A, Initial elastic modulus of stiff alginate-PEG gels for osteogenesis study. B, Stress relaxation tests for stiff alginate-PEG hydrogels. C, Quantification of relaxation time from stress relaxation tests. D, The expression of markers of osteogenesis including OCN, ALP, RUNX2 and BSP. E, Images of alkaline phosphatase staining (ALP, blue), indicating early osteogenic differentiation, for MSCs cultured in alginate-PEG gels for seven days. F, Quantification of ALP positive cells. Student t-tests; **, ***, and **** indicate p < 0.01, p < 0.001, and p < 0.0001, respectively. Scale bar is 50 µm.