Preparation and characterization of thin film surface coatings for biological environments

An approach to the problem of selecting synthetic materials for use in biological media is presented. Firstly, a surface energetic criterion of biocompatibility of foreign surfaces is suggested. This criterion, which is based on an analysis of the surface interactions between a typical biological fluid (i.e. blood) and synthetic surfaces, is founded on the premise that a sufficiently low (but not very low) solid-biological fluid interfacial free energy of the order of 1-3 dyne/cm, is necessary in order to fulfil the dual requirements of maintaining a low thermodynamic driving force for the adsorption of fluid components on the solid surface as well as a mechanically stable solid-fluid interface. In the second part of this investigation, an experimental approach involving the radio frequency (rf) sputter deposition of thin solid films of tightly adhering polymeric compounds on materials with the desired bulk characteristics, is shown to be a promising method of tailoring the surface properties of many types of synthetic materials for use in biological environments. The preparation and surface characterization of thin, solid films of oxidized fluorocarbon coatings (from a Teflon FEP target) by rf sputtering is illustrated. The deposited polymer films were characterized for their surface morphology, thickness, elemental surface chemical composition and their wetting properties in a biological environment, by the techniques of scanning electron microscopy (SEM), ellipsometry, electron spectroscopy for chemical analysis (ESCA) and contact angle measurements, respectively. Based on the ESCA and contact angle results, it emerges that the surfaces of such polymeric coatings possess sufficient mobility to considerably alter their structures between different environments (such as air and water) and thereby present different wetting characters to these environments. The contact angle procedure developed in this investigation permitted the estimation of the relevant wetting properties of such mobile surfaces in an aqueous environment (which is the environment encountered in most biological fluids). In the final part of this investigation, the possibility of effecting a drastic reduction in the solid-water interfacial free energy of the sputtered polymer surfaces by physical and or chemical modification of their surfaces and thereby improving their biocompatibility is illustrated.