Time-dependent corrosion response of cobalt alloy implants to hyaluronic acid–phospholipid vesicles

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in orthopedic implants, where their long-term performance depends on maintaining corrosion resistance in the complex synovial joint environment. While phospholipids are well known for their lubricating properties, their direct influence on electrochemical reactivity of implant alloys remains largely unexplored. This study investigates the corrosion behavior of CoCrMo in a biomimetic medium containing hyaluronic acid-phospholipid vesicles, compared with a standard cell culture solution, across immersion times of 1, 5, 24, 120, and 168 hours. Electrochemical tests, including open circuit potential, anodic polarization, and electrochemical impedance spectroscopy, were complemented by scanning electron microscopy to evaluate surface changes. Results show that phospholipids exert a time-dependent, ambivalent effect: at short immersion times, vesicle adsorption perturbs the passive oxide film, reducing corrosion resistance; with longer exposures, vesicle self-assembly stabilizes the surface, increases capacitance, and enhances the protective nature of the passive layer. This study demonstrates that phospholipids at CoCrMo implant surfaces influence electrochemical behavior locally and dynamically rather than uniformly across the implant's surface. Clinically, these findings highlight that phospholipid adsorption may transiently limit corrosion and reduce metal ion release immediately after implantation, though mechanical loading could disrupt the protective coverage and promote localized tribocorrosion. These insights expand the understanding of implant-biomolecule interactions and suggest new directions for surface engineering and the design of advanced coatings or synthetic synovial fluids to improve implant longevity. STATEMENT OF SIGNIFICANCE: Phospholipids in synovial fluid are widely recognized for their lubricating role, yet their direct electrochemical interactions with metal alloys remain poorly understood. This work is the first to demonstrate the time-dependent, ambivalent influence of hyaluronic acid-phospholipid vesicles on the corrosion resistance of cobalt-chromium-molybdenum (CoCrMo) alloys. Unlike traditional corrosion studies that rely on simple saline or buffer solutions, our approach employs a biomimetic medium to capture clinically relevant interactions. The finding that phospholipids can both destabilize and later stabilize the alloy surface challenges current assumptions and opens a new research direction in implant science. This study provides mechanistic insights with broad implications for implant longevity, surface engineering, and the design of next-generation biomaterials.