In-vivo investigation of electrochemical behavior of Ti and CoCrMo alloy in human synovial fluids

With the development of modern medical techniques, the application of orthopedic implants has been continually increasing in recent years. Passive materials, such as Ti and its alloys as well as CoCrMo alloys, are widely used in artificial joints due to their excellent mechanical proper-ties, corrosion resistance and biocompatibility. Despite their outstanding performance, releasing harmful metal ions remains a critical issue due to tribocorrosion and corrosion of artificial joints operating in human synovial fluids. Although numerous research on the electrochemical behav-ior of metal in simulated body fluids has been conducted, their applicability to the complex hu-man body fluids was never validated. Indeed, the only systematic in-vivo corrosion investiga-tion has shown that simulated body fluids do not represent the body conditions. To understand the corrosion of metals in the human body, a study on the electrochemical behavior of metal directly in human synovial fluids is needed. Thus, this thesis aims to determine the electrochem-ical behavior of pure Ti and CoCrMo alloy in human synovial fluids and understand the corre-sponding mechanisms for a series of patients in different clinical states. To achieve this, a systematic experimental protocol was designed and validated to perform the electrochemical tests on a cohort of 154 patients. A wide palette of electrochemical techniques was used, including open circuit potential (OCP), potentiodynamic methods, electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). After the electrochemical testing, selected surfaces were analyzed using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spec-trometry (EDS) and Auger Electron Spectroscopy (AES). The same tests were conducted in a series of simulated body fluids to contribute to understanding the in-vivo results. The electrochemical results show that the corrosion behavior of Ti and CoCrMo in human syn-ovial fluids varies significantly with patients. The variability can be more than one order of magnitude for some crucial electrochemical parameters, such as corrosion current density. The electrochemical behavior of Ti surface expose to synovial fluids is significantly affected by sur-face roughness. The obtained corrosion rates of both materials correspond well with the metal ion release rates clinically detected in patients with hip and knee implants. For both materials, the corrosion behavior is likely controlled by the combined action of dissolved oxygen and the adsorption of organic molecules. No significant risk of galvanic corrosion between Ti and CoCrMo alloy in human synovial fluids is anticipated by the present results.