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Determination of the shape and compressibility of the Primate Fingertip (distal phalanx)

The first step in performing mechanistic analyses of the primate fingertip is to determine its geometric and material properties. The three dimensional (3D) external geometry of the primate fingertips was determined from accurate epoxy replicas of human and monkey fingertips. Using a videomicroscopy setup we obtained images of orthographic projections of the epoxy replicas at various known orientations. The images were then digitized and processed to determine the boundary of the finger at each orientation. By combining the boundary data for all the different orientations, we were able to reconstruct the 3D external geometry of the fingertip (Perez, Dandekar and Srinivasan, 1992). We have reconstructed several human and monkey fingertips by using this method.

For mechanistic modeling of the human fingerpad, the Poisson's ratio, which is a measure of its compressibility, is required as an input to the mathematical models. The Poisson's ratio for the human fingerpad in vivo is unknown at present. In previous noninvasive experiments on human subjects, we have measured the change in volume of the fingerpad under static indentations with different indentors (Srinivasan, Gulati and Dandekar, 1992). Our results show that the compressibility of the fingertip increases with increases in both the depth of indentation and the contact area with the indentor. The highest change in fingertip volume was about 5%. We have also developed an experimental setup involving a computer controlled linear actuator for fingertip volume change measurements under dynamic conditions (Babiec, 1994). The results show that reductions in fingertip volume are in phase with stimulus variations, with an increase in their mean value over time. The volume changes during the ramp phase increase linearly with indentor displacement and are independent of velocity; during saw tooth stimulations, however, the nature of the hysteresis loops depend on velocity of indentation.

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Last Updated: May 8, 2002 1:45 PM Comments: David Schloerb