Quantifying Hydrogel Elastic Modulus via AFM

The Young’s elastic modulus E of 3D collagen-I gels and 2D PAA gels was assessed with a home-made stand-alone AFM adapted to an inverted optical microscope as described in detail elsewhere [27 (link),36 (link)]. In brief, AFM nanoindentation measurements were performed with low spring constant cantilevers with pyramidal tips (nominal k = 0.03 N/m) (Microlever, Veeco, Santa Barbara, CA, USA), which were calibrated using the thermal noise method [36 (link)]. For each gel location, three force-displacement curves (F-z curves) were acquired at a moderate loading force (~1 nN); E of each F-z curve was computed by least-squares fitting of a contact elastic model, and averaged over the three F-z curves [27 (link)]. The same protocol was applied on at least 9 random gel locations to elicit the final E estimate of the hydrogel.