Prefilled syringes are indispensable in many medical applications, from enteral nutrition to high-dose drug delivery. Like conventional disposable syringes, they consist of a syringe body, a syringe plunger with a seal and a Luer lock connection or an attached cannula. After being filled with the medicinal product or nutrient solution, they are sterilized, if necessary. It is important that they function perfectly even after storage.

Medical applications place high demands on the materials used. In addition to borosilicate glass, plastics such as COC (cycloolefin copolymer) are increasingly being used. This amorphous and biocompatible material is characterized by high transparency, good dimensional stability, barrier properties and chemical resistance and can be sterilized by common sterilization methods such as ETO (ethylene oxide), gamma or electron beams. Unlike borosilicate glass, COC is less susceptible to breakage, which is a significant advantage.

An elastomeric seal is typically inserted into the prefilled syringe plunger. Rubber is the preferred elastomer, but butyl and brominated rubbers and TPEs (thermoplastic elastomers) are also used. These seal the syringe chamber, but tend to press against the inside of the syringe during storage. To prevent this, syringe barrels - and optionally the elastomer seal - are siliconized.

In the case of the syringe barrel, a nozzle dips into the barrel and atomizes silicone oil into fine droplets. It is important that these droplets spread out to form a closed, friction-reducing silicone oil film and also adhere sufficiently to the inner surface of the syringe barrel. The goal is to create a homogeneous silicone oil film with as little silicone oil as possible to minimize any potential interaction with the medicinal product. However, without special surface treatment, the COC will not wet sufficiently with the silicone oil. Pretreatment with Openair-Plasma, for example, can help here.

Using Openair-Plasma to Improve COC Wettability

When energy is applied to a gas, it is ionized and enters the high-energy plasma state. Plasmatreat has harnessed this process for industrial use and developed what is known as Openair-Plasma. Compared to low-pressure plasma technology, Plasmatreat's plasma systems and equipment operate at atmospheric pressure and can be integrated directly into production processes. A vacuum chamber is not required. To generate the Openair-Plasma, Plasmatreat uses a high-voltage discharge in a special plasma nozzle through which the process gas is passed and ionized. Various gases can be used as process gases, with compressed air and pure nitrogen being the most common. When the plasma comes into contact with a substrate surface, it interacts with the molecules on the surface and changes their properties. 

In this particular application of COC syringe barrels, reactive oxygen and nitrogen compounds are chemically bonded into the plastic surface. This treatment increases the surface energy and thus the wetting behavior of the plastic by introducing the molecules. The silicone oil interacts with the plasma-treated inner surface of the syringe barrel and spreads over it to form a homogeneous, thin film.