# How advanced and effective a mechanical arm (only human energy) can be? [closed]

I'm writing about a post-apocalyptic world, and some of my characters got mechanical limbs, etcetera. I want to describe a technology where the implants can work apart from any other energy source - just the human body.

How I can make this "realistic"? Note: the arms are fully functional - except for detecting tact.

Thank you all.

## closed as too broad by Mołot, Hohmannfan, MozerShmozer, TrEs-2b, AifySep 16 '16 at 5:18

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• Probably about 10-100 times strong than humans though it depends on a lot of variables. – Durakken Sep 15 '16 at 21:23
• How about using muscle tissue to power the implants? If these characters are otherwise missing limbs, pull the muscle from another body part. – naomisl Sep 15 '16 at 21:25
• I wonder: what would give energy for the prothesis to work in this case? – Radec Sep 15 '16 at 21:26
• How I can make this "realistic"? - I believe you can't. would require much higher tech level than we have now, hard to get if all they got is what we have minus apocalypse. More details about tech level in your world (before apocalypse, and what's left) would be needed to truly answer your question. – Mołot Sep 15 '16 at 21:54
• @Durakken how can a purely human-powered albeit artificial limb be 1000 times stronger than humans? – JDługosz Sep 15 '16 at 22:50

In the game series Deus Ex, they describe something called bioenergy. Energy required to power augments is stored inside internal 'batteries' that can be drained over time or with heavy augment use. You can read more about how augmentations developed and how they function in the games here.

For the purpose of your story, the scientists of your future would have to have figured out how to engineer synthetic muscle fibres and nanites that could power the fibres through the usual ATP cycle that our bodies already use. If these were efficient enough, then even if the limbs didnt look natural, they'd function the same as normal human limbs.
I wonder though if these nanites would be able to produce enough energy for an artificial limb to outperform human strength, that will depend on your story I suspect. The robotics to provide the same range of motion as a natural limb would be insane, but feasible I think (since the nanites could build the smaller parts).

You could also, like Deus Ex, have several generations and manufacturers of the limbs - thus giving different characters different levels of augmentation. Some better than others.

Edit:
Realised I should also mention that muscle cells fire using an ATP sodium-potassium ion pump. You can imagine each cell acting as a kind of battery. A muscle contraction activates that battery and it discharges. Biochemical actions restore the cells energy but over time there is fatigue (through further complex biochemical processes). The nanites would need to fill the role of the muscle cells and potentially even be able to differentiate to fill the roles of other cells in the body (red blood cells to carry oxygen and carbon dioxide, white blood cells to fight off infections and repair damage, platelets to seal wounds, etc).

To answer your question about glucose, the overall process of oxidizing glucose to carbon dioxide (burning) is known as cellular respiration and can produce about 30 molecules of ATP from a single molecule of glucose. The ATP cycle is how energy is produced in the body, except that glucose isn't necessarily the only source.

Electricity is essentially the flow of electrons, the energy released through cellular respiration is primarily heat I believe (that's why you get hot when you exercise). I can't see how that process would generate electron flow.

• Why nannites? Why ATP cycle? Why can't it be just a fuel cell burning glucose for electricity? – Mołot Sep 15 '16 at 22:40
• This is awesome. I will use the ATP and @Molot comment to build the technology inside my book. – Radec Sep 15 '16 at 23:09
• I'm still pretty new to worldbuilding, so this has honour of being my first accepted answer. Awesome!! – Shaun K Sep 15 '16 at 23:14

Powering prosthetic limbs via muscles elsewhere on the body is something that was developed in the early 20th century and matured in the 70’s. You can search for details and find pictures I’m sure. On TV I recall a guest villain that was a sharpshooter/sniper and had such an arm, and working the rifle with it was quite a spectacle in itself. There was a post-WWII movie about a disabled veteran who was quite adept with his mechanical arm but had self-esteem issues around loved ones and acceptance issues in general. But it also featured first just what he could do, in an early scene before he became a klutz when going home to family and girlfriend.

You may have seen this design: Wikipedia has a section on body-powered arms which are still used it seems because they are much lighter than electrics and offer direct feedback.

So to make them realistic, look at the real technology! Everything from ancient models to the latest body-powered mechanical limbs would probably be in use. I found asking Google for arm prosthesis body powered gives rich results including videos and technical articles.

• Nice answer, awesome historical context. Thank you @JDlugosz! – Radec Sep 15 '16 at 23:10
• I would love to give you an upvote, but I can't :( Thank you! – Radec Sep 15 '16 at 23:19
• Sorry, I didn’t realize that was not available until you get a rep of 15. Try answering some questions, now that you’re here, and you’ll get a few points quickly enough. Then later you can review answers to your own questions and apply any upvotes. Good luck! – JDługosz Sep 15 '16 at 23:36