Polyacrylamide Hydrogels with Rigidity-Independent Surface Chemistry Show Limited Long-Term Maintenance of Pluripotency of Human Induced Pluripotent Stem Cells on Soft Substrates.

In general, cells are cultured and adapted to the in vitro rigidities of plastic or glass ranging between 1 and 10 GPa, which is very far from physiological values that are mostly in the kilopascal range. Stem cells however show a high sensitivity to the rigidity of their culture environment, which impacts their differentiation program. Here, we address the impact of rigidity on the long-term maintenance of pluripotency in human induced pluripotent stem cells (hiPSCs) to determine whether soft substrates could provide a new standard for hiPSC expansion and maintenance. To do this, we set up a fabrication process of polyacrylamide-based culture supports with a rigidity-decoupled surface chemistry. Soft elastic substrates with uniform and reproducible physicochemical properties were designed. The maintenance of pluripotency of two hiPSCs lines on substrates with stiffnesses ranging from 3 to 25 kPa was studied with an identical chemical coating consisting of a truncated recombinant vitronectin with defined surface density. Based on the analysis of cellular adhesion, survival, growth kinetics, three-dimensional distribution, and gene and protein expressions, we demonstrate that below 25 kPa hiPSCs do not maintain pluripotency on long-term culture, while pluripotency and self-renewal capacities are maintained above 25 kPa. In contrast to previous studies, no drift toward a specific germ line lineage was revealed. On soft substrates, cell colonies started to grow in three-dimensional (3D), suggesting that softness allows cells to limit contact with the synthetic matrix and to build their own microenvironment. These observations drastically limit the benefit of using standardized soft substrates to expand hiPSCs, at least with the current culture conditions. The development of a robust technology for the design of soft substrates nevertheless opens up perspectives to fine-tune physicochemical properties of the culture environment in addition to or in replacement of soluble growth factors to finely direct cell fate.