Mar 23 2020

Microwires for Brain Machine Interface

One extremely exciting technology that is in development is the brain-machine interface (BMI). This technology allows for communication between a biological brain and a computer chip. Once perfected, the implications are incredible. Perhaps most exciting is the possibility of  robotic prosthetics that can be controlled with the mind. There are many medical conditions that impair the ability to move, from spinal cord injury to strokes that can literally cause people to be “locked in”. In many conditions the brain is working, but the peripheral nervous system is damaged. With a sufficiently functioning BMI non-functioning limbs or blockages in communication can be bypassed. Amputees could also have fully robotic limbs to replace what’s missing.

From a theoretical perspective, all of the necessary proofs of concept have been done. The brain can learn to control machines, even computers. The brain can map itself to new limbs, and it can incorporate new sensory feedback. With sufficient sensory feedback, control is enhanced, and the sensation that the new artificial limb is part of one’s body can be complete. We can even be made to feel as if we occupy virtual avatars.

Computer hardware and software technology are already powerful enough meet the demands of any such BMI application. Robotics are also functional enough to work, although there is certainly a lot of room for improvement here. All these components are good enough to use right now, and continued incremental advances will just make them better.

The main limiting factor for BMI applications right now is the interface itself – how do we connect the brain to silicon? Scalp surface electrodes are the safest and most convenient. Electrical signals from the brain do make their way through the skull and scalp and can be recorded, but they are greatly attenuated. Only relatively large parts of the brain firing at the same time produce a sufficient signal to produce a detectable wave at the surface. Still, even with this method there are BMI applications, but the discrimination is limited.

Brain surface electrodes are much better, but there are some practical limits. First, the brain pulsates slightly and this can cause the electrodes to move. Second, having wires go from inside to outside the brain is a risk of infection. Third, long term the electrodes can cause scar tissue to form which interrupts the connection. We could put a wireless system entirely inside the skull to eliminate the infection problem, but then we have to power that system and deal with the waste heat.

One novel idea is to put electrodes inside blood vessels inside the brain (in the veins, to avoid the risk of blood clots and strokes). This gets the electrodes inside the scalp close to brain tissue, but does not actually touch the brain and run the risk of scar tissue. There is still the problem of getting the signal outside the body, however.

This is a serious problem with the technology that needs a solution if ever the full potential will be realized. Some researchers are working on soft electrodes, flexible enough to pulsate with the brain so that they don’t move or cause scar tissue to form. If we could make a sufficiently low-power chip that uses very little energy and produces very little waste heat, that would also be a huge help.

A new study reports on a proof-of-concept of yet another approach – using very thin microwires for the interface. Instead of making the wires flexible (or perhaps in addition) these researchers made them very thin – 5 to 25 μm (up to 15 times thinner than a human hair, which apparently is the standard by which all thinness is measured). I won’t go into all the engineering details about how they accomplished this. If you want those details they are well-described in the published article. But here are the important results: The were able to make these very thin wires including insulation and insert them into a rat brain while maintaining their desired positions in three-dimensional space. Such thin wires tend to clump when inserted, so they needed to develop a method to keep them separated. They can insert them with the recording tips at different depths as well.

The result was successful – the ability to record detailed electrical information from the rat’s brain while it went about its business. This seems promising. The researchers admit that the “form factor” needs to be miniaturized from practical use in humans. Of course, the technology needs to be safety tested in humans. I would be particularly interested in long term safety data – how long do the wires survive and what risks do they entail? The idea is that they are so thin any trauma to the brain tissue itself is minimal and without consequence, but that needs to be robustly demonstrated. This technology still leaves us with the problem of communicating to outside the skull.

So again we have a potentially exciting technology but with major hurdles that need to be overcome. In this case it seems the proof-of-concepts are all there, and it really is just a matter of incremental advances in the technology itself. So it is likely more a matter of when than if – but that when can be a long time if the hurdles prove difficult.

I do wonder where this technology will end up in 50 or 100 years. How routine will it be? How specifically with they solve the problems? One interesting speculation comes from one of Clarke’s Odyssey sequels (3001: Final Odyssey). Here we are 1000 years in the future – hopefully it won’t take that long – and humans are routinely fitted with a skull cap. The solution to getting the signals outside the skull is simply to replace the skull with the interface itself. The top of the skull is removed and replaced with a supercomputer that sends microwires down into the brain, creating the interface. The skull cap becomes an artificial brain prosthesis, greatly augmenting mental capability.

A lower tech contemporary version of that is plausible. You could take out a small piece of skull and replace it with a plate that will contain the computer chip that the electrodes communicate to. The plate will potentially have access through the scalp (not a problem as long as the brain cavity is closed off) for recharging and wired communication to external devices. Replacing a small part of the skull is not as big a deal as it may sound. This is actually a routine procedure. It certainly would be worth it in someone who is otherwise paralyzed.

This one study is a baby step, but there seems to be a steady stream of such incremental advances moving this technology forward. It is still difficult to predict which specific solutions will ultimately emerge and how long they will take, but it does seem clear that brain-machine interfaces is likely to be one of those technologies that really shape the future.

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