Surface deflection of primate fingertip under line load.

A study of the biomechanics of the skin and the subcutaneous soft tissues is of fundamental importance in understanding the process of transduction at the mechanoreceptive nerve terminals responsible for the sense of touch. In the present investigation, the fingertips (distal phalanges) of three adult humans and four monkeys were indented in vivo using a line load delivered by a sharp wedge. The resulting skin surface deflection profile was photographed and used as a clue to infer the mechanical nature of the materials that make up the fingertip. It is shown that the modified Boussinesq solution used by Phillips and Johnson (1981), applicable when the fingertip is modeled as an elastic half-space in a state of plane strain, predicts a skin surface deflection profile that can only roughly approximate the empirically observed profiles. As an alternative, a simple model which views the fingertip as an elastic membrane filled with an incompressible fluid (like a 'waterbed') under plane strain conditions is proposed. It is shown that the predictions of this model, which takes into account the finite deformations that occur, agree very well with the photographed profiles in the region of interest (up to about 3 mm from the load).