photolithography.”

Chips and other nanofabrication-created materials start off as a large, pure crystal — typically Silicon. A large block of this crystal, called an ingot, is sliced into small wafers. Photolithography is used to create patterns in the wafer, which tell us where to put our working materials (silicon dioxide, silicon carbide, etc.).

First, the wafer is covered with a “resist” (also known as a “photoresist”), and then spun very quickly to remove excess resist, resulting in a thin, uniform layer. We want to make specific changes, so the underlying wafer needs to be “exposed” in certain areas.

A “mask” and UV light are used to expose the wafer, where the mask blocks off the light except for the areas we want to expose. The exposed areas touched by the UV (or other EM wave) undergo a chemical reaction. Then, the resist is washed off, such that we end up with a wafer with a pattern matching our mask. On those exposed areas (matching our mask), we can add or remove material.

We can do this with many patterns, over many areas, for complex creations.

The Cutting-Edge

Smaller and smaller microfabrication via lithography can be accomplished with smaller wavelengths (higher frequencies) and more powerful lenses. Smaller wavelengths enable smaller etchings, so other frequencies, like UV and X-ray, are commonly used.

Recall that the Neuralink’s threads are as thin as 4 μm, which is still in the realm of microtechnology. Future versions of Neuralink may have sub-micron-level threads, in the realm of nanotechnology, which could potentially be achieved with techniques like ion projection lithography or electron beam lithography.

Electron beam lithography refers to using a focused beam of electrons to draw shapes on electron-sensitive resists, and can make patterns with sub-10 nm resolution. One drawback of this method is low throughput, but even if the Neuralink’s threads were fabricated at such a small scale, there would be other challenges, like having a very small cross-sectional area for the electrical signals to traverse.