Whereas the bulk composition of a polymer generally governs its mechanical properties, its surface chemistry determines how it will interact with biological systems. Since the surfaces of biomedical polymers will be in contact with blood after implantation, control of surface properties is critical in order to prevent complications related to thrombosis and embolization.
PTG developed a novel technology called Surface-Modifying End Groups (SME™) to optimize the surface properties of polyurethanes to the specific physiological environment of the application. This method covalently bonds monofunctional surface-active end groups to polymer molecules during synthesis. For example, when a hydrophobic end group such as poly(dimethylsiloxane) (PDMS) is attached to a polyurethane, the final polymer self-assembles to yield a surface similar to silicone rubber. Hydrophobic surfaces are known to extend blood clotting time and reduce blood platelet adsorption and thrombosis in vivo. SFG validates that there is in fact a difference between the surface chemistry of polymers chemically modified with SME™ and those that are not. In a recent study that appeared in the Journal of the American Chemical Society, PTG's BioSpan® segmented polyurethane modified with PDMS end groups (BioSpan® S) was analyzed using SFG. The results confirmed that the lower surface tension component effectively segregates to the surface in order to minimize the surface free energy of the system. Even at extremely low concentrations, the more hydrophobic silicone tail dominates the surface of BioSpan® S.
For a biopolymer intended to be in contact with living tissues or blood, it is very important to know the structure of the surface in the hydrated state. Because of its ability to study polymers under hydrated conditions, SFG was employed for this purpose. It was demonstrated that aqueous environments stabilize different functional groups of the polymer, so it is important to study the surfaces under the same conditions that the polymer is expected to be employed.
SFG can be attempted on almost any system in which the surface or interface can be accessed by the light. Some key points to consider are: the surface must be ordered, the medium through which the light passes must not interfere appreciably with the light, and the surface or interface must be optically flat.
The first application of SFG was to study catalytic systems in which molecules were adsorbed onto single crystal surfaces. SFG is currently used to study electrochemical surfaces, such as the sub-stoichiometric oxide (sub-oxide) that forms at the interface between silicon and silicon dioxide (a critical performance element of microelectronic devices). Liquid-liquid (oil-water) interfaces are being observed at the University of Oregon using SFG, and the technique has also been employed to examine how polymer surfaces react to bulk mechanical testing. Even the polymer-polymer interface can be probed using SFG, provided one of the polymers is very thin (100 nm) in order to prevent complete absorption of the IR light, and that the materials have different indices of refraction so the light will reflect off the buried interface.