Surface chemical evaluation of thromboresistant materials before and after venous implantation

Candidate biomedical materials were implanted as standard rings (0.7 cm. I.D. × 1 cm. length) in the canine inferior vena cava for evaluation of thrombogenicity. Rings showing suggestive thromboresistance, defined as having zero or minimal thrombus after a 2 hr. implantation, were carefully examined by surface chemical methods to learn which initial surface properties, or changes of these properties in vivo, may correlate with thromboresistance. Before implantation, ring inner surfaces were characterized both by contact angle measurements, which reflect the material’s “wettability” and “critical surface tension”, and by MAIR (multiple attenuated internal reflection) infrared spectra which revealed the chemical constitution of the material’s interfacial zone. After implantation, these same methods revealed the presence of adsorbed organic films and new outermost atomic constitution which had become the actual thromboresistant surface. Contact angle experiments at each stage also determined relative cleanliness of surfaces, presence of transferable surface-active agents, and desorbability of deposited organic films. These surface chemical studies suggest that 3 different mechanisms can tentatively be correlated with thromboresistance of various materials: 1) Intrinsically low-energy surfaces, such as obtained with tallow-polished Stellite, apparently resist thrombus formation because of an erosion- resistant, biocompatible coating of closely packed methyl groups; 2) Intrinsically high-energy surfaces, such as diamond-polished carbon, electrets, and glow-discharged-treated metals rapidly adsorb thin protein films from blood which convert their surfaces to a thromboresistant low-energy character; 3) Materials with desorbable surface-active agents (surfactants, heparin, albumin, etc.) will slowly and continuously leach into adjacent liquid phases and thus shed thrombus precursors as well. In all 3 instances, a proteinaceous layer usually deposits on implanted materials rapidly and spontaneously, and it is this layer which determines subsequent substrate reactivity to blood. Thus, ultimate thromboresistance or thrombogenicity of candidate biomaterials must be intimately related to the nature of the “conditioning” layer of protein initially adsorbed.