Computational modelling of magnesium stent mechanical performance in a remodelling artery: Effects of multiple remodelling stimuli

Composite polymer/wax coatings as a corrosion barrier of bioresorbable magnesium coronary stents

Magnesium and its alloys are suitable materials for biodegradable biomedical implants such as cardiovascular stents. Here we introduce an innovative composite polyelectrolyte multilayer/wax coating applied to commercial coronary Mg-based stents serving as a barrier layer effectively retarding corrosion. This hydrophobic coating, build by layer-by-layer technology, appeared very thin, smooth, homogeneous, strongly adherent and completely covering the surface of the Mg-stent. In-vitro degradation tests showed greater resistance to degradation of coated Mg-stents compared to uncoated and passivated ones. Cytocompatibility studies proved that Mg-stent coated with the composite coating was non-cytotoxic and improved fibroblast cell viability compared to the uncoated Mg-stent.

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.

Multiscale Modeling of Vascular Remodeling Induced by Wall Shear Stress

Objective

Hemodynamics-induced low wall shear stress (WSS) is one of the critical reasons leading to vascular remodeling. However, the coupling effects of WSS and cellular kinetics have not been clearly modeled. The aim of this study was to establish a multiscale modeling approach to reveal the vascular remodeling behavior under the interaction between the macroscale of WSS loading and the microscale of cell evolution.

Methods

Computational fluid dynamics (CFD) method and agent-based model (ABM), which have significantly different characteristics in temporal and spatial scales, were adopted to establish the multiscale model. The CFD method is for the second/organ scale, and the ABM is for the month/cell scale. The CFD method was used to simulate blood flow in a vessel and obtain the WSS in a vessel cross-section. The simulations of the smooth muscle cell (SMC) proliferation/apoptosis and extracellular matrix (ECM) generation/degradation in a vessel cross-section were performed by using ABM. During the simulation of the vascular remodeling procedure, the damage index of the SMC and ECM was defined as deviation from the obtained WSS. The damage index decreased gradually to mimic the recovery of WSS-induced vessel damage.

Results

(1) The significant wall thickening region was consistent with the low WSS region. (2) There was no evident change of wall thickness in the normal WSS region. (3) When the damage index approached to 0, the amount and distribution of SMCs and ECM achieved a stable state, and the vessel reached vascular homeostasis.

Conclusion

The established multiscale model can be used to simulate the vascular remodeling behavior over time under various WSS conditions.

EFFECTS OF COATING PROPERTIES ON CONTROLLED DELIVERY FROM AN EMBEDDED DRUG-ELUTING STENT: A SIMULATION STUDY

The present investigation deals with the effects of biodegradable, biodurable and polymer-free coating of a stent on the release mechanism of the drug in a porous medium. The Brinkman equations for the interstitial fluid, the unsteady convection-diffusion-reaction equation for the transport of free drug in the tissue and the unsteady reaction equations for the bound as well as the internalized drug have been considered. In the coating, the transport of drug has been modeled as a diffusion process. Effects of different percentages of the embedment, convection and various coating properties of the stent on the transport of free drug, its retention and the internalization of the bound drug have been studied. Immersed Boundary Method (IBM) in the staggered grid formulation (IBM-MAC) has been used to tackle numerically the system of nonlinear governing equations. Simulated results predict the fastest release of drug from a biodegradable coating, but the averaged concentrations of all drug forms do reach a quasi-steady state in case of a biodurable coating irrespective of the degrees of embedment. Moreover, for all embedment levels of the stent, a biodegradable coating is superior to that of biodurable and polymer-free coating in the presence/absence of convection for larger times, but this superiority is lost for smaller times. Unlike biodurable coating, it is also predicted that the more the embedment level does not necessarily imply the more the effectiveness of delivery for biodegradable and polymer-free coatings of a stent.

Do we really understand how drug eluted from stents modulates arterial healing?

The advent of drug-eluting stents (DES) has revolutionised the treatment of coronary artery disease. These devices, coated with anti-proliferative drugs, are deployed into stenosed or occluded vessels, compressing the plaque to restore natural blood flow, whilst simultaneously combating the evolution of restenotic tissue. Since the development of the first stent, extensive research has investigated how further advancements in stent technology can improve patient outcome. Mathematical and computational modelling has featured heavily, with models focussing on structural mechanics, computational fluid dynamics, drug elution kinetics and subsequent binding within the arterial wall; often considered separately. Smooth Muscle Cell (SMC) proliferation and neointimal growth are key features of the healing process following stent deployment. However, models which depict the action of drug on these processes are lacking. In this article, we start by reviewing current models of cell growth, which predominantly emanate from cancer research, and available published data on SMC proliferation, before presenting a series of mathematical models of varying complexity to detail the action of drug on SMC growth in vitro. Our results highlight that, at least for Sodium Salicylate and Paclitaxel, the current state-of-the-art nonlinear saturable binding model is incapable of capturing the proliferative response of SMCs across a range of drug doses and exposure times. Our findings potentially have important implications on the interpretation of current computational models and their future use to optimise and control drug release from DES and drug-coated balloons.

Mechanistic evaluation of long-term in-stent restenosis based on models of tissue damage and growth

Development and application of advanced mechanical models of soft tissues and their growth represent one of the main directions in modern mechanics of solids. Such models are increasingly used to deal with complex biomedical problems. Prediction of in-stent restenosis for patients treated with coronary stents remains a highly challenging task. Using a finite element method, this paper presents a mechanistic approach to evaluate the development of in-stent restenosis in an artery following stent implantation. Hyperelastic models with damage, verified with experimental results, are used to describe the level of tissue damage in arterial layers and plaque caused by such intervention. A tissue-growth model, associated with vessel damage, is adopted to describe the growth behaviour of a media layer after stent implantation. Narrowing of lumen diameter with time is used to quantify the development of in-stent restenosis in the vessel after stenting. It is demonstrated that stent designs and materials strongly affect the stenting-induced damage in the media layer and the subsequent development of in-stent restenosis. The larger the artery expansion achieved during balloon inflation, the higher the damage introduced to the media layer, leading to an increased level of in-stent restenosis. In addition, the development of in-stent restenosis is directly correlated with the artery expansion during the stent deployment. The correlation is further used to predict the effect of a complex clinical procedure, such as stent overlapping, on the level of in-stent restenosis developed after percutaneous coronary intervention.