Materials released by secretory cells are stored inside intracellular membrane‐bound vesicles. These moieties are not freely diffusible in the vesicle but remain immobilized in a Ca2+‐crosslinked condensed‐phase polyanionic polymer matrix. During exocytosis a Na+/Ca2+ ion exchange process triggers a volume phase transition resulting in massive swelling and release of the materials to the extracellular space. Here we formulate a simple model to assess Ca2+‐ion binding from the swelling kinetics of polymer networks. We found the diffusivity of the networks (D) exhibits a power‐law dependency on the Ca2+ concentration where D ∝ [Ca2+]−2/3. The model yields an estimate of charge density and ionic affinity of the polymer chains. Studies of post‐exocytic swelling kinetics in airway mucin granules, mast cell granules and granules from the microalga (Phaeocystis globosa) were used to validate predictions from our model. These results suggest that independent of the cell type, from animal to plant cells, a single polyelectrolyte interaction mechanism appear to be responsible for product release in exocytosis.
Rapid hydration and high early strength portland cement and calcium sulfoaluminate (CSA) concretes are commonly used as pavement repair media. Field trials reveal a tendency to crack along the longitudinal axis of the repair. The authors hypothesized that internal curing with saturated lightweight aggregate would prevent or delay restrained shrinkage cracking in the rapid repair media. The fresh properties (slump, setting time), mechanical properties (elastic modulus, compressive and tensile strength), and volume stability (autogenous shrinkage, drying shrinkage, restrained ring shrinkage, creep) of the rapid repair media were evaluated with and without internal curing with saturated lightweight aggregate. Despite delayed hydration and reduced early-age strength, internally cured concretes could retain the requisite minimum 4-hour compressive strength of 4,000 psi. Significant improvements in volume stability were also noted. Drying and autogenous shrinkage were reduced by factors up to 20 % and 50 %. However, detrimentally, creep shrinkage was increased by factors up to 45 %, respectively. The time to cracking in restrained ring shrinkage tests was increased by factors up to 60 %. These results indicate that internal curing can successfully improve volume stability and mitigate restrained shrinkage cracking in rapid repair media without compromising fresh properties or ultimate mechanical strength.
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