Title: Micropipette and Method of OperationField of the inventionThis invention relates to a micropipette and to a method of dispensing liquid by such a pipette.
Backqround to the inventionConventional syringe technology is useful for displacing a wide range of volumes down to approximately 10 lustres (one microlitre). To displace volumes much smaller than this requires a different approach owing to the limitations imposed by manufacturing methods. The invention aims to provide a micropipette capable of displacing volumes in the region of 10 9 to 10 litres with a view to producing a system capable of picking up and transferring a single biological cell, other small particles immersed in liquids, or similar volumes of liquid media.
Summary of the inventionAccording to one aspect of the invention a micropipette comprises a tube open at one end and closed at the other end, the open end of the tube having a small bore up which liquid rises by capillary action when the open end is dipped in the liquid, and means for controlling the temperature of the internal tube volume enclosed by liquid in the tube, whereby control of the temperature affords control over the amount of liquid drawn into the tube and/or dispensed from the tube.
In order to dispense a volume of the order of 10 9 litres it is necessary to produce a tube having a bore of approximately lOAm (10-6 metres) such that a displacement of the liquid meniscus within the tube of 10/Lm will result in a volume displacement of 10 12 m3 m39 litres to a first approximation). It is convenient to produce this tube by drawing glass capillary tube in a suitable apparatus. The small bore preferably extends over a length terminating at the open end of the tube, the remaining length of the tube (which surrounds said enclosed volume) having a larger cross-sectional area.
The means for controlling the temperature of the internal tube volume preferably comprise a heater which in a preferred embodiment takes the form of a filament lamp.
Alternatively, a Peltier device may be used. Such a device relies on the Peltier effect, whereby heat is liberated or absorbed at a junction where an electric current passes from one metal to another. The heater may be controlled in order to raise the temperature of the internal tube volume to a predetermined level, or to a series of such levels, to enable a corresponding predetermined volume, or a series of such volumes, to be dispensed. The means for controlling the temperature may alternatively comprise a cooling element for drawing liquid into the micropipette.
The micropipette may be mounted on a micromanipulator or robot arm attached to a microscope, so that the liquid can be optically examined.
According to another aspect of the invention a method of dispensing a volume of liquid by means of a micropipette tube open at one end and closed at the other end comprises inserting the open end of the tube into the liquid, allowing the liquid to rise intoothe tube by capillary action and controlling the temperature of the internal volume in the closed end of the tube to draw in a controlled volume of liquid and/or to dispense a controlled volume of liquid.
It is desirable to be able to identify a single cell (or group of cells) under a light microscope and then to be able to transfer the cell (or cells) to another vessel for the purpose of further study or further culture, and the micropipette according to the invention enables this to be done. The micropipette is sufficiently small to be mounted on a micromanipulator or robot arm attached to a microscope. When mounted in such a manner the following operations may be carried out.
The dispensing tip (i.e. the open end of the tube) is first introduced into the liquid containing the cells so that some of the liquid is drawn into the tube by capillary action. A small volume of liquid may then be expelled by raising the temperature of the tip to temperature T1. The tip is then placed in close proximity to the desired cell (or cells) and the heater turned off, causing the target cell (or cells) to be drawn into the tube. This may be observed given suitable optics. The tip is then withdrawn and inserted in a second vessel containing a little liquid. The cell may effect further transfers.
The invention will now be further described by way of example with reference to the accompanying drawings in which:Figure 1 shows a glass tube before formation into a closed micropipette tube,Figure 2 shows the closed tube dipped into a liquid medium,Figure 3 shows a graph of temperature variation plotted against volume dispensed,Figure 4 shows a micropipette according to the invention, andFigure 5 shows an alternative to the construction ofFigure 4.
Referring to the drawings, a glass capillary tube 10 (Figure 1) is drawn so as to have a small bore over a length 12 and a larger bore over the remaining length 14.
The end of the length 14 is sealed at 16 (Figure 2) and placed in intimate contact with a filament lamp 18 (Figure 4) serving as a heater. The lamp is a conventional low voltage filament lamp which is cemented to the glass in order to provide both the seal at the end of the tube and the intimate heat transfer. This provides a heater which is readily controllable from a safe low voltage supply, together with a convenient package for handling.
The end 20 of the small bore of the tube is dipped in the liquid 22 to be dispensed, capillary action then draws up the liquid until the internal gas pressure in the broad part of the tip equals the capillary pressure (Figure 2).
The exact position of the meniscus 24 is a function of the diameter of the tube and the surface characteristics of both the glass tube and the liquid.
When the liquid meniscus 24 has reached the equilibrium position the glass tube may be withdrawn from the liquid and placed in a target position for dispensing. The liquid is then slowly expelled by heating the internal gas volume within the tube, by electrical energisation of the lamp 18.
By raising the temperature of the internal gas volume te predetermined levels (T1, T2...) a series of unit volumes of liquid (V1, V2....) may be dispensed (Figure 3).
An alternative arrangement is shown in Figure 5. The pre-formed pipette 25 is mounted on a pin 23 using a suitable medium (eg silicone grease) to effect a gas tight seal. The air space is heated/cooled by use of a Peltier device 21 held in intimate contact with the pipette by use of a spring 26.