Experimental setup and identification of individual fingerprint ridges.

A. Two main mechanically relevant skin layers constitute the epidermis on the human fingertip: the stratum corneum (blue shading) and the viable epidermis (orange). The ridged structure of the fingerprints extends to deeper layers with increasing morphological complexity. Interactions with surfaces (grey shading, flat surface) 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. Identification of specific morphological landmarks, such as the tops and valleys of a ridge, allows the creation of a fine-grained mesh covering the sub-surface structure of a single ridge (blue and orange overlaid meshes). B. Potential ridge deformations during tactile interactions, with arrows indicating the directions of relative deformation. i) Reference mesh (undeformed); ii) Tension and compression of the whole ridge along or orthogonal to the surface axis of the skin; iii) surface (horizontal) shear, where the ridge tilts sideways; iv) ridge (vertical) shear, where the ridge tilts along the axis orthogonal to the surface of the skin. These deformations need not apply to the whole ridge but could also manifest locally, e.g., in individual layers or, as in one of the ridge shear examples, differently across both ridge flanks. C. Detail view of a single OCT frame clearly showing ridged skin structure and sub-surface layers, with mesh covering a single ridge overlaid. D. Ridge widths (n = 153) across all participants based on data obtained using the flat surface 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) as calculated from the same data as above. Markers are the same 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 manually 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 of the top skin layers through the transparent surface, while forces were recorded using a 3-axis force sensor. G. Transparent surface stimuli used in the experiment: a flat surface, a plate embossed with an edge, and a grooved plate. H. Individual images recorded by the OCT scanner display the complex morphology of the skin and its changes in response to tactile features.

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 indentation and then 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.

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. 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.

Ridge deformations for all participants during sliding of the flat 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 Fig. 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 Fig. 4A.

Ridge deformations for all participants during transit of the groove 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 Fig. 4D.