Analysis of stiffness of human fingertip and comparison with artificial fingers

Human fingers have mechanical characteristics which enable human hands to manipulate objects dexterously in complex environments. Research on such characteristics will help in the design of robotic fingers with capability of dexterous manipulation. In this work, the stiffness of human fingertips tissue is investigated for loading and unloading situations at various contact angles and forces on an acrylic block. The stiffness shows strong nonlinearity for each condition. Based on experimental results, we propose two models to express the stiffness of soft fingertips tissue. Also the contact area of human fingertips exhibits a proportional relation with the deformation of fingertips tissue. Subsequently, the stiffness of fingertips can be expressed to vary linearly with the mean pressure which is calculated from the force and the contact area. It is revealed how the natural frequency in peg-in-hole tasks changes depending on the nonlinear stiffness of fingertips tissue and the mass of object. These stiffness properties have an important role in understanding the degree of smoothness and skill in execution of the task by human hands. Finally, these properties obtained from human fingertips are compared with those of artificial fingertips, made of silicone rubber and silicone gel.