Voltage and Wear Debris from Ti-6Al-4V Interact to Affect Cell Viability During In-Vitro Fretting Corrosion

Date of Award

May 2014

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biomedical and Chemical Engineering

Advisor(s)

Jeremy L. Gilbert

Keywords

Cell Viability, Fretting Corrosion, Reduction Reactions, Ti-6Al-4V, Voltage, Wear Debris

Subject Categories

Engineering

Abstract

Fretting crevice corrosion (or mechanically assisted crevice corrosion) is one of the principal mechanisms of degradation of modular designs of orthopedic implants. Titanium–based, cobalt–based and iron–based materials, which owe their resistance to corrosion to the presence of oxide films, are widely used in the orthopedic industry. Fretting of these oxide film–covered alloys abrades the oxide film, which then reforms by electrochemical processes. Two consequences of fretting crevice corrosion are the release of particles and ions as corrosion by–products, and the negative excursion of the voltage of the implant which may, in turn, lead to more deleterious results for the underlying metal and the biological system. Though there are many studies investigating the effects of wear particles and ions on the biological system, the mechanisms of fretting corrosion, and the adverse effects on cells cultured on metals below a threshold potential of −400 mV (vs. Ag/AgCl), there are few studies focused on the direct effects of fretting corrosion on cells in-vitro. The goal of this work is to develop an in-vitro fretting corrosion cell culture test system and to investigate how voltage and wear debris from fretting corrosion interact to affect cell viability. Ti–6Alndash;4V alloy was used as the alloy surface, and MC3T3 prendash;osteoblast cells were investigated in this study. MC3T3 cells were directly plated on the metal surface adjacent to where the fretting corrosion took place. The potential excursion and currents resulting from fretting the alloy with a glass pin were monitored during the experiment or the potential was fixed potentiostatically above −400 mV (−300 mV and −50 mV vs. Ag/AgCl) during fretting. The cell viability was analyzed using the Live / Dead cell assay and the cell morphology was observed using Scanning Electron Microscopy (SEM). The result of this study shows fretting crevice corrosion can statistically significantly decrease the cell viability from 99.1% (no fretting) to 0.5% when the potential from fretting corrosion drops to around −1 V (vs. Ag/AgCl). Under the same fretting corrosion conditions, potentiostatically holding the Ti–6Al–4V sample surface potential to −300 mV or −50 mV (vs. Ag/AgCl), the cell viability increases to 70 % and 38%, respectively. From statistical analysis, cell viability in −300 mV and −50 mV groups, the above–mentioned no fretting group and the fretting but non–potentiostatically held groups are all significantly different with each other (p<0.05). The results indicate that cathodic potential excursions and the wear debris resulting from fretting corrosion both have effects on cell viability, however, the cathodic potential excursions play a more significant role. When the potential remained below the threshold potential for MC3T3 cells during fretting, cells viability dramatically drops to 0.5% compared to 99% of the control group (No fretting, above threshold). While wear particles were observed in all groups of fretting experiments, viable cells were seen adjacent to those wear particles above the threshold potential where 70% or 38% of the cells survived. The current density experienced by the alloys when holding the potential at −300 mV and −50 mV was also monitored. The fretting current density was significantly different between these two groups in the first 1500 seconds (8.53 µA/cm2 for −300 mV and 18.9 µA/cm2 for −50 mV). However, during the rest of the 13 hrs of the test, the fretting current density decreased to 0.737 µA/cm2 and 0.787 µA/cm2 and there was no significant difference between current densities in these two groups. This in-vitro test method is capable of assessing the interactions between cells and fretting corrosion processes and may provide insight into biological processes associated with fretting corrosion.

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