Hypodermic syringe with a selectively retractable needle

A hypodermic syringe with a selectively retractable needle comprising:

• an elongate barrel having an open proximal end and a distal end defining a receiver with an inward shoulder, said barrel having a hollow bore therethrough extending from said proximal end to said distal end;
• an elongate plunger having a proximal end and an open distal end, said plunger having a cavity therewithin extending proximally from said distal end, said plunger being disposed and sized to fit within said bore of said barrel for a slidable movement to define a chamber for receiving and expelling fluids, said plunger having a stopper disposed at said distal end to occlude said open end of said cavity, said stopper being sized and shaped to form a slidably substantially fluid tight seal with said bore of said barrel for forming said chamber;
• an elongate hub having a proximal flange, said hub disposed within and sized for slidable movement within said receiver at said distal end of said barrel with said flange defining a distal end of said chamber in said barrel, said hub having a passageway therethrough;
• a sleeve sized to fit with a clearance about said hub, said sleeve being disposed about said hub between said shoulder and said flange, said sleeve having a sharpened proximal end disposed substantially against a distal surface of said flange, said sleeve having at least one inward projection located distally to said cutting surface;
• an elongate needle having a fluid path therethrough, said needle having a pointed distal end and a proximal end connected to said passageway of said hub so that when said hub is disposed in said receiver in at said distal end of said barrel, said pointed end of said needle extends distally outwardly and said fluid path of said needle is in fluid communication with said chamber of said barrel;
• an elongate spring disposed about said hub sufficiently compressed to provide a bias between said receiver and said flange so that when a force greater than a force required to expel fluid from said chamber is applied to said plunger, said hub is moved distally in said receiver against said bias of said spring a sufficient distance to cause said cutting surface of said sleeve to engage and to cut through said flange and to engage said at least one inward projection on said sleeve against said hub to allow said cutting surface of said sleeve to cut through said stopper to expose said cavity in said plunger thereby to allow said bias of said spring to urge a sufficient movement of said hub into said cavity in said plunger to retract said needle to a position within said syringe where inadvertent contact with said pointed distal end is substantially prevented.