Printed in PLA for use with 1ml syringes though may work with larger sizes. 

Primarily created to keep constant pressure on the plunger when drawing thicker liquids through small needles.  Anyone who does this will know it can take a while to load a syringe resulting in fatigue and slippage when holding for long periods. The vacuum from the vial holds the printed piece in place, freeing up your hands when needed. 

This can also be used to prevent accidental squeezing of the plunger if you are storing filled syringes and want to ensure accidental bumps don't depress them. There are no fixings in place, so this is a passive blocker and it's usefulness may depend on your storage methods.