Proc. Natl Acad. Sci. USA (2017)

Fluids are popular design components for artificial muscles, but usually operate under high pressures that compromise safety and robustness. Now, Shuguang Li and co-workers have come up with an architecture driven by fluids at negative pressures — exploiting ideas from origami metamaterials to build biocompatible muscles capable of complex multiaxial motions.

The design proposed and tested by Li et al. involves a flexible skin filled with fluid and wrapped around a skeletal structure comprising structural voids — like those found in the complex folds of origami. A mechanical model explains the interaction of the three components: pressure differences between the internal and external fluids cause the skin to deform inward towards the voids, which in turn are driven to contract under tension.

The team investigated different skeletal structures, finding that linear zigzag actuators with nylon skins were able to generate actuation stresses of up to 600 kPa, six times larger than that of mammalian skeletal muscle.