Novel Polymeric Valve for Transcatheter Aortic Valve Replacement Applications: In Vitro Hemodynamic Study

A patient-specific echogenic soft robotic left ventricle embedded into a closed-loop cardiovascular simulator for advanced device testing

Cardiovascular medical devices undergo a large number of pre- and post-market tests before their approval for clinical practice use. Sophisticated cardiovascular simulators can significantly expedite the evaluation process by providing a safe and controlled environment and representing clinically relevant case scenarios. The complex nature of the cardiovascular system affected by severe pathologies and the inherently intricate patient-device interaction creates a need for high-fidelity test benches able to reproduce intra- and inter-patient variability of disease states. Therefore, we propose an innovative cardiovascular simulator that combines in silico and in vitro modeling techniques with a soft robotic left ventricle. The simulator leverages patient-specific and echogenic soft robotic phantoms used to recreate the intracardiac pressure and volume waveforms, combined with an in silico lumped parameter model of the remaining cardiovascular system. Three different patient-specific profiles were recreated, to assess the capability of the simulator to represent a variety of working conditions and mechanical properties of the left ventricle. The simulator is shown to provide a realistic physiological and anatomical representation thanks to the use of soft robotics combined with in silico modeling. This tool proves valuable for optimizing and validating medical devices and delineating specific indications and boundary conditions.

Design, manufacturing and testing of a green non-isocyanate polyurethane prosthetic heart valve

The sole effective treatment for most patients with heart valve disease is valve replacement by implantation of mechanical or biological prostheses. However, mechanical valves represent high risk of thromboembolism, and biological prostheses are prone to early degeneration. In this work, we aim to determine the potential of novel environmentally-friendly non-isocyanate polyurethanes (NIPUs) for manufacturing synthetic prosthetic heart valves. Polyhydroxyurethane (PHU) NIPUs are synthesized via an isocyanate-free route, tested in vitro, and used to produce aortic valves. PHU elastomers reinforced with a polyester mesh show mechanical properties similar to native valve leaflets. These NIPUs do not cause hemolysis. Interestingly, both platelet adhesion and contact activation-induced coagulation are strongly reduced on NIPU surfaces, indicating low thrombogenicity. Fibroblasts and endothelial cells maintain normal growth and shape after indirect contact with NIPUs. Fluid-structure interaction (FSI) allows modeling of the ideal valve design, with minimal shear stress on the leaflets. Injection-molded valves are tested in a pulse duplicator and show ISO-compliant hydrodynamic performance, comparable to clinically-used bioprostheses. Poly(tetrahydrofuran) (PTHF)-NIPU patches do not show any evidence of calcification over a period of 8 weeks. NIPUs are promising sustainable biomaterials for the manufacturing of improved prosthetic valves with low thrombogenicity.

Hydrogel-polyurethane fiber composites with enhanced microarchitectural control for heart valve replacement

Polymeric heart valves offer the potential to overcome the limited durability of tissue based bioprosthetic valves and the need for anticoagulant therapy of mechanical valve replacement options. However, developing a single-phase material with requisite biological properties and target mechanical properties remains a challenge. In this study, a composite heart valve material was developed where an electrospun mesh provides tunable mechanical properties and a hydrogel coating confers an antifouling surface for thromboresistance. Key biological responses were evaluated in comparison to glutaraldehyde-fixed pericardium. Platelet and bacterial attachment were reduced by 38% and 98%, respectively, as compared to pericardium that demonstrated the antifouling nature of the hydrogel coating. There was also a notable reduction (59%) in the calcification of the composite material as compared to pericardium. A custom 3D-printed hydrogel coating setup was developed to make valve composites for device-level hemodynamic testing. Regurgitation fraction (9.6 ± 1.8%) and effective orifice area (1.52 ± 0.34 cm2 ) met ISO 5840-2:2021 requirements. Additionally, the mean pressure gradient was comparable to current clinical bioprosthetic heart valves demonstrating preliminary efficacy. Although the hemodynamic properties are promising, it is anticipated that the random microarchitecture will result in suboptimal strain fields and peak stresses that may accelerate leaflet fatigue and degeneration. Previous computational work has demonstrated that bioinspired fiber microarchitectures can improve strain homogeneity of valve materials toward improving durability. To this end, we developed advanced electrospinning methodologies to achieve polyurethane fiber microarchitectures that mimic or exceed the physiological ranges of alignment, tortuosity, and curvilinearity present in the native valve. Control of fiber alignment from a random fiber orientation at a normalized orientation index (NOI) 14.2 ± 6.9% to highly aligned fibers at a NOI of 85.1 ± 1.4%. was achieved through increasing mandrel rotational velocity. Fiber tortuosity and curvilinearity in the range of native valve features were introduced through a post-spinning annealing process and fiber collection on a conical mandrel geometry, respectively. Overall, these studies demonstrate the potential of hydrogel-polyurethane fiber composite as a heart valve material. Future studies will utilize the developed advanced electrospinning methodologies in combination with model-directed fabrication toward optimizing durability as a function of fiber microarchitecture.

A parametric study regarding structural design of a bioprosthetic aortic valve by 3D fluid-structure interaction simulations

Since the introduction of transcatheter aortic valve (AV) implantation as a viable option, surgical bioprosthetic AVs have recently started incorporating shorter struts considering future valve-in-valve procedures. However, the effect of leaflet coaptation geometry on the longevity of these valves remains unexplored. To address this gap, we performed a finite element analysis on bioprosthetic AVs with varying strut heights using a two-way fluid-structure interaction method. To establish a baseline, we used a standard height based on a rendered platform image of the CE PERIMOUNT Magna Ease valve from Edward Lifesciences in Irvine, CA. Bovine pericardium properties were assigned to the leaflets, while normal saline properties were used as the recirculating fluid in hemodynamic simulations. The physiological pressure profile of the cardiac cycle was applied between the aorta and left ventricle. We calculated blood flow velocity, effective orifice area (EOA), and mechanical stress on the leaflets. The results reveal that as the strut height increases, the stroke volume increases, leakage volume decreases, and EOA improves. Additionally, the maximum mechanical stress experienced by the leaflet decreases by 62% as the strut height increases to 1.2 times the standard height. This research highlights that a low-strut design in bioprosthetic AVs may negatively affect their durability, which can be useful in design of next-generation bioprosthetic AVs.

Early Feasibility Study of a Hybrid Tissue-Engineered Mitral Valve in an Ovine Model

Tissue engineering aims to overcome the current limitations of heart valves by providing a viable alternative using living tissue. Nevertheless, the valves constructed from either decellularized xenogeneic or purely biologic scaffolds are unable to withstand the hemodynamic loads, particularly in the left ventricle. To address this, we have been developing a hybrid tissue-engineered heart valve (H-TEHV) concept consisting of a nondegradable elastomeric scaffold enclosed in a valve-like living tissue constructed from autologous cells. We developed a 21 mm mitral valve scaffold for implantation in an ovine model. Smooth muscle cells/fibroblasts and endothelial cells were extracted, isolated, and expanded from the animal's jugular vein. Next, the scaffold underwent a sequential coating with the sorted cells mixed with collagen type I. The resulting H-TEHV was then implanted into the mitral position of the same sheep through open-heart surgery. Echocardiography scans following the procedure revealed an acceptable valve performance, with no signs of regurgitation. The valve orifice area, measured by planimetry, was 2.9 cm2, the ejection fraction reached 67%, and the mean transmitral pressure gradient was measured at 8.39 mmHg. The animal successfully recovered from anesthesia and was transferred to the vivarium. Upon autopsy, the examination confirmed the integrity of the H-TEHV, with no evidence of tissue dehiscence. The preliminary results from the animal implantation suggest the feasibility of the H-TEHV.

Analysis of the Effect of Thickness on the Performance of Polymeric Heart Valves

Polymeric heart valves (PHVs) are a promising and more affordable alternative to mechanical heart valves (MHVs) and bioprosthetic heart valves (BHVs). Materials with good durability and biocompatibility used for PHVs have always been the research focus in the field of prosthetic heart valves for many years, and leaflet thickness is a major design parameter for PHVs. The study aims to discuss the relationship between material properties and valve thickness, provided that the basic functions of PHVs are qualified. The fluid-structure interaction (FSI) approach was employed to obtain a more reliable solution of the effective orifice area (EOA), regurgitant fraction (RF), and stress and strain distribution of the valves with different thicknesses under three materials: Carbothane PC-3585A, xSIBS and SIBS-CNTs. This study demonstrates that the smaller elastic modulus of Carbothane PC-3585A allowed for a thicker valve (>0.3 mm) to be produced, while for materials with an elastic modulus higher than that of xSIBS (2.8 MPa), a thickness less than 0.2 mm would be a good attempt to meet the RF standard. What is more, when the elastic modulus is higher than 23.9 MPa, the thickness of the PHV is recommended to be 0.l-0.15 mm. Reducing the RF is one of the directions of PHV optimization in the future. Reducing the thickness and improving other design parameters are reliable means to reduce the RF for materials with high and low elastic modulus, respectively.

Polymeric prosthetic heart valves: A review of current technologies and future directions

Valvular heart disease is an important source of cardiovascular morbidity and mortality. Current prosthetic valve replacement options, such as bioprosthetic and mechanical heart valves are limited by structural valve degeneration requiring reoperation or the need for lifelong anticoagulation. Several new polymer technologies have been developed in recent years in the hope of creating an ideal polymeric heart valve substitute that overcomes these limitations. These compounds and valve devices are in various stages of research and development and have unique strengths and limitations inherent to their properties. This review summarizes the current literature available for the latest polymer heart valve technologies and compares important characteristics necessary for a successful valve replacement therapy, including hydrodynamic performance, thrombogenicity, hemocompatibility, long-term durability, calcification, and transcatheter application. The latter portion of this review summarizes the currently available clinical outcomes data regarding polymeric heart valves and discusses future directions of research.

Polymeric Heart Valves Will Displace Mechanical and Tissue Heart Valves: A New Era for the Medical Devices

The development of a novel artificial heart valve with outstanding durability and safety has remained a challenge since the first mechanical heart valve entered the market 65 years ago. Recent progress in high-molecular compounds opened new horizons in overcoming major drawbacks of mechanical and tissue heart valves (dysfunction and failure, tissue degradation, calcification, high immunogenic potential, and high risk of thrombosis), providing new insights into the development of an ideal artificial heart valve. Polymeric heart valves can best mimic the tissue-level mechanical behavior of the native valves. This review summarizes the evolution of polymeric heart valves and the state-of-the-art approaches to their development, fabrication, and manufacturing. The review discusses the biocompatibility and durability testing of previously investigated polymeric materials and presents the most recent developments, including the first human clinical trials of LifePolymer. New promising functional polymers, nanocomposite biomaterials, and valve designs are discussed in terms of their potential application in the development of an ideal polymeric heart valve. The superiority and inferiority of nanocomposite and hybrid materials to non-modified polymers are reported. The review proposes several concepts potentially suitable to address the above-mentioned challenges arising in the R&D of polymeric heart valves from the properties, structure, and surface of polymeric materials. Additive manufacturing, nanotechnology, anisotropy control, machine learning, and advanced modeling tools have given the green light to set new directions for polymeric heart valves.

Use of 3D anatomical models in mock circulatory loops for cardiac medical device testing

INTRODUCTION

Mock circulatory loops (MCLs) are mechanical representations of the cardiovascular system largely used to test the hemodynamic performance of cardiovascular medical devices (MD). Thanks to 3 dimensional (3D) printing technologies, MCLs can nowadays also incorporate anatomical models so to offer enhanced testing capabilities. The aim of this review is to provide an overview on MCLs and to discuss the recent developments of 3D anatomical models for cardiovascular MD testing.

METHODS

The review first analyses the different techniques to develop 3D anatomical models, in both rigid and compliant materials. In the second section, the state of the art of MCLs with 3D models is discussed, along with the testing of different MDs: implantable blood pumps, heart valves, and imaging techniques. For each class of MD, the MCL is analyzed in terms of: the cardiovascular model embedded, the 3D model implemented (the anatomy represented, the material used, and the activation method), and the testing applications.

DISCUSSIONS AND CONCLUSIONS

MCLs serve the purpose of testing cardiovascular MDs in different (patho-)physiological scenarios. The addition of 3D anatomical models enables more realistic connections of the MD with the implantation site and enhances the testing capabilities of the MCL. Current attempts focus on the development of personalized MCLs to test MDs in patient-specific hemodynamic and anatomical scenarios. The main limitation of MCLs is the impossibility to assess the impact of a MD in the long-term and at a biological level, for which animal experiments are still needed.

Mild Paravalvular Leak May Pose an Increased Thrombogenic Risk in Transcatheter Aortic Valve Replacement (TAVR) Patients-Insights from Patient Specific In Vitro and In Silico Studies

In recent years, the treatment of aortic stenosis with TAVR has rapidly expanded to younger and lower-risk patients. However, persistent thrombotic events such as stroke and valve thrombosis expose recipients to severe clinical complications that hamper TAVR's rapid advance. We presented a novel methodology for establishing a link between commonly acceptable mild paravalvular leak (PVL) levels through the device and increased thrombogenic risk. It utilizes in vitro patient-specific TAVR 3D-printed replicas evaluated for hydrodynamic performance. High-resolution µCT scans are used to reconstruct in silico FSI models of these replicas, in which multiple platelet trajectories are studied through the PVL channels to quantify thrombogenicity, showing that those are highly dependent on patient-specific flow conditions within the PVL channels. It demonstrates that platelets have the potential to enter the PVL channels multiple times over successive cardiac cycles, increasing the thrombogenic risk. This cannot be reliably approximated by standard hemodynamic parameters. It highlights the shortcomings of subjectively ranked PVL commonly used in clinical practice by indicating an increased thrombogenic risk in patient cases otherwise classified as mild PVL. It reiterates the need for more rigorous clinical evaluation for properly diagnosing thrombogenic risk in TAVR patients.

Single-cell RNA sequencing reveals the diversity and biology of valve cells in cardiac valve disease

From highly aligned extracellular fibrils to the cells, a multilevel ordered hierarchy in valve leaflets is crucial for their biological function. Cardiac valve pathology most frequently involves a disruption in normal structure-function correlations through abnormal and complex interaction of cells, extracellular matrix, and their environment. At present, effective treatment for valve disease is limited and frequently ends with surgical repair or replacement with a mechanical or artificial biological cardiac valve, which comes with insuperable complications for many high-risk patients including aged and pediatric populations. Therefore, there is a critical need to fully appreciate the pathobiology of valve disease in order to develop better, alternative therapies. To date, the majority of studies have focused on delineating valve disease mechanisms at the cellular level. However, the cellular heterogeneity and function is still unclear. In this review, we summarize the body of work on valve cells, with a particular focus on the discoveries about valve cells heterogeneity and functions using single-cell RNA sequencing. We conclude by discussing state-of-the-art strategies for deciphering heterogeneity of these complex cell types, and argue this knowledge could translate into the improved personalized treatment of cardiac valve disease.

Designing a Novel Asymmetric Transcatheter Aortic Valve for Stenotic Bicuspid Aortic Valves Using Patient-Specific Computational Modeling

Bicuspid aortic valve (BAV), the most common congenital heart malformation, is characterized by the presence of only two valve leaflets with asymmetrical geometry, resulting in elliptical systolic opening. BAV often leads to early onset of calcific aortic stenosis (AS). Following the rapid expansion of transcatheter aortic valve replacement (TAVR), designed specifically for treating conventional tricuspid AS, BAV patients with AS were initially treated "off-label" with TAVR, which recently gained FDA and CE regulatory approval. Despite its increasing use in BAV, pathological BAV anatomy often leads to complications stemming from mismatched anatomical features. To mitigate these complications, a novel eccentric polymeric TAVR valve incorporating asymmetrical leaflets was designed specifically for BAV anatomies. Computational modeling was used to optimize its asymmetric leaflets for lower functional stresses and improved hemodynamic performance. Deployment and flow were simulated in patient-specific BAV models (n = 6) and compared to a current commercial TAVR valve (Evolut R 29 mm), to assess deployment and flow parameters. The novel eccentric BAV-dedicated valve demonstrated significant improvements in peak systolic orifice area, along with lower jet velocity and wall shear stress (WSS). This feasibility study demonstrates the clinical potential of the first known BAV-dedicated TAVR design, which will foster advancement of patient-dedicated valvular devices.

Development of Innovative Biomaterials and Devices for the Treatment of Cardiovascular Diseases

Cardiovascular diseases have become the leading cause of death worldwide. The increasing burden of cardiovascular diseases has become a major public health problem and how to carry out efficient and reliable treatment of cardiovascular diseases has become an urgent global problem to be solved. Recently, implantable biomaterials and devices, especially minimally invasive interventional ones, such as vascular stents, artificial heart valves, bioprosthetic cardiac occluders, artificial graft cardiac patches, atrial shunts, and injectable hydrogels against heart failure, have become the most effective means in the treatment of cardiovascular diseases. Herein, an overview of the challenges and research frontier of innovative biomaterials and devices for the treatment of cardiovascular diseases is provided, and their future development directions are discussed.

Visions of TAVR Future: Development and Optimization of a Second Generation Novel Polymeric TAVR

Tissue-based transcatheter aortic valve (AV) replacement (TAVR) devices have been a breakthrough approach for treating aortic valve stenosis. However, with the expansion of TAVR to younger and lower risk patients, issues of long-term durability and thrombosis persist. Recent advances in polymeric valve technology facilitate designing more durable valves with minimal in vivo adverse reactions. We introduce our second-generation polymeric transcatheter aortic valve (TAV) device, designed and optimized to address these issues. We present the optimization process of the device, wherein each aspect of device deployment and functionality was optimized for performance, including unique considerations of polymeric technologies for reducing the volume of the polymer material for lower crimped delivery profiles. The stent frame was optimized to generate larger radial forces with lower material volumes, securing robust deployment and anchoring. The leaflet shape, combined with varying leaflets thickness, was optimized for reducing the flexural cyclic stresses and the valve's hydrodynamics. Our first-generation polymeric device already demonstrated that its hydrodynamic performance meets and exceeds tissue devices for both ISO standard and patient-specific in vitro scenarios. The valve already reached 900 × 106 cycles of accelerated durability testing, equivalent to over 20 years in a patient. The optimization framework and technology led to the second generation of polymeric TAV design- currently undergoing in vitro hydrodynamic testing and following in vivo animal trials. As TAVR use is rapidly expanding, our rigorous bio-engineering optimization methodology and advanced polymer technology serve to establish polymeric TAV technology as a viable alternative to the challenges facing existing tissue-based TAV technology.

Design of an aortic polymeric valve with asymmetric leaflets and evaluation of its performance by finite element method

BACKGROUND

The geometry of leaflets plays a significant role in prosthetic valves' (PVs) performance. Typically, natural aortic valves have three unequal leaflets, which differ in size. The present study aims to design an asymmetric tri-leaflet polymeric valve with one large and two small leaflets based on commissure lengths and leaflet eccentricities.

METHODS

Eccentricity was related to commissure lengths based on the deformation of the free margins for the fully-opened state of leaflets. The polystyrene-block-polyethylene-polypropylene-block-polystyrene polymer characterized the material properties of the leaflets. The Finite Element Method (FEM) was used to evaluate performance parameters, including maximum geometric orifice area (GOA), average GOA, maximum von Mises stress, and leaflet's coaptation surface area (CSA).

RESULTS

Asymmetric valves with no eccentricity provided a low level of GOA because the asymmetric form of small leaflets caused them to close faster than the large leaflet, leading to a sudden drop in the GOA during systole. As the radial curve tends towards a straight line, an undesirable coaptation occurs, and peak stress increases despite higher GOAs. A new radial curve consisting of two straight lines connected by an arc that provided 25.64 mm² coaptation surface area (CAS) and 117.54 mm² average GOA, was proposed to improve coaptation and GOA.

CONCLUSION

The radial curve of leaflets affects the valve's performance more than other geometric parameters. The combination of straight lines and arcs for radial curves was selected as the reference model for asymmetric valves with one large and two small leaflets.

Computational Methods for Fluid-Structure Interaction Simulation of Heart Valves in Patient-Specific Left Heart Anatomies

Given the complexity of human left heart anatomy and valvular structures, the fluid–structure interaction (FSI) simulation of native and prosthetic valves poses a significant challenge for numerical methods. In this review, recent numerical advancements for both fluid and structural solvers for heart valves in patient-specific left hearts are systematically considered, emphasizing the numerical treatments of blood flow and valve surfaces, which are the most critical aspects for accurate simulations. Numerical methods for hemodynamics are considered under both the continuum and discrete (particle) approaches. The numerical treatments for the structural dynamics of aortic/mitral valves and FSI coupling methods between the solid Ωs and fluid domain Ωf are also reviewed. Future work toward more advanced patient-specific simulations is also discussed, including the fusion of high-fidelity simulation within vivo measurements and physics-based digital twining based on data analytics and machine learning techniques.

In silico modelling of aortic valve implants - predicting in vitro performance using finite element analysis

The competing structural and hemodynamic considerations in valve design generally require a large amount of in vitro hydrodynamic and durability testing during development, often resulting in inefficient "trial-and-error" prototyping. While in silico modelling through finite element analysis (FEA) has been widely used to inform valve design by optimising structural performance, few studies have exploited the potential insight FEA could provide into critical hemodynamic performance characteristics of the valve. The objective of this study is to demonstrate the potential of FEA to predict the hydrodynamic performance of tri-leaflet aortic valve implants obtained during development through in vitro testing. Several variations of tri-leaflet aortic valves were designed and manufactured using a synthetic polymer and hydrodynamic testing carried out using a pulsatile flow rig according to ISO 5840, with bulk hydrodynamic parameters measured. In silico models were developed in tandem and suitable surrogate measures were investigated as predictors of the hydrodynamic parameters. Through regression analysis, the in silico parameters of leaflet coaptation area, geometric orifice area and opening pressure were found to be suitable indicators of experimental in vitro hydrodynamic parameters: regurgitant fraction, effective orifice area and transvalvular pressure drop performance, respectively.

Investigation the effect of geometry and position of polymeric heart valves on hemodynamic with fluid-structure interaction numerical method

Computational modeling and numerical simulation of heart valve dynamics incorporating both fluid dynamics and valve structural replications has been challenging. In this study, we developed a double-coupled fluid-structure interaction (FSI) model using arbitrary lagrangian eulerian(ALE) and steered adaptive mesh(SAM). So we were looking to simulate transcatheter aortic valve (TAV) hemodynamic performance throughout entire cardiac cycles [1]. To reach this object, semi-real geometry of aorta and aortic polymeric valves has been created. At model inlet, left ventricular pressure and at the model outlet the elastic porous tube have been considered. Nonlinear finite element way and Sparse solver was utilized to couple fluid and solid equation. Consequently, extensive and comparative simulation were performed to investigate the impact of valve elasticity and valve positions on hemodynamics and solid parameters. Effective Orifice Area(EOA) also has been calculated [1]. The simulation results indicated that the lower of the elastic modulus cause to increase the EOA. Furthermore, the result of valve position showed, whenever the valve is closer to sinuses, a greater EOA and lower stresses impose on the leaflet are achievable.

Hybrid membranes for the production of blood contacting surfaces: physicochemical, structural and biomechanical characterization

Background

Due to the shortage of organs’ donors that limits biological heart transplantations, mechanical circulatory supports can be implanted in case of refractory end-stage heart failure to replace partially (Ventricular Assist Device, VAD) or completely (Total Artificial Heart, TAH) the cardiac function. The hemocompatibility of mechanical circulatory supports is a fundamental issue that has not yet been fully matched; it mostly depends on the nature of blood-contacting surfaces.

Methods

In order to obtain hemocompatible materials, a pool of hybrid membranes was fabricated by coupling a synthetic polymer (polycarbonate urethane, commercially available in two formulations) with a decellularized biological tissue (porcine pericardium). To test their potential suitability as candidate materials for realizing the blood-contacting surfaces of a novel artificial heart, hybrid membranes have been preliminarily characterized in terms of physicochemical, structural and mechanical properties.

Results

Our results ascertained that the hybrid membranes are properly stratified, thus allowing to expose their biological side to blood and their polymeric surface to the actuation system of the intended device. From the biomechanical point of view, the hybrid membranes can withstand deformations up to more than 70 % and stresses up to around 8 MPa.

Conclusions

The hybrid membranes are suitable for the construction of the ventricular chambers of innovative mechanical circulatory support devices.

Transcatheter Heart Valves: A Biomaterials Perspective

Heart valve disease is prevalent throughout the world, and the number of heart valve replacements is expected to increase rapidly in the coming years. Transcatheter heart valve replacement (THVR) provides a safe and minimally invasive means for heart valve replacement in high-risk patients. The latest clinical data demonstrates that THVR is a practical solution for low-risk patients. Despite these promising results, there is no long-term (>20 years) durability data on transcatheter heart valves (THVs), raising concerns about material degeneration and long-term performance. This review presents a detailed account of the materials development for THVRs. It provides a brief overview of THVR, the native valve properties, the criteria for an ideal THV, and how these devices are tested. A comprehensive review of materials and their applications in THVR, including how these materials are fabricated, prepared, and assembled into THVs is presented, followed by a discussion of current and future THVR biomaterial trends. The field of THVR is proliferating, and this review serves as a guide for understanding the development of THVs from a materials science and engineering perspective.

Valve Academic Research Consortium 3: updated endpoint definitions for aortic valve clinical research

Aims 

The Valve Academic Research Consortium (VARC), founded in 2010, was intended to (i) identify appropriate clinical endpoints and (ii) standardize definitions of these endpoints for transcatheter and surgical aortic valve clinical trials. Rapid evolution of the field, including the emergence of new complications, expanding clinical indications, and novel therapy strategies have mandated further refinement and expansion of these definitions to ensure clinical relevance. This document provides an update of the most appropriate clinical endpoint definitions to be used in the conduct of transcatheter and surgical aortic valve clinical research.

Methods and results 

Several years after the publication of the VARC-2 manuscript, an in-person meeting was held involving over 50 independent clinical experts representing several professional societies, academic research organizations, the US Food and Drug Administration (FDA), and industry representatives to (i) evaluate utilization of VARC endpoint definitions in clinical research, (ii) discuss the scope of this focused update, and (iii) review and revise specific clinical endpoint definitions. A writing committee of independent experts was convened and subsequently met to further address outstanding issues. There were ongoing discussions with FDA and many experts to develop a new classification schema for bioprosthetic valve dysfunction and failure. Overall, this multi-disciplinary process has resulted in important recommendations for data reporting, clinical research methods, and updated endpoint definitions. New definitions or modifications of existing definitions are being proposed for repeat hospitalizations, access site-related complications, bleeding events, conduction disturbances, cardiac structural complications, and bioprosthetic valve dysfunction and failure (including valve leaflet thickening and thrombosis). A more granular 5-class grading scheme for paravalvular regurgitation (PVR) is being proposed to help refine the assessment of PVR. Finally, more specific recommendations on quality-of-life assessments have been included, which have been targeted to specific clinical study designs.

Conclusions 

Acknowledging the dynamic and evolving nature of less-invasive aortic valve therapies, further refinements of clinical research processes are required. The adoption of these updated and newly proposed VARC-3 endpoints and definitions will ensure homogenous event reporting, accurate adjudication, and appropriate comparisons of clinical research studies involving devices and new therapeutic strategies.

Valve Academic Research Consortium 3: Updated Endpoint Definitions for Aortic Valve Clinical Research

AIMS

The Valve Academic Research Consortium (VARC), founded in 2010, was intended to (i) identify appropriate clinical endpoints and (ii) standardize definitions of these endpoints for transcatheter and surgical aortic valve clinical trials. Rapid evolution of the field, including the emergence of new complications, expanding clinical indications, and novel therapy strategies have mandated further refinement and expansion of these definitions to ensure clinical relevance. This document provides an update of the most appropriate clinical endpoint definitions to be used in the conduct of transcatheter and surgical aortic valve clinical research.

METHODS AND RESULTS

Several years after the publication of the VARC-2 manuscript, an in-person meeting was held involving over 50 independent clinical experts representing several professional societies, academic research organizations, the US Food and Drug Administration (FDA), and industry representatives to (i) evaluate utilization of VARC endpoint definitions in clinical research, (ii) discuss the scope of this focused update, and (iii) review and revise specific clinical endpoint definitions. A writing committee of independent experts was convened and subsequently met to further address outstanding issues. There were ongoing discussions with FDA and many experts to develop a new classification schema for bioprosthetic valve dysfunction and failure. Overall, this multi-disciplinary process has resulted in important recommendations for data reporting, clinical research methods, and updated endpoint definitions. New definitions or modifications of existing definitions are being proposed for repeat hospitalizations, access site-related complications, bleeding events, conduction disturbances, cardiac structural complications, and bioprosthetic valve dysfunction and failure (including valve leaflet thickening and thrombosis). A more granular 5-class grading scheme for paravalvular regurgitation (PVR) is being proposed to help refine the assessment of PVR. Finally, more specific recommendations on quality-of-life assessments have been included, which have been targeted to specific clinical study designs.

CONCLUSIONS

Acknowledging the dynamic and evolving nature of less-invasive aortic valve therapies, further refinements of clinical research processes are required. The adoption of these updated and newly proposed VARC-3 endpoints and definitions will ensure homogenous event reporting, accurate adjudication, and appropriate comparisons of clinical research studies involving devices and new therapeutic strategies.

Transcatheter Heart Valve Downstream Fluid Dynamics in an Accelerated Evaluation Environment

Transcatheter aortic valve replacements (TAVRs) provide minimally invasive delivery of bioprosthetic heart valves (BHVs) for the treatment of aortic valve disease. While surgical BHVs show efficacy for 8-10 years, long-term TAVR durability remains unknown. Pre-clinical testing evaluates BHV durability in an ISO:5840 compliant accelerated wear tester (AWT), yet, the design and development of AWTs and their accuracy in predicting in vivo performance, is unclear. As a result of limited knowledge on AWT environment and BHV loading, durability assessment of candidate valves remains fundamentally empirical. For the first time, high-speed particle image velocimetry quantified an ISO:5840 compliant downstream AWT velocity field, Reynolds stresses, and turbulence intensity. TAVR enface imaging quantified the orifice area and estimated the flow rate. When valve area and flow rate were at their maximum during peak systole (1.49 cm² and 16.05 L/min, respectively), central jet velocity, Reynolds normal and shear stress, and turbulence intensity grew to 0.50 m/s, 265.1 Pa, 124.6 Pa, and 37.3%, respectively. During diastole, unique AWT recirculation produced retrograde flow and the directional changes created eddies. These novel AWT findings demonstrated a substantially reduced valve fully loaded period and pressure not matching in vivo or in vitro studies, despite the comparable fluid environment and TAVR motion.

Fabrication and in-vitro characterization of a polymeric aortic valve for minimally invasive valve replacement

The valve replacement therapy is the standard treatment for severe heart valve diseases. Nowadays, two types of commercial prosthesis are available: mechanical and biological, but both of them have severe limitations. Moreover, alternative therapeutic approach for valve replacement, based on minimally invasive techniques (MIAVR), motivates the search for new valve materials. In this study a polyurethane-based self-expandable tri-leaflets heart valve compatible with MIAVR procedure is proposed. The device is based on the development, fabrication and characterization of three different elements: the leaflets, the polymeric stent for supporting the leaflets, and the external metallic stent for anchoring the valve to the native aortic root. The polymeric stent and the valve leaflets were fabricated using a thermoplastic silicone-polycarbonate-urethane using 3D printing and spray technology while the external metallic stent was made in nickel titanium (Nitinol) to obtain a self-expandable valve after the crimping process. The three elements were assembled in the completed device and tested by crimping, fatigue and fluid-dynamic test. The novel polymeric valve proposed showed promising results about valve crimping capabilities, durability and fluid dynamic performances. This approach could offer advantages such as low cost and to produce a tailor-made device basing on patient's imaging data. Moreover, the selected biomaterial offers the potential to have a device that could need of permanent anticoagulation and lack of calcification.

Patient-specific in vitro testing for evaluating TAVR clinical performance-A complementary approach to current ISO standard testing

Following in vitro tests established for surgical prosthetic heart valves, transcatheter aortic valves (TAV) are similarly tested in idealized geometries-excluding effects that may hamper TAVR performance in situ. Testing in vitro in pulse duplicator systems that incorporated patient-specific replicas would enhance the testing veracity by bringing it closer to the clinical scenario. To that end we compare TAV hemodynamic performance tested in idealized geometries according to the ISO standard (baseline performance) to that obtained by testing the TAVs following deployment in patient-specific replicas. Balloon-expandable (n = 2) and self-expandable (n = 3) TAVs were tested in an idealized geometry in mock-circulation system (following ISO 5840-3 guidelines) and compared to the measurements in a dedicated mock-circulation system adapted for the five patient-specific replicas. Patient-specific deployments resulted in a decline in performance as compared to the baseline idealized testing, as well as a variation in performance that depended on the design features of each device that was further correlated with the radial expansion and eccentricity of the deployed TAV stent (obtained with CT-scans of the deployed valves). By excluding the deployment effects in irregular geometries, the current idealized ISO testing is limited to characterize the baseline device performance. Utilizing patient-specific anatomic contours provides performance indicators under more stringent conditions likely encountered in vivo. It has the potential to enhance testing and development complementary to the ISO standard, for improved TAV safety and effectiveness.

In Vitro Durability and Stability Testing of a Novel Polymeric Transcatheter Aortic Valve

Transcatheter aortic valve replacement (TAVR) has emerged as an effective therapy for the unmet clinical need of inoperable patients with severe aortic stenosis (AS). Current clinically used tissue TAVR valves suffer from limited durability that hampers TAVR's rapid expansion to younger, lower risk patients. Polymeric TAVR valves optimized for hemodynamic performance, hemocompatibility, extended durability, and resistance to calcific degeneration offer a viable solution to this challenge. We present extensive in vitro durability and stability testing of a novel polymeric TAVR valve (PolyNova valve) using 1) accelerated wear testing (AWT, ISO 5840); 2) calcification susceptibility (in the AWT)-compared with clinically used tissue valves; and 3) extended crimping stability (valves crimped to 16 Fr for 8 days). Hydrodynamic testing was performed every 50M cycles. The valves were also evaluated visually for structural integrity and by scanning electron microscopy for evaluation of surface damage in the micro-scale. Calcium and phosphorus deposition was evaluated using micro-computed tomography (μCT) and inductive coupled plasma spectroscopy. The valves passed 400M cycles in the AWT without failure. The effective orifice area kept stable at 1.8 cm with a desired gradual decrease in transvalvular pressure gradient and regurgitation (10.4 mm Hg and 6.9%, respectively). Calcium and phosphorus deposition was significantly lower in the polymeric valve: down by a factor of 85 and 16, respectively-as compared to a tissue valve. Following the extended crimping testing, no tears nor surface damage were evident. The results of this study demonstrate the potential of a polymeric TAVR valve to be a viable alternative to tissue-based TAVR valves.

Are the dynamic changes of the aortic root determinant for thrombosis or leaflet degeneration after transcatheter aortic valve replacement?

The role of the aortic root is to convert the accumulated elastic energy during systole into kinetic flow energy during diastole, in order to improve blood distribution in the coronary tree. Therefore, the sinuses of Valsalva of the aortic root are not predisposed to accept any bulky material, especially in case of uncrushed solid calcific agglomerates. This concept underlines the differences between surgical aortic valve replacement, in which decalcification is a main part of the procedure, and transcatheter aortic valve replacement (TAVR). Cyclic changes in shape and size of the aortic root influence blood flow in the Valsalva sinuses. Recent papers have been investigating the dynamic changes of the aortic root and whether those differences might be correlated with clinical effects, and this paper aims to summarize part of this flourishing literature. Post-TAVR aortic root remodeling, dynamic flow and TAVR complications might have a fluidodynamic background, and clinically observed side effects such as thrombosis or leaflet degeneration should be further investigated in basic researches. Also, aortic root changes could impact valve type and size selection, affecting the decision of over-sizing or under-sizing in order to prevent valve embolization or coronary ostia obstruction.

New Insights into Valve Hemodynamics

Heart valve diseases are common disorders with five million annual diagnoses being made in the United States alone. All heart valve disorders alter cardiac hemodynamic performance; therefore, treatments aim to restore normal flow. This paper reviews the state-of-the-art clinical and engineering advancements in heart valve treatments with a focus on hemodynamics. We review engineering studies and clinical literature on the experience with devices for aortic valve treatment, as well as the latest advancements in mitral valve treatments and the pulmonic and tricuspid valves on the right side of the heart. Upcoming innovations will potentially revolutionize treatment of heart valve disorders. These advancements, and more gradual enhancements in the procedural techniques and imaging modalities, could improve the quality of life of patients suffering from valvular disease who currently cannot be treated.

In-vitro characterization of self-expandable textile transcatheter aortic valves

OBJECTIVE

This study aims at assessing the global dynamic behavior, closing energy and turbulence characteristics of self-expandable textile (inclined and straight yarn) transcatheter aortic valves (TAV) versus bioprosthetic TAVs.

METHODS

Two self-expandable textile TAVs one with inclined yarn textile and another with straight yarn textile leaflets were assessed in a pulse duplicator and compared with a self-expandable commercial bioprosthetic TAV under physiological pressure and flow. Particle Image Velocimetry and high-speed imaging were performed. Effective orifice areas (EOA), leakage fractions (LF), Pinwheeling indices (PI), closing energy (E), viscous shear stresses (VSS) and Reynolds shear stresses (RSS) were calculated.

RESULTS

(a) EOAs and LFs were 2.27 ± 0.03 cm2, 31.7 ± 0.6%; 2.25 ± 0.08 cm2, 26.6 ± 0.7%; and 1.63 ± 0.01 cm2, 29.1 ± 1.25% for inclined textile, bioprosthetic and straight textile TAV respectively (p < 0.0001). (b) Following same order, PIs were significantly different going from 1.16 ± 0.21%, 8.48 ± 0.8% and 8.865 ± 0.58% with the exception of CoreValve and straight yarn valve (p = 0.37); (c) E is lowest for straight textile TAV (0.0024 ± 0.0017 J), followed by bioprosthetic valve (0.00259 ± 0.0011 J) and then 45° Oriented Yarn Valve (0.00334 ± 0.03 J) (d) At peak systole, the highest RSS distribution was with the Straight textile TAV reaching up to 330Pa. The bioprosthetic TAV shows the smallest range with RSS reaching around 230Pa and the inclined textile TAV up to 280Pa. VSS limits were comparable among the 3 valves ranging between 5.2Pa and 5.7Pa.

CONCLUSION

Hemodynamic similarities were found between the textile self-expandable valves and the bioprosthetic valve. This study constitutes another step towards showing the potential that textile valves have to become an alternative for the biological ones.

Transcatheter Aortic Valve Implantation Current Indications and Future Directions

Degenerative heart valve disease is associated with significant morbidity and mortality and healthcare expenditures. Transcatheter heart valve repair and replacement has introduced a fundamental change in the therapeutic management and transcatheter aortic valve replacement (TAVR) has gained substantial popularity. Favorable results from randomized trials and large real world registries lead to TAVR being considered a standard procedure with high rates of procedural success and low rates of peri-procedural complications. This article aims to review the past evolution, summarize the available evidence, discuss current indications and limitations and venture a glimpse into the future of percutaneous interventions for aortic valve disease.

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.

Polyisobutylene-Based Thermoplastic Elastomers for Manufacturing Polymeric Heart Valve Leaflets: In Vitro and In Vivo Results

Superior polymers represent a promising alternative to mechanical and biological materials commonly used for manufacturing artificial heart valves. The study is aimed at assessing poly(styrene-block-isobutylene-block-styrene) (SIBS) properties and comparing them with polytetrafluoroethylene (Gore-texTM, a reference sample). Surface topography of both materials was evaluated with scanning electron microscopy and atomic force microscopy. The mechanical properties were measured under uniaxial tension. The water contact angle was estimated to evaluate hydrophilicity/hydrophobicity of the study samples. Materials’ hemocompatibility was evaluated using cell lines (Ea.hy 926), donor blood, and in vivo. SIBS possess a regular surface relief. It is hydrophobic and has lower strength as compared to Gore-texTM (3.51 MPa vs. 13.2/23.8 MPa). SIBS and Gore-texTM have similar hemocompatibility (hemolysis, adhesion, and platelet aggregation). The subcutaneous rat implantation reports that SIBS has a lower tendency towards calcification (0.39 mg/g) compared with Gore-texTM (1.29 mg/g). SIBS is a highly hemocompatible material with a promising potential for manufacturing heart valve leaflets, but its mechanical properties require further improvements. The possible options include the reinforcement with nanofillers and introductions of new chains in its structure.

The Influence of Polymer Processing Methods on Polymer Film Physical Properties and Vascular Cell Responsiveness

Implantable vascular devices typically interface with blood and vascular tissues. Physical properties of device materials and coatings, independent of chemical composition, can significantly influence cell responses and implant success. Here, we analyzed the effect of various polymer processing regimes, using a single implant polymer - poly(ε-caprolactone) (PCL), on vascular endothelial cell (EC), smooth muscle cell (SMC), and platelet response. PCL films were formed by varying three parameters: 1) formation method - solvent casting, melt pressing or spin coating; 2) molecular weight - 50 or 100 kDa; and 3) solvent type - dichloromethane (DCM) or tetrahydrofuran (THF). We quantified the relationship of polymer processing choice to surface roughness, wettability, and bulk stiffness; and to EC adhesion, SMC adhesion, and platelet activity state (PAS). Multiple regression analysis identified which processing method signficantly impacted (F-ratio>p-value; p<0.1) polymer physical properties and vascular cell interaction. Film formation method affected PCL roughness (Rq), wettability (°), and stiffness (MPa) with spin coating resulting in the most wettable (81.8±0.7°), and stiffest (1.12±0.07 MPa; p<0.001) polymer film; however, solvent cast films were the roughest (281±66nm). Molecular weight influenced wettability, with the highest wettability on 50 kDa films (79.7±0.7°; p<0.001) and DCM solvent films (83.0±1.0°; p<0.01). The multiple regression model confidently predicted (F-ratio=9.88; p=0.005) wettability from molecular weight (p=0.002) and film formation method (p=0.03); stiffness (F-ratio=4.21; p=0.05) also fit well tofilm formation method (p=0.02). Film formation method impacted SMC adhesion and platelet activity state, but not EC adhesion, with melt press PCL promoting the highest SMC adhesion (18000±1536 SMCs; p<0.05) and PAS (5.0±0.7 %PAS). The regression model confidently fit SMC adhesion (F-ratio=3.15; p=0.09) and PAS (F-ratio=5.30; p=0.05) to polymer processing choices, specifically film formation method (p<0.03). However, only SMC adhesion had a model that fit well (F-ratio=4.13; p=0.05) to the physical properties directly, specifically roughness and wettability (p<0.04).

In vitro hemodynamic assessment of a novel polymeric transcatheter aortic valve

Transcatheter aortic valve replacement (TAVR) is a life-saving alternative to surgical intervention. However, the identification of features associated with poor outcomes, including residual paravalvular leakage (PVL), leaflet calcification, and subclinical leaflet thrombosis, are cause to be concerned about valve durablilty (Mylotte and Piazza, 2015a, 2015b; Dasi et al., 2017; Makkar et al., 2015; Kheradvar et al., 2015a). The aim of this study is to optimize the potential of a hyaluronan (HA) enhanced polymeric transcatheter aortic valve (HA-TAV) that has promised to reduce blood damage causing-turbulent flow while maintaining durability. HA-enhanced linear low-density polyethylene (LLDPE) leaflets were sutured to novel cobalt chromium stents, size 26 mm balloon expandable stents. Hemodynamic performance was assessed in a left heart simulator under physiological pressure and flow conditions and compared to a 26 mm Medtronic Evolut and 26 mm Edwards SAPIEN 3. High-speed imaging and particle image velocimetry (PIV) were performed. The HA-TAV demonstrated an effective orifice area (EOA) within one standard deviation of the leading valve, SAPIEN 3.The regurgitant fraction (RF) of the HA-TAV (11.23 ± 0.55%) is decreased in comparison the Evolut (15.74 ± 0.73%) and slightly higher than the SAPIEN 3 (10.92 ± 0.11%), which is considered trace regurgitation according to valve standards. A decreased number of higher principal Reynolds shear stresses were shown for the HA-TAV at each cardiac phase. The HA-TAV is directly comparable and in some cases superior to the leading commercially available prosthetic heart valves in in-vitro hemodynamic testing.

Principles of TAVR valve design, modelling, and testing

INTRODUCTION

Transcatheter aortic valve replacement (TAVR) has emerged as an effective minimally-invasive alternative to surgical valve replacement in medium- to high-risk, elderly patients with calcific aortic valve disease and severe aortic stenosis. The rapid growth of the TAVR devices market has led to a high variety of designs, each aiming to address persistent complications associated with TAVR valves that may hamper the anticipated expansion of TAVR utility.

AREAS COVERED

Here we outline the challenges and the technical demands that TAVR devices need to address for achieving the desired expansion, and review design aspects of selected, latest generation, TAVR valves of both clinically-used and investigational devices. We further review in detail some of the up-to-date modeling and testing approaches for TAVR, both computationally and experimentally, and additionally discuss those as complementary approaches to the ISO 5840-3 standard. A comprehensive survey of the prior and up-to-date literature was conducted to cover the most pertaining issues and challenges that TAVR technology faces.

EXPERT COMMENTARY

The expansion of TAVR over SAVR and to new indications seems more promising than ever. With new challenges to come, new TAV design approaches, and materials used, are expected to emerge, and novel testing/modeling methods to be developed.