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Ned H. C. Hwang - One of the best experts on this subject based on the ideXlab platform.
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On Accelerated Fatigue Testing of Prosthetic Heart Valves
Frontiers in Biomedical Engineering, 2003Co-Authors: Jia-shing Liu, Ned H. C. HwangAbstract:Accelerated Testing (AT) of prosthetic heart Valves allows simulation of wear and fatigue sustained by the replacement heart Valves, and to estimate the Valves’ life expectancy in human body. At accelerated Test rates, sufficient amounts of data can be collected within a reasonably short time period, after repeated opening and closing cycles, to predict the Valve durability. The U.S. Food and Drug Administration (FDA) Replacement Heart Valve Guidance (Version 4.1, 1994) requires that mechanical heart Valves (MHV) must be Tested at least 600 million cycles (equivalent to 15 years in vivo), while biological heart Valve prostheses (BHV) must be Tested at least 200 million cycles (equivalent to 5 years in vivo) in pulsatile flow simulators. The cyclic Test must meet two basic FDA requirements: 1) the Test Valve open and close fully each cycle; and 2) the average transvalvular pressure is kept at least 100 mmHg at closure. At accelerated Test rates, the Valves were subjected to non-physiologic dynamic force loads and often damaged under excessive conditions, such as cavitation. AT may pinpoint early flaws in the design and in the manufacturing processes, and deflects regions of materials weakness. Hence the design of AT must follow the principles of engineering Testing such as the law of dynamic similarities. One must first identify dimensionless parameters that are physiologically meaningful and those much be specific to heart Valve Testing. The main goal of this paper is to present an AT system and an experimental protocol so that in vitro accelerated Testing may be carried out without creating these excess forces on the Test Valves and to predict the durability of prosthetic heart Valves with physiological considerations.
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Dynamic impact stress analysis of a bileaflet mechanical heart Valve.
The Journal of heart valve disease, 2003Co-Authors: Qi Yuan, Bryan Kok Ann Ngoi, Tony Joon Hock Yeo, Ned H. C. HwangAbstract:Background and aims of the study Mechanical heart Valves (MHV) are widely used to replace dysfunctional and failed heart Valves. The bileaflet MHV is very popular due to its superior hemodynamics. At present, bileaflet MHVs account for about two-thirds of the prosthetic heart Valve market. Since their introduction in 1977, the hemodynamics of bileaflet prostheses has been extensively studied. New technologies used to develop MHV include better design concepts, materials, manufacturing processes, and post-design verification. The study aim was to investigate the dynamic impact stress of a newly designed bileaflet MHV under normal physiological conditions. Methods Pro/Engineer was used to generate a 3-D model of the designed Valve. ANSYS 5.5 and LS-DYNA were used to calculate stress and deformation of the Valve. Due to symmetry, a one-half orifice and one leaflet were modeled using the eight-noded hexahedral elements. When Valve leaflets are in the fully closed position, the static contact stress between leaflet and orifice was predicated under typical heart Valve closing pressure of 80 mmHg. To study the dynamic effects of the closing Valve, LS-DYNA was used to simulate leaflet motion. Typical physiological pressure waveform was employed to initiate this leaflet motion. Two types of Valve were investigated: Test Valve A (size 19, flat leaflet); and Test Valve B (size 19, tapered leaflet 1.5 degrees, with the same thickness at pivot as Valve A). The non-invasive laser sweeping technique was used to measure leaflet closing velocity in a mock flow Test rig. The closing velocity of Test Valve A was compared by experimental and computed results. The corresponding dynamic contact stress on the leaflet was obtained for different modes of loading, simulated under angular velocity, acceleration, and especially under representative pressure waveform. Results The experimental closing velocity of Test Valve A was 1.07 +/- 0.05 m/s; the computed value was 1.130 m/s. During full closure, the leaflets showed a slight rebound, and this was also seen experimentally. For Test Valve B, the computed closing velocity was 1.039 m/s. In the dynamic impact analysis, the physiological pressure waveform was obtained at a normal heart rate of 70 beats/min from the mock flow Test rig. Dynamic stress and displacement of the model Valve were calculated as the Valve was closing. The time step of calculation was determined by the wave propagation velocity and element size. With an interhinge distance of 4.966 mm based on the geometric design of the Valve, maximum dynamic von Mises stress appeared near the hinge of the leaflet (26.92 MPa for Valve A; 22.36 MPa for Valve B). By varying the position of the hinge/pivoting axis (+/- 10%), an optimized Valve geometry could be obtained based on minimal impact stress on the Valve leaflet. Conclusion Based on closing velocity comparison of Valve A, the calculated model and loading conditions were seen to be reasonable. Computational accuracy was satisfied. The tapering feature of the leaflet is designed especially for minimal impact stress at the leaflet contact areas upon impact with the inner walls of the BMHV. These points provide an optimum structure design for the Nanyang Technological University BMHV.
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Bioprosthetic heart Valve leaflet motion monitored by dual camera stereo photogrammetry
Journal of Biomechanics, 2000Co-Authors: Zhi B. Gao, Samir Pandya, Nadeen Hosein, Michael S. Sacks, Ned H. C. HwangAbstract:Abstract Dual camera stereo photogrammetry (DCSP) was applied to investigate the leaflet motion of bioprosthetic heart Valves (BHVs) in a physiologic pulse flow loop (PFL). A 25-mm bovine pericardial Valve was installed in the aortic Valve position of the PFL, which was operated at a pulse rate of 70 beats/min and a cardiac output of 5 l/min. The systolic/diastolic aortic pressure was maintained at 120/80 mmHg to mimic the physiologic load experienced by the aortic Valve. The leaflet of the Test Valve was marked with 80 India ink dots to form a fan-shaped matrix. From the acquired image sequences, 3-D coordinates of the marker matrix were derived and hence the surface contour, local mean and Gaussian curvatures at each opening and closing phase during one cardiac cycle were reconstructed. It is generally believed that the long-term failure rate of BHV is related to the uneven distribution of mechanical stresses occurring in the leaflet material during opening and closing. Unfortunately, a quantitative analysis of the leaflet motion under physiological conditions has not been reported. The newly developed technique permits frame-by-frame mapping of the leaflet surface, which is essential for dynamic analysis of stress–strain behavior in BHV.
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Technical note Bioprosthetic heart Valve lea#et motion monitored by dual camera stereo photogrammetry
2000Co-Authors: Zhi B. Gao, Samir Pandya, Nadeen Hosein, Michael S. Sacks, Ned H. C. HwangAbstract:Dual camera stereo photogrammetry (DCSP) was applied to investigate the lea#et motion of bioprosthetic heart Valves (BHVs) in a physiologic pulse#ow loop (PFL). A 25-mm bovine pericardial Valve was installed in the aortic Valve position of the PFL, which was operated at a pulse rate of 70 beats/min and a cardiac output of 5 l/min. The systolic/diastolic aortic pressure was maintained at 120/80 mmHg to mimic the physiologic load experienced by the aortic Valve. The lea#et of the Test Valve was marked with 80 India ink dots to form a fan-shaped matrix. From the acquired image sequences, 3-D coordinates of the marker matrix were derived and hence the surface contour, local mean and Gaussian curvatures at each opening and closing phase during one cardiac cycle were reconstructed. It is generally believed that the long-term failure rate of BHV is related to the uneven distribution of mechanical stresses occurring in the lea#et material during opening and closing. Unfortunately, a quantitative analysis of the lea#et motion under physiological conditions has not been reported. The newly developed technique permits frame-by-frame mapping of the lea#et surface, which is essential for dynamic analysis of stress}strain behavior in BHV. ( 2000 Elsevier Science Ltd. All rights reserved.
Takeyoshi Dohi - One of the best experts on this subject based on the ideXlab platform.
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Nonsymmetric Leaflet Motion of St. Jude Medical Mitral Valves Simulated with a Computer-Controlled Hydraulic Mock Circulator
Artificial organs, 2008Co-Authors: Yuji Ohta, Takeyoshi Dohi, Kazutoshi Okamoto, Marcel Sonderegger, Hiroshi Matsumoto, Takashi HoriuchiAbstract:: Since the introduction of high-performance prosthetic heart Valves, particularly bileaflet Valves and monoleaflet Valves with larger opening angles, we have observed in vivo complex leaflet motion which has not seen experimentally confirmed yet. We developed a computer-controlled hydraulic mock circulator to study the motion of in vivo leaflets. A high speed CCD camera recorded the Valve movement. In the mitral position, a standard St. Jude Medical Valve was tilted to be horizontal or vertical. The Test Valve was driven with single- or double-peaked flow. The flow rate was set to 5.0 Umin at 70 bpm with a systole/diastole ratio of 0.3. We found the following results: independent of Valve orientation, the Valve showed a nonsymmetric leaflet motion; the Valve showed unpredictable leaflet position during decreasing flow or absence of flow; and the disc closed temporarily at the lower inflow rate between the 2 flow peaks.
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Effect of the Sinus of Valsalva on the Closing Motion of Bileaflet Prosthetic Heart Valves
Artificial organs, 2000Co-Authors: Yuji Ohta, Yukiaki Kikuta, Toshiyuki Shimooka, Yoshinori Mitamura, T. Yuhta, Takeyoshi DohiAbstract:: Conventional bileaflet prosthetic mechanical heart Valves close passively with backflow. Naturally, the Valve has problems associated with closure, such as backflow, water hammer effect, and fracture of the leaflet. On the other hand, in the case of the natural aortic Valve, the vortex flow in the sinus of Valsalva pushes the leaflet to close, and the Valve starts the closing motion earlier than the prosthetic Valve as the forward flow decelerates. This closing mechanism is thought to decrease backflow at Valve closure. In this study, we propose a new bileaflet mechanical Valve resembling a drawbridge in shape, and the prototype Valve was designed so that the leaflet closes with the help of the vortex flow in the sinus. The Test Valve was made of aluminum alloy, and its closing motion was compared to that of the CarboMedics (CM) Valve. Both Valves were driven by a computer controlled hydraulic mock circulator and were photographed at 648 frames/s by a high speed charge-coupled device (CCD) camera. Each frame of the Valve motion image was analyzed with a personal computer, and the opening angles were measured. The flow rate was set as 5.0 L/min. The system was pulsed with 70 bpm, and the systolic/diastolic ratio was 0.3. Glycerin water was used as the circulation fluid at room temperature, and polystyrene particles were used to visualize the streamline. The model of the sinus of Valsalva was made of transparent silicone rubber. As a result, high speed video analysis showed that the Test Valve started the closing motion 41 ms earlier than the CM Valve, and streamline analysis showed that the Test Valve had a closing mechanism similar to the natural one with the effect of vortex flow. The structure of the Test Valve was thought to be effective for soft closure and could solve problems associated with closure.
Jaehun Jung - One of the best experts on this subject based on the ideXlab platform.
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a new synthetic Test circuit for Testing thyristor Valve in hvdc converter
The Transactions of the Korean Institute of Power Electronics, 2012Co-Authors: Jaehun Jung, Yongho Chung, Seungtaek BaekAbstract:This paper proposes a new synthetic Test circuit (STC) to confirm the switching operation of thyristor Valve in HVDC converter. The proposed STC uses a 6-pulse thyristor converter with 2-phase chopper as a high-current source to provide turn-on current to the Test Valve. The operation of proposed STC was verified through theoretical analysis and computer simulations. Based on computer simulations, a hardware scaled model was built and Tested to confirm the feasibility of implementing a real-size Test facility. The proposed system has an advantage of simple structure and operation over the existing system.
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New synthetic Test circuit for Testing thyristor Valve in HVDC converter
2012 IEEE Energy Conversion Congress and Exposition (ECCE), 2012Co-Authors: Jaehun JungAbstract:This paper proposes a new synthetic Test circuit to confirm the switching behavior of thyristor Valve in HVDC converter. The proposed circuit consists of a 2-phase chopper with IGBT switch and auxiliary circuit to supplies low-voltage high current to the Test Valve during on-state. The operation of proposed circuit was verified through theoretical approach and computer simulation. Based on computer simulation results, a hardware scaled model for the proposed circuit was built and Tested to confirm the feasibility of implementing a real Test facility. The proposed synthetic Test circuit could be widely used to Test the thyristor Valve with synthetic manner.
Yuji Ohta - One of the best experts on this subject based on the ideXlab platform.
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Nonsymmetric Leaflet Motion of St. Jude Medical Mitral Valves Simulated with a Computer-Controlled Hydraulic Mock Circulator
Artificial organs, 2008Co-Authors: Yuji Ohta, Takeyoshi Dohi, Kazutoshi Okamoto, Marcel Sonderegger, Hiroshi Matsumoto, Takashi HoriuchiAbstract:: Since the introduction of high-performance prosthetic heart Valves, particularly bileaflet Valves and monoleaflet Valves with larger opening angles, we have observed in vivo complex leaflet motion which has not seen experimentally confirmed yet. We developed a computer-controlled hydraulic mock circulator to study the motion of in vivo leaflets. A high speed CCD camera recorded the Valve movement. In the mitral position, a standard St. Jude Medical Valve was tilted to be horizontal or vertical. The Test Valve was driven with single- or double-peaked flow. The flow rate was set to 5.0 Umin at 70 bpm with a systole/diastole ratio of 0.3. We found the following results: independent of Valve orientation, the Valve showed a nonsymmetric leaflet motion; the Valve showed unpredictable leaflet position during decreasing flow or absence of flow; and the disc closed temporarily at the lower inflow rate between the 2 flow peaks.
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Effect of the Sinus of Valsalva on the Closing Motion of Bileaflet Prosthetic Heart Valves
Artificial organs, 2000Co-Authors: Yuji Ohta, Yukiaki Kikuta, Toshiyuki Shimooka, Yoshinori Mitamura, T. Yuhta, Takeyoshi DohiAbstract:: Conventional bileaflet prosthetic mechanical heart Valves close passively with backflow. Naturally, the Valve has problems associated with closure, such as backflow, water hammer effect, and fracture of the leaflet. On the other hand, in the case of the natural aortic Valve, the vortex flow in the sinus of Valsalva pushes the leaflet to close, and the Valve starts the closing motion earlier than the prosthetic Valve as the forward flow decelerates. This closing mechanism is thought to decrease backflow at Valve closure. In this study, we propose a new bileaflet mechanical Valve resembling a drawbridge in shape, and the prototype Valve was designed so that the leaflet closes with the help of the vortex flow in the sinus. The Test Valve was made of aluminum alloy, and its closing motion was compared to that of the CarboMedics (CM) Valve. Both Valves were driven by a computer controlled hydraulic mock circulator and were photographed at 648 frames/s by a high speed charge-coupled device (CCD) camera. Each frame of the Valve motion image was analyzed with a personal computer, and the opening angles were measured. The flow rate was set as 5.0 L/min. The system was pulsed with 70 bpm, and the systolic/diastolic ratio was 0.3. Glycerin water was used as the circulation fluid at room temperature, and polystyrene particles were used to visualize the streamline. The model of the sinus of Valsalva was made of transparent silicone rubber. As a result, high speed video analysis showed that the Test Valve started the closing motion 41 ms earlier than the CM Valve, and streamline analysis showed that the Test Valve had a closing mechanism similar to the natural one with the effect of vortex flow. The structure of the Test Valve was thought to be effective for soft closure and could solve problems associated with closure.
Jia-shing Liu - One of the best experts on this subject based on the ideXlab platform.
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On Accelerated Fatigue Testing of Prosthetic Heart Valves
Frontiers in Biomedical Engineering, 2003Co-Authors: Jia-shing Liu, Ned H. C. HwangAbstract:Accelerated Testing (AT) of prosthetic heart Valves allows simulation of wear and fatigue sustained by the replacement heart Valves, and to estimate the Valves’ life expectancy in human body. At accelerated Test rates, sufficient amounts of data can be collected within a reasonably short time period, after repeated opening and closing cycles, to predict the Valve durability. The U.S. Food and Drug Administration (FDA) Replacement Heart Valve Guidance (Version 4.1, 1994) requires that mechanical heart Valves (MHV) must be Tested at least 600 million cycles (equivalent to 15 years in vivo), while biological heart Valve prostheses (BHV) must be Tested at least 200 million cycles (equivalent to 5 years in vivo) in pulsatile flow simulators. The cyclic Test must meet two basic FDA requirements: 1) the Test Valve open and close fully each cycle; and 2) the average transvalvular pressure is kept at least 100 mmHg at closure. At accelerated Test rates, the Valves were subjected to non-physiologic dynamic force loads and often damaged under excessive conditions, such as cavitation. AT may pinpoint early flaws in the design and in the manufacturing processes, and deflects regions of materials weakness. Hence the design of AT must follow the principles of engineering Testing such as the law of dynamic similarities. One must first identify dimensionless parameters that are physiologically meaningful and those much be specific to heart Valve Testing. The main goal of this paper is to present an AT system and an experimental protocol so that in vitro accelerated Testing may be carried out without creating these excess forces on the Test Valves and to predict the durability of prosthetic heart Valves with physiological considerations.