Prof. Dr.-Ing Wojciech Kowalczyk

Mechanics and Robotics
University of Duisburg-Essen

Author IDs

  • Layer-Specific Damage Modeling of Porcine Large Intestine under Biaxial Tension
    Bhattarai, A. and May, C.A. and Staat, M. and Kowalczyk, W. and Tran, T.N.
    Bioengineering 9 (2022)
    The mechanical behavior of the large intestine beyond the ultimate stress has never been investigated. Stretching beyond the ultimate stress may drastically impair the tissue microstructure, which consequently weakens its healthy state functions of absorption, temporary storage, and transportation for defecation. Due to closely similar microstructure and function with humans, biaxial tensile experiments on the porcine large intestine have been performed in this study. In this paper, we report hyperelastic characterization of the large intestine based on experiments in 102 specimens. We also report the theoretical analysis of the experimental results, including an exponential damage evolution function. The fracture energies and the threshold stresses are set as damage material parameters for the longitudinal muscular, the circumferential muscular and the submucosal collagenous layers. A biaxial tensile simulation of a linear brick element has been performed to validate the applicability of the estimated material parameters. The model successfully simulates the biomechanical response of the large intestine under physiological and non-physiological loads. © 2022 by the authors.
    view abstract10.3390/bioengineering9100528
  • A literature review on large intestinal hyperelastic constitutive modeling
    Bhattarai, A. and Kowalczyk, W. and Tran, T.N.
    Clinical Biomechanics 88 (2021)
    Impacts, traumas and strokes are spontaneously life-threatening, but chronic symptoms strangle patient every day. Colorectal tissue mechanics in such chronic situations not only regulates the physio-psychological well-being of the patient, but also confirms the level of comfort and post-operative clinical outcomes. Numerous uniaxial and multiaxial tensile experiments on healthy and affected samples have evidenced significant differences in tissue mechanical behavior and strong colorectal anisotropy across each layer in thickness direction and along the length. Furthermore, this study reviewed various forms of passive constitutive models for the highly fibrous colorectal tissue ranging from the simplest linearly elastic and the conventional isotropic hyperelastic to the most sophisticated second harmonic generation image based anisotropic mathematical formulation. Under large deformation, the isotropic description of tissue mechanics is unequivocally ineffective which demands a microstructural based tissue definition. Therefore, the information collected in this review paper would present the current state-of-the-art in colorectal biomechanics and profoundly serve as updated computational resources to develop a sophisticated characterization of colorectal tissues. © 2021 Elsevier Ltd
    view abstract10.1016/j.clinbiomech.2021.105445
  • An in vitro comparison of flow dynamics of the Magna Ease and the Trifecta prostheses
    Marx, P. and Kowalczyk, W. and Demircioglu, A. and Shehada, S.-E. and Wendta, H. and Mourad, F. and Thielmann, M. and Jakob, H. and Wendt, D.
    Minimally Invasive Therapy and Allied Technologies 29 (2020)
    Objectives: We aimed to compare the in vitro flow dynamics of the Perimount Magna Ease™ (PME) and the Trifecta™ (TF) bioprostheses. Material and methods: A new flow chamber was designed to compare the flow patterns of the PME (Edwards Lifesciences, Irvine, CA, USA) and the TF (SJM, St. Paul, MN, USA) aortic valve prostheses. This new channel offered the possibility of 2D-particle-image-velocimetry (2D-PIV) to completely evaluate the flow field downstream from the aortic valve to the middle of the aortic arch. Maximum average velocities, vorticity, shear strength, maximum orifice diameters and jet flow diameters were analyzed. Valve sizes of 21, 23 and 25 mm were evaluated. Results: Average velocity values, shear strength and vorticities were smaller in the flow field of the TF (maximum average velocity: 0.81 ± 0.03m/s, PME 23 mm vs. 0.7 ± 0.02m/s TF 23 mm, P <.001) under pulsatile flow conditions (70 Hz, 70 mL stroke volume). The evaluation of the upper orifice area revealed bigger maximum diameters during the peak flow phase for the TF, but more leaflet-flutter. Conclusions: Our flow chamber allowed a precise and highly sensitive characterization and comparison of complex fluid dynamics of different aortic valve prostheses. Both the Trifecta™ and the Perimount Magna Ease™ showed a good performance on a high level. © 2019, © 2019 Society of Medical Innovation and Technology.
    view abstract10.1080/13645706.2019.1586732
  • Determination of flow parameters of a water flow around an AUV body
    Hoth, J. and Kowalczyk, W.
    Robotics 8 (2019)
    Autonomous underwater vehicles (AUVs) have changed the way marine environment is surveyed, monitored and mapped. Autonomous underwater vehicles have a wide range of applications in research, military, and commercial settings. AUVs not only perform a given task but also adapt to changes in the environment, e.g., sudden side currents, downdrafts, and other effects which are extremely unpredictable. To navigate properly and allow simultaneous localisation and mapping (SLAM) algorithms to be used, these effects need to be detected. With current navigation systems, these disturbances in the water flow are not measured directly. Only the indirect effects are observed. It is proposed to detect the disturbances directly by placing pressure sensors on the surface of the AUV and processing the pressure data obtained. Within this study, the applicability of different learning methods for determining flow parameters of a surrounding fluid from pressure on an AUV body are tested. This is based on CFD simulations using pressure data from specified points on the surface of the AUV. It is shown that support vector machines are most suitable for the given task and yield excellent results. © 2019 by the authors.
    view abstract10.3390/robotics8010005
  • The Fluid Dynamical Performance of the Carpentier-Edwards PERIMOUNT Magna Ease Prosthesis
    Marx, P. and Kowalczyk, W. and Demircioglu, A. and Brault, G.N. and Wendt, H. and Shehada, S.-E. and Tsagakis, K. and El Gabry, M. and Jakob, H. and Wendt, D.
    BioMed Research International 2018 (2018)
    The aim of the present in vitro study was the evaluation of the fluid dynamical performance of the Carpentier-Edwards PERIMOUNT Magna Ease depending on the prosthetic size (21, 23, and 25 mm) and the cardiac output (3.6-6.4 L/min). A self-constructed flow channel in combination with particle image velocimetry (PIV) enabled precise results with high reproducibility, focus on maximal and local peek velocities, strain, and velocity gradients. These flow parameters allow insights into the generation of forces that act on blood cells and the aortic wall. The results showed that the 21 and 23 mm valves have a quite similar performance. Maximal velocities were 3.03±0.1 and 2.87±0.13 m/s; maximal strain Exx, 913.81±173.25 and 896.15±88.16 1/s; maximal velocity gradient Eyx, 1203.14±221.84 1/s and 1200.81±61.83 1/s. The 25 mm size revealed significantly lower values: maximal velocity, 2.47±0.15 m/s; maximal strain Exx, 592.98±155.80 1/s; maximal velocity gradient Eyx, 823.71±38.64 1/s. In summary, the 25 mm Magna Ease was able to create a wider, more homogenous flow with lower peak velocities especially for higher flow rates. Despite the wider flow, the velocity values close to the aortic walls did not exceed the level of the smaller valves. © 2018 Philipp Marx et al.
    view abstract10.1155/2018/5429594
  • Colour reconstruction of underwater images
    Hoth, J. and Kowalczyk, W.
    Underwater Technology 34 (2017)
    Objects look very different in the underwater environment compared to their appearance in sunlight. Images with correct colouring simplify the detection of underwater objects and may permit the use of visual simultaneous localisation and mapping (SLAM) algorithms developed for land-based robots underwater. Hence, image processing is required. Current algorithms focus on the colour reconstruction of scenery at diving depth where different colours can still be distinguished, but this is not possible at greater depth. This study investigates whether machine learning can be used to transform image data. First, laboratory tests are performed using a special light source imitating underwater lighting conditions, showing that the k-nearest neighbour method and support vector machines yield excellent results. Based on these results, an experimental verification is performed under severe conditions in the murky water of a diving basin. It shows that the k-nearest neighbour method gives very good results for short distances between the object and the camera, as well as for small water depths in the red channel. For longer distances, deeper water and the other colour channels, support vector machines are the best choice for the reconstruction of the colour as seen under white light from the underwater images. © 2017, Society for Underwater Technology. All rights reserved.
    view abstract10.3723/ut.34.051
  • Geometric analysis and clinical outcome of two cemented stems for primary total hip replacement with and without modular necks
    Haversath, M. and Wendelborn, C. and Jäger, M. and Schmidt, B. and Kowalczyk, W. and Landgraeber, S.
    Archives of Orthopaedic and Trauma Surgery 137 (2017)
    Introduction: Restoration of the physiological biomechanical principles of the hip is crucial in total hip replacement. The aim of this study was to compare an arthroplasty system with different offset options (a: Exeter®) with a dual-modular stem (b: Profemur Xm®). Materials and methods: A local and an inertial coordinate system were used to assist the description of the components’ assembly in the prosthesis. A resection line of the femoral head in standard position was added to the arthroplasties and geometric parameters were measured. The outcomes of 93 patients were clinically evaluated (a: n = 50, b: n = 43). Preoperative planning was compared to postoperative radiographs (femoral offset, leg-length), and clinical scores (HHS, WOMAC, total range of motion) were assessed preoperatively, and then 1 and 2 years after surgery. Results: The Exeter® offers an offset range from 32.1 to 56.9 mm and the Profemur Xm® a range from 29.3 to 55.3 mm. The leg-length variability of the Profemur Xm® has a range of 25.9 mm, the Exeter® a range of 13.7 mm. The Profemur Xm® offers more possible combinations of offset and leg-length reconstruction. The neck–stem angles of the Exeter® range from 125.2° to 126.3°, of the Profemur Xm® from 127.2° to 142.6°. There was no statistically significant difference in clinical outcome and radiological parameters. Conclusions: We conclude that both stems offer a wide range of options for anatomical reconstruction of the hip resulting in similarly good clinical results. The Profemur Xm® stem has advantages for the reconstruction of hips that deviate from standard anatomy but has the drawback of additional corrosive wear at the stem/neck interface. © 2017, Springer-Verlag GmbH Germany.
    view abstract10.1007/s00402-017-2785-9
  • Effect of the stiffness of bone substitutes on the biomechanical behaviour of femur for core decompression
    Tran, T.N. and Kowalczyk, W. and Hohn, H.P. and Jäger, M. and Landgraeber, S.
    Medical Engineering and Physics 38 (2016)
    Core decompression is the most common procedure for treatment of the early stages of osteonecrosis of the femoral head. The purpose of this study was to compare the biomechanical performance of four different bone graft substitutes combined with core decompression. Subject-specific finite element models generated from computed tomography (CT) scan data were used for a comprehensive analysis. Two different contact conditions were simulated representing states of osseointegration at the interface. Our results showed that the use of a low-stiffness bone substitute did not increase the risk of femoral fracture in the early postoperative phase, but resulted in less micromotion and interfacial stresses than high-stiffness bone substitutes. © 2016 IPEM
    view abstract10.1016/j.medengphy.2016.05.008
  • Simultaneous optimisation of earwig hindwings for flight and folding
    Deiters, J. and Kowalczyk, W. and Seidl, T.
    Biology Open 5 (2016)
    Earwig wings are highly foldable structures that lack internal muscles. The behaviour and shape changes of the wings during flight are yet unknown. We assume that they meet a great structural challenge to control the occurring deformations and prevent the wing from collapsing. At the folding structures especially, the wing could easily yield to the pressure. Detailed microscopy studies reveal adaptions in the structure and material which are not relevant for folding purposes. The wing is parted into two structurally different areas with, for example, a different trend or stiffness of the wing veins. The storage of stiff or more flexible material shows critical areas which undergo great changes or stress during flight. We verified this with high-speed video recordings. These reveal the extent of the occurring deformations and their locations, and support our assumptions. The video recordings reveal a dynamical change of a concave flexion line. In the static unfolded state, this flexion line blocks a folding line, so that the wing stays unfolded. However, during flight it extends and blocks a second critical folding line and prevents the wing from collapsing. With these results, more insight in passive wing control, especially within high foldable structures, is gained. © 2016, Company of Biologists Ltd. All rights reserved.
    view abstract10.1242/bio.016527
  • Colour correction of underwater images
    Hoth, J. and Kowalczyk, W.
    MTS/IEEE OCEANS 2015 - Genova: Discovering Sustainable Ocean Energy for a New World (2015)
    Objects look very different in the underwater environment compared to their appearance in sunlight. High quality images with correct colouring simplify the detection of underwater objects and may allow the use of visual SLAM algorithms developed for land-based robots underwater. Hence, image processing is required to obtain images of high quality and correct colouring. Current algorithms focus on the colour reconstruction of scenery at diving depth which has the advantage that a significant part of sunlight is still present and different colours can still be distinguished. At greater depth the filtering is much stronger such that this is no longer possible. In this study it is investigated whether machine learning can be used to transform image data. In order to obtain images under underwater lighting conditions in a controlled environment a special light source with a defined wavelength is used for illumination of test objects in a laboratory setup. The images are then fed through statistical learning algorithms with or without pre-filters. It is shown that k-nearest neighbour and support vector machines are most suitable for the given task and yield excellent results. Most difficult are the distinction between red and black / dark grey objects as well as coping with reflections. © 2015 IEEE.
    view abstract10.1109/OCEANS-Genova.2015.7271356
  • Direct determination of shape functions for isoparametric elements with arbitrary node configuration
    Hoth, J. and Kowalczyk, W.
    Open Engineering 5 (2015)
    Shape functions have been derived to describe different forms of elements, notably triangles and rectangles in 2-D, and tetrahedrons, cuboids, and triangular prisms in 3-D. There are generalised solutions for some regular node configurations, and hierarchical correction algorithms help with more difficult node distributions. But to this point there is no single formula or set of formulae that allows the direct determination of shape functions for any node configuration without restrictions. This paper shows how a general set of formulae can be derived which is applicable to any isoparametric element type with arbitrary node configuration. This formulation is in such a form that it is clear and concise. The approach is based on the Lagrange polynomial considering up to three Cartesian and four volume coordinates. Additionally, the correction procedure that is inherent in the formulation to guarantee an appropriate evaluation of the generalised shape functions and to fulfil all four isoparametric shape function criteria is discussed. The proof of validity illustrates the correctness of the method. © 2015 J. Hoth and W. Kowalczyk.
    view abstract10.1515/eng-2015-0049
  • Geometric analysis of an expandable reamer for treatment of avascular necrosis of the femoral head
    Landgraeber, S. and Tran, T.N. and Claßen, T. and Warwas, S. and Theysohn, J. and Lazik, A. and Jäger, M. and Kowalczyk, W.
    Archives of Orthopaedic and Trauma Surgery 135 (2015)
    “Advanced core decompression” (ACD) is a treatment option for osteonecrosis of the femoral head (ONFH) that aims at complete removal of the necrotic tissue using a percutaneous expandable reamer and refilling of the head with an osteoconductive bone-graft substitute. The objective of this study was to evaluate if the success of ACD depends on the amount of necrotic tissue remaining after the procedure and how efficiently the necrotic tissue can be removed with the current reamer. Three-dimensional models of proximal femora including ONFH were generated from the preoperative MRIs of 50 patients who underwent ACD. Best-case removal was calculated by geometrical analysis. In 28 of 50 cases, postoperative MRI was used to determine how much necrotic tissue had been removed. Prognostic values and correlations were evaluated in order to assess success or failure of the treatment. The amount of preoperative and remaining necrosis correlates significantly with treatment failure. The larger both volumes are, the more likely it is that treatment will fail. In patients with remaining necrosis of less than 1000 mm3, no treatment failure was observed. The amount of necrosis actually removed differed significantly from the amount calculated as the best possible result. Simulation of the removal procedure showed that complete removal is not possible. These results led to the conclusion that the success of ACD depends on the amount of necrotic tissue remaining in the femoral head after the procedure. Modifications to the instrument are necessary to increase the amount of necrotic tissue that can be removed. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00402-015-2287-6
  • The investigation of systolic and diastolic leaflet kinematics of bioprostheses with a new in-vitro test method
    Wendt, D. and Stühle, S. and Marx, P. and Benedik, J. and Wendt, H. and Stühle, T. and Thoenes, M. and Thielmann, M. and Jakob, H. and Kowalczyk, W.
    Minimally Invasive Therapy and Allied Technologies 24 (2015)
    Objectives: We aimed to investigate leaflet kinematics of bioprostheses with a novel high-speed imaging method. Material and methods: High-speed-imaging (1000Hz) was used to evaluate leaflet kinematics of the Carpentier-Edwards Perimount Magna (PM) and Magna Ease (PME) aortic bioprostheses. Both prostheses (diameter 23 mm) were placed inside a model aorta under pulsatile flow conditions. Frequencies (F) and different stroke volumes (S) were simulated. Maximum aortic valve area (AVA), total ejection time (TET), rapid valve opening time (RVOT) and rapid valve closing time (RVCT) as well as opening (OS) and closing (CS) speeds were evaluated. Results: Both bioprostheses showed different results dependent on flow conditions. The test setup was capable of identifying small AVA-differences between both valves (235 vs 202 mm2, F60/S60; 272 vs 207 mm2 F70/S80), as well as differences in OS and CS (2.36 vs 1.62 mm2/ms; 2.97 vs 2.44 mm2/ms, F80/S60). TET was comparable (638 vs 645 ms F60/S60; 341 vs 343 ms, F90/S60), while results for RVOT and RVCT were equal, and dependent on frequency and stroke volume. Conclusions: The novel evaluation method is sensitive to detect differences between valves, although differences were found to be small. PM has a larger visible AVA associated with higher opening and closing speeds in contrast to PME. © 2015 Informa Healthcare.
    view abstract10.3109/13645706.2015.1078818
  • Experimental and computational studies on the femoral fracture risk for advanced core decompression
    Tran, T.N. and Warwas, S. and Haversath, M. and Classen, T. and Hohn, H.P. and Jäger, M. and Kowalczyk, W. and Landgraeber, S.
    Clinical Biomechanics 29 (2014)
    Background Two questions are often addressed by orthopedists relating to core decompression procedure: 1) Is the core decompression procedure associated with a considerable lack of structural support of the bone? and 2) Is there an optimal region for the surgical entrance point for which the fracture risk would be lowest? As bioresorbable bone substitutes become more and more common and core decompression has been described in combination with them, the current study takes this into account. Methods Finite element model of a femur treated by core decompression with bone substitute was simulated and analyzed. In-vitro compression testing of femora was used to confirm finite element results. Findings The results showed that for core decompression with standard drilling in combination with artificial bone substitute refilling, daily activities (normal walking and walking downstairs) are not risky for femoral fracture. The femoral fracture risk increased successively when the entrance point is located further distal. The critical value of the deviation of the entrance point to a more distal part is about 20 mm. Interpretation The study findings demonstrate that optimal entrance point should locate on the proximal subtrochanteric region in order to reduce the subtrochanteric fracture risk. Furthermore the consistent results of finite element and in-vitro testing imply that the simulations are sufficient. © 2014 Elsevier Ltd.
    view abstract10.1016/j.clinbiomech.2014.02.001
  • A prospective randomized peri- and post-operative comparison of the minimally invasive anterolateral approach versus the lateral approach.
    Landgraeber, S. and Quitmann, H. and Guth, S. and Haversath, M. and Kowalczyk, W. and Kecskemethy, A. and Heep, H. and Jäger, M.
    Orthopedic reviews 5 (2013)
    There is still controversy as to whether minimally invasive total hip arthroplasty enhances the postoperative outcome. The aim of this study was to compare the outcome of patients who underwent total hip replacement through an anterolateral minimally invasive (MIS) or a conventional lateral approach (CON). We performed a randomized, prospective study of 75 patients with primary hip arthritis, who underwent hip replacement through the MIS (n=36) or CON (n=39) approach. The Western Ontario and McMaster Universities Osteoarthritis Index and Harris Hip score (HHS) were evaluated at frequent intervals during the early postoperative follow-up period and then after 3.5 years. Pain sensations were recorded. Serological and radiological analyses were performed. In the MIS group the patients had smaller skin incisions and there was a significantly lower rate of patients with a positive Trendelenburg sign after six weeks postoperatively. After six weeks the HHS was 6.85 points higher in the MIS group (P=0.045). But calculating the mean difference between the baseline and the six weeks HHS we evaluated no significant differences. Blood loss was greater and the duration of surgery was longer in the MIS group. The other parameters, especially after the twelfth week, did not differ significantly. Radiographs showed the inclination of the acetabular component to be significantly higher in the MIS group, but on average it was within the same permitted tolerance range as in the CON group. Both approaches are adequate for hip replacement. Given the data, there appears to be no significant long term advantage to the MIS approach, as described in this study.
    view abstract10.4081/or.2013.e19
  • Advanced core decompression, a new treatment option of avascular necrosis of the femoral head-a first follow-up
    Landgraeber, S. and Theysohn, J.M. and Classen, T. and Jäger, M. and Warwas, S. and Hohn, H.-P. and Kowalczyk, W.
    Journal of Tissue Engineering and Regenerative Medicine 7 (2013)
    Aseptic necrosis of the femoral head (AVN) leads to destruction of the affected hip joint, predominantly in younger patients. Advanced core decompression (ACD) is a new technique that may allow better removal of the necrotic tissue by using a new percutaneous expandable reamer. A further modification is the refilling of the drill hole and the defect with an injectable, hard-setting, composite calcium sulphate (CaSO4)-calcium phosphate (CaPO4) bone graft substitute. Compression tests were performed on seven pairs of femoral cadaver bones. One femur of each pair was treated with ACD, while the opposite side remained untreated. Clinically, the postoperative outcome of 27 hips in 23 patients was performed by physical examination 6weeks after ACD and at average follow-up of 9.69months, and compared with the preoperative results. MRI was used to assess the removal of the necrotic tissue, any possible progression of AVN and evaluation of collapse. In the biomechanical analysis, the applied maximum compression force that caused the fracture did not significantly differ from the untreated opposite side. The overall results of postoperative physical examinations were significantly better than preoperatively. Five hips (18.5%) were converted to a total hip replacement. The follow-up MRIs of the other patients showed no progression of the necrotic area. The first follow-up results of ACD have been encouraging for the early stages of aseptic necrosis of the femoral head. In our opinion, an assured advantage is the high stability of the femoral neck after ACD, which allows quick rehabilitation. © 2012 John Wiley & Sons, Ltd.
    view abstract10.1002/term.1481
  • Numerical simulation of particle distribution in capillary membrane during backwash
    Mansour, H. and Keller, A. and Gimbel, R. and Kowalczyk, W.
    Membranes 3 (2013)
    The membrane filtration with inside-out dead-end driven UF-/MF-capillary membranes is an effective process for particle removal in water treatment. Its industrial application increased in the last decade exponentially. To date, the research activities in this field were aimed first of all at the analysis of filtration phenomena disregarding the influence of backwash on the operation parameters of filtration plants. However, following the main hypothesis of this paper, backwash has great potential to increase the efficiency of filtration. In this paper, a numerical approach for a detailed study of fluid dynamic processes in capillary membranes during backwash is presented. The effect of particle size and inlet flux on the backwash process are investigated. The evaluation of these data concentrates on the analysis of particle behavior in the cross sectional plane and the appearance of eventually formed particle plugs inside the membrane capillary. Simulations are conducted in dead-end filtration mode and with two configurations. The first configuration includes a particle concentration of 10% homogeneously distributed within the capillary and the second configuration demonstrates a cake layer on the membrane surface with a packing density of 0.6. Analyzing the hydrodynamic forces acting on the particles shows that the lift force plays the main role in defining the particle enrichment areas. The operation parameters contribute in enhancing the lift force and the heterogeneity to anticipate the clogging of the membrane. © 2013 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/membranes3040249
  • Comparison of flow dynamics of Perimount Magna and Magna Ease aortic valve prostheses
    Wendt, D. and Sthle, S. and Piotrowski, J.A. and Wendt, H. and Thielmann, M. and Jakob, H. and Kowalczyk, W.
    Biomedizinische Technik 57 (2012)
    The aim of the present study was to evaluate and compare the in vitro and flow dynamics of the Magna (MB) and the Magna Ease aortic valve bioprosthesis (MEB) within the ascending aorta. A 2D-particle-image-velocimetry (2D-PIV) study was performed to compare the flow dynamics induced by each pericardial Carpentier-Edwards Magna and Magna Ease aortic valve prosthesis in the aortic flow field directly behind the valve. Both prostheses (diameter 23 mm) were placed inside an artificial aorta under pulsatile flow conditions (70 Hz and 70 ml stroke volume). The flow field was evaluated according to velocity, shear strength, and vorticity. Both prostheses showed a jet flow type profile with a maximum velocity of 0.97±0.09 m/s for MB and 0.83±1.8 m/s for MEB. Flow fields of both valves were similar in acceleration, peak flow deceleration and leakage phase. Maximum shear strength was 20,285±11,774 l/s 2 for MB and 17,006±8453 l/s 2 for MEB. Vorticity was nearly similar for counterclockwise and clockwise rotation in both prostheses, but slightly higher with MB (251±41 l/s and -250±39 l/s vs. 225±48 l/s and -232±48 l/s). The point-of-interest (POI)-analysis revealed a higher velocity for left-sided aortic wall compared to right-sided at MB (0.12±0.09 m/s vs. 0.18±0.10 m/s, p< 0.001), but was consistent at MEB (0.09±0.05 m/s vs. 0.08±0.04 m/s, p0.508), respectively. Velocity, shear strength and vorticity in an in vitro test set-up are lower with MEB compared to MB, thus resulting in improved flow dynamics with a similar flow field, which might have a positive influence on blood rheology and potential valve degeneration. © 2012 by Walter de Gruyter Berlin Boston 2012.
    view abstract10.1515/bmt-2011-0076
  • Development and In Vitro Characterization of a New Artificial Flow Channel
    Wendt, D. and Stühle, S. and Hou, G. and Thielmann, M. and Tsagakis, K. and Wendt, H. and Jakob, H. and Kowalczyk, W.
    Artificial Organs 35 (2011)
    To date, cardiac valve diseases are considered as a major public health problem and most frequently, the aortic valve is affected. To treat high-risk patients, catheter-based techniques have been developed recently, avoiding open heart surgery and/or cardiopulmonary bypass. Although these sophisticated and rapidly emerging catheter-based technologies do allow a minimally invasive treatment option of high-risk patients on the one hand, further developments and in vitro testing under physiological conditions are necessary, on the other hand, in order to further optimize them for clinical routines. Therefore, we present the concept of a new multifunctional flow channel, offering (i) the possibility of transapical access; (ii) the simulation of physiological flow conditions; and (iii) the evaluation of the fluid flow by 2D particle image velocimetry within a wide range of parameters. © 2011, © the Authors. Artificial Organs © 2011, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.
    view abstract10.1111/j.1525-1594.2010.01166.x
  • Fluid dynamic investigation of the ARS 3F enable sutureless heart valve
    Stühle, S. and Wendt, D. and Hou, G. and Wendt, H. and Thielmann, M. and Jakob, H. and Kowalczyk, W.
    Innovations: Technology and Techniques in Cardiothoracic and Vascular Surgery 6 (2011)
    Objective: Currently, sutureless heart valves (SHV) reveal good clinical results during aortic valve replacement. The aim of this study was to evaluate the fluid dynamics of the ATS 3F Enable SHV in the ascending aorta and their influence on the aortic wall in an in vitro setup. Methods: A two-dimensional particle image velocimetry study with an image rate of 15 Hz was conducted to evaluate the fluid dynamics of the SHV in the aortic flow field. The prosthesis (diameter, 23 mm) was placed inside a silicone mock aorta under pulsatile flow conditions. Velocities, vorticity, and strain rate were obtained and calculated with a fixed frequency (70 Hz) at constant stroke volume (70 mL). Results: 3F Enable showed a jet flow type profile with a maximum velocity of 1.01 ± 0.13 m/s during peak flow phase (PFP). The jet flow was surrounded by ambilateral vortices with a slightly higher percentage of clockwise than counterclockwise vorticity (377 ± 57/s vs 389 ± 76/s), strain rate (370 ± 79/s for elongation vs -370 ± 102/s for contraction) was nearly similar. The point-of-interest analysis revealed a higher velocity for bottom compared with upper aortic wall (0.28 ± 0.07 m/s vs 0.31 ± 0.06 m/s, P = 0.024). All values were lower during acceleration and deceleration phase compared with PFP. Conclusions: The peak flow of the 3F Enable SHV seems to be little higher compared with native aortic valves, thus simulating nearly physiologic conditions. Vorticity and strain rate are high during PFP and low during other phases and might have an influence on the aortic wall as well. Copyright © 2011 by the International Society for Minimally Invasive Cardiothoracic Surgery.
    view abstract10.1097/IMI.0b013e31820c0f0c
  • Numerical simulation of hemodynamics in the ascending aorta induced by different aortic cannulas
    Stühle, S. and Wendt, D. and Jakob, H. and Kowalczyk, W.
    Minimally Invasive Therapy and Allied Technologies 20 (2011)
    There is still a lack of quantitative information concerning optimal blood flow in the aorta and in the carotid arteries during extracorporeal circulation (ECC). Problems are not only based on the location of the aortic cannula, they are furthermore associated with the cannula design itself and the effects on blood cells and aortic wall shear stresses. We simulated a two-phase fluid flow induced by different cannulas in the ascending aorta during ECC. Three commercially available cannulas were examined according to their influence on red blood cells (RBC). Additionally, mass flow in the carotid vessels and wall shear stresses acting on the aortic wall were evaluated. A constant volume flow of blood (3.4 L/min) was applied. Numerical results demonstrate a strong relation between the mass flow rate in the carotid vessels and the geometry of the aortic outflow cannula. RBC distributions both in the aorta and the carotid vessels changed depending on cannula geometry. Maximum blood velocities, shear stresses on the aortic wall, and the fluid mechanical load acting on RBCs varied depending on each cannula design. This numerical approach demonstrates the significant influence of the cannula design on the distribution of RBCs in the carotid vessels during ECC. © 2011 Informa Healthcare.
    view abstract10.3109/13645706.2011.553957
  • Biomechanics

  • fluid dynamics

  • modelling and simulation

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