Impact of pH and chloride content on the biodegradation of magnesium alloys for medical implants: An in vitro and phase-field study

TL;DR

This work decouples the separate effects of pH and chloride content on the corrosion of bioabsorbable Mg alloys using targeted in vitro tests and a variationally consistent diffuse-interface (phase-field) model. The experiments show pH is the dominant driver of corrosion while Cl$^-$ concentration within biomedical ranges has negligible impact on corrosion rates, Mg$^{2+}$ release, or pH evolution. The phase-field framework integrates pH, chloride dependence via the Mg(OH)$_2$ solubility product, and mechano-electrochemical coupling to predict degradation under buffer-free and buffer-regulated conditions, validated against in vitro data. Practical case studies on plates, screws, and porous bone scaffolds demonstrate how mechanical loading accelerates localized degradation and reduces implant lifespan, highlighting the need to account for mechanics in design. The approach offers a cost-effective tool to assess in vitro and in vivo service life of Mg-based biomedical devices and can guide topology optimization to synchronize degradation with bone growth.

Abstract

The individual contributions of pH and chloride concentration to the corrosion kinetics of bioabsorbable magnesium (Mg) alloys remain unresolved despite their significant roles as driving factors in Mg corrosion. This study demonstrates and quantifies hitherto unknown separate effects of pH and chloride content on the corrosion of Mg alloys pertinent to biomedical implant applications. The experimental setup designed for this purpose enables the quantification of the dependence of corrosion on pH and chloride concentration. The in vitro tests conclusively demonstrate that variations in chloride concentration, relevant to biomedical applications, have a negligible effect on corrosion kinetics. The findings identify pH as a critical factor in the corrosion of bioabsorbable Mg alloys. A variationally consistent phase-field model is developed for assessing the degradation of Mg alloys in biological fluids. The model accurately predicts the corrosion performance of Mg alloys observed during the experiments, including their dependence on pH and chloride concentration. The capability of the framework to account for mechano-chemical effects during corrosion is demonstrated in practical orthopaedic applications considering bioabsorbable Mg alloy implants for bone fracture fixation and porous scaffolds for bone tissue engineering. The strategy has the potential to assess the in vitro and in vivo service life of bioabsorbable Mg-based biomedical devices.

Figures (12)

• Figure 1: Schematic illustration of the experimental setup for the Mg alloy wires immersed in different NaCl solutions.
• Figure 2: Underlying electrochemistry and diffuse interface description of the liquid (biological fluid $\phi$ = 0) and solid (biodegradable Mg alloy $\phi = 1$) phases.
• Figure 3: Dependence of (a) solubility product constant $K_\mathrm{sp}$ and (b) chemical stability diagram on the concentration of chloride ions. The solubility experimental data in (a) is taken from Ref. Lorimer1992. The light grey and blue areas stand for the range of chloride ion concentration used in the present study and in common corrosive media for Mg corrosion tests for biomedical applications MEI2020. $[\text{Mg}^{2+}]$ denotes the molar concentration of Mg$^{2+}$ ions.
• Figure 4: Schematic representation of the experimental setup (left) and the corresponding nondimensional computational domain (right) for the Mg alloy wire immersed in different NaCl solutions.
• Figure 5: Comparison between experimental measurements and phase-field predictions for (a) mass loss, (b) mass loss after one day of immersion, (c) average concentration of Mg ions in solution, and (d) bulk pH. The light grey area stands for the standard deviation of the experiments considering data of all three NaCl solutions. The legends in the insets in (c) and (d) apply to the whole computational domain. The white point and grey area in the insets in (c) and (d) stand for the center and final cross-section of the Mg wire after degradation.