Statement of Purpose: The perivascular niche (PVN) plays an important role in the progression of glioblastoma (GBM). Comprised of endothelial cells and stromal cells such as pericytes, the PVN has been shown to contribute to the diffuse invasion of GBM into the brain parenchyma, which leads to expansion of the tumor1. GBM invasion has been shown to occur when tumor cells migrate by co-option, which eventually leads to blood vessel failure, regression, and ultimately the development of necrosis surrounding the vessel. GBM cells then use hypoxic signaling to promote the release of angiogenic factors, which induces new blood vessel formation at the periphery of the tumor. Recently, biomaterial platforms have been developed to study the behavior of GBM cells in artificial perivascular environments. We recently developed a hydrogel culture of glioblastoma cells with endothelial cells and stromal cells in order to model the PVN in a 3D biomaterial2. These hydrogels allow for recapitulation of GBM behavior as observed in vivo, but a distinct limitation of the current model lies in the fact that the vascular structures are developing in the presence of the tumor cells. In the native tumor, tumor cells migrate into pre-existing vascularized environments. There is an opportunity, therefore, to construct a 3D biomaterial model that allows introduction of GBM tumor cells into an already-vascularized PVN. Our lab has developed microfluidic mixing tools to create hydrogels containing gradients in cell density. Here, we demonstrate that we can create hydrogels containing opposing gradients of perivascular cells and U87-MG tumor cells to establish a material model of the GBM PVN at the tumor margins. Such a platform can be used to observe cell-cell interactions that occur during tumor cell invasion along vasculature, as well as to obtain gene expression signatures across the tumor margins.