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I have characters in mind for a science-fantasy setting which are mostly what you'd call mechanical/robotic, but that also have muscles in their major limb joints, as well as a cardiovascular system and lungs in place to support said muscle tissue. The main power source for these beings is a crystalline core implanted into their hull at conception, which delivers a constant electrical charge for, on average, 80+ years.

However, these beings do not sustain their organic tissue through eating food.

Could organic tissue last for that long without sustenance? How long would it take for it to degrade? What could be different about it compared to organic tissue in our world that would prolong its lifespan?

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    $\begingroup$ "Could organic tissue last for that long without sustenance?" - hell no. For time: do they drink? $\endgroup$ – Mołot Jun 20 '17 at 14:14
  • $\begingroup$ Welcome to WorldBuilding.SE McOwen! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jun 20 '17 at 14:16
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    $\begingroup$ Have you considered breathing? It is possible to get most of the elements you need through the air and then convert them into something useful with your sci-fi tech. The ability to breathe out alone would solve a lot of problems (not all though) you are currently facing. $\endgroup$ – Raditz_35 Jun 20 '17 at 14:17
  • $\begingroup$ @Raditz_35 As I said, they do have lungs and a heart, so those could tie into each other and draw necessary substances from the air, I suppose. This would probably mean that their world would have to have a very different atmosphere to Earth, though. $\endgroup$ – McOwen Jun 20 '17 at 14:19
  • $\begingroup$ @McOwen No it doesn't actually. A bit more carbon would be nice, but well ... plants live too. You can solve a lot of problems this way alone. Sadly I have no time today to write a full answer, but since you allowed breathing, this is very very doable. Maybe some minerals and trace elements once in a while, but they could have much more efficient storage than purely biological lifeforms $\endgroup$ – Raditz_35 Jun 20 '17 at 14:22
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This link on starvation describes the various hormones that start up and shut down during various stages of starvation. This is at a macro level, and involves organs like the liver and pancreas. If your cyborgs include these organs, then the results will be similar.

A cascade failure will occur that results in fat being converted to glucose. When that runs out, protein (muscle) will be converted to glucose. Along the way, your cyborg's organs will begin to fail.

If your system relies on artificial organs, they won't fail. If these organs are incapable of responding to the various cellular signals, then the process may not result in muscle conversion to glucose. But that would just accelerate the death of your beings' living tissues, since cells require glucose to survive.

Your cyborgs can survive without eating in the typical mammalian sense. But the living tissues must take in water, nutrients, oxygen, and glucose in a properly maintained balance. In living beings, a complex series of systems do all of this.

If you take those systems out of your cyborgs, then you will need to replace it with some sort of equivalent mechanical system. This reminds me of the scene in the original RoboCop where Robocop is eating baby food. His metal system had a battery. His living systems still needed energy to fuel those living cells. You could have some mechanical organ that combines compressed oxygen with a pre-mixed solution of glucose, water, and nutrients, and then circulates that mix to your tissues.

But the tissues must have these things or they die. Cells don't survive without them. See also cellular metabolism.

Also, cells don't really live all that long on their own, even with proper nutrients. They are constantly dividing and dying, replacing old cells with new. So the tissues your cyborg leaves the factory with will be constantly replaced throughout it's design life with new tissues. This is called Apoptosis.

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  • $\begingroup$ Yes, if you do it like Robocop with parts from a human being. But I think all that you brought up can be solved by creating the right tissue and using the battery wisely and just having really good resource management from the sci-fi tissue in symbiosis with the "mechanical" parts $\endgroup$ – Raditz_35 Jun 20 '17 at 14:44
  • $\begingroup$ The living tissues will still require some source of nutrients, minerals, glucose, water, oxygen, etc. These aren't optional. They are required for biological processes. $\endgroup$ – CaM Jun 20 '17 at 14:47
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    $\begingroup$ I'd argue they are not optional for muscular tissues, at least. en.wikipedia.org/wiki/Metabolism $\endgroup$ – CaM Jun 20 '17 at 14:54
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    $\begingroup$ you can regenerate them with your battery Mm, this I want to see. Go on, how does that work? $\endgroup$ – Draco18s Jun 20 '17 at 14:56
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    $\begingroup$ @Raditz_35 You're making an extraordinary claim. While I'm sure that it's theoretically possible to reconstitute glucose out of cellular waste material (afterall, plants and bacteria manage it), you're trying to do so with electricity as the power source. yourlogicalfallacyis.com/burden-of-proof $\endgroup$ – Draco18s Jun 20 '17 at 15:28
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Muscle cells need to be supplied with glucose and oxygen; the glucose is carried dissolved in blood plasma, while the oxygen is carried by red blood cells.

  • Red blood cells are good for 3 or 4 months, after which they must be replaced. In part, degraded red blood cells are recycled; but certain compounds cannot be recycled. For example, degraded heme is converted to bilirubin (which needs to be removed from the blood stream and excreted) and new heme must be synthesized, ultimately from proteins (and of course some iron) taken as food.

  • The glucose is used as an energy source and needs to be replaced either from an external source or by using up some fat. Eventually all fat will be used up and new fat or glucose will have to be taken up as food.

All in all, biological components are made to work by exchanging matter and energy with the environment. That's just how biology works.

Note that the cyborgs don't necessarily need to eat in the traditional sense; you may imagine that all required replacement substances are supplied ready made in some sort of fuel-like mixture. But they need to take in water and to excrete urine, unless they have high-capacity internal tanks.

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