Recent Designs of Nonfouling Phosphorylcholine Surfaces with Well Defined Structures

MPC is one of the best monomers with which to produce biomimetic surfaces because it can be applied to a wide variety of surface modifications, as shown in Fig. 12. These methods have been applied to improve the bio/blood compatibility of numerous biomedical devices (Iwasaki and Ishihara 2005).

To better understand protein-material and cell-material interactions at the submolecular level, well-defined biomimetic surfaces have recently been produced (Fig. 13).

Alkanethiols terminating in various functional groups have been used to study the physical-organic chemistry of the adsorption of proteins to synthetic surfaces (Kohler at al. 1996; Marra et al. 1997; Tegoulia and Cooper 2000; Wang et al. 2000). It is clear that alkanethiols fixed on a gold surface create a SAM. Tegoulia and Cooper (2000) first reported that adhesion of neutrophils was effectively reduced on a SAM surface with phosphoryl-choline groups. Chen et al. (2005) synthesized disulfide molecules having phosphorylcholine groups and prepared SAMs on a gold surface.

Fig. 12. Surface modifications of a polymer surface with MPC polymers
Fig. 13. Well-defined designs of phosphorylcholine-bearing surfaces

Marra et al. (1997) synthesized the phospholipid monomer 1-palmitoyl-2-[12-(acryloyloxy)dodecanoyl]-sn-glycero-3-phosphorylcholine, prepared as unilamellar vesicles, and fused them onto alkylated glass. Free-radical polymerization was performed in an aqueous solution at 70 °C. It was clear from x-ray photoelectron spectroscopy analysis that the phospholipid assembly had a closely packed monolayer formation. This formation is very stable under static conditions in water and air, as well as in an environment where there is a high shear flow. There are several procedures for the creation of biomembrane-like surfaces with polymerizable phospholipids (Conboy et al. 2003; Feng et al. 2002; Kim et al. 2004).

Surface modification using silane coupling has also been demonstrated with reagents bearing phosphorylcholine. The process was classified into two approaches. One is generated from hydroxyl-terminated monolayers on a silicon wafer (Durrani et al. 1986; Hayward et al. 1984). The other is the chemical or physical adsorption of phospholipids or phospholipid derivatives on alkylsilane monolayers (Hayward et al. 1986a, b; Marra et al. 1997; Tegoulia and Cooper 2000). Kohler et al. (1996) prepared a glass surface that reacted with 3-aminopropyl-trimethoxysilane. The carboxylated phosphatidylcholine was grafted with a coupling agent.

Matsuda and coworkers (2003a) prepared a phosphorylcholine-end-capped poly(N,N'-dimethylacrylamide) [poly(DMEAA)] and a block co-

oligomer with poly(styrene) with the aid of a photoiniferter-based quasi-living polymerization technique. The oligomer exhibits amphiphilic properties and chemisorbs on a gold surface with hydrophobic anchoring. The surface coated with the oligomers reduced plasma protein adsorption and cell adhesion. The authors also explored a surface design for producing one or two phosphorylcholine groups capped at the terminal end of a graft chain of poly(DMEAA) (Matsuda et al. 2003b).

Polymers may have more worth because of their great potential for multiple functionalities. Although preparation of a well-defined surface is normally difficult due to their structural and size distributions, well-defined MPC polymer brushes on silicon wafers have been prepared quite recently by atom transfer radical polymerization (ATRP) (Feng et al. 2004, 2005; Iwata et al. 2004). ATRP is one of the best methods for living radical polymerization because it can be applied to a wide range of monomers. An alternative process, pioneered by Wirth and Tsujii (Huang et al. 1997,1999; Ejaz et al. 1998, 2002), to prepare well-defined polymer brushes on solid surfaces with ATRP is considerably theoretical and deals with experimental interests in the control of surface properties. Iwata et al. (2004) reported the manipulation of protein adsorption on a thin MPC polymer brush surface. First, they treated silicon wafers with 3-(2-bromoisobutyryl)propyl dimethylchlorosilane (BDCS) to form a monolayer that acts as an initiator for ATRP. Silicon-supported BDCS monolayers were soaked in a methanol/ water solution containing Cu(I)Br, bipyridine, and a sacrificial initiator. After MPC was added to the solution, ATRP was allowed to progress for 18 h. The synthetic scheme for poly(MPC) brushes is shown in Fig. 14a. The Mw and thickness of the poly(MPC) brush layer on the silicon surface increased with increasing polymerization time, as shown in Fig. 14b. The dense polymer brushes were obtained by the "grafting from" system. By selective decomposition of the BDCS monolayer by ultraviolet (UV)-light-irradiation, the poly(MPC) brush region and the sizes were well controlled, resulting in fabricating micropatterned poly(MPC) brushes.

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