Finite Element and Density Functional Theory Modeling Effectively Predict Pitting Degradation of Hydroxyapatite-Coated Pure Magnesium

The emergence of degradable orthopedic implants for fracture fixation may abrogate the need for implant removal surgery and minimize pain associated with permanent implants. Magnesium (Mg) and its alloys are being explored as a biomaterial for degradable implants due to mechanical properties similar to those of bone. Previous in vitro studies have determined the degradation rate of pure Mg to be relatively fast when compared to bone regeneration. Hydroxyapatite (HA), the mineral component of bone, may serve as a surface coating on Mg-based implants to effectively slow and control the degradation rate. The objective of this work was to develop and implement a finite element (FE) model that utilizes a damage evolution law for pitting corrosion to predict the degradation of pure Mg (non-coated) and HA-coated pure Mg (coated) materials simulated in physiological conditions. Finite element analysis (FEA) was performed on a cylindrical Mg specimen (25.4 mm diameter, 8 mm height) through Abaqus/Standard software to incrementally monitor the damage value of each Mg element and subsequently delete fully-degraded elements from the simulation. A Fortran user-material (UMAT) subroutine assigned each element a pitting parameter, controlling the rate of degradation throughout the simulation and providing necessary inputs of elastic material properties and degradation model parameters for pure Mg and HA into Abaqus. The simulations allowed for the visualization of both pure Mg and HA-coated pure Mg degradation over a 120-day period, displaying expected degradation trends such as lower corrosion rates for HA-coated Mg and degradation propagating from the edges inward. Simulation results were calibrated with our prior results from a 30-day experimental degradation study via direct comparison with mass loss over time. Additionally, lower length scale, density functional theory (DFT) simulations were performed to provide physical meaning for the model pitting parameter. The FE simulation was extended to model resin-enclosed pure Mg and HA-coated pure Mg degradation, where only the top surface of the specimen was exposed to the corrosion surface, for investigating changes in Mg surface roughness (height) over time. The impacts of this work include the establishment of a computational model of pure Mg and HA-coated pure Mg degradation calibrated using in vitro degradation data to advance the use of Mg-based biomaterials, and more broadly, to predict degradation rates of next-generation orthopedic implants.