1. Field of the Invention
The present invention relates to a safety syringe having a needle retractable into a barrel, especially to a syringe having a piston which acts as a buffer to prevent inadvertent a locking engagement of the needle set.
2. Description of the Prior Art
One conventional type of syringes for subcutaneous injection is the disposable syringe which is discarded after a single use. The disposable syringe is convenient, hygienic and eliminates the risk of injection-related contaminations. However, one problem is that the discarded syringe has an exposed needle which may hurt a medical staff handling it. Even if the use of the conventional disposable syringe is safer with regard to the patient receiving the injection, risks of contamination and injuries still exist for the medical personnel, and more generally for the personnel in charge of treating the used syringes. Therefore, a retractable type syringe has been developed to retract the needle set inside the barrel after use.
A conventional retractable type safety syringe is shown in FIG. 11, wherein a barrel A terminates in a front end with a socket A1. The socket A1 can be used for fixing a needle set B, which includes a needle seat B1 having on a bottom formed with a first engaging portion B2. A plunger C having a pusher rod C1 and a piston C2 slides inside the barrel A. A top of the piston C2 is provided with a second engaging portion C3 engageable with the first engaging portion B2. A gap “d” defines a minimum distance between a top surface of the piston C2 and an opposite end surface of the barrel A without engagement between the first engaging portion B2 and the second engaging portion C3. Accordingly, a force is required to push the pusher rod C1 forwards in the last stage of injection to engage the first engaging portion B2 and the second engaging portion C3. After the plunger C and the needle set B are thereby locked each other, the pusher rod C1 is pulled backwards to retract the needle set B with its needle into the cylinder A. The syringe then can be discarded safely.
Safety syringes provided with spring retracting mechanisms and activating devices for automatically retracting needles are also known in the markets of medical appliances. In this type of device, a force is exerted on an activating device after injection, and the needle set will retract into the barrel by a spring retracting mechanism. Notwithstanding, a buffer gap between the front end of the pusher rod and the needle set is still used to define the disconnection between the pusher rod and the needle set before injection. This gap may result in a dosage error of a fluid to be drawn into the barrel. In addition, due to the resistance caused by the engagement of the first and second engaging portions, the user needs to exert a significant pushing force to discharge the liquid volume corresponding to the buffer gap during the injection operation, which may cause adverse injuries.
Therefore, there is presently a need for an improved retractable safety syringe provided with a mechanism that can ensure a dose of a fluid substance can be accurately drawn in the syringe, and prevent inadvertent locking engagement of the needle with the withdrawal mechanism.