Abstract:
A microdispensing ophthalmic pump is provided for repeatedly delivering doses as small as 5 microliters within an angular operating range. The pump basically comprises a reservoir, a dispensing cap, an actuator and a pump body with a pump mechanism disposed therein. The pump mechanism is regulated by a limited-travel inlet check valve and a biased-closed outlet check valve. A failsafe mechanism is formed between the actuator and dispensing cap to prevent operation of the pump outside the operating range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a fluid medicine delivery device, and, more particularly, the invention is directed to a microdispensing ophthalmic pump for delivering a microdose of ophthalmic fluid. 
     2. Description of the Prior Art 
     U.S. Pat. No. 5,152,435 (hereinafter “the &#39;435 patent)”, entitled “OPHTHALMIC DISPENSING PUMP”, discloses a manually operated dispensing pump capable of delivering a precise quantity of ophthalmic solution to the surface of an eye in a desired spray pattern with an impact pressure on the eye that is comfortably tolerable by an individual and was issued to a co-inventor, Ben Z. Cohen, of this patent. The &#39;435 patent is incorporated by reference herein, including the extensive discussion of the shortcomings of the prior art. The spray pump of the &#39;435 patent is substantial improvement over the prior art, capable of delivering doses of ophthalmic fluid such as 50 microliters in the previously described manner. However, often a dose of much less than 50 microliters of ophthalmic fluid may be required to be delivered in the manner described above. Since a reduction in the size of a dosage inherently decreases the impact force exerted by the dose onto an eye, the administration of fluid by the &#39;435 patent would be even more comfortably tolerable than that disclosed therein with a reduction in the size of the dose the &#39;435 pump could deliver. Also, some medications can have toxic effects, even at doses as small as 50 microliters, and so doses of less than 50 microliters would be better tolerated. 
     It is a primary object of the subject invention to provide a manually operated microdispensing pump for delivering a microdose of ophthalmic solution as small as 5 microliters. 
     Also, it is an object of the subject invention to provide a manually operated microdispensing pump capable of repeatedly administering a full and proper microdose as small as 5 microliters. 
     SUMMARY OF THE INVENTION 
     The above-mentioned objects of the present invention are achieved by a new and improved manually operated microdispensing pump for delivering ophthalmic fluid. In particular, the new and improved manually operated microdispensing pump will enable an individual to repeatedly deliver a predetermined microdose of ophthalmic fluid. 
     In the preferred embodiment, the microdispensing pump of the subject invention is formed to be substantially cylindrical with one end being formed as a reservoir for storing the ophthalmic fluid intended to be dispensed. A pump body is threadedly secured to the reservoir with a cylindrical inner body formed therein which projects along a central axis into the reservoir. A dip tube is provided to communicate fluid from the reservoir to the inner body of the pump body. A pump mechanism is disposed within the inner body which urges fluid from the reservoir and through the pump of the subject invention. The pump mechanism comprises an inlet check valve element for regulating the flow of the fluid from the reservoir into the inner body, a cylindrical piston slidably disposed and sealingly supported within the inner body, an elongated poppet extending from the inner check valve element and through the inner body in a spatial relationship with the piston, an outlet check valve element for regulating flow of the fluid out of the inner body and a spring for urging the cylindrical piston into an upward position in contact with a head formed on the end of the support opposite the inlet check valve element. 
     The microdispensing pump of the subject invention further comprises a dispensing cap mounted onto the cylindrical piston and formed with an outlet chamber which communicates with the inner body, the communication therebetween being controlled by the outlet check valve element, and a slender discharge nozzle communicating the outlet chamber with the periphery of the dispensing cap. An actuator is slidably disposed adjacent the dispensing cap and substantially within the pump body. 
     Once primed with ophthalmic fluid within the inner body, the pump dispenses ophthalmic fluid with a downward translation of the actuator, the dispensing cap and the piston within the inner body. As the piston translates within the inner body, the volume therein is decreased with an accompanying increase in pressure of the ophthalmic fluid contained within the inner body. The check valve elements are both normally closed and contribute to the pressure build-up of the fluid. Eventually, the compressed ophthalmic solution will force the outlet check valve element open, thereby allowing fluid to enter the outlet chamber and the discharge nozzle and force out fluid previously drawn therein. The fluid is delivered in a non-aerosolized jet stream as a series of droplets. A spring is provided to urge the outlet check valve element into a closed position quickly after being forced open. The piston, having completed its downward translation, translates upward into contact with the head of the poppet due to the urging of the spring acting on the piston. As the piston comes into contact with the head of the poppet, the volume within the inner body is increased and the accompanying pressure decreased. The reduction of pressure within the inner body creates a suction effect which urges the inlet check valve element into an open position and draws fluid from the reservoir into the inner body. As pressure builds within the inner body due to the added fluid, the inlet check valve element will be urged into a closed position allowing the pump mechanism to be used again. 
     The new and improved manually operated microdispensing pump of the subject application uses a spring biased outlet check valve element and a limited-travel inlet check valve element to operate under the negligible pressures and strokes associated with the delivery of microdoses of fluid. In the preferred embodiment, a spring is applied to a stainless steel ball to form the outlet check valve, which is biased to a normally closed position. The suction created by the pump mechanism to draw fluid therein may affect the microdose of the pump if fluid disposed in the nozzle and the outlet chamber is drawn into the inner body due to the suction effect. During operation of the pump, the spring urges the outlet check valve element into a closed and seated position prior to suction being created in the inner body and ensures that a proper and full microdose of the ophthalmic fluid is maintained within the nozzle and the outlet chamber, unaffected by the suction effect. 
     An inlet check valve element is provided to regulate the flow of ophthalmic fluid into the pump of the subject invention. Since the delivery of microdoses as small as 5 microliters involves a negligible stroke of the inlet check valve element, a protrusion is disposed opposite the inlet check valve element which restricts the check valve element&#39;s range of motion and prevents the check valve element from simply shuttling during usage. The motion of the inlet check valve element is limited so that in an open position the volume displaced by the inlet check valve element in travelling from a closed position to an open position is less than the volume of the dose being dispensed by the pump. In the preferred embodiment, this volume is the swept volume of an inlet check valve ball and is calculated by taking the product of the clearance between the inlet check valve ball and the protrusion times the cross-sectional area of the inlet check valve ball: (clearance)×[π×(radius of the ball) 2 ]. Although a ball is preferred, any shape inlet check valve element may be used, such as a disk, with the swept volume being determined by the product of the clearance between the inlet check valve element and the protrusion times the largest cross-sectional area of the inlet check valve element measured in a plane perpendicular to the flow of fluid through the check valve. Thus, one feature of the new and improved manually operated microdispensing pump of the subject invention is a valve arrangement sensitive to the negligible strokes associated with microdosing. 
     Prior to initial use, the pump of the subject invention must be primed, wherein air is expelled from the pump mechanism. The pump is primed through the repeated actuation of the pump mechanism which draws fluid therein and forces air thereout. After priming, the re-introduction of air into the pump mechanism is undesired, since air pockets may be formed within the pump mechanism which may render the pump mechanism inoperative. To prevent the entrapment of air within the pump mechanism, the pump of the subject invention includes a failsafe device, a limited volume dip tube and a spherical inlet chamber which function to prevent the introduction and entrapment of air bubbles into the pump mechanism. The failsafe device comprises a ball disposed within an arcuate slotted track formed in the dispensing cap, which cooperates with an actuating block extending from the actuator. To operate the pump of the subject invention, the actuator is urged towards the dispensing cap with the actuating block coming into contact and pressing against the ball disposed within the track, which, under further urging, depresses the dispensing cap and activates the pump mechanism. If the pump were to be operated with the opening of the dip tube exposed to air entrapped within the reservoir, air could possibly be introduced into the pump mechanism. The slot of the failsafe device is formed to guide the ball out of alignment with the actuating block when the dip tube is positioned to be in communication with air trapped in the reservoir, with the ophthalmic fluid being within a predetermined range of fluid levels. Preferably, the slot is formed to allow the pump of the subject invention to operate with the nozzle discharge positioned in a range from approximately 155 to 290 degrees, going clockwise. Outside of this range, the ball will slide within the arcuate slot and prevent actuation of the subject invention pump. 
     To limit the entrapment of air in the pump during priming, the inlet chamber is formed to be substantially spherical to avoid the creation or entrapment of air bubbles therein. Also, during priming, as the pump is actuated with the inlet check valve element not being encompassed by ophthalmic fluid, the inlet check valve element will not provide an adequate seal against its seat and will allow fluid to freely pass the check valve element into the dip tube. This leakage, when the inlet check valve element is in a dry state, may cause an air pocket in the dip tube which prevents ophthalmic fluid from entering the pump mechanism. The air pocket will react to the actuation of the pump by rising and falling within the dip tube corresponding to the existence of suction within the pump mechanism. As a result, ophthalmic fluid is prevented from being drawn into the pump mechanism. To avoid such a problem, the dip tube of the pump of the subject invention is formed to encompass a volume less than the microdose intended to be dispensed by the pump to ensure that the inlet check valve element is submersed in ophthalmic fluid, since the inlet check valve element will not leak when encompassed by ophthalmic fluid. The dip tube has a hollow, substantially cylindric center which contains fluid from its free end to the seat of the inlet check valve element, which will be fully drawn into the pump upon a single actuation. Limiting the volume of the dip tube below the microdose of the pump ensures sufficient fluid will be drawn from the dip tube with a single actuation of the pump which will encompass the inlet check valve element and prevent the formation of an air pocket in the dip tube. Thus, another feature of the new and improved manually operated microdispensing pump of the subject invention prevents the entrapment of air within the pump mechanism. 
     To ensure proper operation of the pump, an annular tapered latch, formed from a resilient plastic, is provided at the base of the actuator and disposed about the inner body and pump mechanism. A corresponding annular shoulder is formed about the inner body with a top surface which comes into contact with the bottom surface of the latch with the downward translation of the actuator. The actuator can translate downward till the bottom surface of the latch is in contact with the annular shoulder without the pump dispensing any fluid. The actuator can further translate downwards, with the latch freely deforming. As the latch continues to deform, the latch generates resistance to further downward translation requiring increasing force to accomplish such translation. The increase in force will eventually build up and overcome a predetermined threshold force, which causes the latch to yield with a great reduction in resistance to even further downward translation. 
     To dispense fluid from the pump, a threshold force must be applied to deform the latch and exceed the yield point, thereby allowing the actuator translation into the pump body such that the pump mechanism is activated through the dispensing cap. The force needed to overcome the latch is much greater than that required to drive the piston a required stroke. Once the latch is overcome, the threshold force will cause the piston to rapidly travel its full stroke. A full and proper dose, as predetermined by the stroke of the pump mechanism, will be ensured through the elimination of a partial pump stroke. Therefore, another feature of the new and improved manually operated microdispensing pump of the subject invention is a latch for ensuring proper dosing. 
     Also, the translation of the dispensing cap into the pump body results in the compression of air trapped therebetween and resistance to downward translation. Vents may be provided to allow the compressed air to escape. The combination of the latch and the vents can be used to establish a threshold force needed to operate the subject invention. The quantity and the size of the vents can be manipulated to add or decrease the threshold force needed to overcome the latch. 
     The deformation of the latch converts the threshold force needed to deform the latch into a rapid actuation of the pump mechanism. An operator of the new and improved pump of the subject invention will not sense the point at which the latch will deform and will continue to apply the threshold force after deformation of the latch. Once deformed, the latch provides no resistance to further translation of the actuator and dispensing cap, which under the applied threshold force will rapidly move and activate the pump mechanism. This rapid activation will cause the pump mechanism to dispense fluid in a non-aerosolized jet stream as a series of droplets which will hit the desired target nearly simultaneously. As an additional feature, the rapid translation of the dispensing cap within the pump body causes the dispensing cap to strike the pump body, which limits the translation of the dispensing body, such that an audible click, tactile click, or any combination thereof, is generated. The audible or tactile click indicates to a user of the subject invention that a dose has been administered. The audible click can be avoided by padding the point of contact either on the dispensing cap or the pump body with a cushioning material, such as rubber or laminated paper. 
     The latch is not necessary to create a jet stream, if the pump can be actuated quickly without it. However, the latch ensures the pump mechanism will be activated with sufficient velocity to create a jet stream. Thus, yet another feature of the new and improved manually operated microdispensing pump of the subject invention is a deformable latch which ensures delivery of fluid from the pump in a jet stream. 
     As with all medical dispensers, precautions must be taken to prevent the introduction of foreign matter which could cause contamination of the dispenser. The spring acting against the outlet check valve element prevents the introduction of foreign matter into the pump mechanism. During fluid administration, the inner body draws fluid through the dip tube as fluid is dispensed. The drawing effect not only affects the inlet check valve element, but also the outlet check valve element. The spring urges the outlet check valve element into a seated position prior to suction being created within the inner body and prevents the drawing of contaminants into the pump through the nozzle. 
     Also, the dispensing cap, along with the discharge nozzle, is disposed within the actuator during non-use. In this position, the nozzle is protected from dirt and debris. The mouth of the discharge nozzle is provided with a conical rim which aids in the separation of the discharging fluid from the nozzle. The rim is encompassed by an annular depression which provides a pocket for collecting undispensed fluid. The annular depression is recessed within the dispensing cap and provides for separation of undispensed fluid from the nozzle, thereby avoiding possible blockage, and from the actuator, thereby avoiding possible gumming on the actuator of undispensed fluid which could contaminate future doses. 
     Although the discussion of the subject invention refers to ophthalmic solutions and administration to a person&#39;s eye, the new and improved manually operated microdispensing pump of the subject invention can be used with any type of fluid, such as lubricants, fragrances, medications and so on, for which a microdose as small as 5 microliters may be required. 
     These and other features of the invention will be better understood through a study of the following detailed description of the invention and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the new and improved pump of the subject invention. 
     FIG. 2 is a cross-sectional view of the new and improved pump of the subject invention in an unactuated position. 
     FIG. 3 is a cross-sectional view of the new and improved pump of the subject invention in a dispensing position. 
     FIG. 4 is a cross-sectional view of the new and improved pump of the subject invention returning to an unactuated position. 
     FIG. 5 is a cross-sectional view of the new and improved pump of the subject invention drawing fluid therein. 
     FIG. 6 is a cross-sectional view of an alternative embodiment of the new and improved pump of the subject invention. 
     FIG. 7 is a cross-sectional view of an alternative embodiment of the new and improved pump of the subject invention. 
     FIGS. 8A-B are respectively is a plan and cross-sectional side view of the latch of the new and improved pump of the subject invention. 
     FIGS. 9A-B are respectively is a plan and cross-sectional side view of the spring fingers of an alternative embodiment of the subject invention. 
     FIGS. 10A-D are cross-sectional views of the operating range of the new and improved pump of the subject invention. 
     FIGS. 11A-D are cross-sectional views of the jet stream dispensed by the new and improved pump of the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the new and improved manually operated microdispensing pump of the subject invention is generally indicated by reference numeral  10  and is capable of delivering a microdose of ophthalmic fluid  11  to a human eye  13 . Referring generally to FIGS. 1-5, the pump  10  comprises a reservoir  12 , a pump body  14 , a pump mechanism  16 , a dispensing cap  18  and an actuator  20 . 
     The reservoir  12  is generally cup-shaped and formed to accommodate fluid. The pump body  14  is mounted onto the reservoir  12  and secured thereto through threaded engagement of threads  22 , formed on neck  24  of the reservoir  12 , and threads  26 , formed on a lower portion  28  of the pump body  14  which is disposed about the neck  24 . An annular seal  25  is disposed between the pump body  14  and the reservoir  12  which prevents fluid from leaking through the threads  22 ,  26 . The pump body  14  comprises a substantially cylindrical outer shell  30 , a substantially cylindrical inner body  32  disposed co-axially within the outer shell  30 , and a transverse bulkhead  34  joining the two cylindrical elements. The outer shell  30  is formed to define a dispensing aperture  36  with sight  38  disposed thereabout. The sight  38  allows a user of the pump  10  to aim and direct the pump&#39;s discharge. 
     The inner body  32  extends from both sides of the bulkhead  34  with one end  40  being open, an opposed end  42  having an inlet channel  44  and an inlet check valve seat  46  formed therein, and a cylindrical inner chamber  48  extending between the two ends  40 ,  42 . A hook-shaped guide  50  depends from the lower ends of the inner body  42  onto which dip tube  52  is mounted. The guide  50  directs the dip tube  52 , which encompasses a volume less than the microdose  11 , to the edge of the reservoir  12  in alignment with the sight  38 . The guide  50  and the dip tube  52  allow an individual to efficiently draw fluid from the reservoir  12 , since the dip tube  52  is fixed and formed to reach deep into the reservoir  12  and communicate with very low levels of fluid. Furthermore, an individual has a tendency to tilt a dispenser forward in administering a fluid; the guide  50  and an end of the dip tube  54  are aligned to consider this tendency. 
     A cylindrical piston  56  is slidably disposed within the inner chamber  48  with an annular seal  58  being in contact with the surface of the inner chamber  48 . The piston  56  is formed with a cylindrical inner chamber  55  having a constant cross-section and a top end  57  forming an opening smaller than the cross-section of the inner surface  55 . A poppet  60  is located within the piston  56  and extends throughout the inner chamber  48 . The poppet  60  is formed with a base  62  having a hemispherical lower surface  64 , which together with the inlet check valve seat  46  from a generally spherical inlet chamber  66 . The inlet channel  44  communicates with the inlet chamber  66  and together with the dip tube  52  form a passageway for fluid to pass into the pump body  14 . An inlet check valve element  67 , preferably a ball, is seated in the inlet check valve seat  46  within the inlet chamber  66 . A protrusion  68  extends from the lower surface  64  of the poppet  60  into close proximity with the inlet check valve element  67 . The protrusion  68  limits the travel of the inlet check valve element  67  within the inlet chamber  66  so that the swept volume of the inlet check valve element  67  is less than the microdose  11 , calculated in a manner previously described. 
     A stem  69  extends from the base  62  through the piston  56  in a spatial relationship, thereby forming an annular flow path  70  therebetween. A head  72  depends from the stem  69  and has a diameter greater than the inner diameter of the piston  56 . A spring  74  is disposed about the base  62  of the poppet  60 , and urges the top of the piston  57  into sealing contact with the head  72 . The inner chamber  48  and the annular flow path  70  receive fluid from the inlet chamber  66  through ports  76  formed in the base of the poppet  62 . An outlet check valve housing  77  is mounted to the piston  56  with a tapered portion  78  being formed therein. The poppet  60  is disposed within the piston  56  by forcing the head  72  through the piston  56 . The piston  56  is preferably made from low density polyethylene, which will allow the head  72 , preferably made from high density polyethylene, to pass through the piston  56  without permanent deformation. 
     The dispensing cap  18  is mounted onto the outlet check valve housing  77 . An outlet chamber  80  is formed within the dispensing cap  18  and communicates with the annular flow path  70  when the head  72  is not in contact with the piston  56 . An outlet check valve element  82 , preferably a ball, is located within the outlet chamber  80  and limits flow from the annular flow path  70  into the outlet chamber  80 . A quick return biasing means  84  urges the outlet check valve element  82  into sealing contact with the tapered portion  78 . Preferably, the quick return biasing means  84  is comprised of a conventional coil spring with a spring force of 2.9 lbs/in., as shown in FIG.  2 . Alternatively, a resilient rubber ball  84 ′ or cantilevered latch spring  84 ″ can also be used, as shown in FIGS. 6-7. 
     A straight walled discharge nozzle  86  is formed to communicate the outlet chamber  80  with the periphery of the dispensing cap  18 . The discharge nozzle  86  is preferably formed to define a length to throat ratio of approximately 7 to 1. The design of the slender discharge nozzle  86  contributes to the formation of a jet stream which is dispensed therefrom. The nozzle  86  is formed with a conical rim  85  and an annular depression  87  about the discharge at the periphery of the dispensing cap  18 . The conical rim  85  aides in the formation of a jet stream which discharges from the nozzle  86  by causing separation of the fluid from the dispensing cap  18  since little surface area is provided about the discharge of the nozzle  86  to which fluid can adhere. If any fluid does adhere, the undispersed fluid collects in the annular depression  87 . The annular depression  87  allows undispensed fluid to collect which will not adhere to the discharge of the nozzle  86 , possibly causing blockage, or to the actuator  20 , possibly causing gumming and contamination of later doses. 
     An upper surface  88  of the inner body  32  and the head of the poppet  72  limit the stroke of the piston  56 . The upper surface  88  represents the lower limit of the stroke whereas the head  72  represents the upper limit. The amount of the microdose can be controlled through the establishment of these limits. 
     A void  90  exists between the upper surface  92  of the bulkhead  34  and the dispensing cap  18 . The void  90 , annular air chamber  94 , air vents  97  and vent  96 , formed within the wall of the inner body  32 , create an atmospheric flow path through which ambient pressure is exposed to the surface of the fluid when the piston  56  is not in contact with the head  72 . The introduction of ambient pressure into the reservoir  12  ensures the surface of the fluid will be under atmospheric pressure and drawn into the dip tube  52  due to a drop in pressure in the inlet chamber  66 , as described below. The reservoir  12  cannot be filled so that the vent  96  is covered by fluid, which would prevent the introduction of atmospheric pressure. The void  90  is vented to atmosphere by the air vents  96 . The air vents  97  also provide pathways for air to escape from the void  90  when the actuator  20  is depressed into the pump body  14  which compresses the air found in the void  90 . 
     The actuator  20  is formed with a skirt  98  disposed between the dispensing cap  18  and the outer shell  30 . Since the skirt  98  is not fixed to the dispensing cap  18  or the outer shell  30 , the actuator  20  is capable of translating therebetween. Normally, the actuator  20  is biased away from the dispensing cap  18  by biasing means  100 . Preferably, the biasing means  100  comprises a conventional coil spring but may also comprise spring member  100 ′ disposed about the lower edge of the actuator, as shown in FIGS. 6,  9 A and  9 B. The spring member  100 ′ is formed with a plurality of inwardly extending resilient spring fingers  101  which urge the actuator  20  away from the dispensing cap  18  when the spring fingers  101  are deformed against the bulkhead  34 . Ridge  104  limits the upward travel of the actuator  20  and contains the actuator  20  within the pump body  14 . A discharge aperture  106  is formed in the skirt  98  which is aligned to be juxtaposed with the dispensing aperture  36  and the discharge nozzle  86  when the actuator  20  is forced into contact with the dispensing cap  18 , as shown in FIG.  3 . The top of the actuator  108  is conveniently formed with an arcuate surface which can comfortably accommodate the tip of a finger of a user of the pump  10 . 
     The inner surface of the actuator  110  and the upper surface of the dispensing cap  112  form a gravity sensitive failsafe mechanism for preventing the introduction of air into the inner chamber  48 . An actuating block  114  extends from the inner surface  110  towards the upper surface of the dispensing cap  112 . The upper surface  112  is formed with an arcuate slot  116  which accommodates ball  118 . The slot  116  is formed to seat the ball  118  below the actuating block  114  when the sight  38  is directed at an angle, rotating clockwise, from approximately 155 degrees to 290 degrees, as shown in FIGS. 10A-D. Referring to FIG. 2, the lower surface of the slot  120  is formed at an angle α, which is preferably 110°, and the upper surface  122  is formed at angle β, measuring 25°. As the pump  10  is turned counterclockwise beyond 155 degrees, the ball  118  will slide up the upper surface  122  and no longer be in alignment with the actuating block  114 . Similarly, if the pump  10  is rotated clockwise beyond 290 degrees, the ball  118  will roll up the lower surface  120  and out of alignment with the actuating block  114 . The range of angles from 155 degrees to 290 degrees was chosen to ensure submersion of the end of the dip tube  54  within the liquid found in the reservoir  12  with fluid being present therein with in predetermined levels. 
     An annular, tapered latch  124 , formed from a resilient plastic, preferably polypropylene, is disposed about the lower end of the actuator  126  about the inner body  32  and is shown in FIGS. 8A and 8B. The latch is formed with a bottom surface  128 . An annular shoulder  130  extends from the bulkhead  34  forming a diameter larger than the inner opening of the latch  124 . The actuator  20  is spaced from the dispensing cap  18  and may be pressed down without either the inner surface  110  or the actuating block  114  coming into contact with the dispensing cap  18 , or the bottom surface  128  of the latch  124  touching the annular shoulder  130 . 
     In operation, the reservoir  12  is filled with a fluid to a level below the vent  96  with the pump  10  being in a vertical position. Initially, the pump  10  must be primed with fluid being urged therethroughout. To do such priming, the pump  10  is activated several times using a normal pump operation. As fluid is drawn into the pump body  14 , air will be expelled, with the pump  10  being primed when no air is within the dip tube  52 , the pump body  14 , or the dispensing cap  18 . The pump process as described below is the same during priming, except the pump medium may include some air. 
     To dispense fluid from the pump  10 , the actuator  20  is depressed into the pump body  14  with the bottom surface  128  of the latch  124  coming into contact with the annular shoulder  130 , as shown in FIG.  3 . The latch  124  freely deforms with further downward translation of the actuator  20 . As the latch  124  continues to deform, the latch  124  generates resistance to further downward translation requiring increasing force to accomplish such translation. The force will eventually build up to a predetermined threshold force which overcomes the latch  124  and causes it to yield. As the threshold force is being reached, the actuating block  114  comes into contact with the ball  118 . The threshold force necessary to overcome the latch  124  ensures the piston  56  will rapidly translate its full stroke. The resistance against downward translation can also be regulated through the size and quantity of the air vents  97 . The depression of the actuator  20  causes the air in the void  90  to compress and requires additional force for further compression and further translation. Since the air vents  97  communicate with the atmosphere and the compressed air in the void  90  is bled thereto, having minimal or none of the air vents  97  results in a slow escape for the compressed air and resistance to translation of the actuator  20 . An increase in the number or size of the air vents  97  allows the compressed air to escape quicker from the void  90  and reduce the resistance against downward translation. The combination of the latch  124  and the vents  97  can be manipulated to establish a threshold force required to operate the pump  10 . 
     As shown in FIG. 3, the actuator  20  must translate the distance S1 for the actuating block  114  to come into contact with the ball  118 . As the distance S1 is translated, the latch  124  and the air vents  97  offer resistance so that a threshold force must be applied to actuate the pump  10 . With the distance S1 translated, the latch  124  will be on the verge of yielding under the threshold force and the ball  118  will be in contact with the actuating block  114 . The distance S2 is equal to the stroke of the piston  56 , and the actuator  20  and the dispensing cap  18  can only travel the distance S2 by having the latch  124  yield and the air of the void  90  overcome. With the application of the threshold force, the latch  124  is quickly deformed with the threshold force continuously being applied thereafter, thereby causing the actuator  20 , along with the dispensing cap  18  and the piston  56 , to quickly travel the distance S2. 
     Referring to FIG. 3, as the piston  56  travels downward the distance S2, fluid within the inner chamber  48  is compressed and forced through the annular flow path  70  about the head  72 , which through the downward travel of the piston  56  is separated from the top of the piston  57 . The fluid rushing past the head  72  will act against the outlet check valve element  82 , with the pressure of the fluid eventually overcoming the bias of the quick return biasing means  84  and causing the outlet check valve element  82  to separate from the tapered portion  78 . In turn, the fluid travelling past the outlet check valve element  82  will force fluid into the discharge nozzle  86  and the microdose  11  out of the nozzle  86 , which is aligned with the discharge aperture  106  and the dispensing aperture  36 . Due to the threshold force required to overcome the latch  124  and the air of the void  90 , the downward travel of the piston  56 , through the distance S2, is rapid, resulting in a rapid surge of fluid through the nozzle  86 . The microdose  11  exiting from the discharge nozzle  86  will from a non-aerosolized jet stream as shown in FIGS. 11A-D. Due to the surface tension of fluid, as the microdose  11  travels away from the pump  10 , it will tend to break into a series of drops with a relatively large droplet and several smaller droplets, which will all hit the eye  13  nearly simultaneously. 
     The yielding of the latch  124  will cause the fluid to surge past the head  72  and the outlet check valve element  82 . As shown in FIG. 4, the quick return biasing means  84  will urge the outlet check valve element  82  into contact with the tapered portion  78 , once the surge of fluid has bypassed the outlet check valve element  82 . The piston spring  74  will urge the piston  56 , the dispensing cap  18  and the actuator  20  upwards, with the biasing means  100  further urging the actuator  10  away from the dispensing cap  18 . Simultaneously, the latch  124  will separate from the annular shoulder  130  and resume its undeformed, annular tapered form. The upward travel of the piston  56  increases the volume of the inner chamber  48  and creates a suction effect. As a result, the inlet check valve element  67  is drawn towards the inner chamber  48  and into contact with the protrusion  68 , as depicted in FIG.  5 . Fluid is then drawn from the dip tube  52  through the inlet channel  44 , the inlet chamber  66  and the ports  76  into the inner chamber  48 . As the inner chamber  48  fills with the drawn fluid, pressure increases therein and the inlet check valve element  67  is forced into a seated position in the seat  46 . 
     The pump  10  can be manually actuated without the latch  124 . The latch  124 , however, ensures the application of the threshold force, which, in turn, ensures the application of a full dose in a jet stream, as described above. 
     Simultaneous to the pumping operation, the vent  96  is exposed to the annular air chamber  94  with the downward travel of the piston  56  and to ambient conditions. As such, the pressure on the surface of the fluid in the reservoir  12  is restored to atmospheric with each actuation of the pump  10 . 
     As is readily apparent, numerous modifications and changes may readily occur to those skilled in the art, and hence it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modification equivalents may be resorted to falling without the scope of the invention as claimed.