biomedical applications of shape memory alloys

Shape memory alloys, and in particular NiTi alloys, are characterized by two unique behaviors, thermally or mechanically activated: theshape memory effectandpseudo-elastic effect. These behaviors, due to the peculiar crystallographic structure of the alloys, assure the recovery of the original shape even after large deformations and the maintenance of a constant applied force in correspondence of significant displacements. These properties, joined with good corrosion and bending resistance, biological and magnetic resonance compatibility, explain the large diffusion, in the last 20 years, of SMA in the production of biomedical devices, in particular for mini-invasive techniques. In this paper a detailed review of the main applications of NiTi alloys in dental, orthopedics, vascular, neurological, and surgical fields is presented. In particular for each device the main characteristics and the advantages of using SMA are discussed. Moreover, the paper underlines the opportunities and the room for new ideas able to enlarge the range of SMA applications. However, it is fundamental to remember that the complexity of the material and application requires a strict collaboration between clinicians, engineers, physicists and chemists for defining accurately the problem, finding the best solution in terms of device design and accordingly optimizing the NiTi alloy properties

Vulnerability analysis aimed at the safeguard of the Ererouyk basilica in Armenia

The basilica of Ererouyk have recently attracted the attention of the Armenian government, which has decided to promote its preservation by commissioning the National University of Architecture and Construction of Armenia to coordinate a group of Italian and Armenian experts. Built in the sixth century, the three-nave basilica of Ererouyk is unique in Armenia for its size, importance, and typology. For this reason, it has interested many scholars since the late 19th century. In this work, the path followed to know in an accurate way the structural response and to evaluate the vulnerability will be described. The first phase of the study consisted in an in-depth analysis of the damages and restoration interventions that the basilica has undergone during its history. Subsequently, the identification of the material properties both using results of tests performed on the basilica material and referring to literature data was performed. Then the seismic input was defined, following Armenian standards. Aware of the huge uncertainties inherent in the behavior of a sack masonry building that has undergone changes over the centuries, it was decided to perform the structural analysis using methods/models of increasing complexity, from linear kinematic approach to nonlinear time history analyses using three-dimensional finite element model with non-linear properties of the masonry. The systematization of all the information collected has allowed giving a complete and exhaustive picture of the vulnerability of the structure, highlighting the necessity to intervene for the improvement of its structural behavior towards seismic action

A fibre flexure-shear model for seismic analysis of RC-framed structures

Whilst currently existing modelling approaches of reinforced concrete behaviour allow a reasonably accurate prediction of flexural response, the determination of its shear counterpart needs further developments. There are various modelling strategies in literature able to predict the shear response and the shear-flexure coupling under monotonic loading conditions. However, very few are the reported models that have demonstrated successful results under cyclic loading, as in the seismic load case. These considerations lead to this research work focused on the development of a flexure-shear model for reinforced concrete beam-column elements. A reliable constitutive model for cracked reinforced concrete subjected to cyclic loading was implemented as bi-axial fibre constitutive model into a two-dimensional Timoshenko beam-column element. Aim of this research work is to arrive at the definition of a numerical model sufficiently accurate and, at the same time, computationally efficient, that will enable implementation within a Finite Element package for nonlinear dynamic analysis of existing non seismically designed RC structures that are prone to shear-induced damage and collapse.info:eu-repo/semantics/publishedVersio

A review on the use of finite element simulations for structural analyses of coronary stenting: What can we do nowadays and what do we need to move forward?

In silico studies to perform structural analyses of coronary stenting have attracted the attention of many researchers in the last 25 years. As a consequence, the finite element models used to describe the fundamental elements of stenting simulations, namely the delivery system (consisting of stent and balloon), the diseased artery, and the deployment procedure have had considerable development, paving the way for the application of numerical analyses in both manufacturing and clinical contexts. Indeed, in accordance with the logic of the 3Rs (refine, reduce, and replace), simulations can play a fundamental role in developing new devices and as a support tool for training/education and operational planning activities for clinical personnel. However, the application of such numerical methodologies in the aforementioned contexts of use requires an adequate level of credibility of the models with respect to the risk associated to their use in the decision-making process. Within this framework, this paper proposes a review of the modeling approaches available today for in silico stenting of coronary arteries and a discussion of their actual or potential application areas. In particular, the attention is focused on the different levels of credibility required by the presented contexts of use with respect to the validation activities of numerical models developed up to now

Nickel-Titanium peripheral stents: can fracture mechanics shed light on their fatigue failure?

The major concern about Nickel-Titanium (Ni-Ti) stents, which are the gold standard in the treatment of occlusive peripheral disease, is fatigue and the consequent fracture in vivo. Indeed, their failure might be responsible for severe drawbacks, among which is the re-occlusion of the treated artery. Although many phenomenological approaches have been proposed to study this topic, the current literature lacks extensive knowledge on the Ni-Ti local damage mechanisms produced by the cyclic loads that promote crack nucleation and lead to the failure of thin struts, such as those of stents. Moreover, due to the super-elastic property of the alloy, the standard approach for interpreting the fracture of metals might be not accurate for this case. This work aims at increasing awareness of fatigue failure in superelastic Ni-Ti thin struts, such as those of stents. To do so, multi-wire specimens, sharing the same dimensions and thermo-mechanical treatment of the stent struts, were fatigue tested under different strain levels and the number of cycles to failure was recorded for each sample. Numerical simulations corroborated the experimental results to gain information on the local stress and strain fields during the fatigue cycles. A fracture mechanics-based fatigue model adopting the cyclic J-integral was here proposed, giving promising results for the interpretation of such failures

Patient-specific cardiovascular superelastic NiTi stents produced by laser powder bed fusion

To date, there is a general lack of customizability within the selection of endovascular devices for catheter-based vascular interventions. Laser powder bed fusion (LPBF) has been flexibly exploited to produce customized implants using conventional biomedical alloys for orthopedic and dental applications. Applying LPBF for cardiovascular applications, patient-specific stents can be produced with small struts (approximately 100-300 µm), variable geometries, and clinically used metals capable of superelastic behaviour at body temperature (eg. equiatomic nickel-titanium alloys, NiTi). Additionally, the growing availability and use of patient-specific 3D models provides a unique opportunity to outline the necessary manufacturing process that would be required for customizable NiTi devices based on patient geometry. In order to fulfil the potential of the patient-specific superelastic stents, process and design know-how should be expanded to the novel material and fine details at the limits of conventional LPBF machines. In this work, a framework for developing a patient-specific superelastic NiTi stent produced by LPBF is demonstrated. At a proof-of-concept stage, the design procedures are shown in a geometry similar to the artery. The stents with 100 µm nominal strut diameter are later produced with a Ni50.8Ti49.2 powder and heat treated. The results confirm the possibility of producing stents with a design suitable for highly complex patient-specific anatomies and having superelastic behavior at body temperature

Investigating Balloon-Vessel Contact Pressure Patterns in Angioplasty: In Silico Insights for Drug-Coated Balloons

: Drug-Coated Balloons have shown promising results as a minimally invasive approach to treat stenotic arteries, but recent animal studies have revealed limited, non-uniform coating transfer onto the arterial lumen. In vitro data suggested that local coating transfer tracks the local Contact Pressure (CP) between the balloon and the endothelium. Therefore, this work aimed to investigate in silico how different interventional and device parameters may affect the spatial distribution of CP during the inflation of an angioplasty balloon within idealized vessels that resemble healthy femoral arteries in size and compliance. An angioplasty balloon computational model was developed, considering longitudinal non-uniform wall thickness, due to its forming process, and the folding procedure of the balloon. To identify the conditions leading to non-uniform CP, sensitivity finite element analyses were performed comparing different values for balloon working length, longitudinally varying wall thickness, friction coefficient on the balloon-vessel interface, vessel wall stiffness and thickness, and balloon-to-vessel diameter ratio. Findings indicate a significant irregularity of contact between the balloon and the vessel, mainly affected by the balloon's unfolding and longitudinal thickness variation. Mirroring published data on coating transfer distribution in animal studies, the interfacial CP distribution was maximal at the middle of the balloon treatment site, while exhibiting a circumferential pattern of linear peaks as a consequence of the particular balloon-vessel interaction during unfolding. A high ratio of balloon-to-vessel diameter, higher vessel stiffness, and thickness was found to increase significantly the amplitude and spatial distribution of the CP, while a higher friction coefficient at the balloon-to-vessel interface further exacerbated the non-uniformity of CP. Evaluation of balloon design effects revealed that the thicker tapered part caused CP reduction in the areas that interacted with the extremities of the balloon, whereas total length only weakly impacted the CP. Taken together, this study offers a deeper understanding of the factors influencing the irregularity of balloon-tissue contact, a key step toward uniformity in drug-coating transfer and potential clinical effectiveness