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

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.