Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Kristi S. Anseth

Second Advisor

Christopher N. Bowman

Third Advisor

Xuedong Liu

Fourth Advisor

Chien-Chi Lin

Fifth Advisor

Joel Kaar

Abstract

Poly(ethylene glycol) (PEG) based biomaterials offer a number of advantages for applications in biomedical technology, including drug delivery and tissue engineering. PEG is notable for both its hydrophilicity and bioinert properties, and is widely used to create hydrogels used for 3D cell culture. There is growing interest in strategies to introduce biological functionality into PEG-based materials, in order to develop platforms to study the complex biochemical and biomechanical cues that govern cell behavior in physiologically relevant context. This thesis explores photopolymerization conditions for hydrogel synthesis that maintain a high degree of bioactivity for proteins in-situ. Such reactions are then utilized to create hydrogels capable of directing cell fate and behavior.

Photopolymerization conditions for the formation of hydrogels were characterized, as these reactions are widely used for protein encapsulation. First, the role of affinity peptides was studied during photoencapsulation of the cytokine transforming growth factor β (TGFβ). When peptides with affinity for TGFβ were included in monomer solutions, they increased the amount of soluble, bioactive protein released from PEG diacrylate hydrogels formed via a chain-growth polymerization. We then studied protein protection during photopolymerization of step-growth networks using a thiol-norbornene reaction. Thiol-ene reactions were shown to be milder than acrylate chain-growth, as they fully maintained the bioactivity of TGFβ and lysozyme. Protein deactivation was correlated to total photoinitiated radical concentrations in order to more fully characterize reaction conditions that maximized protein protection.

We then applied this knowledge to create biomaterials that incorporated covalently linked, bioactive proteins capable of directing complex cellular functions. TGFβ was thiolated via reaction with 2-iminothiolane, a modification that had no impact on bioactivity. This thiolfunctionalized protein could then be readily linked into hydrogels using either thiol-acrylate or thiol-ene polymerizations. When human mesenchymal stem cells were encapsulated into tethered TGFβ hydrogels, the growth factor induced chondrogenic differentiation at levels similar to or exceeding that of soluble delivery. Further, tethered TGFβ hydrogels were used to culture valvular interstitial cells in order to study the combined roles of substrate elasticity and immobilized TGFβ play in activation of myofibroblast phenotype. The photopolymerization conditions and resulting functionalized hydrogels developed in this thesis demonstrate the use of PEG hydrogels to investigate complex cell-material interactions of interest in tissue engineering applications.

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