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Implantable titanium feedthrough reliability is characterized by hermetic integrity, optimum mechanical design, and critical process requirements.
Hermetic integrity of a titanium electrical feedthrough depends on the coefficients of linear thermal expansion (CLTE) of the titanium, the sealing material, and the center conductor metal and on the degree of bonding obtained at seal-to-metal interfaces. Microscopic examination of the bond interface between titanium and the sealing material indicated fusion of the titanium oxide with the polycrystalline ceramic. Dilatometer measurements of titanium, a polycrystalline ceramic, and pure platinum (center conductor metal) showed similar CLTE for titanium and platinum, whereas the polycrystalline ceramic CLTE was less, thus ensuring some degree of compression in the sealing mechanism.
Optimum mechanical design was determined by thermal stress methods, which stressed designs through increasing thermal shocks until loss of hermeticity occurred in 50 percent or more of the specimen population. Two designs, one with mechanical reinforcement and the other without, were subjected to thermal shock ranges as severe as 755 K (900°F) to 78 K (−320°F). The plotted data indicated the design with mechanical reinforcement sustained higher levels of thermal shock without loss of hermeticity.
In vitro experiments on titanium feedthroughs with various surface configurations were conducted at low applied voltages (constant and pulse). The data indicated minimal electrochemical degradation at surfaces with maximum electrical leakage paths obtained by addition of ceramic standoff. Electrochemical degradation was measured in terms of insulation resistance between the titanium housing and the platinum pin and correlated to the accelerated battery depletion of a cardiac pulse generator. The battery life was observed to decrease rapidly as the electrical resistance across the titanium feedthrough decreased from 100 000 Ω.
Certain levels of temperature were shown to affect the grain size of titanium. Titanium (Grade 4) feedthrough housings were exposed to different temperature levels for the same 1-h duration. Scanning electron microscopy (SEM) photomicrographs verified by ASTM grain size determinations indicated substantial grain growth above the 1255 K (1800°F) temperature level. Conservative sealing conditions were recommended at less than the titanium beta transus temperature, 1158 K (1625°F), and less than 1-h exposure.
feedthrough, hermetic integrity, optimum mechanical design, linear thermal expansion coefficients, bond interface, Kryoflex, polycrystalline ceramic, platinum, thermal aging, electrochemical degradation, ceramic standoff, accelerated battery depletion, beta transus temperature, implant materials, titanium
Vice president of engineering, Kyle Technology Corp., Medical Components Div., Roseburg, Ore.