Gentle yet powerful artificial muscles enabled by hierarchical structure engineering

Developing artificial muscles with simultaneous high actuation performance and mild activation conditions is highly needed but still challenging. Here we broke the limitations through a hierarchical structure design of a dual-stimuli-responsive shape memory polymer (SMP) network, achieved by integrating a double-tail quaternary ammonium surfactant with poly(acrylic acid) via ionic complexation. Notably, the double‑tail surfactant exhibits a phase transition around 42 °C, enabling the mild‑temperature‑triggered actuation. The SMP‑based artificial muscle features a hierarchically nanophase‑separated structure, consisting of water‑responsive hydrophilic PAA domains and thermally responsive hydrophobic PAA/surfactant complex crystalline domains, thus decoupling the actuation stress and strain by different stimuli. The nanostructured artificial muscle achieves an actuation stress of up to 6.4 MPa (isometric) and an actuation strain of up to 79% (load-free), both under heating to ~42 oC, surpassing most existing actuators. Furthermore, the system exhibits self-reversible, force-free cyclic actuation. Our work offers a viable route for multi-scale structural engineering toward high-performance soft actuators. Shape memory polymers are promising for artificial muscles, though it is challenge to optimize actuation performance and activation conditions. Here the authors report a system composed of an ammonium surfactant and poly(acrylic acid) network for an artificial muscle optimizing these properties.