Characterization of early extracellular matrix secretions in a three-dimensional thixotropic cell culture system

The extracellular matrix (ECM) provides both molecular and physical cues that affect cell phenotype and function, and hence both normal and abnormal tissue development. The evolution of ECM mechanical properties with time is thus of great importance for cell culture. We have developed a thixotropic polymer–silica nanocomposite gel that may be used to characterize the ECM secretions of cells cultured in a three-dimensional environment. This thixotropic gel can be characterized by its liquefaction stress (τy), defined as the minimum shear stress required to liquefy the gel. When cells were cultivated within these gels, τy was observed to change in a predictable fashion according to the cell type and quantity of ECM secreted. A general trend was observed for different cell lines, where τy increased with increasing ECM concentration when cells were cultivated in gels of lower stiffness, and decreased with increasing ECM concentration when cells were cultivated in gels of higher stiffness. A particular gel stiffness could be identified where increasing the ECM concentration did not change the τy of the gels. We have defined the τy at this point as τECM, which is characteristic for the ECM produced by a specific cell type cultured in the thixotropic gel.