Programmable Helical Hierarchy in Coiled Polymer Artificial Muscles

Simultaneously achieving a large stroke, high payload capacity, and structural programmability in coiled polymer muscles remains challenging due to intrinsic structural and fabrication constraints. Here, we present a multilevel helical fabrication scheme that enables stable, large initial coil pitches and programmable helical hierarchies and chirality within a single polymer fiber, effectively bridging the stroke-payload trade-off and greatly expanding the design space for polymer artificial muscles. Second-order muscles demonstrate superior actuation performance: homochiral muscles achieve a contractile stroke of 88.1% and exhibit a 9-fold increase in payload over first-order muscles at 50% contraction (3.6 vs 0.4 MPa), while heterochiral muscles reach an elongation stroke of 860.7%. Third-order muscles transcend the traditional binary homochiral-heterochiral classification, enabling four chirality combinations with unique actuation modes. A regionally controlled twist-fabrication method allows spatial encoding of the hierarchy and chirality within a single fiber, enabling multifunctional and localized actuation. This programmability is demonstrated in soft and biomimetic robots: a robotic arm driven by a single muscle encoding both flexor and extensor functions, a worm-like robot actuated by regionally inverted chirality within one muscle, and a biomimetic finger combining mixed helical levels to achieve faster and more adaptable wrapping motions.