MDA-231 and HLF tumor spheroid-induced angiogenesis. (a) A schematic illustration of the experimental design. (b) A representative bright-field image of the HUVEC channel and tumor spheroids in the HLF-laden ECM within the microfluidic chip device. (c,d) Confocal images of MDA-231 and HLF tumor spheroid-induced angiogenesis at day 0 (c), day 2 (d), and day 6 (e). (f) An enlarged area of the tumor spheroid-vasculature interface that is highlighted in (e).