Micro-scale testing and micromechanical modelling for high cycle fatigue of CoCr stent material.

This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material. Micro-scale specimens, representative of stent struts, are manufactured via laser micro-machining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterisation of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments.

Computational micromechanics of bioabsorbable magnesium stents.

Magnesium alloys are a promising candidate material for an emerging generation of absorbable metal stents. Due to its hexagonal-close-packed lattice structure and tendency to undergo twinning, the deformation behaviour of magnesium is quite different to that of conventional stent materials, such as stainless steel 316L and cobalt chromium L605. In particular, magnesium exhibits asymmetric plastic behaviour (i.e. different yield behaviours in tension and compression) and has lower ductility than these conventional alloys. In the on-going development of absorbable metal stents it is important to assess how the unique behaviour of magnesium affects device performance. The mechanical behaviour of magnesium stent struts is investigated in this study using computational micromechanics, based on finite element analysis and crystal plasticity theory. The plastic deformation in tension and bending of textured and non-textured magnesium stent struts with different numbers of grains through the strut dimension is investigated. It is predicted that, unlike 316L and L605, the failure risk and load bearing capacity of magnesium stent struts during expansion is not strongly affected by the number of grains across the strut dimensions; however texturing, which may be introduced and controlled in the manufacturing process, is predicted to have a significant influence on these measures of strut performance.

A physical corrosion model for bioabsorbable metal stents.

Absorbable metal stents (AMSs) are an emerging technology in the treatment of heart disease. Computational modelling of AMS performance will facilitate the development of this technology. In this study a physical corrosion model is developed for AMSs based on the finite element method and adaptive meshing. The model addresses a gap between currently available phenomenological corrosion models for AMSs and physical corrosion models that have been developed for more simple geometries than those of a stent. The model developed in this study captures the changing surface of a corroding three-dimensional AMS structure for the case of diffusion-controlled corrosion. Comparisons are made between model predictions and those of previously developed phenomenological corrosion models for AMSs in terms of predicted device geometry and mechanical performance during corrosion. Relationships between alloy solubility and diffusivity in the corrosion environment and device performance during corrosion are also investigated.

Influence of statistical size effects on the plastic deformation of coronary stents.

The dimensions of coronary stent struts are similar to those of the metallic grains of their constituent alloys. This means that statistical size effects (SSEs), which are evident in polycrystals with few grains through their dimensions, can have detrimental effects on the mechanical performance of stent struts undergoing large plastic deformation. Current trends in coronary stent design are towards thinner struts, potentially increasing the influence of SSEs. In order to maintain adequate device performance with decreasing strut thickness, it is therefore important to assess the role of SSEs in the plastic deformation of stents. In this study, finite element modelling and crystal plasticity theory are used to investigate SSEs in the deformation of struts in tension and bending. The relationships between SSEs and microstructure morphology, alloy strain hardening behaviour and secondary phases are also investigated. It is predicted that reducing the number of grains through the strut cross section and increasing the number of grains along the strut length have detrimental effects on mechanical performance. The magnitudes of these effects are predicted to be independent of the uniformity of the studied microstructures, but dependent on alloy strain hardening behaviour. It is believed that model predictions will aid in identifying a lower bound on suitable strut thicknesses in coronary stents for a range of alloys and microstructures.

Comparing coronary stent material performance on a common geometric platform through simulated bench testing.

Absorbable metallic stents (AMSs) are a newly emerging cardiovascular technology which has the potential to eliminate long-term patient health risks associated with conventional permanent stents. AMSs developed to date have consisted of magnesium alloys or iron, materials with inferior mechanical properties to those used in permanent stents, such as stainless steel and cobalt-chromium alloys. However, for AMSs to be feasible for widespread clinical use it is important that their performance is comparable to modern permanent stents. To date, the performances of magnesium, iron, and permanent stent materials have not been compared on a common stent platform for a range of stent performance metrics, such as flexibility, radial strength, and recoil. In this study, this comparison is made through simulated bench testing, based on finite-element modelling. The significance of this study is that it allows potential limitations in current AMS performance to be identified, which will aid in focusing future AMS design. This study also allows the identification of limitations in current AMS materials, thereby informing the on-going development of candidate biodegradable alloys. The results indicate that the AMSs studied here can match the recoil characteristics and radial strength of modern permanent stents; however, to achieve this, larger strut dimensions are required. It is also predicted that the AMSs studied are inferior to permanent stents in terms of maximum absolute curvature and longitudinal stiffness.

A corrosion model for bioabsorbable metallic stents.

In this study a numerical model is developed to predict the effects of corrosion on the mechanical integrity of bioabsorbable metallic stents. To calibrate the model, the effects of corrosion on the integrity of biodegradable metallic foils are assessed experimentally. In addition, the effects of mechanical loading on the corrosion behaviour of the foil samples are determined. A phenomenological corrosion model is developed and applied within a finite element framework, allowing for the analysis of complex three-dimensional structures. The model is used to predict the performance of a bioabsorbable stent in an idealized arterial geometry as it is subject to corrosion over time. The effects of homogeneous and heterogeneous corrosion processes on long-term stent scaffolding ability are contrasted based on model predictions.