Evaluating Poly(Acrylamide‐co‐Acrylic Acid) Hydrogels Stress Relaxation to Direct the Osteogenic Differentiation of Mesenchymal Stem Cells

The aim of this study is to investigate polyacrylamide‐based hydrogels stress relaxation and the subsequent impact on the osteogenic differentiation of human mesenchymal stem cells (hMSCs). Different hydrogels are synthesized by varying the amount of cross‐linker and the ratio between the monomers (acrylamide and acrylic acid), and characterized by compression tests. It has been found that hydrogels containing 18% of acrylic acid exhibit an average relaxation of 70%, while pure polyacrylamide gels show an average relaxation of 15%. Subsequently, hMSCs are cultured on two different hydrogels functionalized with a mimetic peptide of the bone morphogenetic protein‐2 to enable cell adhesion and favor their osteogenic differentiation. Phalloidin staining shows that for a constant stiffness of 55 kPa, a hydrogel with a low relaxation (15%) leads to star‐shaped cells, which is typical of osteocytes, while a hydrogel with a high relaxation (70%) presents cells with a polygonal shape characteristic of osteoblasts. Immunofluorescence labeling of E11, strongly expressed in early osteocytes, also shows a dramatically higher expression for cells cultured on the hydrogel with low relaxation (15%). These results clearly demonstrate that, by fine‐tuning hydrogels stress relaxation, hMSCs differentiation can be directed toward osteoblasts, and even osteocytes, which is particularly rare in vitro. Poly(acrylamide‐co‐acrylic acid) hydrogels are synthesized with controlled stiffness, ranging between 5 and 145 kPa. Hydrogels containing 0% or 18% of acrylic acid exhibit an average relaxation of 15% and 70%, respectively. Hydrogels with the same stiffness (55 kPa) and different stress relaxation (70% and 15%) induce mesenchymal stem cells differentiation into osteoblasts and osteocytes, respectively.