Cell sensing on strain-stiffening substrates is not fully explained by the nonlinear mechanical property

Rudnicki, Mathilda Sophia

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Cell sensing on strain-stiffening substrates is not fully explained by the nonlinear mechanical property

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Cells respond to their mechanical environment by changing shape and size, migrating, or even differentiating to a more specialized cell type. A better understanding of the response of cells to surrounding cues will allow for more targeted and effected designs for biomedical applications, such as disease treatment or cellular therapy. The spreading behavior of both human mesenchymal stem cells (hMSCs) and 3T3 fibroblasts is a function of substrate stiffness, and can be quantified to describe the most visible response to how a cell senses stiffness. The stiffness of the substrate material can be modulated by altering the substrate thickness, and this has been done with the commonly-used linearly elastic gel, polyacrylamide (PA). Though easy to produce and tune, PA gel does not exhibit strain-stiffening behavior, and thus is not as representative of biological tissue as fibrin or collagen gel. Fibroblasts on soft fibrin gel show spreading similar to much stiffer linear gels, indicating a difference in cell stiffness sensing on these two materials. We hypothesize cells can sense further into fibrin and collagen gels than linear materials due to the strain-stiffening material property. The goal of this work is to compare the material response of linear (PA) and strain-stiffening (fibrin, collagen gel) substrates through modulation of effective stiffness of the materials. The two-step approach is to first develop a finite element model to numerically simulate a cell contracting on substrates of different thicknesses, and then to validate the numerical model by quantifying fibroblast spreading on sloped fibrin and collagen gels. The finite element model shows that the effective stiffness of both linear and nonlinear materials sharply increases once the thickness is reduced below 10Ã‚Âµm. Due to the strain-stiffening behavior, the nonlinear material experiences a more drastic increase in effective stiffness at these low thicknesses. Experimentally, the gradual response of cell area of HLF and 3T3 fibroblasts on fibrin and collagen gels is significantly different (p<0.05) from these cell types on PA gel. This gradual increase in area as substrate thickness decreases was not predicted by the finite element model. Therefore, cell spreading response to stiffness is not explained by just the nonlinearity of the material.

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