  The resistance to fracture of reversible biopolymer hydrogels is an important
control factor of the cutting/slicing and eating characteristics of food gels.
It is also critical for their utilization in tissue engineering, for which
mechanical protection of encapsulated components is needed. Its dependence on
loading rate and, recently, on the density and strength of cross-links has been
investigated. But no attention was paid so far to solvent nor to environment
effects. Here we report a systematic study of crack dynamics in gels of gelatin
in water/glycerol mixtures. We show on this model system that: (i) increasing
solvent viscosity slows down cracks; (ii) soaking with solvent increases
markedly gel fragility; (iii) tuning the viscosity of the (miscible)
environmental liquid affects crack propagation via diffusive invasion of the
crack tip vicinity. The results point toward the fact that fracture occurs by
viscoplastic chain pull-out. This mechanism, as well as the related
phenomenology, should be common to all reversibly cross-linked (physical) gels.
