In vitro evaluation of a novel hemodynamically optimized trileaflet polymeric prosthetic heart valve

Research on fatigue optimization simulation of polymeric heart valve based on the iterative sub-regional thickened method

Prosthetic polymeric heart valves (PHVs) have the potential to overcome the inherent material and design limitations of traditional valves in the treatment of valvular heart disease; however, their durability remains limited. Optimal design of the valve structure is necessary to improve their durability. This study aimed to enhance the fatigue resistance of PHVs by improving the stress distribution. Iterative subregional thickening of the leaflets was used, and the mechanical stress distribution and hemodynamics of these polymeric tri-leaflet valves were characterized using a fluid-structure interaction approach. Subregional thickening led to a reduction in stress concentration on the leaflet, with the effective orifice area still meeting ISO 5840-3 and the regurgitant volume achieving a similar value to those in previous studies. The maximum stress in the final iteration was reduced by 28% compared with that of the prototype. The proposed method shows potential for analyzing the stress distribution and hemodynamic performance of subregional thickened valves and can further improve the durability of PHVs.

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 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.

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.

Design consideration of a novel polymeric transcatheter heart valve through computational modeling

Transcatheter heart valve replacement is becoming a more routine procedure, and this is further supported by positive outcomes from studies involving low-risk patients. Nevertheless, the lack of long-term transcatheter heart valve (TAV) durability is still one of the primary concerns. As a result, more research has been focused on improving durability through various methods such as valve design, computational modeling, and material selection. Recent advancements in polymeric valve fabrication showed that linear low-density polyethylene (LLDPE) could be used as leaflet material for transcatheter heart valves. In this paper, a parametric study of computational simulations showed stress distribution on the leaflets of LLDPE-TAV under diastolic load, and the results were used to improve the stent design. The in silico experiment also tested the effect of shock absorbers in terms of valve durability. The results demonstrated that altering specific stent angles can significantly lower peak stress on the leaflets (13.8 vs. 6.07 MPa). Implementing two layers of shock absorbers further reduces the stress value to 4.28 MPa. The pinwheeling index was assessed, which seems to correlate with peak stress. Overall, the parametric study and the computational method can be used to analyze and improve valve durability.

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.

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.

Poor results after pyrocarbon interpositional shoulder arthroplasty

BACKGROUND

This study aimed to describe the clinical outcomes and complications of 10 cases of pyrocarbon interposition shoulder arthroplasty (PISA).

METHODS

The clinical and radiographic records of 10 patients who underwent PISA using the InSpyre shoulder prosthesis (Tornier-Wright) between July 2012 and March 2017 were reviewed. The mean age at surgery was 55 years. Surgical indications included patients aged <60 years with Walch type B glenoid glenohumeral osteoarthritis (n = 7), avascular necrosis (AVN) of the humeral head (n = 1), or secondary severe glenohumeral osteoarthritis with axillary nerve dysfunction (n = 2). Outcomes of interest were postoperative complications and need for revision surgery, preoperative and postoperative patient-reported outcomes (Constant score [CS] and Subjective Shoulder Value [SSV]), and range of motion. The radiographic characteristics of the implants were evaluated.

RESULTS

Among the 10 patients, 5 underwent revision to reverse shoulder arthroplasty during the study period owing to poor clinical outcomes based on the CS and SSV. All 5 revised patients had Walch type B glenoid morphology at the time of the index procedure. The mean time to revision surgery in this subset of patients was 60 months. The remaining 5 patients who did not undergo any revision procedure had significant improvement in mean CS and SSV from 30-65 points and 32%-87%, respectively, but at a shorter duration of follow-up of 35 months.

CONCLUSION

High clinical failure rate and poor results at mean 5-year follow-up were found in younger PISA patients with baseline Walch B glenohumeral osteoarthritis. We would caution against use of PISA in this challenging patient population. PISA yielded more favorable short-term outcomes in patients with humeral-sided deformity or severe secondary glenohumeral osteoarthritis with axillary nerve dysfunction; however, longevity of the implant in this population remains unclear.

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.

Shear-mediated platelet activation in the free flow II: Evolving mechanobiological mechanisms reveal an identifiable signature of activation and a bi-directional platelet dyscrasia with thrombotic and bleeding features

Shear-mediated platelet activation (SMPA) in the "free flow" is the net result of a range of cell mechanobiological mechanisms. Previously, we outlined three main groups of mechanisms including: 1) mechano-destruction - i.e. additive platelet (membrane) damage; 2) mechano-activation - i.e. activation of shear-sensitive ion channels and pores; and 3) mechano-transduction - i.e. "outside-in" signaling via a range of transducers. Here, we report on recent advances since our original report which describes additional features of SMPA. A clear "signature" of SMPA has been defined, allowing differentiation from biochemically-mediated activation. Notably, SMPA is characterized by mitochondrial dysfunction, platelet membrane eversion, externalization of anionic phospholipids, and increased thrombin generation on the platelet surface. However, SMPA does not lead to integrin αIIbβ3 activation or P-selectin exposure due to platelet degranulation, as is commonly observed in biochemical activation. Rather, downregulation of GPIb, αIIbβ3, and P-selectin surface expression is evident. Furthermore, SMPA is accompanied by a decrease in overall platelet size coupled with a concomitant, progressive increase in microparticle generation. Shear-ejected microparticles are highly enriched in GPIb and αIIbβ3. These observations indicate the enhanced diffusion, migration, or otherwise dispersion of platelet adhesion receptors to membrane zones, which are ultimately shed as receptor-rich PDMPs. The pathophysiological consequence of this progressive shear accumulation phenomenon is an associated dyscrasia of remaining platelets - being both reduced in size and less activatable via biochemical means - a tendency to favor bleeding, while concomitantly shed microparticles are highly prothrombotic and increase the tendency for thrombosis in both local and systemic milieu. These mechanisms and observations offer direct clinical utility in allowing measurement and guidance of the net balance of platelet driven events in patients with implanted cardiovascular therapeutic devices.

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.

Numerical evaluation of transcatheter aortic valve performance during heart beating and its post-deployment fluid-structure interaction analysis

Transcatheter aortic valve replacement (TAVR) is a minimally invasive procedure that provides an effective alternative to open-heart surgical valve replacement for treating advanced calcific aortic valve disease patients. However, complications, such as valve durability, device migration, paravalvular leakage (PVL), and thrombogenicity may lead to increased overall post-TAVR morbidity and mortality. A series of numerical studies involving a self-expandable TAVR valve were performed to evaluate these complications. Structural studies were performed with finite element (FE) analysis, followed by computational fluid dynamics (CFD) simulations, and fluid-structure interaction (FSI) analysis. The FE analysis was utilized to study the effect of TAVR valve implantation depth on valve anchorage in the Living Heart Human Model, which is capable of simulating beating heart during repeated cardiac cycles. The TAVR deployment cases where no valve migration was observed were then used to calculate the post-deployment thrombogenic potential via CFD simulations. FSI analysis followed to further assess the post-deployment TAVR hemodynamic performance for different implantation depths. The deployed valves PVL, geometric and effective orifice areas, and the leaflets structural and flow stress magnitudes were compared to determine the device optimal landing zone. The combined structural and hemodynamic analysis indicated that with the TAVR valve deployed at an aft ventricle position an optimal performance was achieved in the specific anatomy studied. Given the TAVR's rapid expansion to younger lower-risk patients, the comprehensive numerical methodology proposed here can potentially be used as a predictive tool for both procedural planning and valve design optimization to minimize the reported complications.

In vitro Assessment of the Impacts of Leaflet Design on the Hemodynamic Characteristics of ePTFE Pulmonary Prosthetic Valves

Prosthetic pulmonary valves are widely used in the management procedures of various congenital heart diseases, including the surgical pulmonary valve replacement (PVR) and right ventricular outflow tract reconstruction (RVOT). The discouraging long-term outcomes of standard prostheses, including homografts and bioprosthetic, constrained their indications. Recent developments in the expanded-polytetrafluoroethylene (ePTFE) pulmonary prosthetic valves provide promising alternatives. In this study, the hemodynamic characteristics of bileaflet and trileaflet ePTFE valve designs were experimentally evaluated. The in vitro tests were performed under the right ventricle (RV) flow conditions by using an in vitro RV circulatory system and particle image velocimetry (PIV). The leaflet kinetics, trans-valvular pressure gradients, effective orifice areas, regurgitant fractions, energy losses, velocity fields, and Reynolds shear stress (RSS) in both prostheses were evaluated. The opening of the bileaflet and trileaflet valve takes 0.060 and 0.088 s, respectively. The closing of the former takes 0.140 s, in contrast to 0.176 s of the latter. The trans-valvular pressure is 6.8 mmHg in the bileaflet valve vs. 7.9 mmHg in the trileaflet valve. The effective orifice area is 1.83 cm2 in the bileaflet valve and 1.72 cm2 in the trileaflet valve. The regurgitant fraction and energy loss of bileaflet are 7.13% and 82 mJ, which are 7.84% and 101.64 mJ in its bileaflet counterpart. The maximum RSS of 48.0 and 49.2 Pa occur at the systole peak in the bileaflet and trileaflet valve, respectively. A higher average RSS level is found in the bileaflet valve. The results from this preliminary study indicate that the current bileaflet prosthetic valve design is capable of providing a better overall hemodynamic performance than the trileaflet design.

Materials and manufacturing perspectives in engineering heart valves: a review

Valvular heart diseases (VHD) are a major health burden, affecting millions of people worldwide. The treatments for such diseases rely on medicine, valve repair, and artificial heart valves including mechanical and bioprosthetic valves. Yet, there are countless reports on possible alternatives noting long-term stability and biocompatibility issues and highlighting the need for fabrication of more durable and effective replacements. This review discusses the current and potential materials that can be used for developing such valves along with existing and developing fabrication methods. With this perspective, we quantitatively compare mechanical properties of various materials that are currently used or proposed for heart valves along with their fabrication processes to identify challenges we face in creating new materials and manufacturing techniques to better mimick ​the performance of native heart valves.

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.

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.

Numerical and in-vitro experimental assessment of the performance of a novel designed expanded-polytetrafluoroethylene stentless bi-leaflet valve for aortic valve replacement

The expanded polytetrafluoroethylene (ePTFE) heart valve can serve as a viable option for prosthetic aortic valve. In this study, an ePTFE bi-leaflet valve design for aortic valve replacement (AVR) is presented, and the performance of the proposed valve was assessed numerically and experimentally. The valve was designed using CAE software. The dynamic behavior of the newly designed bi-leaflet valve under time-varying physiological pressure loading was first investigated by using commercial finite element code. Then, in-vitro tests were performed to validate the simulation and to assess the hemodynamic performance of the proposed design. A tri-leaflet ePTFE valve was tested in-vitro under the same conditions as a reference. The maximum leaflet coaptation area of the bi-leaflet valve during diastole was 216.3 mm2. When fully closed, no leakage gap was observed and the free edges of the molded valve formed S-shaped lines. The maximum Von Mises stress during a full cardiac cycle was 4.20 MPa. The dynamic performance of the bi-leaflet valve was validated by the in-vitro test under physiological aortic pressure pulse. The effective orifice area (EOA), mean pressure gradient, regurgitant volume, leakage volume and energy loss of the proposed valve were 3.14 cm2, 8.74 mmHg, 5.93 ml/beat, 1.55 ml/beat and 98.99 mJ, respectively. This study reports a novel bi-leaflet valve design for AVR. The performance of the proposed valve was numerically and experimentally assessed. Compared with the reference valve, the proposed design exhibited better structural and hemodynamic performances, which improved valve competency. Moreover, the performance of the bi-leaflet design is comparable to commercialized valves available on the market. The results of the present study provide a viable option for the future clinical applications.

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.

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

Transcatheter aortic valve replacement (TAVR) is a minimally-invasive approach for treating severe aortic stenosis. All clinically-used TAVR valves to date utilize chemically-fixed xenograft as the leaflet material. Inherent limitation of the tissue (e.g., calcific degeneration) motivates the search for alternative leaflet material. Here we introduce a novel polymeric TAVR valve that was designed to address the limitations of tissue-valves. In this study, we experimentally evaluated the hemodynamic performance of the valve and compared its performance to clinically-used valves: a gold standard surgical tissue valve, and a TAVR valve. Our comparative testing protocols included: (i) baseline hydrodynamics (ISO:5840-3), (ii) complementary patient-specific hydrodynamics in a dedicated system, and (iii) thrombogenicity. The patient-specific testing system facilitated comparing TAVR valves performance under more realistic conditions. Baseline hydrodynamics results at CO 4-7 L/min showed superior effective orifice area (EOA) for the polymer valve, most-notably as compared to the reference TAVR valve. Regurgitation fraction was higher in the polymeric valve, but within the ISO minimum requirements. Thrombogenicity trends followed the EOA results with the polymeric valve being the least thrombogenic, and clinical TAVR being the most. Hemodynamic-wise, the results strongly indicate that our polymeric TAVR valve can outperform tissue valves.

Comparative Fluid-Structure Interaction Analysis of Polymeric Transcatheter and Surgical Aortic Valves' Hemodynamics and Structural Mechanics

Transcatheter aortic valve replacement (TAVR) has emerged as an effective alternative to conventional surgical aortic valve replacement (SAVR) in high-risk elderly patients with calcified aortic valve disease. All currently FDA-approved TAVR devices use tissue valves that were adapted to but not specifically designed for TAVR use. Emerging clinical evidence indicates that these valves may get damaged during crimping and deployment- leading to valvular calcification, thrombotic complications, and limited durability. This impedes the expected expansion of TAVR to lower-risk and younger patients. Viable polymeric valves have the potential to overcome such limitations. We have developed a polymeric SAVR valve, which was optimized to reduce leaflet stresses and offer a thromboresistance profile similar to that of a tissue valve. This study compares the polymeric SAVR valve's hemodynamic performance and mechanical stresses to a new version of the valve- specifically designed for TAVR. Fluid-structure interaction (FSI) models were utilized and the valves' hemodynamics, flexural stresses, strains, orifice area, and wall shear stresses were compared. The TAVR valve had 42% larger opening area and 27% higher flow rate versus the SAVR valve, while wall shear stress distribution and mechanical stress magnitudes were of the same order, demonstrating the enhanced performance of the TAVR valve prototype. The TAVR valve FSI simulation and Vivitro pulse duplicator experiments were compared in terms of the leaflets' kinematics and the effective orifice area. The numerical methodology presented can be further used as a predictive tool for valve design optimization for enhanced hemodynamics and durability.

A low-cost bioprosthetic semilunar valve for research, disease modelling and surgical training applications

OBJECTIVES

This paper provides detailed instructions for constructing low-cost bioprosthetic semilunar valves for animal research and clinical training. This work fills an important gap between existing simulator training valves and clinical valves by providing fully functioning designs that can be employed in ex vivo and in vivo experiments and can also be modified to model valvular disease.

METHODS

Valves are constructed in 4 steps consisting of creating a metal frame, covering it with fabric and attaching a suture ring and leaflets. Computer-aided design files are provided for making the frame from wire or by metal 3D printing. The covering fabric and suturing ring are made from materials readily available in a surgical lab, while the leaflets are made from pericardium. The entire fabrication process is described in figures and in a video. To demonstrate disease modelling, design modifications are described for producing paravalvular leaks, and these valves were evaluated in porcine ex vivo (n = 3) and in vivo (n = 6) experiments.

RESULTS

Porcine ex vivo and acute in vivo experiments demonstrate that the valves can replicate the performance of clinical valves for research and training purposes. Surgical implantation is similar, and echocardiograms are comparable to clinical valves. Furthermore, valve leaflet function was satisfactory during acute in vivo tests with little central regurgitation, while the paravalvular leak modifications consistently produced leaks in the desired locations.

CONCLUSIONS

The detailed design procedure presented here, which includes a tutorial video and computer-aided design files, should be of substantial benefit to researchers developing valve disease models and to clinicians developing realistic valve training systems.

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.

Biomaterial-driven in situ cardiovascular tissue engineering-a multi-disciplinary perspective

There is a persistent and growing clinical need for readily-available substitutes for heart valves and small-diameter blood vessels. In situ tissue engineering is emerging as a disruptive new technology, providing ready-to-use biodegradable, cell-free constructs which are designed to induce regeneration upon implantation, directly in the functional site. The induced regenerative process hinges around the host response to the implanted biomaterial and the interplay between immune cells, stem/progenitor cell and tissue cells in the microenvironment provided by the scaffold in the hemodynamic environment. Recapitulating the complex tissue microstructure and function of cardiovascular tissues is a highly challenging target. Therein the scaffold plays an instructive role, providing the microenvironment that attracts and harbors host cells, modulating the inflammatory response, and acting as a temporal roadmap for new tissue to be formed. Moreover, the biomechanical loads imposed by the hemodynamic environment play a pivotal role. Here, we provide a multidisciplinary view on in situ cardiovascular tissue engineering using synthetic scaffolds; starting from the state-of-the art, the principles of the biomaterial-driven host response and wound healing and the cellular players involved, toward the impact of the biomechanical, physical, and biochemical microenvironmental cues that are given by the scaffold design. To conclude, we pinpoint and further address the main current challenges for in situ cardiovascular regeneration, namely the achievement of tissue homeostasis, the development of predictive models for long-term performances of the implanted grafts, and the necessity for stratification for successful clinical translation.

A Computational Tool for the Microstructure Optimization of a Polymeric Heart Valve Prosthesis

Styrene-based block copolymers are promising materials for the development of a polymeric heart valve prosthesis (PHV), and the mechanical properties of these polymers can be tuned via the manufacturing process, orienting the cylindrical domains to achieve material anisotropy. The aim of this work is the development of a computational tool for the optimization of the material microstructure in a new PHV intended for aortic valve replacement to enhance the mechanical performance of the device. An iterative procedure was implemented to orient the cylinders along the maximum principal stress direction of the leaflet. A numerical model of the leaflet was developed, and the polymer mechanical behavior was described by a hyperelastic anisotropic constitutive law. A custom routine was implemented to align the cylinders with the maximum principal stress direction in the leaflet for each iteration. The study was focused on valve closure, since during this phase the fibrous structure of the leaflets must bear the greatest load. The optimal microstructure obtained by our procedure is characterized by mainly circumferential orientation of the cylinders within the valve leaflet. An increase in the radial strain and a decrease in the circumferential strain due to the microstructure optimization were observed. Also, a decrease in the maximum value of the strain energy density was found in the case of optimized orientation; since the strain energy density is a widely used criterion to predict elastomer's lifetime, this result suggests a possible increase of the device durability if the polymer microstructure is optimized. The present method represents a valuable tool for the design of a new anisotropic PHV, allowing the investigation of different designs, materials, and loading conditions.

Pledget-Armed Sutures Affect the Haemodynamic Performance of Biologic Aortic Valve Substitutes: A Preliminary Experimental and Computational Study

Surgical aortic valve replacement is the most common procedure of choice for the treatment of severe aortic stenosis. Bioprosthetic valves are traditionally sewed-in the aortic root by means of pledget-armed sutures during open-heart surgery. Recently, novel bioprostheses which include a stent-based anchoring system have been introduced to allow rapid implantation, therefore reducing the duration and invasiveness of the intervention. Different effects on the hemodynamics were clinically reported associated with the two technologies. The aim of this study was therefore to investigate whether the differences in hemodynamic performances are an effect of different anchoring systems. Two commercially available bio-prosthetic aortic valves, one sewed-in with pledget-armed sutures and one rapid-deployment, were thus tested in this study by means of a combined approach of experimental and computational tools. In vitro experiments were performed to evaluate the overall hydrodynamic performance under identical standard conditions; computational fluid dynamics analyses were set-up to explore local flow variations due to different design of the anchoring system. The results showed how the performance of cardiac valve substitutes is negatively affected by the presence of pledget-armed sutures. These are causing flow disturbances, which in turn increase the mean pressure gradient and decrease the effective orifice area. The combined approach of experiments and numerical simulations can be effectively used to quantify the detailed relationship between local fluid-dynamics and overall performances associated with different valve technologies.

Shear-mediated platelet activation in the free flow: Perspectives on the emerging spectrum of cell mechanobiological mechanisms mediating cardiovascular implant thrombosis

Shear-mediated platelet activation (SMPA) is central in thrombosis of implantable cardiovascular therapeutic devices. Despite the morbidity and mortality associated with thrombosis of these devices, our understanding of mechanisms operative in SMPA, particularly in free flowing blood, remains limited. Herein we present and discuss a range of emerging mechanisms for consideration for "free flow" activation under supraphysiologic shear. Further definition and manipulation of these mechanisms will afford opportunities for novel pharmacologic and mechanical strategies to limit SMPA and enhance overall implant device safety.

Fluid dynamic characterization of a polymeric heart valve prototype (Poli-Valve) tested under continuous and pulsatile flow conditions

PURPOSE

Only mechanical and biological heart valve prostheses are currently commercially available. The former show longer durability but require anticoagulant therapy; the latter display better fluid dynamic behavior but do not have adequate durability. New Polymeric Heart Valves (PHVs) could potentially combine the hemodynamic properties of biological valves with the durability of mechanical valves. This work presents a hydrodynamic evaluation of 2 groups of newly developed supra-annular, trileaflet prosthetic heart valves made from styrenic block copolymers (SBC): Poli-Valves.

METHODS

2 types of Poli-Valves made of SBC and differing in polystyrene fraction content were tested under continuous and pulsatile flow conditions as prescribed by ISO 5840 Standard. A pulse duplicator designed ad hoc allowed the valve prototypes to be tested at different flow rates and frequencies. Pressure and flow were recorded; pressure drops, effective orifice area (EOA), and regurgitant volume were computed to assess the behavior of the valve.

RESULTS

Both types of Poli-Valves met the minimum requirements in terms of regurgitation and EOA as specified by the ISO 5840 Standard. Results were compared with 5 mechanical heart valves (MHVs) and 5 tissue heart valves (THVs), currently available on the market.

CONCLUSIONS

Based on these results, PHVs based on styrenic block copolymers, as are Poli-Valves, can be considered a promising alternative for heart valve replacement in the near future.

Physical Characterization and Platelet Interactions under Shear Flows of a Novel Thermoset Polyisobutylene-based Co-polymer

Over the years, several polymers have been developed for use in prosthetic heart valves as alternatives to xenografts. However, most of these materials are beset with a variety of issues, including low material strength, biodegradation, high dynamic creep, calcification, and poor hemocompatibility. We studied the mechanical, surface, and flow-mediated thrombogenic response of poly(styrene-coblock-4-vinylbenzocyclobutene)-polyisobutylene-poly(styrene-coblock-4-vinylbenzocylcobutene) (xSIBS), a thermoset version of the thermoplastic elastomeric polyolefin poly(styrene-block-isobutylene-block-styrene) (SIBS), which has been shown to be resistant to in vivo hydrolysis, oxidation, and enzymolysis. Uniaxial tensile testing yielded an ultimate tensile strength of 35 MPa, 24.5 times greater than that of SIBS. Surface analysis yielded a mean contact angle of 82.05° and surface roughness of 144 nm, which was greater than for poly(ε-caprolactone) (PCL) and poly(methyl methacrylate) (PMMA). However, the change in platelet activation state, a predictor of thrombogenicity, was not significantly different from PCL and PMMA after fluid exposure to 1 dyn/cm(2) and 20 dyn/cm(2). In addition, the number of adherent platelets after 10 dyn/cm(2) flow exposure was on the same order of magnitude as PCL and PMMA. The mechanical strength and low thrombogenicity of xSIBS therefore suggest it as a viable polymeric substrate for fabrication of prosthetic heart valves and other cardiovascular devices.

Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid-structure interaction approach

Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid-structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our hemodynamic shearing device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization.

A Novel Design of a Polymeric Aortic Valve

Introduction

In this paper we propose a novel method for developing a polymeric heart valve that could potentially offer an optimum solution for a heart valve substitute. The valve design proposed will provide superior hydrodynamic performance and excellent structural integrity. A full description of the design process is given together with an analysis of the hemodynamic performance using a 2-way strongly coupled Fluid Structure Interaction (FSI).

Method

A polymeric tri-leaflet heart valve is designed based on a patient's sinus of Valsalva (SOV) geometry. The design strategy aims to improve valve hemodynamic performance as well as valve durability by avoiding stress concentrations in the leaflets and reducing the maximum stress level. The valve dynamics and stress levels are also validated by comparing the predicted data to existing experimental and numerical data.

Results

The stress distribution in the valve structure is fully characterized throughout the simulation and Von Mises stress is found to be up to 5.32 Mpa during diastole. The results show that an effective orifice area (EOA) and a pressure drop of 3.22 cm^2, and 3.52 mmHg, respectively, can be achieved using the proposed design.

Conclusions

The optimized valve demonstrates high hemodynamic performance with no sign of damaging stress concentration in the entire cardiac cycle.

Design of a pulsatile flow facility to evaluate thrombogenic potential of implantable cardiac devices

Due to expensive nature of clinical trials, implantable cardiac devices should first be extensively characterized in vitro. Prosthetic heart valves (PHVs), an important class of these devices, have been shown to be associated with thromboembolic complications. Although various in vitro systems have been designed to quantify blood-cell damage and platelet activation caused by nonphysiological hemodynamic shear stresses in these PHVs, very few systems attempt to characterize both blood damage and fluid dynamics aspects of PHVs in the same test system. Various numerical modeling methodologies are also evolving to simulate the structural mechanics, fluid mechanics, and blood damage aspects of these devices. This article presents a completely hemocompatible small-volume test-platform that can be used for thrombogenicity studies and experimental fluid mechanics characterization. Using a programmable piston pump to drive freshly drawn human blood inside a cylindrical column, the presented system can simulate various physiological and pathophysiological conditions in testing PHVs. The system includes a modular device-mounting chamber, and in this presented case, a 23 mm St. Jude Medical (SJM) Regents® mechanical heart valve (MHV) in aortic position was used as the test device. The system was validated for its capability to quantify blood damage by measuring blood damage induced by the tester itself (using freshly drawn whole human blood). Blood damage levels were ascertained through clinically relevant assays on human blood while fluid dynamics were characterized using time-resolved particle image velocimetry (PIV) using a blood-mimicking fluid. Blood damage induced by the tester itself, assessed through Thrombin-anti-Thrombin (TAT), Prothrombin factor 1.2 (PF1.2), and hemolysis (Drabkins assay), was within clinically accepted levels. The hydrodynamic performance of the tester showed consistent, repeatable physiological pressure and flow conditions. In addition, the system contains proximity sensors to accurately capture leaflet motion during the entire cardiac cycle. The PIV results showed skewing of the leakage jet, caused by the asymmetric closing of the two leaflets. All these results are critical to characterizing the blood damage and fluid dynamics characteristics of the SJM Regents® MHV, proving the utility of this tester as a precise system for assessing the hemodynamics and thrombogenicity for various PHVs.