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

In vitro hemodynamics of fabric composite membrane for cardiac valve prosthesis replacement

This study aimed to investigate the hemodynamics of a novel fabric composite that can be used as a substitute for bovine pericardium. The structure is composed of ultrahigh molecular weight polyethylene (UHMWPE) fabric coated with thermoplastic polyurethane (TPU) membranes on both sides. In vitro experiments were carried out on two composite valve samples with different specifications and a bovine pericardial one with the same dimension and structure. Hemodynamic properties including the effective orifice area (EOA) and regurgitant fraction (RF) were obtained and compared through pulsatile-flow testing in a pulse duplicator. Using the particle image velocimetry (PIV) technique, frames of the downstream velocity field in the aortic valve chamber were captured during cardiac cycles. Then, the field of Reynolds shear stress (RSS), viscous shear stress (VSS), and turbulent kinetic energy (TKE) at peak systole were calculated. A fluid-structure interaction (FSI) model has also been used to verify the pulsatile-flow testing. Compared with the bovine pericardial valve, composite valves have nosuperiority regarding EOA and RF due to their slightly higher rigidity. However, shear stresses of composite valves were lower than those of the bovine pericardial valve indicating more stable blood flows, which means that composite leaflets have the potential to reduce the risks of thrombosis and hemolysis induced by the mechanical contact between the blood flow and leaflets of valve prostheses.

Bioactive surface coating for preventing mechanical heart valve thrombosis

BACKGROUND

Prosthetic heart valves are the only treatment for most patients with severe valvular heart disease. Mechanical valves, made of metallic components, are the most long-lasting type of replacement valves. However, they are prone to thrombosis and require permanent anticoagulation and monitoring, which leads to higher risk of bleeding and impacts the patient's quality of life.

OBJECTIVES

To develop a bioactive coating for mechanical valves with the aim to prevent thrombosis and improve patient outcomes.

METHODS

We used a catechol-based approach to produce a drug-releasing multilayer coating adherent to mechanical valves. The hemodynamic performance of coated Open Pivot valves was verified in a heart model tester, and coating durability in the long term was assessed in a durability tester producing accelerated cardiac cycles. Coating antithrombotic activity was evaluated in vitro with human plasma or whole blood under static and flow conditions and in vivo after surgical valve implantation in a pig's thoracic aorta.

RESULTS

We developed an antithrombotic coating consisting of ticagrelor- and minocycline-releasing cross-linked nanogels covalently linked to polyethylene glycol. We demonstrated the hydrodynamic performance, durability, and hemocompatibility of coated valves. The coating did not increase the contact phase activation of coagulation, and it prevented plasma protein adsorption, platelet adhesion, and thrombus formation. Implantation of coated valves in nonanticoagulated pigs for 1 month efficiently reduced valve thrombosis compared with noncoated valves.

CONCLUSION

Our coating efficiently inhibited mechanical valve thrombosis, which might solve the issues of anticoagulant use in patients and the number of revision surgeries due to valve thrombosis despite anticoagulation.

Mechanical behaviors of high-strength fabric composite membrane designed for cardiac valve prosthesis replacement

Bovine pericardium has been commonly used as leaflets in cardiac valve prosthesis replacement for decades because of its good short-term hemocompatibility and hemodynamic performance. However, fatigue, abrasion, permanent deformation, calcification, and many other failure modes have been reported as well. The degradation of the performance will have a serious impact on the function of valve prostheses, posing a risk to the patient's health. This study aimed to introduce a flexible fabric composite with better mechanical performance such that it can be employed as a substitute material for bioprosthetic valve leaflets. This composite has a multilayered thin film structure made of ultrahigh molecular weight polyethylene (UHMWPE) fabric and thermoplastic polyurethane (TPU) membranes. The mechanical properties of three specifications with different design parameters were tested. The tensile strength, shear behavior, tear resistance, and bending stiffness of the composites were characterized and compared to those of bovine pericardium. A constitutive model was also established to describe the composites' mechanical behaviors and predict their strength. According to the results of the tests, the composite could maintain a flexible bending stiffness with high in-plane tensile strength and tear strength. Therefore, bioprosthetic valve made of this substitute material can withstand harsher loads in the blood flow environment than those made of bovine pericardium. Moreover, all these test results and constitutive models can be used in future research to evaluate hemodynamic performance and clinical applications of fabric composite valve prostheses.

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.

3D Printing of Triamcinolone Acetonide in Triblock Copolymers of Styrene–Isobutylene–Styrene as a Slow-Release System

Additive manufacturing has a wide range of applications and has opened up new methods of drug formulation, in turn achieving attention in medicine. We prepared styrene–isobutylene–styrene triblock copolymers (SIBS; Mn = 10 kDa–25 kDa, PDI 1,3–1,6) as a drug carrier for triamcinolone acetonide (TA), further processed by fused deposition modeling to create a solid drug release system displaying improved bioavailability and applicability. Living carbocationic polymerization was used to exert control over block length and polymeric architecture. Thermorheological properties of the SIBS polymer (22.3 kDa, 38 wt % S) were adjusted to the printability of SIBS/TA mixtures (1–5% of TA), generating an effective release system effective for more than 60 days. Continuous drug release and morphological investigations were conducted to probe the influence of the 3D printing process on the drug release, enabling 3D printing as a formulation method for a slow-release system of Triamcinolone.

Elastomer-Hydrogel Systems: From Bio-Inspired Interfaces to Medical Applications

Novel advanced biomaterials have recently gained great attention, especially in minimally invasive surgical techniques. By applying sophisticated design and engineering methods, various elastomer-hydrogel systems (EHS) with outstanding performance have been developed in the last decades. These systems composed of elastomers and hydrogels are very attractive due to their high biocompatibility, injectability, controlled porosity and often antimicrobial properties. Moreover, their elastomeric properties and bioadhesiveness are making them suitable for soft tissue engineering. Herein, we present the advances in the current state-of-the-art design principles and strategies for strong interface formation inspired by nature (bio-inspiration), the diverse properties and applications of elastomer-hydrogel systems in different medical fields, in particular, in tissue engineering. The functionalities of these systems, including adhesive properties, injectability, antimicrobial properties and degradability, applicable to tissue engineering will be discussed in a context of future efforts towards the development of advanced biomaterials.

Biomaterials Based on Carbon Nanotube Nanocomposites of Poly(styrene-b-isobutylene-b-styrene): The Effect of Nanotube Content on the Mechanical Properties, Biocompatibility and Hemocompatibility

Nanocomposites based on poly(styrene-block-isobutylene-block-styrene) (SIBS) and single-walled carbon nanotubes (CNTs) were prepared and characterized in terms of tensile strength as well as bio- and hemocompatibility. It was shown that modification of CNTs using dodecylamine (DDA), featured by a long non-polar alkane chain, provided much better dispersion of nanotubes in SIBS as compared to unmodified CNTs. As a result of such modification, the tensile strength of the nanocomposite based on SIBS with low molecular weight (Mn = 40,000 g mol-1) containing 4% of functionalized CNTs was increased up to 5.51 ± 0.50 MPa in comparison with composites with unmodified CNTs (3.81 ± 0.11 MPa). However, the addition of CNTs had no significant effect on SIBS with high molecular weight (Mn~70,000 g mol-1) with ultimate tensile stress of pure polymer of 11.62 MPa and 14.45 MPa in case of its modification with 1 wt% of CNT-DDA. Enhanced biocompatibility of nanocomposites as compared to neat SIBS has been demonstrated in experiment with EA.hy 926 cells. However, the platelet aggregation observed at high CNT concentrations can cause thrombosis. Therefore, SIBS with higher molecular weight (Mn~70,000 g mol-1) reinforced by 1-2 wt% of CNTs is the most promising material for the development of cardiovascular implants such as heart valve prostheses.

Synthesis of linear and star-shaped telechelic polyisobutylene by cationic polymerization

Hydroxyl-terminated linear and star-shaped telechelic polyisobutylene have been successfully synthesized by living cationic polymerization using propylene oxide (PO)/Titanium tetrachloride (TiCl₄) as the initiator system. A one-step method to prepare the terminal hydroxyl group was realized by selecting the cheap and beautiful epoxide as the functional initiator, which has the prospect of industrial application. The polymerization mechanism was proposed by the end structure analysis and Gaussian calculation results. At the same time, the living linear macromolecular chain was used as the starting point to react with divinyl compounds for synthesis of star-shaped hydroxyl-terminated polyisobutylene. The effects of initiator-crosslinking agent ratio, arm length, and reaction time on the coupling reaction were studied.

Hydroxyl-terminated linear and star-shaped telechelic polyisobutylene have been successfully synthesized by living cationic polymerization using propylene oxide (PO)/Titanium tetrachloride (TiCl₄) as the initiator system.

Quasiliving carbocationic polymerization of isobutylene using FeCl3 as an efficient and water-tolerant Lewis acid: synthesis of well-defined telechelic polyisobutylenes

A cost-efficient approach for the preparation of well-defined low molecular weight difunctional polyisobutulenes possessing an exo-olefin or hydroxyl group was developed in this study.

Block Copolymer Synthesis by the Combination of Living Cationic Polymerization and Other Polymerization Methods

The foremost limitation of block copolymer synthesis is to polymerize two or more different types of monomers with different reactivity profiles using a single polymerization technique. Controlled living polymerization techniques play a vital role in the preparation of wide range of block copolymers, thus are revolutionary techniques for polymer industry. Polymers with good control over molecular weight, molecular weight distribution, chain-end functionality and architectures can be prepared by these processes. In order to improve the existing applications and create new opportunities to design a new block copolymer system with improved physical and chemical properties, the combination of two different polymerization techniques have tremendous scope. Such kinds of macromolecules may be attended by combination of homopolymerization of different monomers by post-modification techniques using a macroinitiator or by using a dual initiator which allows the combination of two mechanistically distinct techniques. This review focuses on recent advances in synthesis of block copolymers by combination of living cationic polymerization with other polymerization techniques and click chemistry.

Extracellular matrix coating improves the biocompatibility of polymeric heart valves

Prosthetic heart valve replacement is an effective therapy for patients with valvular heart disease. New-type polymer materials provide potential choices of material for preparing prosthetic heart valves. In this study, we focused on enhancing the biocompatibility of polystyrene-block-isobutylene-block-styrene (SIBS) by surface modification with an extracellular matrix (ECM). Experimental results demonstrated that the ECM coating increased the adsorption resistance against protein and platelets. SIBS coated with an ECM adsorbed much less bovine serum albumin and fibrinogen (5.38 μg cm-2 and 31.53 μg cm-2, respectively) than the original material (90.84 μg cm-2 and 132.38 μg cm-2, respectively). The relative platelet adsorption of the ECM-modified SIBS was lower than that of SIBS (0.04 versus 0.10). Moreover, the surface coating could also reduce endothelial cytotoxicity, suppress the immune response, and potentially induce tissue regeneration. In conclusion, ECM coating improved the biocompatibility of SIBS effectively.

Biocompatible Nanocomposites Based on Poly(styrene-block-isobutylene-block-styrene) and Carbon Nanotubes for Biomedical Application

In this study, we incorporated carbon nanotubes (CNTs) into poly(styrene-block-isobutylene-block-styrene) (SIBS) to investigate the physical characteristics of the resulting nanocomposite and its cytotoxicity to endothelial cells. CNTs were dispersed in chloroform using sonication following the addition of a SIBS solution at different ratios. The resultant nanocomposite films were analyzed by X-ray microtomography, optical and scanning electron microscopy; tensile strength was examined by uniaxial tension testing; hydrophobicity was evaluated using a sessile drop technique; for cytotoxicity analysis, human umbilical vein endothelial cells were cultured on SIBS-CNTs for 3 days. We observed an uneven distribution of CNTs in the polymer matrix with sporadic bundles of interwoven nanotubes. Increasing the CNT content from 0 wt% to 8 wt% led to an increase in the tensile strength of SIBS films from 4.69 to 16.48 MPa. The engineering normal strain significantly decreased in 1 wt% SIBS-CNT films in comparison with the unmodified samples, whereas a further increase in the CNT content did not significantly affect this parameter. The incorporation of CNT into the SIBS matrix resulted in increased hydrophilicity, whereas no cytotoxicity towards endothelial cells was noted. We suggest that SIBS-CNT may become a promising material for the manufacture of implantable devices, such as cardiovascular patches or cusps of the polymer heart valve.

An Insight into the Structural Diversity and Clinical Applicability of Polyurethanes in Biomedicine

Due to their mechanical properties, ranging from flexible to hard materials, polyurethanes (PUs) have been widely used in many industrial and biomedical applications. PUs' characteristics, along with their biocompatibility, make them successful biomaterials for short and medium-duration applications. The morphology of PUs includes two structural phases: hard and soft segments. Their high mechanical resistance featuresare determined by the hard segment, while the elastomeric behaviour is established by the soft segment. The most important biomedical applications of PUs include antibacterial surfaces and catheters, blood oxygenators, dialysis devices, stents, cardiac valves, vascular prostheses, bioadhesives/surgical dressings/pressure-sensitive adhesives, drug delivery systems, tissue engineering scaffolds and electrospinning, nerve generation, pacemaker lead insulation and coatings for breast implants. The diversity of polyurethane properties, due to the ease of bulk and surface modification, plays a vital role in their applications.

A New Nanocomposite Copolymer Based On Functionalised Graphene Oxide for Development of Heart Valves

Polymeric heart valves seem to be an attractive alternative to mechanical and biological prostheses as they are more durable, due to the superior properties of novel polymers, and have the biocompatibility and hemodynamics comparable to tissue substitutes. This study reports a comprehensive assessment of a nanocomposite based on the functionalised graphene oxide and poly(carbonate-urea)urethane with the trade name "Hastalex" in comparison with GORE-TEX, a commercial polymer routinely used for cardiovascular medical devices. Experimental data have proved that GORE-TEX has a 2.5-fold (longitudinal direction) and 3.5-fold (transverse direction) lower ultimate tensile strength in comparison with Hastalex (p < 0.05). The contact angles of Hastalex surfaces (85.2 ± 1.1°) significantly (p < 0.05) are lower than those of GORE-TEX (127.1 ± 6.8°). The highest number of viable cells Ea.hy 926 is on the Hastalex surface exceeding 7.5-fold when compared with the GORE-TEX surface (p < 0.001). The platelet deformation index for GORE-TEX is 2-fold higher than that of Hastalex polymer (p < 0.05). Calcium content is greater for GORE-TEX (8.4 mg/g) in comparison with Hastalex (0.55 mg/g). The results of this study have proven that Hastalex meets the main standards required for manufacturing artificial heart valves and has superior mechanical, hemocompatibility and calcific resistance properties in comparison with GORE-TEX.