Stiffness-tunable hydrogels can be obtained by changing the crosslinking degree of gels, stimulating external conditions, changing the molecular weight of materials, and adding nanomaterials. Based on different experimental designs, hydrogels with different characteristics and different stiffness can be obtained.
You may want to refer to the article attached below and see Table 1 which gives the summary of the hydrogels with different stiffness by different design methods.
VitroGel 3D High Concentration (TheWell Bioscience) comes with VitroGel Dilution Solution to adjust the final hydrogel strength from 10 to 4000 Pa. However, it is better to characterize the actual stiffness of the produced gel yourself using an AFM.