Atmospheric-pressure plasma (APP) treatment on polystyrene (PS) substrates by using N(2), He, and He/N(2) gases as working gas were investigated in this study, and photo-induced grafting polymerization of N-isopropylacrylamide (NIPAAm) on APP-treated PS substrates was subsequently applied. From the results of the wetting test, PS substrates treated by He-APP process represented a superior hydrophilicity with a contact angle of less than 10 degrees. Surface analyses by X-ray photoelectron spectroscopy (XPS) indicated that functional groups of the carbonyl group (C=O) and carboxyl group (O-C-O) were generated on the surface of PS substrates after APP processes, which were most likely to enhance the surface hydrophilicity. Additionally, the thermosensitive poly-(NIPAAm) grafted on APP-treated PS substrates exhibited a hydrophilic surface (theta(H2O)=32 degrees) below low critical soluble temperature (LCST), and converted into the hydrophobic surface (theta(H2O)=63 degrees) as the ambient temperature exceeded LCST (>33 degrees C). Our results suggest that the pre-treatment of PS substrates by APP process can increase the surface hydrophilicity and also be beneficial to the photo-induced grafting polymerization of NIPAAm on PS substrates. (C) 2010 Elsevier B.V. All rights reserved.