I'm thinking of the analog to the human body (or animals) ... we have a skeleton, organized and connected via ligaments, and then our "actuators" (muscles) attach to that structure with tendons.
Is there any existing research/samples/demos that mimic this structure?
Elastic materials are favored if you need shock absorption or no seal system.
Both hydraulics and pneumatics suffer from incredibly shitty thermal efficiency. Electric engine can be 99% efficient. Any single pair of gears or a chain drive can be 99% efficient. Typical hydraulic actuator is something between 70 - 40% efficient. Pneumatics are as bad or worse.
They do use rubber air bellows in paper machinery, but it's being replaced by rigid hydraulic cylinder actuators. With pneumatics you can have somewhat accurate force (== pressure), but you never know the accurate position. With hydraulics it's the other way round. As a result, hydraulic actuators can be mounted in any position regardless of gravity. So you can use the same machinery in various positions.
Edit: here is more if you're interested: https://www.machinedesign.com/pneumatics/primer-flexible-act...
(probably-obvious disclaimer: I have zero experience here, so I truly don't know)
You can always read very accurate position with pneumatic actuators. You just can't keep it very accurately in place in dynamically loaded situation. You can also read the force of hydraulic system, but you cannot keep it constant as easily in dynamically loaded situation.
If you want (roughly) certain force and you don't care about precise position, then you have lot's of advantages from pneumatic system. The whole volume of the system acts as "hydro-pneumatic accumulator". Which means that the whole volume of the system is trying to average out the change in force. And you get that free of charge. And also you don't have to invest any money into hydraulic fluid. Air is free and available (almost) everywhere.
If you then insist on hydraulic system, you have to put separate hydro-pneumatic accumulator. But because almost any hydraulic fluid is heavier than any given pneumatic fluid, you have more mass in any given pipe section. That means the system will react slower to any(?) pressure increase. You can read the pressure very accurately, but you cannot smooth it out as fast as pure pneumatic system would.
There are caveats though, sound can travel very fast in hydraulics. And turbulence can eat out pressure very quickly. I'm kinda professional in the business, but still starting out.. :D So take this with pinch of salt.
PS. Hydraulic systems can handle 60 bar safely, while 6 bar pneumatic system can already explode and be quite dangerous. That's sometimes enough of a reason to go hydraulic.
Existing servos / stepper motors / etc obviously do this too, but usually to a much smaller and more predictable degree.
Obviously it still has uses, e.g. as a grasper at the end of a normal robot for a softer grip. But making a whole robot out of it is still largely insane, unless you have esoteric needs. Hence ongoing research, but not application :)
[0]https://techcrunch.com/2008/07/01/darpa-throws-33m-at-biomim...
Doing that with a 3d printer is hard
I just learned from that Wikipedia page that a version of the Shadow Dexterous Hand is actuated with pneumatic muscles [2].
[1] https://en.wikipedia.org/wiki/Pneumatic_artificial_muscles
Hmm, I wonder how well soft robotics could adapt to heavier loads. Even with skeletal components, grip/lift strength is limited by how much pneumatic pressure the silicon can take before bursting.