No abstract

Surface macromolecular architectures with regional dimensional precision, control of the thickness of a graft layer, and blocks of graft chains were attempted using the surface photo-graft copolymerization method pioneered by Otsu et al. This is based on the photochemistry of benzyl N,N-diethyldithiocarbamate, which can be photolyzed into a radical pair (one radical can initiate radical polymerization and the other tends to recombine with the former radical). Ultraviolet light (UV) irradiation of a benzyl N,N-diethyldithiocarbamyl group-immobilized polymer surface in the presence of a vinyl monomer such as N,N-dimethylacrylamide, N-[3-(dimethylamino)propyl]acrylamide, methacrylic acid, or styrene at room temperature allowed precise control of the macromolecular architectures of the grafted surfaces. X-ray photoelectron spectroscopy (XPS) analyses and water contact angle measurements before and after UV irradiation in a monomer solution provided evidence that the graft copolymerization proceeded only during photoirradiation and at photoirradiated portions. Atomic force microscopic (AFM) observations showed that the thickness of the graft-copolymerized layers increased almost linearly with UV irradiation time. Patterned grafted surfaces, which were prepared using the ionic or nonionic hydrophilic monomers listed above under irradiation through a projection mask, were clearly visualized by dye-staining or cell-culturing. The graft copolymerization under irradiation through a projection mask with sequential monomer charges yielded surfaces with regionally dimensionally controlled macromolecular architectures such as di- and triblock graft-copolymerized surfaces. A graft thickness-gradient surface was obtained by using a gradient filter.

An in vivo rat cage implant system was used to identify potential surface chemistries that prevent failure of implanted biomedical devices and prostheses by limiting monocyte adhesion and macrophage fusion into foreign-body giant cells while inducing adherent-macrophage apoptosis. Hydrophobic, hydrophilic, anionic, and cationic surfaces were used for implantation. Analysis of the exudate surrounding the materials revealed no differences between surfaces in the types or levels of cells present. Conversely, the proportion of adherent cells undergoing apoptosis was increased significantly on anionic and hydrophilic surfaces (46 ؎ 3.7 and 57 ؎ 5.0%, respectively) when compared with the polyethylene terephthalate base surface. Additionally, hydrophilic and anionic substrates provided decreased rates of monocyte͞macro-phage adhesion and fusion. These studies demonstrate that biomaterial-adherent cells undergo material-dependent apoptosis in vivo, rendering potentially harmful macrophages nonfunctional while the surrounding environment of the implant remains unaffected.

To directly characterize the thermoresponsive structural changes of a poly(N-isopropylacrylamide) (PNIPAAm) graft layer at the microscopic level, the force-distance curve (f-d curve) was measured on a well-tailored end-grafted PNIPAAm surface in aqueous solution at 25 and 40 °C, using an atomic force microscope (AFM). The PNIPAAm surface was prepared by an iniferter-based photograft polymerization technique. The approach trace of the f-d curve exhibited a steric repulsion profile at 25 °C, while the range of repulsion decreased 1 /10 to 1 /20 at 40 °C, confirming the ascending-heat-induced collapse of the PNIPAAm graft layer. The change in thickness of the graft layer was complementarily measured from the scanning images of the boundary between the grafted and nongrafted regions under well-defined scanning forces. The thermoresponsive characteristics of the PNIPAAm graft layer including its interaction with proteins and the applied-load dependence of the measured graft thickness are discussed.