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Comparison of Precision Grasping Performance between Artificial Fingers With and Without Nails

In this study, we investigated the effect of the presence or absence of fingernails on precision grasping using artificial anthropomimetic fingers. We hypothesized that fingernails improve precision grasping performance by increasing the friction coefficient while suppressing fingertip deformation. To test our hypothesis, we developed artificial fingertips, each composed of bone, nail, skin, and soft tissue, and fabricated three types of artificial fingers with different skin softness grades and artificial fingers without nails as the control condition. Pull-out experiments of cylindrical objects and T-shaped blocks were conducted using the developed artificial fingertips with and without nails, and the pull-out resistance forces were compared. The nail contributed to object grasping stability because the pull-out resistance force was significantly increased by the presence of the nail in the artificial fingertip with soft skin. The rate of increase in the resistance force of the T-shaped block was more significant than that of the cylindrical object because the finger pulp deformation was suppressed by the nail, and the form closure, that is, geometric constraint, was formed for the grasping object. These results suggest that the human nail improves precision grasping performance by forming geometric constraints on the grasped object, suppressing finger pulp deformation.