Lightweight Wearable Fingertip Haptic Device With 3-DOF Directional Force Feedback Based on Fingernail Stimulation.

Wearable haptic devices face a trade-off between providing rich feedback and preserving natural fingertip sensation. Current multi-DOF systems encumber the fingerpad, interfering with fingerpad sensory capabilities, while lightweight devices offer limited 1-DOF feedback. To address this, we present a 5.24 g, three-degree-of-freedom (3-DOF) haptic device that stimulates mechanoreceptors around the fingernail, leaving the fingerpad unobstructed. It uses two string-pulling motors for distal-proximal feedback and an arc-shaped pin motor for radial-ulnar stimulation to generate directional force vectors. This approach is grounded in the physiology of directionally-sensitive slowly adapting type 2 (SA2) mechanoreceptors. Experimental results demonstrate that the proposed 3-DOF approach improves weight and friction discrimination over single-DOF pressure feedback. Moreover, it delivers directional cues during static contact, a capability absent in vibrotactile systems, and achieves higher overall user satisfaction. By preserving fingertip sensation, the proposed device enables simultaneous interaction with virtual and physical objects, making it suitable for mixed reality applications.