Interactive Surfaces Based on Polymer Brushes

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.

Functional polymer brushes via surface-initiated controlled radical polymerizations

Stefano Tugulu

The term polymer brush refers to a well defined arrangement of polymer chains, which are tethered with one end to an interface and usually the surface of a solid substrate. Due to steric repulsion the polymer chains furthermore adopt a defined, stretched chain conformation, which significantly differs from the random walk conformation of free polymer chains in solution or in conventionally solution casted polymer coatings. From a structural point of view polymer brushes can therefore be regarded as the most defined type of polymer coating that can currently be realized. Especially the combination of surface-initiated polymerizations (SIP) with modern, controlled polymerization methods thereby allows tailoring the structure of the polymer brushes almost on the molecular level. Specifically precise control over the thickness, composition, chain architecture, grafting density and via the use of lithographic techniques also over the topography of the brushes can be achieved. This high degree of control over the structural and implicated (physico-)chemical properties of the brush allows at the same time tailoring and fine tuning the interfacial properties of the brush. This thesis describes the systematic exploitation of well-defined polymer brushes as functional coatings with fine-tuned physicochemical properties to direct and control the interaction with and the response of specific biological, bioanalytical and even reactive chemical environments. In detail the use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the fabrication of functional polymer brushes will be presented. These brushes were used as highly defined coatings in various applications ranging from biomedical and bioanalytical applications to biomimetic materials fabrication (Figure 1). Figure 1 Numerous synthetic approaches for the preparation of polymer brushes have been described in the literature. Chapter 2 will therefore present a summary of the most important types of surface-initiated polymerizations, focusing especially on controlled SIP methods and SI-ATRP. Chapter 3 describes a chemoselective and specific immobilization strategy to decorate intrinsically bioinert polymer brushes with proteins of interest in a defined orientation and density. Specifically the resulting substrates represent attractive candidates for the realization of protein microarrays. In view of this application protein-small molecule and protein-protein interactions as well as posttranslational modifications were analyzed within a feasibility study. Due to their well-defined structure polymer brushes represent also attractive candidates for the realization of molecularly defined cell adhesion substrates for tissue engineering or as highly defined biophysical model system to analyze cell adhesion and mechanics. Chapter 4 describes the decoration of intrinsically bioinert polymer brushes with small peptide ligands with defined density to induce integrin specific cell adhesion on the brushes. Peptide functionalized polymer brushes could be readily endothelialized and adhering cell layers have shown to preserve their homeostatic ability to respond to an applied mechanical stimulus in a physiological manner. In Chapter 5 the limitations of high density polymer brushes in biomedical applications were assessed by analyzing their stability under in vitro cell culture conditions. Intrinsically bioinert high density polymer brushes were found to degraft from glass or silicon substrates upon swelling in good solvents, which lead to deterioration of the bioinert properties of the substrate. The degrafting of the brushes might be connected to swelling induced mechanical activation of chemical bonds, which link the brushes to the substrate. Chapter 6 describes the transfer of the developed SIP approach onto flexible PDMS substrates by exploiting the formation of interpenetrating networks of ATRP-initiator siloxane and PDMS. The modification of the PDMS substrates with hydrophilic brushes effectively reduces the unspecific protein adsorption on the substrates. This approach might therefore represent an attractive and facile route for the surface modification of miniaturized PDMS devices for biomedical or bioanalytical applications. In Chapter 7 the systematic development of a reliable and robust protocol for the aqueous SI-ATRP of sodium methacrylate is presented, which gives access to poly(methacrylic acid) (PMAA) brushes with defined molecular weight and grafting density. The resulting brushes were assessed for their ability to direct the mineralization of calcium carbonate. Chapter 8 describes the use of photolithographically patterned poly(methacrylic acid) brushes as biomimetic, acidic macromolecular matrix to fabricate microstructured calcite thin films that are an exact 3D replica of the PMAA brush. The presented strategy relies on three key elements: (i) the use of photolithographic techniques to prepare microstructured PMAA brushes; (ii) the ability of PMAA brushes to stabilize amorphous calcium carbonate (ACC) and (iii) the possibility to convert the metastable ACC phase into a polycrystalline calcite film via a thermal treatment.

EPFL2007

Mechanochemically Responsive Surfaces and Soft Tissue Adhesive Interfaces Generated via Controlled Radical Polymerization

Nariye Cavusoglu Ataman

Thin coatings applied to surfaces dramatically improve the properties of materials. Thus, they have found application in a broad variety of areas including biomedicine, nanotechnology, bioelectronics and optics, amongst others. Polymer brushes prepared via surface-initiated controlled radical polymerization (SI-CRP) techniques represent a versatile class of thin polymer coatings that provide manifold opportunities to engineer surface and interface properties. Densely grafted polymer brushes prepared via SI-CRP possess several attractive material properties such as low friction coefficients and the ability to prevent biofouling, which are related to the stretched conformation of the surface tethered polymer chains. Those distinctive characteristics of polymer brushes make them highly appealing to engineer the properties of functional interfaces. A first aim of this Thesis is to investigate the mechanochemistry of hydrophilic polymer brushes prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) from silica substrates. The second aim of the Thesis is to explore poly(2-hydroxyethyl methacrylate) (PHEMA) based polymer brushes as soft-tissue adhesives in wet tissue environment. This Thesis is divided into four chapters, which are briefly summarized below. Chapter 1 provides a summary of recent literature that has reported detachment or degrafting of densely grafted hydrophilic polymer brushes from substrates upon exposure to aqueous media. The collective results from these reports suggest that swelling-induced stretching of hydrophilic polymer brushes lead to mechanochemical activation of the bonds near the brush-substrate interface and subsequently facilitates hydrolytic chain cleavage. Chapter 2 aims to shed light on the influence of two fundamental structural polymer brush parameters on the degrafting of surface-tethered, densely grafted polymer chains, viz. (i) the chemical composition of the ATRP initiator that is used to grow the polymer brushes and (ii) the effect of surface curvature. To this end, a series of poly(poly(ethylene glycol) methacrylate) (PPEGMA) and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMEMA) brushes were grafted from both flat silica substrates as well as silica nanoparticles. Subsequently, the degrafting of the surface-tethered polymer chains was investigated in cell culture medium and phosphate buffered saline (PBS) using various techniques. The aim of the final two Chapters of this Thesis is less on exploring the chain conformation of polymer chains grafted by SI-CRP, but more to use the ability of SI-CRP to prepare thin polymer films that present a high surface concentration of functional groups to mediate soft tissue adhesion. Chapter 3 describes the preparation of photo-active poly(2-hydroxyethyl methacrylate) (PHEMA) brushes as soft tissue bioadhesives. Tissue adhesion in this Chapter is induced as UV-irradiation of PHEMA brushes converts the initially bio-inert side chain hydroxyl groups of these brushes into (putatively) aldehyde groups, which can react with amine groups present in the tissue. The prepared PHEMA brush coated substrates are biocompatible, which avoids fibrous capsule formation and presents functionalities that can be transformed, on-demand, from a non-reactive, non-adhesive state to a reactive, adhesive state in wet tissue environment. While the photoactivation of the PHEMA side chain functional groups is very efficient, it is not very well defined from a chemistry point of view. Chapter 4 therefore, explores an alternative photoactivation chemistry, which is based on the use of the diazirine motif. These diazirine motifs were incorporated in poly(2-hydroxyethyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate) (P(HEMA-co-PEGMEMA)) copolymer brushes via post-polymerization modification. These diazirine modified brushes are attractive as they may not only be activated by UV-light but also by electrochemical means.

EPFL2016

Polymer Brush Gradients Grafted from Plasma-Polymerized Surfaces

Tugba Bilgiç Tune, Harm-Anton Klok

A new method for generating a surface density gradient of polymer chains is presented. A substrate-independent polymer deposition technique was used to coat materials with a chemical gradient based on plasma copolymerization of 1,7-octadiene and allylamine. This provided a uniform chemical gradient to which initiators for atom transfer radical polymerization (ATRP) were immobilized. After surface-initiated atom transfer radical polymerization (SI-ATRP), poly(2-hydroxyethyl methacrylate) (PHEMA) chains were grafted from the surface and the measured thickness profiles provided direct evidence for how surface crowding provides an entropic driving force resulting in chain extension away from the surface. Film thicknesses were found to increase with the position along the gradient surface, reflecting the gradual transition from collapsed to more extended surface-tethered polymer chains as the grafting density increased. The method described is novel in that the approach provides covalent linkages from the polymer coating to the substrate and is not limited to a particular surface chemistry of the starting material.