Institut des
NanoSciences de Paris

Multiscale mechanics of soft solids

Hydrogels : From slow structural relaxations to ultimate mechanical properties

Tristan Baumberger, Olivier Ronsin, Christiane Caroli, Lise Picaut

We study the mechanical properties of (bio)polymer hydrogels with a physicist’s point of view, that is we consider them as versatile « model systems » :

• As ultra-soft materials, they are suitable for studying high deformability and non-linear elasticity effects on fracture.

• As amorphous disordered elastic networks, they are suitable for studying the role of structural features of the force-transmitting backbone on slow relaxation processes.

• As biocompatible materials with mechanical properties close to those of extracellular matrices and living tissues they can be seen as substrates for drug delivery or tissue engineering applications.

Polymer hydrogels consist of highly hydrated (up to 99 wt% solvent content) networks of polymer chains, crosslinked via covalent (chemical gels) or weak (physical gels) bonds. The archetype of physical gels is gelatin, obtained from the denaturation of the protein collagen followed by its partial, thermoreversible renaturation. Alginate, extracted from see weeds, gels upon addition of specific ions. More generally, biopolymers exhibit a wealth of non-covalent interchain association mechanisms. Biopolymers can also be crosslinked via the formation of interchain covalent bonds. Our favorite chemical gel is gelatin crosslinked via a reaction catalyzed by an enzyme (transglutaminase). Its activity can be conveniently inhibited by heating, providing a straightforward control of the degree of crosslinking.

  • Ultimate properties of hydrogels

- We study the dynamics of crack propagation in soft gels with special interest in the crack front instabilities. The underlying issue is to identify the specific characteristics of fracture of tough-AND-soft materials.

Crack front échelon instability in mixed mode fracture of a strongly nonlinear elastic solid, Europhysics Letters 105 340001 (2014)

Nucleation and Propagation of Quasi-Static Interfacial Slip Pulses,The Journal of Adhesion, 87 (2011) 504 — 529

From thermally activated to viscosity controlled fracture of biopolymer hydrogels, J. Chem. Phys. 130, 061102 (2009)

Magic Angles and Cross-Hatching Instability in Hydrogel Fracture,Phys. Rev. Lett. 100, 178303 (2008)

- We investigate the role of the near-tip environment on the crack dynamics in hydrogels. We are able to speed-up or down the crack growth or trigger instabilities in a controlled way by wetting the tip with a drop of the gel solvent modified by addition of either salt, nanoparticles or an aditional solute. Our aim is to gain insight into the structure of the process zone and the underlying dissipative mechanisms.

Ionic Control of Crack Propagation in Biopolymer Hydrogels, Proc. IUTAM 12 3 (2015)

Cooperative Effect of Stress and Ion Displacement on the Dynamics of Cross-Link Unzipping and Rupture of Alginate Gels, Biomacromolecules 11 1 (2010) 1571–1578

A convective instability mechanism for quasistatic crack branching in a hydrogel, Eur. Phys. J. E 31 1 (2010) 51-58

- Stress singularities also play an important part in polymer extrusion. The no-slip/no-stress transition at the exit of a die through which a liquid is forced to flow generates singular stress and strain rate fields. Although it is agreed to be responsible for the sharkskin extrusion instabilitiy, the extension of the region of quasi-singular elongational stress (the anologous of the process zone at a fracture tip) is poorly known. We revisit this question using gel-forming visco-elastic polymer solutions (alginate or collagen) extruded into a gelling bath.

Ongoing PhD thesis of Lise Picaut

  • Slow relaxations in elastic networks

- Physical gels exhibit mechanical aging and slow stress relaxation. Indeed, alike glassy materials, they are out-of-equilibrium, metastable systems : for instance, gelatin renaturation into the triple-helix collagen is frustrated by the topological constrains induced by gelation. The thermorevesibilty of its physical crosslinks enables the system to slowly relax down its glasslike complex energy landscape.

Interplay between Shear Loading and Structural Aging in a Physical Gelatin Gel, Phys. Rev. Lett. 103, 138302 (2009)

- We investigate this dynamics in physically crosslinked, as well as in hybrid gelatin gels containing a controlled fraction of covalent bonds. This gives access to the effect of decimation of rearrangement paths in the landscape on the relaxation dynamics.

Two-step build-up of a thermoreversible polymer network : From early local to late collective dynamics, Phys. Rev. E 91, 042305 (2015)

- Purely covalent gelatin gels exhibit, above the renaturation temperature, a stress-induced coil to helix transition. We study the resulting stress relaxation dynamics in order to shed light on the effect of network-mediated long-range couplings between helix growth events on individual polymer strands.

Glass-like dynamics of the strain-induced coil/helix transition on a permanent polymer network, to appear in J. Chem. Phys. (2016)

  • Hydrogels as biomaterials

- We study the mechanical properties of biological (tendons) and bioinspired (collagen gels) systems based on self-assembled biopolymers.

Transcription factor EGR1 directs tendon differentiation and promotes tendon repair, J. Clin. Invest. 123 3564 (2013) highlighted in Nature Review of Rheumathology