Biopolymer gels with “physical” cross-links: gelation kinetics, aging, heterogeneous dynamics, and macroscopic mechanical properties

Abstract

Alginate is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links, and displaying a consistent restructuring kinetics. We investigate the kinetics of formation and aging of alginate gels by slow permeation of a curing CaCl 2 agent by means of photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample, while retaining at the same time the spatial resolution of imaging techniques. Specifically, the gelling kinetics displays a peculiar non-diffusive behavior, and the subsequent restructuring of the gel structure shares several features in common with the aging of colloidal gels, in particular for what concerns the occurrence of heterogeneous dynamics effects. A comparative analysis of the gel macroscopic mechanical properties at different aging stages further highlights distinctive effects arising from the non-permanent nature of the bonds.

Spatially: resolved heterogeneous dynamics in a strong colloidal gel

“…One of the crucial advantages of this general approach is that it is highly flexible for what concerns sample illumination: any radiation source, like a spectral lamp [17,18], a LED [19], or even a x-ray source [20,21], whose spatial coherence is suitably controlled (for instance, by stopping-down its emitting aperture with a simple diaphragm) works perfectly. Provided that the amount of scattered light is weak compared to the incident one, moreover, the intrinsic heterodyne detection scheme of the technique (field-mixing of the transmitted and scattered radiation) ensures that the imaging formation method is linear space invariant (LSI).…”

Persistent super-diffusive motion of Escherichia coli chromosomal loci

“…In this scenario, active forces cause a build up of elastic stress, later released in intermittent cascades, similarly to off-equilibrium colloidal materials, and known biological gels 32,36 . This would also be consistent with the observed anti-correlation between apparent diffusion and RCMs ( Supplementary Figs 10 and 12), if we speculate that the constraints inhibiting subdiffusive mobility are the same as those that favour stress-releasing rapid motions.…”

Hierarchical cross-linking in physical alginate gels: a rheological and dynamic light scattering investigation

“…A remarkable exception is provided by the work of Secchi et al 24 , who recently applied advanced DLS methods that combine imaging and scattering 25 to study the kinetics of formation and the aging of alginate gels prepared by diffusion of Ca 2+ from an external reservoir brought in contact with the alginate solution via a porous membrane. A slow relaxation mode is observed, on the time scale of thousands of seconds, together with aging effects (slowing down of the dynamics) and intermittent bursts of dynamical activity.…”

“…The physical picture proposed in Ref. 24 is that of a network that can slowly restructure because the cross-links are not permanent, possibly under the action of internal stress accumulated during the gelation process, in analogy to what observed in colloidal 26,27 and actin 28 gels. Unfortunately, the limited temporal resolution of the experiment by Secchi et al did not allow for a full comparison with mechanical tests: while a broad spectrum of relaxation modes were observed for the latter, only one single relaxation mode was found in light scattering.…”