Photopatterning of thermally sensitive hydrogels useful for microactuators

Hydrogels based on PNIPAAm [poly-(N-isopropylacrylamide)] are an attractive working material for microactuators because of their swelling with thermally induced phase transitions leading to a very high volume expansion. By the employment of co-polymerization with a chromophore, a specially designed polymer has been developed that can be photocrosslinked to a gel and by this photopatterned. The focal point of this work is the preparation and characterization of mircopattems using this co-polymer. The efficiency of the photochemical reaction is quite good and patterning results down to 20 mu m spices have been achieved. The swelling of dot-like patterned hydrogel films on Si/SiO2 substrates in water occurs with similar temperature dependency and a swelling ratio of 3 to 3.5 in the same order of magnitude compared to bulk PNIPAAm-based gels, but the transition time of 2 to 8 s measured at the gelous dots is much shorter than that of macroscopic objects, making the material interesting for microactuator performance. The transition temperature decreases from 32 to 34 degrees C at pure PNIPAAm to 18 to 20 degrees C at the photopatterned gels of the copolymer. (C) 1999 Elsevier Science S.A. All rights reserved.