referring initially to fig1 , an auto - injector 10 in accordance with the present invention is shown being used by a patient ( user ) 12 for a self - administration of a fluid medicament . as can be seen in fig1 , the auto - injector 10 includes a push - button 14 that extends from the external housing 16 of the auto - injector 10 to be depressed by the user 12 to initiate an injection . as envisioned for the present invention , all of the operational components of the auto - injector 10 are maintained out - of - sight , inside the housing 16 . referring now to fig2 it will be seen that the internal components located in the interior 17 of the housing 16 of the auto - injector 10 include a controller 18 that is connected with the push - button 14 . in turn , the controller 18 is connected to a drive mechanism 20 , to a fluid reservoir ( e . g . a fluid medicament cartridge ) 22 and , optionally , to a vacuum system 24 . as envisioned by the present invention , the controller 18 may be an electronic micro - computer of a type well known in the pertinent art . in any event , the purpose of the controller 18 is to coordinate the respective operations of the drive mechanism 20 , the fluid reservoir 22 , the vacuum system 24 and the needle holder 28 . fig2 also shows that the drive mechanism 20 is directly involved with the operations of a connector 26 and a needle holder 28 . to understand the structure and inter - cooperation of these components , however , it is necessary to structurally understand the needle unit ( i . e . needle ) 30 that is to be manipulated by these components . for this purpose , refer to fig3 . there it will be seen that a needle unit 30 includes an elongated , hollow hypodermic tube 32 that has a sharp proximal end 34 and a sharp distal end 36 . further , the needle unit 30 also includes a collar 38 that is positioned and affixed on the tube 32 intermediate the ends 34 and 36 . more specifically , the collar 38 is generally disk - shaped , and it extends in a radial direction from the axis that is defined by the hypodermic tube 32 . fig3 also shows that the distal end 36 of the tube 32 is located at a distance “ l ” from the collar 38 . in fig4 an embodiment of the needle holder 28 is shown , and is generally designated 40 . this particular embodiment 40 of the needle holder 28 is provided to move a needle unit 30 from a magazine 42 , and to then subsequently move it to a storage location 44 . to do this , the embodiment 40 includes an arm 46 that rotates about a point 47 . a grip 48 is located at one end of the arm 46 . thus , as the arm 46 is rotated back and forth in the direction of the arrows 50 , the grip 48 can , in sequence , retrieve a needle unit 30 from the magazine 42 and then move it along a path 52 to a location ( shown as needle unit 30 ′). at this location , the needle unit 30 ′ is positioned to cooperate with the connector 26 . after its cooperation with the connector 26 , needle unit 30 ′ is then moved by the grip 48 along a path 54 to the storage location 44 ( i . e . needle unit 30 ″) where it will be stored for subsequent disposal . fig5 shows another embodiment of the needle holder 28 that is generally designated 56 . for the embodiment 56 of the needle holder 28 , a carousel 58 is employed to move the needle unit 30 into location for cooperation with the connector 26 . more specifically , for the embodiment 56 a needle unit 30 is retrieved from the magazine 42 and moved along path 52 onto the carousel 58 . the carousel 58 then rotates in the direction of the arrow 60 to the location of needle unit 30 ′ where it cooperates with the connector 26 ( see fig7 ). after its cooperation with the connector 267 the needle unit 30 ′ is then moved by the carousel 58 for further movement along a path 54 to the storage location 44 ( shown as needle unit 30 ″). there it will be stored for subsequent disposal . in fig6 , a cassette 61 is shown as yet another embodiment of the needle holder 28 . specifically , the cassette 61 is generally cylindrical shaped , as shown , and it is formed with a plurality of receptacles 63 . as intended for the present invention , individual needle units 30 can be pre - loaded into respective receptacles 63 of the cassette 61 , prior to engaging the cassette 61 with the housing 16 . when used , the cassette 61 is rotated about the axis 65 to present an individual needle unit 30 at the location for cooperation with the connector 26 . thus , the cassette 61 effectively combines the functionality of the needle holder 28 , the magazine 42 and the storage 44 into a single structure . it is to be appreciated that all of the embodiments of the needle holder 28 ( i . e . embodiments 40 and 56 , as well as cassette 61 ) are unitary components of the auto - injector 10 . as such , they can be selectively engaged with the auto - injector 10 and , along with the spent needle units 30 , individually disposed of after they have been used . respective structures for the fluid reservoir 22 , for the drive mechanism 20 , and for the connector 26 will be best appreciated with reference to both fig7 and fig8 a . considering the fluid reservoir 22 first , it will be seen that the reservoir 22 includes a vial 62 for holding a fluid medicament 64 therein . a septum 66 , at one end of the vial 62 , is provided to establish fluid communication with the reservoir 22 whenever it ( i . e . the septum 66 ) is penetrated . it will be appreciated , however , that any other mechanism well known in the pertinent art for establishing a fluid connection can be used for this purpose , such as a luer fitting . the fluid reservoir 22 also includes a bung 67 that can be advanced by a plunger 68 into the vial 62 for purposes of expelling fluid medicament 64 from the reservoir 22 through a penetrated septum 66 . as envisioned for the present invention , the fluid reservoir 22 can be a pre - filled cartridge that can hold either one , or multiple doses of the fluid medicament 64 . further , the plunger 68 can be calibrated to establish a specific dosage of fluid medicament 64 , each time it advances the bung 67 . the drive mechanism 20 for the auto - injector 10 is shown in fig8 a to include a drive rod 70 that is selectively propelled by a compressed spring 72 in a linear direction indicated by the arrow 74 . it will be appreciated by the skilled artisan that the actual mechanism for propelling the drive rod 70 can vary . in this context , the spring 72 is only exemplary . it is important for the present invention , however , that the propulsion of the drive rod 70 by the drive mechanism 20 develops a predetermined momentum for the drive rod 70 ( see operation below ). referring specifically to fig7 , it will be seen that the connector 26 includes a fluid transfer section 76 that is formed with a fluid chamber 78 . further , the fluid transfer section 76 has an input port 80 to the fluid chamber 78 , and it has an output port 82 that is covered by a septum 84 . fig7 also shows that the connector 26 includes a flex hose 86 that has one end connected for fluid communication with the input port 80 of the fluid transfer section 76 . as also shown , the other end of the flex hose 86 is fitted with a spike 88 that can be used to penetrate the septum 66 of the fluid reservoir 22 . for another aspect of the present invention , fig8 a shows that the housing 16 of auto - injector 10 can be formed with a vacuum depression 90 that is placed in fluid communication with the vacuum system 24 ( see fig2 ) via a vacuum hose 92 . in the operation of the auto - injector 10 of the present invention , after setting a desired dose of the fluid medicament 64 to be delivered , the user ( patient ) 12 will position the housing 16 against an injection site 94 . the user 12 then depresses the push - button 14 . after a predetermined time interval , the user 12 removes the auto - injector 10 from the injection site 94 , and the injection of fluid medicament 64 into the user ( patient ) 12 has been completed . at no time , either before , during or after an injection , is any part of a needle unit 30 ever visible to the user 12 . furthermore , other than an earlier loading of the fluid reservoir 22 , engaging the reservoir 22 with connector 26 , and loading a magazine 42 of needle units 30 , only a dose setting may be required before the auto - injector 10 is used . there is nothing for the user 12 to do after the injection has been completed except , perhaps , to put a cover ( not shown ) over the housing 16 . referring back to fig2 , it will be appreciated that as the user 12 depresses the push - button 14 , several mechanisms inside the housing 16 are sequentially activated by the controller 18 . for one , the needle holder 28 ( with either embodiment 40 or , alternatively , embodiment 56 ) retrieves a needle unit 30 from the magazine 42 . the needle holder 28 then moves the needle unit 30 into the position indicated in the drawings as needle unit 30 ′ ( see fig8 a in particular ). prior to this , the connector 26 has been connected in fluid communication with the fluid reservoir 22 . specifically , this connection is made by inserting the spike 88 on flex hose 86 through the septum 66 . at this point , with the needle unit 30 ′ in position , the drive mechanism 20 comes into play . it is an important aspect of the operation of the auto - injector 10 of the present invention , that the drive mechanism 20 propels the drive rod 70 toward the connector 26 ( e . g . fluid transfer section 76 ) with a predetermined momentum . specifically , in accordance with well known impulse and momentum considerations , this predetermined momentum will be determined by the mass of the drive rod 70 and its velocity ( predetermined momentum = m rod v rod ). as intended for the present invention , when the drive rod 70 impacts with the connector 26 , its momentum ( m rod v rod ) is then transferred to the fluid transfer section 76 of the connector 26 . note : the flex hose 86 mechanically isolates the transfer of momentum to only the fluid transfer section 76 . this transfer of momentum will then immediately accomplish several functions . for one , part of the momentum is used to establish fluid communication between the fluid transfer section 76 of the connector 26 and the needle unit 30 . this is accomplished as the proximal end 34 of the hypodermic tube 32 penetrates through the septum 84 . the remaining momentum that is now determined by the velocity ( v f ) of the combined mass ( m combined ) of the fluid transfer section 76 and the needle unit 30 . importantly , the velocity term ( v f ) of this remaining momentum must be sufficient to cause the distal end 36 of the needle unit 30 to penetrate into the user ( patient ) 12 at the injection site 94 ( see fig8 b ). as appreciated by the present invention , the velocity that is necessary for generating the necessary predetermined momentum of the drive rod 70 need not result in an excessively high velocity for the needle unit 30 . to the contrary , the intent here is to generate a so - called “ light touch ” that will guarantee only that an effective penetration of the needle unit 30 is achieved . a benefit here is that the possibility of creating pain or bruising at the injection site 94 is minimized . additional benefits are that by minimizing the final momentum there is less shock to the user 12 , due to reduced recoil , and there is a reduced need for energy input . once the needle unit 30 has penetrated the user ( patient ) 12 at the injection site 94 , the controller 18 will then activate the fluid reservoir 22 . specifically , with this activation , the bung 67 is advanced into the vial 62 to expel fluid medicament 64 into the injection site 94 . when doing this , the fluid medicament 64 traverses the flex hose 86 , and enters the hypodermic tube 32 of needle unit 30 through the fluid transfer section 76 . once the injection has been completed , the needle unit 30 is withdrawn from the injection site 94 . the needle holder 28 then moves the needle unit 30 to storage 44 . at this point , the auto - injector 10 is rearmed and another cycle can then be performed . once the magazine 42 is empty of needle units 30 , it can be disposed of . likewise , when the fluid reservoir 22 has been emptied of fluid medicament 64 , it is ready for disposal . in some instances it may also be desirable to dispose of the connector 26 . further to the above disclosure , it will be appreciated that the vacuum system 24 can be activated during a use of the auto - injector 10 to help stabilize the auto - injector 10 at the injection site 94 and avoid tissue compression . specifically , when a partial vacuum is created in the vacuum depression 90 that is established as housing 16 is positioned against the injection site 94 , skin from the user ( patient ) 12 will be drawn into the depression 90 ( see fig8 b ). this will help stabilize the auto - injector 10 during an injection without the need to push the auto - injector 10 against the skin and , thereby , compress tissue . further , the depth to which the distal end 36 of needle unit 30 will penetrate into the user ( patient ) 12 can be controlled , and varied as desired . in general , penetration depths of up to around one and a half inches are considered typical . in each case , a precise penetration depth is achieved by establishing the distance “ l ” between the collar 38 and distal end 36 of the needle unit 30 ( see fig3 ). more specifically , this distance “ l ” of needle unit 30 , and the location of an adjustable abutment 96 on the housing 16 will establish a travel limit for the collar 38 and needle unit 30 . consequently , a precise penetration depth can be established for the hypodermic tube 32 of the needle unit 30 . referring again to fig8 a , it will be seen that the auto - injector 10 includes a recoil mechanism 98 , such as a spring , that is positioned on the abutment 96 substantially as shown . fig8 a also shows that the auto - injector 10 includes a cocking mechanism 100 that responds to instructions from the controller 18 and is used to rearm the auto - injector 10 in preparation for a subsequent injection cycle . in operation , the recoil mechanism 98 and the cocking mechanism 100 interact with the needle unit 30 in different ways . these different operations are , perhaps , best appreciated by first considering fig8 b . with reference to fig8 b it will be seen that during an injection ( i . e . after an injection cycle has been started ) the spring 98 is depressed between the abutment 96 and the collar 38 of needle unit 30 ( 30 ′). while it is depressed , the spring 98 reacts against the force that is being applied by the drive spring 72 and by the drive rod 70 . the depressed spring 98 , however , does not overcome the combined forces that are applied by spring 72 and drive rod 70 . therefore , during an injection , the distal end 36 of needle unit 30 remains inserted at the injection site 94 . indeed , it may be desirable for the distal end 36 to remain inserted at the injection site 94 , even after the injection of fluid medicament 64 is completed . if so , the controller 18 can be programmed to delay the activation of cocking mechanism 100 for the withdrawal of the distal end 36 of needle unit 30 from the injection site 94 . this delay can be for any predetermined period of time ( e . g . five seconds ). as indicated above , the cocking mechanism 100 is used to return the drive rod 70 and its drive spring 72 from their respective positions shown in fig8 b ( i . e . during an injection ) to those shown in fig8 a ( i . e . preparatory to an injection ). it also happens that this action returns the fluid transfer section 76 and the needle unit 30 to their positions shown in fig8 a . to help insure that this withdrawal is accomplished without complication , the depressed spring 98 assists in lifting the distal end 36 of needle unit 30 from the injection site 94 . the needle unit 30 can then be separated from the fluid transfer section 76 . importantly , the now - used needle unit 30 can be removed from its location between the fluid transfer section 76 and the abutment 96 , and replaced with a new sterile needle unit 30 . the sequence of operation can then be repeated , until the supply of needle units 30 that has been loaded into the auto - injector 10 is exhausted . while the particular injection system with hidden needles as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated , it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims .
an auto - injector confines all functional components inside an enclosed housing , to keep its needle out - of - sight , at all times . within the housing , a needle holder positions a needle at a location . a drive mechanism then simultaneously engages the needle with a fluid source and accelerates it with a predetermined momentum for insertion into a patient . after fluid delivery , the withdrawn needle is moved to storage within the housing for subsequent disposal .