Abstract:
A pipette assembly for automatic pipetting machines including a stepped mandrel and tip for providing a minimum contact seal between the pipette tip and mandrel. The mandrel includes a first cylindrical portion with a first exterior diameter and a second cylindrical portion with a second exterior diameter. Raised bands on the first and second cylindrical portions contact the interior wall of the pipette tip to form seals. Because only a portion of the mandrel, specifically the seal portions, contacts the pipette tip, lower forces are required to insert the tip onto the mandrel.

Description:
This application is a reissue application of U.S. Pat. No. 6,973,845, which was derived from U.S. patent application Ser. No. 09/764,691, filed on Jan. 18, 2001, which is herein incorporated by reference in its entirety for all purposes.  
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an apparatus for handling chemical and biological substances, and more particularly to pipetting systems. 
     2. Background Information 
     The use of manual, semiautomatic, or automatic pipette devices for the transfer and dispensing of precise quantities of fluids in analytical systems is well known as is the use of disposable pipette tip members. Disposable tips accommodate the serial use of such pipette devices in the transfer of different fluids without carryover or contamination. 
     A proper seat between the pipette device and disposable tip is essential. Most pipetting systems require a proper seal to create a vacuum for receiving and dispelling samples. Additionally, many analytical processes require very small sample sizes, for example, in the range of 1 to 250 micro liters. If the seal is not air-tight, the pipette device may not pick up the precise amount of sample that the device was set to receive. Therefore, the pipette device may receive and dispel too much or too little sample which could impact the quantitative or qualitative result of the assay. Also, many samples are very expensive and are wasted by unintended oversampling. This results in premature depletion of the sample and, thus, added cost. 
     Commercially available pipetting devices use several techniques for picking up and discarding disposable pipette tips. Some companies use specially designed mandrels for engagement with disposable pipette tips. These mandrel ends are generally tapered or cylindrical in shape to accommodate pipette tips. Both tapered and cylindrical mandrel ends provide a good seal with the pipette tip and work well to align the tip with the mandrel. However, large insertion forces are required for insertion of the pipette tip onto the tapered or cylindrical mandrel end. 
     With the tapered mandrel end, the engaged portion of the pipette tip continues stretching as the pipette tip travels farther up the mandrel end which results in an exponential increase in the insertion force required as the pipette tip travels farther up the mandrel. With the cylindrical mandrel end, the engaged portion of the pipette tip is held in the stretched position as the pipette tip travels farther up the mandrel end which results in a roughly linear increase in the insertion force required as the pipette tip travels farther up the mandrel. 
     To accommodate the large insertion forces required with cylindrical or tapered mandrel ends for automatic pipetting devices, many systems require high-inertia instrument structures to effectively attach and shuck disposable pipette tips. These high-inertia to instrument structures tend to be large and very expensive. Therefore, it is desirable to have a pipetting device and custom molded tip design that minimizes the force necessary to attach and shuck disposable pipette tips, thereby eliminating the need for massive and expensive high-inertia instrumentation systems. 
     To minimize the force necessary to attach and shuck disposable pipette tips, one pipetting device uses a substantially cylindrical mandrel in conjunction with custom molded pipette tips that have molded rings which act as seals between the mandrel end and the pipette tips. This prior art pipetting device is shown in  FIG. 6 . As shown in  FIG. 6 , the molded rings  10  of the pipette tip  100  engage the mandrel  200  to form seals. During insertion of the mandrel  200  into the tip  100 , only a substantially constant insertion force is required because only the molded rings  110  contact the surface of the mandrel during insertion and no additional surfaces contact the mandrel as insertion continues. While these pipette tips work fairly well with the devices for which they were designed, many of the tips are damaged during manufacturing. The single-piece core pin which forms the interior of the pipette tip must be pulled out of the tip during molding. Because the seals are on the interior wall of the pipette tip and extend inwardly toward the center axis of the pipette tip, the core pin must contact and pull upon the seals before it can be removed from the pipette tip during molding. This contact can damage the seals thus reducing the percentage of pipette tips that pass quality control testing and thereby resulting in increased manufacturing cost for the pipette tips. Furthermore, pipette tips with only minor damage may pass quality control testing, but may not be able to secure properly to the mandrel because of the damage caused by the molding process. Since most pipetting systems require a proper seal to create a vacuum for receiving and dispelling sample, the pipette device may not pick up the precise amount of sample that the device was set to receive if the pipette tip is not properly secured to the mandrel. Therefore, the pipette device may receive and dispel too much or too little of the sample which could impact the quantitative or qualitative result of the assay. Expensive samples may be wasted as a result. 
     Another problem with the pipetting device shown in  FIG. 6  results because the seals on the pipette tip are resilient. The resilient seals may improperly twist upon insertion of the mandrel into the pipette tip and prevent proper sealing. For example, if significant friction is encountered by a seal as it contacts the mandrel during insertion, the seal may twist or “roll” against the mandrel instead of sliding upon the mandrel. After such twisting, the seal may be deformed and may not properly seat against the mandrel, thereby preventing a proper seal from forming between the mandrel and the pipette tip. 
     For the foregoing reasons there is a need for a low insertion force custom tip and mandrel design in which the seals are positioned on the pipettor mandrel. This will reduce the need for large and costly high-inertia instrumentation. In addition, it will reduce manufacturing costs associated with pipette tips with molded rings acting as seals since fewer will be damaged during molding. 
     SUMMARY 
     The present invention is directed to an apparatus that satisfies the need for a low insertion force custom pipette tip and pipettor mandrel design in which the seals are positioned on the pipettor mandrel. 
     The pipette mandrel of the present invention is an elongated hollow metallic structure that includes a lead-in portion with a first cylindrical portion adjacent to the lead-in portion. The first cylindrical portion has a first exterior diameter with a first raised band positioned upon the first exterior diameter. Additionally, the mandrel may include a second cylindrical portion with a second exterior diameter adjacent to the first cylindrical portion. The second cylindrical portion also includes a second raised band positioned upon the second exterior diameter. Both the first and second raised bands are non-resilient and stationary, being integrated as part of the mandrel. 
     The pipette tip of the present invention includes a collar portion and an adjacent conical head. The conical head is the receptacle portion for receiving fluids. The collar portion is used to connect the pipette tip to the mandrel. The collar portion has an interior cylindrical wall which is defined by a first step portion having a first interior diameter. When the pipette tip is fully inserted onto the mandrel, the first raised band on the mandrel contacts the first step portion of the pipette tip. The interior cylindrical wall of the pipette tip may also have a second step portion having a second interior diameter that may contact a second raised band on the mandrel when fully inserted. Thus, the first raised band on the cylindrical portion contacts the interior wall of the pipette tip to form the first seal. Additionally, the second raised band on the cylindrical portion may contact the pipette tip to form a second seal. At a minimum, the second band is useful in aligning the pipette tip on the mandrel. Because only the seal portions of the mandrel contact the pipette tip, lower forces are required to insert the pipette tip onto the mandrel and remove the pipette tip from the mandrel. Additionally, placement of the seals on the mandrel as opposed to the pipette tips reduces manufacturing costs associated with pipette tips while adding almost no additional cost to mandrel manufacturing. Furthermore, placement of the non-resilient seals upon the mandrel provide for a consistent and reliable seal between the mandrel and the pipette tips. 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a pipettor assembly in accordance with the present invention. 
         FIG. 2A  is an enlarged cross-sectional view of the pipettor assembly showing the portion of the mandrel encircled by line B of  FIG. 1 . 
         FIG. 2B  is another cross-sectional view of the pipettor assembly shown in  FIG. 2A  with the mandrel more fully inserted into the pipette tip. 
         FIG. 3  is a cross-sectional view of the pipettor assembly which illustrates an alternative embodiment of the invention in which an internal molded ring on the pipette tip is the positive stop. 
         FIG. 4  is a perspective view of the pipette tip which illustrates an alternative embodiment of the invention in which the collar portion of the pipette tip has external ribs. 
         FIG. 5  is a cross sectional view of the pipette tip shown in  FIG. 5  taken along line A-A shown in  FIG. 3 . 
         FIG. 6  is a cross-sectional view of a prior art pipette tip with molded rings acting as seals on the pipette tip. 
     
    
    
     DESCRIPTION 
     Referring to  FIG. 1 , a pipettor assembly  10  in accordance with the present invention includes a pipette tip  12  and a pipettor mandrel  30  having a distal end  31  and a proximate end  33 . The pipette tip  12  is generally made from polypropylene, and has an elongated, truncated, body portion with a collar portion  16  and a conical head  14 . Referring to  FIG. 2A , the collar portion  16  includes a mouth  18  defined by a rim  19  and a funnel-shaped first taper  20 . The collar portion  16  further includes a second step  22  which is a cylinder-shaped portion having a substantially constant interior diameter. The second step  22  is defined between the first taper  20  and a second taper  23 . The second taper  23  is also funnel-shaped and feeds into a first step  24 . Like the second step  22 , the first step  24  is also a cylinder-shaped portion having a substantially constant interior diameter. The interior diameter of the first step  24  is smaller than the interior diameter of the second step  22 . After the first step  24 , the collar portion ends in a positive stop  26 . The positive stop is a flange between the collar portion  16  and conical head  14 . 
     The exterior of the mandrel  30  is defined by a tapered lead in  38  on the distal end  31  of the mandrel, followed by a first band  40 , a first cylindrical portion  42 , a second cylindrical portion  44 , and a second band  46 . The diameter of the lead in  38  gradually increases from the distal end  31  up to the diameter of the first band  40 . The first band  40  is a raised portion on the mandrel  30  adjacent to the lead in  38  upon the first cylindrical portion  42 . The first cylindrical portion  42  is an elongated portion of the mandrel  30  extending from the first band  40 . The diameter of the first band  40  is slightly larger than the diameter of the first cylindrical portion  42 . On the opposite end of the first cylindrical portion  42  from the first band  40 , the mandrel  30  tapers into the second cylindrical portion  44 , which has a larger diameter than the first cylindrical portion  42 . Like the first band  40 , the second band  46  is a raised portion upon the mandrel  30 . The second band  46  is positioned upon the second cylindrical portion  44  and has a diameter slightly larger than that of the second cylindrical portion  44 . 
     Insertion of the mandrel  30  into the pipette tip  12  is now described with reference first to  FIG. 2A . The mouth  18  of the collar portion  16  of the pipette tip  12  is designed to receive the mandrel  30 . Upon insertion of the mandrel into the pipette tip, the mandrel lead in  3 $8 moves axially towards the positive stop  26  of the pipette tip  12  in the direction of the arrow  31 . Initially, the mandrel lead in  38  enters the taper  20  of the pipette tip  12 . Next, the first band  40  of the mandrel  30  enters the taper  20  of the pipette tip  12 , and then enters the second step  22  of the pipette tip  12 . The diameter of the first band  40  is smaller than the diameter of the second step  22  of the pipette tip  12 . Thus, as the lead in portion  38 , first band  40 , and first cylindrical portion  42  enter the second step  22 , the first band  40  only occasionally contacts the interior cylindrical walls of the second step  22  of the pipette tip  12 . The occasional contacts with the interior cylindrical walls may adjust orientation of the pipette tip  12 , causing the pipette tip to properly align with the mandrel  30  during insertion. Additional alignment occurs when the first band  40  of the mandrel  30  moves past the second taper  23  and into the first step  24  of the pipette tip  12 . As shown in  FIG. 2A , sealing occurs when the first band  40  of the mandrel  30  fully engages the first step  24  of the pipette tip  12  causing a portion of said first step  24  to be displaced because the diameter of the first seal is slightly larger than the diameter of the first step. When the first step  24  is displaced, it presses against the first band  40  to form an air-tight seal between the mandrel and tip. Thus, at this point, the pipette tip  12  will stay on and seal. Sealing continues to occur as the first band  40  moves in the direction of the arrow  31  toward the positive stop  26 . 
     The first cylindrical portion  42  of the mandrel  30  does not generally contact the interior cylindrical wall of the pipette tip  12  as the mandrel is inserted because the diameter of the first cylindrical portion is less than the interior diameter of both the second step  22  and the first step  24  of the pipette tip  12 . However, there may be some incidental contact between the first cylindrical portion  42  and the first step  24 , depending upon manufacturing tolerances, but this incidental contact does not contribute any significant resistance during insertion. Because only a portion of the mandrel  30 , specifically the first band  40 , contacts the pipette tip  12 , roughly constant insertion forces are required to insert the mandrel into the tip once the first band fully engages the mandrel. This constant insertion force provides an advantage over other pipettor assemblies where a greater portion of the mandrel contacts the tip. 
     Final alignment occurs when the second cylindrical portion  44  and the second band  46  of the mandrel  30  enters the taper  20  and second step  22  of the pipette tip  12 . As shown in  FIG. 2B , a second seal may be formed, depending upon tolerances, if the second band  46  of the mandrel  30  engages the second step  22  of the pipette tip  12  causing a portion of said  1  second step  22  to stretch. As with the first seal, roughly constant insertion forces are required if a second seal is formed because only the second band  46  contacts the interior wall of the second step  22  of the pipette tip  12 . The mandrel  30  is fully inserted into the tip  12  when the mandrel lead in  38  abuts the positive stop  26  on the pipette tip  12 . 
     As with insertion, the forces required to remove the mandrel  30  from the tip  12  are roughly constant during removal. During removal, if a second seal has been formed between the second band  46  and the second step  22  of the pipette tip  12 , contact is maintained between the second band  46  and the interior wall of the second step  22  of the pipette tip  12  until the second band clears the second step and enters the first taper portion  20  of the mouth  18  of the pipette tip. The second cylindrical portion  44  of the mandrel  30  does not continually contact the interior wall of the first taper  20  of the pipette tip  12  as the mandrel is removed because the diameter of the first taper of the pipette tip is larger than the diameter of the second cylindrical portion  44 . There may be some incidental contact between the second cylindrical portion  44  and the first taper  20 , but this incidental contact does not contribute any significant resistance during removal. 
     The first seal is maintained during removal until the first band  40  clears the first step  24  of the pipette tip  12 . The first band  40  then enters the second taper  23  followed by the second step  22  of the pipette tip  12 . As the first band  40  is removed from the tip  12 , the first band of the mandrel  30  does not generally contact the interior wall of the second step  22  or first taper  20  since the diameters of the second step and first taper are both larger than the diameter of the first band. There may be some incidental contact between the first cylindrical portion  42  and the second step  22  or first taper  20 , but this incidental contact does not contribute any significant resistance during insertion. Therefore, removal forces are similar to the roughly constant insertion forces. 
     Since the seals for the pipettor assembly  10  are on the mandrel and not on the interior wall of the pipette tip, greater manufacturing yields of the pipette tips can be attained. As discussed previously, a core pin which forms the interior of the pipette tip must be pulled out of the tip during manufacturing. When the seals are on the interior wall of the pipette tip as with some prior pipette tips, the core pin must be dragged across the seals in order to remove the core pin from the mold, thus increasing the likelihood of damage to the seals. In contrast, during removal of the core pin from the pipette tips of the present invention, the core pin is  1  pulled out of the pipette through portions of the pipette tip with increasingly greater diameters, thereby eliminating any drag. Thus, fewer pipette tips are damaged during manufacturing when the seals are positioned on the mandrel and not the pipette tip. 
     Furthermore, since the seals for the pipettor assembly  10  are on the non-resilient mandrel  30  and not on the resilient interior wall of the pipette tip  12 , there is no twisting of the seals upon insertion of the tip onto the mandrel. As discussed previously, when the seals are resilient and located on the pipette tip, they may improperly twist upon insertion of the mandrel into the pipette tip and prevent proper sealing. However, the present invention avoids this problem by integrating non-resilient seals onto the mandrel. When such seals are positioned on the mandrel  30  and not the pipette tip  12 , twisting of the seals upon insertion of the tip onto the mandrel is eliminated and a proper seal is consistently formed between the mandrel and the pipette tip. 
     Another embodiment of the present invention further improves manufacturability of the pipette tips. In this embodiment, shown in  FIGS. 3-5 , the exterior of the collar portion  16  is defined by external ribs  17  that run parallel to the axis of the pipette tip. As shown in  FIGS. 4 and 5 , the external ribs  17  are positioned along a section of the collar portion  16  adjacent to the conical head portion  14  of the pipette tip  12 . These ribs improve the flow of plastic into the tip during molding thereby improving the ease of manufacturing the tips. At the same time, by adding ribs, the wall of the collar portion  16 , particularly the first step  24 , may be made thinner. By thinning this wall, the forces required to insert or remove the mandrel  30  from the tip  12  are lowered because the wall of the collar portion  16  is easier to displace by the first band  40  on the mandrel during insertion or removal. As shown in  FIG. 3 , this embodiment also includes a molded internal ring  27  in the pipette tip  12  which is a positive stop for the mandrel  30  when it is inserted into the tip. This molded internal ring also functions as a “puller ring” that facilitates molding by keeping the tip on the core pin when the mold opens. Other puller rings  29  are included on the conical head  14  of the pipette tip  12 . These puller rings  29  on the conical head  14  of the pipette tip  12  may also be included in other embodiments of the invention, such as that shown in  FIG. 2A , to facilitate molding of the pipette tip. 
     The previously described versions of the present invention have many advantages including, but not limited to low insertion, sealing, and removal forces, and higher manufacturing yields for the custom molded pipette tips. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.