Mechanical mismatch injures neurons each time the soft tissue moves. To prevent this, microelectronic meshes should be cushioned with hydrogels or similar materials.
At cortical parenchyma, just below where webbed collagen of the arachnoid layer thickens into pia, we find many immune cells called astrocytes. This is the glia limitans.
Glia limitans is a gatekeeping layer, like an army posted just inside castle gates. Nothing enters vulnerable brain tissue without the astrocytes noticing.
If we put something stiff into glia limitans, the surrounding tissue pulses with heartbeats. This mismatch causes fluid shear, akin to stirring a pot of water, which astrocytes interpret as an (extremely severe) infection[1].
The resulting inflammatory fallout kills surrounding neurons, and leftover scar tissue electrically insulates (blinds/mutes) electrodes.
We probably can't remove astrocyte cells. The immune system is important, but also astrocytes might be involved in more than "just" immunity[2].
Also, astrocytes are sensing something important! An oscillating rigid probe is like raking steel velcro across your skin all day.
However, immunosuppressants do help a lot and are in fact probably required for the immediate post-insertion inflammation at this sampling density. They degrade CNS[3] wound healing in some cases, but a soft implant probably isn't catastrophically injurious.
For whatever inflammatory chemicals appear, we can prevent their adhesion (so they don't stick around causing more inflammation) by using zwitterionic chemistry; I'll cover this in depth later. In short, zwitterionics prefer to be coated in water molecules than amino acids, so proteins bounce off.
So implants with enough surface area (>100 radius circles) need materials with brainlike elasticity.
Mesh implants are long, thin strips of electronics, sometimes curled so they have a springiness[4] which allows long-distance stretching.
Above a few square millimeters, mesh implants need bulky high-bandwidth/power cabling. (Wires eventually branch, but long-distance transmission only works on thick conductors.)
So we need to pull the cabling away from tissue; how about vertically, suspended in a jelly-like cushion material?
This way, we pay to solve material issues with chemical R&D. ML-assisted molecular search (of the sort used for novel drug discovery) makes this much easier than it was a few years ago.
In the next post, I'll walk through the chemistry of hydrogels.
The electrical components of meshes (transistors, wires, capacitors) are all rigid, and by my estimate we're a very long way from making the electrically conductive parts as stretchy as brain tissue; OLEDs use thin sheets to be flexible, but they're not stretchy.
More specifically, OLEDs use long pi conjugated molecules for conductivity and bandgaps; hydrogels can use this same chemistry, but it's supremely difficult to carry a chip pattern across the hydration stage. Wet hydrogels are incompatible with nearly all modern lithography tooling.
Mechanical mismatch injures neurons each time the soft tissue moves. To prevent this, microelectronic meshes should be cushioned with hydrogels or similar materials.
At cortical parenchyma, just below where webbed collagen of the arachnoid layer thickens into pia, we find many immune cells called astrocytes. This is the glia limitans.
Glia limitans is a gatekeeping layer, like an army posted just inside castle gates. Nothing enters vulnerable brain tissue without the astrocytes noticing.
If we put something stiff into glia limitans, the surrounding tissue pulses with heartbeats. This mismatch causes fluid shear, akin to stirring a pot of water, which astrocytes interpret as an (extremely severe) infection[1].
The resulting inflammatory fallout kills surrounding neurons, and leftover scar tissue electrically insulates (blinds/mutes) electrodes.
We probably can't remove astrocyte cells. The immune system is important, but also astrocytes might be involved in more than "just" immunity[2].
Also, astrocytes are sensing something important! An oscillating rigid probe is like raking steel velcro across your skin all day.
However, immunosuppressants do help a lot and are in fact probably required for the immediate post-insertion inflammation at this sampling density. They degrade CNS[3] wound healing in some cases, but a soft implant probably isn't catastrophically injurious.
For whatever inflammatory chemicals appear, we can prevent their adhesion (so they don't stick around causing more inflammation) by using zwitterionic chemistry; I'll cover this in depth later. In short, zwitterionics prefer to be coated in water molecules than amino acids, so proteins bounce off.
So implants with enough surface area (>100
Mesh implants are long, thin strips of electronics, sometimes curled so they have a springiness[4] which allows long-distance stretching.
Above a few square millimeters, mesh implants need bulky high-bandwidth/power cabling. (Wires eventually branch, but long-distance transmission only works on thick conductors.)
So we need to pull the cabling away from tissue; how about vertically, suspended in a jelly-like cushion material?
This way, we pay to solve material issues with chemical R&D. ML-assisted molecular search (of the sort used for novel drug discovery) makes this much easier than it was a few years ago.
In the next post, I'll walk through the chemistry of hydrogels.
Astrocytes sense pressure changes with sodium channels, the same ones used by pain-sensing nerve endings and bone-growth-promoting mechanoreceptors.
I say this mostly as a heuristic generalization from this article, though myelin non-plasticity in particular might not be as worrisome for neuropil.
(Central nervous system, which has quite different mechanisms than peripheral nervous system.)
The electrical components of meshes (transistors, wires, capacitors) are all rigid, and by my estimate we're a very long way from making the electrically conductive parts as stretchy as brain tissue; OLEDs use thin sheets to be flexible, but they're not stretchy.
More specifically, OLEDs use long pi conjugated molecules for conductivity and bandgaps; hydrogels can use this same chemistry, but it's supremely difficult to carry a chip pattern across the hydration stage. Wet hydrogels are incompatible with nearly all modern lithography tooling.
Human astrocytes extend below signaling neurons, far enough that physical / laser ablation would kill important machinery.