Simple method for preparation of ultra-thin poly(N-isopropylacrylamide) hydrogel layers and characterization of their thermo-responsive properties

A reactive polymer of N-isopropylacrylamide (co-PNIPAAm) was prepared by copolymerization with a vinyl monomer carrying a protected isocyanate group. Thin layers of co-PNIPAAm were formed on a surface carrying hydroxyl groups using a spin-coater and were cross-linked and immobilized on the surface by heating. These layers were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and surface plasmon resonance (SPR). The thickness of the co-PNIPAAm layers could be controlled from 4 to 35 nm by changing the concentration of the polymer solutions and the spin-coating conditions. Very little amount of γ-globulin was adsorbed onto the co-PNIPAAm layers even when thicknesses as low as 4 nm were analyzed. In addition, the swelling ratios of the co-PNIPAAm layers were about 4.6 and 1.3 at 25 °C and at 40 °C, respectively. These results indicate that the co-PNIPAAm can be very useful in preparing thermo-sensitive layers on substrates and thus various biomedical applications may be useful.