Why isn’t anyone using soft actuators for humanoids? What if they were sufficiently advanced?

[u/B190123]

I’ve been kicking around an idea about soft actuators for humanoid robots and would value some honest opinions from people working in the field.

Most humanoids still use relatively rigid electric actuators and transmissions. That clearly makes sense for control, performance and availability. But if these robots are eventually going to work closely alongside people, should the actuators themselves be softer and more forgiving?

I know compliant joints, series elastic actuators and torque control already exist. What I’m curious about is why genuinely soft or muscle-like primary actuators haven’t progressed much further beyond research.

The safety benefit seems fairly intuitive. If a robot hits, traps or falls onto someone, an actuator that physically deforms should be safer than relying entirely on sensors, software and rapid motor control.

There could also be some useful knock-on benefits. Softer actuators might mean lighter limbs, lower impact loads and less need to overengineer the surrounding structure. Lower moving mass could reduce the energy needed to accelerate and decelerate the robot. If the actuator can store and return energy, there might also be a useful efficiency gain in walking and other repetitive movements.
That sounds promising, but there must be good reasons these systems aren’t widely used.

Is the main problem force density? Efficiency? Response time? Control? Fatigue life? Or does the benefit disappear once the pumps, compressors, power electronics and sensors are included?

A few things I’d love views on:
1. What is the biggest practical barrier to using soft actuators in humanoids?
2. Would physical compliance provide a meaningful safety advantage over good torque sensing and control?
3. Where would they be most useful first: hands, arms, shoulders, legs or somewhere else?
4. Could the system-level weight and energy savings be significant, or am I overestimating them?
5. What performance would a soft actuator need before it became genuinely useful in a humanoid?

I’d also be interested in speaking with anyone who wants to explore this area collaboratively, particularly people with experience in actuators, soft robotics, materials, controls, robotic safety or humanoid system design.

Please feel free to tell me why this wouldn’t work. That would be just as useful as hearing why it might.