Patent Publication Number: US-2011054398-A1

Title: Medical syringe with retractable needle

Description:
BACKGROUND 
     The present disclosure relates to medical syringes and more particularly to a safety syringe with retractable needle. 
     According to present statistics, a high number of accidental injuries of medical personnel are caused by the sharp exposed tip of a syringe needle, after injection of the patient. Such needle may be highly contaminated and result in sickness or disease to the medial personnel beyond the physical discomfort of the needle penetration. 
     Syringes with retracting needles which allow for safe and easy disposal of the syringe while preventing reuse, are known. One example is described in U.S. Pat. No. 6,010,486 “Retracting Needle Syringe”. According to that device, a spring-preloaded needle carrier is triggered to retract into the piston at the end of the piston stroke. The device described in U.S. Pat. No. 6,010,486 is, however, somewhat complicated and difficult to manufacture and assemble. Accordingly, there is a need for a lower cost alternative to that design. 
     SUMMARY 
     The present in invention provides numerous advantages relative to known syringes having retractable needles. The syringe can be manufactured with a reduced part count and simplified functionality of the components because of combined locking and sealing functions. The parts are of a design that can be easily mass produced and automatically assembled. Only minimal changes would be required to conventional tooling for the manufacturer of syringes. Moreover, no special user training is required. 
     In general, the present disclosure is directed to a retracting needle syringe of the type comprising a syringe barrel having a longitudinal axis and proximal and distal ends; a piston coaxially disposed in the barrel, having proximal and distal ends and a cavity with an opening toward the distal end. A needle assembly is situated within the distal end the barrel, having a hub, a hollow needle fixed in the hub with a tip extending forward of the barrel, and a spring seated between the distal end of the barrel and the hub, whereby a serum volume is defined between the hub and the distal end of the piston. A membrane is interposed between the hub and the opening at the distal end of the piston. A locking member is lodged between the barrel and the needle assembly, thereby holding the needle assembly within the distal end of the barrel to compress the spring while the needle projects from the barrel. With serum in the serum volume and the piston in a proximally retracted position, the piston is manually displaceable toward the needle assembly through functional positions in which the piston (a) forces the serum out of the serum volume through the needle, (b) urges the membrane against the hub, and (c) dislodges the locking member thereby releasing the needle assembly such that the spring displaces the hub and at least a portion of the membrane into the cavity and withdraws the needle tip into the distal end of the barrel. 
     In one embodiment, the hub has a piercing surface facing the opening at the distal end of the piston, and the membrane covers the opening. Preferably, the functional positions (a), (b), and (c) are sequential, such that between functional positions (b) and (c) of the piston, the piercing surface severs the membrane. Initially, the locking member is in a deformed state bearing on the periphery at the proximal end of the hub. A pusher on the piston is axially aligned with the locking member and displaceable along the periphery of the hub. This displacement drives the piercing surface through the membrane and thereafter dislodges the locking member thereby releasing the needle carrier into the cavity. 
     In another embodiment, the locking member is in a deformed state at the distal end of the barrel, bearing on the periphery of the distal end of the hub. The pusher is axially aligned with the hub, whereby displacement of the pusher surface to the actuation position urges the hub distally, such that the hub dislodges the locking member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a longitudinal section view of a syringe according to an aspect of the present invention, as taken out of a sterile storage pouch; 
         FIG. 2  is an enlarged view of the distal region of the syringe of  FIG. 1 ; 
         FIG. 3  is a schematic representation of the position of an alternative piston within the barrel, ready for filling with serum after the protective cover has been removed from the needle; 
         FIG. 4  shows the position of the piston within the barrel, as the forward chamber is filled with serum prior to injection; 
         FIG. 5  is an enlarged view of the distal region of the barrel showing the moment just before the continued displacement of the piston following completion of the injection, releases a locking member on the needle carrier; 
         FIG. 6  shows the displacement of the needle carrier into the central cavity of the piston by a compression spring seated between the nose of the barrel and the needle carrier, after release of the locking member shown in  FIG. 5 ; 
         FIG. 7  shows a first alternative embodiment of the needle carrier having a cup-shaped face with sharp rim for penetrating a membrane barrier covering the cavity at the distal end of the piston; 
         FIG. 8  shows a second alternative embodiment of the needle carrier; 
         FIG. 9  shows the orientation of the retracted needle carrier associated with the embodiment of  FIG. 8 ; 
         FIG. 10  shows another embodiment, in which the barrier covering the cavity at the distal end of the piston has dedicated weakened areas for penetration by the needle carrier during the retraction mode; 
         FIG. 11  shows yet another embodiment of the barrier covering the cavity at the distal end of the piston; 
         FIG. 12  represents a series of steps in the assembly of a syringe according to the embodiment of  FIGS. 1-10 ; 
         FIG. 13  shows the steps for completing a piston sub-assembly to be inserted into the barrel sub-assembly according to  FIG. 12 ; 
         FIG. 14  shows the last three steps of the assembly sequence of  FIG. 12  in greater detail; 
         FIG. 15  shows in greater detail, the steps for installing the disk within the barrel; 
         FIG. 16  is a view similar to  FIG. 3 , for an embodiment having a pre-filled vial of serum within the syringe; 
         FIG. 17  is a detail view of the needle assembly associated with  FIG. 16 , locked in place at the nose of the barrel; 
         FIG. 18  shows the locking member associated with  FIG. 17  in the compressed and uncompressed conditions; 
         FIG. 19  shows an alternative disk for use in the embodiment of  FIGS. 1-15 , incorporating an aspect of the embodiment of  FIG. 17 ; and 
         FIG. 20  shows the distal end of another embodiment of the syringe with an alternative disk having a reduced diameter. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a syringe  10  having a substantially cylindrically hollow barrel  12  with a piston  14  situated therein that enters the proximal end of the barrel and extends forward to a location spaced from the reduced diameter nose portion  16  at the distal end of the barrel. A needle carrier  18  is situated primarily in the nose  16  of the barrel, and has a hollow needle  20  extending distally to a tip  22 . The carrier  18  has a generally funnel-shaped inlet  24  at its proximal end, which may be part of the needle or provide the inlet to a passageway within the carrier  18  in fluid communication with needle  20  affixed within the needle carrier. A helical spring  26  is in compression between the nose  16  and the carrier  18 , held in the compressed state by a lock member  28  engaging the carrier  18  and the barrel  12 . 
     In the condition shown in  FIGS. 1 and 2 , where the syringe has been taken out of the sterile storage pouch, a relatively small chamber  30  is formed between the carrier  18  and spaced apart distal end of piston  14 . The piston has an axially extending cylindrical cavity  34  with a distal opening that is sealed by a frangible barrier  36 . An annular fluid seal  32  is situated between the piston and the barrel. 
     The protective cover  38  can be readily removed by pulling it axially, thereby disengaging the frictionally held base  40  of the cover, off the outside of nose  16 , thereby exposing the needle  20  just prior to use on the subject. 
     As is more evident in  FIG. 2 , the nose  16  of the barrel is also substantially cylindrical, but with a small diameter distal end  42 , followed by a larger diameter, longitudinally extending bore  44  which enlarges to form a proximal shoulder  46 , and which then tapers outwardly at  48  in the proximal direction, to define an annular support surface  50  facing piston  14 . 
     The needle carrier  18  has an elongated inner hub  52  which at its proximal end is integrated with a larger diameter outer hub  54 , thereby defining an annular recess  56  therebetween. The spring  26  is compressed between a distal seat  58 A formed by an internal shoulder on the nose  16 , and proximal seat  58 B at the end wall of recess  56 , against the outer hub  54 . 
     The proximal end of the needle carrier  18  defines a piercing surface  60  which, when the needle carrier is released as will be described below, is driven in the proximal direction by the expanding spring  26  to penetrate the frangible barrier  36  such that the needle assembly  18  enters the cavity  34 . In the illustrated embodiment, the membrane  36  as installed is a generally cup-shaped piece of rubber or plastic, having a rim portion within an annular notch  62  near the distal end of the piston  14  where it is trapped by an annular seal ring  32 , which slidingly bears on the inside diameter of the barrel  12 . Preferably, the distal end of the piston is formed as a pusher, and particularly a pushing ring  64 , which may have a tapered surface, but in any event is axially aligned with the lock member  28 . 
     The cavity  34  can have a larger diameter opening  66  for facilitating the sliding of the cavity wall over the outside diameter or surface  68  of the outer hub  54 , whereby the pusher  64  at the periphery of the barrier  36  can intersect with the locking member  28  as the piston  14  is pushed distally completely through chamber  30 . 
     The lock member  28  is preferably an annular metal or similar washer-like ring or disk with an inner opening having a diameter slightly smaller than the outer diameter  68  of the outer hub  54 , compressively deformed at a bend angle oblique to the axis of the syringe as shown at  70 . The disk is rigidly supported on its distal side, preferably toward its periphery, at support surface  50 , producing a radially inward holding force against the outer hub  54 . This press fit at  68  holds the spring  26  in compression as shown in  FIG. 2 . 
       FIG. 3  shows the condition of the syringe when ready for filling. This view also shows that the piston  14  may not be of the same diameter over its entire length, but could have a proximal portion  14 A that closely conforms to the inner diameter of the barrel and may or may not have wings or other diskrete structure for guiding and thereby facilitating sliding within the barrel, and a proximal portion  14 B that is of smaller diameter. The proximal end of the piston  14  has a thumb pad  72 , by which the piston  14  can be gently moved from the condition shown in  FIGS. 1 and 2  to the condition shown in  FIG. 3 . The barrier  36  at the distal end of the piston is at or near abutment with the proximal end or piercing face  60  of the needle carrier, thereby establishing a reference volume for the chamber, as indicated at  30 ′. This position is assured by the engagement of female and male detent members  76 ,  78  at the proximal end of the barrel, where a flange  74  extends outwardly for convenient engagement by the user&#39;s fingers as the thumb pushes on the thumb pad  72 . 
     Before injection, the piston  14  is withdrawn by overcoming the detents  76 ,  78 , to a position represented by  FIG. 4  thereby filling the chamber  30 ″ with serum to the extent of the necessary volume as observed by scales (not shown) on the transparent barrel. The needle is then inserted into the patient&#39;s flesh and the serum injected by pushing on the thumb tab  72 . During such injection, the piston  14  will pass through the position shown in  FIG. 2  and reach the position shown in  FIG. 5 . The push ring  64  travels along the outer diameter of the outer hub  54 , with continued displacement resulting in a lever effect by which the contact of the pusher  64  on the proximal surface of the lock member, at a radial position between the interference at  68 ′ and the support at  50 , increases the oblique angle to the extent that the inner end of the lock member loses the radial force against the needle carrier member  18 . 
     Before the pusher ring  64  contacts the lock member  28  the piercing surface  60  on the outer hub has penetrated the barrier  38 , such that the proximal end of the carrier  18  enters the cavity  34 . The release of the lock member  28  decompresses the spring such that the spring displaces the needle carrier into the cavity and withdraws needle tip  22  into the distal end of the barrel, as shown in  FIG. 6 . 
     It is preferable that the axial force of the piston  14  resulting from the user&#39;s pressure on the thumb pad break the barrier  36  and produce the initial entry of the needle carrier  18  into the cavity  34 . Although the release of the lock member  28  could optionally drive the piercing surface  60  through the barrier and displace the needle carrier into the cavity simultaneously, performing these two functions in sequence minimizes the possibility of the carrier  18  hanging up during the piercing if this piercing function is dependent only on the force of the spring. Tighter tolerance would be needed for maintaining a precise coaxial relationship between the piercing face  60  and the opening of the cavity  34 . 
     It should be appreciated that the locking disk as fabricated is initially completely flat, but because the disk ID is slightly smaller and the OD of the outer hub is slightly larger compared to the respective mating diameters of the needle carrier  18  and the cavity  34 , the disk  28  as installed assumes a frusto conical shape. In this position the disk is very stable in supporting both mechanical (skin and tissue penetration by the needle tip) and hydraulic (vacuum during serum filling) coaxial forces. The spring  26  in the fully compressed state provides sufficient force to resist the hydraulic force generated by the relatively low injection pressure. However, when the piston approaches the end position, two events will take place. First, the piercing phase  60  of the needle carrier  18  will penetrate the barrier and open the cavity, and subsequently the pusher  64  will load the disk  28  close to the inner diameter. This will force the disk to pivot around the support  50  and subsequently its inner diameter will expand, thereby releasing the needle carrier  18 . 
     A small clearance at the needle carrier inner hub  52  can be provided which will reverse the above described sequence of releasing and gate opening and at the same time reduce the force required. With the small gap the needle carrier will initially move together with the piston and the only force needed corresponds to the preload of the disk whereas without the gap it must additionally overcome the friction between the needle carrier and disk bore. After the needle carrier is seated the barrier will be penetrated. 
     It should be further appreciated that during the injection of serum, the inner diameter of disk  28  as press fit on hub  16  at  68  and the circular line contact with the step or shoulder  50  of the barrel form a front seal for chamber  30  whereas the membrane  36  and seal rings such as  32  associated with the piston form a back seal for chamber  30 . This assures that all serum in chamber  30  passes through the needle  20 . For this reason, the disk  28  is preferably made of a hard rubber or polymeric material such as Polypropylene, Nylon-6 or Acetal. 
       FIG. 6  shows the piston  14  in its maximum distal insertion within the barrel, and the needle carrier  18  fully within the cavity while the tip of the needle  22  is within the nose  16  of the barrel. 
     The piercing surface  60  as shown in  FIGS. 1 and 2  is simply a flat circle, but as shown in  FIG. 7 , an alternative carrier  84  has an outer hub  86  with a dish shaped proximal face  88  which includes an annular sharp edge  90  that can more easily pierce the membrane  36 . 
     Whereas in the embodiments shown in  FIGS. 6 and 7 , the needle carrier  18  and needle  20  remain substantially coaxial with the axis of the syringe after retraction,  FIGS. 8 and 9  show another embodiment in which a further safety feature is implemented. In this embodiment, the needle carrier  92  has a dish face  94  at the proximal end, similar to that shown in  FIG. 7 , except that the dish and cutting edge are angled obliquely relative to the axis. Upon retraction, as shown in  FIG. 9 , the spring force remains axial and is applied axially against the distal, flat fact of the  92 ′ of the needle carrier. Because the diameter of the inner hub  92  is smaller than the diameter of the spring, the axial force on face  92 ′ urges the rim at  94  to square up with the vertical wall of the cavity but due to the angle of the dish  94  the inner hub and its coaxial needle assume an angle relative to the axis. In the event the syringe is accidentally dropped tip down, and the inertial force should overpower the relatively low force of the fully expanded spring, the tip of the needle will hit a solid wall such as seat  58 A instead of being re-exposed through the exit bore of the nose. 
     In another embodiment shown in  FIG. 10 , the membrane  96  serves the dual function of providing the fluid seal  98  between the piston and the barrel, and the barrier  104  at the opening of the cavity. The membrane is substantially cup-shaped, with an enlarged rim portion defining the seal  98  with the radial inner portion mounted in the annular notch  100  immediately behind the push ring portion of the piston. The barrier portion  104  preferably includes weakened circular groove or similar region  102  aligned with the piercing surface of the outer hub  54 . It can be appreciated that the membrane has an enlarged rim portion  98  in the notch  100 , base portion  104  overlying the push ring and opening thereby constituting the barrier, and weakened region  102  within the piercing surface of the outer hub. 
     In yet another embodiment, shown in  FIG. 11 , the barrier is a distinct plug  114  retained by compressive deformation against a cylindrical wall defining the opening at  112  and displaceable by the needle carrier through the cavity as the piston is displaced into overlapping relation with the outer hub. In this embodiment, the push ring  106  is not covered by a membrane and the fluid seal  108  is distinct from the membrane. 
       FIGS. 12-14  show the ease with which the syringe such as described previously with respect to  FIGS. 1-10 , can be readily assembled. With particular reference to  FIG. 12 , starting from the left, a syringe barrel  12  is selected and a needle protection cover  38  is installed over the nose portion  16  of the barrel. The washer-like locking disk  28  is then placed in the barrel and a special tool  116  is inserted in the barrel to push the locking disk until it reaches the shoulder or step  50  at the bottom of the main portion of the barrel. 
     As shown in the sequence of  FIG. 15 , initially, the disk  28  is in the form of a flat washer, at the entrance to the barrel  12 , per  FIG. 15(   a ). The disk diameter is preferably slightly larger than the mouth of the barrel especially at the detent  78 , but smaller than the main bore of the barrel. In order to move further down into the bore, the disk must deform, per  FIGS. 15  ( b ) and ( c ). The installation tool  116  contacts the disk at the central hole while the disk is supported closer to the outside diameter. This will cause the disk to bend (basically pivoting around the barrel wall) and once it assumes a conical shape it will continue to slide through the bore per  FIG. 15(   d ) until it reaches the stop or shelf  50  per  FIG. 12 . Because of the elasticity of the material the central bore will temporarily increase in diameter while the outside diameter will shrink to fit through the narrower bore. 
     Subsequently, as also shown in  FIG. 14(   a ) the needle carrier return spring  26  is dropped through the central bore  118  of tool  116 , whereupon the tool  116  is removed. As also shown in  FIG. 14(   b ) another tool  120  is then inserted into the barrel, while holding the hub  52 ,  54  of the needle assembly. The hub is held inside the tool by a small protrusion or the like  122  in a position such that the tip  22  of the needle is fully hidden within the main cylindrical bore of the barrel. A push rod  124  is displaced axially through the tool  120 , displacing the hub of the needle carrier until it seats on the step or shoulder  46  of the barrel. As previously described, the displacement of the outer hub  54  in the aperture of the locking disk  28  loads the disk and thus captures the needle assembly in the condition shown at (c) of  FIG. 14 . Once the needle assembly is locked in place, the installation tool  120  and associated push rod  124  can be retracted. 
     The partially assembled syringe is then ready to receive the piston or plunger, which is preferably assembled as shown in  FIG. 13 . A round membrane disk  126  is stamped out of a thin plastic band  128 . The disk  126  is pulled over the proximal tip of the piston, assuming a dish or cup shape. The rubber seal  32  is then placed over the annular groove  62  thereby providing a membrane closure to the cavity  34  of the piston. The piston with closure is then inserted into the barrel as shown in  FIG. 14(   c ) to arrive at the completed syringe as shown in  FIG. 1 . Typically, the syringe is placed inside a protective pouch which can be firmly welded and the entire package subject to sterilization. 
       FIGS. 16-18  show another embodiment of a syringe  130  according to the inventive concept. This embodiment is suitable for use with pre-filled syringes. The syringe has a barrel  132  with nose  134  in which the needle assembly  136  is situated. The piston  138  has a proximal end  140  with axial cavity  142 . A serum volume  144  is defined between the piston  140  and the needle assembly  136 . In this embodiment, the serum is contained within a closed membrane defining a vial  146  situated in the serum volume  144 . Unlike the previously described embodiment, this membrane does not initially cover or seal the cavity  142  relative to the serum volume  144 , but as will be described, the membrane defining the vial  146  is frangible, or equivalently deformable, so that all or a portion of the vial enters the cavity  142  along with the needle assembly  136  following injection of the serum to the needle. The preferred configuration of the needle assembly  136  during injection through needle  148  is shown in  FIGS. 17 and 18 . 
     As with the previously described embodiments, the piston  138  is manually displaceable toward the needle assembly  136  through several functional positions. First, the piston  138  forces the serum out of the serum volume through needle  148 . This is achieved by compressing the vial  146  until the proximal end of the needle  148  penetrates the vial and continued displacement of the piston empties the vial through the needle. During this injection step, the needle assembly  136  remains in a locked position within the barrel nose  134 . This is shown in  FIG. 17 , wherein the portion  146 ′ of the membrane  146  has been penetrated by the proximal end  150  of needle  148 . 
     The needle assembly  136  has an outer hub  152  and an inner hub  154 , as in the previously described embodiment. Preferably, the distal portion  156  of the outer hub  152  defines a seat for spring  160  which is compressed with respect to a distal seat formed in nose  134 . The spring  160  as compressed therebetween preloads the needle assembly for displacement toward the piston when the injection has been completed. The proximal end of the outer hub  152  preferably has a central recess  158  from which the proximal end  150  of the needle projects. Upon pressurization of the vial  146 , a lobe or the like  146 ′ conforms to the recess and assures penetration by the needle  150 . Preferably, the proximal face of the outer hub  152  defines a piercing surface, preferably with a sharp annular edge such as indicated at  186 . This edge does not perforate the membrane  146  during injection. 
     In the second position the piston urges the membrane  146  against the piercing surface  186  at the conclusion of the injection step. Preferably, the emptied vial is at least partially urged into the piston cavity  142  by the piercing surface  186 . In the last step or position, the needle carrier follows at least a portion of the vial into the cavity  142 . 
     During injection and preferably until at least some of the membrane  146  has entered the cavity  142 , the needle assembly  136  is held in place by lock member  162  lodged between a tapered or similarly ramped groove  164  near the distal end of the inner hub  154 , and a conical or similarly ramped or tapered surface  166  on the distal end of the barrel nose  134 . One suitable locking member is shown in  FIG. 18 . The condition  162 ′ corresponds to the compressed state as shown in  FIG. 17 , whereas the condition  162 ″ is the natural or neutral condition. Thus, when the locking ring is in the condition  162 ′, it is under radially inward compression, which if released will expand into the condition shown at  162 ″. 
     At the same time, or after the injection event, the piston moves the carrier assembly  136  slightly in the distal direction, enabling the compressed locking ring  162  to expand and “jump out” of the groove  164 . The spring  160  is now free to act against seat  156 , release the needle carrier and displace it into the cavity, thereby withdrawing the needle tip into the distal end of the barrel. The maximum displacement of the piston  138  toward the needle assembly  136  can optionally directly act against the sharp rim  186 , thereby cutting a central portion of the membrane out of the vial whereby that central portion of the membrane and the carrier assembly are retracted into the cavity  142 . 
     It should be appreciated that during injection the volume of the vial is reduced corresponding to the reduction in volume of serum contained therein. As a consequence, the membrane begins to deform and eventually collapses. The deformation at the distal end of the vial enables the membrane to enter the recess  158  for penetration by the needle end  150 . At the same time, the proximal end of the vial facing the piston also begins to conform to the open end of the cavity  142 . The friction between the vial wall and the rand of the piston is sufficient to prevent the vial from completely entering the cavity until the pressure in the vial has dropped to near zero (because of the serum drain). The vial then becomes flaccid and can fold and be pushed by the retracting needle assembly into the piston cavity together with the needle assembly. To facilitate complete entry of the vial into the cavity, a small venting hole can be provided at the proximal end of the piston cavity. Another option is to shape the contact surface between piston and the vial as several concentric rings and by that increase friction to prevent the vial from entering the piston cavity before all or nearly all of the serum has been injected. Another possibility would be to configure and size the cutting edge  186  and the ID of the piston cavity in such a way that when the piston reaches its fully inserted (bottom) position, the vial will be sheared off. With reference to  FIG. 20 , an additional embodiment exists wherein the metal disk  171  has an outer diameter that is smaller than the inner diameter of the barrel  12  to minimize or eliminate contact between the disk  171  and the barrel surface during installation. Here, the barrel is fit with a recess  188  at the distal end of the chamber  30  for engaging the disk  171 . 
     It should be understood that, as used herein, the “piercing surface” at the proximal end of the needle carrier or hub, performs the broad function of assuring that the membrane passes through (or pierces) the piston cavity opening and thus enters the cavity. In some embodiments, a membrane at the opening is merely cut, whereas in other embodiments all or part of the membrane is pushed into the cavity. 
       FIG. 19  shows one embodiment  168  of how a metal locking member  170  would preferably be employed in a combination of the concepts discloses in  FIGS. 1-15  and  17 . With the plastic disk  28  of  FIGS. 1-15 , the disk is both a locking member and a front seal for the serum volume or chamber  30 , to resist the fluid pressure exerted on the serum by the piston during injection. Plastic has the advantage of being somewhat flexible and pliable, facilitating assembly through the barrel and conforming as a seal to the hub  54 , but care should be taken to assure that the pressure differentials acting on the disk during drawing of serum and during injection do not excessively deform and thus undermine the operation of the disk. A solid metal disk locking member  170  would provide a stronger locking action to better resist pressure differentials, but due to increased rigidity could be more difficult to install and the force necessary to stretch the inner bore and release the needle carrier  18  could be too high, i.e., outside of the force level applied with normal syringes. 
     However if the disk  170  is provided with inner and outer cut outs  172 ,  174 , installation would be easier while the stronger locking action would be retained. As used herein, “perforations” encompasses cut outs, holes, etc. The inner diameter edge  176  need not be stretched but merely bent around the shelf  50  in the barrel. In a similar manner the outer diameter edge  178  need not be compressed but only bent about the mating diameter of the installation tool  116  during the assembly. The perforations render the disk pressure insensitive, so it can resist maximum vacuum when serum is drawn and also maximum pressure during injection, without collapsing. Such disk provides excellent locking at reduced installation and actuation forces but such disk does not also provide the fluid seal function at the front of the serum chamber. 
     As shown in  FIG. 19 , the front seal  180  in this embodiment can be the same or similar to the front seal  156  in the embodiment of  FIG. 17 , i.e., a flexible formation carried on the hub  182  of needle assembly  184  and engaging the inside surface of the barrel. This seal  180  can in the alternative be a simple elastomeric insert or “O” ring but, when integrated in the needle carrier as shown, the part count is minimized. 
     The strongest advantage of this perforated metal disk is that by being absolutely insensitive to internal pressure, it will not release the needle carrier regardless internal pressure, but it will be released by the full distal displacement of the piston.