The light way towards bioactive and antifouling coatings

This thesis centered on the study and development of control light-triggered techniques for the creation of polymer brushes. The designed structures are applied as antifouling and bioactive coatings. 

Chapter 1 provides a general introduction to the problem of protein fouling and ways to curb it. This chapter focuses on the antifouling polymer brushes and on methods and techniques to create those structures on the surface. This chapter also dives into mechanisms of the control polymerizations such as ATRP and RAFT as main techniques for the creation of polymer brushes. The light-triggered control polymerization mechanism and techniques applied in the next chapters are also discussed in detail.

Hierarchical bioactive surfaces are introduced in Chapter 2 for future application in biosensing and tissue engineering. Those surfaces are created by visible light-induced surface-initiated living radical polymerization employing tris[2-phenylpyridinato-C2,N]iridium(III) as a photocatalyst. The hierarchical antifouling diblock copolymer structures consist of N-(2-hydroxypropyl)-methacrylamide (HPMA; first block) and carboxybetaine methacrylate (CBMA; second block). The living nature of the polymerization is shown by a linear increase in layer thickness over time (as measured by atomic force microscopy, AFM), and by the possibility for reinitiation of the polymerization to create a patterned second block of the polymer. The chemical structure of the brushes is confirmed by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection infrared spectroscopy (IRRAS) measurements. The block copolymer brushes demonstrate excellent antifouling properties when exposed to single-protein solutions or to bovine serum. The second carboxybetaine block of the hierarchical antifouling structures can effectively be biofunctionalized with an anti-fibrinogen antibody. The coated surfaces show a high affinity and specificity to fibrinogen while preventing non-specific adsorption from other proteins in bovine serum.

We used the gained knowledge of controlled photopolymerization for further exploration of applications of surface-initiated photoinduced electron transfer–reversible addition-fragmentation chain transfer (SI-PET-RAFT) polymerization in water (Chapter 3). This novel process proceeds in an aqueous environment under atmospheric conditions without any prior degassing, and without the use of heavy metal catalysts, with Eosin Y and triethanolamine as catalysts for the synthesis of antifouling polymer brushes. The versatility of the method is shown by using three chemically different monomers: oligo(ethylene glycol) methacrylate, HPMA, and CBMA. In addition, the light-triggered nature of the polymerization allows the creation of complex three-dimensional structures. The composition and topological structuring of the brushes is confirmed by XPS and AFM. The kinetics of the polymerizations are followed by measuring the layer thickness with ellipsometry. The polymer brushes demonstrate excellent antifouling properties when exposed to single-protein solutions and complex biological matrices such as diluted bovine serum. This method thus presents a new simple and robust approach for the manufacturing of antifouling coatings for biomedical and biotechnological applications.

We further push the SI-PET-RAFT boundaries for the creation of PLL-HPMA bottlebrushes as antifouling coatings in Chapter 4. The poly(HPMA) side chains, grown by PET-RAFT polymerization, provide antifouling properties to the surface. In this chapter such brushes are prepared in three different ways, and subsequently investigated for their antifouling potential. First, the PLL-HPMA coatings are synthesized in a bottom-up fashion through a grafting-from approach. In this route, the PLL is self-assembled onto a surface, after which a polymerization agent is immobilized, and finally HPMA is polymerized from the surface. In the second explored route the PLL is modified in solution by a RAFT agent to create a macroinitiator. After self-assembly of this macroinitiator, the HPMA is polymerized from the surface by RAFT. In the third and last route, the whole PLL-HPMA bottlebrush is synthesized in solution. To this end, HPMA is polymerized from the macroinitiator in solution and the PLL-HPMA is then self-assembled onto the surface in just one step (grafting-to). Additionally, in this third route, we also design and synthesize a bottlebrush polymer with a PLL backbone and HPMA side chains that contain 5% CBMA monomers that allow for additional (bio)functionalization in solution or after surface immobilization.

These three routes are evaluated with respect to the following terms: ease of synthesis, scalability, ease of characterization, and a preliminary investigation of their antifouling performance. All three coating procedures result in coatings that show good antifouling properties in single-protein antifouling tests. This method thus presents a new, simple, versatile, and highly scalable approach for the manufacturing of PLL-based bottlebrush coatings that can be synthesized partly or completely on the surface or in solution, depending on the desired production process and/or application.

In Chapter 5, SI-PET-RAFT was applied for the creation of antifouling polymer brushes on gold surfaces. The living nature of this method allowed the creation of random and diblock copolymer brushes based on HPMA and CBMA. The introduction of the CBMA into the polymer brushes opens the route for further brush functionalization by versatile active ester chemistry. The chemical composition of the brushes was confirmed by XPS, and the polymer brush thickness was determined by spectroscopic ellipsometry. The polymer brushes demonstrate good antifouling properties against undiluted human serum monitored by quartz crystal microbalance with dissipation (QCMD-D) and surface plasmon resonance (SPR) spectroscopy in real-time. This approach further widens the road towards building highly antifouling and still functional copolymer brushes in a scalable, robust, oxygen-tolerant, and the heavy metal-free way that opens up applications in biosensing and tissue engineering.

Chapter 6 discusses and highlightes previous chapters with a focus on open questions in antifouling, polymer brushes, and surface modification research. It also looks at the future of this research, in particular to perfect antifouling and functional monomers, and surface-independent approaches for the surfaces modification.