Colloidal packing fraction in sedimented aluminum adjuvants dictate their resuspension rheology.

Vaccine formulations containing aluminum adjuvants can be difficult to resuspend after prolonged storage, rendering them unsuitable for administration. While earlier studies suggested that the increased flocculation generated loosely packed sediments, there is a lack of mechanistic understanding on how particle packing affects resuspendability. In this study, we observe an exponential correlation between the weight fraction of sediments and the number of inversions required for complete resuspension of aluminum phosphate adjuvants formulated with varying ionic strengths (0 and 150 mM sodium chloride), protein types (bovine serum albumin and lysozyme), and protein concentrations (0, 0.5, and 1 mg/mL). Oscillatory shear rheology indicates that an increase in packing fraction enhances the strength of the adjuvant gel microstructure and its yield stress, while confocal microscopy images show that sediments with higher packing fractions are more homogeneous. These sediments possess greater inter-aggregate bond density, which strengthens the microstructure and resists its fluidization into a homogenous suspension. Salt-free formulations containing bovine serum albumin form sediments with the highest packing fractions. Particulate gels continue settling under gravity and increasing in packing fraction until the yield stress is high enough to resist the gravitational stress. We speculate that formulations containing BSA form large and polydisperse secondary aggregates which lowers the yield stress and allows the aggregates to pack most efficiently, thereby increasing the packing fraction and resuspension difficulty.