Functional Materials for 3D-Printed Soft Robotics: Conductors, Actuators, and Sensing Networks: A Review

3D printing is transforming soft robotics by enabling complex geometries, rapid iteration, and cofabrication of structure, actuation, and sensing. Central to this progress are functional materials that provide stretchable conductivity, stimuli-responsive actuation, and reliable sensing when deposited by additive processes. This review surveys recent advances in conductive materials for printed soft systems (conductive polymer composites [CPCs], liquid metals, conductive hydrogels, and metalized inks), printed actuators (pneumatic networks, stimuli-responsive hydrogels, dielectric elastomers, magneto-active and shape-memory composites), and sensing networks (resistive, capacitive, optical, and multimodal printed arrays). The paper emphasizes how material choices interact with printing modalities (direct ink writing [DIW], multimaterial fused deposition [FDM], inkjet/aerosol jet, and DLP/SLA), and further identifies constraints such as mechanical mismatch, postprocessing, long-term stability, energy supply, and multimaterial interfacial reliability that currently limit translation. Finally, highlighting promising directions such as printable self-healing conductive hydrogels, multimodal printed skins, embedded energy and computation, magneto-responsive inks for untethered actuation, and community standards for materials characterization and durability benchmarking.