Programmable microscale stiffness pattern of flat polymeric substrates by temperature-memory technology
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esearch Letter
Programmable microscale stiffness pattern of flat polymeric substrates by temperature-memory technology Yi Jiang, Helmholtz-Zentrum Geesthacht, Kantstr. 55, 14513 Teltow, Germany; Institute of Chemistry, University of Potsdam, 14476 Potsdam, Germany Ulrich Mansfeld, and Karl Kratz, Helmholtz-Zentrum Geesthacht, Kantstr. 55, 14513 Teltow, Germany Andreas Lendlein, Helmholtz-Zentrum Geesthacht, Kantstr. 55, 14513 Teltow, Germany; Institute of Chemistry, University of Potsdam, 14476 Potsdam, Germany Address all correspondence to Andreas Lendlein at [email protected] (Received 10 January 2019; accepted 12 February 2019)
Abstract Temperature-memory technology was utilized to generate flat substrates with a programmable stiffness pattern from cross-linked poly(ethylene-co-vinyl acetate) substrates with cylindrical microstructures. Programmed substrates were obtained by vertical compression at temperatures in the range from 60 to 100 °C and subsequent cooling, whereby a flat substrate was achieved by compression at 72 °C, as documented by scanning electron microscopy and atomic force microscopy (AFM). AFM nanoindentation experiments revealed that all programmed substrates exhibited the targeted stiffness pattern. The presented technology for generating polymeric substrates with programmable stiffness pattern should be attractive for applications such as touchpads, optical storage, or cell instructive substrates.
Introduction Polymeric substrates comprising local mechanical stiffness pattern or nanostructural features are intensively investigated in the context of applications such as haptic displays (touchpads), stretchable electronics, mechanical and optical data storage devices,[1–8] or as instructive cell substrates guiding mechanosensitive (stem) cells.[9–15] While individual cells can react to structural features of few nanometers in size and mechanical differences in the Pascal (Pa) range,[9,14] the tactile sensitivity of a human finger is only capable of detecting structural features above 10 nm and local mechanics in the kPa regime.[16–20] Mechanically patterned surfaces can be realized by variation of the polymer’s chemical composition (e.g., phase separated blends)[2] or crosslinking density (e.g., hydrogels).[9,21] Another approach is based on placing rigid microstructures or defined closed cavities (pores)[22] under soft/elastic polymeric materials and applying pressure.[11] In most technical applications (i.e., touchpads or data storage) the programmability of the material’s surface is a central requirement. Thermally-triggered shape-memory polymers (SMPs)[23] are a class of smart polymers that allow the realization of programmable surface structures. Here, thermal transitions (e.g., melting Tm or glass transition Tg temperature) are utilized as molecular switches that fix/stabilize a deformed (temporary) shape during a heating–deformation–cooling procedure, called programming. The obtained temporary shape is long-term stable until the programmed materials are again exposed to temperatures abo
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