Tunable surface wrinkling on shape memory polymers with application in smart micromirror

Abstract

Surfaces with tunable topological features enable important applications, such as optical devices, precision metrology, adhesion, and wetting. In this study, we demonstrate a facile method to fabricate and control the surface morphologies by combining thin film wrinkling and thermal expansion. This approach utilizes self-assembled surface wrinkling induced by shape recovery of shape memory polymers and localized thermal expansion caused by Joule heating. Recovering the prestrain in the SMP substrate induces global wrinkling of the thin film on the substrate. Joule heating in the SMP by a heating wire embedded in the substrate induces thermal expansion of the substrate in a localized area, which leads to disappearance of the wrinkling pattern. This effect is reversed when heating is stopped, leading to reversible and repeatable tuning of the surface morphology in a controllable localized surface region. With metal coating, the SMP surface can be switched from specular to diffuse reflectance with respond to external Joule heating. Finally, we demonstrate a smart micromirror device with its diffuse reflectance tunable between 13.5% to 81.9% in visible light region. This approach provides a method to modulate surface diffusivity by controlling its surface morphologies, with potential applications in optical display and optical microelectromechanical (MEMS) devices. Conflicts of interestThere are no conflicts of interest to declare.

Materials with Electroprogrammable Stiffness

“…[ 183 ] While this work presented another option for tunable dry adhesion via Joule heating and stiffness programming, other electrothermal materials have been implemented for adhesion control, albeit without electrically controlled heaters. For example, the surface topology of SMPs can be changed via heat, [ 231 ] and Wang et al. implemented external heating to introduce specific wrinkling patterns into an SMP on a PDMS substrate, which then displayed programmable and reversible adhesion to a substrate.…”

Application and Development of Shape Memory Micro/Nano Patterns

“…A relatively hard skin supported by softer internal tissues will form wrinkles under compressive stress, and wrinkles on the surface are caused by the application of transverse compressive strain or stress; [77] SMPs based surface wrinkles have unique aspects that are different from other micropatterns. [78][79][80][81][82] Surface wrinkles can be formed by metal coating processing on SMP substrate. [83] Xie et al [84] obtained 3D arbitrary images by controlling the spatial distribution of wrinkles and the angle of diffraction, which is a breakthrough in the research and development of microoptics devices.…”

Island Effect in Stretchable Inorganic Electronics

“…[ 33–40 ] Herein, the elastic stretchability refers to the value of elongation of the device as elastically stretched to a strain level, below which the device can reversibly return to the load‐free state, even after thousands of cycles without material failure, for example, fatigue failure of ductile materials, fracture of brittle materials and delamination at adhesive interfaces. Among various structural designs of stretchable inorganic electronics, [ 41–53 ] the “island‐bridge” design represents a widely used strategy, where the bridge‐like deformable interconnects in the intermediate regions (i.e., trenches) between the island‐like non‐stretchable elements (e.g., commercial chips) provide the stretchability, due to the contrast of their tensile stiffness. [ 15,54–57 ] Recent works have reported substrate designs with programmable stiffness to limit the local deformation of the substrate as a physical protection of functional components, which could provide larger stretchabilities of the system.…”