Dusko Paripovic
While the bulk structure of a material determines its mechanical properties, its surface plays a key role when interactions with the environment are important. Nowadays materials in bio- and nanotechnology are required to possess very complex functionalities and to interact specifically to the stimuli they receive from their surroundings. Thus, their surface modifications are becoming very demanding. In recent years surface initiated controlled radical polymerization techniques have emerged as very versatile strategies that provide a powerful toolbox to tackle the challenges that are set to material scientists. The attractiveness of these methods lies in the possibility to create well defined surface arrangements of polymer chains, which we refer to as polymer brushes, with precise control over thickness, composition and architecture. In addition, these techniques can be applied to very challenging substrates with respect to the chemical composition and geometry, which also includes porous materials. This Thesis, therefore, exploits the possibilities that surface-initiated controlled radical polymerization techniques offer to finely tune interfacial properties of materials and render them capable to interact with chemical and biological species in a desired way. Its paramount aim is to demonstrate how the functionalities and geometry of a polymer brush matrix can be employed to influence inorganic film deposition using synthetic, biomimetic and purely biological mechanism or enhance polymer brushed stability in aqueous media. This Thesis is divided into five distinct chapters, which will be briefly summarized in following paragraphs. Chapter 1 describes the preparation of polymer brushes and strategies that are used to control their chemical composition and architecture. Additionally, it will give an overview on the use of brushes as cell adhesive surfaces as well as templates for the production of nanoparticles and thin inorganic films. Chapter 2 explores a new strategy to guide the chemical solution deposition of thin, microstructured metal films. The proposed strategy is based on the use of poly(2-(methacryloyloxy)ethyl ammonium chloride) (PMETAC) brushes grown via surface-initiated atom transfer radical polymerization as a template. Thin films are prepared by first loading the PMETAC brushes with an appropriate precursor, followed by its reduction by NaBH4 and finally an oxygen plasma treatment to remove the stabilizing polymer brush matrix and generate the desired thin metallic film. Of particular interest is the question whether the thickness and lateral dimensions of the deposited metallic films can be controlled by micropatterning the polymer brush template and varying the brush thickness. In addition, this concept will be extended to the preparation of bimetallic (gold/palladium) films using a single polymer brush matrix. A similar strategy is explored in Chapter 3 to control the deposition of thin calcium carbonate films using a biomimetic process. The process is characterized by the stabilization of amorphous calcium carbonate, the geometry and localication of which can be controlled by the poly(methacrylic acid) brush template. A special focus of this Chapter is to examine the influence of the polymer brush thickness on the mineral deposition and to determine to which extent is the mineral phase is integrated into the polymer brush film. The aim of Chapter 4 is to develop a general approach for the modification of implantable materials with a polymer brush based coating that promotes bone formation. The proposed coatings are prepared via surface-initiated copolymerization of 2-hydoxyethyl methacrylate and 2-(methacryloyl)ethyl phosphate, which results in a thin hydrogel layer that mimics the negatively charged matrix macromolecules involved in natural bone formation. To further emulate the properties of the natural bone extracellular matrix, the coatings will be functionalized with the cell adhesive GGGRGDS peptide. The ability of the proposed coatings to induce bone formation is investigated in in vitro experiments on MC3T3-E1 pre-osteoblasts and evaluated using an ALP and alizarin red assays. Finally, Chapter 5 focuses on studying the parameters that could prevent the hydrolytic cleavage of Si-O bond, which tethers the polymer brushes grafted from SiOx in aqueous media. In the first part of the study, the length of the organosilane initiator's alkyl spacer will be varied, in order to evaluate to which extent it can insulate the site that anchors the hydrophilic polymer brush to the substrate. In the second part, a short, hydrophobic block will be introduced between the silicon oxide substrate and the hydrophilic polymer in order to more efficiently protect the underlying Si-O bond from the surrounding aqueous media and prevent the chain detachment.
EPFL2011
