How to cushion the brain from high g-forces

Question

Let's say you have a cyborg who's replaced their whole body, everything but the brain, with machinery. Next, you put them on a spaceship pulling dozens or even hundreds of gs. How are they going to survive? What's the best way to protect a disembodied brain from arbitrarily high g-forces?

This question pertains specifically to space travel, however it might also be useful in answering the question of whether or not a cyborg is physically capable of dodging a bullet (the eternal question that's stumped sci-fi writers for ages lol).

Answer 1

Fill the rest of the skull

Woodpeckers can supposedly hammer trees using 1000g, about 80 million times throughout their lives, without receiving concussions or eventually developing dementia. By copying their strategies, it is possible to reduce the risk for human beings.

I cannot promise full protection, but this is the best you're gonna get.

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1. Compress the jugular vein

Doctors have engineered a collar that reduces blood flow to the brain. It allows slightly more blood to build up in the skull, but no adverse affects have been observed. There is hope that it will prevent concussions in football players.

2. Strengthen the neck muscles

This can be achieved through steroids, training, etc., but it may be important to keep the head attached to the body in these circumstances :)

3. Adapt the bone structure

This will likely require a couple invasive surgeries, but it will help to some extent. Woodpeckers have skulls that act as shock absorbers; this would reduce damage from both moving the head relative to the neck, and moving the brain into the skull. The inner layer of bone is somewhat spongy, and thickest toward the back of the head.

4. Transplant some stem cells

While @CortAmmon is correct that the brain is incredibly squishy, you can also consider growing tissues in the cerebrospinal fluid using the patient's own cells. This will prevent the body from rejecting it; if there is sufficient technology / brain surgery is safer and less risky during the time of space travel, it may be possible to put some tissues in the spaces within the brain - further cushioning it.

Answer 2

The physics simply won't let you do this.

The usual approach to protecting against high-G forces is to immerse the squishy organic thing in liquid. However, the tricky bit is that the liquid must be as close to the same density as the brain as possible. Our body already does this. The brain is protected with a layer of liquid in this way. However, the 90–100G's that occur when a football player collides with another demonstrate the limits of this. The liquids are not quite the right density, and a concussion results.

Sustaining those G forces is even harder. In the football case, there's no much time to shift the fluids around. In a sustained case, there's time for the blood to respond to the forces. Hydrostatic forces are defined by $P=\rho a\Delta x$ where $P$ is the pressure exerted, $\rho$ is the density of the fluid, $a$ is the acceleration and $\Delta x$ is the difference in position of two points in the direction of the acceleration. At $a=1000m/s^2$ (roughly 100 gees) and for water ($\rho=1.0\frac{g}{cm^3}$), you find that the hydrostatic force is 1,000,000 pascals/m (9.8 atmospheres/m). Across a brain ($\Delta X=.15m$) we see a force of roughly 1.4 atmospheres. That is a lot, but it probably doesn't mean much to most people, so let me convert that into a unit that more people are used to seeing. Systolic blood pressure is ideally around 120mmHg. High blood pressure can drive it higher. Exercise can get us up to around 220mmHg. The hydraulic forces we are talking about here are on the order of 1100mmHg!

Now this won't immediately cause an aneurysm. If you get the densities just right, those forces should be counteracted by the fluid pressure around it. However, this will have all sorts of strange effects. The body really is not designed to be subjected to those forces.

For example, what happens if we pull more gees? What if we pull 300gees? That's three times the force, so you'll see three times the pressure: 4.2atm. That's the equivalent of going 40ft underwater! Anyone who's a diver knows what that means: we're going to have to pay attention to "the bends." Nitrogen can enter the blood at very high concentrations on one side of the brain, get shuffled along to the other side of the brain in its natural flow, and then emit bubbles of nitrogen on the other side! This could be resolved by using special mixtures like heliox which don't have nitrogen, but it shows just how strange this could be. Professional divers, such as underwater welders, often have strange neurological symptoms associated with the effects the pressure has on them, even with fancy mixtures!

Finally, there's the elephant in the room: not everything in the brain is the same density. Even if you tried to do this perfectly, you'd still have the fundamental issue that different tissues have different densities. This means that, no matter what density fluid you pick, you're going to get it wrong for something. Get it right for the blood, the grey matter is too dense and moves. Get it right for the grey matter, and the blood now applies enough pressure for aneurysms.

To pull this off, your cyborg would have to replace virtually all of the brain itself. Stick to nothing but neurons in a vat whose density is exactly equal to that of the liquid in the neurons, and deal with the difficulty of feeding those neurons (most of the mass of the brain is actually helper cells whose sole job is to keep the neurons fed and happy).

Answer 3

Freeze the brain temporarily

A fast-freezing unit included in the brain could make it more resistant to percussions, assuming the metal skull is resistant as well.

Artificial vessels of freezing fluid can be introduced through the brain for faster access to central parts.

The freezing has to be fast and well controlled to prevent ice "blades" to form and preserve cells intact. This is usual in food preserving nowadays. For instance, strawberries and fish need particular care to keep the flesh in a good condition.

While the brain is frozen, an alternate electronic brain can take over, with limited functionalities of course.

Answer 4

Dozens easily without the rest of body keeping oxygenated fluid in the brain is easy. Use artificial blood and add some shock absorbing material around the fake skull and you're good.

Hundreds is more difficult, Woodpeckers can withstand 1200 gees, but only as impacts by having a combination of shock absorbers around the skull and almost no room for the brain to move within the skull. but the human brain is just too big to be helped by making the skull tighter. The one possible way you might do it is you flooded the fluid and brain with a Non-Newtonian liquid so that under high G's the fluid acts more like a gel, that will let them sustain much higher gees provided it is rarely used, however if they are under those gees for long of too often the brain will run out of oxygen becasue the fluid is not moving enough.

Answer 5

Disembodied brain g tolerances are not well studied so this answer will require some speculation.

Snake strikes can subject the snake's head to approximately 30 gs, and snakes are capable of striking multiple times in rapid succession. This implies that moderately complex brains can withstand high g loads. It is thought that the flexibility of the snake's skull distributes the force of the strike over a longer period of time. So perhaps attach your brain case to a suspension system. This is especially feasible if your ship has one large main engine - all significant accelerations will be in the same axis. In addition, make your brain case signifcantly larger than a normal skull to give the cushioning liquid inside of it more time to slow the brain. Basically you are creating multiple systems to spread the shock out as much as possible.

John Stapp demonstrated that a fit individual can walk away from a brief 30 g acceleration and survive over 45 gs. A layered suspension system should allow short accelerations above 100 gs for your cyborg ship. Sustained acceleration of that magnitude is likely not feasible.

For sustained acceleration you will likely want to have high precision adjustable pressure pumps replace the heart. Trained pilots can withstand sustained g loads on the order of 10 g before they begin to black out. With high power, high precision pumps you should be able to maintain blood flow at higher g loads, I guesstimate 20 gs as the upper limit - where you start to worry about deformation of the brain tissue itself.

Extended periods of highly elevated blood pressure may have negative consequences. Perhaps a lining or treatment for the brain's remaining blood vessels would be useful.