Real-time imaging of cancer cell chemotaxis in paper-based scaffolds.

Cellular migration is the movement of cells, cultured as a monolayer; cellular invasion is similar to migration, but requires the cells to move through a three-dimensional material such as basement membrane extract or a synthetic hydrogel. Migration assays, such as the transwell assay, are widely used to study cellular movement because they are amenable to high-throughput screens with minimal experimental setup. These assays offer limited information about cellular responses to gradients in vivo because they oversimplify the threedimensional (3D) environment of a tissue. There are a number of invasion assays that support 3D cultures, some of which provide experimental control over the spatial and temporal gradients imparted on the culture. These assays, in their current form, are difficult to setup and maintain, and often require specialized laboratory equipment or engineering expertise. Here we describe a paper-based invasion assay in which cellular movement can be monitored in real-time with fluorescence microscopy. These assays are easily prepared and utilize materials commonly found in any laboratory: a single sheet of paper. These sheets are wax patterned to contain channels in which cells suspended in a hydrogel are seeded and cultured. Cell-containing sheets of paper are placed in a custom-built holder that allows gradients to form along the length of the channels. In this work, we compare the invasion of cells cultured in the presence and absence of an oxygen gradient. Our result support previous findings that oxygen is a chemoattractant, and selectively directs cellular movement in a 3D culture environment.