  Hydrogels have the ability to undergo large deformations and yet fail like
brittle materials. The development of biocompatible hydrogels with high
strength and toughness is an ongoing challenge in many applications. We
crosslinked bovine gelatin using glutaraldehyde (control) and methylglyoxal
(MGO) and assessed changes in their fracture toughness. Swelling experiments
show ~710% retention of water in MGO hydrogels as compared to ~450% in control
specimens. We used FTIR to identify the presence of chemical groups that may be
involved in the crosslinking of gelatin gels. Scanning electron micrographs of
lyophilized MGO hydrogels show large pores with plate-like intact walls that
help retain water as compared to control specimens. Monotonic compression tests
demonstrate nonlinear stress-strain behaviors for both hydrogel groups. MGO
samples had 96% higher moduli as compared to control hydrogels that had moduli
of 4.77+-0.73 kPa (n=4). A first order Ogden model fit the stress-strain data
well as compared to Mooney-Rivlin and neo-Hookean models. We used cavitation
rheology to quantify the maximum pressure for bubble failure in the hydrogels
using blunt needles with inner radii of 75, 150, 230, and 320 {\mu}m
respectively. Pressures inside the bubbles increased linearly with time and
dropped sharply following a critical value. Bubbles in MGO gels were small and
penny-shaped as compared to large spherical bubbles in control samples. We used
the critical pressures to quantify the fracture energies of the hydrogels. MGO
treatment increased the fracture energy by 187% from 13.09 J/m2 for control
gels. Finally, we discuss the challenges in using the Ogden and Mooney-Rivlin
models to compute the failure energy for gelatin hydrogels.

Biological Physics;Soft Condensed Matter