Tangential versus normal displacement of skin: biomechanics and perceived intensity

We investigated the relative effectiveness of tangential versus normal displacements of skin for eliciting tactile sensations. Subjects adjusted the magnitude of slow tangential and oblique displacements of a cylindrical, flat-ended 1 mm diameter probe glued to the skin in order to match the perceived intensity of a reference displacement that indented the skin normal to the surface. At both the forearm and finger pad, subjects chose tangential displacements with magnitudes only 0.3 to 0.6-times the magnitude of the normal stimulus, indicating significantly higher sensitivity to tangential displacements (p<0.005). These findings are in rough agreement with a basic analysis of how normal and tangential loads transmit energy to mechanoreceptors at different depths in the tissue. Based on measurements of the mechanical impedance of the skin to normal and tangential displacements, these results were also expressed in terms of forces. At the forearm, subjects were more sensitive to tangential forces than normal force. However, at the fingerpad, sensitivity to tangential forces was lower than sensitivity to normal force, due to the approximately five-fold greater stiffness of the fingerpad to tangential traction. These results provide guidance for development of tactile displays: (1) When an actuator is limited primarily in terms of peak displacement (e.g., the maximum strain of a ceramic peizoelectric actuator) then tangential stimulation is a superior choice for both body sites we tested. (2) When an actuator is limited primarily in terms of peak force (e.g., the stall torque of a DC micromotor) tangential stimulation is the superior choice for the hairy skin, but normal stimulation is the better choice on the fingerpad. Key Words touch tactile display shear normal tangential actuator strain energy density mechanics SAI mechanoreceptor depth