Effect of fingerprints orientation on skin vibrations during tactile exploration of textured surfaces

Abstract

In humans, the tactile perception of fine textures is mediated by skin vibrations when scanning the surface with the fingertip. These vibrations are encoded by specific mechanoreceptors, Pacinian corpuscules (PCs), located about 2mm below the skin surface. In a recent article, we performed experiments using a biomimetic sensor which suggest that fingerprints (epidermal ridges) may play an important role in shaping the subcutaneous stress vibrations in a way which facilitates their processing by the PC channel. Here we further test this hypothesis by directly recording the modulations of the fingerpad/substrate friction force induced by scanning an actual fingertip across a textured surface. When the fingerprints are oriented perpendicular to the scanning direction, the spectrum of these modulations shows a pronounced maximum around the frequency v/λ, where v is the scanning velocity and λ the fingerprints period. This simple biomechanical result confirms the relevance of our previous finding for human touch.

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Figures and Tables

Figure 1 (A) Experimental set-up. The substrate (in grey) consists of a 50 mm long glass slide patterned with 1D rough textures (see ref. 14 for details). It is mounted on a double cantilever apparatus which allows one to monitor both the normal and tangential forces (denoted N and F, respectively) acting on it. The forefinger (male, 37 years old) is constrained in a fixed position at 45° with respect to the surface. Once in place, the substrate is brought in contact at a prescribed load N. It is then moved at a constant speed v = 5 mm/s using a DC-current motor along either distal (mode 1, right) or radial (mode 2, left) direction. (B) Typical images of the contact zone between the fingertip and a smooth substrate for 4 different loads (0.2, 0.4, 0.8, 1.6 N). The finger is pointing upward. The arrows correspond to the scanning direction in mode 1 (blue) and mode 2 (red). The white bar is 2 mm long. (C) Comparison of the normalized power spectra of the tangential force F obtained in mode 1 (blue, shifted vertically for clarity) and mode 2 (red). The different graphs corresponds to loads N = 0.2, 0.4, 0.8, 1.2, 1.7 N. (D) Linear/linear plot of the spectra for N = 0.4 N. The maximum of the normalized power spectrum in mode 1 occurs at the spatial frequency 2 mm⁻¹. Inset: spectrum amplitude at this particular spatial frequency as a function of N.

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