Synthetic fascia for stiff and tough 4D printed multifunctional structures that detect and tolerate damage

Creating shape-morphing structures with time-responsive materials is a key goal of 4D printing, but combining high stiffness (E) and toughness (K) in a single material remains difficult. Soft materials stretch but lack strength, while stiff materials resist deformation but are brittle. Nature overcomes this trade-off in skeletal muscle by surrounding strong fibers with soft, tough tissue for protection and support. Inspired by this strategy, we develop a multi-material printing method that combines stiff synthetic muscle with a soft, stretchable adhesive to form a composite structure. Here, we show that this design greatly improves toughness without sacrificing stiffness, enables controlled actuation, and maintains function after multiple fractures. We demonstrate these properties in a damage-tolerant actuator, a lifting robot with record performance, and a lattice that detects and withstands extreme loads while remaining operational.