AFM measurement procedure

AFM was performed to measure hydrogel stiffness as previously described³¹. Briefly, indentations were performed using a pyrex-nitride probe with a pyramid tip (spring constant ~0.04 N/m, 35º half-angle opening, NanoAndMore USA Corporation, cat # PNP-TR) connected to a MFP-3D Bio Atomic Force Microscope (Oxford Instruments) mounted on a Ti-U fluorescent inverted microscope (Nikon Instruments). Probes were calibrated using the Igor 6.34A software (WaveMetrics). Samples were then loaded on the AFM, submersed in PBS, and indented at a velocity of 2 μm/s with a trigger force of 2 nN. ~20 force measurements were performed over a 90 μm x 90 μm region per gel. Measurements were made each day for three separate gels per condition. Elastic modulus was calculated based on a Hertz-based fit using a built-in code written in the Igor 6.34A software.