To ensure easy administration the plunger as well as the barrel in a pre-filled syringes are lubricated with silicone oil. In order to ensure a homogeneous distribution of the silicone oil, e.g. Dow Corning 36® on the barrel of a pre-filled syringe a nozzle dives into the barrel and the heated oil is sprayed on.
Mostly the 1 ml syringe is used and for this type about 400 µg silicone are sprayed into one syringe. Typical method of measuring of the amount is by weighing the syringe before and after the application. Quality and function of the silicone layer can be only measured indirectly by using force displacement measurement, this is also influenced by the diameter of the plunger and the barrel as well as of the filling of the syringe.
Silicone oil layer amount (weight) and homogeneity can be quickly assessed and documented with a cutting-edge device called the Layer Explorer. The measurement is non-contact and preserves the silicone layer for further tests.
As we see in many examples, silicone oil once applied forms not a static but a rather dynamic layer. Re-distribution of silicone by gravity is observed and can be visualized by the Layer Explorer. Furthermore the corrugation of the Silicone layer and the formation of up to 50 µm sized silicone mountains with a height of up to 1,300 nm are observable over longer storage times. Therefore the age of a syringe really matters as well as temperature storage conditions.
Interaction with the drug also affects the layer and changes its uniformity and therefore the performance of the device. This is well known for spray-on siliconized syringes, but also an important quality factor baked-on cartridges.
Several studies have shown that the inhomogeneous distribution of silicone oil pre-filled syringe lubrication can promote the release of silicone oil droplets into the formulation. They can now be classified by the updated USP chapters USP <1790> and USP <1787> as intrinsic particles. Silicone oil droplets could induce aggregation of the proteins to form formulation-inherent protein particles.
Homogeneous distribution of silicone is a critical factor for the function of a pre-filled syringe and more critical of an auto-injector cartridge. Holes in the silicone layer will result in a higher force displacement. The spring of an auto-injector could be insufficiently strong and the plunger could stop, causing a malfunction of the injection device. Control over the siliconization before filling can save precious product and minimize the chance of stability issues.
Visualize interaction of the silicone layer with your product at normal and accelerated conditions