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This article provides an overview of the connection between the microstructural state and the mechanical response of various bioresorbable polylactide (PLA) devices for medical applications. PLLA is currently the most commonly used material for bioresorbable stents and sutures, and its use is increasing in many other medical applications. The non-linear mechanical response of PLLA, due in part to its low glass transition temperature ( T g  ≈ 60 °C), is highly sensitive to the molecular weight and molecular orientation field, the degree of crystallinity, and the physical aging time. These microstructural parameters can be tailored for specific applications using different resin formulations and processing conditions. The stress–strain, deformation, and degradation response of a bioresorbable medical device is also strongly dependent on the time history of applied loads and boundary conditions. All of these factors can be incorporated into a suitable constitutive model that captures the multiple physics that are involved in the device response. Currently developed constitutive models already provide powerful computations simulation tools, and more progress in this area is expected to occur in the coming years.

Stent induced hemodynamic changes in the coronary arteries are associated with higher risk of adverse clinical outcome. The purpose of this study was to evaluate the impact of stent design on wall shear stress (WSS), time average WSS, and WSS gradient (WSSG), in idealized stent geometries using computational fluid dynamics. Strut spacing, thickness, luminal protrusion, and malapposition were systematically investigated and a comparison made between two commercially available stents (Omega and Biomatrix). Narrower strut spacing led to larger areas of adverse low WSS and high WSSG but these effects were mitigated when strut size was reduced, particularly for WSSG. Local hemodynamics worsened with luminal protrusion of the stent and with stent malapposition, adverse high WSS and WSSG were identified around peak flow and throughout the cardiac cycle respectively. For the Biomatrix stent, the adverse effect of thicker struts was mitigated by greater strut spacing, radial cell offset and flow-aligned struts. In conclusion, adverse hemodynamic effects of specific design features (such as strut size and narrow spacing) can be mitigated when combined with other hemodynamically beneficial design features but increased luminal protrusion can worsen the stent’s hemodynamic profile significantly.

The field of percutaneous coronary intervention has witnessed many progressions over the last few decades, more recently with the advancement of fully degradable bioabsorbable stents. Bioabsorbable materials, such as metallic alloys and aliphatic polyesters, have the potential to yield stents which provide temporary support to the blood vessel and allow native healing of the tissue to occur. Many chemical and physical reactions are reported to play a part in the degradation of such bioabsorbable materials, including, but not limited to, corrosion mechanisms for metals and the hydrolysis and crystallization of the backbone chains in polymers. In the design and analysis of bioabsorbable stents it is important to consider the effect of each aspect of the degradation on the material’s in vivo performance. The development of robust computational modelling techniques which fully capture the degradation behaviour of these bioabsorbable materials is a key factor in the design of bioabsorable stents. A critical review of the current computational modelling techniques used in the design and analysis of these next generation devices is presented here, with the main accomplishments and limitations of each technique highlighted.

Transcatheter aortic valve replacement (TAVR) represents an established recent technology in a high risk patient base. To better understand TAVR performance, a fluid–structure interaction (FSI) model of a self-expandable transcatheter aortic valve was proposed. After an in vitro durability experiment was done to test the valve, the FSI model was built to reproduce the experimental test. Lastly, the FSI model was used to simulate the virtual implant and performance in a patient-specific case. Results showed that the leaflet opening area during the cycle was similar to that of the in vitro test and the difference of the maximum leaflet opening between the two methodologies was of 0.42%. Furthermore, the FSI simulation quantified the pressure and velocity fields. The computed strain amplitudes in the stent frame showed that this distribution in the patient-specific case is highly affected by the aortic root anatomy, suggesting that the in vitro tests that follow standards might not be representative of the real behavior of the percutaneous valve. The patient-specific case also compared in vivo literature data on fast opening and closing characteristics of the aortic valve during systolic ejection. FSI simulations represent useful tools in determining design errors or optimization potentials before the fabrication of aortic valve prototypes and the performance of tests.

This paper reviews the current state of the art for coronary stent materials and surface coatings, with an emphasis on new technologies that followed on from first-generation bare metal and drug-eluting stents. These developments have been driven mainly by the need to improve long term outcomes, including late stent thrombosis. Biodegradable drug-eluting coatings aim to address the long term effects of residual durable polymer after drug elution; the SYNERGY, BioMatrix, and Nobori stents are all promising devices in this category, with minimal polymer through the use of abluminal coatings. Textured stent surfaces have been used to attached drug directly, without polymer; the Yukon Choice and BioFreedom stents have some promising data in this category, while a hydroxyapatite textured surface has had less success. The use of drug-filled reservoirs looked promising initially but the NEVO device has experienced both technical and commercial set-backs. However this approach may eventually make it to market if trials with the Drug-Filled Stent prove to be successful. Non-pharmacological coatings such as silicon carbide, carbon, and titanium–nitride-oxide are also proving to have potential to provide better performance than BMS, without some of the longer term issues associated with DES. In terms of biological coatings, the Genous stent which promotes attachment of endothelial progenitor cells has made good progress while gene-eluting stents still have some practical challenges to overcome. Perhaps the most advancement has been in the field of biodegradable stents. The BVS PLLA device is now seeing increasing clinical use in many complex indications while magnesium stents continue to make steady advancements.

Biodegradable implants have demonstrated clinical success in simple applications (e.g., absorbable sutures) and have shown great potential in many other areas of interventional medicine, such as localized drug delivery, engineered tissue scaffolding, and structural implants. For endovascular stenting and musculoskeletal applications, they can serve as temporary mechanical support that provides a smooth stress-transfer from the degradable implant to the healing tissue. However, for more complex device geometries, in vivo environments, and evolving load-bearing functions, such as required for vascular stents, there are considerable challenges associated with the use of biodegradable materials. A biodegradable stent must restore blood flow and provide support for a predictable appropriate period to facilitate artery healing, and subsequently, fail safely and be absorbed in a controllable manner. Biodegradable polymers are typically weaker than metals currently employed to construct stents, so it is difficult to ensure sufficient strength to keep the artery open and alleviate symptoms acutely while keeping other design parameters within clinically acceptable ranges. These design challenges are serious, given the general lack of understanding of biodegradable polymer behavior and evolution in intimal operating conditions. The modus operandi is mainly empirical and relies heavily on trial-and-error methodologies burdened by difficult, resource-expensive, and time-consuming experiments. We are striving for theoretical advancements systematizing the empirical knowledge into rational frameworks that could be cast into in silico tools for simulation and product development optimization. These challenges are evident when one considers that there are no biodegradable stents on the US market despite more than 30 years of development efforts (and currently only a couple with CE mark). This review summarizes previous efforts at implementing biodegradable stents, discusses the specific challenges involved, and presents recently developed material-modeling frameworks that can benefit this exciting field.

The great success of stents in treating cardiovascular disease is actually undermined by their long-term fatigue failure. The high variability of stent failure incidence suggests that it is due to several correlated aspects, such as loading conditions, material properties, component design, surgical procedure, and patient functional anatomy. Numerical and experimental non-clinical assessments are included in the recommendations and requirements of several regulatory bodies and they are thus exploited in the analysis of stent fatigue performance. Optimization-based simulation methodologies have been developed as well, to improve the fatigue endurance of novel designs. This paper presents a review on the fatigue issue in metallic stents, starting from a description of clinical evidence about stent fracture up to the analysis of computational approaches available from the literature. The reported discussion on both the experimental and numerical framework aims at providing a general insight into stent lifetime prediction as well as at understanding the factors which affect stent fatigue performance for the design of novel components.

Percutaneous coronary interventions (PCI) were proposed in the late 1970s as an alternative to surgical coronary artery bypass grafting for the treatment of coronary artery disease. Important technological progress has been made since. Balloon angioplasty was replaced by bare metal stents, which allowed to permanently scaffold the coronary vessel avoiding acute recoil and abrupt occlusion. Thereafter, the introduction of early generation drug-eluting stents (DES) has significantly improved clinical outcomes, primarily by markedly reducing the risk of restenosis. New generation DES with thinner stent struts, novel durable or biodegradable polymer coatings, and new limus antiproliferative agents, have further improved upon the safety and efficacy profile of early generation DES. The present article aims to review the impact of technological advances on clinical outcomes in the field of PCI with coronary stents, and to provide a brief overview on clinical margins of improvement and unmet needs of available DES.

Coronary angioplasty initially employed balloon dilatation only. This technique revolutionized the treatment of coronary artery disease, although outcomes were compromised by acute vessel closure, late constrictive remodeling, and restenosis due to neointimal proliferation. These processes were studied in animal models, which contributed to understanding the biology of endovascular arterial injury. Coronary stents overcome acute recoil, with improvements in the design and metallurgy since then, leading to the development of drug-eluting stents and bioresorbable scaffolds. These devices now undergo computer modeling and benchtop and animal testing before evaluation in clinical trials. Animal models, including rabbit, sheep, dog and pig are available, all with individual benefits and limitations. In smaller mammals, such as mouse and rabbit, the target for stenting is generally the aorta; whereas in larger animals, such as the pig, it is generally the coronary artery. The pig coronary stenting model is a gold-standard for evaluating safety; but insights into biomechanical properties, the biology of stenting, and efficacy in controlling neointimal proliferation can also be gained. Intra-coronary imaging modalities such as intravascular ultrasound and optical coherence tomography allow precise serial evaluation in vivo , and recent developments in genetically modified animal models of atherosclerosis provide realistic test beds for future stents and scaffolds.

Stenting treatments for the management of disease in the heart, arterial and venous systems, biliary ducts, urethras, ureters, oesophageal tract and prostate have made enormous technical advances since their introduction into clinical use. The progression from metallic to polymer based bio-absorbable stents, coupled with the advances in additive manufacturing techniques, present a unique opportunity to completely re-envision the design, manufacture, and supply chain of stents. This paper looks at current stenting trends and proposes a future where the stent supply chain is condensed from ~150 days to ~20 min. The Cardiologist therefore has the opportunity to become a designer, manufacturer and user with patients receiving custom stents specific to their unique pathology that will be generated, delivered and deployed in the Cath-lab. The paper will outline this potentially revolutionary development and consider the technical challenges that will need to be overcome in order to achieve these ambitious goals. A high level overview of the generating eluting stents in situ program—GENESIS—is outlined including some early experimental work.