Transcatheter Heart Valves: A Biomaterials Perspective

Engineering of Bioprosthetic Heart Valves with Synergistic Zwitterionic Surface Modification and Zirconium Cross-linking for Improved Biocompatibility and Durability

Bioprosthetic heart valves (BHVs) are frequently utilized in surgeries for heart valve replacement to address valvular heart disease (VHD). Despite their widespread use, BHVs still face challenges in clinical applications, such as thrombosis, calcification, immune responses, poor re-endothelialization, infection, component degradation, and mechanical failure, which are largely due to the heterogeneous cross-linking effects. To address these issues, we propose a synergistic engineering strategy based on sequential zwitterionic surface modification and zirconium cross-linking to improve the biocompatibility and durability of BHVs. After surface modification via ring-opening reactions of zwitterionic epoxy copolymers (PGSB) on collagen fibers of decellularized porcine pericardium (D-PP), the zwitterionic PGSB significantly promoted the uniform transfer of zirconium ions (Zr4+) and further coordinated with Zr4+ to achieve homogeneous cross-linking between collagen fibers. Compared to conventional glutaraldehyde (GA)-cross-linked PP, PGSB/Zr-PP showed enhanced anti-thrombotic performance, attenuated immune rejection, accelerated endothelialization, and over 95% reduction in calcification after 90 days of subcutaneous implantation, collectively indicating improved biocompatibility. Furthermore, this homogeneously cross-linked PGSB/Zr-PP exhibited undetectable component degradation and simultaneous improvements in both strength and toughness, all of which are essential for improving the durability of BHVs. Intriguingly, the zwitterionic sulfobetaine groups could be converted into bactericidal quaternary ammonium groups upon coordination with Zr4+, resulting in strong antibacterial and anti-biofilm activities beneficial for preventing life-threatening prosthetic valve endocarditis. More importantly, PGSB/Zr-PP met the ISO 5840-3 standards required for BHV applications in terms of hydrodynamic performance and 200-million-cycle durability. These results demonstrate that PGSB/Zr-PP would be a promising alternative to GA-cross-linked BHVs. STATEMENT OF SIGNIFICANCE: Mainstream glutaraldehyde-cross-linked BHV face persistent clinical challenges, including thrombosis, calcification, immune response, poor re-endothelialization, infection, component degradation, and mechanical failure. Although various non-glutaraldehyde cross-linkers have been investigated, few strategies effectively address these challenges due to the heterogeneous nature of cross-linking. Herein, we present a synergistic engineering strategy based on sequential zwitterionic surface modification and zirconium cross-linking. This strategy produces homogeneously cross-linked BHVs with comprehensive improvements in anti-thrombogenicity, immune compatibility, endothelialization, resistance to calcification and infection, enzymatic stability, and mechanical strength. Notably, the aortic BHV fabricated via this method met the ISO 5840-3 standards for hydrodynamic performance and durability, demonstrating its long-term clinical potential.

An oxidized chondroitin sulfate-crosslinked and CuCDs-loaded decellularized bovine pericardium with improved anti-coagulation, pro-endothelialization and anti-calcification properties for BHVs

With the growth of the elderly people and the development of transcatheter aortic valve replacement (TAVR) technology, bioprosthetic heart valves (BHVs) originating from the decellularized bovine pericardium (DBP) have become a favourable option for severe valvular heart disease (VHD). However, currently, available commercial bioprosthetic heart valves prepared from glutaraldehyde (GA)-crosslinked xenografts have limited durability because of various factors, including severe cytotoxicity, inflammatory response, poor pro-endothelialization ability and calcification. Therefore, the development of valve materials with better performance is urgent. In this work, we first synthesized Cu-doped carbon dots (CuCDs) with excellent biocompatibility and high stability using sodium citrate, ethylenediamine and copper chloride. Subsequently, oxidized chondroitin sulfate (OCS) was used to crosslink the decellularized bovine pericardium to obtain OCS-BP followed by loading CuCDs onto the surface of this OCS-fixed BP sample through amide bonds formed by an EDC/NHS-catalyzed reaction between the functional groups on CuCDs and OCS-BP to prepare the BHV (CuCDs-OCS-BP) with specific properties. Relevant experiments conducted both in vivo and in vitro indicate that CuCDs-OCS-BP with good stability showed improved mechanical properties, compliance and flexibility, encouraging HUVEC-cytocompatibility, excellent anti-blood cell adhesion, antithrombogenic properties, anti-inflammatory and anti-calcification properties, and a good endothelialisation ability due to the catalytic generation of endogenous nitric oxide. Overall, CuCDs-OCS-BP is a promising material for BHVs.

A Biocompatible, Magnetic-Responsive Shape Memory Silicone Composite for Active Flow Controlling Valve

A magnetically responsive shape memory silicone composite is developed for fabricating an active valve in biomedical fluidic systems. The composite consists of magnetic neodymium-iron-boron (NdFeB) microparticles (MPs) and poly(glycerol-dodecanoate) (PGD) MPs in silicone. PGD has tunable transition temperatures (Tg) of 42-50 °C, empowering shape programmability to the composite, while the NdFeB MPs make the composite responsive to a magnetic field gradient. The composite can be reprogrammed into a compact form for ease delivery and subsequently recovered to its original shape stimulated by external heat that is generated by an oscillation magnetic field (Bh) applied to the NdFeB MPs. Because of the dramatically reduced stiffness above Tg, the valve shape can be actuated to a different one by exerted torques induced by a simultaneously applied actuation magnetic field (Ba). By tuning the magnetic density and direction of Ba, the shape changing extent, e.g., bending angles, which determine the fluid flow resistance, can be modulated. This works provides a proof-of-concept for a programmable, remotely controlled valve for fluid regulation. The adaptability, biocompatibility, and capability for minimally invasive delivery highlight the potential of the valve for applications in drug delivery, embolization, and flow management in blood vessels and eyes.

In Silico Trials of Prosthetic Valves Replicate Methodologies for Evaluating the Fatigue Life of Artificial Leaflets to Expand Beyond In Vitro Tests and Conventional Clinical Trials

Background: Fatigue failure of artificial leaflets significantly limits the durability of prosthetic valves. However, the costs and complexities associated with in vitro testing and conventional clinical trials to investigate the fatigue life of leaflets are progressively escalating. In silico trials offer an alternative solution and validation pathway. This study presents in silico trials of prosthetic valves, along with methodologies incorporating nonlinear behaviors to evaluate the fatigue life of artificial leaflets. Methods: Three virtual patient models were established based on in vitro test and clinical trial data, and virtual surgeries and physiological homeostasis maintenance simulations were performed. These simulations modeled the hemodynamics of three virtual patients following transcatheter valve therapy to predict the service life and crack propagation of leaflets based on the fatigue damage assessment. Results and Conclusions: Compared to traditional trials, in silico trials enable a broader and more rapid investigation into factors related to leaflet damage. The fatigue life of the leaflets in two virtual patients with good implantation morphology exceeded 400 million cycles, meeting the requirements, while the fatigue life of a virtual patient with a shape fold in the leaflet was only 440,000 cycles. The fatigue life of the leaflets varied considerably with different implant morphologies. Postoperative balloon dilation positively enhanced fatigue life. Importantly, in silico trials yielded insights that are difficult or impossible to uncover through conventional experiments, such as the increased susceptibility of leaflets to fatigue damage under compressive loading.

Materials Advances in Devices for Heart Disease Interventions

Heart disease encompasses a range of conditions that affect the heart, including coronary artery disease, arrhythmias, congenital heart defects, heart valve disease, and conditions that affect the heart muscle. Intervention strategies can be categorized according to when they are administered and include: 1) Monitoring cardiac function using sensor technology to inform diagnosis and treatment, 2) Managing symptoms by restoring cardiac output, electrophysiology, and hemodynamics, and often serving as bridge-to-recovery or bridge-to-transplantation strategies, and 3) Repairing damaged tissue, including myocardium and heart valves, when management strategies are insufficient. Each intervention approach and technology require specific material properties to function optimally, relying on materials that support their action and interface with the body, with new technologies increasingly depending on advances in materials science and engineering. This review explores material properties and requirements driving innovation in advanced intervention strategies for heart disease and highlights key examples of recent progress in the field driven by advances in materials research.

Biomimetic multilayered polymeric heart valve featuring ultra-thin thickness and exceptional durability

The transcatheter aortic valve replacement technique has rapidly developed in clinical practice, with its indications expanding to low-risk populations and younger patients. The durability of valve materials has emerged as a critical requirement from a clinical perspective and a research focus in interventional valves. Polymeric heart valves exhibit a promising future because of their enhanced durability and reduced cost. Herein, we developed biomimetic multilayered polymeric heart valves (BMPHVs) utilizing thermoplastic silicone-polycarbonate polyurethane and ultra-high molecular weight polyethylene fibers, selected to replicate the fibrosa, spongiosa, and ventricular layers in natural heart valves. The construction process involved the fabrication of ultra-strong, ultra-fine fiber textiles and leaflet laser cutting utilizing edge-sealing technology. BMPHVs exhibit superior durability than bovine pericardial valves. These findings facilitate the development of a new category of advanced synthetic materials characterized by long-term durability for valve replacement.

The importance of tissue science and valve design in relation to durability and hemodynamics of the DurAVR aortic heart valve

Introduction

Clinical evidence highlighting the efficacy and safety of transcatheter aortic valve replacement (TAVR) and the 2019 Food and Drug Administration (FDA) approval for TAVR in low-risk (younger) patients has created a demand for durable and long-lasting bioprosthetic heart valve (BHV) leaflet materials. Over the life of an implanted BHV mechanical stress, immunogenicity, calcification, and hemodynamic dysfunction lead to failure via structural valve deterioration (SVD). Consequently, the durability of the bioprosthetic materials selected for valve manufacture is of utmost importance.

Technology

The ADAPT™ tissue engineering process, an anti-calcification preparation that transforms xenograft tissue (bovine pericardium) into a durable valve bioscaffold, shows significant clinical benefits in mitigating the interrelated mechanisms leading to SVD. The novel acellular, biostable and non-calcifying biomaterial has recently been molded into a single-piece 3D biomimetic valve (DurAVR™) with excellent early clinical results and the potential to meet the growing demand of durable BHVs for the treatment of aortic stenosis.

Discussion

The unique design of the DurAVR biomimetic valve in combination with the superior biostability of ADAPT tissue could advance the BHV space by providing superior performance and durability to aortic stenosis patients in need of TAVR.

Recent advancements in polymeric heart valves: From basic research to clinical trials

Valvular heart diseases (VHDs) have become one of the most prevalent heart diseases worldwide, and prosthetic valve replacement is one of the effective treatments. With the fast development of minimal invasive technology, transcatheter valves replacement has been exploring in recent years, such as transcatheter aortic valve replacement (TAVR) technology. In addition, basic research on prosthetic valves has begun to shift from traditional mechanical valves and biological valves to the development of polymeric heart valves. The polymeric heart valves (PHVs) have shown a bright future due to their advantages of longer durability, better biocompatibility and reduced cost. This review gives a brief history of the development of polymeric heart valves, provides a summary of the types of polymer materials suitable for heart leaflets and the emerging processing/preparation methods for polymeric heart valves in the basic research. Besides, we facilitate a deeper understanding of polymeric heart valve products that are currently in preclinical/clinical studies, also summary the limitations of the present researches as well as the future development trends. Hence, this review will provide a holistic understanding for researchers working in the field of prosthetic valves, and will offer ideas for the design and research of valves with better durability and biocompatibility.

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.

Surface activation of Hastalex by vacuum argon plasma for cytocompatibility enhancement

Here, we present surface analysis and biocompatibility evaluation of novel composite material based on graphene oxide traded as Hastalex. First, the surface morphology and elemental analysis of the pristine material were examined by atomic force and scanning electron microscopies, and by energy-dispersive and X-ray photoelectron spectroscopies, respectively. The Hastalex surface was then modified by plasma (3 and 8 W with exposure times up to 240 s), the impact of which on the material surface wettability and morphology was further evaluated. In addition, the material aging was studied at room and elevated temperatures. Significant changes in surface roughness, morphology, and area were detected at the nanometer scale after plasma exposure. An increase in oxygen content due to the plasma exposure was observed both for 3 and 8 W. The plasma treatment had an outstanding effect on the cytocompatibility of Hastalex foil treated at both input powers of 3 and 8 W. The cell number of human MRC-5 fibroblasts on Hastalex foils exposed to plasma increased significantly compared to pristine Hastalex and even to tissue culture polystyrene. The plasma exposure also affected the fibroblasts' cell growth and shape.

In silico fatigue optimization of TAVR stent designs with physiological motion in a beating heart model

BACKGROUND AND OBJECTIVE

The rapid expansion of TAVR to younger, low-risk patients raises concerns regarding device durability. Necessarily, extended stent lifetime will become more critical for new generation devices. In vitro methods commonly used for TAVR stent fatigue testing exclude the effects of the beating heart. We present a more realistic in silico stent fatigue analysis utilizing a beating heart model in which TAVR stents experience complex, nonuniform dynamic loading.

METHODS

Virtual TAVR deployments were simulated in the SIMULIA Living Heart Human Model of a beating heart using stent models of the self-expandable nitinol 26-mm CoreValve and Evolut R devices, and a 27-mm PolyV-2. Stent deformation was monitored over three cardiac cycles, and fatigue resistance was evaluated for the nitinol stents using finite element analysis via ABAQUS/Explicit. The average strain and strain amplitude of each stent element were tracked, and established thresholds were applied to determine potential fatigue failure. Fatigue performance of control stents was compared to parametrically modified models with a 20% increase or decrease in strut width.

RESULTS

Stents with reduced strut width applied lower radial force against the contracting myocardium of the beating heart, resulting in larger displacements and higher strain values. Formulas relating in vivo strain to stent design do not account for this. In all models, there were elements in which strains exceeded fatigue failure. The PolyV-2 stent had far fewer failing elements since its struts were optimized to reduce the strain in stent joints, achieving better fatigue resistance in the stent crown and waist elements. Different stent sections showed markedly different fatigue resistance due to the varying loading conditions.

CONCLUSIONS

Our analysis indicates that previous studies underestimate strain amplitudes that may cause stent failure. This study demonstrates the utility of advanced in silico analysis of devices deployed within a beating heart that mimics in vivo loading, offering a cost-effective alternative to human or animal trials and establishing a platform to assess the impact of device design on device durability. The limited fatigue life of TAVR stents indicated here highlights a clinical complication that may eventually develop as younger, lower-risk TAVR patients, age.

Flaw-insensitive fatigue resistance of chemically fixed collagenous soft tissues

Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance.

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.

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.

Increased TGFβ1 and SMAD3 Contribute to Age-Related Aortic Valve Calcification

Aims

Calcific aortic valve disease (CAVD) is a progressive heart disease that is particularly prevalent in elderly patients. The current treatment of CAVD is surgical valve replacement, but this is not a permanent solution, and it is very challenging for elderly patients. Thus, a pharmacological intervention for CAVD may be beneficial. In this study, we intended to rescue aortic valve (AV) calcification through inhibition of TGFβ1 and SMAD3 signaling pathways.

Methods and Results

The klotho gene, which was discovered as an aging-suppressor gene, has been observed to play a crucial role in AV calcification. The klotho knockout (Kl^–/–) mice have shorter life span (8–12 weeks) and develop severe AV calcification. Here, we showed that increased TGFβ1 and TGFβ-dependent SMAD3 signaling were associated with AV calcification in Kl^–/– mice. Next, we generated Tgfb1- and Smad3-haploinsufficient Kl^–/– mice to determine the contribution of TGFβ1 and SMAD3 to the AV calcification in Kl^–/– mice. The histological and morphometric evaluation suggested a significant reduction of AV calcification in Kl^–/–; Tgfb1^± mice compared to Kl^–/– mice. Smad3 heterozygous deletion was observed to be more potent in reducing AV calcification in Kl^–/– mice compared to the Kl^–/–; Tgfb1^± mice. We observed significant inhibition of Tgfb1, Pai1, Bmp2, Alk2, Spp1, and Runx2 mRNA expression in Kl^–/–; Tgfb1^± and Kl^–/–; Smad3^± mice compared to Kl^–/– mice. Western blot analysis confirmed that the inhibition of TGFβ canonical and non-canonical signaling pathways were associated with the rescue of AV calcification of both Kl^–/–; Tgfb1^± and Kl^–/–; Smad3^± mice.

Conclusion

Overall, inhibition of the TGFβ1-dependent SMAD3 signaling pathway significantly blocks the development of AV calcification in Kl^–/– mice. This information is useful in understanding the signaling mechanisms involved in CAVD.

A Novel Crosslinking Method for Improving the Anti-Calcification Ability and Extracellular Matrix Stability in Transcatheter Heart Valves

More than 200,000 patients with aortic diseases worldwide undergo surgical valve replacement each year, and transcatheter heart valves (THV) have been more widely used than ever before. However, THV made by the glutaraldehyde (Glut) crosslinking method has the disadvantage of being prone to calcification, which significantly reduces the durability of biomaterials. In this study, we applied a novel crosslinking method using ribose in THV for the first time, which can decrease calcification and increase the stability of the extracellular matrix (ECM). We incubated the bovine pericardium (BP) in ribose solution at 37°C by shaking for 12 days and confirmed that the structure of the BP was more compact than that of the Glut group. Moreover, the ribose method remarkably enhanced the biomechanical properties and provided reliable resistance to enzymatic degradation and satisfactory cellular compatibility in THV. When the BP was implanted subcutaneously in vivo, we demonstrated that ECM components were preserved more completely, especially in elastin, and the immune-inflammatory response was more moderate than that in the Glut treatment group. Finally, the ribose-cross-linked materials showed better anti-calcification potential and improved durability of THV than Glut-cross-linked materials.

Comparison of performance of self-expanding and balloon-expandable transcatheter aortic valves

Objective

To evaluate the flow dynamics of self-expanding and balloon-expandable transcatheter aortic valves pertaining to turbulence and pressure recovery. Transcatheter aortic valves are characterized by different designs that have different valve performance and outcomes.

Methods

Assessment of transcatheter aortic valves was performed using self-expanding devices (26-mm Evolut [Medtronic], 23-mm Allegra [New Valve Technologies], and small Acurate neo [Boston Scientific]) and a balloon-expandable device (23-mm Sapien 3 [Edwards Lifesciences]). Particle image velocimetry assessed the flow downstream. A Millar catheter was used for pressure recovery calculation. Velocity, Reynolds shear stresses, viscous shear stress, and pressure gradients were calculated.

Results

The maximal velocity at peak systole obtained with the Evolut R, Sapien 3, Acurate neo, and Allegra was 2.12 ± 0.19 m/sec, 2.41 ± 0.06 m/sec, 2.99 ± 0.10 m/sec, and 2.45 ± 0.08 m/sec, respectively (P < .001). Leaflet oscillations with the flow were clear with the Evolut R and Acurate neo. The Allegra shows the minimal range of Reynolds shear stress magnitudes (up to 320 Pa), and Sapien 3 the maximal (up to 650 Pa). The Evolut had the smallest viscous shear stress magnitude range (up to 3.5 Pa), and the Sapien 3 the largest (up to 6.2 Pa). The largest pressure drop at the vena contracta occurred with the Acurate neo transcatheter aortic valve with a pressure gradient of 13.96 ± 1.35 mm Hg. In the recovery zone, the smallest pressure gradient was obtained with the Allegra (3.32 ± 0.94 mm Hg).

Conclusions

Flow dynamics downstream of different transcatheter aortic valves vary significantly depending on the valve type, despite not having a general trend depending on whether or not valves are self-expanding or balloon-expandable. Deployment design did not have an influence on flow dynamics.