@article{SchaefferHerrmannSchratzenstalleretal., author = {Schaeffer, Leon and Herrmann, David and Schratzenstaller, Thomas and Dendorfer, Sebastian and B{\"o}hm, Valter}, title = {Preliminary theoretical considerations on the stiffness characteristics of a tensegrity joint for the use in dynamic orthoses}, series = {Journal of Medical Robotics Research}, journal = {Journal of Medical Robotics Research}, publisher = {World Scientific}, doi = {10.1142/S2424905X23400081}, abstract = {Early motion therapy plays an important role for effective long-term healing of joint injuries. In many cases, conventional dynamic orthoses fail to address the intricate movement possibilities of the underlying joints, limited by their simplistic joint representations, often represented by revolute joints, enabling rotations by only one axis. In this paper, a two-dimensional compliant tensegrity joint for use in biomedical applications is investigated. It consists of two compressed members and five compliant tensioned members. Relative movement possibilities are realized by the intrinsic compliance of the structure. In the development of these systems, the first step is the determination of the static stable equilibrium. This analysis is conducted in this paper by considering the potential energy approach or by using the geometric nonlinear finite element method. The mechanical behavior of the structure is assessed with a specific emphasis on its mechanical compliance. The primary objective of this study is the investigation of the influence of structural parameters on the overall stiffness and movability of the structure. The results underscore the significant effect of member parameters on the stiffness and movability of the compliant tensegrity joint, particularly under varying load magnitudes. These findings provide insights for optimizing the joint's performance, contributing to its potential application in advanced orthotic and exoskeleton devices.}, language = {en} } @article{EigenbergerFelthausSchratzenstalleretal., author = {Eigenberger, Andreas and Felthaus, Oliver and Schratzenstaller, Thomas and Haerteis, Silke and Utpatel, Kirsten and Prantl, Lukas}, title = {The Effects of Shear Force-Based Processing of Lipoaspirates on White Adipose Tissue and the Differentiation Potential of Adipose Derived Stem Cells}, series = {cells}, volume = {11}, journal = {cells}, number = {16}, publisher = {MDPI}, address = {Basel}, doi = {10.3390/cells11162543}, pages = {13}, abstract = {Autologous lipotransfer is a promising method for tissue regeneration, because white adipose tissue contains a heterogeneous cell population, including mesenchymal stem cells, endothelial cells, immune cells, and adipocytes. In order to improve the outcome, adipose tissue can be processed before application. In this study, we investigated changes caused by mechanical processing. Lipoaspirates were processed using sedimentation, first-time centrifugation, shear-force homogenization, and second-time centrifugation. The average adipocyte size, stromal vascular cell count, and adipocyte depot size were examined histologically at every processing step. In addition, the adipose derived stem cells (ADSCs) were isolated and differentiated osteogenically and adipogenically. While homogenization causes a disruption of adipocyte depots, the shape of the remaining adipocytes is not changed. On average, these adipocytes are smaller than the depot adipocytes, they are surrounded by the ECM, and therefore mechanically more stable. The volume loss of adipocyte depots leads to a significant enrichment of stromal vascular cells such as ADSCs. However, the mechanical processing does not change the potential of the ADSCs to differentiate adipogenically or steogenically. It thus appears that mechanically processed lipoaspirates are promising for the reparation of even mechanically stressed tissue as that found in nasolabial folds. The changes resulting from the processing correspond more to a filtration of mechanically less stable components than to a manipulation of the tissue.}, language = {en} } @article{ObermaierLehleSchmidetal., author = {Obermaier, Lisa and Lehle, Karla and Schmid, Stefanie and Schmid, Christof and Schratzenstaller, Thomas}, title = {Introduction of a new ex vivo porcine coronary artery model: Evaluation of the direct vascular injury after stent implantation with and without dogbone effect}, series = {European Surgical Research}, volume = {63}, journal = {European Surgical Research}, number = {4}, publisher = {Karger}, address = {Basel}, issn = {1421-9921}, doi = {10.1159/000527883}, pages = {285 -- 293}, abstract = {Introduction: Neointimal hyperplasia after percutaneous coronary intervention remains a major determinant of in-stent restenosis (ISR). The extent of mechanical vessel injury correlates with ISR. A new ex vivo porcine stent model was introduced and evaluated comparing different stent designs. Methods: Coronary arteries were prepared from pig hearts from the slaughterhouse and used for ex vivo implantations of coronary stents. One basic stent design in two configurations (dogbone, DB; non-dogbone, NDB) was used. Vascular injury was determined according to a modified injury score (IS). Results: Standardized experimental conditions ensured comparable vessel dimensions and overstretch data. DB stents caused more severe IS compared to NDB stents. The mean IS and the IS at the distal end of all stents were significantly reduced for NDB stents (ISMean, DB, 1.16 ±0.12; NDB, 1.02 ±0.12; p=0.018; ISDist, DB, 1.39 ±0.28; NDB, 1.13 ±0.24; p=0.03). Discussion/Conclusion: The introduced ex-vivo model allowed the evaluation of different stent designs exclude unfavorable stent designs.}, language = {en} } @article{GeithNothdurfterHeimletal., author = {Geith, Markus A. and Nothdurfter, Laurenz and Heiml, Manuel and Agrafiotis, Emmanouil and Gruber, Markus and Sommer, Gerhard and Schratzenstaller, Thomas and Holzapfel, Gerhard A.}, title = {Quantifying stent-induced damage in coronary arteries by investigating mechanical and structural alterations}, series = {Acta Biomaterialia}, volume = {116}, journal = {Acta Biomaterialia}, number = {October}, publisher = {Elsevier}, doi = {10.1016/j.actbio.2020.08.016}, pages = {285 -- 301}, abstract = {Vascular damage develops with diverging severity during and after percutaneous coronary intervention with stent placement and is the prevailing stimulus for in-stent restenosis. Previous work has failed to link mechanical data obtained in a realistic in vivo or in vitro environment with data collected during imaging processes. We investigated whether specimens of porcine right coronary arteries soften when indented with a stent strut shaped structure, and if the softening results from damage mechanisms inside the fibrillar collagen structure. To simulate the multiaxial loading scenario of a stented coronary artery, we developed the testing device 'LAESIO' that can measure differences in the stress-stretch behavior of the arterial wall before and after the indentation of a strut-like stamp. The testing protocol was optimized according to preliminary experiments, more specifically equilibrium and relaxation tests. After chemical fixation of the specimens and subsequent tissue clearing, we performed three-dimensional surface and second-harmonic generation scans on the deformed specimens. We analyzed and correlated the mechanical response with structural parameters of high-affected tissue located next to the stamp indentation and low-affected tissue beyond the injured area. The results reveal that damage mechanisms, like tissue compression as well as softening, fiber dispersion, and the lesion extent, are direction-dependent, and the severity of them is linked to the strut orientation, indentation pressure, and position. The findings highlight the need for further investigations by applying the proposed methods to human coronary arteries. Additional data and insights might help to incorporate the observed damage mechanisms into material models for finite element analyses to perform more accurate simulations of stent-implantations.}, language = {en} } @inproceedings{WiesentGeithWagner, author = {Wiesent, Lisa and Geith, Markus A. and Wagner, Marcus}, title = {Simulation of Fluid-Structure Interaction between injection medium and balloon catheter using ICFD}, series = {11th European LS-DYNA Conference 2017, 9 - 11 May, Salzburg, Austria}, booktitle = {11th European LS-DYNA Conference 2017, 9 - 11 May, Salzburg, Austria}, isbn = {978-3981621549}, abstract = {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.}, subject = {Koronare Herzkrankheit}, language = {en} } @article{WiesentSchultheissSchmidetal., author = {Wiesent, Lisa and Schultheiss, Ulrich and Schmid, Christof and Schratzenstaller, Thomas and Nonn, Aida}, title = {Experimentally validated simulation of coronary stents considering different dogboning ratios and asymmetric stent positioning}, series = {PlOS One}, volume = {14}, journal = {PlOS One}, number = {10}, publisher = {PLOS}, doi = {10.1371/journal.pone.0224026}, pages = {1 -- 25}, abstract = {In-stent restenosis remains a major problem of arteriosclerosis treatment by stenting. Expansion-optimized stents could reduce this problem. With numerical simulations, stent designs/ expansion behaviours can be effectively analyzed. For reasons of efficiency, simplified models of balloon-expandable stents are often used, but their accuracy must be challenged due to insufficient experimental validation. In this work, a realistic stent life-cycle simulation has been performed including balloon folding, stent crimping and free expansion of the balloon-stent-system. The successful simulation and validation of two stent designs with homogenous and heterogeneous stent stiffness and an asymmetrically positioned stent on the balloon catheter confirm the universal applicability of the simulation approach. Dogboning ratio, as well as the final dimensions of the folded balloon, the crimped and expanded stent, correspond well to the experimental dimensions with only slight deviations. In contrast to the detailed stent life-cycle simulation, a displacement-controlled simulation can not predict the transient stent expansion, but is suitable to reproduce the final expanded stent shape and the associated stress states. The detailed stent life-cycle simulation is thus essential for stent expansion analysis/optimization, whereas for reasons of computational efficiency, the displacement-controlled approach can be considered in the context of pure stress analysis.}, subject = {Stent}, language = {en} } @article{GeithSwidergalHochholdingeretal., author = {Geith, Markus A. and Swidergal, Krzysztof and Hochholdinger, Bernd and Schratzenstaller, Thomas and Wagner, Marcus and Holzapfel, Gerhard A.}, title = {On the importance of modeling balloon folding, pleating, and stent crimping: An FE study comparing experimental inflation tests}, series = {International Journal for Numerical Methods in Biomedical Engineering}, volume = {35}, journal = {International Journal for Numerical Methods in Biomedical Engineering}, number = {11}, publisher = {Wiley}, doi = {10.1002/cnm.3249}, abstract = {Finite element (FE)-based studies of preoperative processes such as folding,pleating, and stent crimping with a comparison with experimental inflation tests are not yet available. Therefore, a novel workflow is presented in which residual stresses of balloon folding and pleating, as well as stent crimping, and the geometries of all contact partners were ultimately implemented in an FE code to simulate stent expansion by using an implicit solver. The numerical results demonstrate that the incorporation of residual stresses and strains experienced during the production step significantly increased the accuracy of the subsequent simulations, especially of the stent expansion model. During the preoperative processes, stresses inside the membrane and the stent material also reached a rather high level. Hence, there can be no presumption that balloon catheters or stents are undamaged before the actual surgery. The implementation of the realistic geometry, in particular the balloon tapers, and the blades of the process devices improved the simulation of the expansion mech-anisms, such as dogboning, concave bending, or overexpansion of stent cells. This study shows that implicit solvers are able to precisely simulate the mentioned preoperative processes and the stent expansion procedure without a preceding manipulation of the simulation time or physical mass.}, subject = {Stent}, language = {en} } @misc{WiesentHupkeBalketal., author = {Wiesent, Lisa and Hupke, Constantin and Balk, Christian and Schultheiss, Ulrich and Schratzenstaller, Thomas}, title = {Optimization of the cardiovascular stent design towards improved expansion behaviour and radial stiffness properties}, series = {Biomedizinische Technik}, volume = {63}, journal = {Biomedizinische Technik}, number = {s1}, doi = {10.1515/bmt-2018-6031}, abstract = {- 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)}, subject = {Kardiovaskul{\"a}res System}, language = {en} } @misc{GeithSwidergalSchratzenstalleretal., author = {Geith, Markus A. and Swidergal, Krzysztof and Schratzenstaller, Thomas and Holzapfel, Gerhard A. and Wagner, Marcus}, title = {Numerical analysis of stent delivery systems during pre- and intraoperative processes}, series = {15. Deutsches LS-DYNA Forum, 15.-17.10.2018, Bamberg}, journal = {15. Deutsches LS-DYNA Forum, 15.-17.10.2018, Bamberg}, language = {en} } @article{GeithSommerSchratzenstalleretal., author = {Geith, Markus A. and Sommer, Gerhard and Schratzenstaller, Thomas and Holzapfel, Gerhard A.}, title = {Biomechanical and structural quantification of vascular damage: A unique investigation of stent implantation}, series = {Artery Research}, volume = {20}, journal = {Artery Research}, number = {Issue C}, doi = {10.1016/j.artres.2017.10.025}, pages = {50}, language = {en} } @article{ZentgrafNuetzelMuehlbaueretal., author = {Zentgraf, Jan and N{\"u}tzel, Florian and M{\"u}hlbauer, Nico and Schultheiss, Ulrich and Grad, Marius and Schratzenstaller, Thomas}, title = {Surface Treatment of Additively Manufactured Polyetheretherketone (PEEK) by Centrifugal Disc Finishing Process: Identification of the Key Parameters}, series = {Polymers}, volume = {16}, journal = {Polymers}, publisher = {MDPI}, address = {Basel}, issn = {2073-4360}, doi = {10.3390/polym16162348}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:898-opus4-73959}, abstract = {Polyetheretherketone is a promising material for implants due to its good mechanical properties and excellent biocompatibility. Its accessibility to a wide range of applications is facilitated by the ability to process it with an easy-to-use manufacturing process such as fused filament fabrication. The elimination of disadvantages associated with the manufacturing process, such as a poor surface quality, is a main challenge to deal with. As part of the mass finishing process, centrifugal disc finishing has demonstrated good results in surface optimization, making it a promising candidate for the post-processing of additively manufactured parts. The objective of this study is to identify the key parameters of the centrifugal disc finishing process on the waviness of additively manufactured PEEK specimens, which has not been investigated previously. The waviness of the specimen was investigated by means of confocal laser scanning microscopy (CLSM), while weight loss was additionally tracked. Six parameters were investigated: type, amount and speed of media, use of compound, amount of water and time. Type of media, time and speed were found to significantly influence waviness reduction and weight loss. Surface electron microscopy images demonstrated the additional effects of deburring and corner rounding. Results on previous studies with specimens made of metal showed similar results. Further investigation is required to optimize waviness reduction and polish parts in a second post-processing step.}, language = {en} } @article{WiesentSchultheissLullaetal., author = {Wiesent, Lisa and Schultheiß, Ulrich and Lulla, Philipp and Noster, Ulf and Schratzenstaller, Thomas and Schmid, Christof and Nonn, Aida and Spear, Ashley}, title = {Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents}, series = {PLoS ONE}, volume = {15}, journal = {PLoS ONE}, number = {12}, publisher = {PLOS}, doi = {10.1371/journal.pone.0244463}, pages = {1 -- 30}, abstract = {Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 ± 0.63 N) or the heat treated condition (5.87 ± 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 ± 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41\% in the as-built and by 59\% in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures.}, subject = {Koronarendoprothese}, language = {en} }