Syringes are used for injecting fluids and for withdrawing fluids from fluid carrying sources. In an effort to reduce the transfer of communicable diseases, safety features were added to commercially available syringes to minimize accidental contact or sticking with used needle tips.
Principally among these safety features are tip protectors and syringes with retractable carriages. Broadly speaking, in the tip protector technology, a protective element is mounted over a needle and configured to cover the needle tip of the needle subsequent to an injection to block the needle tip. The protective element may be activated manually to cover the needle tip or automatically by way of releasing a spring to then push the protective element over the needle tip.
In the retractable carriage technology, the syringe is fitted with a movable carriage at a distal end of the syringe barrel. The carriage may incorporate a fixed needle or a Luer tip for mounting a needle hub with a needle. After an injection, the carriage can be retracted into the interior cavity of the syringe barrel along with the needle to prevent needle stick. More particularly, following an injection, the carriage is typically engaged by a plunger and retracted into the interior cavity of the barrel by pulling onto the plunger in the opposite or proximal direction. Alternatively, the carriage is disengaged from the barrel by the plunger and a spring automatically retracts the carriage into the interior cavity of the barrel.
Although the prior art safety features for syringes are useful, the safety syringes described elsewhere herein are better alternatives. Among other things, the prior art safety devices have shortcomings in that the air cannot be completely expelled from the syringe barrel prior to aspirating fluid without triggering the safety mechanism by the plunger. This premature triggering, when attempting to fill the device, makes the prior art syringe ineffective and frustrates the health care worker trying to use it.