Engineering 3D systems with tunable mechanical properties to mimic the tumor microenvironment

The extracellular matrix (ECM) network provides biophysical cues that regulate many physiologically relevant cellular functions, particularly cell migration. Cells are known to sense and respond to biophysical cues including mechanical and structural properties of the tumor microenvironment (TME). This includes understanding how ECM stiffness in general or when presented in spatial gradients alters cell behavior. The latter is critical given that cell migration can be directed along gradients of stiffness in a process called durotaxis or mechanotaxis. Durotaxis has been implicated in contributing to cell migration that governs cancer invasion. Research probing the influence of ECM stiffness in controlling cancer cell migration has mostly been conducted by tuning the bulk elastic modulus of the 2D substrates. Soft-stiff interfaces have also been used to control mechanical properties since cells have the ability to sense the interfaces as far as hundreds of microns away from them. This approach can be used to both create uniform as well as spatial gradients of stiffnesses. Soft-stiff interfaces are useful in manipulating the effective local stiffness without altering the bulk properties. In addition, they allow non-expert end users to generate gradients of stiffness, because the surface, which is manufactured before introduction of the ECM and cells, controls the stiffness gradients and not some patterning process that occurs co-temporal to the introduction of cells. Much work has been carried out in 2D to study cellular responses to stiffness cues leveraging the interactions at the soft-stiff interfaces. However, 3D in vitro models mimicking TME are becoming a standard for studying cell behavior and how stiff-soft interfaces operate in 3D environments is unknown. Finally, there is opportunity to tune the stiff-soft interfacial interaction through surface chemistry, but the design principles associated with this surface chemistry is currently unknown. In this thesis, I present a novel method to tune the mechanical properties of the 3D microenvironment by leveraging the interactions at the soft-stiff interface. We examine the migration of cells embedded in a collagen gel sandwiched between two stiff substrates (glass). We study how surface attachment of collagen to the stiff substrate and the thickness of the collagen fiber network influence the cell migration speed and cell morphology. The findings of this study suggest that these perturbations result in different cell responses. Cell migration appears to be controlled by the gel thickness, whereas cell morphology is tuned by the surface chemistry at the stiff-soft interface and the gel thickness. Our study also suggest that durotaxis can be elicited by leveraging the thickness of the gels. In the future, we will design 3D structures to elicit durotactic responses using soft-stiff interfaces