Aqueous channel topology governs resuspension in aluminum phosphate colloidal formulations

Aluminum adjuvants sediment during storage and must be resuspended prior to vaccine administration, yet the sediment structures dictating resuspension remain poorly resolved. Here, we used lumogallion-stained aluminum phosphate (AP) adjuvants containing lysozyme and bovine serum albumin (BSA), with and without salt, as model formulations to connect sediment microstructure to resuspension behavior. After 28 days of quiescent settling, confocal laser scanning microscopy was used to image unperturbed sediment networks and residual adjuvant clusters before and after inversion. BSA formulations were hard to resuspend and formed dense, homogeneous sediments with disconnected aqueous pores, while lysozyme and control formulations formed interconnected aqueous networks that were easy to resuspend. Resuspension difficulty increased when aqueous channel connectivity decreased relative to AP connectivity, indicating that the solvent phase topology governs how far inversion-driven fluid stresses stream into sediments. Furthermore, difficult-to-resuspend formulations generated higher numbers of residual ridged clusters after inversion. These results identify sediment topology as an image-based descriptor of adjuvant resuspension that captures outliers from bulk measurements such as sediment packing, because poorly connected aqueous channels and persistent residual clusters are found in formulations resistant to resuspension. Preserving connected solvent pathways may therefore improve the resuspendability of aluminum-adjuvanted vaccine formulations.