A physical corrosion model for bioabsorbable metal stents

Strength and ductility loss of Magnesium-Gadolinium due to corrosion in physiological environment: Experiments and modeling

We propose a computational framework to study the effect of corrosion on the mechanical strength of magnesium (Mg) samples. Our work is motivated by the need to predict the residual strength of biomedical Mg implants after a given period of degradation in a physiological environment. To model corrosion, a mass-diffusion type model is used that accounts for localised corrosion using Weibull statistics. The overall mass loss is prescribed (e.g., based on experimental data). The mechanical behaviour of the Mg samples is modeled by a state-of-the-art Cazacu-Plunkett-Barlat plasticity model with a coupled damage model. This allowed us to study how Mg degradation in immersed samples reduces the mechanical strength over time. We performed a large number of in vitro corrosion experiments and mechanical tests to validate our computational framework. Our framework could predict both the experimentally observed loss of mechanical strength and the ductility due to corrosion for both tension and compression tests.

Phase-field modeling of pitting and mechanically-assisted corrosion of Mg alloys for biomedical applications

A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution, naturally predicting the transition from activation-controlled to diffusion-controlled bio-corrosion. In addition to uniform corrosion, the presented framework captures pitting corrosion and accounts for the synergistic effect of aggressive environments and mechanical loading in accelerating corrosion kinetics. The model applies to arbitrary 2D and 3D geometries with no special treatment for the evolution of the corrosion front, which is described using a diffuse interface approach. Experiments are conducted to validate the model and a good agreement is attained against in vitro measurements on Mg wires. The potential of the model to capture mechano-chemical effects during corrosion is demonstrated in case studies considering Mg wires in tension and bioabsorbable coronary Mg stents subjected to mechanical loading. The proposed methodology can be used to assess the in vitro and in vivo service life of Mg-based biomedical devices and optimize the design taking into account the effect of mechanical deformation on the corrosion rate. The model has the potential to advocate further development of Mg alloys as a biodegradable implant material for biomedical applications. STATEMENT OF SIGNIFICANCE: A physically-based model is developed to simulate the corrosion of bioabsorbable metals in environments that resemble biological fluids. The model captures pitting corrosion and incorporates the role of mechanical fields in enhancing the corrosion of bioabsorbable metals. Model predictions are validated against dedicated in vitro corrosion experiments on Mg wires. The potential of the model to capture mechano-chemical effects is demonstrated in representative examples. The simulations show that the presence of mechanical fields leads to the formation of cracks accelerating the failure of Mg wires, whereas pitting severely compromises the structural integrity of coronary Mg stents. This work extends phase-field modeling to bioengineering and provides a mechanistic tool for assessing the service life of bioabsorbable metallic biomedical devices.

Linking the effect of localised pitting corrosion with mechanical integrity of a rare earth magnesium alloy for implant use

This study presents a computational framework that investigates the effect of localised surface-based corrosion on the mechanical performance of a magnesium-based alloy. A finite element-based phenomenological corrosion model was used to generate a wide range of corrosion profiles, with subsequent uniaxial tensile test simulations to predict the mechanical response to failure. The python-based detection framework PitScan provides detailed quantification of the spatial phenomenological features of corrosion, including a full geometric tracking of corroding surface. Through this approach, this study is the first to quantitatively demonstrate that a surface-based non-uniform corrosion model can capture both the geometrical and mechanical features of a magnesium alloy undergoing corrosion by comparing to experimental data. Using this verified corrosion modelling approach, a wide range of corrosion scenarios was evaluated and enabled quantitative relationships to be established between the mechanical integrity and key phenomenological corrosion features. In particular, we demonstrated that the minimal cross-sectional area parameter was the strongest predictor of the remaining mechanical strength (R² = 0.98), with this relationship being independent of the severity or spatial features of localised surface corrosion. Interestingly, our analysis demonstrated that parameters described in ASTM G46-94 showed weaker correlations to the mechanical integrity of corroding specimens, compared to parameters determined by Pitscan. This study establishes new mechanistic insight into the performance of the magnesium-based materials undergoing corrosion.

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Corrosion profile (uniform/localised) generation with phenomenological degradation model.

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Automated phenomenological tracking of corrosion features by PitScan.

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Linking mechanical key parameters and geometrical corrosion features including in-vitro data.

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Identification of profile independent corrosion features and model fitting.

An enhanced phenomenological model to predict surface-based localised corrosion of magnesium alloys for medical use

This study developed an enhanced phenomenological model for the predictions of surface-based localised corrosion of magnesium alloys for use in medical applications. The modelling framework extended previous surface-based approaches by considering the role of β-phase components throughout the material volume to better predict spatial and temporal aspects of surface-based corrosion in magnesium alloys. This enhanced surface-based corrosion model offers many advantages as it (i) captures multi-directional pitting, (ii) captures various pit morphologies, (iii) eliminates mesh sizing effects, (iv) reduces computational cost through custom time controls (v) offers control of pit sizing and (vi) produces corrosion rates that are independent of pitting parameter values. The model was fully implemented in three dimensions within the finite element framework and shows excellent potential to enable robust predictions of the long-term performance of magnesium-based implants undergoing corrosion.

Finite Element Analysis of the Non-Uniform Degradation of Biodegradable Vascular Stents

Most of the studies on the finite element analysis (FEA) of biodegradable vascular stents (BVSs) during the degradation process have limited the accuracy of the simulation results due to the application of the uniform degradation model. This paper aims to establish an FEA model for the non-uniform degradation of BVSs by considering factors such as the dynamic changes of the corrosion properties and material properties of the element, as well as the pitting corrosion and stress corrosion. The results revealed that adjusting the corrosion rate according to the number of exposed surfaces of the element and reducing the stress threshold according to the corrosion status accelerates the degradation time of BVSs by 26% and 25%, respectively, compared with the uniform degradation model. The addition of the pitting model reduces the service life of the BVSs by up to 12%. The effective support of the stent to the vessel could reach at least 60% of the treatment effect before the vessel collapsed. These data indicate that the proposed non-uniform degradation model of BVSs with multiple factors produces different phenomena compared with the commonly used models and make the numerical simulation results more consistent with the real degradation scenario.

A review of current challenges and prospects of magnesium and its alloy for bone implant applications

Medical application materials must meet multiple requirements, and the designed implant must mimic the bone structure in shape and support the formation of bone tissue (osteogenesis). Magnesium (Mg) alloys, as a "smart" biodegradable material and as "the green engineering material in the twenty-first century", have become an outstanding bone implant material due to their natural degradability, smart biocompatibility, and desirable mechanical properties. Magnesium is recognised as the next generation of orthopaedic appliances and bioresorbable scaffolds. At the same time, improving the mechanical properties and corrosion resistance of magnesium alloys is an urgent challenge to promote the application of magnesium alloys. Nevertheless, the excessively quick deterioration rate generally results in premature mechanical integrity disintegration and local hydrogen build-up, resulting in restricted clinical bone restoration applicability. The condition of Mg bone implants is thoroughly examined in this study. The relevant approaches to boost the corrosion resistance, including purification, alloying treatment, surface coating, and Mg-based metal matrix composite, are comprehensively revealed. These characteristics are reviewed to assess the progress of contemporary Mg-based biocomposites and alloys for biomedical applications. The fabricating techniques for Mg bone implants also are thoroughly investigated. Notably, laser-based additive manufacturing fabricates customised forms and complicated porous structures based on its distinctive additive manufacturing conception. Because of its high laser energy density and strong controllability, it is capable of fast heating and cooling, allowing it to modify the microstructure and performance. This review paper aims to provide more insight on the present challenges and continued research on Mg bone implants, highlighting some of the most important characteristics, challenges, and strategies for improving Mg bone implants.

Simulation of corrosion and mechanical degradation of additively manufactured Mg scaffolds in simulated body fluid

A simulation strategy based in the finite element model was developed to model the corrosion and mechanical properties of biodegradable Mg scaffolds manufactured by laser power bed fusion after immersion in simulated body fluid. Corrosion was simulated through a phenomenological, diffusion-based model which can take into account pitting. The elements in which the concentration of Mg was below a certain threshold (representative of the formation of Mg(OH)₂) after the corrosion simulation were deleted for the mechanical simulations, in which Mg was assumed to behave as an isotropic, elastic-perfectly plastic solid and fracture was introduced through a ductile failure model. The parameters of the models were obtained from previous experimental results and the numerical predictions of the strength and fracture mechanisms of WE43 Mg alloy porous scaffolds in the as-printed condition and after immersion in simulated body fluid were in good agreement with the experimental results. Thus, the simulation strategy is able to assess the effect of corrosion on the mechanical behavior of biodegradable scaffolds, which is critical for design of biodegradable scaffolds for biomedical applications.

A Physical Approach to Simulate the Corrosion of Ceramic-Coated Magnesium Implants

Magnesium-based biodegradable materials are currently of great interest in various biomedical applications, especially those related to the treatment of bone trauma and the manufacturing of bone implants. Due to the complexity of the degradation process of magnesium, several numerical models were developed to help predict the change of the implant’s integrity in the body using in vitro tests. In this study, experimental in vitro tests and finite element methods are combined to calibrate a diffusion-based model of the uniform galvanic corrosion of high purity magnesium (HP-Mg). In addition, and for the first time, the impact of a porous coating layer generated by the Micro Arc oxidation (MAO) method is investigated and incorporated into the model. The calibrated model parameters are validated using the same immersion test conditions on a near-standard of treatment screws geometry made of HP-Mg.

Estimation of degradation velocity of biocompatible damaged stents due to blood flow

OBJECTIVE

Bioresorbable materials represent a promising technology for the treatment of coronary disease. Among the different materials employed, magnesium stents display favourable mechanical properties. One of the main uncertainties regarding use is their behaviour when deployed on coronary bifurcations, especially when their retardant coating has been damaged during the implantation process. This paper analyses the temporal evolution of the degradation of a damaged magnesium stent inserted into a coronary bifurcation.

METHODS

The rate of erosion-corrosion and the effect of the flow configuration on the mass transfer coefficient were estimated on the basis of previous experimental studies and numerical simulations. This coefficient has been employed to reproduce the conditions that can appear in real stent configurations, and computational fluid dynamics simulations were performed.

RESULTS

The diffusion coefficient for this particular case has been calculated from the mass transfer coefficient and the Sherwood number. The results of the simulation show how the presence of the inner artery wall has a positive effect, preventing a premature degradation of the stent, and how the distal strut is protected by the presence of the proximal struts.

CONCLUSIONS

This study demonstrates the usefulness of the proposed methodology to evaluate the temporal evolution of the degradation of struts made of magnesium alloys. In addition, this methodology can be applied to a study of different materials and geometric configurations.

SIGNIFICANCE

The proposed technique can contribute to expanding existing knowledge concerning bioresorbable stent flow-corrosion, thus improving their design and implantation.

Damage evolution of biodegradable magnesium alloy stent based on configurational forces

Magnesium alloy has attracted most of the recent attention as a candidate for stent material due to their biocompatible and biodegradable nature. However, their corrosion behavior in the human body is still a major issue in research today. In this paper, a strategy to simulate damage evolution in biodegradable magnesium alloy stent is given by introducing a configurational damage model. In the framework of continuum thermodynamics, one can characterize the development and evolution of local damage of materials by establishing internal variables in phenomenological method. We believe that corrosion can damage alloy in two different ways: surface corrosion and stress corrosion. Surface corrosion is described using uniform damage, when the structure is exposed in a corrosion environment; Configurational force is used to describe stress corrosion when the structure is exposed in a stimulating environment. We then select global damage and radial resistance force to perform the changes of macroscopic mechanical properties during stent degradation. Finally, the well performance of the proposed model is demonstrated through several numerical examples. This model has the potential to assist stent design and development in the future.

Determination of the Entire Stent Surface Area by a New Analytical Method

Stenting is a widely used treatment procedure for coronary artery disease around the world. Stents have a complex geometry, which makes the characterization of their corrosion difficult due to the absence of a mathematical model to calculate the entire stent surface area (ESSA). Therefore, corrosion experiments with stents are mostly based on qualitative analysis. Additionally, the quantitative analysis of corrosion is conducted with simpler samples made of stent material instead of stents, in most cases. At present, several methods are available to calculate the stent outer surface area (SOSA), whereas no model exists for the calculation of the ESSA. This paper presents a novel mathematical model for the calculation of the ESSA using the SOSA as one of the main parameters. The ESSA of seven magnesium alloy stents (MeKo Laser Material Processing GmbH, Sarstedt, Germany) were calculated using the developed model. The calculated SOSA and ESSA for all stents are 33.34%(±0.26%) and 111.86 mm (±0.85 mm), respectively. The model is validated by micro-computed tomography (micro-CT), with a difference of 12.34% (±0.46%). The value of corrosion rates calculated using the ESSA computed with the developed model will be 12.34% (±0.46%) less than that of using ESSA obtained by micro-CT.

Development of a micro-scale method to assess the effect of corrosion on the mechanical properties of a biodegradable Fe-316L stent material

The application of biodegradable materials to stent design has the potential to transform coronary artery disease treatment. It is critical that biodegradable stents have sustained strength during degradation and vessel healing to prevent re-occlusion. Proper assessment of the impact of corrosion on the mechanical behaviour of potential biomaterials is important. Investigations within literature frequently implement simplified testing conditions to understand this behaviour and fail to consider size effects associated with strut thickness, or the increase in corrosion due to blood flow, both of which can impact material properties. A protocol was developed that utilizes micro-scale specimens, in conjunction with dynamic degradation, to assess the effect of corrosion on the mechanical properties of a novel Fe-316L material. Dynamic degradation led to increased specimen corrosion, resulting in a greater reduction in strength after 48 h of degradation in comparison to samples statically corroded. It was found that thicker micro-tensile samples (h > 200 μm) had a greater loss of strength in comparison to its thinner counterpart (h < 200 μm), due to increased corrosion of the thicker samples (203 MPa versus 260 MPa after 48 h, p = 0.0017). This investigation emphasizes the necessity of implementing physiologically relevant testing conditions, including dynamic corrosion and stent strut thickness, when evaluating potential biomaterials for biodegradable stent application.

The degradable performance of bile-duct stent based on a continuum damage model: A finite element analysis

Biomedical magnesium alloy stents have become a hot bed of research focus in interventional therapy for nonvascular diseases. In this study, a numerical model for a balloon-expandable bile duct stent made of magnesium alloy with laser sculpture is developed to predict the effects of the degradation of the stent on the biomechanical behavior in the stent-bile duct coupling system. Based on a continuum damage model, the degradable model of the stent is built to understand its performance in an idealized bile duct as it is subject to corrosion over time. The degradation model developed in this study addresses the uniform corrosion and pitting corrosion. By means of the secondary development function of commercial numerical software ANSYS, the finite element analysis procedures were written to control the degradation process based on the technology of element "birth and death," and it is shown how the three-dimensional model and approach give the possibility of analyzing for the degradation mechanism of a magnesium alloy stent in the bile duct or other nonvascular cavities.

Analysis of Vascular Mechanical Characteristics after Coronary Degradable Stent Implantation

PURPOSE

To explore the effect of vascular stress changes on endothelial function recovery and vascular restenosis inhibition, under the condition of dynamic degradation process of the degradable stent.

METHODS

Fitting the material parameters of the hyperelastic vascular constitutive relationship, the stress distribution of the intima of the blood vessel before the stent was implanted and during the dynamic degradation was calculated by numerical simulation. In vitro culture experiments were carried out, and the stretch ratios of the silicone chamber were set to 0%, 5%, 10%, and 15%, respectively, to simulate the effects of different degradation stages on the growth state of endothelial cells.

RESULTS

After the stent was completely degraded, the circumferential intimal stress (strain) of the vessel was recovered to 0.137 MPa, 5.5%, which was close to the physiological parameters (0.122 MPa, 4.8%) before stent implantation. In vitro experiments showed that the endothelial cell survival rate was the highest under the condition of circumferential stress (strain) of 0.1 MPa, 5%, and all adhesion growth could be achieved.

CONCLUSIONS

With the occurrence of degradation process of the stent, the circumferential stress (strain) of the intima was recovered to a range close to physiological parameters, which promotes the growth of endothelial cells. The recovery of intimal function can effectively inhibit the process of vascular restenosis. The results can provide a theoretical basis and experimental platform for the study of coronary intervention for the treatment of vascular restenosis.

In vivo and in vitro evaluation of a biodegradable magnesium vascular stent designed by shape optimization strategy

The performance of biodegradable magnesium alloy stents (BMgS) requires special attention to non-uniform residual stress distribution and stress concentration, which can accelerate localized degradation after implantation. We now report on a novel concept in stent shape optimization using a finite element method (FEM) toolkit. A Mg-Nd-Zn-Zr alloy with uniform degradation behavior served as the basis of our BMgS. Comprehensive in vitro evaluations drove stent optimization, based on observed crimping and balloon inflation performance, measurement of radial strength, and stress condition validation via microarea-XRD. Moreover, a Rapamycin-eluting polymer coating was sprayed on the prototypical BMgS to improve the corrosion resistance and release anti-hyperplasia drugs. In vivo evaluation of the optimized coated BMgS was conducted in the iliac artery of New Zealand white rabbit with quantitative coronary angiography (QCA), optical coherence tomography (OCT) and micro-CT observation at 1, 3, 5-month follow-ups. Neither thrombus or early restenosis was observed, and the coated BMgS supported the vessel effectively prior to degradation and allowed for arterial healing thereafter. The proposed shape optimization framework based on FEM provides an novel concept in stent design and in-depth understanding of how deformation history affects the biomechanical performance of BMgS. Computational analysis tools can indeed promote the development of biodegradable magnesium stents.

A numerical corrosion-fatigue model for biodegradable Mg alloy stents

Biodegradable magnesium alloys have attracted research interest as matrix materials for next-generation absorbable metallic coronary stents. Subject to cyclic stresses, magnesium alloy stents (MAS) are prone to premature failures caused by corrosion fatigue damage. This work aimed to develop a numerical continuum damage mechanics model, implemented with the finite element method, which can account for the corrosion fatigue of Mg alloys and the applications in coronary stents. The parameters in the resulting phenomenological model were calibrated using our previous experimental data of HP-Mg and WE43 alloy and then applied in assessing the performance of the MAS. The results indicated that it was valid to predict the degradation rate, the damage-induced reduction of the radial stiffness, and the critical location of the MAS. Furthermore, this model and the numerical procedure can be easily adapted for other biodegradable alloy systems, for instance, Fe and Zn, and used to achieve the optimal degradation rate while improving fatigue endurance. STATEMENT OF SIGNIFICANCE: Subject to cyclic stresses, magnesium alloy stents are prone to premature failures caused by corrosion fatigue damage. This work aimed to develop a numerical continuum damage mechanics model, implemented with the finite element method, which can account for the corrosion fatigue of Mg alloys and the applications in coronary stents. The results indicated that it was valid to predict the degradation rate, damage-induced reduction of the radial stiffness, and the critical location of the Mg alloy stent; therefore, these stents can be easily adapted to other biodegradable alloy systems such as Fe and Zn.

Continuum Modeling and Simulation in Bone Tissue Engineering

Bone tissue engineering is currently a mature methodology from a research perspective. Moreover, modeling and simulation of involved processes and phenomena in BTE have been proved in a number of papers to be an excellent assessment tool in the stages of design and proof of concept through in-vivo or in-vitro experimentation. In this paper, a review of the most relevant contributions in modeling and simulation, in silico, in BTE applications is conducted. The most popular in silico simulations in BTE are classified into: (i) Mechanics modeling and scaffold design, (ii) transport and flow modeling, and (iii) modeling of physical phenomena. The paper is restricted to the review of the numerical implementation and simulation of continuum theories applied to different processes in BTE, such that molecular dynamics or discrete approaches are out of the scope of the paper. Two main conclusions are drawn at the end of the paper: First, the great potential and advantages that in silico simulation offers in BTE, and second, the need for interdisciplinary collaboration to further validate numerical models developed in BTE.

Effect of Temperature on the Corrosion Behavior of Biodegradable AZ31B Magnesium Alloy in Ringer’s Physiological Solution

In this work, the corrosion behaviors of the AZ31B alloy in Ringer’s solution at 20 °C and 37 °C were compared over four days to better understand the influence of temperature and immersion time on corrosion rate. The corrosion products on the surfaces of the AZ31B alloys were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) provided information about the protective properties of the corrosion layers. A significant acceleration in corrosion rate with increasing temperature was measured using mass loss and evolved hydrogen methods. This temperature effect was directly related to the changes in chemical composition and thickness of the Al-rich corrosion layer formed on the surface of the AZ31B alloy. At 20 °C, the presence of a thick (micrometer scale) Al-rich corrosion layer on the surface reduced the corrosion rate in Ringer’s solution over time. At 37 °C, the incorporation of additional Mg and Al compounds containing Cl into the Al-rich corrosion layer was observed in the early stages of exposure to Ringer’s solution. At 37 °C, a significant decrease in the thickness of this corrosion layer was noted after four days.

Mechanical analysis of a novel biodegradable zinc alloy stent based on a degradation model

BACKGROUND

Biodegradable stents display insufficient scaffold performance due to their poor Young's Modulus. In addition, the corresponding biodegradable materials harbor weakened structures during degradation processes. Consequently, such stents have not been extensively applied in clinical therapy. In this study, the scaffold performance of a patented stent and its ability to reshape damaged vessels during degradation process were evaluated.

METHODS

A common stent was chosen as a control to assess the mechanical behavior of the patented stent. Finite element analysis was used to simulate stent deployment into a 40% stenotic vessel. A material corrosion model involving uniform and stress corrosion was implemented within the finite element framework to update the stress state following degradation.

RESULTS

The results showed that radial recoiling ratio and mass loss ratio of the patented stent is 7.19% and 3.1%, respectively, which are definitely lower than those of the common stent with the corresponding values of 22.6% and 14.1%, respectively. Moreover, the patented stent displayed stronger scaffold performance in a corrosive environment and the plaque treated with patented stents had a larger and flatter lumen.

CONCLUSION

Owing to its improved mechanical performance, the novel biodegradable zinc alloy stent reported here has high potential as an alternative choice in surgery.

Magnesium degradation under physiological conditions - Best practice

This review focusses on the application of physiological conditions for the mechanistic understanding of magnesium degradation. Despite the undisputed relevance of simplified laboratory setups for alloy screening purposes, realistic and predictive in vitro setups are needed. Due to the complexity of these systems, the review gives an overview about technical measures, defines some caveats and can be used as a guideline for the establishment of harmonized laboratory approaches.

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Physiological conditions are mandatory for mechanistic understanding of magnesium degradation.

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Guidelines and caveats for experimental setups are reviewed.

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Media composition is essential for reliable experiments.

A strain-mediated corrosion model for bioabsorbable metallic stents

This paper presents a strain-mediated phenomenological corrosion model, based on the discrete finite element modelling method which was developed for use with the ANSYS Implicit finite element code. The corrosion model was calibrated from experimental data and used to simulate the corrosion performance of a WE43 magnesium alloy stent. The model was found to be capable of predicting the experimentally observed plastic strain-mediated mass loss profile. The non-linear plastic strain model, extrapolated from the experimental data, was also found to adequately capture the corrosion-induced reduction in the radial stiffness of the stent over time. The model developed will help direct future design efforts towards the minimisation of plastic strain during device manufacture, deployment and in-service, in order to reduce corrosion rates and prolong the mechanical integrity of magnesium devices.

STATEMENT OF SIGNIFICANCE

The need for corrosion models that explore the interaction of strain with corrosion damage has been recognised as one of the current challenges in degradable material modelling (Gastaldi et al., 2011). A finite element based plastic strain-mediated phenomenological corrosion model was developed in this work and was calibrated based on the results of the corrosion experiments. It was found to be capable of predicting the experimentally observed plastic strain-mediated mass loss profile and the corrosion-induced reduction in the radial stiffness of the stent over time. To the author's knowledge, the results presented here represent the first experimental calibration of a plastic strain-mediated corrosion model of a corroding magnesium stent.

Computational Modeling of the Mechanical Performance of a Magnesium Stent Undergoing Uniform and Pitting Corrosion in a Remodeling Artery

Coronary stents made from degradable biomaterials such as magnesium alloy are an emerging technology in the treatment of coronary artery disease. Biodegradable stents provide mechanical support to the artery during the initial scaffolding period after which the artery will have remodeled. The subsequent resorption of the stent biomaterial by the body has potential to reduce the risk associated with long-term placement of these devices, such as in-stent restenosis, late stent thrombosis, and fatigue fracture. Computational modeling such as finite-element analysis has proven to be an extremely useful tool in the continued design and development of these medical devices. What is lacking in computational modeling literature is the representation of the active response of the arterial tissue in the weeks and months following stent implantation, i.e., neointimal remodeling. The phenomenon of neointimal remodeling is particularly interesting and significant in the case of biodegradable stents, when both stent degradation and neointimal remodeling can occur simultaneously, presenting the possibility of a mechanical interaction and transfer of load between the degrading stent and the remodeling artery. In this paper, a computational modeling framework is developed that combines magnesium alloy degradation and neointimal remodeling, which is capable of simulating both uniform (best case) and localized pitting (realistic) stent corrosion in a remodeling artery. The framework is used to evaluate the effects of the neointima on the mechanics of the stent, when the stent is undergoing uniform or pitting corrosion, and to assess the effects of the neointimal formation rate relative to the overall stent degradation rate (for both uniform and pitting conditions).

Mathematical modelling of the degradation behaviour of biodegradable metals

A mathematical model for the biodegradation of magnesium is developed in this study to inspect the corrosion behaviour of biodegradable implants. The aim of this study was to provide a suitable framework for the assessment of the corrosion rate of magnesium which includes the process of formation/dissolution of the protective film. The model is intended to aid the design of implants with suitable geometries. The level-set method is used to follow the changing geometry of the implants during the corrosion process. A system of partial differential equations is formulated based on the physical and chemical processes that occur at the implant-medium boundary in order to simulate the effect of the formation of a protective film on the degradation rate. The experimental data from the literature on the corrosion of a high-purity magnesium sample immersed in simulated body fluid is used to calibrate the model. The model is then used to predict the degradation behaviour of a porous orthopaedic implant. The model successfully reproduces the precipitation of the corrosion products on the magnesium surface and the effect on the degradation rate. It can be used to simulate the implant degradation and the formation of the corrosion products on the surface of biodegradable magnesium implants with complex geometries.

Absorbable magnesium-based stent: physiological factors to consider for in vitro degradation assessments

Absorbable metals have been widely tested in various in vitro settings using cells to evaluate their possible suitability as an implant material. However, there exists a gap between in vivo and in vitro test results for absorbable materials. A lot of traditional in vitro assessments for permanent materials are no longer applicable to absorbable metallic implants. A key step is to identify and test the relevant microenvironment and parameters in test systems, which should be adapted according to the specific application. New test methods are necessary to reduce the difference between in vivo and in vitro test results and provide more accurate information to better understand absorbable metallic implants. In this investigative review, we strive to summarize the latest test methods for characterizing absorbable magnesium-based stent for bioabsorption/biodegradation behavior in the mimicking vascular environments. Also, this article comprehensively discusses the direction of test standardization for absorbable stents to paint a more accurate picture of the in vivo condition around implants to determine the most important parameters and their dynamic interactions.