Flow behavior of concentrated tricalcium phosphate suspensions in oil through injection for softgel encapsulation

Softgel encapsulation is an important manufacturing process used to produce convenient oral dosage forms of pharmaceuticals and dietary supplements. An important step in this process involves the injection of a fill material into a gelatin lined mold, and poor flow of the fill material during the injection may lead to undesired defects. We investigated the effect of fill composition on the injection behavior of an oil-based tricalcium phosphate (TCP) suspension containing lecithin by rheological measurements, capillary breakup extensional rheometry (CaBER), and high-speed flow visualization experiments. Shear rheology for TCP suspensions in soybean oil at concentrations of 27.8 wt% and 55.6 wt% showed shear thinning behaviors both with and without the addition of lecithin. Adding up to 0.5 wt% lecithin led to decreases in shear viscosity, while additions up to 1 wt% caused a slight increase. Results from CaBER and breakoff studies indicated that the length at which flow breakoff occurs was closely related to the Ohnesorge number, a dimensionless parameter representing the relative importance of viscous to surface-tension forces during breakoff, with higher values correlated to longer breakoff lengths. An optimal lecithin concentration exists for minimized TCP suspension viscosity which shortens flow breakoff times and improves softgel capsule seaming quality.