Active Stimuli-Responsive Polymer Surfaces and Thin Films: Design, Properties and Applications

We report a novel approach for the temperature-triggered development of water-soluble photoresists based on photocleavable poly(N-isopropylacrylamide) copolymers. These copolymers are soluble in an aqueous environment below their Lower Critical Solution Temperature (LCST). Upon UV irradiation, the photocleavable groups are deprotected resulting in an increased LCST. Thus, the illuminated parts of spin-coated copolymer layers dissolve at higher temperatures than the surrounding areas, leading to pattern development. The photoresist can finally be completely removed at low temperature. We demonstrate the applicability of this novel photolithographic approach by the patterning of fluorescent proteins. Development of photopatterning technology is of importance for microelectronics, sensor design, microfluidics, biotechnology, bioanalytics, and tissue engineering. Thereby, different compounds undergoing photoinduced degradation or cross-linking are implemented as photoresists for fabrication of structured surfaces. In fact, the design of photosensitive compounds determines the applicability of any particular sort of photolithography. For example, water-soluble photoresists are of particular interest for the in situ patterning of proteins and cells under biologically relevant conditions. Recently, various photoresists that are water-soluble and can be developed in an aqueous environment have been designed. However, most of them undergo immediate development upon illumination. This causes an undesired contamination of the surrounding due to the uncontrolled release of dissolved photoresists. Here, we demonstrate a novel approach for the design of photoresists which can be fully processed in aqueous environment but which development is triggered by temperature. Our design is based on random copolymers of poly(N-isopropylacrylamide) (PNIPAM) with photocleavable groups. In an aqueous environment, PNIPAM (homopolymer) reversibly changes its solubility at the Low Critical Solution Temperature (LCST ) 33 °C). Due to its thermoresponsive properties, PNIPAM has been already applied for cell culturing, directed protein adsorption, control of biomolecular motors, protein purification, and drug delivery. In our approach, we incorporated hydrophobic 2-nitrobenzyl photocleavable groups into the PNIPAM chains and prepared random poly(2-nitrobenzyl acrylate-co-N-isopropylacrylamide) copolymers (poly(NIPAMNBA)) resulting in a lowered LCST. Upon UV irradiation, the LCST of the copolymers increases due to the formation of poly(acrylic acid-co-N-isopropylacrylamide) (poly(NIPAMAA)). Consequently, when photostructured copolymer layers † Max-Planck-Institute of Molecular Cell Biology and Genetics. ‡ Leibniz Institute of Polymer Research Dresden. (1) Beh, W. 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