SYRINGES AND WINDSCREEN WIPERS
posted 24th August 2022
You know that horrid juddering noise that windscreen wipers make sometimes? This is caused by alternate sticking and slipping of rubber on glass. The same phenomenon occurs with the rubber plunger in a glass syringe, albeit with more serious consequences including uncomfortable injections, variability in dosing, and wastage of drug.
Billions of syringes are used each year and one thing they all have in common is the need for low and constant friction between the syringe and plunger. The trend of installing syringes in autoinjectors makes this even more critical.
Many prefilled syringes have to be made from glass to ensure drug compatibility and sterility. Rubber plungers have high, and variable, friction with glass so glass syringes are almost always lubricated with a coat of silicone.
Sprayed-on silicone oil suffers from the challenges of ensuring a consistent coating down the syringe, migration of the silicone oil whilst in storage, and potential interaction with the drug.
One way manufacturers have overcome these problems with sprayed-on silicone is to coat the inside of the glass in a layer of silicone oil emulsion. This can then be baked at about 300 °C to induce a chemical reaction between the silicone and the glass to give a thin, lubricious silicone layer that is well bonded to the surface. This is important to avoid adverse interactions with drugs.
The trouble with this is that this temperature is far in excess of the approximately 150 °C maximum temperature of the UV adhesives used to secure, or "stake", a needle in the syringe. So this treatment is limited to syringes without a staked needle, giving manufacturers a choice between (i) a sticky syringe with a conveniently staked needle, and (ii) a syringe that is lubricious but imposes an increased burden on the end user to select the right needle.
If this choice is unacceptable for your drug delivery system, there are fortunately now some other options.
Completely re-designing the manufacturing process to apply the silicone before the needle is glued in may not be a viable option, but plasma activation technology has been used for several decades in the field of microfluidics to promote adhesion between silicone and glass, and could be applied in syringe coatings. Treating the glass with plasma generates reactive radicals on the surface which can react with and bond to silicone molecules to give an adherent film, without the need for elevated temperatures
For example, Stevanato's Alba syringe systems use a similar technology to achieve a bonded silicone coating and are now available with staked needles.
Naturally, there are risks associated with moving to a new syringe system and a different manufacturing process, not to mention increased costs. Depending on the application, your device may be well served by alternative approaches to bonding silicone coatings like a UV-activated ozone process, or indeed by an unbonded silicone oil coating.
If plasma treatment does not provide a satisfactory solution, then there are other options such as cyclo-olefin polymer syringes with various different barrier and lubrication coatings such as those from SiO2 Materials Science or Mitsubishi Gas and Chemical.
Written by Omar Shah, Consultant Engineer (Materials & Mechanical).
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