The evaluation of the force to expel oily injection vehicles from syringes.

Measurement of the force needed to expel oily injection vehicles from all-glass syringes through hypodermic needles has shown that the force required to maintain various rates of expulsion is in agreement with a modified form of the Poiseuille equation. The flow from disposable plastic syringes with rubber plunger tips fits this equation only if a correction is made to take account of the resistance to movement of the plunger (which is effectively zero for all-glass syringes). This resistance or binding force was shown to be increased by exposure to vegetable oils, and its magnitude was inversely dependent on vehicle viscosity. This increase was caused by swelling of the rubber plunger tips due to uptake of the oily vehicles by the rubber. No such increase in binding force was observed with a viscous aqueous vehicle.