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Fig. 1. Cell and matrix contraction. (A) Cells were imaged in brightfield mode 3-5 µm above the cell-matrix interface (i) and in fluorescence at the cell-matrix interface to observe particle motion during contraction (see inset pseudo-colored beads) to determine cellular and matrix deformations, respectively, that were then used to generate matrix (ii) and cellular (iii) principal strain maps. Note the difference in matrix strain between soft and stiff gels reflected by decreased deformation of the contracted cell. (B) The maximum cellular strain 
cell is similar to the maximum matrix strain for soft matrices out, resulting in small in values. Hard substrates only allow the cell to deform itself during contraction. The transition between these regimes occurs at E*, where internal and external strain appear equivalent. Curve fits to guide the eye are of the form y=a+bEmxn/(Em+E1/2m) with m=6 and n=0 (matrix), n=1 (cell). Error bars, ± s.d. for at least five cells in duplicate studies. Paired t-tests were used to identify significant differences between in of cells on softer versus stiffer gels (*P<0.05) and between cell strains on different gels. In the latter case, no two cell strains were significantly different. (C) Schematic of strained states for soft gels (E* gels) and stiff gels that respectively yield in<out, in=out, and in>out. (D) The contractile `work' done by cardiomyocytes on the substrate peaks at E*.
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