Solvent control of crack dynamics in a reversible hydrogel

Abstract

1The resistance to fracture of reversible biopolymer hydrogels is an important control factor of the cutting/slicing and eating characteristics of food gels 1 . It is also critical for their utilization in tissue engineering, for which mechanical protection of encapsulated components is needed 2,3 . Its dependence on loading rate 4 and, recently, on the density and strength of cross-links 3 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.Gelatin gels are constituted of denatured (coil) collagen chains, held together by crosslinks made of segments of three-stranded helices stabilized by hydrogen bonds 5 . This network, swollen by the aqueous solvent, which controls its (undrained) bulk modulus, is responsible for the finite shear modulus µ, of order a few kPa. Hence, hydrogels can be considered incompressible. One estimates average mesh sizes ξ ∼ (kT /µ) 1/3 of order 10 nm, i.e. coil segments involving a few 100 units (residues) 6 . Moreover, in the presence of pressure gradients, the solvent diffuses through the network. This poroelastic behaviour 7,8 controls e.g. slow solvent draining in or out of the gel under applied stresses.They are thermoreversible, i.e., in contrast with chemical, covalently cross-linked gels, their network "melts" close above room temperature. This behavior, assignable to their small cross-link binding energy, leads to the well studied 5 slow aging (strengthening) of µ, and to their noticeable creep under moderate stresses 9 . When stretched at constant strain rate, gelatin gels ultimately fail at a strain ∼ 1 which, though rather poorly reproducible, is clearly rate-dependent 4 . In order to get insight into the nature of the dissipative processes at play, one needs to investigate the propagation of cracks independently from their (stochastic) nucleation 10 . Here we study the fracture energy G(V ) needed to propagate a crack at constant velocity V in notched long thin plates (see Fig. 1) of gels differing by the glycerol content of their aqueous solvent.

Preferential hydration fully controls the renaturation dynamics of collagen in water-glycerol solvents

“…For gelatin gels, this process occurs without any scission of the polymer chains, but via their stress-induced disentanglement and pull-out [16,17]. Fig.2 displays the stress relaxation curves of the two gels, the compositions of which was chosen so that they exhibit the largest viscosity contrast (η 60 /η 0 = 11, see inset of Fig.…”

Magic Angles and Cross-Hatching Instability in Hydrogel Fracture

“…After an initial transient, the crack reaches a steady regime at a velocity V which increases with the "energy release rate" G imposed by the opening ∆. We found that, in this slow, subsonic regime (V < 30 mm/s):with G 0 ≃ 2.5 J.m −2 , Γ ≃ 10 6 , and were able to assign this strong V -dependence to the fact that, in such a reversible gel, fracture occurs via scissionless chain pull-out, the high dissipation being due to viscous drag [10,11]. The results reported here were obtained on gels with a 5 wt.% content of gelatin in the solvent (pure water except when otherwise specified).…”

Fatigue fracture of tough hydrogels

“…The stress-stretch curve of a hydrogel or an elastomer often depends on loading history due to various inelastic processes such as the Mullins effect [37,38], viscoelasticity [39][40][41][42][43], poroelasticity [42][43][44][45][46] and viscoplasticity [47]. In particular, the Mullins effect has been considered as the major factor to explain the softening of double network hydrogels over consecutive loading cycles [48][49][50][51][52][53][54], and has been attributed to progressive damage of the polymer networks [50,55].…”