The LLS is an ensemble of transparent polyacrylamide microgels (Figure 1A). To facilitate cell adhesion and migration, we conjugated type I collagen on the surface of the LLS, namely COL1-LLS. For better visualization of the LLS architecture and presence of collagen on the microgel surface, a fluorescent anti-collagen antibody was used (Figure 1B). While the COL1-LLS supports single cells and tumor spheres in 3D (Figure 1C&D), the interstitial space between the particles provides random paths for T cell migration. We co-culture CAR T cells and tumor spheres in 3D in COL1-LLS to evaluate anti-tumor function via trafficking, expansion, and cytotoxic killing.