Surface Characterization of Poly(N-isopropylacrylamide) Grafted Tissue Culture Polystyrene by Electron Beam Irradiation, Using Atomic Force Microscopy, and X-Ray Photoelectron Spectroscopy

To understand features of polymers grafted by electron beam (EB) irradiation method, we investigated topology of poly(N-isopropylacrylamide) (PIPAAm) grafted tissue culture polystyrene (TCPS) (PIPAAm-TCPS) prepared by EB irradiation, using atomic force microscopy (AFM) in air and under aqueous conditions. Furthermore, surfaces properties of PIPAAm-TCPS surfaces before and after cell culture were also examined for evaluation of functionality of the surface as biomaterials, using XPS analysis. Three types of PIPAAm-TCPSs with different graft densities (1.0±0.1, 1.6±0.1, and 2.0±0.1 μg/cm² of the grafted) were obtained (abbreviated as 11PIPAAm-, 16PIPAAm-, and 20PIPAAm-TCPS) by using different initial monomer concentration (20, 55, and 65 wt%). Contact angles (cos value) of the surfaces increased with an increase in density of the grafted polymer. AFM observation in air clearly revealed that original TCPS surface possesses scratched and grooved topology (ca. 10 nm height of the scratch), while PIPAAm-TCPSs surfaces exhibited nanoordered PIPAAm particle-like domains. The size of the particles also increased proportionally initial IPAAm monomer concentration. The 11PIPAAm-and 16PIPAAm-TCPS surfaces having ca. 10–30 nm and ca. 40–50 nm size of the particles also displayed scratched and grooved topology featured in basal TCPS. However, the larger sizes of the particles (ca. 40–100 nm) formed on 20PIPAAm-TCPS surfaces adequately conceals the topological feature of the basal TCPS surfaces. The AFM images indicate that the graft polymer is as ultra thin as the scratch and grooves featured on basal TCPS are discernible, and the grafted PIPAAm layer become thicker with an increase of the monomer concentration. For 16PIPAAm-TCPS surfaces, the nanoordered particles were also observable in aqueous conditions at 20 °C and 37 °C. Comparison between the images obtained at 20 °C and 37 °C suggest that the domains are not likely to exhibit significant swelling and shrinking by temperature change, although the topology of PIPAAm grafted onto clover glass surface (50 μm thickness of the gel layer) were dramatically changed by temperature change in early reports. This difference should be due to ultra thin thickness of the grafted PIPAAm, which is subject to more restricted molecular motion by basal hydrophobic TCPS interfaces, as we reported previously. XPS C1s and N1s spectra of 16PIPAAm-TCPS surface after removal of cells suggest that proteins and/or peptides components possibly remained on the surfaces. Based on results from XPS analysis, we further discuss surface properties of 16PIPAAm-TCPS as biomaterials, comparing those of PIPAAm grafted polystyrene prepared by a radio frequency plasma method used in recent reports.