Abstract:
A disposable needleless injection device includes an integral unit that is dimensioned and arranged to be grasped in the hand of a user. The system is spring-loaded and is manufactured and shipped with the spring in a pre-cocked condition. An integral ampule is fillable by manipulation of a thrust rod/shaft which extends longitudinally through the device and is able to be grasped by a user. Once the unit is filled with a selected medication, it is held proximate the skin in order to inject the selected dosage. The unit is constructed from a maximum of eight component parts and is assembled in a matter of moments by unskilled personnel.

Description:
PRIORITY CLAIM 
     This patent application claims the benefit of the filing date of U.S. provisional patent Application No. 60/195,389, filed on Apr. 7, 2000, entitled LOW COST DISPOSABLE NEEDLELESS INJECTOR SYSTEM FOR VARIABLE AND FIXED DOSE APPLICATIONS, the entire contents of which is hereby expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to needleless hypodermic drug delivery devices and methods. The present invention relates more particularly to a low cost, disposable, spring actuated needleless injection device which utilizes a high pressure liquid stream to inject a medicament or other liquid through the skin and also relates more particularly to a method for using and manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     Needleless injection devices which administer intramuscular and/or subcutaneous medications without the use of a needle are well known. Among the many advantages of such needleless injection devices are the reduction of pain and apprehension commonly associated with hypodermic needles, the elimination of needle stick injuries, and the reduction of environmental pollution associated with contaminated needle disposal. Moreover, needleless injection devices are useful in a wide range of drug therapies, including the administration of vaccines, hormone therapies and local anesthetics. Further, it is well known that such needleless injection devices are useful in the administration of insulin to the diabetic population, where individuals frequently require a number of daily injections. 
     Injectable medications fall into two different general categories, namely: unit dose drugs such as vaccines and analgesics; and variable dose drugs such as insulin, where the dose size must be adjusted specifically so as to meet the immediate needs of the individual at the time of administration. When a variable dose is required, as in the case of the administration of insulin, a very accurate amount of medication must be transferred to a variable dose ampule of the needleless injector. Insulin doses are typically marketed in 3 ml and 5 ml syringe cartridges, as well as being provided in bulk in a standard 10 ml medication vial. 
     The use of needleless injection devices has recently become of great interest, particularly by people of limited physical abilities such as the elderly, the very young and the infirm. Such persons with limited physical abilities may find the use of conventional needle syringes either difficult or impossible. Therefore, the simplified injection process associated with needleless injectors makes their use very desirable among such people. 
     The principles of needleless injection and the advantages of such needleless drug delivery systems over conventional hypodermic needle injection systems have long been known. However, very few needleless injection devices have achieved commercial success in the marketplace. This lack of acceptance by the user community can be attributed, at least in part, to a number of factors, chief among which are: mechanical designs which have the potential to inflict serious injury if an injector device is inadvertently fired without a medicament container or ampule in place, undesirably complex filling techniques, and the high cost of such contemporary injection devices. This cost disadvantage is particularly troublesome for those individuals who must self-administer a large number of daily injections, such as diabetics. 
     One existing needleless injection device is described in U.S. Pat. No. 4,874,367 to Edwards. It employs a sealed ampule that is prefilled with a selected amount of medication. The prefilled ampule is attached to a separate spring-loaded firing mechanism which, when triggered, propels a ramrod from the front of the mechanism and against a plunger located in the ampule. The ramrod drives the plunger against the medication, producing a high pressured jet for injection purposes. The plunger expels the medication from a discharge orifice and into the patient&#39;s subcutaneous tissue. 
     Although effective in some respects, this contemporary needleless injection device is severely limited in practical applications. In order to cock the firing mechanism, the user is required to force the ramrod back into the firing mechanism by pushing the device against a solid surface, such as a table top, until the ramrod latches behind a trigger mechanism. Thus, the strength of an individual user imposes a strict limit upon the spring force that can be utilized in the device. Many elderly, very young or infirm people simply do not have the physical strength required to cock the firing mechanism of such a contemporary needleless injection device. 
     Moreover, employing a spring force which is low enough to be practical for the elderly, the very young and the infirm to cock the device results in the spring force being inadequate to produce effective and reliable injection pressures for most adults. That is, such a device would generally lack the ability to penetrate the skin and subcutaneous tissue sufficiently to insure proper, reliable, operation thereof. 
     In addition, the firing mechanism, having a spring actuated ramrod which extends outside of the device body, has the potential to inflict serious injury if inadvertently fired without the ampule in place. For example, firing such a device without having the ampule attached thereto may result in harm to a person who is inadvertently struck with the rapidly moving ramrod. 
     Moreover, the fixed dose ampule of contemporary needleless injection devices such as those of the &#39;367 patent must be prefilled at the factory and then attached to the injector when required for usage. In actual practice, however, this procedure is not practical for the simple reason that drug products cannot generally be stored in plastic containers for the extended periods of time which are typically experienced by such factory prefilled ampules. The only approved material for long term liquid medication storage is type-1 glass, which is used for virtually all drug products. However, due to the dynamics of needleless injection, in which the ampule is subjected to very high pressures during the ejection process, glass is not a suitable material for the ampule because it is too easily shattered. Consequently, it is desirable to have an ampule which may be coupled to a conventional drug vial or other medication container at the time of use, and then be filled therefrom with an accurate dose of medication. The ampule should be made of a high strength plastic material. 
     Indeed, the needleless injection device of U.S. Pat. No. 4,874,367 is formed of durable materials and utilizes comparatively expensive manufacturing techniques, so as to assure long time reliable use thereof. As such, this device is comparatively expensive to manufacture. The expense associated with the manufacture of this device precludes the sale and use of this device as a single use, disposable needleless injector. 
     U.S. Pat. No. 4,913,699 to Parsons overcomes some of the aforementioned deficiencies associated with contemporary needleless injectors. This patent describes a disposable needleless injection device having a firing mechanism that operates to release compressed gas from a storage compartment. The compressed gas acts upon a piston which drives a plunger that ejects a selected dosage of medication through an aperture in the discharge end of the device. However, the medication to be administered must first be drawn into a chamber provided in the interior of the injector before being dispensed. Thus, although being pre-cocked, and loadable (with medicine), the device is rather complicated to use. In addition, no provision is made for filling the medication chamber directly from standard medication containers. In order to fill the medication chamber, a complex liquid transfer system is required. 
     The device disclosed in the &#39;699 patent is relatively complex. It is manufactured from materials able to withstand the pressures associated with a compressed gas activation system. Indeed, this needleless injection device is formed of comparatively durable materials and utilizes comparatively expensive manufacturing techniques, so as to assure long term reliable operation thereof in light of the aforementioned pressures. As such, this device is comparatively expensive to manufacture. The expense associated with the manufacture of this device precludes its sale and use as a single use, disposable item. 
     Thus, although the aforementioned contemporary needleless injection systems are compact and reliable, they are too complex and expensive to manufacture in order to be considered disposable. 
     In view of the foregoing, it is clear that there is a need in the art for a needleless hypodermic injection device which has an enhanced simplicity of design and which can be manufactured from a small number of low cost components in order to be implemented as a truly disposable system. Furthermore, there is a need for a needleless hypodermic injection system which includes a medication ampule which is capable of being filled with an accurate dose of medication and which does not impose a risk of injury due to needle use and which does not contribute to the contamination hazards attendant with needle disposal. 
     In this regard, it is desirable to provide a convenient, low cost and disposable needleless injector device that is configured to be conveniently and comfortably grasped in one hand of a user in a manner which facilitates self-administration of a desired medication. The system should comprise a firing mechanism and an ampule cooperating in a novel design having simplicity in both structure and function. The ampule should be such that the user may fill it with a selected dosage conveniently and accurately from existing medication vials, so as to facilitate both variable and fixed dose applications. The injector firing mechanism should be conveniently and safely operated without the need for a user to force the apparatus against a piece of furniture or the like in order to cock a spring. There should be no substantial danger associated with firing the device without an ampule in place. 
     SUMMARY OF THE INVENTION 
     The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a housing and a spring injector mechanism disposed at least partially within the housing. Preferably, the spring injector mechanism and/or the housing are configured to be used only one time. 
     Preferably, the needleless injector of the present invention is provided with a spring injector mechanism which is already compressed or cocked when purchased, such that the user does not have to cock the device. Thus, the needleless injector of the present invention is particularly suitable for the elderly, the very young and the infirm. 
     Further, the needleless injector of the present invention is preferably configured such that it is generally suitable for only a single use. 
     Thus, according to the present invention, a low cost, disposable needleless injector is provided, so as to facilitate the administration of injections such as vaccines, hormones, local anesthetics and insulin. 
     It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings, wherein: 
     FIG. 1 is a semi-schematic side view of the low cost, disposable needleless injector of the present invention, as it is provided by a supplier, i.e., cocked and having an empty ampule; 
     FIG. 2 is a semi-schematic top view of the needleless injector of FIG. 1; 
     FIG. 3 is a semi-schematic cross-sectional end view of the needleless injector taken along line  3  of FIG. 2; 
     FIG. 4 is a semi-schematic cross-sectional end view of the needleless injector taken along line  4  of FIG. 2; 
     FIG. 5 is a semi-schematic cross-sectional side view of the needleless injector of FIG. 1; 
     FIG. 6 is a semi-schematic side view of one housing section; i.e., the housing section opposite that which is shown in FIG. 1; 
     FIG. 7 is a semi-schematic side view of the shaft of FIG. 1; 
     FIG. 8 is a semi-schematic side view of the plunger of FIG. 1; 
     FIG. 9 is a semi-schematic side view of the piston of FIG. 5; 
     FIG. 10 is a semi-schematic perspective view of the safety slide of FIG. 1; 
     FIG. 11 is a semi-schematic perspective view of the trigger of FIG. 1; 
     FIG. 12 is a semi-schematic perspective view of an exemplary one of the two elastomeric washers or cushions of FIG. 5; 
     FIG. 13 is a semi-schematic side view of the ampule of FIG. 1; 
     FIG. 14 is a semi-schematic cross-sectional side view of the needleless injector of FIG. 1, wherein the shaft is positioned within the ampule at a location corresponding to approximately 0.05 ml of medication being present in the ampule; 
     FIG. 15 is a semi-schematic cross-sectional side view of the needleless injector of FIG. 1, wherein the shaft is positioned within the ampule at a location corresponding to approximately 0.30 ml of medication being present in the ampule; 
     FIG. 16 is a semi-schematic cross-sectional side view of the needleless injector of FIG. 1, showing the configuration of the trigger, spring, piston, and shaft after the trigger has been depressed so as to actuate the spring injector mechanism; 
     FIG. 17 is a semi-schematic cross-sectional view showing the shoulder  29  and the cushions,  31  and  32 ; 
     FIG. 18 is a semi-schematic perspective view of a package for a single needleless injector according to the present invention; 
     FIG. 19 is a semi-schematic perspective view of a volume efficient cluster of packages for containing a plurality of the needleless injectors of the present invention, wherein a plurality of single needleless injector packages have been formed to one another; 
     FIG. 20 is a semi-schematic perspective view of the cluster of packages of FIG. 19, which has been rotated 180° about the longitudinal axis thereof; 
     FIG. 21 is a semi-schematic end view of the cluster of packages of FIGS. 19 and 20, showing the distal end thereof; and 
     FIG. 22 is a semi-schematic end view of the cluster of packages of FIGS. 19 and 20, showing the proximal end thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the construction and functions of the invention, as well as the sequence of steps for operating the invention in connection with the illustrated embodiment. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. 
     According to the present invention, a low cost, disposable needleless injector is provided so as to facilitate the administration of drug therapies such as vaccines, hormones, local anesthetics and insulin. The disposable needleless injector of the present invention is pre-cocked at the factory, so as to eliminate any need for a user to cock the needleless injector. Thus, the disposable needleless injector of the present invention is well suited for use by the elderly, the very young and the infirm, as well as any other persons who may find cocking of such devices difficult and/or unsafe. This device is also very easy for healthcare workers and the like to use on a patient. Thus, the present invention is well suited for both the self-administration of medication and for the injection of others, such as by healthcare workers. 
     The needleless injector of the present invention is specifically configured so as to be suitable for only a single use. Such configuration of the present invention is accomplished, at least in part, via the selection of particular materials which provide safe and reliable operation for a single use, but which are not suitable for indefinitely repeated use thereof. Single use of the needleless injector of the present invention is further facilitated by the mechanical design thereof, which readily facilitates cocking of the device at the factory, but which substantially inhibits cocking by a user. Indeed, cocking of the present invention by a user is virtually impossible. 
     Further, the needleless injector of the present invention preferably has an ampule permanently attached thereto, such that repeated use is inhibited and such that any hazard associated with firing of the device with an ampule not attached thereto is substantially mitigated. 
     More particularly, the low cost, disposable needleless injector of the present invention comprises a housing and a spring injector mechanism disposed at least partially within the housing. The spring injector mechanism and/or the housing are configured specifically so as to be used only a single time. For example, the spring injector mechanism is configured so as to inhibit cocking thereof by a user, such as by providing a shaft which cannot easily be grasped and pulled in a manner which is necessary to effect cocking of the needleless injector. Thus, the spring injector mechanism is configured so as to require a specially designed machine tool to facilitate cocking thereof. Of course, this machine tool is not available to the general public. 
     Those skilled in the art will appreciate that various locks, catches, latches, detents and the like may be utilized to prevent re-cocking of the spring injector mechanism. For example, actuation of the spring injector mechanism may trip a latch which prevents further movement, i.e., re-cocking, of a shaft of the spring injector mechanism. Such a latch would be disposed within the housing, and thus not be easily disengaged by a user. In this manner, a user is substantially inhibited from re-cocking the spring injector mechanism, so as to facilitate reuse thereof. Those skilled in the art will appreciate that various other, different methods for mechanically locking at least a portion of the spring injector mechanism in the fired position, after a single use thereof, are likewise suitable for inhibiting re-use of the present invention. 
     As stated above, the housing and/or the spring injector mechanism are configured so as to be unsuitable for indefinitely repeated use of the present invention. This may alternatively be accomplished, for example, by utilizing materials which inhibit repeated use thereof. Thus, the needleless injector of the present invention is preferably formed of materials which are insufficiently durable so as to facilitate repeated use thereof. That is, the low cost disposable needleless injector of the present invention is formed of materials which are sufficiently durable as to safely and reliably facilitate a single use thereof, but which will not withstand the forces and pressures associated with repeated use thereof. 
     For example, the housing is preferably formed of an inexpensive and sufficiently durable (for a single use) polymer material, such as glass loaded acrylonitrile-butadiene-styrene (ABS) or such as polycarbonate. If the housing is formed of glass loaded acrylonitrile-butadiene-styrene (ABS), the acrylonitrile-butadiene-styrene (ABS) preferably comprises approximately 15% to approximately 20% glass. This particular formulation of acrylonitrile-butadiene-styrene (ABS) has been found to be sufficiently durable to reliably and safely facilitate a single use of the present invention, while being insufficiently durable as to facilitate indefinitely repeated use thereof. Further, both acrylonitrile-butadiene-styrene (ABS) and polycarbonate are sufficiently inexpensive as to facilitate the provision of a truly disposable device. The housing is preferably formed of two separate housing sections which are substantially mirror images of one another, so as to further mitigate cost. Preferably, multiple cavity injection molds are used to form the housing. 
     As discussed in detail below, the plunger and the two resilient cushions, all of which are preferably simultaneously co-molded to the shaft, are all formed of Santoprene. While Santoprene is sufficiently durable for a single use of the present invention, Santoprene is not sufficiently durable for indefinitely repeated use thereof. Repeated use of the present invention will result in degradation of the Santoprene components thereof and thus render the present invention unsuitable for further use. In particular, the cushions will tend to deform substantially with each use, such that they rapidly loose their ability to function as shock absorbers. 
     A trigger for the needleless injector of the present invention is preferably formed of stamped stainless steel sheet. Such a stamped stainless steel trigger is sufficiently durable as to facilitate single use thereof. The stamped stainless steel trigger is also sufficiently inexpensive as to facilitate the construction of a truly disposable device. The trigger may be polished, if desired, so as to enhance the appearance thereof, since the cost associated with such polishing is negligible. 
     The trigger is preferably formed in a stair-stepped configuration, so as to eliminate the need for a pivot pin therefor and thus further reduce costs, as discussed in detail below. 
     The spring injector mechanism preferably comprises a spring disposed within the housing, a piston configured to be moved by the spring and a shaft configured to be moved by the piston. The spring preferably comprises a closed, but not ground, music wire helical spring. Eliminating grinding of the spring further reduces cost without impairing the reliability or safety of the present invention. 
     The piston preferably comprises a die cast zinc, copper and aluminum alloy. The proportions of zinc, copper and aluminum are selected to provide the mass necessary to eject fluid from an ampule with the necessary force as to effect a subcutaneous injection. The use of such an alloy provides the necessary mass to drive the shaft in a manner that assures proper operation of the present invention, e.g., the development of pressure within the ampule of approximately 3,000-3,500 psi. The use of this alloy also reduces costs sufficiently to facilitate the construction of a disposable device. 
     The shaft preferably comprises polycarbonate having approximately 15% glass or a glass loaded polymer of equivalent performance such as Amodel (a federally registered trademark of Amoco Oil Company of Chicago, Ill.). The use of such a polycarbonate shaft provides sufficient durability to facilitate a single use of the present invention, while inhibiting indefinitely repeated use thereof. Further, this polycarbonate shaft reduces the cost of the needleless injector, so as to facilitate the construction of a truly disposable device. 
     According to the present invention, the shaft is a single piece or unitary construction member and extends completely through the housing and into the ampule. The shaft extends from the proximal end of the housing such that the spring injector mechanism may be cocked by pulling the shaft proximally. As mentioned above, the shaft is preferably configured so as to inhibit grasping thereof, in order to similarly inhibit cocking of the spring injector mechanism by a user. Rather, the spring injector mechanism is cocked at the factory by a machine which is specifically configured to grasp the proximal end of the shaft and pull the proximal end of the shaft proximally with respect to the housing. 
     Such single piece or integral construction of the shaft has a further advantage, in that it readily facilitates filling of the ampule with a desired quantity of medication in a simple and accurate manner. The ampule may be filled by manipulating the proximal end of the shaft. Manipulating the proximal end of the shaft similarly manipulates the distal end thereof, so as to draw fluid into the ampule or expel fluid from the ampule in a very precise manner. 
     The ampule is permanently attached to the housing, such that access to the distal end of the shaft is inhibited, thereby preventing the spring injector mechanism from being cocked by pushing the distal end of the shaft against a surface (as is done according to some contemporary needleless injectors) Permanently attaching the ampule to the housing further mitigates a danger associated with dry firing or actuating the needleless injector without medicine in the ampule. Although the shaft is driven forcible in the distal direction when the needleless injector is dry fired, the permanently attached ampule prevents the moving shaft from striking anything other than the ampule itself. 
     The ampule is preferably threadedly attached to the housing, and then either adhesively bonded or sonically welded thereto. Permanent attachment of the ampule to the housing not only enhances safety by mitigating the ability to fire the needleless injector with the ampule removed and facilitates single use of the needleless injector by inhibiting refilling of the ampule due to sterility concerns, but also reduces the cost of the present invention. The cost of the present invention is reduced by eliminating the need to form threads within the housing which are suitable for repeated attachment and removal of the ampule. That is, according to the present invention, the threads may merely be injected molded as a part of the two housing sections and need not be formed or touched up via machining or the use of a tap. Rather, the threads which are integrally injected molded with the two housing sections are sufficient for a single use of the present invention, particularly when the ampule is further attached to the housing via adhesive bonding and/or sonic welding. Indeed, the housing need not comprise any threads for attaching the ampule thereto, but rather may alternatively utilize any desired injection moldable structure for this purpose. For example, threads may be eliminated from both the housing and the ampule and the ampule may merely be inserted into the housing and permanently bonded thereto. 
     The needleless injector of the present invention is more particularly described below with reference to FIGS. 1-16 of the drawings, which depict a presently preferred embodiment thereof. FIGS. 17-21 depict a volume efficient package for the needleless injector of the present invention, so as to substantially reduce the transportation and storage cost associated therewith, and thereby further facilitate the provision of a truly disposable device. As shown in FIG. 1, the needleless injector of the present invention is provided to a user in a substantially ready-to-use condition, requiring only that the ampule be filled with a particular medicament prior to injection. 
     Referring now to FIGS. 1-13, the low cost disposable needleless injector  10  of the present invention generally comprises a housing  8  and a spring injector mechanism  9  disposed partially within the housing. The housing  8  is generally cylindrical, approximately four inches in length excluding the ampule, and approximately one-half inch in diameter. The ampule is approximately one and five-eighths inch in length and approximately three-eighths inch in diameter. 
     The housing  8  is shaped such that it is easily grasped in the hand of a user, and when appropriately oriented, presents a depressable trigger mechanism in the vicinity of the user&#39;s thumb or finger. Indeed, the housing and trigger are configured such that when the housing is grasped within a user&#39;s hand, the trigger may be depressed either by the thumb, a finger or by merely squeezing the device in the user&#39;s hand or fist. The trigger has sufficient surface area and leverage to make the present invention easily actuatable by the elderly, the very young and the infirm. Moreover, the configuration and positioning of the trigger readily facilitates actuation by persons in a debilitated or weakened state and is even suitable for use in emergencies since a user does not have to take time to carefully hold and/or position the device in a particular manner. The device may be held in any manner which is convenient for the person administering the injection. It is merely necessary that the device be held in close contact to the skin, preferably generally perpendicularly thereto, and that the trigger then be depressed. The trigger is positioned to facilitate use of the present invention for the self-administration of medicine, as well as to facilitate the administration of medicine to patients by healthcare workers in any desired manner, e.g., with a thumb, finger, or by squeezing the entire device in a person&#39;s hand or fist. 
     The housing  8  preferably comprises first and second housing sections which are preferably injection molded either from acrylonitrile-butadiene-styrene (ABS) loaded with approximately 15% to approximately 20% glass or is alternatively injection molded from polycarbonate. The housing  8  has a proximal end and a distal end as labeled in FIG.  1 . All references to proximal and distal herein use these terms as defined in FIG.  1 . It is the distal end of the device (more particularly, the distal end of the ampule  50 ) which is pressed against the skin of a person who is to receive an injection. 
     Forming the housing  8  as two separate housing sections reduces the complexity and cost associated with the molding process, since an unthreading mold would be required if a unitary construction housing were to be provided with female threads in order to receive the ampule. As those skilled in the art will appreciate, such unthreading molds are considerably more expensive than simple release molds, and would thus undesirably increase the cost of the needleless injector of the present invention. 
     An additional advantageous feature afforded by forming the housing as two separate housing sections is appreciated during the assembly process, wherein the device may be assembled by simply placing the components into one of the two housing sections and then positioning the other housing section thereover. The two housing sections are bonded to one another using adhesive, sonic welding, or any other desired method. 
     More particularly, the assembly process comprises inserting a shaft  25  through the piston  15 . The resilient washers  31  and  32  and the plunger  26  have previously been co-molded or over-molded to the shaft of the shaft  25 . The spring  13  is then slid over the sleeve  17  of the piston  15  so as to define the spring injector mechanism  9 . The spring injector mechanism  9  is then placed into one of the two housing sections. The safety slide  42  and the trigger  40  are similarly placed in the same housing section. The other housing section is then mated thereto and bonding is effected. The ampule may then be screwed into the housing and bonded thereto. Alternatively, both the ampule and the housing may lack threads, such that the ampule may simply be slid into the housing and bonded thereto. Those skilled in the art will appreciate that various different mechanical locking means for permanently attaching the ampule to the housing are likewise suitable. Thus, the ampule may be mechanically locked to the housing and/or bonded thereto. Then the needleless injector of the present invention may be cocked utilizing a machine which holds the housing in place, while pulling the shaft  25  proximally, by grasping the ball  27  thereof. With the shaft  25  held in the cocked position, the trigger  40  is moved to a position wherein the sear  37  thereof inhibits distal movement of the piston  15  and the safety slide  42  is moved distally, so as to prevent undesirable movement of the trigger  40 . 
     The spring is preferably a closed, but not ground, music wire helical spring  13  (best shown in FIGS.  5  and  14 - 16 ) which is disposed within the housing such that the spring  13  has a cocked or compressed configuration, as shown in FIGS. 5,  14  and  15  and also has a fired or uncompressed configuration as shown in FIG.  16 . The spring  13  preferably has a spring constant of approximately 24 lbs./in. The spring is preferably approximately 3 inches long and develops approximately 28 lbs. of force when compressed so as to cock the spring injection mechanism. The spring  13  is sized to slide freely within a bore  22  of the housing. The proximal end of the spring  13  bears upon shoulder  14  formed within the housing and the distal end of the spring  13  bears upon a piston  15  (best shown in FIG.  9 ). 
     The piston  15  comprises a sleeve  17  which is sized to be received within the spring  13 . The sleeve  17  preferably has an outside diameter which is substantially less than the inside diameter of the coil formed by the spring  13 , such that the sleeve  17  moves freely along the inside of the spring without substantially frictionally engaging the spring  13 . 
     The piston further comprises a head  19  which defines a shoulder  20  against which the distal end of the spring  13  abuts. The head  19  of the piston  15  is sized to slide freely within the bore  22 . The piston optionally further comprises a conically tapered portion  21  of the head  19 . However, those skilled in the art will appreciate that the exact configuration, size and dimensions of the piston  15  will be determined, as least in part, by the mass thereof which is required to eject fluid from an ampule  50  with the necessary force to reliably perform an injection. Thus, the mass of the piston  15  is determined by both its dimensions and the materials from which it is formed, as discussed above. 
     The ampule  50  is preferably substantially transparent, so as to facilitate viewing of the quantity of liquid contained therein. A scale is typically provided upon the ampule so as to provide a numerical indication of the quantity (typically in milliliters) of fluid contained therein. 
     The spring injector mechanism  9  further comprises an elongated shaft  25  (best shown in FIG. 7) having a plunger  26  (best shown in FIG. 8) formed upon the distal end thereof and having a cocking knob  27  formed upon the proximal end thereof. The plunger  26  is preferably formed of an elastomer or thermosetting rubber such as Santoprene manufactured by Sonoco or the equivalent. 
     The plunger  26  is secured firmly to the shaft  25  via a shaft retaining heads  90   a  and  90   b  formed at the distal end of a portion  91  of the shaft  25  having a reduced diameter, as discussed in detail below. 
     The gripping ball  27  is preferably formed upon the proximal end of the shaft  25  by forming a reduced diameter neck  28  formed in the shaft  25  near the proximal end thereof. The shaft  25  further comprises a portion of increased diameter or shoulder  29  formed thereon and disposed within the housing. The shoulder  29  is preferably formed integrally with the shaft  25 . Alternatively, the shoulder  29  may be formed separately from the shaft  25  and be attached thereto. Resilient cushions  31  and  32  are preferably disposed upon either side of the shoulder  29 . Resilient cushion  31  cushions the impact of the piston  15  when the piston moves forward and strikes the shoulder  29  of the shaft  25  so as to cause the shaft  25  to eject fluid, as described in detail below. Similarly, resilient cushion  32  cushions the impact of the shoulder  29  of the shaft  25 , when the shoulder  29  strikes a proximal portion of the ampule  50  during the fluid injection process, as also discussed in detail below. Both resilient cushions,  31  and  32 , thus cooperate to mitigate noise and recoil when the device is operated. The mitigation of noise and recoil is important, so that the device is perceived as user friendly. Those skilled in the art will appreciate that excessive noise and recoil may be associated with pain or discomfort, and are thus undesirable. 
     As shown in FIG. 1, the needleless injector  10  of the present invention is configured as it is typically packaged and received by a user. The ampule  50  is empty, i.e., contains no medication when configured for variable dose applications, and the shaft  25  is disposed partially within the ampule  50  so as to reduce the overall length of the device to facilitate volume efficient packaging thereof. 
     The first and second resilient cushions,  31  and  32 , as well as the plunger  26 , are all preferably injection molded to the shaft  25  during a single injection co-molding process, and thus all preferably comprise the same resilient thermosetting rubber material, e.g., Santoprene (a federally registered trademark of Monsanto Company of St. Louis, Mo.). The plunger  26  and the first and second cushions,  31  and  32 , may alternatively be formed of polyvinyl chloride (PVC) or silicone. With particular reference to FIG. 8, the plunger  26  preferably comprises a central bore  24  ending in distally located hemispherical chambers  34   a  and  34   b.  The bore  24  is configured to receive the reduced diameter portion  91  of the shaft  25  and the chambers  34   a  and  34   b  are configured to receive the heads  90   a  and  90   b  of the shaft  25 , in a manner which facilitates secure attachment of the plunger  26  to the shaft of the shaft  25 . The heads  90   a  and  90   b  of the plunger  25  preferably define a Christmas tree or conically barbed protrusion, such as those commonly used to facilitate the attachment of an elastomeric component to a more rigid member. 
     The piston  15  is held in the cocked position thereof, against the force of spring  13  by the trigger  40 , which is preferably formed of stamped stainless steel. Preferably a rib  36  is formed longitudinally along a substantial portion of the length of the trigger  40 , so as to enhance the structural strength thereof. As those skilled in the art will appreciate, the use of such a rib  36  allows the trigger  40  to be formed of substantially thinner sheet stainless steel, thereby further reducing the cost thereof. 
     As best shown in FIG. 11, the trigger is formed in a stair-step like fashion, so as to define a sear  37 , a lower portion  38 , a middle portion  39  and an upper portion  40 . This stair-step configuration of the trigger  35  facilitates reliable use thereof without the need for a pivot pin, so as to further mitigate costs. 
     Safety slide  42  (best shown in FIG. 10) is slideably attached to the housing such that the safety slide has a distal position wherein movement of the trigger  40  is inhibited so as to likewise inhibit actuation of the spring injection mechanism, and also has a proximal position, wherein the trigger is free to move, as discussed in detail below. The distal position at the safety slide  42  thus mitigates the likelihood of inadvertent actuation of the spring injector mechanism. The safety slide  42  preferably has ridges  43  formed thereon, so as to facilitate easy operation thereof. 
     A flange  48  extends distally from the safety slide  42 . When the safety slide  42  is in its safe or distalmost position, the flange  48  covers a portion of the trigger  40 , so as to inhibit actuation of the injector spring mechanism, as discussed in detail below. 
     Female detents  47  (there is preferably one female detent on each side of the safety slide, cooperate with corresponding male detents formed upon the first and second housing sections to releasably lock the safety slide in the safe position (the distal most position) thereof, so as to inhibit inadvertent movement of the safety slide away from the safe position thereof. 
     The ampule  50  (best shown in FIGS.  13  and  14 - 16 ) has a chamber  51  formed longitudinally therein for containing medication. The ampule  50  is preferably permanently attached to the housing via threads  53  formed upon the proximal end thereof and complementary threads  54  (best shown in FIG. 6) formed at the distal end of the housing. In addition to threadedly attaching the ampule  50  to the housing, the ampule is preferably adhesively bonded and/or sonically welded to the housing, so as to assure permanent attachment thereof. 
     As used herein, permanent attachment of the ampule to the housing is defined as attachment of the ampule to the housing via bonding, such as adhesive bonding or sonic welding, and/or via mechanical fastening, in a manner which substantially inhibits removal of the ampule from the housing by a user. 
     Indeed, according to the present invention the ampule is preferably attached to the housing in such a manner that the ampule is not likely to be removed from the housing by a user without damaging the device and rendering it unuseable. 
     The ampule  50  further comprises a tip  60  formed at the distal most end thereof. The tip  60  has a bore formed therein so as to facilitate fluid communication of the medicine from the chamber  51  and therethrough during the injection process. 
     A Luer like threaded fitting or lug  62  is preferably formed proximate the distal end of the ampule, so as to facilitate filling thereof. The ampule is preferably formed of polycarbonate and optionally comprises a contemporary ampule such as those commonly used in needleless injection devices. 
     Generally, the ampule is filled by affixing a transfer coupler to the front end of the ampule and using the transfer coupler to access the contents of either a standard drug vial, standard syringe cartridge, or the like. Once the contents of the medication container are accessed, the injection device is filled by manipulating the shaft  25 . The shaft  25  is withdrawn or moved proximally so as to create a suction in the chamber  51  of the ampule  50 , so as to effect extraction of medication from the medication container. 
     For example, a user may move the shaft  25  to its distal most position, thereby forcing substantially all of the air from the ampule. The user may then move the shaft  25  proximally, so as to produce suction within the chamber  51  of the ampule  50  and thus effect withdrawal of medication from a vial into the chamber  51 . Typically, the ampule will be filled within slightly more medication then is necessary for the desired injection, so that any air in the ampule can be ejected by simply holding the needleless injector vertical, with the ampule uppermost, and then pushing the shaft back into the ampule chamber slightly, so as to effect ejection of any air within the ampule, as well as a small quantity of medicine, as those skilled in the art will appreciate. The user may then verify that the correct dosage has been withdrawn into the ampule by viewing the position of the plunger  26  within the ampule, relative to the graduations that are preferably provided upon the transparent ampule. Since the ampule is generally overfilled slightly, the user may reduce the quantity of medicine contained within the ampule by simply pushing the shaft  25  distally. 
     With particular reference to FIG. 6, the housing preferably comprises a second housing section  12 , which is substantially a mirror image of the first housing section  11 . One distinction between the first and second,  11  and  12 , housing sections is that one of the two housing sections has first, second, third and fourth guide pins,  71 ,  72 ,  73  and  74 , as well as guide slat  75  formed thereon. These guide members comprise male guide members, while on the other one the two housing sections,  12  and  11 , complimentary female guide members are formed. The male and female guide members engage one another, so as to facilitate proper alignment of the two housing sections and so as to maintain the two housing sections in such proper alignment during sonic welding of the first and second,  11  and  12 , housing sections to one another. 
     Each housing section,  11  and  12 , comprises a groove  77  within which the safety slide  42  is slideably disposed. A male detent  82  is preferably formed within the groove  77  of each housing section so as to engage a corresponding female detent  47  of the safety slide  42  in a manner which releasably locks the safety slide  42  in the safe position, as discussed above. Safety slide  42  comprises first  78  and second  79  flanges which are received within the slots  77  of the first and second housing sections to facilitate slidable movement of the safety slide  42 . 
     Both the first and second housing sections further comprise a tapered slot  80  within which the trigger  35  is pivotally disposed, as shown in FIGS.  5  and  14 - 16 . The tapered slot  80  facilitates pivoting of the trigger  35 , in a see-saw like fashion, about angled edge  81  (FIG. 11) of the trigger such that when the distal end of the trigger  35  is depressed or pushed toward the housing, then the proximal end of the trigger  35 , including the sear  37  thereof, moves upwardly, so as to effect disengagement of the sear  37  from the piston and thereby actuate or fire the spring injection mechanism to effect an injection. 
     The needleless injector cannot easily be used more than one time due to the difficulty associated with re-cocking thereof and/or due to the unsuitability of the materials and/or design for repeated reuse. 
     The needleless injector of the present invention is formed utilizing a minimum number of components, such that it may be manufactured in a simple fashion by unskilled workers. 
     Having described the structure of the low cost, disposable needleless injector in detail above, it may be beneficial to likewise describe the operation thereof. Operation of the needleless injector is described below with reference to FIGS. 14-16. 
     With particular reference to FIG. 14, the needleless injector  10  is shown with the shaft  25  positioned within the ampule  50  in a manner which facilitates the injection of a comparatively small amount, e.g., 0.05 ml, of medicine. Similarly, FIG. 15 shows the shaft  25  positioned within the ampule  50  in a manner which facilitates the injection of a comparatively larger amount of medicine, e.g., approximately 0.30 ml. Thus, the shaft  25  can be moved to various positions within the ampule  50 , so as to facilitate the injection of various different quantities of medication, as is necessary for variable dose usage. 
     In both FIGS. 14 and 15, the spring  13  is compressed intermediate the shoulder  14  of the housing and the shoulder  20  of the piston  15 . The piston  15  is maintained in this cocked position by the sear  37  of the trigger  40 . The safety slide  42  prevents actuation of the spring injector mechanism by preventing the trigger  40  from being depressed toward the housing and thereby preventing the sear  37  from disengaging the piston  15 . Thus, the safety slide  42  tends to prevent accidental actuation of the spring injector mechanism, which might result in accidental injection of the medicine. In any event, accidental actuation of the spring injector mechanism of the present invention will render the device unusable, since the low cost disposable needleless injector of the present invention is specifically configured for only a single use thereof. 
     The ampule  50  of the needleless injector of the present invention is loaded with medicine by attaching a vial or the like to the distal end of the ampule  50  and moving the shaft  25  proximally, so as to draw medicine from the vial into the ampule  50 , according to well known principles. 
     Since the ampule is permanently affixed to the needleless injector of the present invention, there is no need for a user to have to insert, attach, or otherwise adapt the ampule to an injector, in order to perform an injection. 
     Preferably, the shaft  25  is placed in its distal most position prior to attaching the ampule  50  to the vial, such that there is very little or no air within the chamber  51  of the ampule  50 . The quantity of medicine with which the ampule  50  is filled can be read accurately from a scale (FIG. 13) formed upon the ampule  50 . The ampule  50  may be slightly over filled, if desired. 
     After filling the ampule, the injection site upon the person receiving the injection and/or the distal end of the ampule  50  are sterilized, such as with alcohol, and the injection is then administered. 
     The injection is administered by holding the needleless injector  10  generally perpendicular to the skin at the injection site and then depressing the trigger  40 , such as with the thumb of the hand holding the device. Prior to depressing the trigger, the safety slide  42  must be moved proximally, so as to allow the sear  37  to disengage the piston  15 . 
     After the trigger  40  has been depressed, and the sear  37  disengages the piston  15 , the spring  13  moves from its compressed position (as shown in FIGS. 14 and 15) to its uncompressed or extended position as shown in FIG.  16 . As the spring  13  moves toward its uncompressed position, the spring  13  urges the piston  15  distally. As the piston  15  moves distally, the piston  15  strikes the first resilient washer  31  and causes the shaft  25  to move distally along with the piston  15 . The first resilient washer  31  cushions the impact of the piston  19  with respect to the shoulder  29  of the shaft  25 . 
     It will be noted that there is a gap between the piston  15  and the shoulder  29 , as shown in both FIGS. 14 and 15. This gap is the distance that the piston  15  must travel before it strikes the cushion  31 , which is located just proximal of the shoulder  29 . The length of the gap is within a range of gap lengths which are defined by the position of the plunger  26  within the ampule, as determined by the quantity of medication to be injected. Each gap length within this range of gap lengths is suitable for allowing the piston  15  to accelerate sufficiently before striking the cushion  31 , so as to generate the desired pressure (typically approximately 3,000 psi-5,000 psi) within the chamber  51  of the ampule  50 . 
     As the spring  13  continues to move toward its extended position, the plunger  26  expels the medicine from the chamber  51  of the ampule  50 , thereby effecting the injection. According to the preferred embodiment of the present invention, the spring  13  remains partially compressed in its fully extended position (as shown in FIG. 16) and therefore has a preload. 
     It is important to appreciate that after the injection has been effected, it would be extremely difficult for a user to re-cock the spring injector mechanism so as to effect the performance of another injection with the needleless injector of the present invention. The proximal end of the shaft  25  is specifically configured to inhibit grasping thereof, as is necessary to effect re-cocking of the device. 
     Further, permanent attachment of the ampule  50  to the housing prevents the attachment of a new, sterile ampule to the housing, as would be desirable in the performance of another injection. 
     Referring now to FIG. 17, the shoulder  29  is shown to be integrally formed with the shaft  25 . Those skilled in the art will appreciate that various other means of forming such a shoulder upon a shaft are likewise suitable. For example, the should may alternatively be formed by adding a separately formed structural member to the shaft  25 . The cushions  31  and  32  are molded directly to the shaft  25  and the shoulder  29 , as discussed in detail above. 
     Referring now to FIGS. 18-22, volume efficient packaging for the needleless injector of the present invention is shown. As those skilled in the art will appreciate, it is important to reduce the various costs associated with use of the needleless injector of the present invention, so as to make disposability thereof economically feasible. One important aspect of such cost reduction involves the use of volume efficient packaging, so as to mitigate transportation and storage costs associated with the present invention. 
     The packaging system of the present invention has been specifically designed so as to minimize the cost and the volume associated with the packaging. Thus, according to the present invention, each package, as shown in FIG. 18 requires only slightly more volume than the needleless injector itself. Further, a cluster of packages as shown in FIGS. 19 and 22 comprises interleaved individual packages which further minimize wasted space. 
     Wasted space is minimized in the cluster of packages by inserting or interleaving one row of packages along with another row thereof. Thus, space between adjacent packages within a row, which is normally wasted, is efficiently utilized according to the present invention. That is, a needleless injector in one row of packaging according to the present invention is disposed efficiently intermediate two adjacent needleless injectors of the other row thereof. 
     With particular reference to FIG. 18, each individual package  100  for a needleless injector  10  according to the present invention, comprises a cradle  101  and a cover  102 . The cover  102  is preferably bonded, such as via adhesive bonding to a planar surface  104  of the cradle  101 . The cradle  101  defines a cavity  103 . The cover  102  completely covers and hermetically seals the cavity  103 , so as to facilitate the provision of a sterile environment for the needleless injector contained therein. Both the cradle  101  and the cover  102  are preferably transparent, so that the presence of a needleless injector within the package  100  is easily detected. 
     Each cradle  101  is preferably formed by vacuum forming sheet polymer material. Each cover  102  is preferably formed by cutting, die stamping or otherwise forming individual covers from roll or sheet polymer. 
     According to the preferred embodiment of the present invention, each corner of the cradle  101  comprises a bevel  105  which exposes a corresponding corner  107  of the cover  102 , so that the cover  102  may be easily grasped and peeled away from the cradle  101 , as shown in FIG.  18 . 
     The cradle preferably further comprises a plurality of ribs  109 , which enhance the structural strength thereof, and thus facilitate the use of thinner material in the construction of the cradle, so as to further reduce the cost thereof. 
     The cavity  103  of the cradle  101  generally conforms in shape to the shape of the needleless injector, so as to minimize the cost of the packaging, as well as the volume thereof. 
     When a plurality of such packages are placed in a row, i.e., side by side, then a gap is formed between adjacent packages. As discussed below, the present invention takes advantage of this gap between adjacent packages so as to further enhance the volumetric efficiency associated with the packaging of a plurality of needleless injectors. 
     With particular reference to FIGS. 19-22, a plurality of packages, such as those of FIG. 18, are formed together in a cluster  110 . The cluster may comprise either a plurality of individual packages, which have been attached to one another, or may alternatively comprise one or more pluralities of individual packages which are formed together, integrally with one another. For example, the cluster  110  may comprise two pluralities of packages or rows,  113  and  114 , wherein each row,  113  and  114 , is formed integrally and separately from each other row,  114  and  113 , and then the two rows are attached to one another or interleaved. Thus, each row,  113  and  114 , of the cluster  110 , may be formed separately by vacuum forming or the like, and then the rows may be interleaved. This interleaving of the two rows enhances the volume efficiency of the completed multiple needleless injector package is thus more volume efficient and less costly. 
     The covers for each cradle in a row are either separately formed or are formed of one piece of material which is perforated or scored, such that the individual cover associated with a particular needleless injector may be peeled away from the cradle so as to facilitate the removal of a desired particular needleless injector. The use of such separate covers for each needleless injector facilitates the storage of the remaining needleless injectors in a sterile environment after one or more of the needleless injectors has been removed from such a cluster package. 
     The clusters may be formed such that they are easily broken apart, so as to form individual packages or so as to form smaller clusters. Scores or perforations may be utilized so as to facilitate such breaking apart of a cluster. A cluster so scored or perforated may be formed so as to break apart into smaller clusters having any desired number of individual packages. 
     The cradles are preferably formed from a transparent polymer material such as polyethylene terephalate glycol (PETG). The covers are preferably comprised of a material such as Tyvek (a federally registered trademark of Dupont de Nemurs and Company of Wilmington, Del.). 
     According to the present invention, all of the components of the present invention, including the components of the packaging, are suitable for gamma sterilization. The needleless injector of the present invention is packaged, either automatically or manually, by placing one needleless injector in the cavity of each cradle and then sealing the cover to the upper flat surface  104  of each cradle, so as to provide a hermetic seal. After hermetically sealing a needleless injector in each package of a cluster (or in a non-cluster, single product package), then the needleless injector(s) and associated package(s) are gamma sterilized. Gamma sterilization is performed after sealing of the needleless injectors within their packages so as to assure maintenance of proper sterilization thereof, until the packages are opened by the user. 
     According to an alternative configuration of the present invention, the needleless injector may be provided to the user with the ampule anesceptically filled with a desired medicament. When the needleless injector is provided with the ampule prefilled, then the proximal most portion of the shaft  25  is preferably cut off or otherwise removed, so as to inhibit movement thereof, which might tend to undesirably force fluid from the ampule. That is, that portion of the shaft  25  which would otherwise extend from the proximal end of the housing is removed, so as to prevent the shaft  25  from being undesirably manipulated during handling. In this manner, fixed dose applications of the present invention are facilitated. 
     It is understood that the exemplary low cost, disposable needleless injector described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. For example, various different configurations of the spring, piston and shaft are contemplated. For example, those skilled in the art will appreciate that the piston may have various different configurations, shapes and/or dimensions which facilitate compression of the spring, locking of the spring in the compressed configuration thereof by the trigger, and which have the necessary mass so as to assure proper ejection of a fluid from the ampule when the shaft is struck thereby. 
     Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.