Influence of nonlinear mechanics of the PIC matrix on the spreading of hASCs.

Hydrogels with low critical stress, cells are able to adhere and pull the PIC fibers, stiffen them and accumulate enough traction force for the observed morphological changes.

A wide variety of cellular processes, including mechanosensing, cell-cell communication, and functional binding sites involve cell anchoring to the ECM fibers and straining them that induces a stiffening response of the matrix. The storage modulus (i.e. the stiffness in the linear viscoelastic regime) of PIC gels falls into the category of soft biomaterials, which means that primarily the difference in critical stress governs the real mechanical properties of the microenvironment of encapsulated cells.
A lower critical stress implies that the material is more sensitive to cell-induced matrix stiffening. For gels that display large stiffening indices, i.e. gels that are more responsive towards contractile  stresses applied by cell, this effect is further enhanced. In these fibrous gels, cells are able to adhere and pull the PIC fibers, stiffen them and accumulate enough traction force for the observed morphological changes. Culturing in gels with a higher σc, on the other hand, leads to the opposite. These materials remain relatively static; i.e. the cells are able to anchor due to the sufficient amount of adhesive peptides present but are unable to generate sufficient traction force to spread. Therefore,
cells tend to maintain the rounded morphology and do not spread out. 

Figure: Influence of nonlinear mechanics of the PIC matrix on the spreading of hASCs. (a) Representative bright field images of hASCs. (b) Representative fluorescence images of hASCs; nuclei are stained with DAPI (in blue), F-actin is stained using Texas Red Phalloidin (in red). (c) Cell outlines of ten representative cells. All images were taken 3 days after cell encapsulation; cell density: 200,000 cells mL–1; scale bars: 70 μm. (d) Quantified circularity of the cells in panel c for gel S1–S6. (e) Averaged circularity as a function of the corresponding critical stress σc of the gels. (f) Quantified cell proliferation using a PicoGreenTM assay, normalized by day 0. Note that the colors in d-f match those in Figure 2b-d. For each sample, 10 cells were analyzed. Statistics: n.s. = not significant (p > 0.05), ** p < 0.01, *** p < 0.001. 

Reference: Synthetic extracellular matrices with nonlinear elasticity regulate cellular organization
Kaizheng Liu, Silvia M. Mihaila, Alan Rowan, Egbert Oosterwijk, and Paul Kouwer
Biomacromolecules DOI: 10.1021/acs.biomac.8b01445 Publication Date: 04 Jan 2019