Optimization of Engineered Heart Valve Tissue Extracellular Matrix

ABSTRACTOPTIMIZATION OF ENGINEERED HEART VALVE TISSUE EXTRACELLULAR MATRIXGeorge Carl Engelmayr, Jr., PhDUniversity of Pittsburgh, 2005Prosthetic heart valves, whether biologically-derived or mechanical, have improved the quality of life of millions of patients worldwide since their introduction in the 1960's. However, while currently available prosthetic valves perform sufficiently well in the short term, the side-effects of anticoagulation therapy (mechanical valves) and the structural degeneration of bioprosthetic and allograft valves represent significant drawbacks in the long-term. The limitations of these non-viable devices are especially pronounced in pediatric patients suffering from congenital valvular lesions, as none of the current prosthetic valves have the capacity to grow in tandem with the somatic growth of the patient.Tissue engineered heart valves (TEHV) are conceptually appealing for use in the surgical repair of valvular lesions because they harbor a living cell population potentially capable of orchestrating tissue self-repair, growth, and resistance to infection. Since the mid-1990's, significant progress has been made toward the development of a functional TEHV, culminating in long-term implantation studies in sheep. In these previous studies, TEHV were constructed by seeding vascular-derived smooth muscle and endothelial cells, or bone marrow-derived mesenchymal stem cells (BMSC), onto bioresorbable polymer scaffolds. The resultant TEHV were subsequently cultivated in a pulsatile flow loop bioreactor in which TEHV could be exposed to graduated increases in mechanical stimulation prior to implantation.Importantly, it was demonstrated through these previous studies that mechanical stimulation is critical to the development of a functional TEHV. In the absence of mechanical stimulation, TEHV exhibited significantly reduced extracellular matrix (ECM) formation, and thus upon scaffold degradation retained insufficient structural integrity for acute hemodynamic function. However, because the various mechanical stimuli (e.g., cyclic flexure, tension, and fluid flow) were coupled in the pulsatile flow loop bioreactor, it could not be deduced how the individual modes of mechanical stimulation contributed to the overall TEHV developmental response. Such information is essential, both for developing rationally designed mechanical conditioning regimens, and importantly for potential clinical applications, for quantifying the sensitivity of the tissue formation process to perturbations in these factors.To establish biomechanical end-points for evaluating TEHV, in the current study mathematical models were developed to predict the effective stiffness of TEHV biomaterials from the properties and structure of the individual constituents. It was found that the effective stiffness of the nonwoven polymer scaffolds could accurately be predicted from the spring-like tensile properties and orientations of the scaffold fibers, and that the primary mechanical effect of ECM deposition was an increase in the number of fiber-fiber bond points. Toward quantifying the independent and coupled effects of mechanical stimulation on TEHV development, a novel flex-stretch-flow (FSF) bioreactor was developed in which multiple TEHV specimens could be subjected to well-defined combinations of mechanical stimuli. Finally, the FSF bioreactor was used to elucidate the independent and coupled effects of cyclic flexure and laminar flow on TEHV tissue formation by BMSC. The combination of flexure and flow was found to synergistically accelerate tissue formation and BMSC differentiation, thus paving the way toward rational designs for TEHV conditioning regimens utilizing novel cell sources.