The number of patients requiring replacement heart valves due to valvular heart disease is forecast to rise from 290,000 in 2003 to 850,000 in 2050. Currently, treatment of these patients is dominated by either surgically implanted mechanical or bioprosthetic valves. Unfortunately, none of these solutions are optimal, they simply replace native valvular disease with “prosthetic valvular disease”—mechanical valves require lifelong anticoagulation, and bioprostheses have a limited durability of approximately 15 yr. As such, since the 1950s, researchers have sought solutions which may overcome these drawbacks. Flexible leaflet polymeric prosthetic heart valves (PHVs) have been proposed as a potential solution—able to provide good longterm durability and function without the need for anticoagulation, however, no valve has been clinically successful, and in the past 40 years of development no valve has reached clinical trials [1]. As such, the process by which we design polymeric valves should be seriously considered. The design of polymeric valves may be split into a structural valve design and material selection problem. The use of material selection indices to aid the quantitative design of engineering components was formalized by Ashby et al. [2], and in this report, we propose the use of a novel material performance index (PI) to evaluate potential polymer materials for PHVs. We identify the leaflets as the critical component for the valves, and this report is focused on material selection for leaflets. We limit our search of the material “kingdom” to elastomeric materials whose mechanical properties are similar to the biological tissues. This allows us to produce quasi-physiological flow. We proceed by producing a shortlist of materials based upon hemocompatibility (minimum of inflammation and thrombogenicity) and biostability (resistance to oxidation and hydrolysis). A number of elastomers and coatings fulfill these criteria. In the realm of PHV, a number of “biocompatible” and “biostable” polymers have been proposed: polyhedral oligomeric silsesquioxane polycarbonate urethane, polystyrene (PS)-block-polyisobutyleneb-PS (and cross linked), Elasteon, cellulose, PS-b-polyethylenepropylene-b-PS, and polyethylene-hydraluron. Within a group such as this we seek to determine the most suitable polymer to fulfill the PHV role. Durability and calcification are the two failure routes which have plagued all flexible leaflet polymeric valves tested in vitro or in vivo. Durability is a function of material and valve design. Although various leaflet forms and dimensions can result in reduced leaflet stresses, the first order determinant of the stresses induced in a leaflet when loaded during diastole is a function of the leaflet’s thickness; thicker leaflets have lower stresses. Concurrently, we must be mindful of the hemodynamics of the valve. In general, the reduction in leaflet thickness leads to a lower mean pressure gradient [3]. Given these conflicting criteria regarding leaflet thickness, we must seek a compromise, leading to the use of a material PI [2]. We must define a relationship for hemodynamics and durability using leaflet thickness and appropriate material parameters. For a trileaflet valve to successfully function the curvature of each leaflet must be reversed, in theory, TPGmax / Et where TPGmax is the maximum transvalvular pressure gradient, E is the Young’s Modulus (or flexural modulus), and t is the leaflet thickness. Using this relation for hydrodynamics, Haworth [4] used tear strength as the parameter for prediction of lifetime. The use of tear strength failed in part because it describes failure at a stress state several magnitudes greater than those found in physiological scenarios. Parfeev et al. [5] improved upon this by comparing polymers based upon wear limits after 10 cycles at 22 Hz. We performed frequency sweep dynamic mechanical analysis of candidate polymers showing that they undergo a transition at 15–20 Hz, making testing at such a high frequency unacceptable for predicting low frequency behavior. Since the 1980s, fatigue testing and prediction of rubber component failure has received significant research in other fields. In particular, the use of crack growth models to predict long term behavior has been successfully employed in the cyclic straining of various rubber components [6]. We now propose a novel material PI based on cyclic fatigue theory, for the improved design of polymeric PHVs.
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Jan 5, 2024
Hong Truong Luu + 4
Open Access