What Is a Syringe Plunger?

A syringe plunger is a part of a syringe that enables the injection of a fluid. The plunger is designed so that it can pass through the most constricted area of the syringe without coming in contact with the head of the needle holder. This design also minimizes the clamping and friction forces on the needle holder. A spring holds the plunger in place and allows it to retract.

There are two types of plungers. The first type is a non-siliconized type. This type of plunger is designed for barrels that are not silicone-coated. It provides excellent container closure integrity and maintains consistent injection performance over time. This plunger is biocompatible and reduces the amount of sub-visible particles. It also helps protect the drug from leaks.

Another type of plunger is a vent-tube style, which allows the plunger to operate with high-speed filling lines. These plungers are designed to provide consistent performance and minimize the risk of leaking and degrading drug integrity. One example of such a product is the Aptar Pharma PremiumCoat 1-3mL plunger, which is able to deliver high-viscosity drugs.

When a syringe is in the first position, a small amount of fluid remains in the variable chamber, and the needle holder is retracted. In the second position, the needle holder is pulled up and a small amount of fluid remains in the septum. The fluid in the retraction cavity is then discharged by depressing the plunger.