The gelation of alginate is driven by an ion-exchange mechanism in which divalent cations, such as calcium ions (Ca 2+ ), replace the sodium ions (Na + ) and crosslink the negatively charged carboxylate groups on the alginate’s backbone. While crosslinked alginate is often assumed to be uniform, experimental observations reveal that a gelation wave propagates inward from the point where the divalent crosslinker (typically CaCl 2 ) is introduced, resulting in structural and compositional gradients. In this work, we investigated the dynamics of the gelation wave that forms when sodium alginate gels are exposed to CaCl 2 . Using time-resolved imaging and conductivity measurements, we measured the propagation speed of the gelation front under conditions that yield gradient vs. uniform gel structure. Our results indicate that gelation kinetics are mainly dominated by Ca 2+ transport into the alginate but are further influenced significantly by electrostatic interactions and ion-pairing hydration phenomena. To interpret these findings, three modeling frameworks were evaluated: (1) reaction-diffusion models explaining the propagation of Ca 2+ transport and ion-pairing, (2) Poisson-Nernst-Planck (PNP) electro-diffusion models that address charge separation and ion migration profiles, and (3) Born-modified PNP (BMPNP) model, incorporating Born hydration energies and pairing effects to match shifts in experimentally observed Donnan potentials. The hypothesis is that using a simple reaction-diffusion model is insufficient to fully describe ion-exchange kinetics in alginate crosslinking. Understanding the gelation process, particularly when gradients arise, requires electro-diffusion effects and ion-specific interactions. This work bridges classical biochemical methods with electrostatic biophysical methods, offering a novel framework to predict and control hydrogel structures in various applications, including biomaterials, drug delivery systems, wearable sensors, and soft robotics.
Pirmohammadi et al. (Sun,) studied this question.