Femtosecond Laser Direct Writing of Gecko-Inspired Switchable Adhesion Interfaces on a Flexible Substrate

Abstract

Biomimetic switchable adhesion interfaces (BSAIs) with dynamic adhesion states have demonstrated significant advantages in micro-manipulation and bio-detection. Among them, gecko-inspired adhesives have garnered considerable attention due to their exceptional adaptability to extreme environments. However, their high adhesion strength poses challenges in achieving flexible control. Herein, we propose an elegant and efficient approach by fabricating three-dimensional mushroom-shaped polydimethylsiloxane (PDMS) micropillars on a flexible PDMS substrate to mimic the bending and stretching of gecko footpads. The fabrication process that employs two-photon polymerization ensures high spatial resolution, resulting in micropillars with exquisite structures and ultra-smooth surfaces, even for tip/stem ratios exceeding 2 (a critical factor for maintaining adhesion strength). Furthermore, these adhesive structures display outstanding resilience, enduring 175% deformation and severe bending without collapse, ascribing to the excellent compatibility of the micropillar's composition and physical properties with the substrate. Our BSAIs can achieve highly controllable adhesion force and rapid manipulation of liquid droplets through mechanical bending and stretching of the PDMS substrate. By adjusting the spacing between the micropillars, precise control of adhesion strength is achieved. These intriguing properties make them promising candidates for various applications in the fields of microfluidics, micro-assembly, flexible electronics, and beyond.

Keywords: mushroom-shaped micropillars; switchable adhesion; two-photon polymerization.

Figure 1

Schematic FsLDW of BSAI on a flexible substrate. (i) PDMS spin-coated onto the ITO substrate, (ii,iii) FsLDW and developing of the mushroom-shaped micropillars based on IP-PDMS, (iv) Peeling the micropillar arrays film off the substrate, and (v) illustration of sample bending and stretching along with promising applications.

Figure 2

Geometries of micropillars with different tip diameters. (a) Design dimensions of the micropillars. (b) SEM images of micropillar arrays from top view. (c) SEM images of micropillar arrays from side view. (d) AFM topographies on the tip of a mushroom-shaped micropillar.

Figure 3

Deformation characterizations of the micropillar arrays on a flexible substrate. (a) Schematic and corresponding real experiment snapshots of bending BSAI with increased curvatures. (b) Schematic and corresponding real experiment snapshots of bending the sample with increased stretching strains. (c) Illustration of a beetle equipped with flexible BSAI. (d) A photograph of a beetle specimen equipped with flexible BSAI. (e) The state of beetle specimen equipped with (right) and without (left) flexible BSAI on an inclined glass.

Figure 4

Force characterizations in non-stretched BSAIs. (a) Illustration of the custom adhesion platform. (b) Stress–strain curves of a single micropillar on the flexible substrate. (c) Adhesion values of the non-stretched samples under different preloads and the corresponding morphologies of the micropillars. (d) The evolutions of the corresponding contact signatures of the non-stretched samples under different preloads. The regions enclosed by the blue dashed line represent the efficient contact areas, while the regions enclosed by the yellow dashed line indicate the collapse areas of the BSAIs.

Figure 5

Adhesion characterizations in stretched BSAIs. (a) Adhesion force graphs with respect to time of the samples under different stretching strains. (b) Adhesion values of the samples under varied stretching strains (c) The evolutions of the corresponding contact signatures with corresponding stretching strains. The region enclosed by the blue dashed line represents the efficient contact area. (d) The application of BSAI in non-destructive transfer of a small electronic device.

Figure 6

Wettability characterizations of the BSAIs. (a) Schematic of a water droplet suspended by the vertical component of surface tension. (b) Morphologies of the droplets on samples of different stretching strains. (c) Results of advancing and receding contact angles with related stretching strains. (d) Schematic and corresponding snapshots of picking up and releasing a dyed water droplet by the BSAI.