Serpentine Shape Memory Alloy-Based Skin-Attachable Haptic Interface for Multimodal Tactile Feedback in Wearable Systems
Conventional haptic interfaces are constrained by rigid mechanical structures, limiting wearability and multimodal feedback. This study presents a skin-attachable haptic device based on a serpentine shape memory alloy (SMA) structure, addressing these bottlenecks through material and design innovation. Nitinol was micromachined via ultraviolet laser into a serpentine geometry, integrated with four actuators in an opposed layout around a central UV-cured probe. The system, encapsulated in a 3D-printed flexible finger cap, enables multimodal actuation: single-set activation produces lateral traction, two adjacent sets yield diagonal sliding, cyclic four-set activation induces rotation, and full activation simulates normal press, generating 11 distinct tactile modes. Wireless control via an ESP32 WROOM module (WiFi) eliminates cables. Safety is ensured by limiting driving current to 0.8 A and single-drive duration to 0.1 s, keeping skin contact temperature below 36°C. Experimental results show stable tactile forces of 0.5–0.85 N (exceeding the 0.1 N finger threshold) with response times ≤0.5 s. Durability tests demonstrated >95% displacement retention after 100 cycles at conventional frequency, with reversible high-frequency decay. Subject tests achieved 100% recognition accuracy for low-frequency patterns. Practical validation in VR interaction, navigation, and assistive technology confirmed utility, including millimetre-level positioning accuracy and target location by blindfolded users in under 15 s. This work provides a lightweight, wireless, multimodal haptic paradigm, though cooling efficiency and material fatigue require further optimization.