Skin nonlinearities and their effect on user perception for rotational skin stretch

Given that skin is a nonlinear, anisotropic and viscoelastic material, we are interested in exploring the relationship between skin mechanical properties and skin stretch perception, for the case of rotational displacements applied to the skin on a user´s limbs. Studies were conducted with 10 naive subjects first to characterize the nonlinear stiffness of the skin with respect to applied rotations and subsequently to characterize the skin viscoelastic response and hysteresis. Despite substantial subject-to-subject variability, when results are normalized by each subject´s maximum torque, the torque/displacement results are fairly consistent across subjects, at low and high speeds, and can be fit with a third order polynomial. For roughly one half of the subjects, a similar nonlinearity is discernible in the perceived versus actual rotation; other subjects produced nearly linear results across the range of positive and negative rotations. Viscoelastic and hysteresis effects in the skin were also evident. However, while subjects can clearly distinguish between slow and rapid movements, the speed of the applied motion does not significantly affect their perception of rotation.