Open Access

- Development of a FEA Tool for a realistic stent simulation - investigation on minor modification on the stent design on the expansion behaviour - analysis of three stent designs: classical stent design with pronounced dogbone effect, two modified stent design (non-dogbone-design)

Customized additively manufactured (laser powder bed fused (L-PBF)) stents could improve the treatment of complex lesions by enhancing stent-artery conformity. However, geometric irregularities inherent for L-PBF stents are expected to influence not only their mechanical behavior but also their interaction with the artery. In this study, the influence of geometrical irregularities on stent-artery interaction is evaluated within a numerical framework. Thus, computed arterial stresses induced by a reconstructed L-PBF stent model are compared to those induced by the intended stent model (also representing a stent geometry obtained from conventional manufacturing processes) and a modified CAD stent model that accounts for the increased strut thickness inherent for L-PBF stents. It was found that, similar to conventionally manufactured stents, arterial stresses are initially related to the basic stent design/topology, with the highest stresses occurring at the indentations of the stent struts. Compared to the stent CAD model, the L-PBF stent induces distinctly higher and more maximum volume stresses within the plaque and the arterial wall. In return, the modified CAD model overestimates the arterial stresses induced by the L-PBF stent due to its homogeneously increased strut thickness and thus its homogeneously increased geometric stiffness compared with the L-PBF stent. Therefore, the L-PBF-induced geometric irregularities must be explicitly considered when evaluating the L-PBF stent-induced stresses because the intended stent CAD model underestimates the arterial stresses, whereas the modified CAD model overestimates them. The arterial stresses induced by the L-PBF stent were still within the range of values reported for conventional stents in literature, suggesting that the use of L-PBF stents is conceivable in principle. However, because geometric irregularities, such as protruding features from the stent surface, could potentially damage the artery or lead to premature stent failure, further improvement of L-PBF stents is essential.

Re-narrowing of a coronary vessel after stent implantation, known as in-stent restenosis (ISR), is a predominant problem in the treatment of atherosclerosis. ISR is caused, e.g., by vessel wall injury during stent implantation, malpositioning, over- or undersizing of the stent, and associated adverse alteration of natural blood flow. Advances in metal additive manufacturing, particularly in laser powder bed fusion (L-PBF), are enabling the generation of micro-scale L-PBF lattice structures and thus potentially coronary stents. By enabling new or even patient-specific stent designs, L-PBF stents could improve the conformity of the implanted stent and the vessel wall, thus potentially reducing ISR rates in the future. Research in the field of L-PBF stents is still in its early stages. Previous studies have mainly focused on the analysis of stent design requirements and basic functionality of L-BF stents. Studies regarding the determination of the specific mechanical behavior of L-PBF stents but also regarding their numerical analysis are currently not available. Due to their similar topology, L-PBF stents resemble L-PBF lattice structures with a low structural density. Therefore, it is reasonable to transfer the findings in the field of L-PBF lattice structures to L-PBF stents. L-PBF lattice structures exhibit process-related geometric irregularities that (negatively) affect their morphology and their mechanical behavior. Therefore, for an accurate (numerical) evaluation of L-PBF lattice structures and thus of L-PBF stents, their mechanical behavior must be determined first, and the influence of the process-related geometric irregularities must be analyzed or considered within the numerical models. Furthermore, the mechanical and morphological behavior of filigree L-PBF stents can be altered by post-processing steps (surface, heat treatment). However, studies on L-PBF lattice structures are mainly limited to as-built structures. Therefore, the aim of this doctoral thesis is to determine the effects of L-PBF process-related geometric irregularities and different post-processing conditions on the mechanical behavior of L-PBF 316L stents, as well as to develop a numerical methodology for their numerical evaluation. In a first step, a finite element analysis (FEA) for the prediction of stent deformation during crimping and expansion was developed and validated using extensive experimental data from conventionally manufactured stents. These models accurately predicted the expansion behavior of two different stent designs with different expansion behavior, as well as different positioning of the stent on the balloon catheter. In the second step, the mechanical behavior of L-PBF 316L was determined using uniaxial tensile tests on standard flat tensile specimens with variable specimen thickness and orientation angle. For each specimen configuration, as-built and heat treated specimens were considered. In the as-built condition, besides the anisotropic mechanical properties of L-PBF 316L already known from the literature, a significant increase in strength with increasing specimen thickness was observed, which stagnated at a specimen thickness of t > 1.5 mm, thus reaching a saturation value. Heat treatment resulted in homogenization but no recrystallization of the microstructure. Thus, the melt pool boundaries and substructures were dissolved, and residual stresses were reduced, whereas the elongated and oriented grains and thus the anisotropic microstructure were preserved. Accordingly, the specimen thickness- and direction-dependent mechanical properties of L-PBF 316L were still observed after heat treatment. Thus, for a reliable structural mechanical evaluation of L-PBF parts, their mechanical properties must be determined using test specimens that are comparable in size, orientation angle, and post-treatment condition to the later L-PBF part. In a final step, the mechanical behavior of L-PBF stents was determined and the expansion behavior of L-PBF stents under different post-processing conditions was evaluated by FEAs. The generation of L-PBF miniature tensile specimens of comparable cross section to stent struts and their experimental evaluation is challenging and highly error-prone. Therefore, a combined experimental-numerical approach was developed for the inverse determination of the mechanical behavior of L-PBF 316L stents based on experimental testing and FEA of uniaxial compression of L-PBF stents. The stent models were reconstructed from computed tomography (CT) scans of real L-PBF stents. In this way, process-related geometric irregularities were depicted enabling an accurate prediction of the stent structure-property relationship. Thus, the macroscopic mechanical behavior of L-PBF 316L stents could be determined for the first time and subsequently described numerically by a material model. Morphological analysis of the L-PBF stents further revealed significant discrepancies between the actual L-PBF stents and its computed aided design (CAD) model due to process-related geometric irregularities (surface roughness, strut waviness, enlarged and inhomogeneous strut diameters, internal defects). Numerical expansion analysis of the L-PBF stent models showed that L-PBF stents can exhibit comparable expansion behavior to conventional stents only after surface and heat treatment. However, subsequent analysis of deformation and stress states showed that L-PBF stents, both in the as-built condition and after surface and heat treatment, may exhibit critical local stress/strain concentrations, especially in the areas of pronounced geometric irregularities. Improvements in the L-PBF process, post-processing steps, and stent design are therefore essential to minimize process-related geometric irregularities and thus their strength-reducing effects, ultimately ensuring the structural safety of L-PBF stents. One possible improvement approach is to manufacture the stents on special µ-L-PBF systems that have explicitly been optimized to produce filigree structures. In this way, a higher geometric accuracy and low surface roughness could already be achieved in the as-built condition of L-PBF stents, and the subsequent required surface treatment could be reduced to a minimum. Furthermore, the fatigue strength, the damage behavior, the interaction of the stent with the blood vessel as well as the biocompatibility of L-PBF 316L stents should be investigated. To effectively use numerical models for the development of L-PBF stents, the potential of synthetic L-PBF stent models should also be investigated. The synthetic stent models represent a statistics-based modification of the original stent CAD model (e.g., local variations of strut cross section along strut length). In this way, the effects of L-PBF process-related geometric irregularities could be represented statistically and thus without explicit reconstruction from CT scans. The development of L-PBF stents is a very complex interdisciplinary task in the fields of manufacturing technology, material science, design development and numerical simulation. To establish L-PBF as a reliable alternative to conventional stent fabrication, further research in this area is essential. By providing a method to determine the mechanical properties of L-PBF stents as well as their numerical analysis, this doctoral thesis could contribute to the further development of L-PBF 316L stents, as well as define necessary research aspects for further work.

Arteriosclerosis is a major health issue worldwide. While it is commonly treated by the implantation of an balloon-expandable stent, micro injuries may occur during stent deployment, and induce in-stent restenosis, whose consequence can be fatal. Studying this undesirable phenomenon is usually limited as experimental data is hard to obtain on ethical ground. Numerical simulation are performed to better understand this problem. To construct a more realistic simulation of a balloon-expandable stent, a partitioned strongly-coupled FSI simulation of the balloon deployment was set up using the ICFD solver of LS-DYNA, - a quite innovative approach. The complex balloon configuration as well as the interaction of the injection medium and the balloon structure was considered. The balloon structure consisting of shell elements was obtained from preliminary balloon folding and pleating simulations. The balloon consists of a flexible thin walled polyamide. The injection fluid is implemented using volume elements. Balloon deployment was initiated by a pressure boundary condition inducing a volume flow into the balloon. The initial feasibility analysis showed promising result including a continuous balloon deployment and a reasonable development of the fluid pressure and velocity field. However, applying this FSI approach to a more complex balloon structure led to a non convergent solution. The non-convergence could be mainly reduced to mechanical factors including the low wall thickness of the balloon (< 0.05 mm) and the flexibility of the polyamide. Further, the ICFD solver shows less accuracy concerning the FSI conditions when dealing with thin flexible structures as well as enclosed volumes. A shell thickness of 0.06 mm is believed to result in a convergent solution.