Abstract:
A retractable safety syringe includes a needle set, a barrel and a plunger. The needle set is attached to the barrel, and has a first engaging portion at one end. The plunger is mounted with a compressive piston and terminates in a second engaging portion. The piston contacts with a shoulder of the barrel as the plunger is pushed forward. To withdraw the need set inside the barrel, the plunger is pushed forward while the piston contacts with the shoulder of the barrel so as to engage the first and second engaging portion. The plunger is thereby locked with the needle set for withdrawal inside the barrel.

Description:
CROSS REFERENCE OF RELATED APPLICATION 
   This is a continuation-in-part application of the U.S. patent application having a Ser. No. 10/747,053 and a filing date of Dec. 30, 2003, now abandoned which is a continuation-in-part application of the U.S. patent application having a Ser. No. 10/301,577 and a filing date of Nov. 22, 2002 now abandoned. 

   BACKGROUND OF THE INVENTION 
   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 A 1 . The socket A 1  can be used for fixing a needle set B, which includes a needle seat B 1  having on a bottom formed with a first engaging portion B 2 . A plunger C having a pusher rod C 1  and a piston C 2  slides inside the barrel A. A top of the piston C 2  is provided with a second engaging portion C 3  engageable with the first engaging portion B 2 . A gap “d” defines a minimum distance between a top surface of the piston C 2  and an opposite end surface of the barrel A without engagement between the first engaging portion B 2  and the second engaging portion C 3 . Accordingly, a force is required to push the pusher rod C 1  forwards in the last stage of injection to engage the first engaging portion B 2  and the second engaging portion C 3 . After the plunger C and the needle set B are thereby locked each other, the pusher rod C 1  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. 
   SUMMARY OF THE INVENTION 
   A primary object of the present invention is to provide a retractable safety syringe having a buffer piston which can prevent inadvertent locking engagement of a needle set with a plunger. 
   A secondary object of the present invention is to provide a retractable safety syringe having a buffer piston which reaches its course end at a dosage volume reference of the barrel without locking engagement between a plunger and a needle set of the syringe. A user thereby can accurately control a dose of fluid substance drawn in the syringe and injected into a receiving body. The engagement of the needle set and the plunger can be operated after the syringe is taken out of the body of the patient, once all the dose of fluid substance has been discharged. 
   To achieve the above objectives, the retractable safety syringe of the present invention is comprised of a needle set, a barrel and a plunger. The needle set has a first engaging portion. An end of the barrel is formed with a needle-set socket. The needle set is connected to the needle-set socket. The plunger has a piston made of an elastic compressible material. In some embodiment, the piston may have a hollow space therein, and is in a shape easy to be compressed and restored. The plunger terminates with a second engaging portion, which may be arranged at the front end of the piston. 
   According to the present invention, the piston can be pushed inside the barrel till the top surface of the piston contacts a shoulder of the barrel without locking engagement between the first engaging portion and the second engaging portion. At the contact with the shoulder of the barrel, pushing further the plunger compresses the piston so that the projecting second engaging portion engages with the first engaging portion. 
   The present invention provides several kinds of embodiments of various shapes and structures of pistons. The piston can be configured so as to exert a force suitable for promoting the disconnection of the needle set from the barrel once the first and second engaging portions are connected each other. The present invention also provides other variant embodiments of the way the second engaging portion is arranged on the plunger. 
   The present invention will be apparent after reading the detailed description of the preferred embodiments in reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view showing an embodiment of a retractable safety syringe of the present invention; 
       FIG. 2  is a schematic sectional view of the embodiment of  FIG. 1 . 
       FIG. 2A  is a partially enlarged view of a mechanism for locking a needle set with a plunger according to the embodiment of  FIG. 2 ; 
       FIG. 2B  is a partially enlarged view of the locking mechanism of  FIG. 2  in a state where a piston contacts with a shoulder of the barrel without locking engagement between the needle set and the plunger; 
       FIG. 3  is a schematic sectional view of the embodiment of  FIG. 1  with the plunger being locked with the needle set according to an embodiment of the invention; 
       FIG. 3A  is a partially enlarged view of  FIG. 3  showing the locking engagement between the needle set and the plunger; 
       FIG. 4  is a schematic sectional view of the embodiment of  FIG. 1  showing the needle set withdrawn within the barrel; 
       FIG. 5  is a schematic sectional view of another embodiment of the present invention; 
       FIG. 5A  is a partially enlarged view of the embodiment of  FIG. 5  showing a mechanism for locking the needle set with the plunger; 
       FIG. 6  is a schematic sectional view of another variant embodiment of the present invention; 
       FIG. 6A  is a partially enlarged view of the embodiment of  FIG. 6  showing a mechanism for locking the needle set with the plunger; 
       FIG. 7  is a schematic sectional view of another variation of the retractable safety syringe according to the present invention; 
       FIG. 7A  is a partially enlarged view of the embodiment of  FIG. 7  showing a mechanism for locking the needle set with the plunger; 
       FIG. 8  is a schematic sectional view of another variation of a mechanism for locking a needle set with a plunger according to an embodiment of the retractable safety syringe of the present invention; 
       FIG. 8A  is a partially enlarged view of the mechanism for locking a needle set with a plunger generally shown in  FIG. 8 ; 
       FIG. 9  is a schematic sectional view of another embodiment of the locking mechanism according to the present invention; 
       FIG. 9A  is a partially enlarged view of the locking mechanism of  FIG. 9 ; 
       FIG. 10  is a schematic sectional view of a retractable safety syringe with an interchangeable needle according to an embodiment of the present invention; 
       FIG. 10A  is a partially enlarged view of the locking mechanism generally shown in  FIG. 10 ; and 
       FIG. 11  is a schematic sectional view of a conventional retractable type safety syringe. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 ,  2  and  2 A, the retractable safety syringe of the present invention comprises: a needle set  40 , a barrel  50  having a generally cylindrical shape, and a plunger  60 . The needle set  40  at least includes a needle  41  and a needle seat  42  for fixing the needle  41 . A bottom of the needle seat  42  has a first engaging portion  43  which is formed by at least a detent according to an exemplary embodiment of the invention. A lower end of the first engaging portion  43  connects with an inner bottom wall of the needle seat  42 , and extends into a generally conical upper end which centrally forms a tapered opening. The first engaging portion  43  thus is centrally hollow communicating with the needle seat  42  and the needle  41  to form a passage for a fluid substance. 
   The plunger  60  is movable in translation along a hollow interior  52  of the barrel  50  to draw in or deplete a volume of fluid. The plunger  60  has a portion formed with a pusher rod  62 , and axially extends into a piston shaft  63  on which is flexibly sleeved a piston  61 . According to an embodiment, the piston  61  is made of an elastic material and defines an inner hollow space  612 , and is in a shape that can be compressed and restored. The piston shaft  63  terminates with a second engaging portion  64  having a conical tip protruding out of the top surface  611  of the piston  61  in this embodiment. According to this embodiment, a portion of the engaging portion  64  may be in contact with the top surface  20  of the piston  61 . The first engaging portion  43  and the second engaging portion  64  are engageable with each other to form a locking mechanism. 
   As shown in  FIGS. 1 ,  2  and  2 A, the piston  61  in this embodiment is composed of two conical elastic portions stacking one over the other, and having their round peripheries closely clung to the inner wall of the barrel  50 . In operation for drawing in a liquid substance in the syringe, a doctor or a nurse pushes on the end of the pusher rod  62  to move the top surface  611  of the piston  61  to contact a shoulder  521  formed from an inner surface of the interior  52  of the barrel  50 . When the top surface  611  of the piston  61  reaches the shoulder  521 , the first engaging portion  43  is not yet connected with the second engaging portion  64  (referring to  FIG. 2B ). The user thus can draw a dose of a fluid substance in the syringe starting from a volume reference, which may be indicated as a “0” mark on the barrel. To inject the dose of substance confined in the syringe, the user inserts the needle  41  in the body receiving the injection, and pushes on the pusher rod  62  until the top surface  611  of the piston  61  contacts with the shoulder  521  in the interior  52  to fully discharge the dose of substance contained in barrel  50 . When the top surface  611  of the piston  61  reaches the shoulder  521 , the first engaging portion  43  is not yet connected with the second engaging portion  64 . After completion of the injection, the user removes the syringe from the body receiving the injection. To ensure the used syringe is safely discarded, a user pushes further the pusher rod  62  while the top surface  611  of the piston  61  abuts against the shoulder  521 , which presses and deforms the piston  61  to advance the piston shaft  63  and engage the first engaging portion  43  with the second engaging portion  64 , as shown in  FIGS. 3 and 3A . The plunger  60  thereby locks with the needle set  40  for withdrawal into the barrel  50 . When the pushing force exerted on the pusher rod  62  is released, the restoring force of the piston  61  can help to overcome the connection force of the needle set  40  with a needle-set socket  51  of the barrel  50  as the user withdraws the needle set  40  into the cylinder  50  by pulling rearwards the pusher rod  62 . The needle set  40  retracted into the barrel  50  thus will not be exposed out of the barrel  50 , and the syringe can be safely  5  discarded. 
   Many variations of the foregoing embodiments can be envisaged within the scope of the invention.  FIGS. 5 and 5A  show another embodiment, in which the piston, illustrated with reference number  61   a , has at least two round-disk shaped portions stacking on each other. FIGS.  6  and  6 A show a variant embodiment in which the piston  61   b  includes a reduced middle neck portion.  FIGS. 7 and 7A  show another variation in which the piston  61   c  forms a post with a thread on its surface. 
   Further, other embodiments may be envisaged with respect to the second engaging portion  64 . In the foregoing illustration, the implementation of the second engaging portion is protruding out of the top surface  611  of the piston  61 . Notwithstanding, the implementation can also be disposed at a different position relative to the top surface of the piston depending on the design or use requirements.  FIG. 8  shows an embodiment in which a front portion of the engaging portion  64  protrudes out of the top surface  611  of the piston  61 , while the remaining portions of the engaging portion  64  are positioned in the hollow space  612  of the piston  61 . As shown in  FIG. 9 , the engaging portion  64  could be totally disposed within the piston  61  (i.e., the engaging portion  64  is totally covered by the piston  61 ). When a force is exerted to press and deform the piston  61 , the engaging portion  64  protrudes out of the top surface  611  of the piston  61  to connect with the first engaging portion  43 . Therefore, no matter the position of the engaging portion  64  is completely exposed to the outside of the piston  61  (as shown in  FIG. 2 ), partially exposed to the outside of the piston  61  (as shown in.  FIG. 8A ) or completely received in the piston  61  (as shown in  FIG. 9A ), once a force is exerted to press and deform the piston  61 , the engaging portion  64  extends out of the top surface  611  of the piston  61  to connect with the first engaging portion  43 . 
   A person skilled in the art will appreciate that the first engaging portion and the second engaging portion can also be implemented with other mutually engageable shapes without departing from the scope of this invention. 
   Again referring to  FIG. 10 , the needle set  40  of the present invention further can include an adapter  44 . The adapter  44  is mounted at an end of the barrel  50 , and is configured to detachably connect with the needle seat  42  carrying the needle  41 . The detachable connection between the needle seat  42  and the adapter  44  can be, for example, a Luer-lock type connection effected by rotation, or a Luer-slip type connection by direct slipping engagement. In this embodiment, the first engaging portion  43  is formed at the bottom of the adapter  44 . With the adapter  44 , the needle  41  can thus be changed depending on the required use.  FIG. 10  shows an example of rotatively connecting the needle seat. Other retractable syringes with different shapes and connecting modes of the needle seat are also possible with the present invention. 
   The structures disclosed herein can be used with various retractable safety syringes, and include at least the following advantages:
     1. The restoring force of the piston under compression can help to overcome the connection force between the needle set and the needle-set socket, which may facilitate the retraction of the needle set into the barrel.   2. The syringe is configured with an initial volume reference which corresponds to an end of the injecting course of the piston of the plunger without locking engagement between the plunger and the needle set, which allows to accurately control the dose drawn in the syringe, and further ensures that it is fully expelled during the injection.   3. The piston acts as a buffer at its end course. The user has to exert an additional pushing force compressing the piston to connect the needle set with the plunger. Inadvertent locking engagement between the plunger and the needle set thus can be prevented.   

   As stated in the above disclosed, the present invention can surely attain its expected objects to provide a retractable safe syringe which is of simple structure, convenient for use and industrially valuable.