Seamless Mechanical Programming Through Hybrid Epoxy-Acrylate Printing for Soft-Rigid Robots

Soft robotic systems require the integration of compliant and stiff components, yet soft-rigid interfaces remain prone to failure under cyclic deformation due to discrete material junctions. In contrast, biological systems rely on functionally graded materials to distribute stress and optimize performance. Here, we implement hybrid epoxy-acrylate printing (HEAP) to create covalently integrated polymer networks with spatially programmed mechanical properties. HEAP is a multi-wavelength 3D printing strategy that enables control over polymer network architecture within a single resin. We achieve both continuous hardness gradients spanning Shore-A 8 to Shore-D 80. These gradients eliminate weak interfacial boundaries and maintain mechanical integrity under large, repeated strains. Compared to commercial multi-material printing, HEAP reduces interfacial failure by >5× while preserving near-intrinsic stretchability of soft domains. This capability enables single-shot fabrication of soft-rigid robotic systems that undergo large, cyclic deformations, including functionally graded pneumatic bellows with integrated tubing interfaces and spatially programmable deformation modes, bioinspired soft robotic hands with embedded bone-like structures, and an augmented undulating walker that achieves more than a twofold increase in locomotion speed relative to an all-soft counterpart. These results establish a generalizable materials and manufacturing framework for mechanically graded polymer systems, enabling robust soft-rigid integration in robotics and beyond.