Scaffold stiffness altered the invasion kinetics of breast cancer spheroids. A.) MDA-MB-231 spheroids were cultured in low and high stiffness gelMA/coll hydrogels for 0, 24, 48, or 72 hours, fixed, and then stained with DAPI to visualize cell nuclei. Shown is a compressed z-stack, scale bar=100 μm. B.) Cell invasion in soft gels occurred sooner and the number of invading cells was greater than in stiffer environments. * denotes p<0.05 relative to 2 kPa scaffolds, n=7–14 spheroids per stiffness. C.) The rate of invasion in 2 kPa scaffolds was linear while invasion in 12 kPa gels was delayed and appeared exponential in growth once initiated. * denotes p<0.05 relative to 2 kPa scaffolds, n=7–14 spheroids per stiffness.