MMP9 and breast cancer: Notably, breast cancer cells grown on stiffer 2D silicone substrates, such as 1.1 MPa and 14.3 kPa versus 5.2 kPa, demonstrated higher transendothelial migration in a soft 3D hydrogel in vitro, which was concomitant with enhanced MMP9 release, implying that the increased migration in reaction to stiffness was maintained and perhaps facilitated through MMP9 [276].