Patent Publication Number: US-2012029471-A1

Title: Two chamber syringe with locking mechanism

Description:
INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to syringe injectors. 
     2. General Background 
     Syringes are commonly used in the medical field for the injection or withdrawal of liquid medications. Syringes typically have a hollow glass or plastic barrel with an internal piston. By moving the piston, a user can create a positive or negative pressure inside the barrel, thereby transmitting fluid out of or into the barrel through a small opening opposite the piston. 
     Syringes are often used in intravenous therapy where the syringe may directly puncture the vein, or more commonly, may be used in conjunction with a catheter. When a catheter is used, one side of the catheter remains in the vein, while the other side remains outside the skin. The external portion of the catheter typically includes a coupler for connection to a syringe. 
     After injection in either procedure, a small amount of medication is typically left behind. When a syringe is used, the medication remains within the tip of the syringe. When a catheter is used in conjunction with a syringe, the unadministered medication remains in both the tip of the syringe and in the catheter. 
     This leftover medication is problematic for several reasons. First, it necessarily means that the entire amount of medicine drawn into the syringe does not reach the patient. Second, many medications are time sensitive and should not remain in the catheter until a subsequent medicine flushes it through. 
     In a catheter system, these problems are solved using a second liquid to immediately flush the remaining medication out of the catheter and into the patient. Generally, a second syringe prefilled with a flushing solution provides the second liquid. 
     While many different liquids may be used to flush the catheter, the most commonly used liquid is a 0.9% concentration of sodium chloride (saline solution). The saline solution is injected from a syringe into the catheter, thereby flushing any stranded medication into the patient. Thus, the saline flush ensures that a full dosage of medication has been timely delivered. 
     This method for purging the catheter has certain disadvantages. For instance, by using a separate syringe for each injection, there is an increased chance of medical error. Most medicines are colorless (like the saline solution), and it is easy to accidentally administer medication when intending to flush the line or vice versa. This risk is increased when clinicians carry medicines for multiple patients at one time. 
     The likelihood of error is compounded in an emergency, when it may be necessary to inject several medications quickly and in a specific order. In such situations, a separate saline flush is necessary between every individual medication injection, so the risk of error is high, and the consequences of a mistake may be grave. 
     Finally, the clinician may be distracted by a separate medical need during the time between the injection of medication and the saline flush. Without some reminder, the clinician may forget that he or she has not flushed the line. 
     Even if all precautions are taken and the two injections are made in the proper order, drawbacks remain. With each breach of the catheter&#39;s seal for injection, the patient is potentially exposed to bacteria, increasing the risk of infection. By requiring a clinician to access the system once for the medication and a second time for the flush, the risk of infection is doubled. 
     Using a second syringe for the saline flush also wastes resources. Attaching a second syringe to the catheter takes time, and since a clinician may perform a saline flush more than one hundred times per day, this lost time adds up quickly. Finally, requiring a second syringe unnecessarily increases the already significant costs related to manufacturing, shipping, storage, and disposal of syringes. 
     Syringes adapted to deliver multiple fluids for sequential injection have been described in the prior art. However, due to design limitations, no syringe has become widely accepted that allows routine medication administration and subsequent catheter flushing from a single syringe. Some prior art syringes include a “standard” syringe that is separated by an intermediate sliding stopper into two chambers. The sliding stopper receives motive force communicated through an intermediate fluid from a primary stopper (part of a plunger assembly of the standard syringe) against which an external force is applied. Examples of such prior art devices may be found in U.S. Pat. Nos. 5,720,731, 6,997,910 and 7,101,354, which describe multiple embodiments of a conventional syringe adapted to deliver multiple fluids and a displaceable valved stopper which partitions a conventional syringe. Other sequential delivery syringes have been developed, such as U.S. Pat. No. 6,723,074, which uses a piercing member to open an internal chamber. 
     The previously described syringes adapted to deliver multiple fluids have not been widely adopted because they generally require the delivery of two fluids that are both prefilled during manufacturing. Rather, current standard practice is to use two separate syringes—one prefilled with saline and one empty syringe that is filled with medication shortly before administration. Requiring hospitals to change their practice to using previously described multi-fluid syringes is unfeasible because the hospitals would be required to keep an unreasonable number of prefilled medicine syringes in stock to accommodate the varied number of doses and types of medications required for routine patient care. The applicants&#39; invention solves all of these problems, and does so with a simple design that makes storage easy and keeps manufacturing costs to a minimum. The present invention includes all the functionality of a standard syringe (including the ability to depress and pull back on the plunger when withdrawing medicines from a multidose vial) independent of the flush chamber. The design of the syringe takes advantage of basic fluid mechanics to keep the flush and the medicine from contacting each other during use. The present invention advances the state of the art by providing a cost-effective single syringe that both administers medication and flushes the intravenous system. By using a single syringe for both purposes, a clinician need only access the intravenous catheter once, thereby decreasing the rate of error and infection. Additionally, the presence of the saline or other solution in the syringe after injection alerts the clinician of the need to flush the system, thus reducing the chance that the flush would be forgotten. Finally, the extra cost and time associated with a second “flush-only” syringe would be eliminated. 
     SUMMARY OF THE INVENTION 
     The present invention is a two-chambered syringe with an outer barrel having an open end for slidably receiving an inner barrel/first piston. A second piston is slidably movable in the inner barrel/first piston. A latching mechanism locks and unlocks the inner barrel/first piston to the second piston. In the locked configuration, the second piston is prevented from substantially all longitudinal movement relative to the inner barrel/first piston, and in the unlocked configuration, the second piston may move longitudinally within the inner barrel. The invention may be used, for example, to administer a medicine from the outer barrel and then administer a flushing solution from the inner barrel. Thus, the invention may be used as a traditional syringe to withdraw medicine from a bottle, either before or after the administration of a second flushing solution contained in the syringe. 
     A cost-effective single syringe that both administers medication and flushes the intravenous system is needed to improve the standard of care. It is desirable to allow caregivers to follow their standard syringe filling procedures; to not rely on the fluid in the distal chamber to expel the primary fluid (medicine) from the syringe; to include a physical locking mechanism such that the intermediate fluid cannot be expelled accidentally while depressing the plunger during routine filling; to allow filling of the distal chamber from the proximal end (during manufacturing), which enables complete filling of the distal chamber without trapping any large/non injectable air bubbles; to utilize basic fluid mechanics to keep the two fluids separate when disposed within the syringe; and to not limit the volume of medicine that can be filled into the proximal chamber. 
     Described herein are syringe devices, systems and methods. 
     In general, a syringe includes a cartridge and a second chamber. The cartridge includes a first chamber, a second end, and a locking mechanism. The second end is movable within the first chamber between a first position and a second position. The locking mechanism has a locked configuration and an unlocked configuration and prevents movement of the second end within the first chamber while in the locked configuration. The locking mechanism includes: a flexible arm having a first end coupled to the second end of the cartridge and a second free end; a tab coupled to the second free end; and a groove in the inner surface of the chamber configured to receive the tab. The cartridge is moveable within the second chamber. 
     This and other embodiments may include one or more of the following features. The tab can include a ramped surface such that when the second end is rotated within the inner surface of the cartridge, the ramped surface allows the tab to slide more easily out of the groove. The tab can include two ramped surfaces such that the second end may be rotated in two directions within the first chamber. The syringe can further include an adjacent groove adapted to receive the tab when the locking mechanism is in the unlocked configuration. The tab can be substantially trapezoidal shaped. The tab can be substantially semi-circular shaped. The syringe can further include a second groove configured to receive the tab when the second end is in the second position. The second groove can extend around the circumference of the cartridge. The syringe can further include a first ridge on the inner surface of the cartridge, wherein the ridge is configured to prevent the withdrawal of the second end from the first chamber. The syringe can further include a second ridge on the outer surface of the second end of the cartridge and a second ridge, wherein the first ridge is configured to engage with the second ridge such that withdrawal of the second end from the first chamber is prevented. The cartridge can further include a first end that defines a conduit in liquid communication with the first chamber. The first end of the cartridge can be coupled to the first chamber such that when the second end of the cartridge is rotated within the first chamber, the first end is not rotated. The first chamber can have a noncircular cross section. The syringe can further include indicia that signify when the locking mechanism is in the locked configuration and when the locking mechanism is in an unlocked configuration. The syringe can further include a ridge coupled to an end of the groove, wherein the ridge is configured to prevent the tab from reentering the groove after it has been released. The second end of the cartridge can further include a handle sized and configured to move the second end within the first chamber. 
     In general, a method of using a syringe having a cartridge having a first chamber, a second end moveable within the first chamber, and a locking mechanism, wherein the syringe further includes a second chamber having an outlet, includes: expelling a liquid from the second chamber through the outlet by moving the cartridge within the second chamber toward the outlet; rotating the second end of the cartridge with respect to the first chamber to release the locking mechanism; and expelling a second liquid from the first chamber through the outlet by moving the second end of the cartridge within the first chamber toward the outlet. 
     This and other embodiments may include one or more of the following features. The second end can include a tab having a ramp, and rotating can include rotating the tab against a groove defined by the inner wall of the first chamber such that the tab is released from the groove. The rotating step can further include further rotating the tab after the tab is released by the groove such that the tab is received by a second adjacent groove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a two-chambered syringe according to an embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of a two-chambered syringe according to an embodiment of the present invention. 
         FIG. 3  is a perspective view of the embodiment depicted in  FIG. 1 . 
         FIG. 4  is a perspective view of the embodiment depicted in  FIG. 2 . 
         FIG. 5  is a side cross-sectional view of the embodiment depicted in  FIG. 1 , with the inner barrel/first piston full of a liquid such as a saline solution. 
         FIG. 6  is a side cross-sectional view of the embodiment depicted in  FIG. 1 , with the second piston partially depressed, thereby expelling some of the liquid. 
         FIG. 7  is a perspective view of the inner barrel/first piston and sealing ring depicted in  FIG. 1 . 
         FIG. 8  is a perspective view of the inner barrel/first piston and sealing ring according to an alternative embodiment of the present invention. 
         FIGS. 9-14  are perspective views of the proximal end of the second piston and sealing ring according to alternative embodiments of the present invention. 
         FIGS. 15(   a )-( g ) are side cross-sectional views of various stages of operation of the two-chambered syringe depicted in  FIG. 1 .  FIG. 15(   a ) depicts the syringe as delivered to the clinician.  FIG. 15(   b ) depicts the front chamber being filled with air.  FIG. 15(   c ) depicts the air being injected into a medicine bottle.  FIG. 15(   d ) depicts the withdrawal of medicine from a bottle into the front chamber.  FIG. 15(   e ) depicts the administration of the medicine to a patient.  FIG. 15(   f ) depicts unlocking the front chamber from the back chamber.  FIG. 15(   f ) depicts the administration of the prefilled flush solution. 
         FIG. 16  is a perspective cut away view of the inner barrel/first piston showing the raised track and rear lip. 
         FIGS. 17(   a ) and ( b ) are perspective views of a portion of the inner barrel of a two-chambered syringe according to an embodiment of the present invention. 
         FIG. 18  is a side cross-sectional view of the embodiment depicted in  FIG. 1 , with the inner barrel/first piston full of a liquid such that there is a liquid-air interface at or within the conduit. 
         FIGS. 18A  and B represent the balance of forces inside a conduit of a syringe. 
         FIGS. 18C-F  illustrate multiple embodiments of the shape of a conduit of a syringe. 
         FIG. 19  is a side cross-sectional view of an embodiment of a syringe including an end cap. 
         FIGS. 20A-20D  illustrate multiple embodiments of a method of filling a syringe. 
         FIGS. 21A-21H  illustrate multiple embodiments of the shape of a bubble in the conduit of a syringe. 
         FIGS. 22A-23  are perspective views of an embodiment of a syringe including a locking mechanism. 
         FIGS. 24-25  are perspective views of an embodiment of a syringe including a locking mechanism and additional grooves. 
         FIG. 26A  is a perspective view of an embodiment of a syringe including a locking mechanism and a ridge.  FIG. 26B  is a cross-section of  FIG. 26A . 
         FIGS. 27-28  illustrate various embodiments of a syringe wherein a first end may be fixed with respect to the outer barrel. 
         FIG. 29  is a perspective view of an embodiment of a syringe including indicia. 
         FIG. 30  illustrates a groove of a locking mechanism including a ridge. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Described herein are syringe devices, systems and methods. In general, the syringe may include a second chamber and a cartridge movable within the second chamber. The cartridge includes a cartridge chamber (first chamber), a first end that defines a conduit in fluid communication with the cartridge chamber and the second chamber, a liquid disposed within the cartridge chamber such that there is a liquid-air interface at or within the conduit, wherein the liquid has a fluid property such that the liquid-air interface, cooperating with the conduit and the fixed volume of the cartridge, prevents movement of the liquid out of the conduit, a second end, movable within the cartridge chamber, and a locking mechanism having a locked configuration and an unlocked configuration, the locking mechanism preventing movement of the second end within the cartridge chamber while in the locked configuration. In general, the methods of filling a syringe during manufacturing may include the steps of injecting a liquid into a cartridge chamber through a conduit of the cartridge and creating a liquid-air interface within the conduit, wherein the liquid-air interface, cooperating with the conduit, prevents movement of the liquid out of the conduit. In general, the methods of using a syringe may include the steps of drawing a second liquid into the second chamber through the proximal outlet by moving the cartridge distally within the first chamber and creating a second liquid-air interface within the conduit, wherein the second liquid-air interface, cooperating with the conduit and locked prefilled cartridge, prevents movement of the first liquid out of the conduit and prevents movement of the second liquid into the conduit. 
     The syringe devices, systems, methods, and any combination thereof described herein provide at least the following advantages. First, the syringe described herein does not rely on an intermediate fluid in the distal chamber to expel the primary fluid (medicine) from the syringe. Not using an intermediate fluid to expel the primary fluid avoids mixture that can occur between the two fluids when the syringe is used in standard fashion. Ensuring that the two fluids are not mixed ensures that the patient receives the correct fluids during treatment. As a result, the syringe is more versatile and reliable. 
     Further, the syringe cartridge includes a conduit designed to keep two fluids in a multi-chamber syringe separate from one another. Keeping the two fluids separate from one another avoids mixing of the two fluids. As noted above, avoiding mixing ensures that the patient receives the correct fluid during treatment, making the syringe more versatile and reliable. 
     Moreover, prefilled saline flush cannot be inadvertently expelled during routine use. This is an advantage, because, when using a syringe, a caretaker will typically eject air from the proximal chamber prior to drawing a medicine into the proximal chamber, as described later. In pushing the air from the proximal chamber in a prior art syringe lacking a locking mechanism, the caretaker could easily cause the plunger to contact the proximal end of the syringe barrel which would prematurely open the valve, thereby accidentally expelling the flushing liquid. Accidental expelling of the intermediate fluid can cause mixing of the two fluids in the syringe. A physical locking mechanism in conjunction with the other syringe features, as described herein, will therefore keep the fluids from mixing, even in similar stresses and situations. As discussed above, avoiding mixing ensures that the syringe is more versatile and reliable. 
     Additionally, the syringe described herein provides the advantage that it allows the filling of the distal chamber from the proximal end during manufacturing. Filling the distal chamber from the proximal end during manufacturing enables complete filling of the distal chamber without trapping any large/non injectable air bubbles. Some prior art syringes that are adapted to deliver multiple fluids require filling procedures that include placing an intermediate sliding stopper into a conventional syringe barrel, then filling the distal chamber from the distal end with a liquid, such as saline, and subsequently installing the plunger assembly. By filling the distal chamber with saline before installing the conventional syringe plunger, the prior art syringe has the disadvantage of reliance on the compressibility of the gas trapped in the distal chamber for a successful installation of the plunger. Therefore, by allowing the filling of the distal chamber without trapping large air bubbles, the syringe described herein can provide more accurate and reliable administration. Further, filling the distal chamber during manufacturing gives the caretaker the ability to fill the proximal chamber with a necessary amount of medicine at the time of administration. Some prior art syringes that are adapted to deliver multiple fluids require that they be provided to a caregiver with prefilled distal (saline) and proximal (medicine) chambers. Such a requirement is not desirable, as many patients require different doses of the same medication. If hospitals were to adopt the use of syringes prefilled with medication, it would cause a tremendous storage and utilization problem. Thus, giving the caretaker the ability to fill the proximal chamber with a necessary amount of medicine at the time of administration advantageously allows patients to receive varying amounts of medications. 
     Additionally, the syringe described herein allows caregivers to follow their standard syringe filling procedures. The most common procedure a clinician uses to fill an empty syringe with medication includes the steps of (1) fitting a syringe with a needle (metal or plastic) to penetrate the seal on a medicine bottle; (2) pulling the handle of the syringe back (distally) to draw air into the syringe of equal or greater volume than the medicine that is to be withdrawn; (3) inserting the air filled syringe with attached needle into the medicine bottle; (4) depressing (pushing proximally) the plunger to inject the air into the medicine bottle; (5) pulling the handle of the syringe back (distally) to draw medicine from the bottle into the syringe; and (6) withdrawing the needle/syringe from the medicine bottle and removing the needle from the syringe. Prior syringes that are adapted to deliver multiple fluids cannot be used in this procedure for at least the reason that during Step 4, after injecting all the air from the proximal chamber of the syringe into the medicine bottle, the plunger will often collide with the internal surface of the inside of the syringe barrel. This collision causes the displaceable valved stopper to open and remain open. Once the valve is open, pulling back on the plunger would cause medicine to flow through the open valve and mix with the contents of distal chamber. Alternatively, if the forward force were continually applied, after the valve was opened, the contents of the distal chamber would flow through the open valve into the medicine bottle. Neither one of these scenarios is acceptable. The syringe described herein, including a physical locking mechanism and separate cartridge (including an inner barrel) which is adapted to use fluid mechanics to keep fluids separate, is ideally suited for a caregiver&#39;s standard filling procedure. 
     A further advantage of the syringe described herein is that it does not limit the volume of medicine that can be filled into the proximal chamber. A disadvantage of some prior syringes that are adapted to deliver multiple fluids by sectioning a standard syringe into two compartments is that the volume of medicine that can be filled into the proximal chamber is limited by the presence of the distal chamber. In general the greater the diameter of the syringe barrel, the less exact a measurement of volume can be made by reading the fluid meniscus against gradations marked on the outside of the syringe. The accuracy required is generally related the total volume of medicine to be administered, the smaller the dose of medicine the more accurate measurement is needed. To solve this problem clinicians use a wide range of syringe sizes depending on the amount of medication to be administered. Syringes from 1 ml to 60 ml are the most commonly used sizes. In the prior art syringes that are adapted to deliver multiple fluids the distal chamber defined by the sliding stopper takes up space within the standard syringe barrel (the effective volume for medication is decreased by the distal chamber volume by about a factor of 2 for a given syringe size) and therefore clinicians would have to use a relatively larger syringe barrel size and therefore less accurate measurements to attempt to administer the same volume of medicine. The syringe described herein includes a separate cartridge that includes the distal chamber, and therefore does not negatively impact the potential size of the proximal chamber and its capability to hold a volume of medicine. 
     The present invention is a two-chambered syringe with three basic components: (i) an outer barrel  10  for holding a liquid  20 , (ii) an inner barrel/first piston  30  for holding a flushing liquid  52 , and (iii) a second piston  60 . See  FIG. 5 . The syringe also includes a latching mechanism for controlling the movement of the second piston  60  in the inner barrel/first piston  30 . See  FIGS. 3 and 4 . 
     The barrels and pistons may be constructed of polypropylene or other similar inert, nonreactive semi-flexible material. Both barrels  10 ,  30  are generally circular cylinders. The inner barrel/first piston  30  acts as both a barrel and a piston. That is, it both holds liquid like a barrel, and may be used as a plunger to expel liquid from the outer barrel  10 . See  FIGS. 5 and 6 . 
     For purposes of this patent, the proximal end of the syringe is the end typically comprising a first conduit  20 , while the distal end is the end of the syringe typically comprising the second piston  60  and a gripping handle  64 . See  FIGS. 1 and 2 . 
     The outer barrel  10  has an outer barrel distal open end  14  adapted for receiving the inner barrel/first piston  30 . See  FIG. 1 . The inner barrel/first piston  30  is slidably contained in outer barrel  10  in a liquid-tight relation, similar to the piston or plunger in syringes common to the art. See  FIGS. 1-6  and  15 . 
     In one embodiment, a proximal end  16  of the outer barrel  10  may comprise an adapter  18 , such as a luer connector device as disclosed in U.S. Pat. No. 4,452,473 or other locking means common in the art. See  FIG. 1 . The adapter  18  allows a connection between the present invention and an intravenous system. An outer barrel open proximal end  22  is at the proximal end  16  of the outer barrel  10  and may contain a first conduit  20 . See  FIG. 1 . The distal end of first conduit  20  is in communication with the proximal end  16  of the outer barrel  10 , providing a passageway for fluid from either the outer barrel  10  or the inner barrel/first piston  30 . See  FIG. 1 . 
     The inner barrel/first piston  30  has an inner barrel/first piston proximal end  40  slidably received within the outer barrel open distal end  14 . See  FIG. 1 . It also includes a hollow projection  42  that extends proximally out of the inner barrel/first piston  30 . See  FIGS. 1 ,  9 - 14 . The hollow projection  42  defines a second conduit  44  through which liquid flows from the inner barrel/first piston  30  to the outer barrel  10 . See FIGS.  1  and  6 - 8 . The hollow projection  42  has a flared tip  48  that secures a first sealing ring  46 , as shown in  FIGS. 7 and 8 . The flared tip  48  may take many different forms, as shown in  FIGS. 9-14 . 
     The first sealing ring  46  comprises a sealing ring conduit  45  through which extends the hollow projection  42 . See  FIGS. 7-14 . The first sealing ring  46  is substantially the same diameter as both the inner barrel/first piston outer wall  32  and the outer barrel inner wall  24 , creating a liquid tight seal between the inner barrel/first piston  30  and the outer barrel  10 . See  FIG. 6 . Thus, the only fluid connection between the inner barrel/first piston  30  and the outer barrel  10  is through the second conduit  44  and the sealing ring conduit  45 . The sealing ring  46  may be constructed of an elastic material such as natural or synthetic rubber. 
     The flushing liquid  52  is inside the inner barrel/first piston  30 . See  FIGS. 5 ,  5 - 6 . The flushing liquid  52  may be a saline solution, or any other suitable solution, such as heparin, when anticoagulation is desired, or antibiotics, when a line infection is being treated. 
     The flushing liquid  52  occupies substantially all of the space defined by the inner barrel/first piston inner wall  50 , and initially extends partially through the second conduit  44  defined by the hollow projection  42 . See  FIG. 6 . Because the flushing liquid  52  only extends partially through the second conduit  44 , the flushing liquid  52  remains isolated from any liquid later drawn into the outer barrel  10 . 
     The second piston  60  is slidably placed within the inner barrel/first piston  30 . See  FIGS. 3-5  and  15 . The second piston  60  comprises a second piston proximal end  66  further comprising a solid projection  70  that fits through an aperture  76  in a second sealing ring  72 , thereby attaching the second piston  60  to the second sealing ring  72 . See  FIGS. 1 ,  3 . The second sealing ring  72  is of substantially equal diameter to the inner barrel/first piston inner wall  50 , and is created from an elastic rubber-like material that provides a liquid-tight seal for the inner barrel/first piston  30 . See  FIG. 3 . Alternatively, this liquid-tight seal may be created by a similar rubber-like sealing material  61  placed around the periphery of the proximal end of the second piston  60 . See  FIG. 17 . The second piston  60  moves in and out of the lumen of inner barrel/first piston  30 , thereby dispensing liquid from or drawing liquid into the inner barrel/first piston  30 . See  FIG. 3 . 
     Extending distally from second piston proximal end  66  is a piston rod  62 . See  FIGS. 3 and 4 . A gripping handle  64  is placed at the most distal end of the second piston  60 . 
     The two-chambered syringe further comprises a latching mechanism that can alternate between an unlocked configuration and a locked configuration. See generally  FIGS. 3-4  and  7 - 8 . In the locked configuration, the second piston  60  is longitudinally locked relative to the inner barrel/first piston  30 . See  FIG. 15(   b ). In this configuration, the second piston  60  will not move longitudinally relative to the inner barrel/first piston  30 . See  FIGS. 5 and 15(   a )- 15 ( e ). However, a longitudinal force applied to the second piston  60  will be transferred proximally and the inner barrel/first piston  30  will move relative to the outer barrel  10 . 
     In the unlocked configuration, the second piston  60  is free to move longitudinally relative to the inner barrel/first piston  30 . See  FIGS. 6 and 15(   f )- 15 ( g ). Thus, the contents of the inner barrel/first piston  30  are ejected through the second conduit  44  when the second piston  60  is depressed. When the second piston  60  is retracted, the inner barrel/first piston  30  will provide sufficient suction to draw in the contents of the outer barrel  10  through the second conduit  44 . 
     In one embodiment, the latching mechanism comprises a projection  68 , extending outward radially from near the second piston proximal end  66 . See  FIGS. 1 and 3 . In this embodiment, the projection is constructed of a polypropylene or other similar inert, nonreactive semi-flexible material the same as or similar to that comprising the barrels and pistons of the syringe. While the radial width of the projection  68  shown in  FIGS. 1 and 3  is small relative to the distance around piston rod  62 , the same principle preventing movement of the piston rod  62  would apply regardless of the radial width or shape of projection  68 . See  FIG. 3 . 
     This projection fits snugly into a groove  34  cut into the inner barrel/first piston inner wall  50 , thereby allowing the second piston  60  to only move according a path of movement defined by groove  34 . See  FIGS. 3 and 6 . 
     The groove  34  includes a longitudinal portion  39  extending longitudinally along the inner barrel/first piston inner wall  50 , ending at the inner barrel/first piston proximal end  40 . See  FIG. 6 . Near the distal end of the inner barrel/first piston  30 , the longitudinal portion  39  makes a substantially right angle and continues circumferentially around the inner barrel/first piston inner wall  50  as a radial portion  37 . See  FIGS. 3 ,  6 , and  8 . In one embodiment, the radial portion  37  of the groove  34  extends less than one half of one revolution of the perimeter around the inner barrel/first piston inner wall  50 . See  FIGS. 7 and 8 . 
     In one embodiment, the groove  34  continues to substantially the distal end of inner barrel/first piston  30 , outlining a track ultimately leading to a projection entry point  36 . See  FIGS. 3 ,  6 ,  7 . The projection entry point  36  serves as an entrance to the groove  34  for the projection  68 , simplifying the assembly process for the syringe and reducing the cost of construction. In the alternate embodiment, shown in  FIG. 8 , the second piston  60  with protrusion  68  would be installed into the inner barrel by applying sufficient pressure to temporarily flex the plastic allowing a press-fit construction. See  FIGS. 3 and 8 . 
     When the second piston  60  is in the fully extended position, the projection  68  will lie in the radial portion  37  of the groove  34 . See  FIG. 5 . From this position, the second piston  60  may be axially rotated, and the projection  68  will slide along the radial portion  37  of the groove  34 . Additionally, the second piston  60  and the inner barrel/first piston  30  are longitudinally locked together, and in this fixed position the two components function collectively as one piston relative to the outer barrel  10 . See  FIGS. 5 and 15(   a )- 15 ( e ). The syringe may then be used in the same manner as a conventional one-chambered syringe, as described later herein. 
     In yet another embodiment, instead of comprising a track defined by an indented groove on the inner barrel/first piston  30 , the syringe comprises a track defined by a raised track outlining the same path previously defined by the groove  34 . See  FIGS. 2-4 , and  16 . Correspondingly, the second piston  60  comprises an indentation  69  instead of the projection  68 . See  FIGS. 2-4 . In this configuration raised track  35  fits snugly into indentation  69 , thus defining a track for the second piston  60  to follow when in the unlocked position. See  FIGS. 2 and 4 . In this embodiment, the track need not extend longitudinally the entire length of the inner barrel to accomplish the locking feature. 
     To ensure the saline does not leak backwards out of the flush chamber, the second piston  60  may additionally comprise breakaway guard  75 , which provides a cover over the indentation  69 . The breakaway guard  75  may be a layer of plastic that is capable of being punctured by raised track  35  when the operator applies sufficient force. The operator of the syringe will feel the resistance and subsequent release as the breakaway guard is punctured. See  FIGS. 2 ,  4 , and  16 . The need for this guard may be circumvented by making a rear lip  31  large enough to prevent backward flow of the flush solution. The lip  31  of the inner barrel enables a unidirectional press fit construction (due to the sloped angle of the lip  31 ) in which the second plunger may be easily slid into the inner barrel, but cannot be easily removed. Thus, the second piston  60  is effectively trapped between the raised track  35  and the lip  31  thus preventing the second piston from moving longitudinally with respect to the inner barrel/first piston when the second piston is in the locked configuration. See  FIGS. 15(   a )- 15 ( f ). 
     Other latching mechanisms may be used, some of which are described further below with respect to  FIGS. 22-30 . For purposes of this patent, “latching mechanism” refers generically to any structure that can lock and unlock the inner barrel/first piston  30  relative to the second piston  60 . See  FIG. 1 . 
     One advantage of applicant&#39;s device is that the syringe may function as a traditional syringe, independent of the internal flush chamber in the inner barrel/first piston  30 . See  FIGS. 15(   b )- 15 ( e ). Additionally, this syringe may be used to dispense a flush solution without filling the outer chamber with a second liquid or gas. 
     In operation, the syringe will typically first be in the locked position so medicine withdrawn from a bottle fills the outer chamber  10 . See  FIG. 15(   a )-( d ). When medication is administered directly to a vein, a clinician using a traditional syringe will often confirm that a vein has been pierced by drawing a small amount of blood into the syringe, prior to injection of the medication. This device allows for this normal operation to be performed when the device is in the locked configuration. See  FIGS. 15(   b )- 15 ( c ). 
     Because the flushing liquid  52  does not extend through the second conduit  44 , it will not mix with fluid drawn into the outer chamber  10 . In a separate embodiment (shown in  FIG. 5 ), flushing liquid  52  extends only partially through the second conduit  44 , but not enough to mix with fluid drawn into outer chamber  10 . The two fluids will not come in contact with each other due to basic fluid mechanics. That is, surface tension of the fluid drawn into the outer chamber  10  prevents it from entering the second conduit  44 . The flushing liquid  52  does not move through the second conduit because as it completely fills the inner barrel/first piston  30 , the negative pressure created inside the outer barrel  10  when fluid is drawn in, is not great enough to displace the flushing liquid  52  from the inner barrel/first piston  30 . 
     Next, while the syringe is still in the locked configuration, the contents of the outer barrel  10  may be delivered to a patient by depressing the second piston  60 . See  FIGS. 15(   e )- 15 ( f ). After injecting the medication, the operator may axially rotate the second piston  60  until the longitudinal portion  39  of either the groove  34  or the track  35  defines the path of movement. See  FIGS. 6 and 15(   f )- 15 ( g ). In the embodiments shown in  FIG. 2  and  FIG. 8  the clinician may confirm this alignment upon feeling that the axial rotation is halted by forward projection  67 . In the embodiments shown in  FIG. 1  and  FIG. 3 , a clinician may confirm this alignment by rotating the second piston  60  until an indicating mark on second piston  60  is longitudinally in line with a mark on the inner barrel/first piston  30  or the outer barrel  10 , as described below with respect to  FIG. 29 . From this position, the second piston  60  may be longitudinally moved down the length of the inner barrel/first piston  30 , thereby emptying the contents of the inner barrel/first piston  30  into the outer barrel  10  and then into the catheter. See  FIGS. 6 and 15(   f )- 15 ( g ). 
     In the embodiment shown in  FIG. 2 , after the outer barrel is dispensed the second plunger may be rotated axially until the forward protrusion  67  meets the raised track  35 , impeding further rotation. From this position, proper alignment of the track and indentation is assured because the forward protrusion  67  is adjacent to the indentation  69 . Next, the operator would depress the second piston  60  a second time, emptying the contents of the inner barrel through second conduit  44 . See  FIGS. 6 and 15(   f )- 15 ( g ). Preferably, at this point in the process, the medication from the outer barrel  10  is already expelled into the intravenous system, and thus the contents of the inner barrel/first piston  30  may be used to flush any remaining medication into the patient. 
     As shown in  FIG. 18 , and as described above, the syringe described herein includes an outer barrel  10  and an inner barrel  30  that is movable within the outer barrel  10 . The inner barrel  30  has a first end which defines a conduit  44  that is in fluid communication with the outer barrel  10 . The inner barrel  30  is movable within the outer barrel  10  such that the inner barrel  30  can act as a piston. A second piston  60  can be located within the inner barrel  30  so as to draw or flush fluid in or out of the inner barrel  30 . The inner barrel  30 , piston  60 , and conduit  44  can together be called a “cartridge” of the syringe. The syringe may further include a locking mechanism on the inner barrel  30  (e.g., having groove  34  as shown in  FIG. 18 ). The locking mechanism prevents movement of the piston  60  within the inner barrel  30  while in the locked configuration. The outer barrel  10  defines a proximal chamber and the inner barrel  30  defines a distal chamber. The proximal chamber will typically hold a medicine, while the distal chamber will typically hold a prefilled flushing liquid such as saline. 
     The inner barrel  30  is thus adapted to hold a liquid  52  in the distal chamber. A liquid-air interface  1801  is created within the conduit  44 , which, along with the fixed volume created by the locking mechanism, prevents movement of the liquid out of the conduit  44 . Further, the liquid  52  and the liquid of the outer barrel  10  will not come in contact with each other due to basic fluid mechanics. That is, the physical properties of the conduit  44  and the surface tension of the fluid drawn into the outer barrel  10  prevent the fluid from entering the second conduit  44 , while the physical properties of the conduit  44  and the surface tension of the liquid  52  within the inner barrel  30  prevent the liquid  52  from exiting the inner barrel  30  through the conduit  44 . Moreover, the flushing liquid  52  does not move through the conduit  44  because it completely fills the inner barrel  30 , which is locked with a fixed volume. As a result, the negative pressure created inside the outer barrel  10  when fluid is drawn in is not great enough to displace the liquid  52  from the inner barrel. 
     After a clinician fills the outer barrel  10  with a fluid or medicine, larger air bubbles in the inner barrel  10  are removed by a standard process of tapping the side of the syringe to cause the air bubbles to coalesce into one large bubble which is expelled by orienting the syringe such that the air bubble is near the syringe tip and then the plunger is depressed to expel this air. However, the air that was originally in the conduit  44  as a result of filling of the inner barrel  30  remains in place due to the force of the surface tension and the fixed volume of the inner barrel  30 . The standard process of removing the larger air bubbles does not dislodge the retained air within the conduit. When the outer barrel  10  is filled with a fluid, the distal end of the proximal chamber includes a second liquid-air interface  1802 , forming a bubble or air pocket  1803  that plugs the conduit  44 . The gas-filled space defined by the inner wall of the conduit  44  and the two fluid interfaces act as a plug to keep the two fluids separate until the inner barrel  30  is unlocked. 
     Because the liquid  52  does not extend through or extends only partially through the second conduit  44 , it will not mix with fluid drawn into the outer chamber  10 . That is, the bubble  1803  acts as a barrier between the fluids in the two chambers. The bubble is preferably small, having a volume of about 0.01 ml to 0.1 ml. In some embodiments, the volume is about 0.024 ml. This bubble is of substantially similar size to, or smaller than, micro bubbles which routinely form in conventional syringes used to administer medicine to patients. In fact, most conventional prefilled saline syringes contain incidental gas bubbles larger than about 0.024 ml. Clinically, small bubbles are unavoidable and completely harmless. Once injected into the patient they are broken up in the capillary bed and absorbed from the circulation without any effects to the patient. A large bubble is commonly defined as a bubble having a volume greater than 50 ml. A large bubble can behave differently from a small bubble and can be dangerous to a patient if injected into their blood stream. In contrast, the small bubble formed in the conduit  44  is not dangerous to a patient and can be beneficial for creating the valve feature, as described herein. 
     The bubble  1803  is held into position by forces that include surface tension, buoyancy, gravity, resistance to flow, the shape of the conduit, and the fixed volume of the cartridge chamber. Thus, the force required to dislodge the bubble depends on the dimensions of the conduit  44 , the fluid viscosity and the compliance of the saline chamber. It also depends on the surface tension, contact angle of the fluid, wettability of the surface, and shape of the conduit  44 .  FIGS. 18A and 18B  represent the balance of forces in the conduit  44  when a liquid-air interface  1810  is formed between air  1812  (e.g. the bubble  1803 ) and liquid  1814 . The buoyancy force, Fb, is directed upward with respect to the Earth&#39;s surface. The surface tension, Fγ, is directed along the contact angle at the edge of the liquid-air interface  1810 . The gravity force, Fg, is directed downward with respect to the Earth&#39;s surface. The pressure in the fluid, P 1 , is applied equally and perpendicular to all surfaces, including the liquid-air interface  1810  and the inner walls of the conduit  44 . Likewise, the pressure in the air, P 2 , is applied equally to all surfaces, including the liquid-air interface  1810  and the inner walls of the conduit  44 . 
     The syringe system is nearly rigid; the volume contained within the inner barrel  30  of the syringe is constant if the second piston  60  is fixed in position by the locking mechanism. As a result, the bubble  1803  in the conduit  44  will remain in position. When the locking mechanism is unlocked, however, and the second piston  60  is depressed, the bubble&#39;s rear surface is disrupted by the forward flow of liquid  52 , causing the bubble to be propelled forward into the outer chamber  10  and/or out through the proximal outlet of the outer chamber  10 . 
     The stability of the bubble position is related to the force of attachment to the wall of the conduit  44 . This can be measured as a pressure needed to detach and move the bubble. The pressure needed to move the bubble out of the conduit is a function of the following variables: dimensions of the conduit  44 , the fluid viscosity, the compliance of the inner barrel  30 , the surface tension, contact angle of the fluid and wettability of the surface. These dependencies are detailed below. 
     In some embodiments, the conduit  44  between the proximal and distal chambers has a cylindrical shape, with radius R and length L. When the volume of the bubble is greater than that of a sphere equal to 4/3πR 3 , the bubble elongates in the conduit into a cigar shape, as shown in  FIGS. 18 and 21E . The external force holding the bubble stationary depends on the bubble half length, H and the bubble radius, R in the following way: 
     
       
      
       F∝H 
       2 
       R  
      
     
     This equation assumes that 
     
       
         
           
             
               
                 H 
                 2 
               
               
                 R 
                 2 
               
             
             &gt; 
             1 
           
         
       
     
     Thus, the length of the bubble 2H is maximum when 2H is equal to L, and the resistive force of the bubble will increase more when length is increased than when radius is increased. Accordingly, the dimensions of the conduit can be chosen such that the bubble keeps the fluid in the first chamber and the fluid in the second chamber apart. In one particular embodiment, the conduit has a diameter of about 0.069 inches. In one particular embodiment, the conduit has a length of about 0.4 inches. 
     Viscosity is a fluid property that describes it&#39;s resistance to flow. It is also known as the ‘thicknesses’ of the fluid. A higher force will be required to attain the same fluid velocity for a higher viscosity fluid. The resistance of the bubble  1803  is slightly increased with increased viscosity. Most medicines will not have a viscosity difference from saline of a magnitude that would significantly affect the resistance. 
     The position of the bubble  1803  will also be a function of the compliance of the distal chamber (of inner barrel  30 ), which can be affected by the relative displacement of the locking mechanism while in the locked configuration. If the locking mechanism is not designed or built with the appropriate rigidity, excess motion (wiggle of the locking mechanism) is capable of producing a change in volume of the distal chamber, this can cause the bubble to be dislodged from the conduit  44 . If the bubble can be dislodged from the conduit  44 , then a small amount of mixing of the saline with the medicine may occur. The maximum displacement can be determined by the following equation: 
     
       
         
           
             
               maximum 
                
               
                   
               
                
               displacement 
             
             = 
             
               
                 R 
                 
                   R 
                   distal 
                 
               
                
               L 
             
           
         
       
     
     where R proximal  is the radius of the proximal chamber and R, L as defined earlier. 
     The compliance of the proximal chamber (of inner barrel  30 ) will also be influenced by the compliance of the second end of the cartridge, for example by the compliance of a rubber plunger, C plunger  if the rubber plunger comprises part of the conduit. If the rubber plunger compresses significantly under pressure, it can reduce the proximal chamber volume and dislodge the bubble. The compressive volume change over the expected range of pressure should be less than an amount equal to the current conduit  44  volume. 
       Δ V=ΔP·C   plunger   &lt;πR   2   L.  
 
     The surface tension of a fluid is a measure of how readily the fluid surface is attracted to another surface. It is a property of a fluid that is related to the surface free energy, and affects the contact angle. The force or pressure needed to dislodge the bubble  1803  from the conduit  44  is increased with increasing surface tension. 
     Contact angle is classically measured by placing a drop on a horizontal surface and measuring the angle of the drop edge. The contact angle is determined from the position of the interfaces between solid, liquid and gas at equilibrium. If a droplet of water spreads on a solid surface, the contact angle is very small and the surface is considered hydrophilic. If the droplet rounds up, the contact angle is greater than 90°, and the surface is hydrophobic. The contact angle of the fluid used is preferably less than 90° to maintain the bubble seal between the two chambers. 
     The wettability of a surface is directly related to the contact angle, and is another indication of the balance of forces within the liquid that are cohesive, and those between the liquid and the surface that are adhesive. A hydrophilic contact angle is indicative of a strong attraction between the fluid and the surface, a surface that is considered wetting. 
     Although the conduit  44  is described above as being cylindrical, the shape of the conduit  44  can be varied to optimize the ability of the conduit  44  to keep air in the conduit  44 , and thus to keep the liquids in the proximal and distal chamber separate. For example, as shown in  FIG. 18C , the conduit can be cylindrical and include rounded indentations  1812  into the inner circumference of the conduit  44 . As shown in  FIG. 18D , the conduit  44  can be cylindrical and include sharp indentations  1814  into the inner circumference of the conduit  44 . As shown in  FIG. 18E , the conduit  44  can include two opposing straight-walled conical portions  1816   a ,  1816   b . A straight cylindrical portion  1818  can be located between the two opposing conical portions  1816   a ,  1816   b . As shown in  FIG. 18F , the conduit  44  can include two opposing curved-walled conical portions  1820   a ,  1820   b . A straight cylindrical portion  1822  can be located between the two opposing conical portions  1820   a ,  1820   d.    
     In some embodiments, as shown in  FIG. 19 , the syringe further includes a removable end cap  1900  coupled to the outlet  1901  of the outer barrel  10  and to the conduit or conduit  44  of the cartridge. As shown, the end cap includes an end surface  1902  within the conduit  44 . The end surface  1902 , cooperating with the conduit  44 , prevents movement of the liquid  52  out of the conduit  44  and out of the inner barrel  30 . As shown, the syringe includes a gas bubble  1803  disposed in the conduit  44  between the end surface  1902  of the end cap and the liquid-air interface  1801 . In some embodiments, the syringe is designed to be filled with saline by the manufacturer, capped with an end cap  1900  and shipped to the customer. The customer will then remove the cap, and fill the proximal chamber of the outer barrel  10  with medicine as desired. 
     As described above, in some embodiments, the syringe is designed to be filled with a flushing liquid, such as saline, by the manufacturer of the syringe. In general, as shown in  FIGS. 20A and 20B  or  20 C and  20 D, a method of filling a syringe cartridge includes the steps of injecting a liquid  52  into the inner barrel  30  through conduit  44  and creating a liquid-air interface  2001  within the conduit  44 . The liquid-air interface is maintained during packaging and shipping. The inner barrel chamber is filled up to the distal end of the conduit  44  or beyond, such that the inner barrel  30  is completely filled with fluid and a small volume of air remains in the conduit  44 . 
     The liquid  52  may be injected into the cartridge chamber via a needle or nozzle  2002  positioned within the outlet of the outer barrel  10  and the conduit  44  of the cartridge. In some embodiments, the inner barrel  30  can hold about 1 to 10 ml of liquid, such as 2 to 3 ml of liquid. Ideally the inner barrel  30  holds the smallest volume of fluid, such as saline, that can still effectively flush an intravenous catheter line, for example. In one particular embodiment, the cartridge is prefilled with 2.5 ml of saline. In some embodiments, the syringe may be offered in a complete line of syringes of different volumes. For example, the range of syringe sizes may include syringes that are capable of holding 1, 3, 6, 12, 15, 30, and/or 60 ml of an injectable liquid such as medicine in the proximal chamber. Each syringe size may have a common flush size in the distal chamber, for example 2.5 ml. Alternatively, each syringe size may include a distal chamber having a different volume. 
     As shown in  FIGS. 20A and 20B , filling the distal chamber of the inner barrel  30  includes injecting a liquid, such as a flushing liquid, into the inner barrel  30  through the outlet of the outer barrel  10  and the conduit  44  of the inner barrel  30 . The inner barrel is positioned at the proximal end of the outer barrel  10 . As shown in  FIG. 20A , the proximal end of the second piston  60  is positioned at the proximal end of the inner barrel  30 . As the liquid  52  is injected into the inner barrel  30  of the cartridge, the volume of liquid  52  increases. The liquid pushes the proximal end  2003  of the piston  60  in the distal direction such that the distal chamber of the inner barrel  30  expands as it is filled with liquid  52 , as shown in  FIG. 20B . In some embodiments, the piston  60  may be pulled distally to assist in filling the cartridge chamber. In some embodiments, the inner barrel  30  will be filled until the locking mechanism is engaged and the proximal end of the piston  60  is locked with respect to the inner barrel  30 . 
     As shown in  FIGS. 20C and 20D , in some embodiments, the inner barrel  30  is filled independently from the outer barrel (not shown). The liquid  52  may be injected into the cartridge chamber via a needle or nozzle  2102  positioned within the conduit  44 . Once filled, the inner barrel  30  may then be placed within the outer barrel, such that the inner barrel  30  is movable within the outer barrel. As shown in  FIG. 20C , the proximal end  2103  of the piston  60  is positioned at the distal end of the inner barrel  30 . As the liquid  52  is injected into the inner barrel  30  of the cartridge, the liquid  52  fills the volume of the chamber within the inner barrel, as shown in  FIG. 20D . In this embodiment, the needle does not completely occlude the conduit  44  such that air is allowed to escape as the distal chamber of the inner barrel  30  is filled with fluid. Alternatively, the liquid  52  can move the proximal end  2103  of the cartridge plunger distally to expand the distal chamber as the inner barrel  30  is filled, as described above. These filling techniques could also be utilized when the cartridge is disposed (to start) within the outer barrel. 
     In an alternative embodiment, the distal chamber may be filled through the distal end of the inner barrel  30 , which may be prior to inserting the second piston  60  into the inner barrel. In this embodiment, the conduit  44  may be temporarily occluded while the distal chamber is filled through the open distal end of the inner barrel  30 . Once the distal chamber is filled, the piston  60  may be positioned within the inner barrel  30 , and in some embodiments locked in place with respect to the inner barrel  30  by the locking mechanism. Once the distal chamber is closed off by the piston  60 , the occlusion from the conduit  44  such that air trapped in the distal chamber during the filling and positioning of the piston  60  may escape. 
     Once a caregiver receives a syringe having a prefilled distal chamber, filling the proximal chamber (of the outer barrel  10 ) follows the standard operation for filling a syringe, which includes the steps of (1) fitting a syringe with a needle (metal or plastic) to penetrate the seal on a medicine bottle; (2) pulling the handle of the syringe back (distally) to draw air into the syringe of equal or greater volume than the medicine that is to be withdrawn; (3) inserting the air filled syringe with attached needle into the medicine bottle; (4) depressing (pushing proximally) the plunger to inject the air into the medicine bottle; (5) pulling the handle of the syringe back (distally) to draw medicine from the bottle into the syringe; and (6) withdrawing the needle/syringe from the medicine bottle and removing the needle from the syringe. 
     The syringe may then be connected to the patient or patient line at a luer port for injection of the medicine. The handle is depressed to inject the medicine, then the cartridge is unlocked and the handle depressed further to inject the saline. The syringe is removed and discarded. 
     In general, a method of using a syringe includes the steps of drawing a second liquid (such as medicine) into the outer barrel  10  through the proximal outlet by moving the inner barrel  30  distally within the outer barrel  10  and creating a second liquid-air interface within the conduit. As described above, the second liquid-air interface and the first liquid-air interface define a bubble which, cooperating with the conduit  44 , prevent movement of the first liquid out of the conduit  44  and prevent movement of the second liquid into the conduit  44 . In some embodiments, the method further includes the steps of (a) expelling the second liquid (such as medicine) from the outer barrel  10  through the proximal outlet by moving the inner barrel  30  proximally within the outer barrel  10 , (b) releasing the locking mechanism from a locked configuration to an unlocked configuration to allow movement of the piston  60  within the inner barrel  30 , and (c) expelling the first liquid (such as saline) from the inner barrel of the cartridge through the proximal outlet by moving the piston  60  proximally within the inner barrel  30 . In some embodiments, the step (c) of expelling the first liquid includes expelling the gas bubble from the conduit, along with the first liquid, through the proximal outlet. As described above, the gas bubble within the conduit disposed between the first liquid-gas interface and the second liquid-gas interface is a small bubble, safe for injection into a patient. 
     The different stages of use described above will have varying effects on the bubble  1803  formed in the conduit  44 .  FIG. 21A  represents the shape of the air pocket  2210  in the conduit  44  during shipping. As shown, only one liquid-air interface  1801  will be present, as no medicine or additional fluid will have been added to the proximal chamber. 
       FIG. 21B  represents the shape of the air pocket  2210  in the conduit  44  when the proximal chamber is filled with air prior to ejecting the air into the bottle containing fluid (for later uptake of fluid into the proximal chamber). Although there is a slight negative pressure created in the proximal chamber as air is draw in from the open atmosphere, the effect is negligible, and the shape of the liquid-air interface  1801  remains substantially unchanged. 
       FIG. 21C  represents the shape of the air pocket  2210  during injection of air into the bottle. The arrow  2112  shows the net direction of force on the air pocket  2210  resulting from pressure generated as the proximal chamber is depressed. There is a distal deformation of the liquid-air interface  1801  resulting from the force. 
       FIG. 21D  represents the shape of the air pocket  2210  during withdrawal of medication into the proximal chamber from the bottle. The arrow  2114  shows the net direction of force on the air pocket  2210 . The negative pressure in the proximal chamber causes a proximal deformation of the liquid-air interface  1801  without disrupting the liquid-air interface  1801 . 
       FIG. 21E  represents the air bubble  1803  formed after fluid has been filled into the proximal chamber. A net zero force is on the bubble  1803  such that the liquid-air interface  1802  formed near the proximal chamber has the same curvature in the opposite direction as the liquid-air interface  1801  formed near the distal chamber. 
       FIG. 21F  represents the shape of the air bubble  1803  as the fluid from the proximal chamber is administered to the patient. As the fluid is released, the bubble  1803  experiences a net distal force as a result of the pressure in the proximal chamber, represented by the arrow  2116 . As a result, both the liquid-air interface  1802  near the proximal chamber and the liquid-air interface  1801  near the distal chamber move distally. 
       FIG. 21G  represents the shape of the air bubble  1803  after the contents have been dispensed from the proximal chamber. Due to the surface tension of the bubble, a small amount of liquid remains in conduit on the proximal side of the bubble  1803 . As a result, the liquid-air interface  1802  on the proximal side remains intact. 
       FIG. 21H  represents the shape of the bubble  1803  as the fluid  52  in the distal chamber is expelled. The net direction of force on the bubble  1803  is proximal, as shown by arrow  2118 . As the solution  52  is discharged, there is initial proximal deformation of both the liquid-air interface  1802  on the proximal side as well as the liquid-air interface  1801  on the distal side. The force  2118  is great enough in the proximal direction that the bubble  1803  will eventually be displaced. 
     As discussed above, the syringe can include a locking mechanism to prevent movement of the piston  60  within the inner chamber  30 . In some embodiments, as shown in  FIGS. 22A and 22B , the syringe includes a locking mechanism  2200  having a flexible arm  2207  with a first end connected to the piston  60  and a free end having a tab  2210 , and a groove  2211  configured to receive the tab  2210 . In some embodiments, the flexible arm  2207  is coupled to the proximal end of the piston  60  while the groove is defined by the inner barrel  30 . In some embodiments, the flexible arm  2207  of the locking mechanism may be coupled to the distal handle of the piston  60  as shown in  FIG. 22A . 
     The geometry of the groove  2211  is such that it receives the tab  2210  and holds the tab  2210  in place, preventing movement of the piston  60  with respect to the distal chamber. The geometry of the groove  2211  is such that the tab  2210  can be moved in and out of the groove  2211  in the circumferential direction, i.e. by rotating the piston  60  with respect to the cartridge chamber. The tab  2210  cannot be moved in and out of the groove  2211  in the axial direction (i.e. distally or proximally). Once the tab  2210  is rotated out of the groove  2211 , however, the locking mechanism is in the unlocked configuration, and the tab  2210  may be moved distally or proximally with respect to the groove, allowing the piston  60  to be moved distally or proximally with respect to the inner barrel  30 . 
     The flexible arm  2207  is configured such that it has an equilibrium configuration and a bent configuration. In the equilibrium configuration, the tab  2210  extends beyond the outer circumferential surface of the piston  60 . In the bent configuration, the free end of the flexible arm  2207  is bent inward such that the tab  2210  is within or flush with the outer surface of the piston  60 . When the flexible arm  2207  is in the bent configuration, the piston  60  can move with respect to the inner barrel  30 . When the flexible arm  2207  is in the equilibrium configuration, the tab is able to extend beyond the outer circumferential surface of the piston  60 . It is in the equilibrium configuration that the tab  2210  will be received by the groove  2211  and that the locking mechanism  2200  is in the locked configuration. The flexible arm  2207  is biased toward the equilibrium configuration such that once the tab  2210  reaches a groove  2211 , the tab  2210  will spring into the groove  2211 , thereby locking the locking mechanism  2200 . To release the tab  2210  from the groove  2211 , the tab  2210  is rotated out of the groove  2211 . 
     As shown in  FIG. 23 , the tab  2210  includes a single ramped surface  2300 . As the piston  60  is rotated counter-clockwise (toward the top of the Figure), for example, the ramped surface  2300  interacts with the edge of the groove  2211  such that the groove pushes the tab down along the ramped surface  2300 , allowing the flexible arm  2207  to transition from the equilibrium configuration to the bent configuration. Once the piston  60  is rotated sufficiently such that the tab  2210  is released from the groove  2211 , the locking mechanism become unlocked, and the piston  60  may be moved proximally or proximally with respect to the inner chamber  30 . In some embodiments, as shown in  FIGS. 22A and 22B , the tab  2210  includes two ramped surfaces. Although shown as distinct ramped surfaces, the surfaces can be more continuous such that the tab is approximately semi-circular in shape. As a result of the ramped surfaces, the tab as shown in  FIGS. 22A and 22B  may be rotated in both the clockwise direction or in the counter-clockwise direction to release the locking mechanism. As shown in  FIG. 23 , the tab  2210  includes a single ramped surface  2300  and is therefore substantially trapezoidal shaped. The tab  2210  may include any suitable number of ramped surfaces and have any suitable geometry. 
     In some embodiments, as shown in  FIG. 24 , the syringe further includes an additional adjacent groove  2401  adjacent to the locking mechanism  2200 . The adjacent groove  2401  is configured to receive the tab  2210  once it is rotated out of groove  2211  and when the locking mechanism  2200  is in the unlocked configuration. When the tab  2210  springs from the bent configuration back into a partial equilibrium configuration, it will be received by the adjacent groove  2401 . This will signify that the locking mechanism  2210  is unlocked and that the piston  60  can be moved with respect to the inner barrel. Furthermore, when the tab  2210  is received by the adjacent groove  2401 , the adjacent groove  2401  can give a tactile or audible signal to the user confirming that the release of the locking mechanism  2200  is complete. The adjacent grooves are located on the inner surface of the inner barrel  30 . The adjacent grooves may include a tapered depth such that the adjacent groove permits movement (in the proximal direction) of the piston  60  with respect to the inner barrel  30 . The tapered depth may function to slowly transition the flexible arm  2207  from the equilibrium configuration to the bent configuration as the piston  60  is moved proximally with respect to the inner barrel  30 . 
     In some embodiments, the locking mechanism  2200  may include two grooves  2210  and two flexible arms  2207  on opposite sides of the inner barrel  30  from one another. In this embodiment, the syringe may include two adjacent grooves  2401  on opposite side from one another and about 90 degrees from the grooves  2211 . The syringe may alternatively include any suitable number of locking mechanisms and adjacent grooves. 
     In some embodiments, as shown in  FIG. 25 , the syringe further includes an additional groove  2500  positioned on the inner surface of the inner barrel  30 , proximal to the locking mechanism  2200 . The groove  2500  is configured to receive but never release the tab  2210  as the proximal end of the piston  60  reaches the proximal end of the inner barrel  30 . In some embodiments, the syringe may include a single additional groove  2500  or multiple additional grooves  2500  distributed around the circumference of the inner barrel  30 . Alternatively, a single additional groove  2500  may extend all the way around the circumference of the inner barrel  30 , as shown in  FIG. 25 . The geometry of the additional groove  2500  may be configured such that once the additional groove  2500  receives the tab  2210 , the tab  2210  cannot be released from the additional groove  2500 . This prevents the withdrawal of the piston  60  once the flush liquid has been expelled from the inner barrel  30  and prevents reuse of the syringe which could be harmful to patients. 
     In some embodiments, as shown in  FIGS. 26A and 26B , the syringe further includes a first ridge  2600  on the inner surface of the inner barrel  30 . The ridge is configured to prevent the withdrawal of the piston  60  from the inner barrel  30  in the distal direction when the locking mechanism  2200  is in the unlocked configuration. In some embodiments, the syringe further includes a second ridge  2601  on the outer surface of the proximal end of the piston  60 . The first and second ridges are configured such that the first ridge  2600  prevents movement of the second ridge  2601  in the distal direction, thereby preventing the withdrawal of the second end from the cartridge when in the unlocked position. The ridges may extend around the circumference of the syringe, as shown, or may only partially extend around the circumference of the syringe. Alternatively, the syringe may include a series of ridges distributed around the circumference of the syringe. 
     The syringe as described above may be configured such that the inner barrel  30  and the piston  60  are first moved together proximally within the outer barrel  10  to expel the liquid from the proximal chamber, such as medicine. Once the liquid is expelled from the proximal chamber, and the cartridge is positioned toward the proximal end of the outer barrel  10 , the locking mechanism  2200  may be released by rotating the piston  60 . While the piston  60  is rotated, the inner barrel  30  remains fixed with respect to the outer barrel  10 . The inner barrel  30  may be fixed with respect to the outer barrel  10  in one of several variations. In a first variation, as shown in  FIG. 27 , inner barrel  30  is movable within outer barrel  10 . As shown, outer barrel  10  has a non-circular cross section, such as an oval cross section. The inner barrel  30  of cartridge includes a proximal end  2701  having an outer diameter that is also non-circular (e.g. oval). As such, the inner barrel  30  is movable in the distal and proximal direction within the outer barrel  10 . The inner barrel  30  cannot, however, rotate within the outer barrel  10  due to the non-circular cross sections. The inner diameter of the inner barrel  30  can have a circular cross section. The piston  60  and sealing ring  72  also have circular cross sections such that the piston  60  can be placed within the inner barrel  30  and is movable in the distal and proximal directions within the inner barrel  30 . Further, the piston  60  has a circular cross-section and can therefore rotate within the inner barrel  30  due to the circular cross sections. Thus, the piston  60  can therefore be rotated within the inner barrel to unlock the locking mechanism  2200 . 
     In a second variation, as shown in  FIG. 28 , the syringe includes a male/female locking mechanism comprising male portion  2800  and female portion  2801 . Once the inner barrel  30  is moved proximally within the outer barrel  10  to expel the liquid from the proximal chamber and the inner barrel  30  is positioned toward the proximal end of the outer barrel  10 , the male portion  2800  (coupled to the proximal  2803  of the cartridge) is moved into and received by female portion  2801  (coupled to the proximal end of the outer barrel  10 ). Once the male portion is fitted within the female portion, the male/female locking mechanism prevents rotation of the first end  2803  of the cartridge within the outer barrel  10 . Therefore, the locking mechanism  2200  of the inner barrel  30  may be released by rotating the piston  60  with respect to the inner barrel  30  while the inner barrel  30  remains fixed with respect to the outer barrel  10 . 
     In a third variation, the syringe may include a screw mechanism, such as a luer lock, such that the inner barrel  30  is screwed into the proximal end of the outer barrel  10  and locked into place to prevent further rotation. 
     In a fourth variation, there may be sufficient friction between outer surface of the inner barrel  30  and the inner surface of the outer barrel  10  such that as the piston  60  is rotated within the inner barrel  30 , the inner barrel  30  remains fixed. This may be accomplished by having the inner barrel include a rubber stopper  72  (as shown in  FIG. 1 ). The friction between the rubber stopper  72  and the inner surface of the outer barrel  10  can be sufficiently greater than the friction between the piston  72  and the inner barrel  30 , such that as the piston  60  is rotated, the inner barrel  30  will not rotate with respect to the outer barrel  10 . In such an embodiment the friction between the outer surface of the inner barrel  30  and the inner surface of the outer barrel  10  must remain appropriate for longitudinal movement of the cartridge. 
     In some embodiments, the syringe may further include lock state indicia that aid a user of the syringe by signifying when the locking mechanism  2200  is in the locked configuration and/or when the locking mechanism  2200  is in an unlocked configuration. The syringe may also bear warning not to prematurely rotate the piston  60  prior to the desired time of expelling the flush liquid, and/or any other suitable indication or warning. The lock state indicia may be printed onto a surface of the syringe or may be printed on a label coupled to the syringe. In the case of a label coupled to the syringe, the outer surface of the syringe may include a groove or recess sized to receive the label. As shown in  FIG. 29 , the syringe may include lock state indicia  2901  and  2902  that indicate when the locking mechanism is in the locked configuration and the unlocked configuration. As shown, the indicia  2901  (coupled to the inner barrel  30 ) and  2902  (coupled to the piston  60 ) do not line up when the locking mechanism  2200  is locked. As the piston  60  is rotated and the tab  2210  is released from the groove  2211 , indicium  2902  will be moved to line up with indicium  2901  indicating that the locking mechanism is unlocked. As shown, the lock state indicia may be lines or other suitable symbols. The lock state indicia may alternatively include characters or words. For example, indicium  2902  may be the word “LOCKED” while indicium  2901  may be the prefix “UN” such that when the locking mechanism is unlocked and indicium  2901  is aligned with indicium  2902 , together they spell the word “UNLOCKED”. 
     In some embodiments, as shown in  FIG. 30 , groove  2211  coupled to the inner surface of the inner barrel  30  of the cartridge may include a ridge  3001 . The ridge  3001  is sized and configured to prevent the tab (not shown) of the locking mechanism from reentering the groove. For example, as shown, tab  2210  would be rotated out of groove  3000  by rotating the tab  2210  in the counter-clockwise direction (toward the bottom of the Figure). If one were to continue to rotate the tab  2210  in the counter-clockwise direction, the tab  2210  could reenter the groove  2211  from the opposite side, thereby returning the locking mechanism  2200  to the locked configuration. In some instances, this may not be desirable and therefore ridge  3001  may function to stop the tab  2210  from reentering the groove  2211  from that side, thereby maintaining the locking mechanism  2200  in the unlocked configuration. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments, which are presented for purposes of illustration and not of limitation. 
     The examples and illustrations included herein show, by way of illustration and not of limitation, specific embodiments in which the subject matter may be practiced. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. Such embodiments of the inventive subject matter may be referred to herein individually or collectively by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept, if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description.