Our bodies are composed of complex tissues and organs, and each tissue is governed by the careful coordination of cells, solutes, and extracellular matrix components. As such, the tissue engineering field has sought to develop tools to study tissue physiology at the molecular and cellular level. Biomaterials play a critical role in mimicking the extracellular matrix in design and function, acting as the scaffolding from which cells can attach, proliferate, and differentiate to form complex tissues. This dissertation focuses on light-assisted patterning of these materials for investigating cellular interactions within the tissue microenvironment. The stiffness of the extracellular matrix has been implicated in governing cell fate (e.g. proliferation, migration, and differentiation) in vivo, thus we developed digital plasmonic patterning (DPP) -- a laser-based patterning system -- to control stiffness on a two-dimensional (2D) hydrogel substrate in vitro. Cells exhibited durotaxis, or migration to the stiffer patterns, as well as alignment onto the patterns. We built on this research by studying cellular migration in a three-dimensional (3D) collagen hydrogel. We used ultrafast laser-induced degradation (ULID) to spatially pattern channels (void spaces) in the collagen gel. Endothelial cells responded to the void spaces by migrating, aligning, and eventually forming tube-like structures similar to early blood vessel formation. To enable the fabrication of more complex hydrogel structures, we turned to UV light-based 3D printing. First, we printed hydrogels with precise concave architectures and seeded breast cancer cells. Cells aggregated into spheroids over several days and developed hypoxic and necrotic cores by day 10, hallmarks of the tumor microenvironment. These results suggest a new way to study tumor progression. We furthered our study of cancer progression by developing a co-culture 3D printed in vitro model of glioblastoma (GBM) and its blood vessels. Results showed GBM proliferating, invading, and ultimately coopting the vasculature, and moreover demonstrated a similar response to FDA approved drugs as the clinical outcome. In summary, we demonstrated the vast utility of light-assisted biopatterning for understanding cellular interactions in their microenvironment and later applied these methods to develop in vitro models for drug screening

The mechanical properties of the extracellular matrix (ECM) can dictate cell fate in biological systems. In tissue engineering, varying the stiffness of hydrogels-water-swollen polymeric networks that act as ECM substrates-has previously been demonstrated to control cell migration, proliferation, and differentiation. Here, "digital plasmonic patterning" (DPP) is developed to mechanically alter a hydrogel encapsulated with gold nanorods using a near-infrared laser, according to a digital (computer-generated) pattern. DPP can provide orders of magnitude changes in stiffness, and can be tuned by laser intensity and speed of writing. In vitro cellular experiments using A7R5 smooth muscle cells confirm cell migration and alignment according to these patterns, making DPP a useful technique for mechanically patterning hydrogels for various biomedical applications.

Stem cells regulate their fate by binding to, and contracting against, the extracellular matrix. Recently, it has been proposed that in addition to matrix stiffness and ligand type, the degree of coupling of fibrous protein to the surface of the underlying substrate, that is, tethering and matrix porosity, also regulates stem cell differentiation. By modulating substrate porosity without altering stiffness in polyacrylamide gels, we show that varying substrate porosity did not significantly change protein tethering, substrate deformations, or the osteogenic and adipogenic differentiation of human adipose-derived stromal cells and marrow-derived mesenchymal stromal cells. Varying protein-substrate linker density up to 50-fold changed tethering, but did not affect osteogenesis, adipogenesis, surface-protein unfolding or underlying substrate deformations. Differentiation was also unaffected by the absence of protein tethering. Our findings imply that the stiffness of planar matrices regulates stem cell differentiation independently of protein tethering and porosity.