Liquid dispensing apparatus and method
A multiwell plate for microarraying comprising a self-sealing lower membrane and optionally also a self-sealing upper membrane. Spotting is performed by pushing a pin down through the liquid and then on to pierce the self-sealing lower membrane. A slide having an upper spotting surface is arranged under the well plate. The liquid sample forced through the lower membrane by the pin tip can thus be deposited directly onto the spotting surface. The pin is then withdrawn upwards through the lower membrane, which automatically reseals preventing further loss of liquid. The optional resealable upper membrane also prevents loss of sample liquid by evaporation and spillage. By contrast to the prior art, there is the major advantage that the pin head does not have to traverse between the well plate and slide to collect sample liquid, thus dramatically increasing operational speed for an automated microarraying apparatus employing the multiwell plate. Excellent spot reproducibility is also observed.
The invention relates to an apparatus for and method of dispensing liquid. More especially, but not exclusively, the invention relates to dispensing liquid from well plates as widely used in the field of chemistry and biotechnology for microarraying and other applications.
Microarraying is a technique in widespread use. Conventional microarraying is based on standard multi-well plates having a 4.5 mm grid and 384 wells. However, larger array sizes of 1536 wells are becoming more widely used, these larger arrays conform to a 2.25 mm grid. Liquid samples are stored in the wells of a well plate. The liquid may be assays or any other biological or chemical sample of interest. To spot the liquid from a well, a pin is dipped in the well to retrieve an amount of the liquid. The pin carrying an amount of the sample liquid is then moved across to a spotting surface of a microscope slide or other suitable surface. A spot of liquid is deposited on the slide by bringing the pin into close proximity, or by physically contacting the tip of the pin, with the slide surface.
These pin designs have in common that they rely on capillary action to gain a reservoir of sample liquid sufficient for many spot depositions. This avoids having to dip into the well for each spot.
Most microarray pins in the prior art float vertically in a common head. They rest in the lowest position by gravity or spring biasing. The head tends to over travel by a small amount and the pins will lift in the head by the over travel.
Regardless of the pin design, spotting is carried out with the following basic steps. The pin is moved to above the well plate. The pin is dipped in a well of the well plate to retrieve some liquid. The pin carrying the liquid is moved over to above the spotting surface. The retrieved liquid is deposited from the pin onto the spotting surface, either with only one spot, or with several spots for a pin that carries a reservoir of sample liquid. The pin is moved back to the well plate to retrieve more liquid for further spotting.
Design effort has been concentrated on speeding up the head transit times so that the time taken between dipping in the well plate and spot deposition on the slide is reduced. As mentioned further above, the pins are also sometimes designed to use capillary action for storing a charge of liquid in the pin sufficient for depositing a number of spots. This also speeds up the spotting procedure by reducing the number of times the head needs to be traversed between the well plate and the slide.SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a well plate with a self-sealing lower membrane. Spotting is performed by pushing a pin down through the liquid and then on to pierce the self-sealing lower membrane. A spotting surface is arranged under the well plate. The liquid sample forced through the lower membrane by the pin tip can thus be deposited directly onto the spotting surface. The pin is then withdrawn upwards through the lower membrane, which automatically reseals preventing further loss of liquid.
By contrast to the prior art, the pin head does not have to traverse between the well plate and spotting surface to collect sample liquid, thus dramatically increasing operational speed. Moreover, experiments have proved that the spot size obtained by deposition through the self-sealing membrane is highly consistent. In prior art systems which rely on one dip of the pin into the well to put down many spots, larger variances in the spot size tend to occur as the pin becomes dry through evaporation and deposition.
The well plate may also be provided with a self-sealing upper membrane, thereby fully enclosing the liquid in the well. The pin then pierces first through the upper membrane and then on through the liquid and the lower membrane. When the pin is withdrawn, the upper membrane self seals in the same manner as the lower membrane. Consequently, loss of sample from the wells by evaporation is prevented. This is especially useful for valuable or toxic samples and has the further advantage of greatly reducing the risk of sample contamination. Additionally, the upper membrane wipes the shank of the pin as it is withdrawn. This cleaning action cleans the pin while at the same time reducing loss of sample liquid. Moreover, the upper membrane ensures that accidental dropping of the well plate will not result in spillage.
Well plates may be provided in a variety of sizes and configurations. Standard 96, 384 or 1536 geometries may be provided. Specially sized well plates may also be developed to suit specific applications, or as a proprietary measure.
According to a second aspect of the invention, there is provided a head apparatus for operation with the multi-well plate of the first aspect of the invention. The head apparatus comprises a pin head and a mounting frame adapted to hold a multiwell plate beneath the pin head. A motor stage is operable to drive the pins of the pin head down through the multiwell plate. The pins can thus be actuated through the wells and through the self-sealing membrane to deposit a sample directly onto a spotting surface held below the head apparatus. The pin head and a well plate held thereto can thus be moved around together by a robotic guidance system, instead of moving the head independently of the well plate as in the prior art.
The pins are preferably fixed in the body portion so that their tips lie in a common plane distal the body portion. Slidably mounted pins are not necessary, resulting in a considerable cost saving.
Alternatively the pins may be individually actuatable and addressable, for example using conventional pin array addressing mechanisms.
Advantageously, the head may further comprise an abutment arranged to stop the pin tips being advanced beyond a plane defined by the abutment. The abutment is designed to contact the spotting surface simultaneously with the pins for spotting. The abutment thus defines the maximum travel of the pins during spot deposition. The pins are preferably constrained so that they either stand off slightly from the spotting surface or only just contact it at their points of maximum travel.BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:
In operation, to dispense an amount of sample, a pin is driven down from the position shown by pin 10′. The pin travels down through the upper membrane 16, then through the sample liquid and on through the lower membrane 14 until in close proximity with an upper surface of the spotting surface 16. This position is shown by pin 10 in
Experiments have shown that the quantity of sample carried through the membrane is highly consistent, providing spots of 80 micrometer diameter on a glass spotting surface, with very low variance. Spot size can be varied by using pins with varying tip diameters. In the experiments, the tips were not pressed against the spotting surface, but rather brought into a nominally zero stand-off or offset with the spotting surface using an abutment arrangement described further below. Liquid deposition is thus driven by surface tension and fluid flow effects, or by throwing the liquid off the tip by deceleration. In the experiments, the pins were traditional surgical needles made of 316 grade stainless steel modified by flattening the needle tips to a diameter of 50 micrometers. In the experiments, the lower membrane resealed, apparently perfectly, with no compromise to the sealing properties over a test with 1000 piercing actions in a single piercing position. No loss of sample (other than through the spot deposition) or damage to the membrane was detectable.
Other needle tip shapes and dimensions may be used to provide dosage control of the amount of deposited liquid. For example, fluted needle tips may be used.
Although the membranes are shown as single sheets adhesively bonded to the well plate main body, it will be understood that the upper or lower membranes may be segmented. For example, individual membrane sections could be provided for each well or for groups of wells. Further, the membranes could be mechanically clamped onto the surfaces of the well plate main body, for example by a plate perforated with a grid of holes. Moreover, it will be understood that the upper membrane is optional. The upper membrane could be dispensed with altogether. Alternatively, the upper membrane could be fitted initially, after filling the well plate with sample liquid, to prevent evaporation and spillage during transport and storage. The upper membrane could then be removed immediately prior to microarraying. It will thus be understood that the upper membrane need not be a self-sealing membrane in all cases.
Contrary to conventional designs, the pins 10 are fixed in a body part 31 of the head in order to allow them to be pushed through one or more membranes, as is required with the multiwell plates described herein. Fixing the pins is a great advantage since it would be very costly to build a head with 384 gravity located pins for a conventional multiwell plate. The small mass of the pins makes them sensitive to variations in the fit of the pins in their guide holes. As the number of pins is increased, it becomes increasingly difficult to avoid some pins falling freely and other pins sticking in their holes. Adding pin biasing springs has been proposed-to overcome this problem, but this is not ideal since it increases impact forces on the pin tips, thereby increasing pin damage and wear rates. In any case, the biasing would have to be heavy if the pins were to be able to penetrate one or membranes.
In use, the z-axis linear motor drives the body 31 down within the mounting frame 33 so that the pins 10 fixed in the body 31 pass through the wells of well plate 30 until the abutment 44 touches the spotting surface 16 whereupon the liquid is transferred.
It will be appreciated that although particular embodiments of the invention have been described, many modifications/additions and/or substitutions may be made within the spirit and scope of the present invention.
5. A well plate comprising:
- an upper surface;
- a lower surface;
- an array of wells extending between the upper surface and the lower surface;
- a lower membrane extending over the lower surface to form a self-sealing, liquid-tight base for the wells.
6. A well plate according to claim 5, wherein the lower membrane is formed of a single sheet of material.
7. A well plate according to claim 5, wherein the lower membrane is formed of multiple sheets.
8. A well plate according to claim 7, wherein there is one sheet for each of the wells.
9. A well plate according to claim 5, further comprising an upper membrane extending over the upper surface to form a top for the wells.
10. A well plate according to claim 9, wherein the upper membrane is made of self-sealing material.
11. A well plate according to claim 5, further comprising a removable cover extending over the upper surface to form a top for the well plate.
12. A well plate according to claim 5, wherein the array of wells conforms to a square grid having a grid spacing of one of: 2.25 and 4.5 millimeters.
13. A well plate according to claim 5, wherein the array of wells has one of: 96, 384 and 1536 wells.