Comment by adrian_b

Most of what you have said is not different from what I have said.

All the rotating parts bigger than some tens of micrometers have only a limited rotation angle, where the limits are enforced by the solid connections between the 2 mobile parts, e.g. tendons, nerves and blood vessels.

The bacterial flagella and the rotating enzymes, which are powered by ionic currents, cannot be scaled to greater sizes. Already the flagella of nucleated cells (eukaryotes) are no longer based on rotating motors, but on contractile proteins, which must be attached at both ends on the mobile parts, limiting the relative movement.

Unlimited rotation is an absolutely necessary condition for a wheel that is used in locomotion, otherwise it is no longer a wheel.

A wheel used in locomotion that would have limited rotation would be just a leg that happens to have the shape of a wheel, because like a leg it would have to be raised from the ground for the forward motion, eliminating the exact advantage in efficiency that wheeled vehicles and tracked vehicles have over legs (i.e. that backward and forward movement are simultaneous and not separated in time during a step cycle, and no energy is wasted with a vertical oscillation of the leg).

The distinction between electromagnetic motors and biological motors is definitely one of principle and not an implementation detail. The only resemblance is that both are motors.

It is true that you can claim that when analyzing both chemical reactions and the interactions between the mobile parts of an electromagnetic motor they can be eventually reduced to electromagnetic interactions. Nevertheless such an assertion is completely useless, because most things that matter to us in the surrounding world can be reduced to electromagnetic interactions. Knowing this is not helpful at all for classifying them and understanding the differences between them.

The contraction of a protein caused by a chemical transformation and the magnetic forces that appear either between electrical currents through conductors or between electrical currents and ferromagnetic materials are very different phenomena and knowing that both of them have as primary cause electromagnetic interactions is of absolutely no help for understanding how they work or for designing either kind of motors.

Electromagnetic motors that are not extremely small need ferromagnetic materials. The only ferromagnetic material that is known to be synthesized by living beings is magnetite. Magnetite crystals can be good enough for sensing the magnetic field of the Earth, but they would be a very poor material for motors.

An easier to evolve rotating biological motor would be a rotating hydraulic motor, e.g. powered by pumped blood or lymph. This could work if the wheel would become non-living after being grown, to no longer need nerves and blood vessels. However it would be very difficult for a living being to seal the space between an axis and the rotating wheel in such a way so that blood or lymph would not spill out through the interstice.