Abstract:
A pipettor includes a wash chamber between an upper plate, a lower plate, an upper seal, and a lower seal. The upper and lower seal retain each other and are located between the upper and lower plates. The pipettor includes a gasket located below the lower plate, a pipette tip retained by the gasket, and a piston with a tapered tip that passes through the plates, seals, and gasket into the pipette tip. The upper and lower seal isolate the piston from the wash chamber. The pipettor includes a channel defined by the piston, extending into the pipette tip, and an actuator that advances and retracts the piston. When the actuator retracts the piston such that the piston loses sealing contact with the lower seal and the wash chamber is supplied with wash fluid and pressurized, wash fluid passes through the groove in the upper seal and into the channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from U.S. Provisional Application No. 61/792,453, filed Mar. 15, 2013 for “WASH THROUGH PIPETTOR” by Richard J. Shoeneck et al. 
    
    
     BACKGROUND 
     The present invention relates to pipettors for high throughput screening, and specifically to the use of a wash through pipettor for dispensing and aspirating. 
     Laboratories use pipettors to transfer liquids from one container to another. Automated pipettors for high throughput screening typically transfer liquid from one array of containers to another array of containers. The container arrays are most commonly 96 well or 384 well microtiter plates. The 96 array format uses 8 rows by 12 columns using 9 mm grid spacing. The 384 format uses twice the density with 16 rows by 24 columns using 4.5 mm grid spacing. When these pipettors are used, the tips need to be discarded or washed when changing liquids. The current method of washing the pipette tips is to aspirate wash fluid, and then dispense it into a waste container. The wash cycle is repeated many times to dilute the residue on the inside of the tips to an acceptable level. 
     Another concern with pipettors is dispense volume accuracy, especially at volumes less than 1 uL. When dispensing small volumes, the wetting and break-off behavior of the very small droplet at the tip is influenced by many factors. If the liquid prefers to wet to the target container (either dry or containing liquid already), then the liquid column in the pipette tip will experience a downward force as the liquid wets out into the target well. The amount of liquid dispensed is variable depending upon the wetting action as compared to the forces that will keep the rest of the liquid in the tip. Conversely, if the liquid is repelled from the target, then it may be pushed back up into the tip. The forces between the tips and containers are often random and variable. Current pipettors all have a sizeable volume of air above the liquid that acts as a coupling member between the piston and the liquid. The compressibility of the air and the vapor pressure therein create a weak coupling between the piston position and the fluid position. 
     SUMMARY 
     A pipettor includes an upper plate, a lower plate, and a wash chamber defined by a space between the upper plate and the lower plate. The pipettor further includes an upper seal with a groove and a lower seal, the upper seal located below the upper plate and retained by the lower seal, and the lower seal located above the lower plate and retained by the upper seal. The pipettor further includes a gasket located below the lower plate, a pipette tip retained by the gasket, and a piston with a tapered tip that passes through the upper plate, the upper seal, the lower seal, the lower plate, and the gasket into the pipette tip. The upper seal and the lower seal isolate the piston from the wash chamber. A channel is defined by the piston and extends into the pipette tip. The pipettor further includes an actuator for advancing and retracting the piston. When the actuator retracts the piston such that the piston loses sealing contact with the lower seal and the wash chamber is supplied with a wash fluid and pressurized, the wash fluid passes through the groove in the upper seal and into the channel defined by the piston. 
     A method of operating a pipettor includes retracting a piston with a tapered tip such that the piston passes through a lower plate and a lower seal and loses sealing contact with the lower seal, the lower seal retaining an upper seal with a groove and the upper seal retaining the lower seal. The method further includes supplying a wash chamber surrounding the lower seal and the upper seal with wash fluid and pressurizing the wash chamber such that the wash fluid passes through the groove of the upper seal into a channel defined by the piston, the channel passing through a lower plate below the lower seal, through a gasket below the lower plate, and into a pipette tip retained by the gasket. The method further includes advancing the piston such that the wash fluid flows through the channel, filling the pipette tip and such that the piston seals off the channel from the wash chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of the wash through pipettor of the present invention. 
         FIG. 2  is a cross-sectional view of the wash through pipettor of the present invention along line  2 - 2  in  FIG. 1 . 
         FIG. 3A  is an exploded view of the sealing mechanism of the wash through pipettor of the present invention. 
         FIG. 3B  is a side view of the sealing mechanism of the wash through pipettor of the present invention. 
         FIG. 4A  is a cross-sectional view of the sealing mechanism of the wash through pipettor of the present invention along line  4 - 4  in  FIG. 3B . 
         FIG. 4B  is a zoomed-in view of the cross-sectional view of the sealing mechanism in  FIG. 4A . 
         FIG. 5  is a cross-sectional view of a pipette tip of the wash through pipettor of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a front view of wash through pipettor  10 .  FIG. 2  is a cross-sectional view of wash through pipettor  10  along line  2 - 2  in  FIG. 1 . As shown in  FIG. 1  and  FIG. 2 , wash through pipettor  10  includes piston assembly  12 , actuator assembly  14 , and tip tray  16 . Actuator assembly  14  includes actuator frame  18 , actuator plate  20 , linear bearings  22 , ball screw nut  24 , ball screw  26 , actuator motor  28 , and tip tray clamp  30 . Piston assembly  12  includes an array of pistons  32  with tapered tips  36 , piston plate  34 , top seal plate  38 , upper seal  40 , wash plate upper seal retainer  42 , wash chamber  44 , wash plate  46 , lower seal  48 , cylinder block  50 , thru tip gasket  52 , supply port  56 , and tip gasket retainer  58 . Tip tray  16  includes pipette tips  54  and tip holder  60 . 
     Pistons  32  are connected to piston plate  34 . Pistons  32  with tapered tips  36  pass through top seal plate  38 , upper seal  40 , wash plate upper seal retainer  42 , wash chamber  44 , wash plate  46 , lower seal  48 , cylinder block  50 , and thru tip gasket  52 . Pistons  32  define fluid flow channels  61 . The volume capacity of each fluid flow channel  61  is determined by the cross section area of each piston  32  multiplied by the maximum stroke length. The maximum stroke length is the sum of the thickness of cylinder block  50  and the length that pistons  32  are allowed to advance down into the interior of pipette tips  54 . 
     Wash plate  46  includes wash chamber  44  supplied with wash fluid via supply port  56 . Upper seal  40  is retained between top seal plate  38  and wash plate upper seal retainer  42 . Lower seal  48  is retained between wash plate  46  and cylinder block  50 . Upper seal  40  and lower seal  48  are designed to seal around pistons  32 . Thru tip gasket  52  is designed not to seal around pistons  32 . Thru tip gasket  52  is designed to seal against the top rim of pipette tips  54 . Thru tip gasket  52  is retained by tip gasket retainer  58 . Tip tray  16  is loaded into tip tray clamp  30 , which presses tip tray  16  against thru tip gasket  52  to form a seal between pipette tips  54  and thru tip gasket  52 . 
     Actuator assembly  14  holds piston assembly  12  and tip tray  16 . When actuator motor  28  turns ball screw  26 , actuator motor  28  drives ball screw nut  24  to position actuator plate  20  along linear bearings  22 . Actuator plate  20  is bolted to piston plate  34  to move pistons  32  through top seal plate  38 , upper seal  40 , wash plate upper seal retainer  42 , wash chamber  44 , wash plate  46 , lower seal  48 , cylinder block  50 , and thru tip gasket  52 . Retracting pistons  32  away from pipette tips  54  is an aspirate motion. During an aspirate motion, fluid or gas is aspirated into pipette tips  54 . Advancing pistons  32  toward pipette tips  54  is a dispense motion. During a dispense motion, fluid or gas is dispensed from pipette tips  54 . 
       FIGS. 3A and 3B  are an exploded view and a side view illustrating the sealing mechanism of wash through pipettor  10 .  FIG. 4A  is a cross-sectional view of the sealing mechanism of wash through pipettor  10  along line  4 - 4  in  FIG. 3B .  FIG. 4B  is a zoomed-in view of the cross-sectional view of the sealing mechanism in  FIG. 4A . The sealing mechanism of wash through pipettor  10  includes pistons  32  with tapered tips  36 , top seal plate  38 , upper seal  40  with groove  62 , lower seal  48 , and cylinder block  50 . Gap  64  provides a space between top seal plate  38  and cylinder block  50 . Upper seal  40  is retained by lower seal  48  and lower seal  48  is retained by upper seal  40 . 
     A wash cycle in wash through pipettor  10  begins by retracting pistons  32  until tapered tips  36  lose sealing contact with lower seal  48 , acting as a mechanical valve by opening fluid flow channels  61  to wash chamber  44 . Wash fluid, such as purified water, is supplied to supply port  56 . Wash chamber  44  is pressurized, upper seal  40  becomes tighter and lower seal  48  becomes looser, and wash fluid floods wash chamber  44 . Wash fluid flows through gap  64  into fluid flow channels  61 . Groove  62  of upper seal  40  ensures that wash water can get between upper seal  40  and lower seal  48  into fluid flow channels  61 . Tapered tips  36  of pistons  32  increase wash fluid flow into fluid channels  61  as pistons  32  are retracted partially to fully from lower seal  48 . 
     Wash fluid flows through fluid flow channels  61 , passing through wash plate  46 , lower seal  48 , cylinder block  50 , and thru tip gasket  52 , and washing out pipette tips  54 . In an alternative embodiment, lower seal  48  may be a check valve with a cracking pressure suitable for opening with high pressure wash water, but remaining sealed for normal aspirate and dispense pressures, allowing wash fluid to flow into fluid channels  61  without retracting pistons  32 . Wash through pipettor  10  may be positioned over a waste container to catch waste water as it flows out of pipette tips  54 . The waste container may also be a bath type container where pipette tips  54  are submerged for cleaning the outside of pipette tips  54 , with or without sonication. 
     After sufficiently washing the inside of each channel of fluid flow channels  61 , supply port  56  is closed. Pistons  32  are advanced to pass through lower seal  48 , closing off fluid flow channels  61  from wash chamber  44 . Pistons  32  are further advanced to dispense excess wash fluid into the waste container. If the aspirate volume for the next operation is known, pistons  32  may be advanced a partial stroke sufficient for the needs of the next aspiration. This will save time and avoid drawing contaminants higher up in fluid flow channels  61 . When the wash water dispense action is finished, the wash cycle is complete. At the end of a wash cycle, each channel of fluid flow channels  61  is filled with wash fluid  64  from upper seal  40  to the end of pipette tips  54 . 
       FIG. 5  is a cross-sectional view of a pipette tip of pipette tips  54  of wash through pipettor  10 , including wash fluid  64 , air gap  66 , and sample fluid  68 . An aspirate cycle of wash through pipettor  10  begins after the conclusion of a wash cycle. Pistons  32  are retracted to aspirate air gap  66 . Air gap  66  is required to separate wash fluid  64  from aspirated sample liquid  68 . The size of air gap  66  will determine the rigidity of the coupling between the piston motion and the fluid motion. If air gap  66  is too small, the risk of wash fluid  64  and aspirated sample liquid  68  mixing is increased. If air gap  66  is too large, the coupling will become less rigid. When dispensing large volumes, air gap  66  can be made larger to guard against accidental dilution. When dispensing small volumes, air gap  66  is kept as small as possible. Since fluid flow channels  61  are mostly liquid filled, the issue of a varying vapor pressure is avoided. This is most significant when fluid flow channels  61  are filled with dry gas, and aspirating a liquid causes the gas to absorb vapor, creating a slow increase of vapor pressure. This issue is a common challenge with typical pipettors as the vapor pressure of the gas in each channel of fluid flow channels  61  varies when pipette tips are open to atmosphere as opposed to filled with sample liquids. 
     After air gap  66  is created, wash through pipettor  10  is positioned with pipette tips  54  dipped in a source plate. The source plate may be a plate with a plurality of wells, such as a microplate. Pistons  32  are retracted further to aspirate sample liquid  68  from the source plate. Pipette tips  54  are then retracted from the source plate. The velocity of retraction may be adjusted to influence the break-off behavior of the liquid at pipette tips  54  to increase aspiration accuracy. 
     After the aspiration cycle is complete, the dispense cycle begins by positioning pipette tips  54  over the target plate. There are many variations of dispensing methods that include pre-dispensing a small volume prior to making physical contact with the target. The cycle may include pre-moving the fluid up and down within pipette tips  54  to avoid a dry meniscus causing sample fluid  68  to not move freely within fluid flow channels  61 . Sample fluid  68  is dispensed by advancing pistons  32  through fluid flow channels  61  toward and/or into pipette tips  54 . After the dispense cycle is complete and sample fluid  68  makes contact with the target, pipette tips  54  are retracted away from the target plate. The velocity of retraction may be adjusted to influence the break-off behavior of the liquid at pipette tips  54  to increase dispensing accuracy. The dispense cycle is often repeated several times for a single aspirate cycle. After the dispensing cycles are complete, the wash cycle is performed. 
     In alternative embodiments, compressed air may be supplied through supply port  56  to push out wash liquid  64 , or to perform drying of fluid flow channels  61 . Wash cycles may include alternating wash liquid with air to operate wash through pipettor  10  with the advantages of the wash through cleaning, but avoid using wash liquid  64  with air gap  66 . When aspirating and dispensing in the air only mode, pistons  32  are capable of advancing far into pipette tips  54 , minimizing the volume of air in fluid flow channels  61 , causing the coupling between the piston motion and the fluid motion to be more rigid and improving dispense volume accuracy. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.