Electricity in the human body -- Is there enough to power a prosthesis?

[Doma Noemi]

TL;DR Summary

Electricity in the human body

Can we use the electricity from the muscle to power an engine (like a prosthesis)?

 

[berkeman]

Welcome to PhysicsForums.

Interesting question. I believe the answer is "no", but will let others chime in. It's probably more practical to use battery power and some convenient recharging mechanism (like proximity, non-contact recharging).

 

[hutchphd]

I agree with @berkeman that there is probably not enough "electricity" available. At rest about 20% of the ~100 Watts we process go into neuronal activity. That's not very much power even if we could somehow tap it.
More interesting to my mind would be a system that could use the ATP-ADP cycle somehow. I have no idea whether that is possible or under investigation. Seems possible?

 

[TeethWhitener]

Summary:: Electricity in the human body

Can we use the electricity from the muscle to power an engine (like a prosthesis)?

Why not just use the muscle itself? The average person has a basal metabolic rate of 1-1.5 kcal/min, which works out to about 70-80 W. The neural activity portion of this is (I see @hutchphd beat me to it) 20% of that, or about 15W. The electrical power supplied to a muscle is a small fraction of this. However, during vigorous exercise, you can put out another 100-200 W of useful power on top of your basal metabolic rate (more if you're an elite athlete). I believe that most of the self-powered prosthetics coming onto the market attempt to tie into residual muscle for power (seems corroborated by a quick google search of "powered prosthesis," though I didn't do a deep dive). More advanced models use neural signals for information (move this finger, wiggle this toe, etc.), rather than for power.

 

[BillTre]

Most of the "electrical" power in the body is in the charge difference between the two side of cell membranes throughout the body. Cell membranes are only a few nanometers thick.
Being largely made of lipids, the membranes act as insulators with proteins in the membrane regulating ion (charges) flows across the membrane.
The membrane can easily store the charge because it acts as a capacitor.
The voltages across the membrane are on the order of 0.05 to .2 Volts.
This is not a reasonable source of energy to make use of. It can be measured in small local cellular areas with microelectrodes and occilloscopes, but there no way to extract enough electrical energy to do any significant work.

Another possible source of "bio-electricity" would be small current flows from one area of the body to another. An example would be the small currents that cone out of the cut stump of an amputated salamander leg. These are probably carried initially by ions leaving the would opening.
These currents are so small that their detection requires special probes to measure.
The spatial separation would allow using electrodes (one at the cut stump and one in the body as a "ground") to make something like a battery, but both the currents and voltages are very small.

Things like electric eels use a stack of modified muscle cells (like a stack of coins), where the membrane proteins controlling membrane permeability are located on only one side of the stacked cells. The permeability properties of these proteins is neurally controlled. When all the cells in the stack are stimulated to open these channel proteins, ions flow through the stack of cells (like a series of small batteries) in one direction.
This can result in hundreds of volts separated by distances upto about four feet (length of a large electric eel).
This could be a source of useful electric power but it is intermittent and under neural control.
In a physiological set-up (a short term lab set-up to do an experiment) the discharges could be controlled by stimulating nerves.
Something like this would be your best option for gathering electric power from biological sources.

 

[Averagesupernova]

The obvious solution although not likely the one you are after is to use the movement from another part of the body to activate a gripper on the end of a severed arm for instance.