The role of skin mechanics in contact force variation under different friction conditions

When grasping objects, humans actively adjust grip force in response to surface slipperiness and motion. Previous studies have showed that corrective actions occur after tactile afferents signal surface friction or slip events. However, the influence of the mechanical behavior of the skin on the development of contact forces is poorly understood. In this study, using contact kinematics derived from a natural reach-and-grasp task, we applied a glass surface onto restrained fingers via a robotic manipulator under low- and high-friction conditions. Contact forces were measured with a force sensor, and skin deformations were captured using a high-speed camera. As expected, the normal force remained unaffected by friction, however, interestingly the tangential force rose more slowly and peaked lower under low friction. This resulted in a higher normal-to-tangential force ratio, resembling friction-dependent scaling of grip-to-load force ratio observed in active grasping. The skin partially slipped throughout contact development, with the proportion of the slipped area first decreasing and then increasing. The time course of tangential force correlated with the extent of skin slip, both varying with friction. The findings demonstrate that skin mechanics potentially influences the grip stabilization during the initial phase of object handling, which doesn't involve feedback-driven grip force adjustments.