Well-Defined Biomimetic Extra Cellular Matrix Models
Substrate elasticity can guide the differentiation of adult human stem cells as reported recently by Engler et al. in Cell . These striking results were obtained by cell culture of human mesenchymal stem cells (MSCs) on collagen coated cross-linked poly(acrylamide) (PA) substrates with varying stiffness. Firstly introduced by Pelham and Wang in 1997 , these synthetic polymer matrices are a great experimental tool to study the impact of micro mechanical environment on cells.
Despite their convincing mechanical properties these PA gels lack biocompatibility and are not biodegradable preventing any potential in vivo usage. This inspired this project to establish novel biocompatible and biodegradable materials that can be used to mimic the ECM and that enable in vivo applications. As starting point natural constituents of the human ECM such as collagen and glycosaminoglycans (GAG) were used to fabricate mechanically tunable and biocompatible substrates.
A tunable and stable hydrogel has been created by chemically modifying hyaluronic acid (HA) and covalently cross-linking. The effective Young’s modulus E as measured by AFM can be finely tuned in the range of 0.1 kPa up to 150 kPa by varying cross-linker and HA concentration. Figure 1 shows the HA concentration dependence of the elasticity E. The data fits well to a scaling law used to describe cross-linked polymer networks (red solid line).
To compare the novel HA hydrogels with the conventional PA system MSCs were spread on collagen coated substrates exhibiting different stiffness (1, 11, and 34 kPa). As shown in Figure 2 both systems yield similar morphologies on the respective elasticities demonstrating the applicability of the HA substrates.
 Engler, A.J. et al., Cell, 2006. 126: p. 677-689.
 Pelham, R.J. and Y.L. Wang, PNAS, 1997. 94(25): p. 13661-13665.
 Discher, D.E at al., Science, 2005. 310(5751): p. 1139-1143