Fig 1.
Spectrum of mechanical responses.
Digital rendering and example SEM images showing a range of deformation outcomes in order of decreasing micropillar bending stiffness.
Fig 2.
Summary of the experimental design.
(a) Surface of PDMS replica finger with fingerprint ridges molded from human fingers. (b) Overview of geometrical parameters, namely micropillar diameter, aspect ratio and interpillar spacing, that were varied in this experiment. (c) Rendering of replica finger showing the sliding path across substrate carrier. The inset shows example SEM images of the phenomena we refer to as pair-wise, clustering, and matting. A complete schematic and photographs of the testing apparatus can be found in the S1 Fig.
Fig 3.
Survivability of tested micropillar arrays.
Panel (a) displays the smallest diameter micropillars (d = 6 μm) that were the most prone to deformation and the only diameter to display lateral and ground collapse phenomena at both 4:1 and 6:1 aspect ratios. In contrast, panel (b) shows the outcome for micropillar arrays with the largest diameter (d = 36 μm) for both materials. The medium diameter (d = 18 μm) micropillars for the more compliant material (E = 10 MPa) are shown in (c) while the medium diameter micropillars composed of the stiffer polymer (E = 10 MPa) are shown in subpanel (d).
Fig 4.
Representative SEM images showing the deformation of micropillar arrays.
Panel (a) shows an example of “clustering” lateral collapse by the 6 μm diameter elements for the compliant (E = 10 MPa) material. In contrast, panel (b) shows the stiffer material (E = 1000 MPa) with the same geometry exhibiting pairwise lateral collapse. Both images (a) and (b) were taken in the bidirectional region of the travel path of the replica finger. To illustrate the role that unidirectional versus bidirectional travel can play in the deformation mode of micropillars, images (c) (unidirectional) and (d) (bidirectional) are taken from the same sample.
Fig 5.
Topology of human finger pads.
Panels (a) through (c) are SEM images of a paraffin mold used to cast the PDMS replica finger probes used in this study, shown at sequentially higher magnification. Note that because this mold is a negative image from which the replica finger probe is cast, pits and depressions within the lower regions represent asperities that protrude from the fingerprint ridges of the probe. Panels (d) and (e) are SEM images of a 30:1 PDMS probe at the conclusion of testing. Panel (f) shows profilometry traces of fingerprint molds from two authors to demonstrate the general dimension and variability of human fingerprints where the single ridge rectangular insets are then plotted in (g) with equally scaled axes to convey the true extent of microroughness in contact with the micropillar arrays.
Fig 6.
Mechanical analysis of PDMS-micropillar interactions.
(a) Free-body diagram with associated moments, reactions, forces and deformation. (b) 3D models of the micropillar cases from Fig 4A and 4B along with deflections and forces to produce those deflections. (c) 3D model showing same for the d = 18 μm, s = d = 18, μm, E = 1000 MPa case. (d-e) SEM images showing the isolated nature of deformation for the case shown in (c).
Fig 7.
Resulting mechanical outcomes for all samples tested.
Mechanical outcomes for all E = 10 MPa micropillar samples are shown on the left while all E = 1000 MPa micropillar results are shown on the right. The legend in the foreground assigns reds of increasing saturation for increasingly severe adhesion/deformation. Green indicates the tested micropillars remained in pristine condition while yellow was chosen for ploughing because micropillars survived despite injurious results for the replica finger.