Star poly(ethylene glycol) as a tunable scaffold for neural tissue engineering
Creators: name: Zustiak, Silviya Petrova
Publisher: name: Maryland Shared Open Access Repository
Year: 2009
Subjects: degradable
diffusion
neuron
PEG
scaffold
tissue engineering

description: The primary focus of this work was to develop a novel synthetic hydrogel scaffold as an in vitro model to enable future detailed studies of how neurons grow in environments with controllable diffusion profiles of soluble cues and tunable neuron-matrix interactions. The development of in vitro models that enable elucidation of the mechanisms of system performance is a recently emerging goal of tissue engineering. The design of three-dimensional (3D) scaffolds in particular, is motivated by the need to develop model systems that better mimic native tissue as compared to conventional two-dimensional (2D) cell culture substrates. An ideal scaffold is degradable, porous, biocompatible, with mechanical properties to match those of the tissues of interest and with a suitable surface chemistry for cell attachment, proliferation, and differentiation.  Although naturally derived materials are more versatile in providing complex biological cues, synthetic polymers are preferable for the design of in vitro models as they provide wider range of properties, controllable degradation rates, and easier processing. Most importantly, their mechanical properties can be decoupled from their biological properties, a crucial issue in interpreting cell responses. The synthetic material provides the structural backbone of the scaffold while biochemical function is added via incorporation of ligands or proteins aimed at triggering specific cell behaviors.  As presented in this dissertation, we have developed and characterized a new synthetic 3D hydrogel scaffold from cross-linked poly(ethylene glycol) (PEG). PEG was selected because it is hydrophilic, non-toxic, biocompatible, and inert to protein adhesion. The chosen cross-linking chemistry was a highly specific reaction that occurred under physiological conditions so that cells could be embedded within the gel prior to cross-linking. Controllable degradability was imparted via series of hydrolytically degradable PEG cross-linkers. Thorough analysis demonstrated the independent tuning of the mechanical, biochemical and biological properties of the developed hydrogel.  Because soluble cues such as neurotrophic factors are an effective means for promoting nerve regeneration, the diffusion of biomolecules through the PEG hydrogel were also explored in depth via two methods: fluorescence correlation spectroscopy (FCS) and bulk diffusion experiments. This is the first demonstration of FCS to delineate protein diffusivity within a cross-linked synthetic hydrogel and describe local and dynamic protein-polymer interactions that occur within these systems. Further, since PEG is inert, short ligands such as RGD were used to promote cell adhesion and new insights into how these ligands impact hydrogel mechanical and transport properties were established. Finally, to test the utility of the developed material as an in vitro model, neuronal cell-matrix interactions were studied by tuning hydrogel properties and assessing cell viability and neurite outgrowth.  We believe that this work is major step in building an in vitro model for gaining an understanding of the key parameters that guide nerve regeneration and have the potential to lead to the development of better strategies to treat peripheral nerve injuries.