SELECTION AND EVALUATION OF BLOOD-COMPATIBLE AND TRIBOLOGICALLY COMPATIBLE JOURNAL BEARING MATERIALS

A critical issue in the Cleveland Clinic Foundation (CCF) Innovative V entricular Assist System (IVAS) blood pump is the selection of materia ls for the blood-lubricated journal bearing. Under normal operating co nditions, the journal bearing geometry creates a thick blood film that separates the rotating and stationary surfaces. However, since start- up and certain transients could cause temporary contact, the material pair selected for these surfaces must be both tribologically and blood compatible. Combinations of two biocompatible alloys were tested: a t itanium-zirconium-niobium alloy (Ti-13Zr-13Nb) and a zirconium-niobium alloy (Zr-2.5Nb). A standard pin-on-disk tester was used, with the co ntact surfaces lubricated by glycerol/saline mixtures simulating the v iscosity range of blood. One test series evaluated start-up conditions ; the other modeled a high-speed rub that might occur if the fluid fil m broke down. Results showed that the preoxidized Zr-2.5Nb pin/Ti-13Zr -13Nb disk combination was superior at all sliding velocities; a self- mated Zr-2.5Nb pair also showed promise. The oxide film on a self-mate d Ti-13Zr-13Nb pair, and a Ti-13Zr-13Nb pin and Zr-2.5Nb disk combinat ion did not show adequate wear life. More work remains to explain dist inct performance differences of certain combinations, with more data n eeded on mechanical properties of thin, hard coatings on softer metal substrates.