The stiffness of the GelMA hydrogels was measured using an atomic force microscope (AFM) (Asylum MFP-3D, Asylum Research, Santa Barbara, CA, USA) in contact force mode using silicon nitride cantilevers with a spring constant of 0.12 N/m (DNP-S10, Bruker, CA, USA). The tip geometry was triangular with a tip radius of 10 nm. Three samples were prepared for each sterilization method and three force maps of 10 μm × 10 μm were collected on each sample. Each force map was fitted to the linear region of the force curve to obtain the Young’s modulus using the Hertz Model and assuming the sample was isotropic and linearly elastic [16 (link)].