TEMPERATURE CONTROL SYSTEM FOR MICROFLUIDIC DEVICE

Abstract

A heating system for a EWOD device using a single, spatially-structured temperature control element, used to create a zone on the device with a specific temperature profile. The heating system uses multiple contact regions between the temperature control element and the device. One or more of the contact regions are separated from the temperature control element by one or more thermally resistive layers whose purpose is to restrict heat flow from the temperature control element to the device, and further to restrict lateral flow of heat between adjacent contact regions. The heating system can use one or more materials with different thermal resistance to alter the heat flow to different regions of the device. The spatial location of the contact regions is also used to determine the temperature profile within the device. The device has an optional temperature control element which offsets the low temperature point from the inlet temperature. This invention also describes methods to process multiple droplets within the multiple temperature zones.

Claims

1. A temperature control element for a microfluidic device, the microfluidic device comprising first and second substrates spaced apart from another to define a fluid gap therebetween; wherein the temperature control element comprises: a controllable heating and/or cooling element; a plurality of contact elements arranged to be disposed, in use, against a first substrate of the microfluidic device so as to provide thermal transfer between the heating/cooling element and the first substrate of the microfluidic device at a plurality of discrete contact regions whereby thermal transfer between the temperature control element and the first substrate of the microfluidic device occurs preferentially at the contact regions; and a first thermally resistive material disposed between the heating and/or cooling element and at least some of the contact elements; wherein the contact regions are so shaped, sized and/or distributed so as to provide, in use, a desired spatial temperature profile within the microfluidic device.

2. A temperature control element as claimed in claim 1 wherein the first thermally resistive material has a thermal conductivity of less than 50 W/mK.

3. A temperature control element as claimed in claim 1, wherein the contact elements have a thermal conductivity greater than the thermal conductivities of the first thermally resistive material.

4. A temperature control element as claimed in claim 1 wherein the contact elements have a thermal conductivity equal or substantially equal to the thermal conductivity of the first thermally resistive material.

5. A temperature control element as claimed in claim 1 wherein the first thermally resistive material is disposed between the heating and/or cooling element and all of the contact elements.

6. A temperature control element as claimed in claim 1 and comprising a second thermally resistive material (454) disposed between the heating and/or cooling element (351) and one or more other contact elements (355), the first and second thermally resistive materials having different thermal conductivity to one another.

7. A temperature control element as claimed in claim 1 wherein the contact regions are shaped, sized and/or distributed so as to provide, in use, a temperature gradient within fluid gap of the microfluidic device.

8. A temperature control element as claimed in claim 1 wherein the contact regions are of variable width so as to provide, in use, a second temperature gradient along the length of the contact regions

9. A microfluidic device as claimed in claim 1 wherein the contact regions are shaped, sized and/or distributed so as to provide, in use, a linear temperature gradient within the fluid gap of the microfluidic device.

10. A temperature control element as claimed in claim 2, wherein the spacing between neighbouring contact elements varies over the temperature control element.

11. A temperature control device comprising a plurality of temperature control elements as claimed in claim 1 which provide, in use, a defined temperature gradient over an area larger than any individual temperature control element.

12. A microfluidic device comprising first and second substrates spaced apart from another to define a fluid gap therebetween; the microfluidic device further comprising a temperature control element as defined in claim 1 arranged such that contact elements of the temperature control element are disposed against the first substrate of the microfluidic device.

13. A microfluidic device as claimed in claim 12 and further comprising a second temperature control element arranged in thermal contact with the second substrate of the microfluidic device.

14. A microfluidic device as claimed in claim 12, wherein the first substrate of the device has a thermal conductivity of 2 W/mK or below.

15. A microfluidic device as claimed in claim 12, and configured to move a fluid droplet contained in the fluid gap along the spatial temperature profile defined by the temperature control element.

16. A microfluidic device as claimed in claim 12, and further comprising a sensor for measuring one or more properties of a fluid droplet contained in the fluid gap.

17. A reader for a microfluidic device, the microfluidic device comprising first and second substrates spaced apart from another to define a fluid gap therebetween; wherein the reader comprises a temperature control element as defined in claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0049] In the annexed drawings, like references indicate like parts or features:

[0050] FIG. 1 is a schematic diagram depicting a conventional EWOD device in cross-section;

[0051] FIG. 2 shows a block diagram of the EWOD device, cartridge and reader system according to this invention;

[0052] FIG. 3(a) shows in cross section an EWOD device according to a first embodiment of this invention;

[0053] FIG. 3(b) shows in cross section a temperature control element according to an embodiment of this invention;

[0054] FIG. 4 shows a schematic representation of the temperature profile within an EWOD device according to a first embodiment of this invention

[0055] FIG. 5 shows in cross section an EWOD device according to a second embodiment of this invention

[0056] FIG. 6 shows in cross section an EWOD device according to a third embodiment of this invention

[0057] FIG. 7 shows a schematic diagram of an EWOD device according to a fourth embodiment of this invention

[0058] FIG. 8 shows a schematic plan view diagram of an EWOD device according to a fifth embodiment of this invention

[0059] FIG. 9 shows a schematic plan view diagram of an EWOD device according to a sixth embodiment of this invention

DESCRIPTION OF REFERENCE NUMERALS

[0060] 4 liquid droplet [0061] 6 contact angle [0062] 16 First hydrophobic coating [0063] 20 First insulator layer [0064] 22 Second insulator layer [0065] 26 Second hydrophobic coating [0066] 28 Reference electrode [0067] 32 Spacer [0068] 34 Non-polar fluid [0069] 35 Fluid gap [0070] 36 Top substrate [0071] 38A and 38B Array Element Electrodes [0072] 40 Reader [0073] 41 EWOD device [0074] 42 Cartridge upper part [0075] 44 Cartridge lower part [0076] 46 Cartridge input structure [0077] 48 Cartridge output holes [0078] 49 Cartridge [0079] 50 Heating system located inside reader [0080] 301 Simplified representation of EWOD device according to this invention [0081] 304 Liquid droplet contained within an EWOD device according to this invention [0082] 335 Fluid gap within an EWOD device according to this invention [0083] 336 Upper substrate of an EWOD device according to this invention [0084] 350 Temperature control element according to a first embodiment of this invention [0085] 351 Heating and/or cooling element [0086] 352 Thermally resistive layer according to a first embodiment of this invention [0087] 355 Contact elements between thermally resistive layer and EWOD device according to this invention [0088] 372 Lower substrate of an EWOD device according to this invention [0089] 374 Temperature profile of an EWOD device according to a first embodiment of this invention [0090] 452 A first thermally resistive layer according to a second embodiment of this invention [0091] 454 A second thermally resistive layer according to a second embodiment of this invention [0092] 556 Additional temperature control element according to a third embodiment of this invention [0093] 658 Light source according to a fourth embodiment of this invention [0094] 660 Light filter according to a fourth embodiment of this invention [0095] 662 Dichroic beam splitter according to a fourth embodiment of this invention [0096] 664 Light detector/imager according to a fourth embodiment of this invention [0097] 666 Light filter according to a fourth embodiment of this invention

DETAILED DESCRIPTION OF INVENTION

Embodiment 1

[0098] FIG. 2 shows an exemplary droplet microfluidic handling system according to a first embodiment of the present invention. The system is in two parts comprising a cartridge 49 and a reader 40.

[0099] The cartridge 49 is shown comprised of the EWOD device 41, which may optionally be mounted into a housing, e.g. a plastic housing to form a microfluidic cartridge 49. The housing may, for example, be comprised of upper 42 and lower 44 plastic parts, though this is a matter of design and many arrangements are possible. Typically the cartridge 49 includes input structures 46 (e.g. holes) for entry of fluids and may optionally also include output structures 48 whereby liquid may be ejected from the device. The input and output structures are connected fluidically to the fluid gap 35 (not shown in FIG. 2) of the EWOD device whereby fluid may be input to and/or extracted from the EWOD device. The EWOD device typically contains a dispensing structure for generating liquid droplets 4 in the fluid gap, for example by means of electro-wetting. Typically the cartridge may be configured to perform an assay, test or sample manipulation function. Typically the cartridge may be disposable and for one-time use.

[0100] The EWOD device is in most regards of a standard and well-known construction, for example as shown in FIG. 1. The EWOD device typically comprises a lower substrate 72, a top substrate 36, a spacer 32 and a non-polar fluid 34 (e.g. an oil) as a surrounding medium within which the liquid droplets 4 are constrained and may be manipulated. In operation the EWOD device is configured to perform droplet operations in accordance with a sequence configurable according to the requirements of the application. The droplet manipulation sequence is executed by selectively actuating the element electrodes 38 to perform multiple droplet operations in series and/or parallel. Typical droplet operations, described in detail in the prior art references, include: [0101] Moving droplets (from one array element to another), [0102] Mixing droplets together (by merging and agitation), [0103] Splitting droplets into two halves, [0104] Dispensing of a small droplet from a large reservoir droplet, and [0105] Inputting droplets onto the array from large input reservoirs, which may interface the device with the outside world.

[0106] The reader 40 performs the functions of supplying electrical control and power signals to the EWOD device. To enable these functions an electrical connection is provided between the EWOD device and the reader. Optionally the EWOD device may also include built in sensor functions for sensing the presence or absence of droplets at element electrodes, or for sensing properties of the liquid droplets, e.g. chemical properties or temperature. In this case the reader 40 may also perform the function of reading output signals generated by the EWOD device. The reader may further comprise apparatus to measure other aspects of the droplet that are pertinent to the assay under test, for example optical equipment to measure optical properties of the liquid droplets such as absorption, reflection or fluorescence. An optical measurement function may typically be used to readout the result of an assay or biochemical test. The reader 40 may further comprise a heating system 50 to control the temperature profile of the EWOD device 49.

[0107] In accordance with the present invention, the heating/cooling system for the EWOD device is designed to create distinct thermal zones within the device. Each thermal zone may have a defined temperature profile. A thermal zone may be heated or cooled (relative to ambient temperature) to a single constant temperature. Alternatively a thermal zone may be heated or cooled to produce a gradation of temperature across the zone.

[0108] The heating or cooling for a thermal zone is provided by a temperature control element 350. A given temperature control element may be either a source of or a sink of heat. There are a number of possible physical implementations for realizing temperature control elements that may be employed. Such physical implementations may include, for example, one or more of the following: resistive (Joule) heaters, Peltier-effect based heaters and/or coolers, optical means of heat generation (e.g. lasers), magnetic type heaters (e.g. conduction), heaters or coolers based on the convective, conductive or radiative transfer of heat in or out of the temperature control elements etc.

[0109] An arrangement of a heating system in accordance with a first and most general embodiment of the present invention is shown in FIG. 3(a). In this embodiment the temperature control element 350 is intended to create a zone of increased temperature in the EWOD device, and is formed of a heating element 351, a thermally resistive layer 352 and a heat transfer part constituted by one or more contact elements 355. The heating element 351 is located separate from the EWOD device 301 and below its lower substrate 372. The heating element 351 may be comprised of thin resistive heater bars or some other heating element such as a Peltier device. The surface of Peltier devices is typically formed using high thermal conductivity surfaces such as aluminium oxide with a thermal conductivity of approximately 30 W/mK.

[0110] The heating element 351 is in thermal communication with the thermally resistive layer 352. The thermally resistive layer typically has significantly lower (10-100) thermal conductivity than the surface of the temperature control element and the contact regions. Suitable materials include polymers, plastics and ceramics and certain low conductivity metals. In this embodiment the layer is made from a high temperature polymer such as poly(ether ether ketone) which has thermal conductivity of approximately 0.25 W/mK

[0111] The thermally resistive layer 352 is in thermal communication with the contact elements 355. The contact regions provide locations where heat can flow from the thermally resistive layer to the device. The contact regions are spatially located to transfer heat in a controlled way to the device. In this embodiment, the contact regions are more densely positioned on the right hand side of the device compared to the left hand side. This embodiment leads to a greater average flow of heat into the device at the right hand side compared to the left hand side. This leads to a higher temperature on the right hand side of the device compared to the left hand side, as shown schematically by the temperature profile 374 in FIG. 4. Intermediate contact regions are positioned along the length of the device in order to provide the required temperature profile along the length of the device.

[0112] The contact regions in FIG. 3(a) may extend into the plane of the paper, so that that a droplet moving from left to right along the device in FIG. 3(a) experiences the same temperature profile regardless of its position into the plane of the figure. Alternatively, discrete contact regions may be arranged along lines extending into the plane of the figure.

[0113] The contact regions may in principle have any value of thermal conductivity. It may however be advantageous if the contact regions are made from a material with relatively high thermal conductivity compared to the thermally resistive layer, such as metals and high thermal conductivity ceramics, in order to eliminate or reduce temperature variations along a direction into the plane of FIG. 3.

[0114] In one embodiment, shown in FIG. 3(b), the contact regions are made from aluminium, with thermal conductivity of approximately 237 W/mK. The thermally resistive layer is made from polyether ether ketonePEEK, and is 5 mm thick. The temperature control element has 5 contact regions, with the spacing between contact regions increasing along the device, eg 1 mm, 2.5 mm, 3.5 mm, 8 mm so that the heat transferred into the EWOD device (in a case where the temperature control element has a heating element) decreases from left to right in FIG. 3(b). Each contact region has a thickness (in a direction parallel to the fluid gap) of 1 mm except for the first contact region (at the left in FIG. 3(b)) which has a greater thickness (eg 3 mm) to provide greater heat flow.

[0115] The thermal contact between the heating element and the thermally resistive layer, the thermally resistive layer and the contact regions, and the contact regions and the device is particularly important since the transfer of heat is primarily conductive, as opposed to convective or radiative. To improve the thermal contact a flexible, thermally conductive medium may be positioned at the interface between different layers. This may take the form of conductive pastes, foams, pads, or thin layers of fluids such as oils.

[0116] Typically and preferably, the upper substrate 336 of the EWOD device and lower substrate 372 of the EWOD device may be comprised of a material having a relatively low thermal conductivity. A preferred material for the upper substrate 336 and lower substrate 372 may be glass with thermal conductivity 1-2 W/mK. The glass may be of thickness less than 1 mm and may be of a type typically used in the manufacture of liquid crystal displays. Alternatively, the upper substrate 336 and lower substrate 372 may be made from other materials including, but not limited to silica, sapphire and plastics and the like. The low thermal conductivity of the aforementioned materials is advantageous for this embodiment since it limits lateral heat flow between adjacent contact regions.

[0117] An advantage of the heating system according to the arrangement of the first embodiment is that it generates a temperature gradient through the use of a single temperature control element. This reduces the complexity of the temperature control system since it only has to control a single element.

[0118] A further advantage is that it is possible to provide any desired temperature profile by suitably choosing the spatial location, size and/or shape of the contact regions and/or the thermal resistance of the thermally resistive layer 352. In this way a substantially linear temperature profile or temperature gradient can be achieved. This is in contrast to two-heater systems where the temperature gradient is substantially nonlinear. It should be noted that if for some reason a non-linear temperature gradient is desirable such a temperature profile may be provided by appropriate spatial location, size and shape of the contact regions of the present embodiment. (While all contact regions are shown as having the same size and the same shape in FIG. 3(a) the invention is not limited to this and, in other embodiments, one contact region may have a different shape and/or a different size to another contact region (as shown in FIG. 3(b)).)

[0119] A further advantage is the heating system can be made very small. This is an important consideration for microfluidic devices and has implications for yield and cost.

2.SUP.nd .Embodiment

[0120] A heating system in accordance with a second embodiment of the present invention is shown in FIG. 5. This second embodiment is an extension of the first embodiment whereby the thermally resistive layer is composed of a first thermally resistive material 452 and second thermally resistive material 454 with different thermal resistances. This serves the purpose of allowing a different heat flow from the temperature control element in the two different regions of the device. In this embodiment, the thermal resistance of the first thermally resistive material 452 is much lower than the thermal resistance of the second thermally resistive material 454. This allows a greater heat flow at the edges of the heating system compared to the middle. In many cases there will be greater heat loss from the EWOD device at its edges (assuming that the fluid layer is heated to above ambient temperature), and provision of the first thermally resistive material 452 can compensate for this increased heat loss and enable a uniform temperature profile to be created across the device in the vicinity of this heater.

[0121] An advantage of this second embodiment is that it provides a zone of relatively uniform temperature on the device. This is useful if many droplets are required to be stored at the same temperature, or if multiple parallel droplet operations are being performed which require uniform temperature.

3.SUP.rd .Embodiment

[0122] A heating system in accordance with a third embodiment of this invention is shown in FIG. 6. This third embodiment is an extension of either of the first or second embodiments wherein an additional temperature control element 556 is provided on the upper substrate of the EWOD device. The additional temperature control element 556 may be controlled independently from the first temperature control element. The purpose of the additional temperature control element is to control the temperature of the device above or below the ambient temperature, or to control the initial temperature of the droplets.

[0123] An advantage of this embodiment is that it allows a wider range of temperature profiles to be generated within the device.

[0124] The additional temperature control element 556 may be a conventional (planar) heating/cooling element, or it may be another temperature control element 350 of the invention.

4.SUP.th .Embodiment

[0125] In accordance with a fourth embodiment of the present invention a method is provided for changing the temperature of at least one droplet contained within the fluid gap 335 of the EWOD device. To achieve said function, a heating system in accordance with any of the preceding embodiments is used in conjunction with droplet operations conducted by the EWOD device. In an exemplary scheme, a droplet 304, is moved by means of the electro-wetting force through the thermal zone defined by the temperature control (ie, by the heating element, thermally resistive layer and the contact regions). Optionally and preferentially, the thermal zone presents a temperature gradient profile in the direction of movement of the droplet. Multiple droplets may be moved in a parallel flow across the thermal profile zone, shuttled backwards and forwards within the zone, or moved in other combinations of the above movements. The method described in the present fourth embodiment may for example be used to move a droplet through a positive temperature gradient for the purpose of measuring the melt curve of a DNA amplicon as part of a Polymerase Chain Reaction (PCR) assay. Optionally, the fluorescence of the droplet may be monitored by optical means as it traverses through the temperature gradient. An example fluorescence detection system is shown schematically in FIG. 7. A light source 658 such as a white light source, LED or laser is used to illuminate the droplet from the side of the upper substrate. The light from this source may be forced to pass through an optical filter 660 such as a band pass filter so as to illuminate the sample with only a limited range of wavelengths, and thereafter forced to reflect from a dichroic mirror 662 onto the device. Light that fluoresces from the liquid droplet may pass through the dichroic mirror 662 and be collected at the sensor (photodetector) 664. The sensor may be a single element collector such as a photodiode or photo multiplying device, or it may contain multiple elements so as to image a large area containing one or multiple droplets within the device. Imagers may be CCD, CMOS or other imaging systems. An optical filter 666 may be positioned in front of the photodetector. This filter may only allow fluorescent light to reach the photodetector and block any other stray light. In a typical melt-curve measurement the spatial location at which the fluorescence reduces corresponds to the melting temperature of the DNA amplicon, and the melting temperature of the DNA amplicon can be determined from knowledge of the temperature gradient across the EWOD device. This temperature is characteristic of the DNA amplicon itself, and can be used to quantify the efficacy of the amplification assay. Other optical arrangements to measure the fluorescent signal can be devised including off-normal illumination and detection, with and without the dichroic mirror, and are included in this embodiment.

5.SUP.th .Embodiment

[0126] A heating system in accordance with a fifth embodiment of this invention contains a plurality of individual heating elements provided on either the upper or lower substrates of the EWOD device as shown in plan view in FIG. 8. Each individual heating element 350 has contact elements 355 as described in embodiment 2, so arranged with appropriate thickness and thermal resistances to produce a defined temperature profile across the surface of the heating element, for example a relatively uniform temperature profile. The plurality of such individual heating elements are so located and spaced from one another such that, together, they produce a defined temperature profile across the whole area above the plurality of heating elements, for example a relatively uniform temperature profile. In this case the spacing between heating elements is chosen such that the temperature in the regions between heating elements remains relatively constant.

[0127] An advantage of this embodiment is that it allows a relatively uniform, or otherwise defined, temperature profile to be generated over a large area, ranging in size from the area of a single heating element up to the whole area above a plurality of heating elements.

6.SUP.th .Embodiment

[0128] A heating system in accordance with a sixth embodiment of this invention contains contact elements that vary in width along their length, as shown in plan view in FIG. 9. The variable width of the contact elements 355 produces a variable rate of heat flow to the device along the length of the contact elements, according to their width. A wider contact element allows greater heat flow to the device and so produces a higher temperature. Elements can for example be wedge-shaped being narrower at one end compared to the other end. This causes a lower rate of heat flow at the narrow end compared to the other end. Such variable width contact elements produce a temperature gradient along their length, i.e. a temperature gradient into the plane of the paper of FIG. 3(a). This second temperature gradient can have a larger or smaller temperature range than the first temperature gradient (produced by the spacing of the contact elements in the plane of the paper of FIG. 3(a)). The combination of this second temperature gradient and the first temperature gradient causes a two-dimensional temperature gradient.

[0129] An advantage of this embodiment is that it can be used to produce a droplet pathway with a very fine temperature resolution across a wide temperature range. For example, if the temperature difference between adjacent pixels in the direction of the first temperature gradient is (T2T1), the second temperature gradient can provide a number of intermediate temperatures between T1 and T2 over a number of pixels, N. The temperature resolution of a heating system in accordance with this embodiment would then be defined by (T2T1)/N.

[0130] The invention has been described above with reference to embodiments intended to create a zone of increased temperature in the EWOD device. The invention may alternatively be used to create a zone of reduced temperature in the EWOD device, by replacing the heating element of the described embodiments by a cooling element.

[0131] Where embodiments of invention as described above are used to create a zone containing a spatial temperature gradient, the spatial temperature gradient may be a 1-D temperature gradientthat is, a droplet moving in one direction in the fluid gap of the EWOD device (for example to the right or left in FIG. 3) would experience a temperature gradient whereas a droplet moving in a perpendicular direction in the fluid gap of the EWOD device (for example into or out of the plane of the paper in FIG. 3) would experience no temperature gradient. In principle however the invention may be used to create a zone in which a droplet moving in any direction in the plane of the fluid gap would experience a temperature gradient.

INDUSTRIAL APPLICABILITY

[0132] The described embodiments could be used to provide an enhanced EWOD device. The EWOD device could form a part of a lab-on-a-chip system. Such devices could be used in manipulating, reacting and sensing chemical, biochemical or physiological materials. Applications include healthcare diagnostic testing, material testing, chemical or biochemical material synthesis, proteomics, tools for research in life sciences and forensic science.