Fabrication of Biocompatible Microstructures to Support Tissue Regeneration
Cardiac fibrosis is considered to be an independent risk factor in the outcome of congestive heart failure. Heart transplantation is the only treatment for patients who are at the end stage of this condition. Shortage of donor organs has created a need for therapeutic alternatives. This dissertation investigates new strategies for cardiac repair systems that could reduce pathological fibrosis and promote growth of myocytes at the site of injury. Design of successful engineered therapies for tissue regeneration relies on discerning how cell behavior can be modulated by chemical and physical cues. Recent studies have shown that external physical cues such as stiffness and geometry can affect cell morphology and function. In this work, the combinatorial effect of stiffness and micro-scale topographical cues on proliferation and gene expression is investigated in 2D and 3D. The 2D system consists of fibroblasts grown on "micropegged" polydimethylsiloxane (PDMS) substrates of different stiffness. The 3D system consists of fibroblasts encapsulated with poly(ethylene glycol) dimethacrylate (PEGDMA) "microrods" of different stiffness in matrigel to create a 3D culture with micro-scale cues of defined mechanical properties in the physiological range.
Fibroblasts cultured on micropegged substrates have reduced collagen expression compared to fibroblasts cultured on flat substrates. Cells on stiffer micropegged substrates exhibit down regulation of important regulators of ECM synthesis but there is no down-regulation of these markers when cells are cultured on the softer micropegged substrates. Similarly, three-dimensional cultures with stiffer microrods show reduced fibroblast proliferation and down-regulation of collagen and other important regulators of ECM synthesis, but three dimensional cultures with soft microrods do not show significant difference on fibroblast proliferation and expression of some ECM regulators compared to cultures with no microrods.
To determine whether microrods can influence myocardial repair and remodeling in vivo, adult female Sprague-Dawley rats undergo left anterior descending (LAD) artery occlusion for thirty minutes followed by reperfusion. Microrods suspended in PBS are injected into the left ventricle (LV) two days after myocardial infarction (MI). Five weeks after the injections echocardiography reveals an increase in ejection fraction (EF) in the microrod group compared to the PBS control. Histology analysis shows a trend on decreased amount of scar formation and increased wall thickness of the microrod treated group compared to the PBS control.
Furthermore, it is shown that microrods can be utilized as growth factor delivery devices. Mechano growth factor (MGF) is expressed rapidly after tissue damage and prevents apoptosis in the myocardium; thus is considered a suitable therapeutic candidate for cardiac repair. Here, it is shown that microrods can be loaded with MGF and that the peptide can be eluted from the microrods for a period of days. Bioactivity of MGF is confirmed by analyzing stem cell migration and myoblast proliferation. Interestingly, it is also observed that myoblast's interaction with microrods can enhance proliferation even after seven days of culture. These studies indicate that microrods protect rapid degradation of MGF, promote stem cell migration and enhance myoblast proliferation.
Results from this work will help create more tunable in vitro tissue engineered platforms and develop new therapeutic approaches for in vivo tissue regeneration.