Retractable Syringe with a Cutting Crown

Abstract

A retractable syringe comprises a cutting head mounted on a plunger of the syringe. When the plunger is advanced to expel fluid from the syringe, the cutting crown cuts a retention mechanism which holds a needle unit of the syringe, allowing a drive mechanism to drive the needle unit into the syringe. The cutting crown is shaped to reduce the force which has to be applied to cut the retention mechanism. Furthermore, a disposable needle assembly for attachment to a syringe unit comprises a needle and connector element for connecting the needle to the syringe unit. The connector element encircles the needle and has a thread on its outer surface for mating with the syringe unit. When the needle assembly is mated with the syringe, a central bore of substantially constant bore extends from the tip of the needle into the interior chamber of the syringe.

1 . An elongate needle assembly for attachment to a syringe unit having a plunger movable within a barrel, to form the syringe assembly, the needle assembly comprising:
 a hub with an outwardly-facing threaded surface for mating with an inwardly-facing threaded surface of the syringe,   a needle extending from a first end of the hub,   a neck extending from a second end of the hub and terminating at an end face, the neck having a smooth outer surface suitable as a guiding surface for leading the inwardly-facing threaded surface of the syringe to the outwardly-facing threaded surface of the hub, and   wherein the needle assembly defines a central passage of substantially constant bore extending from the end face and along a central axis of the needle to a tip of the needle.   
     
     
         2 . The needle assembly of  claim 1 , wherein the needle is encased in a protector, the protector being formed as a single piece together with the hub, the protector being connected to the hub by a frangible portion. 
     
     
         3 . The needle assembly of  claim 1 , wherein the protector further comprises a wing extending from an outer wall of the protector, whereby a user can deliver rotational force to the protector about a longitudinal axis of the protector. 
     
     
         4 . A syringe assembly which is a combination of a needle assembly according to  claim 1 , and a syringe unit having an inwardly-facing threaded surface mated with the outwardly-facing threaded surface of the needle hub,
 the central passage of the needle assembly meeting a passage of the syringe unit having substantially the same bore and communicating with a chamber within a barrel of the syringe unit.   
     
     
         5 . A method of forming a syringe assembly accordingly to  claim 4 , comprising mating the needle assembly with the syringe unit.