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The passive dissolution kinetics of titanium were measured in various media to establish the mechanisms controlling the phenomenon. Titanium fibers were immersed in three different media: simulated interstitial electrolyte (SIE), human serum in SIE (serum/SIE), and 8.0 mM ethylenediaminetetraacetic acid in simulated interstitial electrolyte (EDTA). The concentration of titanium released as a function of immersion time was measured using electrothermal atomic absorption spectrophotometry. Dissolution and surface chemistry measurements were also made using titanium thin films in EDTA. Changes in the oxide surface chemistry of the fibers and films were determined in parallel with the dissolution measurements using Auger electron spectroscopy (AES) and X-Ray photoelectron spectroscopy (XPS). Solution ligands enhanced the magnitude of dissolution, with EDTA > serum/SIE > SIE. The oxide surface chemistry became hydroxylated and incorporated elements from the electrolyte (P, Ca) as a function of time. The surface chemistry changes were most pronounced during the first 10 days of immersion, where the oxide thickness increased most rapidly, and anions and cations exchanged with acidic and basic hydroxyl groups on the oxide surface. To reflect these changes in surface chemistry, the dissolution kinetics were empirically fit by a two-phase model. The first phase (t < 10 d) was dictated by equilibration of the surface with the solution, and the second phase (t > 15 d) by mass diffusion obeying Fickian kinetics. The relevance of these mechanisms for passive dissolution in clinical situations is addressed.
titanium, passive dissolution, oxide chemistry, surface chemistry, proteins
Associate Professor, Northwestern University, Chicago, IL
Professor, University of Pennsylvania, Philadelphia, PA