Single-use pneumatic safety syringe providing gas-driven needle retraction

A syringe having a retractable needle, the syringe comprising:

• a syringe barrel;
• a needle disposed at a distal end of the syringe barrel, the needle having a needle header for holding the needle in place within the syringe barrel;
• a plunger slideable within the syringe barrel for injecting medicament and having a retraction lumen for receiving the needle when the needle is retracted;
• a needle carrier sealingly engaged within a distal end of the plunger, the needle carrier having a distal bearing surface against which gas under pressure may bear and being initially retained in a fixed position relative to the plunger, the needle carrier being slideable within the retraction lumen in response to force applied by gas under pressure;
• a gas release cell located within the syringe barrel distally of the needle carrier;
• gas release trigger means located within the syringe barrel distally of the needle carrier for rupturing the gas release cell in response to the application of a post-injection force to the plunger by a user to release compressed gas in an area confined by the syringe barrel and the needle carrier;
• and
• an engagement ring frictionally engaged within the syringe body distally of the plunger and proximally of both the gas release cell and the gas release trigger means, the engagement ring being initially retained in a fixed position and slideable in the distal direction upon the application of the post-injection force to the plunger by the user to cause the gas release trigger means to rupture the gas release cell,
• wherein the needle carrier is engageable with the needle header to retract the needle into the plunger lumen under force applied by released gas when the gas release cell is ruptured.