Agarose hydrogels consist of connected semi-flexible strands and feature a waterlogged microstructure. Despite intensive use in biotechnology and numerous studies on the elastic properties of agarose hydrogels, little is known about the poro-visco-elastic behavior and the microstructural changes of such fibrillar networks under uniaxial compression. A CRPP team studied the mechanical response of centimeter-sized pre-molded agarose cylinders under uni-axial compression over a wide range of loading speed (0.1 µm/s – 103µm/s) and agarose concentration (0.5 g% – 23 g%). A spatio-temporal analysis of the variations in the volume of the cylinder under compression made it possible to measure without ambiguity the Poisson’s ratio ν in the linear regime. The hydrogel is compressible for strain rates lower than 0.7 %s-1. The response is typical of a deformation mode either dominated by the bending of the semi-flexible strands (enthalpic regime), or by the stretching of the network (entropic regime) at higher agarose concentrations c > 3 g%. The transition to the nonlinear regime occurs for a critical compressive deformation εc of only a few percent as the result of the mechanical buckling of either the semi-flexible strands (enthalpic regime) or the semi-flexible pores (entropic regime). Dynamic shear rheology experiments under low amplitude shear oscillations further show a drop in the elastic shear modulus of the compressed hydrogel and thus confirm the mechanical buckling of the microstructure, regardless of the topology or the connectivity of the network. The buckling of the network induced either by the mechanical compressive deformations or the effects of thermal aging are finally shown to influence in a noticeable way the temporal relaxation of the normal stress. This contribution shows for the first time how much the buckling of semi-flexible networks under low compressive strain influences the poroviscoelastic behavior and the complex non-linear mechanical response of biopolymer hydrogels.

Figure : Uniaxial compression of an agarose gel cylinder in water (a,b) and spatio-temoral representation of the diameter changes D(ε) of the compressed hydrogel (c). Poisson’s coefficient ν (d) and critical compressive strain ε (e) of the hydrogel under compression as a function of the agarose concentration.