Date of Award

2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Bioengineering

First Advisor

Jason A. Burdick

Abstract

In order to effectively incorporate stem cells into tissue engineering solutions, a deeper understanding of the microenvironment factors that influence their behaviors is necessary. Specifically, the inherent mechanics of the extracellular matrix (ECM) have been shown to profoundly effect multiple stem cell behaviors such as their morphology, proliferation, differentiation, and secretion of factors. The effect of matrix mechanics on stem cells has been investigated using a wide range of material systems; however, many of these systems lack the mechanical complexity that native tissues possess in terms of their spatial and temporal properties, as well as context (2D vs. 3D). In order to determine the effect of heterogeneous and dynamic mechanical signals on stem cells, a sequential crosslinking technique was developed that allowed for formation of hydrogels with a wide range in mechanical properties in terms of magnitude, context, and spatiotemporal presentation. Hydrogels with tunable mechanics were synthesized using methacrylate hyaluronic acid (MeHA) in a sequential process: 1) Michael-type `addition' crosslinking using dithiothreitol to consume a fraction of the methacrylate groups, and 2) UV-initiated `radical' crosslinking using controlled UV light exposure in the presence of a photoinitiator to consume unreacted methacrylates.

Using this approach, we demonstrated local control of stem cell morphology, proliferation, and differentiation (adipogenesis and osteogenesis) in both patterned and gradient systems on 2D hydrogels. We further investigated the effects of mechanics in a 3D context using non-porous and porous presentations of controlled mechanics. In the non-porous system, cell behavior was shown to be dependent on mechanics as threshold responses were observed related to the ability of hMSCs to adopt a spread or rounded morphology within the hydrogel. In the 3D macroporous system, mechanics were spatially and temporally modulated and hMSC morphology, proliferation, differentiation, and secretion of angiogenic and cytokine factors were shown to be dependent on the local and temporal presentation of mechanical signals.

This dissertation work emphasizes the importance of the magnitude, context, and presentation of mechanical signals and highlights this sequential crosslinking process as a model system for future investigations into heterogeneous, dynamic microenvironments, as well as a novel platform for developing future tissue engineering strategies.

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