A comparison of flow field structures of two tri-leaflet polymeric heart valves

Computational Modeling of Aortic Stenosis With a Reduced Degree-of-Freedom Fluid-Structure Interaction Valve Model

In this study, a novel reduced degree-of-freedom (rDOF) aortic valve model is employed to investigate the fluid-structure interaction (FSI) and hemodynamics associated with aortic stenosis. The dynamics of the valve leaflets are determined by an ordinary differential equation with two parameters and this rDOF model is shown to reproduce key features of more complex valve models. The hemodynamics associated with aortic stenosis is studied for three cases: a healthy case and two stenosed cases. The focus of the study is to correlate the hemodynamic features with the source generation mechanism of systolic murmurs associated with aortic stenosis. In the healthy case, extremely weak flow fluctuations are observed. However, in the stenosed cases, simulations show significant turbulent fluctuations in the ascending aorta, which are responsible for the generation of strong wall pressure fluctuations after the aortic root mostly during the deceleration phase of the systole. The intensity of the murmur generation increases with the severity of the stenosis, and the source locations for the two diseased cases studied here lie around 1.0 inlet duct diameters (Do) downstream of the ascending aorta.

Biomaterials in Valvular Heart Diseases

Valvular heart disease (VHD) occurs as the result of valvular malfunction, which can greatly reduce patient's quality of life and if left untreated may lead to death. Different treatment regiments are available for management of this defect, which can be helpful in reducing the symptoms. The global commitment to reduce VHD-related mortality rates has enhanced the need for new therapeutic approaches. During the past decade, development of innovative pharmacological and surgical approaches have dramatically improved the quality of life for VHD patients, yet the search for low cost, more effective, and less invasive approaches is ongoing. The gold standard approach for VHD management is to replace or repair the injured valvular tissue with natural or synthetic biomaterials. Application of these biomaterials for cardiac valve regeneration and repair holds a great promise for treatment of this type of heart disease. The focus of the present review is the current use of different types of biomaterials in treatment of valvular heart diseases.

Mechanical considerations for polymeric heart valve development: Biomechanics, materials, design and manufacturing

The native human heart valve leaflet contains a layered microstructure comprising a hierarchical arrangement of collagen, elastin, proteoglycans and various cell types. Here, we review the various experimental methods that have been employed to probe this intricate microstructure and which attempt to elucidate the mechanisms that govern the leaflet's mechanical properties. These methods include uniaxial, biaxial, and flexural tests, coupled with microstructural characterization techniques such as small angle X-ray scattering (SAXS), small angle light scattering (SALS), and polarized light microscopy. These experiments have revealed complex elastic and viscoelastic mechanisms that are highly directional and dependent upon loading conditions and biochemistry. Of all engineering materials, polymers and polymer-based composites are best able to mimic the tissue-level mechanical behavior of the native leaflet. This similarity to native tissue permits the fabrication of polymeric valves with physiological flow patterns, reducing the risk of thrombosis compared to mechanical valves and in some cases surpassing the in vivo durability of bioprosthetic valves. Earlier work on polymeric valves simply assumed the mechanical properties of the polymer material to be linear elastic, while more recent studies have considered the full hyperelastic stress-strain response. These material models have been incorporated into computational models for the optimization of valve geometry, with the goal of minimizing internal stresses and improving durability. The latter portion of this review recounts these developments in polymeric heart valves, with a focus on mechanical testing of polymers, valve geometry, and manufacturing methods.

Development of an Off-the-Shelf Tissue-Engineered Sinus Valve for Transcatheter Pulmonary Valve Replacement: a Proof-of-Concept Study

Tissue-engineered heart valves with self-repair and regeneration properties may overcome the problem of long-term degeneration of currently used artificial prostheses. The aim of this study was the development and in vivo proof-of-concept of next-generation off-the-shelf tissue-engineered sinus valve (TESV) for transcatheter pulmonary valve replacement (TPVR). Transcatheter implantation of off-the-shelf TESVs was performed in a translational sheep model for up to 16 weeks. Transapical delivery of TESVs was successful and showed good acute and short-term performance (up to 8 weeks), which then worsened over time most likely due to a non-optimized in vitro valve design. Post-mortem analyses confirmed the remodelling potential of the TESVs, with host cell infiltration, polymer degradation, and collagen and elastin deposition. TESVs proved to be suitable as TPVR in a preclinical model, with encouraging short-term performance and remodelling potential. Future studies will enhance the clinical translation of such approach by improving the valve design to ensure long-term functionality.

Off-the-shelf tissue engineered heart valves for in situ regeneration: current state, challenges and future directions

INTRODUCTION

Transcatheter aortic valve replacement (TAVR) is continuously evolving and is expected to surpass surgical valve implantation in the near future. Combining durable valve substitutes with minimally invasive implantation techniques might increase the clinical relevance of this therapeutic option for younger patient populations. Tissue engineering offers the possibility to create tissue engineered heart valves (TEHVs) with regenerative and self-repair capacities which may overcome the pitfalls of current TAVR prostheses.

AREAS COVERED

This review focuses on off-the-shelf TEHVs which rely on a clinically-relevant in situ tissue engineering approach and which have already advanced into preclinical or first-in-human investigation.

EXPERT COMMENTARY

Among the off-the-shelf in situ TEHVs reported in literature, the vast majority covers pulmonary valve substitutes, and only few are combined with transcatheter implantation technologies. Hence, further innovations should include the development of transcatheter tissue engineered aortic valve substitutes, which would considerably increase the clinical relevance of such prostheses.

Physiological vortices in the sinuses of Valsalva: An in vitro approach for bio-prosthetic valves

PURPOSE

The physiological flow dynamics within the Valsalva sinuses, in terms of global and local parameters, are still not fully understood. This study attempts to identify the physiological conditions as closely as possible, and to give an explanation of the different and sometime contradictory results in literature.

METHODS

An in vitro approach was implemented for testing porcine bio-prosthetic valves operating within different aortic root configurations. All tests were performed on a pulse duplicator, under physiological pressure and flow conditions. The fluid dynamics established in the various cases were analysed by means of 2D Particle Image Velocimetry, and related with the achieved hydrodynamic performance.

RESULTS

Each configuration is associated with substantially different flow dynamics, which significantly affects the valve performance. The configuration most closely replicating healthy native anatomy was characterised by the best hemodynamic performance, and any mismatch in size and position between the valve and the root produced substantial modification of the fluid dynamics downstream of the valve, hindering the hydrodynamic performance of the system. The worst conditions were observed for a configuration characterised by the total absence of the Valsalva sinuses.

CONCLUSION

This study provides an explanation for the different vortical structures described in the literature downstream of bioprosthetic valves, enlightening the experimental complications in valve testing. Most importantly, the results clearly identify the fluid mechanisms promoted by the Valsalva sinuses to enhance the ejection and closing phases, and this study exposes the importance of an optimal integration of the valve and root, to operate as a single system.

An immersogeometric variational framework for fluid-structure interaction: application to bioprosthetic heart valves

In this paper, we develop a geometrically flexible technique for computational fluid-structure interaction (FSI). The motivating application is the simulation of tri-leaflet bioprosthetic heart valve function over the complete cardiac cycle. Due to the complex motion of the heart valve leaflets, the fluid domain undergoes large deformations, including changes of topology. The proposed method directly analyzes a spline-based surface representation of the structure by immersing it into a non-boundary-fitted discretization of the surrounding fluid domain. This places our method within an emerging class of computational techniques that aim to capture geometry on non-boundary-fitted analysis meshes. We introduce the term "immersogeometric analysis" to identify this paradigm. The framework starts with an augmented Lagrangian formulation for FSI that enforces kinematic constraints with a combination of Lagrange multipliers and penalty forces. For immersed volumetric objects, we formally eliminate the multiplier field by substituting a fluid-structure interface traction, arriving at Nitsche's method for enforcing Dirichlet boundary conditions on object surfaces. For immersed thin shell structures modeled geometrically as surfaces, the tractions from opposite sides cancel due to the continuity of the background fluid solution space, leaving a penalty method. Application to a bioprosthetic heart valve, where there is a large pressure jump across the leaflets, reveals shortcomings of the penalty approach. To counteract steep pressure gradients through the structure without the conditioning problems that accompany strong penalty forces, we resurrect the Lagrange multiplier field. Further, since the fluid discretization is not tailored to the structure geometry, there is a significant error in the approximation of pressure discontinuities across the shell. This error becomes especially troublesome in residual-based stabilized methods for incompressible flow, leading to problematic compressibility at practical levels of refinement. We modify existing stabilized methods to improve performance. To evaluate the accuracy of the proposed methods, we test them on benchmark problems and compare the results with those of established boundary-fitted techniques. Finally, we simulate the coupling of the bioprosthetic heart valve and the surrounding blood flow under physiological conditions, demonstrating the effectiveness of the proposed techniques in practical computations.

Polymeric trileaflet prosthetic heart valves: evolution and path to clinical reality

Present prosthetic heart valves, while hemodynamically effective, remain limited by progressive structural deterioration of tissue valves or the burden of chronic anticoagulation for mechanical valves. An idealized valve prosthesis would eliminate these limitations. Polymeric heart valves (PHVs), fabricated from advanced polymeric materials, offer the potential of durability and hemocompatibility. Unfortunately, the clinical realization of PHVs to date has been hampered by findings of in vivo calcification, degradation and thrombosis. Here, the authors review the evolution of PHVs, evaluate the state of the art of this technology and propose a pathway towards clinical reality. In particular, the authors discuss the development of a novel aortic PHV that may be deployed via transcatheter implantation, as well as its optimization via device thrombogenicity emulation.

Fluid Dynamic Investigation of the ATS 3F Enable Sutureless Heart Valve

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.

Experimental Technique of Measuring Dynamic Fluid Shear Stress on the Aortic Surface of the Aortic Valve Leaflet

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. The exact cause and mechanism of the progression of AV calcification is unknown, although mechanical forces have been known to play a role. It is thus important to characterize the mechanical environment of the AV. In the current study, we establish a methodology of measuring shear stresses experienced by the aortic surface of the AV leaflets using an in vitro valve model and adapting the laser Doppler velocimetry (LDV) technique. The valve model was constructed from a fresh porcine aortic valve, which was trimmed and sutured onto a plastic stented ring, and inserted into an idealized three-lobed sinus acrylic chamber. Valve leaflet location was measured by obtaining the location of highest back-scattered LDV laser light intensity. The technique of performing LDV measurements near to biological surfaces as well as the leaflet locating technique was first validated in two phantom flow systems: (1) steady flow within a straight tube with AV leaflet adhered to the wall, and (2) steady flow within the actual valve model. Dynamic shear stresses were then obtained by applying the techniques on the valve model in a physiologic pulsatile flow loop. Results show that aortic surface shear stresses are low during early systole (<5dyn/cm2) but elevated to its peak during mid to late systole at about 18–20 dyn/cm2. Low magnitude shear stress (<5dyn/cm2) was observed during early diastole and dissipated to zero over the diastolic duration. Systolic shear stress was observed to elevate only with the formation of sinus vortex flow. The presented technique can also be used on other in vitro valve models such as congenitally geometrically malformed valves, or to investigate effects of hemodynamics on valve shear stress. Shear stress data can be used for further experiments investigating effects of fluid shear stress on valve biology, for conditioning tissue engineered AV, and to validate numerical simulations.

Design and validation of a novel bioreactor to subject aortic valve leaflets to side-specific shear stress

Hemodynamic stresses are presumed to play an important role in the development of calcific aortic valve disease (CAVD). The elucidation of the shear stress mechanisms involved in the pathogenesis of CAVD has been hampered by the complexity of the native unsteady and side-specific valvular flow environment. To address this gap, this article describes the design and validation of a novel device to expose leaflet samples to time-dependent side-specific shear stress. The device built on a double cone-and-plate geometry was dimensioned based on our previous single-sided shear stress device that minimizes secondary flow effects inherent to this geometry. A fluid-structure interaction (FSI) model was designed to predict the actual shear stress produced on a tissue sample mounted in the new device. Staining was performed on porcine leaflets conditioned in the new bioreactor to assess endothelial integrity and cellular apoptosis. The FSI results demonstrated good agreement between the target (native) and the actual side-specific shear stress produced on a tissue sample. No significant difference in endothelial integrity and cellular apoptosis was detected between samples conditioned for 96 h and fresh controls. This new device will enable the investigation of valvular response to normal and pathologic hemodynamics and the potential mechano-etiology of CAVD.

Design of a novel polymeric heart valve

Polymeric heart valves could offer an optimum alternative to current prostheses, by joining the advantages of mechanical and bioprosthetic valves. Though a number of materials suitable for this application have recently become available, significant improvements in the valve design are still needed. In this paper, a novel polymeric heart valve design is proposed and its optimization procedure, based on the use of finite elements, is described. The design strategy was aimed at reducing the energy absorbed during the operating cycle, resulting in high hydrodynamic performances and reduced stress levels. The efficacy of the design strategy was assessed by comparing the valve dynamics and stress levels predicted numerically during the cycle with those of an existing and well qualified polymeric valve design. The improved hydrodynamic performance of the proposed design was confirmed experimentally, by in vitro testing in a pulse duplicator.

Role of Computational Simulations in Heart Valve Dynamics and Design of Valvular Prostheses

Computational simulations are playing an increasingly important role in enhancing our understanding of the normal human physiological function, etiology of diseased states, surgical and interventional planning, and in the design and evaluation of artificial implants. Researchers are taking advantage of computational simulations to speed up the initial design of implantable devices before a prototype is developed and hence able to reduce animal experimentation for the functional evaluation of the devices under development. A review of the reported studies to date relevant to the simulation of the native and prosthetic heart valve dynamics is the subject of the present paper. Potential future directions toward multi-scale simulation studies for our further understanding of the physiology and pathophysiology of heart valve dynamics and valvular implants are also discussed.

Dynamic Hemodynamic Energy Loss in Normal and Stenosed Aortic Valves

Aortic valve (AV) stenosis, if untreated, leads to heart failure. From a mechanics standpoint, heart failure can be interpreted as the failure of the heart to generate sufficient power to overcome energy losses in the circulation. Thus, energy efficiency-based measures for evaluating AV performance and disease severity have the advantage of being a direct measure of the contribution of the AV hydrodynamic characteristics toward heart failure. We present a new method for computing the rate of energy dissipation as a function of systolic time, by modifying the Navier–Stokes momentum equation. This method preserves the dynamic term of the Navier–Stokes momentum equation, and allows the investigation of the trend of the rate of energy dissipation over time. This method is applied to a series of in vitro experiments, where a trimmed porcine valve is exposed to various conditions: varying stroke volumes (50 ml to 90 ml) at the fixed heart rate; varying heart rates (60–80 beats/min) at fixed stroke volume; and varying stenosis levels (normal, mild stenosis, moderate stenosis). The results are: (1) energy dissipation waveform has a distinctive pattern of being skewed toward late systole, due to flow instabilities during deceleration phases; (2) increasing heart rate and stroke volume increases energy dissipation, but the normalized shape of the energy dissipation waveform is preserved across heart rates and stroke volumes; (3) increasing stenosis level increases energy dissipation, and also alters the normalized shape of the energy dissipation waveform. Since stenosis produces a signature energy dissipation waveform shape, dynamic energy dissipation analysis can potentially be extended into a clinical tool for AV evaluation.

A novel nanocomposite polymer for development of synthetic heart valve leaflets

A novel nanocomposite polymer with a polycarbonate soft segment (PCU) and polyhedral oligomeric silsesquioxanes (POSS) nanoparticle (POSS-PCU) has been selected for a synthetic heart valve due to its superior biocompatibility and in vivo biostability. However, the development of synthetic heart valves from polymeric materials requires an understanding of the basic mechanical and surface properties of the polymer. In this study, the mechanical properties of POSS-PCU, including tensile strength, tear strength and hardness, were tested and compared to control (PCU). The surface property was analyzed using contact angle measurement and the resistance to platelet adhesion was also investigated. POSS-PCU (hardness 84+/-0.8 Shore A) demonstrated significantly higher tensile strength 53.6+/-3.4 and 55.9+/-3.9Nmm(-2) at 25 and 37 degrees C, respectively) than PCU (33.8+/-2.1 and 28.8+/-3.4Nmm(-2) at 25 and 37 degrees C, respectively). Tensile strength and elongation at break of POSS-PCU was significantly higher than PCU at both 25 and 37 degrees C (P<0.001). POSS-PCU showed a relatively low Young's modulus (25.9+/-1.9 and 26.2+/-2.0Nmm(-2)) which was significantly greater in comparison with control PCU (9.1+/-0.9 and 8.4+/-0.5Nmm(-2)) at 25 and 37 degrees C, respectively, with 100mum thickness. There was no significant difference (P>0.05) in tear strength between POSS-PCU and PCU at 25 degrees C. However, tear strength increased significantly (P<0.001) (at 37 degrees C) as the thickness increased from 100microm (51.0+/-3.3Nmm(-1)) to 200microm (63+/-1.5Nmm(-1)). The surface of POSS-PCU was significantly less hydrophilic than that of PCU.

Flow visualization techniques in a mock ventricle supported by a nonpulsatile left ventricular assist device

Little is known about flow patterns in ventricles supported by continuous flow left ventricular assist devices (LVADs), and valuable information can be obtained with simple flow visualization experiments. We describe the application of several experimental techniques for the in vitro study of ventricular flow patterns (e.g., unsteadiness, vortical motions, stagnation regions) in the presence of a continuous flow LVAD. We used dye streaks, particle paths, and hydrogen bubble techniques to visualize fluid flow in an idealized, static, transparent mock ventricle attached to a Jarvik 2000 continuous flow LVAD. We recorded ventricular flow behavior at various pump speeds while independently adjusting pump flow (by varying the afterload) to emulate in vivo pump flow at various phases of the cardiac cycle. Changes in ventricular flow behavior at different pump flow rates may be of clinical relevance, because continuous flow pumps are extremely sensitive to inflow and outflow pressures and instantaneous pump flow varies significantly at different points throughout the cardiac cycle. Further work is needed to quantitatively compare the flow behavior of different continuous flow devices in a pulsatile ventricular model.

Fluid mechanics of artificial heart valves

1. Artificial heart valves have been in use for over five decades to replace diseased heart valves. Since the first heart valve replacement performed with a caged-ball valve, more than 50 valve designs have been developed, differing principally in valve geometry, number of leaflets and material. To date, all artificial heart valves are plagued with complications associated with haemolysis, coagulation for mechanical heart valves and leaflet tearing for tissue-based valve prosthesis. For mechanical heart valves, these complications are believed to be associated with non-physiological blood flow patterns. 2. In the present review, we provide a bird's-eye view of fluid mechanics for the major artificial heart valve types and highlight how the engineering approach has shaped this rapidly diversifying area of research. 3. Mechanical heart valve designs have evolved significantly, with the most recent designs providing relatively superior haemodynamics with very low aerodynamic resistance. However, high shearing of blood cells and platelets still pose significant design challenges and patients must undergo life-long anticoagulation therapy. Bioprosthetic or tissue valves do not require anticoagulants due to their distinct similarity to the native valve geometry and haemodynamics, but many of these valves fail structurally within the first 10-15 years of implantation. 4. These shortcomings have directed present and future research in three main directions in attempts to design superior artificial valves: (i) engineering living tissue heart valves; (ii) development of advanced computational tools; and (iii) blood experiments to establish the link between flow and blood damage.

In vitro hemodynamic investigation of the embryonic aortic arch at late gestation

This study focuses on the dynamic flow through the fetal aortic arch driven by the concurrent action of right and left ventricles. We created a parametric pulsatile computational fluid dynamics (CFD) model of the fetal aortic junction with physiologic vessel geometries. To gain a better biophysical understanding, an in vitro experimental fetal flow loop for flow visualization was constructed for identical CFD conditions. CFD and in vitro experimental results were comparable. Swirling flow during the acceleration phase of the cardiac cycle and unidirectional flow following mid-deceleration phase were observed in pulmonary arteries (PA), head-neck vessels, and descending aorta. Right-to-left (oxygenated) blood flowed through the ductus arteriosus (DA) posterior relative to the antegrade left ventricular outflow tract (LVOT) stream and resembled jet flow. LVOT and right ventricular outflow tract flow mixing had not completed until approximately 3.5 descending aorta diameters downstream of the DA insertion into the aortic arch. Normal arch model flow patterns were then compared to flow patterns of four common congenital heart malformations that include aortic arch anomalies. Weak oscillatory reversing flow through the DA junction was observed only for the Tetralogy of Fallot configuration. PA and hypoplastic left heart syndrome configurations demonstrated complex, abnormal flow patterns in the PAs and head-neck vessels. Aortic coarctation resulted in large-scale recirculating flow in the aortic arch proximal to the DA. Intravascular flow patterns spatially correlated with abnormal vascular structures consistent with the paradigm that abnormal intravascular flow patterns associated with congenital heart disease influence vascular growth and function.

Fluid Dynamic Assessment of Three Polymeric Heart Valves Using Particle Image Velocimetry

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this paper we characterize the in vitro velocity and Reynolds Shear Stress (RSS) fields inside and downstream of three different prototype trileaflet polymeric heart valves. The fluid dynamic differences are then correlated with variations in valve design parameters. The three valves differ in leaflet thickness, ranging from 80 to 120 mum, and commisural design, either closed, opened, or semi-opened. The valves were subjected to aortic flow conditions and the velocity measured using three-dimensional stereo Particle Image Velocimetry. The peak forward flow phase in the three valves was characterized by a strong central orifice jet of approximately 2 m/s with a flat profile along the trailing edge of the leaflets. Leakage jets, with principle RSS magnitudes exceeding 4,500 dyn/cm(2), were observed in all valves with larger leaflet thicknesses and also corresponded to larger leakage volumes. Additional leakage jets were observed at the commissural region of valves with the open and the semi-open commissural designs. The results of the present study indicate that commissural design and leaflet thickness influence valve fluid dynamics and thus the thrombogenic potential of trileaflet polymeric valves.