VSL-Skin: Individually Addressable Phase-Change Voxel Skin for Variable-Stiffness and Virtual Joints Bridging Soft and Rigid Robots

Abstract

Soft robots exhibit compliance but lack load support and pose retention, while rigid robots provide structural capacity but sacrifice adaptability. Existing variable-stiffness approaches operate at segment or patch scales, preventing precise spatial control over stiffness distribution and virtual joint placement. This paper presents the Variable Stiffness Lattice Skin (VSL-Skin), the first system enabling individually addressable voxel-level morphological control with millimeter-scale precision. The system achieves three unprecedented capabilities: nearly two orders of magnitude stiffness modulation across axial 15-1200 N/mm, shear 45-850 N/mm, bending 8*10^2-3*10^4 N/deg, and torsional modes with millimeter-scale spatial control; the first demonstrated 30% axial compression in phase-change systems while maintaining structural integrity; and autonomous component-level self-repair through thermal cycling that eliminates fatigue accumulation and enables programmable sacrificial joints for predictable failure management. Selective voxel activation creates six canonical virtual joint types with programmable compliance while preserving structural integrity in non-activated regions. The platform incorporates closed-form design models and finite element analysis for predictive synthesis of stiffness patterns and joint placement. Experimental validation demonstrates 30% axial contraction, thermal switching in 30-45 second cycles, and cut-to-fit integration that preserves addressability after trimming. The row-column architecture enables platform-agnostic deployment across diverse robotic systems without specialized infrastructure. This framework establishes morphological intelligence as an engineerable system property, fundamentally advancing autonomous reconfigurable robotics.