(438f) A Comparison of Protein-Repellent Properties of Zwitterionic and Cationic Fluorinated Block Copolymer Surfaces

Protein and cell-repellant polymers are important in surface modification of biomaterials and biosensors, and in ultrafiltration and marine antifouling technologies [1,2]. Hydrophilic coatings, such as those prepared using the nonionic poly(ethylene glycol) (PEG) containing polymers, have been widely studied. Based on these studies, it has become evident that a high degree of hydration and conformational flexibility are important attributes of a protein-repellant polymer surface. In recent years, there is a significant interest in the use of ionic polymers for creating antifouling surfaces, because of the ability of ions to bind water molecules. Ionic polymers have also been explored for biomedical applications such as bactericidal coatings, coatings for tissue culture substrates, and coatings for drug delivery and gene delivery [2].

In this work, different zwitterionic and cationic block copolymers were synthesized from the same partially-fluorinated block-copolymer precursor, using a polymer analogous reaction approach. The zwitterionic block copolymers contained the carboxybetaine and the sulfobetaine groups. Surfaces of thin films cast from these polymers were investigated for surface wettability, surface energy, and protein adsorption. The ionic block copolymers were found to have good film-forming properties, which makes them suitable for simple coating techniques such as dip-coating, spin-coating, or spraying.

All of the ionic block copolymer surfaces were hydrophilic, as expected. The influence of cationic and zwitterionic groups on the dispersion and polar components of surface energy was studied using the surface energy models of Owens, Wendt and Kaelble, and van Oss, Chaudhury and Good. Composition depth profiles were determined using X-ray photoelectron spectroscopy (XPS). Because of the introduction of the low surface energy perfluoroalkyl groups in the polymer microstructure, thermally annealed coatings of the block copolymers showed surface segregation of the ionic mers, in spite of the high surface energy of polar molecules. Surface composition and wettability were also found to be affected by the number of methylene groups separating the cationic and anionic centers in the zwitterions.

Electrokinetic measurements were used to probe the net charge on the block copolymer surfaces, and the influence of pH and ionic strength on net surface charge. The effect of surface charge, on the adsorption of fluorescently-labeled, positively and negatively charged protein molecules, was studied. Protein interactions with the ionic block copolymer surfaces were strongly mediated by the repulsive steric-hydration force and the attractive electrostatic force between the protein molecules and the surface. The steric-hydration force on the block copolymer surfaces was found decrease in the following order: carboxybetaine > sulfobetaine > cationic. The zwitterionic block copolymers, particularly the carboxybetaine polymer, that are developed in the present work are expected to function as stable low-fouling surface-modifiers in different biological environments.

References

1. Krishnan, S.; Weinman, C. J.; Ober, C. K. Advances in Polymers for Anti-biofouling Surfaces. J. Mater. Chem. 2008, 18, 3405−3413.
2. Vendra, V. K.; Wu, L.; Krishnan, S. Polymer Thin Films for Biomedical Applications. In Nanomaterials for the Life Sciences Vol. 5: Nanostructured Thin Films and Surfaces; Kumar, C. S. S. R., Ed.; Wiley-VCH: Weinheim, 2010; pp. 1−54.