Abstract:
A blowout springless manual air displacement pipette including a mechanical assist for generating a mechanical force opposing a return spring force on a plunger unit to aid a pipette user in locating and maintaining the plunger unit at a “home” position within a pipette body and ready for immersion of a pipette tip in a liquid to be drawn into the tip.

Description:
BACKGROUND 
     The present invention relates to manual air displacement pipettes and more particularly to an improved manual pipette including a moveable plunger and a mechanical assist for aiding a pipette user in manually locating and maintaining the plunger at a “home” position ready to aspirate a predetermined volume of liquid. 
     U.S. Pat. Nos. 3,827,305 and 4,909,991, for example, describe commercially available single channel manual air displacement pipettes. Each such pipette includes an elongated hand-holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit is supported for axial movement in the pipette body between a first or upper stop position in which an end portion of the plunger unit extends from an upper end of the pipette body. A pipette user grips the pipette body with his or her thumb over the exposed end of the plunger unit. Downward thumb action on the plunger unit moves the plunger unit downward from its upper stop position against the upward bias of a return spring to a second or a lower stop position at which all fluid is expelled from a tip secured to the pipette. Adjacent the lower stop position is a “home” position for the plunger unit to which the plunger unit is returned by the pipette user at the beginning of each aspiration operation with the pipette. 
     In the commercially available pipettes described in the foregoing patents, the home position is defined by a “soft” stop. As described in such patents, the soft stop comprises a second relatively stiff spring mechanism within the pipette body which is activated when the plunger unit reaches the home position. In this regard, and as depicted in FIG. 4 a  herein, as the pipette user manually moves the plunger unit from its upper stop position by pressing downwardly with his or her thumb on the exposed end of the plunger unit, the pipette user can “feel” an increased resistance to movement of the plunger unit associated with an activation of the second spring assembly opposing further downward movement of the plunger unit. The position of the plunger unit where the user feels the activation of the second spring mechanism defines the home position for the plunger unit. Continued movement of the plunger unit beyond the home position to the lower stop position is resisted by a combination of the return spring and the second spring mechanism. 
     Thus, in pipeting liquids with such commercially available pipettes, the pipette user grasps the pipette housing with his or her thumb on top of the exposed end of the plunger unit. Exerting downward thumb pressure on the plunger unit, the user moves the plunger unit away from the upper stop position against the force of the return spring. The user detects the home position for the plunger unit during movement of the plunger unit away from the first stop position by sensing the start of an increase in the downward force required to move the plunger unit. Such increase force is the result of movement of the plunger unit against the return spring and the second spring mechanism, commonly referred to as a “blowout” spring mechanism. Accurate sensing of the start of the increase in the downward force required to move the plunger unit is a delicate operation requiring great care to be exercised by the pipette user. Thus, with his or her thumb on top of the exposed end of the plunger unit, the user very carefully senses and then manually maintains the plunger unit at the home position. In practice, a significant portion of the total time associated with a pipeting operation is occupied by the pipette user manually maintaining the plunger unit at the home position ready for insertion of a tip extending from the pipette into the liquid which is to be aspirated by the pipette. Then, with the tip inserted in the liquid, the user manually controls the rate of return of the plunger unit from the home position to the upper stop position. 
     For accuracy and repeatability of operation of the pipette, it is important that the pipette user always bring the plunger unit to the exact same home position and that the pipette user manually control the rate of return of the plunger unit to the upper stop position in a repeatable manner for each pipette operation. This is necessary in order that the same desired volume of liquid will be drawn into the pipette tip during each repeated operation. It should be appreciated that such manual operation of a pipette places substantial physical and mental strain upon the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. In extreme cases, the physical hand and wrist strain associated with extensive and prolonged manual pipette operation can contribute to or produce repetitive strain injuries such as tendinitis and carpel tunnel syndrome. 
     Similar physical and mental stress problems are associated with other manual pipettes which include different mechanisms for defining the plunger unit home position. Examples of such different mechanisms are described in U.S. Pat. No. 4,041,764 and in German patent applications 239 539 A1 and 239 540 A1. Specifically, U.S. Pat. No. 4,041,764, describes a magnetic detent which is engaged between an upper stop and the pipette user exerting an increased axial force on a push button when it is desired to move the piston beyond the home position against the force of a return spring. The manual forces which a user of the pipette of U.S. Pat. No. 4,041,764 must exert on its pipette piston (plunger) in moving the piston from its upper stop position to and through a home position to a lower stop position are depicted in FIG. 4 b  herein. 
     The German patent applications, on the other hand, each describe a hollow piston pipette with ferromagnetic systems at upper and lower stops. The lower stop is a “hard” bottom stop for the hollow piston in that no piston movement beyond the lower stop is permitted. A user of the hollow piston pipette does not have to “feel” a “soft” stop defining a home position for the hollow piston. Rather, the lower stop defines the home position for the hollow piston pipette. Thus, in the operation of the hollow piston pipette, the user simply grasps the pipette body and by exerting a downward thumb force on an activating knob drives the hollow piston to the lower stop. To aspirate liquid into a tip connected to a lower cone of the hollow piston pipette, the user simply releases the activating knob and allows a compression spring to move the hollow piston from the lower stop to the upper stop. The ferromagnetic systems of the upper and lower stops interact with a magnetized locking piece to control operation of a disk seal in opening and closing the aperture of the hollow piston. For example, since the retaining force of the ferromagnetic system of the lower stop is greater than that of the locking piece and the axial motion of the locking piece is limited by a stop, the disk seal lifts away from a flange on the hollow piston and frees the aperture of the hollow piston so that a first cylinder-piston system communicates with a lumen of the pipette tip through the hollow piston and holes leading to a ventilation channel to atmosphere. 
     It is to be noted that in all of the foregoing manual pipettes, the pipette user is required to continuously apply steady downward force with his or her thumb to maintain the pipette plunger unit in its home position ready for insertion of a tip of the pipette into the liquid to be drawn into the tip by controlled upward movement of the plunger unit from the home position to its upper stop position. 
     Recognizing the physical and mental strain associated with repeated and prolonged operation of a manual pipette by a pipette user and to significantly reduce the physical and mental strain associated with the operation of manual pipettes and eliminate the need for the pipette user to physically maintain a pipette plunger in a home position, a latch mechanism operable as a pipette plunger reaches the home position has been recently developed and is described and illustrated in U.S. Pat. No. 5,364,596 assigned to the assignee of the present invention. As described in U.S. Pat. No. 5,364,596, the latch mechanism releasably maintains a plunger in the home position without any user exerted force on the plunger in opposition to the force of the return spring. Such an improved manual pipette may further include a velocity governor for automatically controlling the rate of return movement of the piston from the home position to the upper stop position for the plunger upon a release of the latch mechanism. 
     While such improved manual pipettes including latch and velocity governors improve the repeatability and reliability of operation of manual pipettes and reduce the physical and mental strain on pipette users where repeatability of operation is essential, they introduce significant increases in the manufacturing costs for manual pipettes which are reflected in increased prices for such improved manual pipette over their more simple predecessors. 
     More recently, an improved manual pipette which is of simple construction and low in manufacturing cost has been developed which provides a significant reduction in the physical and mental strain on a pipette user over the course of a series of pipette operations where repeatability of operation is essential. That manual pipette is described and illustrated in U.S. Pat. No. 5,700,959 assigned to the assignee of the present invention. 
     As previously stated, in prior conventional manual pipettes, the pipette user must exert a relatively strong downward thumb force on the plunger unit to retain it in the “home” position in opposition the return spring and a relatively strong “blow out” spring defining the “soft” stop. With the manual pipette described in the &#39;959 patent however, rather than requiring the user to carefully sense the exact start of a sudden increase in a force opposing downward movement of a plunger unit in locating the “home” position for the plunger unit and rather than requiring the user to manually exert a strong downward force to maintain the plunger unit in its “home” position against the return and blow out springs, the manual pipette of the &#39;959 patent includes a magnet assist mechanism. The magnet assist mechanism generates a downward magnetic force in opposition to the return spring force as the unit reaches and is at the home position. The magnetic force is less than the upward force generated by the return spring and does not latch the plunger unit at the home position as is the &#39;596 patent. Rather, the opposition force generated by the magnet assist is (i) reflected in a reduction in the downward force required to move the plunger unit as it approaches the home position to aid the pipette user in sensing the home position, and (ii) reduces the manual force that the pipette user must exert to maintain the plunger unit in the home position. The magnet assist thereby substantially reduces the physical and mental strain on the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. 
     While the magnet assist mechanism described in the &#39;959 patent is simple in construction, low in cost and significantly reduces the problems associated with conventional manual pipettes with respect to physical and mental strain, some pipette users have expressed the desire to be able to exercise complete manual control over the travel of the plunger particularly in the region of its axial travel where the magnet assist mechanism is operational to aide in locating and maintaining the pipette plunger at its “home position”. The present invention not only satisfies that request but also provides a manual pipette which does not require or utilize a blow out spring or a latch mechanism to define the “home position” for the plunger unit. 
     SUMMARY OF INVENTION 
     Like prior conventional manual pipettes, the present invention comprises a hand holdable pipette body having a return spring biased plunger unit supported therein for axial movement from a first or upper stop position. As with prior manual pipettes, a pipette user holding the pipette of the present invention presses on a plunger control knob to move the plunger unit downward from the first stop position against the upward force of the return spring to a second or lower stop position wherein all fluid contained in a pipette tip secured to the pipette body is expelled from the tip. The pipette user then allows the return spring to return the plunger to a “home” position adjacent the lower stop position. The “home” position is defined by a “soft” stop and is the starting position to which the plunger unit is returned for the start of each successive aspiration operation with the pipette. In prior conventional manual pipettes, the pipette user must exert a relatively strong downward thumb force on the plunger unit to retain it in the “home” position in opposition the return spring and a relatively strong “blow out” spring defining the “soft” stop. In particular, any downward movement of the plunger unit beyond the “home” position activates the “blow out” spring which generates a strong upward force in opposition to such downward movement of the plunger unit. The pipette user senses or “feels” the start of the increase in the return force which provides the user an indication that the plunger unit has reached and is at the “home” position. 
     The pipette of the present invention, however, does not include a blow out spring. Further, with the present invention, rather than requiring the user to carefully sense the exact start of a sudden increase in a force opposing downward movement of a plunger unit in locating the “home” position for the plunger unit and rather than requiring the user to manually exert a strong downward force to maintain the plunger unit in its “home” position against a return spring and a blow out spring, the pipette of the present invention includes a mechanical assist mechanism. As the plunger unit reaches and is at the home position, the mechanical assist mechanism generates a lateral force which is translated by the plunger into a mechanical force in opposition to the return spring force. The mechanical opposition force is less than the upward force generated by the return spring and is reflected in a relatively small change in the downward force required to move the plunger unit as it approaches the home position and aids the pipette user in sensing the home position. Further, the opposition force generated by the mechanical assist reduces the manual force that the pipette user must exert to maintain the plunger unit in the home position. The mechanical assist thereby substantially reduces the physical and mental strain on the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. 
     Still further, the absence of a blow out spring in the pipette of the present invention means that a major operating force associated with all prior commercially available air displacement pipettes is eliminated in the present invention. That is, the large user generated downward force required to effect “blow out” of all liquid from the tip of the pipette in opposition to the strong blow out spring common in commercial air displacement pipettes. Thus, using a simple relatively low cost construction which eliminates the blow out spring common to air displacement pipettes, the present invention significantly reduces the problems associated with conventional manual pipettes with respect to physical and mental strain with only a minor increase in manufacturing cost. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side view of a manual air displacement pipette, partially in section, and including one form of the mechanical assist of the present invention. The manual pipette is illustrated at a position just below the upper stop or start position for a plunger unit included in the pipette. 
     FIG. 2 is a fragmentary side view of the manual pipette of FIG. 1 showing the plunger unit at its home position with the mechanical assist operative to aide a pipette user in maintaining the plunger unit at the home position. 
     FIG. 3 is an enlarged cross-sectional side view of the mechanical assist illustrated in FIGS. 1 and 2. 
     FIGS. 4 a, b  and  c  are graphs depicting the magnitude of the actuating force which a pipette user must exert on a plunger unit in moving the plunger unit from its upper stop to its home position and then to its lower stop position. FIG. 4 a  depicts the actuating force associated with a standard manual pipette. FIG. 4 b  depicts the actuating force associated with the pipette described in U.S. Pat. No. 4,041,764. FIG. 4 c  depicts the actuating forces associated with the manual pipette with the mechanical assist of the present invention illustrated in FIGS. 1 and 2. 
     FIGS. 5 a ,  5   b ,  5   c ,  5   d  and  5   e  are diagrammatic enlarged sectional side views similar to FIG. 3 illustrating different versions of the mechanical assist which may be included in the manual pipette of the present invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to FIGS. 1 and 2, a preferred form of the manual air displacement pipette of the present invention is illustrated and represented by the numeral  10 . The pipette  10  comprises a pipette body  12  preferably formed from a plastic material. The body  12  is axially elongated and shaped to be hand holdable with a liquid end  14  contiguous with and extending axially from a lower end of the body  12  to receive a disposable pipette tip  15 . A plunger unit  16 , upwardly biased by a return spring  18 , is supported for axial movement within the pipette body  12  between an upper stop  20  and a lower stop  24 . At the upper stop  20 , an upper end of an enlarged portion  33  of a plunger  34  of the plunger unit  16  engages the upper stop with an end portion of the plunger unit  16  extending from an upper end of the pipette body  12  to receive a control knob  22 . The body  12  and control knob  22  are shaped such that when a pipette user grips the body  12 , his or her thumb extends over the top of the control knob such that thumb action of the user will exert a downward force on the plunger unit  16  to move the plunger unit downward from the upper stop  20  against the action of the spring  18  to the lower stop  24 . At the lower stop  24 , a bottom stop member  36  moveable with the plunger unit  16  engages an annular shoulder  45  within the pipette body  12  and defines the lower stop to limit further downward movement of the plunger unit within the pipette body. 
     Also located within the pipette body  12  is a mechanical assist mechanism  26  for aiding in (i) locating the “home” position of the plunger unit  16  and (ii) holding the plunger unit at a “home” position against the continuous upward spring bias of the return spring  18 . 
     Parenthetically, the “home” position is the axial position of the plunger unit  16  in the pipette body  12  where the pipette  10  is ready for its tip  15  to be immersed in a liquid for pickup by the pipette  10  and subsequent dispensing into a receptacle. It is also the return position for the plunger unit  16  during repeated pipette operations in drawing liquid into and dispensing liquid from a series of disposable tips such as the tip  15 . In that regard, the pipette  10  includes a pipette tip ejector  27  such as the improved ejector described in U.S. Pat. No. 5,614,153 issued Mar. 25, 1997 and assigned to the assignee of the present invention. As is common practice in the pipeting of liquids, following each pipette operation, the disposable tip is ejected from the pipette and replaced with a new tip to insure against contamination of the series of liquids samples dispensed by the pipette. 
     As represented in FIGS. 1,  2  and  3 , the mechanical assist mechanism  26  is designed to generate a counter force to the upward force of the return spring  18 . The counter force is less than the upward force generated by the return spring. In this regard, as the piston unit  16  approaches its “home” position in the illustrated version of the mechanical assist  26 , the counter force preferably is sensed by the pipette user as a slight increase and then a reduction in the downward manual hand force which must be exerted to move the piston unit. Alternatively, the counter force may be sensed as a slight reduction or increase in the downward manual force moving the piston unit. Any of foregoing forces sensed by the pipette user signals the user of the approach of the “home” position and aids in the exact locating of the home position. Further, the counter force generated by the mechanical assist  26  substantially reduces the manual hand force which must be generated by the pipette user to maintain the plunger unit  16  at the home position ready for aspiration of liquid into the pipette tip  15  during repeated operations with the pipette. Still further, since the counter force generated by the mechanical assist mechanism  26  is less than the upward force generated by the return spring  18 , the pipette user maintains manual control over the position of the plunger unit  16  within the pipette body  12  both at the home position for the plunger unit  16  as well as during the initial upward movement of the plunger unit from the home position toward the upper stop position. After the initial movement of the piston unit  16  from the home position either in an upward or downward direction, the plunger is free of influence of the mechanical assist mechanism  26  and is only subjected to the upward influence of the return spring  18 , since the manual pipette of the present invention does not incorporate a blow out spring. This means that the pipette user maintains complete control over the rate of upward movement of the plunger unit during aspiration of the liquid into the pipette tip  15 . Accordingly, it is much easier for the pipette user to (i) maintain the pipette plunger at the exact same home position during a series of aspiration operations and (ii) allow the plunger unit to return to the upper stop position at the same velocity profile during successive aspiration operations with the manual pipette. 
     The above-described regulation of the manual force which the pipette user is required to generate in operating the manual pipette of the present invention including the mechanical assist  26  illustrated in FIGS. 1-3 is depicted in FIG. 4 c . The advantages afforded by the present invention may be appreciated by a comparison of FIG. 4 c  with the graphs of FIGS. 4 a  and  4   b  depicting the plunger unit activation forces associated with prior art manual pipettes including “soft” stops defining a “home” position. As depicted in FIG. 4 c , as the plunger unit in the manual pipette of the present invention is moved from its upper stop position, the manual force which the pipette user must generate is that which is required to overcome the return spring  18  and is depicted at  80  in each of FIGS. 4 a ,  4   b  and  4   c . However, as the plunger unit in the manual pipette of the present invention approaches its home position, the mechanical assist  26  illustrated in FIGS. 1-3 generates a controlled counter force which is reflected first as a slight and gradual increase in the manual force followed by a slight and gradual reduction in the manual force  80  as shown at  82  until the “home” position is reached. To maintain the plunger unit at the “home” position the pipette user needs only exert the reduced force indicated at  83 . Only then, and only if the pipette user desires to effect a “blow out” of liquid in the tip of the pipette  10  is the user required to exert an increased downward force as shown at  84  in opposition to the return spring  18 . However, as indicated above, since the manual pipette of the present invention does not include a blow out spring, the only element of the manual pipette  10  exerting an upward force on the piston unit  16  as it travels downward from the home position is the return spring  18 . This means that the manual force which is generated by the user of the pipette  10  of the present invention in effecting blow out is a downward manual force which slightly exceeds the upward force generated by the return spring. That downward manual force is depicted at  84  and is substantially less that the downward manual force required to effect blow out in a conventional manual pipette as depicted in FIG. 4 a . Thus, FIG. 4 c  clearly reflects (1) the reduction in the manual force on the plunger unit which signals the pipette user of the approach of the “home” position, (2) the reduction in the manual force required to maintain the plunger unit at the home position as compared to the operation of the prior art manual pipettes depicted in FIGS. 4 a  and  4   b  and (3) the significant reduction in the manual force which a user of the pipette of the present invention is required generate to effect blow out. 
     Referring more specifically to FIGS. 1 and 2, the plunger unit  16  comprises axially elongated plunger  34  terminating at its upper end in the control knob  22  and at its lower end in the bottom stop member  36 . The member  36  is secured to the upper end of a piston  38  moveable axially with the plunger  34  within the liquid end  14 . The return spring  18  surrounds the piston  38  with an upper end bearing on an underside  51  of an annular flange  52  extending outwardly from an upper end of the bottom stop member  36  and a lower end bearing on a seal retainer  40  for a fluid tight seal  41  seated on a shoulder  42  inside the liquid end  14  around the piston  38 . Thus confined, the return spring  18  continuously exerts an upward force on the piston  38 , the member  36  and hence the plunger  34  to continuously urge the plunger unit  16  upward toward the upper stop  20 , the upper stop being defined by an axially adjustable shoulder  44  within the body  12  of the pipette. 
     As illustrated most clearly in FIGS. 1 and 2, the bottom stop position for the plunger unit  16  in the pipette  10  is defined by the annular flange  52  at a top of the bottom stop member  36  which is designed to engage the shoulder  45  defining the lower stop  24  as the plunger unit is moved downward in response to downward manual force exerted by the pipette user on the push button  22 . 
     Likewise, FIGS. 1-3 most clearly illustrate that for the pipette  10 , the “home” position for the plunger unit  16  is defined by the mechanical assist  26 . In that regard, the illustrated version of the mechanical assist  26  comprises cam  48  and a cam follower  54 . The cam  48  is located on the plunger  34  below the enlarged portion  33  thereof while the cam follower  54  is secured to a inner sidewall of a cylinder  50  secured to the housing  12  and axially receiving the plunger unit  16 . In FIGS. 1-3, the illustrated version of the cam follower  54  comprises a spring loaded ball  56  mounted and captured within in a tubular housing  58  extending inwardly from the cylinder  50  toward the plunger  34 . An outer surface of the ball  56  is exposed to and normally spaced from the plunger  34 . However, as the plunger  38  is depressed by the pipette user during dispensing of liquid from the pipette, the ball will engage and ride over the cam  48 . As this occurs, the spring biased ball  56  exerts a lateral force on the plunger  34  which is reflected as depicted in FIG. 4 c  as a change in the plunger actuation force providing the user with the indication that the plunger is approaching and has arrived at the “home” position as previously described. Specifically, the illustrated version of the cam  48  comprises an annular sleeve member  60  which from an upper end  61  gradually increases and then decreases in annular thickness to form an annular depression  62  and then again gradually increases and decreases in annular thickness terminating at a lower end  63  of the sleeve. The ball  56  riding into the annular depression  62  provides the user with the indication that the plunger is in its above-described “home” position. 
     FIGS. 5 a ,  5   b ,  5   c ,  5   d  and  5   e  diagrammatically illustrate different embodiments of the mechanical assist mechanism  26 . In FIG. 5 a , the cam follower  50  is in the form a leaf spring member  64  carried by the plunger  34  for engaging the cam  48  secured to or forming an inside of the cylinder  50  and having an inner surface like the sleeve  61 . In FIG. 5 b , the cam follower  50  in the form the leaf spring member  64  is secured to the inside of the cylinder  50  to engage the surface of the cam  48  comprising the sleeve  61  as previously described. In both embodiments, the leaf spring  64  riding on the surface  61  including the depression  62  generates a lateral force on the plunger  34  which is reflected as changes in the downward plunger force providing the user of the pipette of the present invention with an indication of the approach of and arrival of the plunger at the “home” position. 
     In FIGS. 5 c ,  5   d  and  5   e , the mechanical assist mechanisms  26  comprise detent mechanisms resembling somewhat the cam and cam follower structures of FIGS. 5 a  and  5   b . In each illustrated detent mechanism, a spring loaded member extending either the plunger  34  or cylinder  50  rides on the other to exert a lateral force thereon and ride into and out of a depression defining the “home” position for the plunger in the associated pipette. In FIG. 5 c , the leaf spring  64  extends from the plunger  34  to ride on the cylinder into a depression  62 ′ while in FIG. 5 d , the leaf spring  64  extends from the cylinder to ride on the plunger and into an annular depression  62 ″. In FIG. 5 e , the previously described and illustrated (FIG. 3) spring loaded ball  56  rides on the surface of the piston into the depression  62 ″. 
     While differing in detail, in each embodiment of the mechanical assist mechanism  26 , the user is assisted in locating and maintaining the plunger in the “home” position as previously described. 
     While particularly preferred embodiments of the present invention has been illustrated and described herein above, it is to be appreciated that changes and modifications may be made in the preferred embodiment without departing from the spirit of the present invention. Accordingly, the present invention is to be limited in its scope only by the following claims.