Vented piston and push-rod subassembly for use in a syringe barrel

Claim 1

In a syringe of the type having a hollow cylindrical barrel open at both ends and with its open front end adapted to receive a needle, the improved vented piston and push-rod subassembly for insertion into its open rear end in plug-forming relation thereto which comprises: a cup-shaped piston formed from an elastic material having a hollow interior with a rear-opening entryway bordered at the front end by a front forwardly-facing annular sealing surface and at the rear end by a rear rearwardly-facing annular sealing surface, said piston also having a continuous annular rib encircling same effective upon insertion into the barrel to form a fluid and air-tight seal in wiping contact therewith, said piston further including means for admitting both fluids and air into the hollow interior thereof ahead of said continuous annular rib, said means comprising a discontinuous rib spaced forwardly of the continuous one and a medial section of reduced cross section between said ribs with an opening therethrough; and, a push-rod terminating at its forward end in a necked-down head cooperating therewith to define a front rearwardly-facing annular sealing surface and a rear forwardly-facing annular sealing surface spaced therebehind, said head being sized for insertion through the entryway into the piston so as to form a plug therefor while cooperating therewith to define a fluid collection chamber within the hollow interior thereof, the front forwardly-facing annular sealing surface of said piston and the front rearwardly-facing annular sealing surface of said push-rod cooperating upon the application of a force to the latter in a direction to retract same to define a continuous annular fluid and air-tight seal therebetween effective to aspirate fluid into said barrel when mounted therein, the rear rearwardly-facing annular sealing surface of said piston and the rear forwardly-facing annular sealing surface of said push-rod also cooperating upon the application of a force to the latter in a direction to extend same to define a continuous annular fluid and air-tight seal therebetween effective to discharge fluid from the open front end of said barrel when in place therein, and all four of said sealing surfaces between said piston and push-rod cooperating when no force is applied to the latter in either direction to define a continuous annular fluid-tight seal between said front sealing surfaces effective to vent air rearwardly past the piston from the hollow interior thereof while retaining all fluids collected in its fluid collection chamber.