The HexSOWS: a Modular Soft and Robust Skin Concept for Multimodal Tactile Sensing

Distributed soft sensing requires flexible, conformable, and mechanically robust skins to discriminate pressure and strain over large areas. We present the Hexagonal Soft Optical Waveguide Sensor (HexSOWS), a modular skin concept that can cover large areas via reversible compliant interlocking between modules. Each module comprises a central pressure-sensing hexagon and three peripheral strain sensors, enabling pressure mapping and strain detection in three directions at 120° intervals. Two versions were developed and compared: a TPU-based version for mechanical robustness and an all-silicone version with titanium dioxide (TiO2) coating for enhanced optical shielding and flexibility. Mechanical characterization showed that the TPU-based version exhibited higher indentation toughness (16.47 ± 3.25 mJ), while the all-silicone version demonstrated greater tensile toughness (4.54 ± 0.79 MPa) and higher sensitivity. Convolutional neural networks used for contact regression achieved standardized RMSE values of 0.12 (TPU) and 0.10 (all-silicone). These results highlight the critical balance between material properties and sensing performance, with the modularity of HexSOWS enabling adaptation to diverse mechanical and sensing requirements, thus advancing robotic skin design for safe and effective environmental interaction.