The mature central nervous system has a very limited capacity for self-renewal and repair following injury. Neural stem cells (NSCs), however, provide a promising new therapeutic option and can be readily expanded in vitro . Towards the development of an effective therapy, greater understanding and control is needed over the mechanisms regulating the differentiation of these cells into function-restoring neurons. In vivo, the neural stem cell niche plays a critical role in directing stem cell self-renewal and differentiation. By understanding and harnessing the power of this niche, a tissue engineered system with encapsulated neural stem cells could be designed to encourage neuronal differentiation and ultimately regeneration of damaged neural tissue. Poly(ethylene glycol)-based hydrogels were used here as a platform for isolating and investigating the response of neural stem cells to various matrix, soluble, and cellular components of the niche. When covalently modified with a cyclic RGD peptide, the synthetic scaffold was demonstrated to support attachment and proliferation of a human NSC line under conditions permissive to cell growth. Under differentiating conditions, the scaffold maintained appropriate lineage potential of the cells by permitting the development of both neuronal and glial populations. Expansion and differentiation of NSCs was also observed in a more biomimetic, three dimensional environment following encapsulation within a degradable hydrogel material. To simulate the soluble signals in the niche, fibroblast growth factor and nerve growth factor were tethered to the hydrogel and shown to direct NSC proliferation and neuronal differentiation respectively. Finally, as an example of the cell-cell interactions in the niche, the pro-angiogenic capacity of encapsulated neural stem cells was evaluated both in vitro and in vivo. Ideally, the optimal scaffold design will be applied to guide NSCs in a therapeutic application. Toward this goal, a novel method was developed for encapsulation of the cells within injectable hydrogel microspheres. This technique was optimized for high cell viability and microsphere yield and was demonstrated with successful microencapsulation and delivery of neural stem cells in rodent model of ischemic stroke.

Hyaluronic acid (HA) is one of the primary chemical building blocks of the extracellular matrix and thus is an attractive material for biomedical applications. FDA approved HA-based materials are available as dermal fillers, joint viscosupplements, vitreous substitutes, and abdominal adhesion barriers. The engineering of new HA-based materials and applications is an active area of research. Here we develop several new types of HA-based hydrogels with unique and useful properties. To address the challenge of delivering hydrophobic drugs from hydrophilic hydrogel matrices we have grafted HA hydrogels with [Beta]-cyclodextrin to create hydrogels capable of binding poorly water soluble drugs. To create HA hydrogels with unique anisotropic swelling behavior we have developed a dual-crosslinking technique in which a super-swelling chemically crosslinked hydrogel is patterned with low-swelling photocrosslinked domains. When this dual-crosslinked hydrogel is swelled it contorts into a new shape because of differential swelling among photopatterned regions. To address the challenge of creating hydrogel scaffolds with biomimetic branched porosity we have invented a "crystal templating" technique. This technique grows dendritic crystals throughout a biopolymer solution, crosslinks the biopolymer around the crystals, and washes the crystals away to yield a hydrogel with a dendritic macroporous network. Lastly, we invented a method for patterning a substrate with a microarray of hydrogel compartments. A microarray of living cells is obtained when cells are seeded on the hydrogel patterned substrate. This method addresses the need for an inexpensive, simple method for obtaining living cell microarrays that does not require clean room labs and lithographic expertise. Each of these new materials were based on hyaluronic acid hydrogels but the methods are generalizable to hydrogels of other polymers too. In conclusion, the novel methods in this dissertation are a significant contribution to the engineering of HA-based materials. / text

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