Synergistic Roles of Hard and Soft Structures in Fingertip Tactile Sensing

Touch is often attributed to the skin, but what if hard structures above the contact surface play a larger role than we thought? While most haptic research focuses on soft tissue, we explore how the fingernail and distal phalanx bone contribute to texture perception, combining human exploration data, finite element analysis (FEA), and experiments with a soft robotic fingertip equipped with Hall-effect sensors. Analysis of human exploration revealed a predominant use of lateral movements, which informed the robotic and simulation procedures. FEA showed that adding rigid elements altered internal stress patterns within the soft tissue during lateral sweeping, increasing strain variability in regions relevant for sensing. Lateral sweeping consistently produced higher strain variability than longitudinal motion, with Shannon entropy ratios up to 1.96 across probe regions. Robotic texture classification experiments quantified these effects: Adding only a nail improved discrimination accuracy by 22%, adding only a bone by 51%, and integrating both features increased performance by more than 88% compared to the soft-only model. These results demonstrate that the nail and distal phalanx bone provide complementary mechanical benefits that enhance texture-related signal richness and can improve the efficacy of soft tactile systems. This study can lead to a design principle for soft robotic tactile systems in prosthetics, surgical robotics, and dexterous manipulation where sensing performance must be maximized without increasing sensor complexity.