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Cells in vivo generate mechanical traction on the surrounding 3D extracellular matrix (ECM) and neighboring cells. Such traction and biochemical cues may remodel the matrix, e.g., increase stiffness, which, in turn, influences cell functions and forces. This dynamic reciprocity mediates development and tumorigenesis. Currently, there is no method available to directly quantify single-cell forces and matrix remodeling in 3D. Here, we introduce a method to fulfill this long-standing need. We developed a high-resolution microfabricated sensor that hosts a 3D cell-ECM tissue formed by self-assembly. This sensor measures cell forces and tissue stiffness and can apply mechanical stimulation to the tissue. We measured single and multicellular force dynamics of fibroblasts (3T3), human colon (FET) and lung (A549) cancer cells, and cancer-associated fibroblasts (CAF05) with 1-nN resolution. Single cells show notable force fluctuations in 3D. FET/CAF coculture system, mimicking cancer tumor microenvironment, increased tissue stiffness by three times within 24 hours.

Fluorescent microscopy employs monochromatic light which can affect the cells being observed. We reported earlier that fibroblasts relax their contractile force in response to green light of typical intensity. Here we show that such effects are independent of extracellular matrix and type of cell. In addition, we establish a threshold light that invokes minimal effect on cells. We cultured fibroblasts on soft 2D elastic hydrogels embedded with fluorescent beads to trace substrate deformation. The beads move towards cell center when cells contract, but they move away when cells relax. We use relaxation/contraction ratio, λ r , as a measure of cell response to light. The cells were exposed to green (wavelength, λ = 545-580 nm) and red (λ = 635-650 nm) light with a range of intensities. We find red light with intensity less than ~57 W/m 2 results in λ r = 1, i.e., cells maintain force homeostasis. Higher intensities and smaller wavelengths result in widespread force-relaxation in cells with λ r > 1. We suggest the use of λ > 650 nm light with low intensity (I ≤ 57 W/m 2 ) for time-lapse imaging of cells and tissues in order to avoid lightinduced artifacts in experimental observations.

Cells in vivo generate mechanical forces (traction) on surrounding 3D extra cellular matrix (ECM) and cells. Such traction and biochemical cues may remodel the matrix, e.g. increase stiffness, which in turn influences cell functions and forces. This dynamic reciprocity mediates development and tumorigenesis. Currently, there is no method available to directly quantify single cell traction and matrix remodeling in 3D. Here, we introduce a method to fulfil this long-standing need. We developed a high-resolution microfabricated sensor which hosts a 3D cell-ECM tissue formed by self-assembly. It measures cell forces and tissue-stiffness and can apply mechanical stimulation to the tissue. We measured single and multicellular force dynamics of fibroblasts (3T3), human colon (FET) and lung (A549) cancer cells and cancer associated fibroblasts (CAF05) with 1 nN resolution. Single cells show significant force fluctuations in 3D. FET/CAF co-culture system, mimicking cancer tumor microenvironment, increased tissue stiffness by 3 times within 24 hours.

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