This study investigates the impact of tunable stress relaxation in hydrogels on the behavior of mesenchymal stem cells (MSCs) in 3D culture. The researchers developed a method to control the stress relaxation rate of alginate hydrogels, independent of their initial elastic modulus and degradation properties. They found that faster stress relaxation enhanced cell spreading, proliferation, and osteogenic differentiation of MSCs. Notably, MSCs cultured in rapidly relaxing hydrogels with an initial elastic modulus of 17 kPa formed a mineralized, collagen-1-rich matrix similar to bone. The effects of stress relaxation were mediated through integrin-based adhesion, actomyosin contractility, and mechanical clustering of adhesion ligands. These findings highlight the importance of stress relaxation as a key characteristic of cell-ECM interactions and suggest its potential as a design parameter for biomaterials in cell culture and tissue engineering.This study investigates the impact of tunable stress relaxation in hydrogels on the behavior of mesenchymal stem cells (MSCs) in 3D culture. The researchers developed a method to control the stress relaxation rate of alginate hydrogels, independent of their initial elastic modulus and degradation properties. They found that faster stress relaxation enhanced cell spreading, proliferation, and osteogenic differentiation of MSCs. Notably, MSCs cultured in rapidly relaxing hydrogels with an initial elastic modulus of 17 kPa formed a mineralized, collagen-1-rich matrix similar to bone. The effects of stress relaxation were mediated through integrin-based adhesion, actomyosin contractility, and mechanical clustering of adhesion ligands. These findings highlight the importance of stress relaxation as a key characteristic of cell-ECM interactions and suggest its potential as a design parameter for biomaterials in cell culture and tissue engineering.