Microfluidic generation of biomaterial gradients for control of neurite outgrowth

Enhancing and directing axon regeneration through an injury site represents a prime goal for therapies following nerve or spinal cord injury. The objective of this thesis is to develop a system to create durotactic and haptotactic gradients with a biomaterial scaffold for control of neural cell behavior. Gradients of mechanical properties and adhesion are generated in a 3D collagen gel using microfluidics. To spatially control the mechanical properties, gradients of genipin - a naturally occurring, cell-tolerated crosslinking agent - are created in 3D through a fibrillar collagen gel using a simple source-sink network. Durotactic gradients of mechanical properties are evaluated by measuring genipin-induced fluorescence, which we demonstrated can be correlated to the rheological properties of the collagen gel. Neurite growth from chick dorsal root ganglia was assayed in gradients and appropriate controls. Neurite growth was biased down a gradient of stiffness of ~60Pa/mm towards a more compliant region, and neurites were significantly longer in this direction than up the gradient and than in uniform conditions without crosslinking. Haptotactic gradients are generated by first grafting laminin derived peptides, IKVAV and YIGSR, onto the collagen backbone using 1-ethyl-3-(3-Dimethylaminopropyl) carbodiimide (EDC). Peptide-grafted collagen solutions are mixed with untreated collagen in the network Neurite outgrowth was enhanced in all laminin peptide-grafted collagen gradient conditions compared to the untreated collagen controls. Neurite growth was more responsive to gradients of YIGSR than IKVAV. Enhanced growth was observed in all YIGSR-grafted conditions with the greatest bias in the steepest gradient of (24.7 ??g/ml/mm). Enhanced growth was also observed in IKVAV-grafted collagen conditions, but stunted growth occurred in collagen with uniform IKVAV presentation, implying IKVAV may be too 'sticky' for neurite outgrowth at the highest concentration studied. Since both peptides are present on the whole-length laminin molecule, the results suggest that gradients of laminin would be sub-optimal because of the contradiction in the response to the two peptides. These results demonstrate that neurite growth can be enhanced and directed by durotactic and haptotactic gradients, and suggest that including a combination of these gradients in regenerative therapies may accelerate nerve and spinal cord regeneration.Ph. D.Includes bibliographical references (p. 115-122).by Harini Sundararaghava