Patent Application
20130161193
The present invention relates to a system for loading fluid into a microfluidic device. In particular it relates to a system that controls the volume of fluid loaded into such a device. It further to relates to a metered fluid loading system for Electrowetting-On-Dielectric (EWOD) devices.
Microfluidics is a rapidly expanding field concerned with the manipulation and precise control of fluids on a small scale, often dealing with sub-microlitre volumes. There is growing interest in its application to chemical or biochemical assay and synthesis, both in research and production, and applied to healthcare diagnostics (“lab-on-a-chip”). In the latter case, the small nature of such devices allows rapid testing at point of need using much smaller clinical sample volumes than for traditional lab-based testing.
A microfluidic device can be identified by the fact that it has one or more channels (or more generally gaps) with at least one dimension less than 1 millimeter (mm). Common fluids used in microfluidic devices include whole blood samples, bacterial cell suspensions, protein or antibody solutions and various buffers. Microfluidic devices can be used to obtain a variety of interesting measurements including molecular diffusion coefficients, fluid viscosity, pH, chemical binding coefficients and enzyme reaction kinetics. Other applications for microfluidic devices include capillary electrophoresis, isoelectric focusing, immunoassays, enzymatic assays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, PCR amplification, DNA analysis, cell manipulation, cell separation, cell patterning and chemical gradient formation. Many of these applications have utility for clinical diagnostics.
Many techniques are known for the manipulation of fluids on the sub-millimetre scale, characterised principally by laminar flow and dominance of surface forces over bulk forces. Most fall into the category of continuous flow systems, often employing cumbersome external pipework and pumps. Systems employing discrete droplets instead have the advantage of greater flexibility of function.
Electrowetting on dielectric (EWOD) is a well-known technique for manipulating discrete droplets of fluid by application of an electric field. It is thus a candidate technology for microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, (R. B. Fair, Microfluid Nanofluid (2007) 3:245-281). This review notes that methods for introducing fluids into the EWOD device are not discussed at length in the literature. It should be noted that this technology employs the use of hydrophobic internal surfaces. In general, therefore, it is energetically unfavourable for aqueous fluids to fill into such a device from outside by capillary action alone. Further, this may still be true when a voltage is applied and the device is in an actuated state. Capillary filling of non-polar fluids (e.g. oil) may be energetically favourable due to the lower surface tension at the liquid-solid interface.
A few examples exist of small microfluidic devices where fluid input mechanisms are described. U.S. Pat. No. 5,096,669 (Lauks et al.; published Mar. 17, 1992) shows such a device comprising an entrance hole and inlet channel for sample input coupled with an air bladder which pumps fluid around the device when actuated. It is does not describe how to input discrete droplets of fluid into the system nor does it describe a method of measuring or controlling the inputted volume of such droplets. Such control of input volume (known as “metering”) is important in avoiding overloading the device with excess fluid and helps in the accuracy of assays carried out where known volumes or volume ratios are required.
US20100282608 (Srinivasan et al.; published Nov. 11, 2010) describes an EWOD device comprising an upper section of two portions with an aperture through which fluids may enter. It does not describe how fluids may be forced into the device nor does it describe a method of measuring or controlling the inputted volume of such fluids. Related application US20100282609 (Pollack et al.; published Nov. 11, 2010) does describe a piston mechanism for inputting the fluid, but again does not describe a method of measuring or controlling the inputted volume of such fluid.
A basic concept of the invention is an integrated fluid input mechanism for delivering a metered discrete volume of fluid into a microfluidic device.
Such a device may employ EWOD or AM-EWOD methods for fluid control.
In some embodiments the fluid input mechanism may operate by the displacement of a body of liquid or gas (e.g. air) to force fluid into the microfluidic device. In other embodiments the fluid input mechanism may operate by the expansion of a body of gas (e.g. air) to force fluid into the device.
In some embodiments the system may provide feedback via a fluid sensor, in order to verify that correct fluid input has occurred as part of an error detection method. In other embodiments temperature feedback may provide the operator with analogue control of the volume of fluid input.
In further embodiments closure means are provided for closing and sealing the system following initial fluid application and prior to actuation of the fluid input mechanism.
Advantages of the invention include:
Metering of the input fluid volume improves assay accuracy (ensuring sample and reagent are mixed in correct ratios), helps prevent overloading the device with too much fluid and hence leaves enough space for multiple assays or multiple assay operations on the same device
System is easy to use by a semi-skilled operator e.g. in a point of need setting—it does not rely on operator skill for accurate dispensing of sample and reagent, or prevention of leakage (a safety hazard)
Input mechanism is integrated into the microfluidic device for simplicity. The whole assembly is easy to fabricate at low cost, for example by injection moulding. This is important if the device is to be single-use disposable for diagnostic applications.
Provides means of sealing the device so that biological samples are enclosed within it and do not provide a contamination hazard
Less prone to leaks than systems using external pumping mechanisms
Methods for error detection to confirm correct filling
Analogue control of input volume in some embodiments
According to an aspect of the invention, a microfluidic system is provided which includes a microfluidic device; and a metered fluid loading system formed integrally with the microfluidic device and configured to load a discrete metered volume of fluid into the microfluidic device upon actuation.
According to another aspect, the metered fluid loading system includes a fluid input mechanism configured to actuate the metered fluid loading system.
In accordance with another aspect, the fluid input mechanism includes a bistable membrane actuator.
According to yet another aspect, the bistable membrane actuator is configured to actuate the metered fluid loading system as a result of being deformed from a first bistable state to a second bistable state.
In accordance with another aspect, the fluid input mechanism includes a deformable membrane actuator.
According to still another aspect, the deformable membrane actuator is configured to actuate the metered fluid loading system as a result of being deformed from an undeformed state to a deformed state.
In still another aspect, the fluid input mechanism further includes a limiter configured to limit an extent of deformation of the deformable membrane actuator.
According to another aspect, the fluid input mechanism includes a heater which effects an expansion of a body of gas to actuate the metered fluid loading system.
According to yet another aspect, the metered fluid loading system further including a temperature sensor as a feedback mechanism to control the expansion of the body of gas.
In accordance with another aspect, the metered fluid loading system includes a reservoir including an input channel through which the fluid is coupled from the reservoir to a gap of the microfluidic device.
According to still another aspect, the fluid input mechanism is operative to displace or expand a body of liquid or gas within the reservoir upon being actuated to force the fluid from the reservoir to the microfluidic device.
In accordance with yet another aspect, the system further includes a seal for forming an airtight seal between the fluid input mechanism and the reservoir.
According to yet another aspect, the fluid input mechanism is hinged to the reservoir permitting the fluid input mechanism to be opened and closed, and wherein when the fluid input mechanism is in an open position fluid which is to be loaded into the microfluidic device may be placed in the input channel, and when the fluid input mechanism is closed an airtight seal between the fluid input mechanism and the reservoir is formed.
In still another aspect, the system further includes a holding mechanism for holding the fluid input mechanism in the closed position.
In yet another aspect, the fluid input mechanism is coupled to the reservoir by sliding engagement.
According to yet another aspect, the reservoir includes a plurality of input channels.
In accordance with another aspect, the reservoir further includes a vent column.
According to still another aspect, the microfluidic device includes sensing elements to detect a presence of the fluid loaded by the metered fluid loading system.
In accordance with another aspect, the microfluidic device includes at least one hydrophobic surface in contact with the fluid loaded in the microfluidic device.
According to still another aspect, the microfluidic device is configured to draw fluid from the fluid loaded in the microfluidic device by controlling a hydrophobicity of a surface of the microfluidic device.
According to yet another aspect, the microfluidic device is an electrowetting on dielectric (EWOD) device.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In the annexed drawings, like references indicate like parts or features:
a,
3
b and 3c show a cross section through a first embodiment of the invention
a and 5b show a cross section through a second embodiment of the invention illustrating a bistable actuator
a and 6b show a cross section through a third embodiment of the invention illustrating a deformable membrane actuator
a and 10b show a cross section through a seventh embodiment of the invention illustrating an implementation of a further closure method
a and 11b show a cross section through an eighth embodiment of the invention illustrating an implementation of a yet further closure method
a and 12b shows a ninth embodiment of the invention illustrating an combined sealing and actuation element
4 Conducting fluid droplet
6 Contact angle theta
8 Fluid
10 Droplet
16 Hydrophobic surface
20 Insulator layer
26 Hydrophobic layer
28 Electrode
32 Spacer
34 Non-conducting fluid
36 Top substrate
38 Electrode
42 Electrode array
44 Lower substrate
46 Reservoir
48 Input channel
50 Fluid input mechanism
52 Bistable actuator
54 Deformable membrane actuator
56 Reservoir with limiter
60 Device controller box
62 Actuation post
70 Lid
72 Heater
74 Temperature sensor
80 Clip
82 Seal
84 Hinge
90 Sliding hood
92 Runner
100 Sliding fluid input mechanism
110 Combined seal and deformable membrane actuator element
112 Deformable membrane actuator sub-element
114 Seal sub-element
120 Fill column
122 Vent column
130 High resolution electrode array
132 Low resolution electrode array
The device includes a lower substrate 44 with a plurality of electrodes 38 (e.g., 38A and 38B) disposed upon it. These electrodes may have voltages applied directly or via a layer of thin-film electronics situated beneath the electrode layer (not shown) for example as in U.S. application Ser. No. 12/830,477 an “Array Element Circuit and Active Matrix Device, filed on Jul. 6, 2010. The droplet 4, consisting of a conducting (e.g. ionic or polar) material is constrained in a plane between the lower substrate 44 and a top substrate 36. The top substrate 36 has an electrode 28 thereon and a hydrophobic layer 26. A suitable gap between the two substrates may be realised by means of a spacer 32, and a non-conducting liquid 34 (e.g. dodecane oil) may be used to occupy the volume not occupied by the droplet 4. An insulator layer 20 disposed upon the lower substrate 44 separates the conductive electrodes 38A, 38B from a hydrophobic surface 16 upon which the droplet 4 sits with a contact angle θ 6. By appropriate design and operation, different voltages may be applied to different electrodes (e.g. electrodes 38A and 38B). The hydrophobicity of the surface 16 can be thus be controlled, thus facilitating droplet movement in the lateral plane between the two substrates.
a-3c illustrate a first embodiment of the invention. In addition to the EWOD components (insulator layers 20 and hydrophobic layers 26 are not shown for clarity) there is provided a metered fluid loading system for inputting fluids, and namely a discrete metered volume of fluid, into the EWOD device. The system preferably is formed integrally with the EWOD device, for example formed on the same lower substrate 44. The entire assembly may be fabricated at low cost, for example by injection moulding. The metered fluid loading system includes a reservoir 46 containing an input channel 48. Such a reservoir 46 is illustrated in projection view in
a illustrates a first stage of the fluid input process where the fluid 8 substantially resides in the input channel 48 of the reservoir 46. When the fluid input mechanism 50 is activated the fluid 8 is forced into the gap between top substrate 36 and lower substrate 44 as illustrated in
The reservoir 46 may be formed of any suitable material that is compatible with the applied fluids. Compatibility implies that the material is not damaged or dissolved by the fluid 8, nor is the fluid contaminated by the material or substantially adheres to the material. For example, many engineering polymers may be used including PMMA (Poly(methyl methacrylate)), Nylon, PTFE (Polytetrafluoroethylene), PET (Polyethylene terephthalate), Polypropylene. It may be advantageous that the shallow horizontal section of the input channel 48 is narrower than the gap between top substrate 36 and bottom substrate 44. This would help this feature to act as a capillary stop for fluid in the channel so that fluid does not enter the gap between top substrate 36 and bottom substrate 44 until the fluid input mechanism 50 is actuated. The manner in which the fluid input mechanism 50 may be actuated is further discussed below.
It should be appreciated that such a system including a reservoir 46 and fluid input mechanism 50 in accordance with the invention could be applied to any other form of microfluidic system where fluid control is achieved by methods other than EWOD.
a-5b illustrate a second embodiment of the invention providing a specific implementation of the fluid input mechanism. The fluid input mechanism includes a bistable actuator 52 which may readily adopt either a first or second shape. For example, the first shape (first bistable state) may be a convex section of a sphere or ellipsoid (as shown in
a-6b illustrate a third embodiment of the invention providing a further implementation of the fluid input mechanism. The fluid input mechanism includes a deformable membrane actuator 54 which is made of a suitable material capable of being deformed on application of an external force. This force may be applied for example by an external rigid elongate element (not shown) pressed against the actuator 54. In some applications it is useful if this deformation is reversible on removal of the force. Suitable materials from which the actuator 54 may be made include elastomers such as silicone rubbers, natural rubber, nitrile rubbers, or fluoroelastomers such as copolymers of hexafluoropropylene (HFP) and vinylidene fluoride (VDF or VF2). The actuator 54 is coupled to a reservoir 56 which differs from the reservoir of previous embodiments in that the top of the input channel 48 is structured as to form a limiter which limits the extent of deformation of the actuator 54. In the embodiment of
The air in the input channel 48 will approximately follow the ideal gas law:
PV=nRT
where P is pressure, V is volume, n is the number of moles of the gas, R is the ideal gas constant and T is the temperature.
As this is a trapped pocket of air then n is fixed. As the total system is not sealed, no increase in air pressure can be sustained. Therefore, if the temperature T is increased then the volume of air will increase in proportion i.e.
V0/V1=T0/T1 or ΔV/V0=ΔT/T0
where ΔV & ΔT are changes in V & T and V0 & T0 and V1 & T1 are initial and final values respectively.
This expansion of the air will cause the fluid 8 to be displaced. This in turn forces fluid 8 into the gap between top substrate 36 and bottom substrate 44 thus achieving input of fluid into the device. The volume of fluid input is thus known and controllable by appropriate controller of the heater 72 so as to introduce a metered volume of fluid. Of course in another embodiment, a gas other than air may occupy the fluid input channel.
In the foregoing embodiments the fluid input mechanism 50 (52,54,70) is sealed to the reservoir 46. Naturally this sealing must occur after the fluid is placed in the input channel.
a-10b illustrate a seventh embodiment of the invention showing a further implementation of sealing the fluid input mechanism 50 (52,54,70) to the reservoir 46 in any of the foregoing embodiments. This differs from the embodiment of
a-11b illustrate an eighth embodiment of the invention showing a further implementation of sealing the fluid input mechanism to the reservoir 46 which may be utilized in conjunction with any of the foregoing embodiments. In this case the fluid input mechanism is a sliding fluid input mechanism 100 attached to the reservoir 46 via a runner (not shown) on the reservoir 46 so as to be in sliding engagement.
Previous embodiments have illustrated a reservoir with a single input channel. However, it is also possible to form multiple input channels within the same reservoir block. This may be useful to allow the separate input of sample and several reagents. The element 110 of
In any of the foregoing embodiments the electrode array 42 may also include sensing elements which would detect the presence of the droplet 10. Further, such elements may measure the impedance of the fluid that is present (for example, as described in the aforementioned U.S. application Ser. No. 12/830,477). Use of such sensing elements would provide confirmation that filling had occurred correctly and serve as an error detection mechanism. For example, if filling of the fluid was insufficient or had failed entirely, then detection of this could be used to automatically trigger, or indicate to an operator, a repeat of the filling operation. In the case of multiple input channels being present where one was malfunctioning, an alternative channel could automatically be selected.
It will be further apparent that the microfluidic device described could form part of a complete lab-on-a-chip system as described in prior art. Within such a system, the fluids input and manipulated in the device could be chemical or biological fluids, e.g. blood, saliva, urine, etc. or any test reagent, and that the whole arrangement could be configured to perform a chemical or biological test or to synthesize a chemical or biochemical compound.
Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. For example, while the above embodiments rely primarily on the displacement or expansion of air or other suitable gas to force the fluid from the input channel into the microfluidic device, a suitable liquid may also be employed. Suitable liquids include those which within the input channel remain separated from the fluid which is to be loaded into the microfluidic device. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
The microfluidic 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, chemical or biochemical material synthesis, proteomics, tools for research in life sciences and forensic science.
