A full body cyborg is basically a brain encased in a mechanical body. The brain, being the only organic component, needs to be maintained.

The brain, being the most complex organ, requires a steady supply of blood and a safe sterilized environment. The sterile part is simple enough, you wouldn't having cyborgs walking around without appropriate artificial skulls and sterilisation. The blood part brings more complications, like keeping the organs alive and making sure the brain is in hospitable conditions (hormones and such). Mechanical limbs do not require such upkeep, instead they need monthly checkups to make sure they stay in working order. Basically, only the brain needs those organs.

What this implies is that the organs don't need to be at their full size like in regular humans. This makes it easier to fit the necessary parts like batteries and hydraulics. Artificial organs are an option of course but based on modern technology need to be quite large just to clean blood. Grown or fitted organs would make this much simpler (why reinvent the wheel?). Against rejection the organs can be kept in a biocompatible bag or the mechanical components can be coated with cells from the body to avoid rejection. But back to the question at hand. In short: I'd like an estimate of the minimum size required for organs to support a human brain.

  • $\begingroup$ Er, DOES the brain need blood? I thought there was a barrier that kept blood out, and only passed nutrients and select chemicals in. $\endgroup$
    – Jedediah
    Nov 28, 2021 at 15:55
  • $\begingroup$ Unfortunately, since we don't have the technology to do this today, we can only make some estimates as to what the realistic requirements would be. Does the minimal platform need to be mobile? Independent of external supplies? You could make it pretty small with external power and nutrient solutions (food) and gas (air) inputs. regular dialysis (liver) reduces this even further. How far down the rabbit hole do you want to get? $\endgroup$
    – DWKraus
    Nov 28, 2021 at 15:56
  • $\begingroup$ @DWKraus I’m only asking about the organ size necessary for the brain. The cyborgs have a human frame, are normal sized and eat conventional food (and change batteries). Does that answer your question? $\endgroup$ Nov 28, 2021 at 16:48
  • $\begingroup$ @LiveInAmbeR I've already put in an answer, Take a look and I can modify it based on feedback. Normal food? $\endgroup$
    – DWKraus
    Nov 28, 2021 at 16:50
  • $\begingroup$ @DWKraus Normal food. $\endgroup$ Nov 28, 2021 at 21:24

2 Answers 2


Ballpark: Twice the size of the brain (with appropriate inputs)

I'll lay out a few assumptions and factors I'm considering. Since the tech hasn't been invented yet, we're only guesstimating. We aren't maximizing independence, we're minimizing living parts, so compromises will be made accordingly.

  • Liver/kidney/bladder: Wastes can be kept to a minimum by giving the cyborg a sterilized, purified solution containing only the exact mix of nutrients needed for the current use. if we're keeping the biologicals to a minimum, it's easier to excrete any wastes (continuously) rather than try to metabolize them. It's easier to continuously get new inputs from an external reservoir rather than try to recycle new materials. You can either constantly add and remove these materials (ideal for minimal biology), or for more independence have a device filter the wastes, metabolize and convert the toxins, then mix with new nutrients to supply needs.
  • heart/lungs: It is likely that in the future a simple pump can be used mechanically that will perform the circulation for your cyborg more efficiently and with more redundancy than a biological heart. COVID has certainly proved systems like ECHMO are safe and effective at maintaining life support with current technology, and with small organs and future tech, mechanicals will certainly do this job. Organs adapted to near-pure oxygen and external scrubbers combined with reduced chemical energy needs (see below) will mean that if you DO want a biological lung, it won't need to be much larger than that of a sparrow (and more efficient structures than mammalian lungs are certainly engineerable).
  • Blood/Immunology: A small amount of bone marrow material will be needed unless you are willing to have periodic (about every 15 days) transfusions to replace lost red blood cells. You are likely to need at least macrophages to aid in the recycling of red cells and tissues as they need to be replaced. But I've seen patients with tiny amounts of stem cells colonized to the liver performing this function, so volume is likely to be quite small for such a tiny body. Cancer will need to be monitored carefully without a significant immune system, but the risks will be minimized with appropriate radiation shielding and keeping the nutrients rich in antioxidants and free of damaging carcinogens.
  • Direct stimulation/power (TMS): Currently only used significantly for treating depression, cells can be directly stimulated with energy through various means, bypassing the need for quite a bit of metabolism. while the brain can't be retrofitted, it's likely the other tissues can get much or possibly ALL of their energy needs met non-chemically if they are designed to do so. This greatly reduces the need for biological components to perform these functions.

PS: Based on OP feedback, if the cyborg eats normal food, an autoclave will be needed in the design to guarantee a sterile food intake. A food processing system will also be needed, and an external disposal for food processing waste. The risk of spores or other resistant organisms entering the sterile system is very great. And if we're digesting external food biologically, then you'd need more stomach, at least a decent approximation of an intestine (less extensive, but still...) more liver to handle random toxic materials, and a considerable increase in an immune system to handle invasive organisms. A blood-brain barrier would again be needed since sterility is no longer assured. To handle the shocks of infection, consider doubling the capacity of pump and lungs (even if mechanical) to deal with infection of these structures to deal with unpredictable contingencies. I'd say we'd be looking in that case at something about three times the size of the brain total.

Easier to eat normal food, then flush it into a toilet and subsist on nutrient solution to stay alive. But hey, it's your world, and I don't know that I'd want to be simply a brain in a jar running a body without eating. It does feel like a shame to let food go to waste, and theoretically increases independence.


S(min) = 0

Given the apparent technological achievement of the culture that is able to make and use cyborgs, I think it would be quite fair to assume that the problem of biological organs has been solved. And in the real world, we're really quite close. Connecting a brain to a machine implies an extremely robust technological base, especially in the area of miniaturisation. As you mention in your query, artificial organs are a possibility and that's the possibility I'd advocate, but you complain of size...

Review of Organ Systems:

  • Integument: skin and supporting tissues are not needed at all because the body is 100% mechanical
  • Musculoskeletal: again, bones, muscles, tendons &c are not needed at all because their functions are assumed by mechanical means
  • Digestive: the only organ system that needs to be fed is the brain and the neural net interface. More later.
  • Kidney/Urinary/Liver: this system handles waste disposal. More later.
  • Reproductive: unnecessary due to the body being mechanical, it can be dispensed with
  • Cardiovascular/lymphatic: sole purpose is to pump blood around. More later.
  • Respiratory: necessary for gas exchange & some acid/base equilibrium. More later.

Replacing the Necessaries:

We focus on the brain & neural network. We definitely need to feed the brain constantly; we need to keep it oxygenated; we need to remove wastes; we need to emulate some other functions like hormones. We can do away with the whole digestive system simply by supplying the cyborg body with a storage container that holds a concoction of appropriate nutrients (carbs, proteins, vitamins, minerals, etc). We can do away with the kidney & liver by use of what is essentially a dialysis machine: it removes impurities & filters wastes. We can do away with the CV system by means of a mechanical pump, artificial vessels and an artificial blood substitute. Likewise, we can do away with the respiratory system by means of an oxygen concentrator and an ECMO device.

Various synthetic hormones can similarly be stored in the cyborg's body and released to the brain through the artificial circulatory system as needed.

The essential breakthrough in the transformation from significantly biologically reliant cyborg prototypes to a minimally biological concept comes by means of the advances in miniaturisation. Being able to discern, categorise, separate, connect & programme all of the nerve connexions that exit (and enter) the brain has been a boon for all other areas of biocybernetics & engineering in general.

The Conundrum Solved:

The basic problem that "Borg" type cyborgs have is that they are primarily biological in nature. Therefore the various organ systems all need to be "full sized". This of course means that there is really no room inside the body for anything extra. This is why Borg have things sticking off their bodies. Even when we replace the musculoskeletal system with mechanical systems, the organs still need to be pretty close to full size just to maintain themselves! This, plus the fact that the mechanical systems need some kind of battery and some kind of fluid reservoirs, this would likely make the cyborg's body somewhat larger.

It is not until the microrevolution occurs that machines we already have, like the ECMO, like the dialysis machine, can be made small enough, literally crafted on a molecular level. This will allow for the manufacture for a relatively small & integrated unit to be housed within the cyborg body and still leave a whole lot of room for all the mechanicals, weapons systems, auxiliary processing systems, energy storage systems, etc.


The minimum size for the cyborg's vital organs is zero, because we've done away with the need for reliance on biological organs.

Technically, the brain itself is a vital organ and must be full size.


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