Soft Skins With Reversible Thickness Morphing: Materials, Mechanisms, and Applications.

Soft skins with reversible thickness morphing represent a distinct and underexplored class of adaptive material interfaces. Unlike conventional soft actuators that achieve motion through bending, elongation, or twisting, these systems enable out-of-plane deformation, producing localized protrusion, retraction, and programmable contact mechanics without rigid support structures. This review reframes thickness modulation not merely as an actuation outcome, but as a material-architecture strategy that couples energy transduction, geometry, and compliance to enable new modes of haptic interaction, morphological adaptation, and operation in confined or unstructured environments. We present a comprehensive synthesis of thickness-morphing soft skins, covering actuation stimuli, material platforms, structural architectures, fabrication strategies, modeling frameworks, and system-level integration. Particular emphasis is placed on hierarchical elastomer composites, origami- and kirigami-inspired designs, electrohydraulic and multimodal hybrid systems, and emerging data-driven control approaches that expand the functional design space. Despite rapid progress, key challenges remain in durability under cyclic loading, energy efficiency and autonomy, scalable manufacturing, and integration of sensing, actuation, and computation. Addressing these challenges will enable self-powered, fault-tolerant, and computationally intelligent soft skins capable of embodied perception and safe autonomous operation, positioning thickness morphing as a foundational design axis for next-generation haptics and soft robotic systems.