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TECHNICAL PAPERS: Fluids/Heat/Transport

3-D Finite-Element Models of Human and Monkey Fingertips to Investigate the Mechanics of Tactile Sense

[+] Author and Article Information
Kiran Dandekar, Balasundar I. Raju, Mandayam A. Srinivasan

The Touch Lab Department of Mechanical Engineering, and The Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139

J Biomech Eng 125(5), 682-691 (Oct 09, 2003) (10 pages) doi:10.1115/1.1613673 History: Received July 11, 2002; Revised April 28, 2003; Online October 09, 2003
Copyright © 2003 by ASME
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References

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Srinivasan,  M. A., and Dandekar,  K., 1996, “An Investigation of the Mechanics of Tactile Sense Using Two-Dimensional Models of the Primate Fingertip,” J. Biomech. Eng., 118, pp. 48–55.
Srinivasan,  M. A., 1989, “Surface Deflection of Primate Fingertip Under Line Load,” J. Biomech., 22, pp. 343–349.
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Srinivasan,  M. A., Gulati,  R. J., and Dandekar,  K., 1992, “In Vivo Compressibility of the Human Fingertip,” Adv. Bioeng., 22, pp. 573–576.
Tregear, R. T., 1966, Physical Functions of Skin, Academic Press, New York, NY.
Gulati, R. J., and Srinivasan, M. A., 1997, Determination of Mechanical Properties of the Human Fingerpad in Vivo Using a Tactile Stimulator, MIT, Research Laboratory of Electronics Technical Report No. 605.
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Figures

Grahic Jump Location
Experimental setup used for extracting borders from the epoxy replicas of monkey and human fingertips
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Extraction of the fingertip boundary from images of the fingertip replicas. The raw image shown in the left panel is the image as acquired by the frame grabber. The 2-D histogram is shown in the middle panel and shows a clear distinction between the gray-scale values of the fingertip and the background. The extracted border is shown in the right panel.
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The algorithm for reconstruction of the 3-D fingertip. Four extracted fingertip boundaries at 0, 45, 90, and 120 deg are shown on the left. An axial cross-section reconstructed from the extracted borders is shown in the middle. As shown on the right, the set of all such cross-sections define the external geometry of the monkey fingertip and a solid can be generated by generating a patch over the axial cross-sections.
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3-D view of the monkey (top) and human (bottom) fingertip meshes along with the nail. The hidden lines are removed for clarity. The scale is not the same for the two fingertips (monkey fingertip width ∼8–10 mm; human fingertip width ∼16–20 mm).
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Three slices of the monkey fingertip model are shown. The elements are shaded to show the five layers used in the model.
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Deformed meshes of the monkey (top) and human (bottom) fingertips indented by a sharp wedge aligned perpendicular to the axis of the finger. The wedge was indented into the fingertip up to a depth of 2.0 mm in increments of 0.5 mm. The contours of constant vertical displacement (in steps of 0.2 mm) of the fingertips under 2 mm indentation by the line load are shown.
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Surface deformation predicted by the homogeneous monkey (top) and human (bottom) fingertip models are compared with the experimental data from Srinivasan 3. The element size for the monkey and human models were approximately 0.4 and 0.8 mm, respectively. Vertical displacements plotted for two depths of indentations of 1 mm and 2 mm show that homogeneous models do not accurately model the biomechanical behavior of the fingertips.
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Effect of layer stiffness on the surface deformation. The elastic modulus of the bone was held constant at 108 and the elastic moduli of other layers were varied successively by a factor of 10. The predicted vertical displacement is compared with the experimentally observed data for a monkey fingertip indented to a depth of 2.0 mm.
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Surface deformation of the monkey fingertip under line loads predicted by the finite-element model and comparison with observed surface profile data for four monkey fingertips, each indented to 4 different depths (0.5 mm to 2.0 mm at 0.5 mm steps). The empirical data from Srinivasan 3 are shown with crosses and the finite-element predictions are shown by solid lines. For each monkey fingertip, the material model used in the finite-element models is listed on the top of the plot.
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Surface deformation of the human fingertip under line loads predicted by the finite-element model. The empirical data shown with crosses for three human subjects is from Srinivasan 3. Same format as Fig. 9.
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Neural response of an SA-I afferent to indentation by a 3 mm-wide rectangular bar predicted by the high-resolution monkey fingertip model based on four candidate mechanical quantities: strain energy density, maximum compressive strain, mean normal stress, and vertical compressive stress. The quantities were computed for both the layered and the homogeneous models. The x axis is the location of the center of the bar as it was indented at various locations on the fingerpad. The numbers above each panel indicate the correlation coefficient between the recorded neural response and the predicted response for each mechanical quantity for the layered (“lay”) and homogeneous (“hom”) models. The layered model was able to model the neural response better than the homogeneous model for all the four quantities. Strain energy density was able to predict the neural response better than the other quantities.
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The variation in contact area and maximum contact pressure for constant force of indentation of step 5 at various locations on the monkey fingertip model
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Neural response of SA-I afferent to indentation by step shapes predicted by the monkey fingertip model. Strain energy density computed at a typical receptor location (0.75 mm below the skin surface) is matched with spatial response profiles recorded previously by Srinivasan and LaMotte 11. Good fits between the predicted and measured data (same scaling and threshold values for all the three step shapes) indicate that strain energy density is a good candidate be the relevant stimulus for SA-I mechanoreceptors.

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