Experimental setup.

A. Two major mechanically relevant skin layers constitute the epidermis on the human fingertip: the stratum corneum (blue shading) and the viable epidermis (orange). The papillary ridges making up the fingerprints at the skin surface extend to deeper layers, where morphological complexity increases. Interactions with surfaces (grey shading) take place at the outer boundary of the stratum corneum, while mechanoreceptors are located at the border between the epidermis and the dermis (red shading), at distinct landmarks associated with the ridge structure (dark red and blue circles). Identification of morphological landmarks related to the ridge structure allows the creation of a fine-grained mesh covering most of the sub-surface structure of a single papillary ridge (blue and orange overlaid meshes). B. Potential deformations of the tracked ridge structure, including the stratum corneum and the bulk of the viable epidermis, during tactile interactions, with arrows indicating the directions of relative deformation. i) Reference mesh (undeformed); ii) Tension or compression of the whole structure along or orthogonal to the surface axis of the skin; iii) horizontal (surface) shear, where the ridge structure tilts sideways; iv) vertical shear, where the ridge structure tilts along the axis orthogonal to the surface of the skin. These deformations might affect different parts separately, e.g. via shearing in different directions across both ridge flanks as shown on the far right (see darker shading to highlight a single ridge flank). C. Detail view of a single OCT frame showing ridged skin structure and clear boundary between the stratum corneum and viable epidermis. A mesh covering the stratum corneum and the upper part of the viable epidermis (without the intermediate ridge) is overlaid spanning a single papillary ridge. The border between the viable epidermis and dermis is less clearly delineated, but some deeper feature are resolved less well. D. Ridge widths (n = 153) across all participants. Each dot corresponds to a unique ridge with the participant mean indicated by the horizontal line. E. Thickness of stratum corneum (blue) and viable epidermis (yellow). Markers as in D. F. The experimental apparatus consisted of a finger holder to which the left middle finger of the participant was secured. A horizontal plate with a smaller inlaid transparent surface could be moved to indent the fingertip. A motorized linear stage moved the plate in the distal/proximal direction across the fingertip. An OCT scanner recorded images through the transparent surface, while forces were recorded using a 3-axis force sensor. G. Illustration of the three transparent PMMA surfaces: a flat surface, one embossed with a rounded edge, and a grooved one. H. Individual images recorded by the OCT scanner using the three surfaces shown in panel G. Figure 1—video 1. Recordings of sub-surface skin deformations. Example recordings demonstrating stick-to-slip with the flat plate, and interactions with a groove and an edge, respectively. Each sequence shows first the raw data, and then the processed sequence with a single tracked ridge overlaid.

Ridge deformation under normal load.

A. Example frames showing a single ridge unloaded (0 N; left) and maximally loaded (3.5 N; right). B. Examples of ridge deformation under static normal load for different load conditions. Shown is the unloaded ridge outline (grey shaded regions) with the deformed ridge mesh superimposed (blue and orange), under 1 N, 2 N, and 3.5 N loading. C. Examples of ridge deformation under static normal load for different participants. Shown is the unloaded ridge outline (grey shaded regions) with the ridge mesh under 3.5 N loading superimposed. Arrows indicate the principal compressive axis for each facet, with colors indicating different orientations relative to vertical. In the lower panels, the magnitude of principal tensile (e1, D.) and compressive (e2, E.) strains as well as area change (ea, F.) as a function of normal load. Thin lines denote individual participants, and thick lines show the average. Data from five participants with seven individual ridges tracked each.

Ridge deformations during sliding.

A. Measurements during repeated movements of the flat plate along the distal-proximal axis for a single participant. Top: Tangential (purple) and normal (red) load as a function of time. The normal load was set at 0.5 N at the start of the trial by adjusting the indentation and then was not further controlled. Tangential load alternates between positive and negative values depending on the movement direction of the plate. Middle: Horizontal velocity of the plate (dash-dotted grey line) and average velocity of each tracked fingerprint ridge (thin black lines) during all eight transitions of the flat plate. Two phases are evident: when the ridge is moving along with the plate (stick, indicated by green shading) and when the ridge is stationary, but the plate is moving (slip: indicated by pink shading). Bottom: Vertical velocity of all tracked ridges along the normal axis, which is close to zero. B. Example frames showing a single-tracked ridge for the stick and slip phases of the two movement directions. White arrows point to presumed collagen fiber bundles anchoring the skin to the bone. C. Average ridge meshes (black lines) during each of the four phases calculated over all tracked ridges and all time points assigned to each phase for the same participant as in A, B. Colored lines indicate individual sample meshes (n = 3773; blue: stratum corneum, yellow: viable epidermis). D. Examples showing ridge deformation across the four phases for three participants. Shown are the ridge outline for the previous phase (grey shaded regions) with the ridge mesh for the current mesh superimposed. Arrows indicate the principal compressive axis for each facet, with colors indicating different orientations relative to vertical. E. Average magnitude of maximal shear strains as a proxy for overall deformation in the stick-to-slip transition compared to the movement reversal. Thin lines denote individual participants (averaged over facets and movement directions), and thick lines show the grand average. Asterisks denote statistically significant differences (paired Wilcoxon tests). Strains are about a third higher during stick-to-slip transitions than during movement reversals. F. Histograms of average principal strain angles for all mesh facets and participants in stick-to-slip transitions (left) and movement reversals (right). White bars denote angles that are within 22.5° of the coordinate axes (horizontal or vertical) and therefore denote tension or compression without considerable shear. Grey bars denote angles within 22.5° of the diagonal and therefore denote horizontal shear. Positive angles denote shear acting in the same direction as the plate movement, while negative angles denote shear in the direction opposite to the plate movement. G.Change in principal strain angles when transitioning to slip (top) or stick (bottom) phases, separated by ridge flank (light and dark grey). Movement reversals cause a 90-degree shift in the strain angle, while stick-to-slip transitions cause little change, with no differences between ridge flanks evident. Figure 3—figure supplement 1. Ridge deformations for all participants during sliding of the flat plate.

Ridge deformations and skin strains during transit of small tactile features.

A. Top row: Illustration of identified movement phases with the edge feature in different locations relative to the tracked fingerprint ridge. Bottom row: Average ridge deformation and associated principal compressive strain orientation for a single participant. Note that all strains are calculated with respect to the mesh shown in the left-most column, which represents the ridge during full slip before the interaction with the tactile feature. B. Histograms of principal strain orientations across all ridge facets and participants for the approach (2), central (3), and withdrawal (4) phases. Red bars denote distal ridge flanks and blue bars denote proximal ones. Darker shading denotes orientations close to diagonal, indicating shear, while lighter shadings denote angles aligned vertically or horizontally and therefore denote no shear. C. Average magnitude of principal tensile (e1, left panel) and compressive (e2, middle panel) strains as well as area change (right panel) relative to the stereotypical ridge under full sliding (phase 1). Thin lines denote individual participants (n = 9), and thick lines show the average. Asterisks denote statistically significant differences between the stratum corneum and the viable epidermis (Bonferroni corrected paired Wilcoxon tests). Ridges are most deformed when directly under the edge feature when strains are higher in the viable epidermis than the stratum corneum. While the stratum corneum is incompressible, the area of the viable epidermis expands when directly under the ridge. D-F. Same as in A-C, but for a small groove transitioning over the ridge. Note that the findings broadly reflect those obtained with the edge feature, but with the direction of compressive and tensile strains reversed. Figure 4—figure supplement 1. Ridge deformations for all participants during transit of the edge feature. Figure 4—figure supplement 2. Ridge deformations for all participants during transit of the grooved feature.

Ridge deformations for all participants during sliding of the lat plate.

Ridge deformation across the four phases for all participants. Shown are the ridge outline for the previous phase (grey shaded regions) with the ridge mesh for the current phase superimposed. Arrows indicate the principal compressive axis for each facet, with colors indicating different orientations relative to vertical. Same style as Figure 3D.

Ridge deformations for all participants during transit of the edge feature.

Ridge deformation and associated principal compressive strain orientation for all participants. Note that all strains are calculated with respect to the mesh shown in the left-most column (phase 1), which represents the ridge during full slip before the interaction with the edge feature. Same style as Figure 4A.

Ridge deformations for all participants during transit of the grooved feature.

Ridge deformation and associated principal compressive strain orientation for all participants. Note that all strains are calculated with respect to the mesh shown in the left-most column (phase 1), which represents the ridge during full slip before the interaction with the edge feature. Same style as Figure 4D.