The effect of ECM properties and microstructure confinement on tumor invasion. (a) “Double-sheet” composite structure was used to introduce a gradient rigidity on the uniform surface. The rapid cytoskeleton remodeling of HeLa cells can be seen with elongated shape and many actin cytoplasmic protrusions on the PDMS substrate with different stiffness modulation.88 Reprinted with permission from I. E. Palama et al., Integr. Biol. 4, 228 (2012). Copyright 2012 The Royal Society of Chemistry. (b) Sequential frames showing the displacement of two cells confined in the narrow channel, one cell with amoeboid and the other with mesenchymal morphology.105 Reprinted with permission from D. Irimia and M. Toner, Integr. Biol. 1, 506 (2009). Copyright 2009 The Royal Society of Chemistry. (c) The microchannel with subcellular dimension strongly affects the migration phenotype and the spatial cytoskeletal keratin organization related with the tumor cells invasive potential.106 Reprinted with permission from C. G. Rolli et al., PLoS ONE 5, e8726 (2010). Copyright 2010 Public Library of Science.