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A platform of thin polymer coatings was introduced for the functional modulation of immobilized bioactive molecules at solid/liquid interfaces. The approach is based on covalently attached alternating maleic acid anhydride copolymers with a variety of comonomers and extended through conversion of the anhydride moieties by hydrolysis, reaction with functional amines, and other conversions of the anhydride moieties. We demonstrate that these options permit control of the physicochemical constraints for bioactive molecules immobilized at interfaces to influence important performance characteristics of biofunctionalized materials for medical devices and molecular diagnostics. Examples concern the impact of the substrate-anchorage of fibronectin on the formation of cell-matrix adhesions, the orientation of endothelial cells according to lateral anti-adhesive micropatterns using grafted poly(ethylene oxide), and the spacer-dependent activity of immobilized synthetic thrombin inhibitors.

Macroporous scaffolds with adaptable mechanical and biomolecular properties can be instrumental in enabling cell-based therapies. To meet these requirements, a cryostructuration method was adapted to prepare spongy hydrogels based on chemically cross-linked star-shaped poly(ethylene glycol) (starPEG) and heparin. Subzero temperature treatment of the gel forming reaction mixtures and subsequent lyophilization of the incompletely frozen gels resulted in macroporous biohybrid cryogels showing rapid swelling, porosity of up to 92% with interconnected large pores (30–180 μm), low bulk stiffness, and high mechanical stability upon compression. The applicability of the cryogel scaffolds was investigated using human umbilical vein endothelial cells. Cell attachment and three-dimensional spreading resulted in evenly distributed viable cells within the macroporous starPEG-heparin materials, demonstrating the significant translational potential of the developed three-dimensional cell carriers.

The surface properties of poly(dimethyl siloxane) (PDMS) layers screen printed onto silicon wafers were studied after oxygen and ammonia plasma treatments and subsequent grafting of poly(ethylene -alt-maleic anhydride) (PEMA) using X-ray photoelectron spectroscopy (XPS), roughness analysis, and contact angle and electrokinetic measurements. In the case of oxygen-plasma-treated PDMS, a hydrophilic, brittle, silica-like surface layer containing reactive silanol groups was obtained. These surfaces indicate a strong tendency for "hydrophobic recovery" due to the surface segregation of low-molecular-weight PDMS species. The ammonia plasma treatment of PDMS resulted in the generation of amino-functional surface groups and the formation of a weak boundary layer that could be washed off by polar liquids. To avoid the loss of the plasma modification effect and to achieve stabilization of the mechanically instable, functionalized PDMS top layer, PEMA was subsequently grafted directly or after using gamma-APS as a coupling agent on the plasma-activated PDMS surfaces. In this way, long-time stable surface functionalization of PDMS was obtained. The reactivity of the PEMA-coated PDMS surface caused by the availability of anhydride groups could be controlled by the number of amino functional surface groups of the PDMS surface necessary for the covalent binding of PEMA. The higher the number of amino functional surface groups available for the grafting-to procedure, the lower the hydrophilicity and hence the lower the reactivity of the PEMA-coated PDMS surface. Additionally, pull-off tests were applied to estimate the effect of surface modification on the adhesion between the silicone rubber and an epoxy resin.

Heparin and four-armed, end-functionalized polyethylene glycol (starPEG) were recently combined in sets of covalently linked biohybrid hydrogel networks capable of directing various therapeutically relevant cell types. To extend the variability and applicability of this novel biomaterials platform, the influence of size and molar ratio of the two building blocks on the hydrogel properties was investigated in the present study. Heparin and starPEG were converted in various molar ratios and in different molecular weights to tune swelling, stiffness and pore size of the obtained polymer networks. Hydrogels with a range of elastic moduli could be generated by controlling either the crosslinking density or the chain length of the starPEG, whereas altering the molecular mass of heparin did not significantly affect hydrogel strength. The concentration of heparin in the swollen gels was found to be nearly invariant at varying crosslinking degrees for any given set of building blocks but adjustable by the size of the building blocks. Since heparin is the base for all biofunctionalization schemes of the gels these findings lay the ground for an even more versatile customization of this powerful new class of biomaterials. OPEN ACCESSPolymers 2011, 3 603

Thin films of poly(octadecene-alt-maleic anhydride) on top of Si wafers and glass plates were modified by reactions with different functional amines to be used in future studies on the relevance of certain molecular surface properties for the covalent immobilization of proteins. For that aim, a strategy was developed and applied to convert the anhydride moieties of the copolymer by functional amines into side chains bearing hydrophilic groups of acidic (carboxylic acid, sulfonic acid), basic (amines), or neutral (poly(ethylene oxide) (PEO), glucose) character. The modification of the copolymer films was achieved through the two-step formation of a cyclic imide, which was very stable in aqueous solution. Depending on the reactivity of the applied amine, the adjustment of the reaction time was suitable for the preparation of partially converted surfaces of the polymer film. Degrees of modification between 5 and 30% (according to X-ray photoelectron spectroscopy data) were obtained. Annealing the modified polymer films induced efficient back-formation of the anhydride groups. By reaction of the layered polyanhydrides with highly crosslinked diamines, amine-functionalized polymer films were produced that were capable of binding secondary polyanhydride layers.

New thermoresponsive graft copolymers were synthesized by the cationic ring-opening polymerization of 2-methyl-2-oxazoline (MeOxa) or 2-ethyl-2-oxazoline (EtOxa), initiated by the random copolymers of chloromethylstyrene (CMS) and N-isopropylacrylamide (NIPAAm) using the “grafting from” method with a yield of 66−94%. The polymers were characterized by NMR, GPC, and DSC, and the conformational transition (lower critical solution temperature, LCST) of macroinitiators and graft copolymers was determined by the turbidity and DSC measurements. The transition temperature of the graft copolymers could be fine-tuned through the composition of the macroinitiator and the graft copolymer. An increasing quantity of the hydrophobic comonomer chloromethylstyrene in the macroinitiator lowered its LCST, while in the graft copolymer an increasing content of the hydrophilic segment of poly(2-methyl-2-oxazoline) or poly(2-ethyl-2-oxazoline) raised the transition temperature. For graft copolymers with a high content of long poly(2-alkyl-2-oxazoline) grafts, stabilized aggregates with a thermoresponsive core can be formed at the LCST instead of precipitation of the material.

Chitosans are natural aminopolysaccharides, whose low cytotoxicity suggests their potential use for nonadhesive, antibacterial coatings on biomaterials implant surfaces. Here, the antiadhesive behavior and ability to kill bacteria upon adhesion ("contact killing") of chitosan coatings were evaluated for two strains of Enterococcus faecalis, isolated from clogged biliary stents. Chitosan coatings covalently grafted or applied as chitosan/kappa-carrageenan multilayers were characterized by ellipsometry, scanning force microscopy (SFM), X-ray photoelectron spectroscopy (XPS), and electrokinetic measurements. Decreases in initial bacterial deposition rates and the number of bacteria adhering in a more advanced state of the adhesion process were observed on both types of modified surfaces, with more pronounced effects on highly hydrated multilayers. Adhesion of negatively charged enterococci was slightly enhanced on chitosan-terminated multilayers, but antibacterial effect was absent on kappa-carrageenan-terminated multilayers. Thus, the efficacy of multilayers remains an interesting interplay between the promoting effect of cationically charged groups on adhesion of negatively charged bacteria and, on the other hand, their antibacterial effects.