A tunable hydrogel system and pulsatile flow bioreactor for the development of tissue engineered vascular grafts

A Tunable Hydrogel System and Pulsatile Flow Bioreactor for the Development of Tissue Engineered Vascular Grafts by Melissa K. McHale The prevalence of coronary artery disease combined with a paucity of suitable vessel substitutes act as driving forces for cardiovascular tissue engineering research. In this thesis poly(ethylene glycol) diacrylate (PEGDA) hydrogels were investigated as a biomaterial for tissue engineered vascular grafts (TEVG). The global objectives for the work were two-fold. First, a thorough characterization of the hydrogels was warranted to determine material properties and cell interaction characteristics. The second objective was to develop a pulsatile flow culture system for TEVG that was capable of achieving physiologically relevant fluid flow parameters. Bulk properties of PEGDA hydrogels formed from a range of polymer molecular weights and solution concentrations were characterized. Resultant materials demonstrate tunable stiffness and strength, and network properties that are appropriate for supporting viability of encapsulated cells. Human coronary artery smooth muscle cells seeded on top of these PEGDA hydrogels exhibit changes in attachment, proliferation, and morphology that can be directly correlated to the rigidity of the substrate material. In general, stiffer materials encourage greater attachment, a higher rate of proliferation, and the development of a mature, spread morphology. Finally, these responses were shown to be independently modulated by changing either the hydrogel material properties or the peptide directed bio-adhesiveness of the substrate. The tissue bioreactor presented in this work is capable of imparting physiological fluid flow (120 mL/min), shear (5-10 dynes/cm ), pressure waveforms (120/80 mmHg), and pulse rates (60 or 120 bpm). Cell-laden hydrogel constructs cultured for up to 8 wk in this system responded to mechanical stimulation with increases in cell and extracellular matrix (ECM) content and positive modulations to material properties. Though the magnitude of ECM accumulation is quite low, changes in hydrogel stiffness and the presence of degrading enzymes indicate that the encapsulated vascular cells are working towards a more biologically appropriate surrounding. Since all parameters for appropriate TEVG culture are not yet understood, this device will serve as an important tool in the development of a small diameter vessel substitute. ACKNOWLEDGMENTS My sincere gratitude is extended to the members of my thesis committee. Dr. Jennifer West, Dr. Jane Grande-Allen, and Dr. Mary Ellen Lane serve as fine examples of excellence in science and education. I thank you for your advice and mentorship as I worked to complete this thesis. My research has been supported by members of the West Lab past and present, especially Mariah Hahn who worked with me in setting up the bioreactor, and April Smith, Logan Hsu, and Alicia Allen who helped keep all the experiments pumping along. I'm grateful to James Moon and Jordan Miller for their infinite technical discussions, and Marcella Estrella for assistance with everything from pumps to parties. Thanks to everyone who helped when I asked, especially in this last semester when I needed it most. I am fortunate to have wonderful friends like, Kim, who taught me everything I needed to know to get started in research and Dana, who traveled this journey with me all the way from ABE. We're finally done. I could not have reached this point without the loving support of my family. Thanks, Mom. Most importantly, thank you to my wonderful husband Martin for his encouragement and patience; and for bringing Lily to the lab for picnics when I couldn't make it home. I am forever indebted. This thesis was financially supported by a Whitaker Foundation Graduate Fellowship and by grants from the NIH. TABLE OF CONTENTS ABSTRACT ii ACKNOWLEDGMENTS . i TABLE OF CONTENTS v LIST OF TABLES . v LIST OF FIGURES ix LIST OF EQUATIONS v