(550g) Chemical Cross-Linking of Polyelectrolyte Nanofilms to Control Mechanical and Cell-Contacting Properties

Nanofilms comprised of select polyelectrolytes and constructed by layer-by-layer (LbL) assembly show great promise as surface coatings due to their highly tunable film properties, such as thickness and viscoelasticity, as well as their ability to effectively decouple a material's surface from bulk properties. Nanofilm properties are controlled by physical means such as solution chemistry, pH, and ionic strength, and also by chemical means such as cross-linking of functional groups within the polyelectrolytes. This work focuses on changes in mechanical and cell-contacting properties of nanofilm coatings brought about by different types and extents of chemical cross-linking. EDC-NHS chemistry is employed to form cross-links of controlled extent, both homogeneously throughout the film and in an inhomogeneous manner where increased film rigidity is sought near to the film's surface. Cross-link extent and location are monitored in situ via Fourier transform infrared (FTIR) spectroscopy within the attenuated total reflection (ATR) mode, and film mechanical properties are evaluated using quartz crystal microgravimetry with dissipation monitoring (QCM-D). Cell-contacting properties are investigated using human hepatocellular carcinoma (HepG2) cells. Cross-linking is seen to result insignificantly increased film viscous and elastic moduli, with values for films subjected to non-uniform cross-linking intermediate between fully cross-linked and native films. HepG2 cells adhere strongly to both fully and partially cross-linked films. FTIR spectra confirm the presence of a spatially non-uniform distribution of cross-links, demonstrating the possibility of inhomogeneous films whose bulk and surface properties are effectively decoupled. In conclusion, chemical cross-linking represents a facile and highly effective means of controlling polyelectrolyte nanofilm mechanical and cell contacting properties.