Patent Application
20100101670
The present invention generally relates to the field of microfluids. More specifically, the invention relates to a microvalve for use in a microfluidic device and to a method of manufacturing an electrical microvalve.
Lab-on-a-chip and micro-total-analysis systems have been experiencing a huge increase in interest in the biomedical and chemistry area during the last decade. Lots of work has been done towards the development of new technologies enabling labs to be shrunk and integrated onto single chips. This emerging technology has proven to be very promising, and is often referred as microfluidics. Microfluidics allows fluid flow control and mix of fluids on chips using microchannels, in which fluids are injected. Such chips integrate many functions on a single substrate which not only allows an entire experiment to be built on a chip, but also allows a large amount of parallel experiments to be performed using very small volumes of fluids in a limited amount of time.
Microfluidic circuits require microvalves, i.e. tiny valves that are the key building blocks for making complex microfluidics integrated circuits. Microvalves are used to direct and pump fluids. Typically, the microvalve is used to block the passage of the fluid in the microchannel. Many configurations of microvalve have been investigated in prior art references.
One of the types of microvalve is the pressure actuated flexible microvalve which is also referred to as pneumatic valve and which is key component of so called multilayer soft lithography (MSL) microfluidic circuits. In such a type of microvalve, the microvalve typically includes a flexible membrane, which is forced to block a channel by applying a pressure thereto. Upon release of the pressure, the membrane recedes and allows passage of the fluid in the microchannel. The pressure can be transmitted pneumatically using gases or hydraulically using liquids. Although effective, this technology is bulky, as it requires a separate source of pressure for every single independent microvalve. European Patent Application 0 845 603, filed by Xerox Corporation describes such an air-actuated microvalve system and a method of production of such microvalves.
Another type of microvalve also commonly known is the electrically actuated microvalve. Such microvalve uses electricity to function. Electrically actuated microvalves are basically composed of two electrodes, separated by an elastomeric substance. This type of microvalve includes two subcategories: the normally open microvalve and the normally closed microvalve. The normally open microvalve is located along the microchannel and requires electricity to close the microchannel, while the normally closed microvalve is located adjacent to the microchannel and requires electricity to open the microchannel. International Patent Application WO 2006/044458 to University of Virginia Patent Foundation depicts and describes an example of a normally closed electrically actuated microvalve, while United States Patent Application 2003/0080442 to Fluidigm Corp. and United States Patent Application 2002/0109114 to California Institute of technology describe a normally-open electrically actuated microvalve. Another interesting reference in the art is United States Patent Application 2006/0118895 to Fluidigm Corp., which describes both normally open and normally closed electrically actuated microvalve. However, in this Fluidigm patent, the design of the microvalve causes important stress on electrodes and elastomeric material, which is not desirable, as it seriously reduces the lifetime of the microvalve, necessitates high voltage (HV) for opening and closing the valve, and slows down the actuation speed.
However, there are numerous problems with both the normally open and the normally closed electrically actuated microvalves of the prior art. More particularly, for the normally open electrically actuated microvalve, the microchannel within which the sample fluid is to flow in is molded within the elastomeric substance. Furthermore, when a conductive liquid fills the valve, normally open valves have the drawback of stopping proper operation under DC actuation and require high-frequency AC actuation. Another drawback, is that the normally open electrically actuated microvalve of the prior art, such as those described in United States Patent Application US2002/0109114 to Fluidigm Corp. necessitates a very high voltage of 1600V for closure which makes its use quite impractical. As for the normally-closed electrically actuated microvalve, even though its manufacturing is simpler than for the direct electrically actuated microvalve, it needs to be rigid to prevent flow of liquid which makes it difficult to actuate it, thus also requiring excessively high voltages.
More recently, direct electrical actuation of valves has been shown, which allows high-density integration of microfluidics. However, because the electrical fields are applied directly to conductive solutions, a DC voltage cannot be used and high frequency AC voltages are required. In the example published by Bansal et al., titled “A class of low voltage, elastomer-metal ‘wet’ actuators for use in high density microfluidics” in Lab on a chip volume 7, pages 164-16, the valves are 5 μm deep only, large (600 μm in diameter) and slow (up to 5 s for closing), and have not been used with pressurized liquids. Faster actuation and deeper channels necessitate higher voltages, but excessive heating is likely to become an issue. The fabrication process hitherto requires multiple difficult processing steps and makes it difficult to produce disposable chips. Finally, the conduits are too small to manipulate cells.
There is therefore a need for an electrical microvalve that is simpler to manufacture, which can function over a longer period of time, which can be actuated with lower voltages, DC current, and for which the actuation does not depend on the composition of the sample fluid.
In accordance with an embodiment of the present invention, there is provided a microvalve for a microchannel. The microvalve comprises first and second electrodes. The first electrode is affixed to a portion of the microchannel, while the second electrode is located over the microchannel, forms a membrane demonstrating substantially no resilience, and is substantially aligned with the first electrode. Upon electrical actuation of the first and second electrodes, the membrane is forced within the microchannel so as to obstruct the microchannel. A lid adapted to support the membrane may also be provided.
In accordance with another embodiment, the present invention relates to a method of manufacturing a microvalve. The method of the present invention proceeds with affixing a first electrode on a microchannel. Then, the method pursues with a step of applying a dielectric substance covering at least a portion of the microchannel overlooking the first electrode. Afterwards, the method includes a step of affixing a second electrode over the dielectric substance in such a manner that the second electrode is substantially aligned with the first electrode.
In accordance with yet another embodiment, the present invention relates to a microfluidic circuit. The microfluidic circuit comprises multiple microchannels and at least one microvalve affixed to one of the multiple microchannels. The at least one microvalve is adapted to indirectly actuate a flexible valve which regulates a flow of fluid in another one of a multiplicity of microchannels.
These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
a-e are manufacturing steps of a microchannel of the microvalve of
a-b are manufacturing steps of a second electrode of the microvalve of
a-f are manufacturing steps of a membrane in accordance with an embodiment of the present invention.
a-c are partial cross-sectional side views of a microchannel in accordance with other embodiments of the present invention.
a-b are perspective views of examples of uses of the microvalve of
a-b are cross-sectional views of another embodiment of the microvalve of the present invention.
a-b are cross-sectional views of another embodiment of the present invention.
Miniaturization, integration and parallelization (MIP) has driven the (micro) electronic revolution and has started to bear strongly on the life sciences, and already revolutionized gene expression profiling with DNA microarrays and genotyping with high throughput sequencers. The cell is the minimal physiological functional unit, yet of extraordinary complexity as it contains 23000 genes (for humans) and many more different proteins and protein machines. Cells have recently become an important focus of the drug discovery processes following the increasing rate of failures of drugs in late clinical trials or even following market introduction. High throughput cell assays can now be performed automatically in 96 or 384 well plates and is called high content screening (HCS) because it can provide insight on multiple biochemical pathways. HCS is an extension of high throughput screening (HTS) which examines individual bimolecular interactions outside of the cell. HCS is challenging because it requires a tight control of environmental parameters, the delivery of multiple reagents, advanced microscopy, and multi-parameter readouts; consequently it is expensive. Yet HCS represents an annual market value of hundreds of millions of USD, with a rapid growth rate of above 20% annually. The pressure on identifying adverse side effects of drugs early in the drug development process fuels a rapidly rising demand for HCS in the pharmaceutical and biotech industries. There are no intrinsic biological barriers to the further miniaturization and parallelization of HCS and of cellular assays within microfluidic systems, except for the lack of a microfluidic technology that supports MIP on a large scale. For those reasons, the present invention proposes a new microvalve, and the application of this microvalve to microfluidic systems that renders the latter scalable, and that may be used for cell assays and HCS. Furthermore, the present invention provides a novel indirect control architecture where electrostatic elastomeric valves (electrical microvalves, embedded in a control chip) regulate the pressure of fluid in a manifold connected to flexible membrane valves which control the flow of sample fluids. This architecture permits integration of microelectronic integrated circuits (ICs) with microfluidics and hence opens the door to large scale MIP of microfluidics.
With the present invention, electronic microfluidic systems will allow performing cellular assays and HCS with greater flexibility, with much higher throughput, and ultimately at a fraction of the cost of current technologies. We believe that the availability of electronic microfluidic cell chips with thousands of addressable microcompartments will transform drug screening, cell biology and medicine in a similar manner that DNA chips and high throughput sequencers have transformed, and are still transforming, them.
From a terminology standpoint, microfluidics concerns the manipulation and transport of minute amount of liquids. Many microfluidic pumping technologies have emerged in the last 15 years including electro osmosis, electrophoresis, dielectrophoresis, capillary systems, MSL and droplet-based microfluidics. Many strategies are unreliable (e.g. sensitive to the composition of the solution, or to changes in surface chemistry, both of which are difficult to control when using complex biological solutions) and not suitable for integration because they depend on macroscopic peripherals. To date microfluidics have not replaced conventional equipments, except in few niche applications.
The present invention provides a microvalve for a microchannel, which overcomes some of the problems known in the art. Furthermore, the microvalve of the present invention is composed of elements that are affixed to the microchannel and surrounding surface. Also, the present invention provides for a microvalve which reduces the need for high actuation voltages, is amenable to large-scale integration, and is much more resistant over time due to its intrinsic design. Finally, the present invention provides and uses a microvalve, which relies on a membrane demonstrating substantially no resilience to indirectly actuate a flexible valve controlling sample fluid flow.
A general embodiment of the present invention will now be described.
Reference is now made concurrently to
When the membrane 14 is forced against the microchannel, it may not spring back to the open position by itself because of adhesion forces between the membrane and the microchannel surface and because of the lack of resilience of the membrane 14. The application of a pressure to the microchannel will however detach the membrane 14 from the microchannel surface, and press it against the cover lid and thereby open the microchannel. The use of membranes such as described may appear unpractical because a sample microchannel may remain closed for lack of pressure. However, because of indirect actuation, as described below, the use of membranes 14 becomes practical, and offers a surprisingly attractive solution to make electrical valves. In addition, the microvalve surface can be made rough or ruguous so as to reduce the adhesion forces between the membrane 14 and the microchannel surface.
It will be apparent to those skilled in the art that due to its design and the selected materials, the microvalve of the present invention requires from the electrical source 21 a lower electrical voltage than microvalves of the prior art.
In general, the method of manufacturing some aspects of the microvalve of the present invention consists of affixing the first electrode 20 on a portion of the surface 16 of the microchannel 10, applying the dielectric substance 13 in such a manner that it covers at least a portion of the microchannel 10 while overlapping the first electrode 20, and affixing the second electrode 18 over the dielectric substance 13.
More particularly,
Reference is now made to
Reference is now made to
The second electrode 18 is fabricated directly on the dielectric substance 13 by first depositing a thin metal layer 86 of electrode material such as Cr/Au as shown in
The metal layer 86 is patterned using lithography and wet etching. Once the second electrode 18 is deposited on the dielectric substance 13, the lid 22, with cavities matching the microvalve 10 location, is bonded to the base 12 using oxygen plasma surface activation. The base 12 and the lid 22 may be made of for example PDMS or glass. Alternatively, it may be possible to replace the second electrode 18 by a conductive polymer or elastomer electrode.
In another embodiment of the present method of manufacturing, after step 4b where the metallic bottom electrode is etched using a wet chemical solution, a dielectric insulation thin layer is deposited via sputtering, evaporation or chemical vapor deposition. The layer could be silicon dioxide (SiO2), silicon nitride (Si3N4), tantalum pentoxide (Ta2O5) or any highly resistive material exhibiting a high breakdown voltage. Steps 5a to 5b are similar as previously described. Before step 5c, there is an additional step which consists in patterning an electrode on the elastic substance. This electrode is the second electrode of the microvalve. In order to increase adhesion of metal onto the substance, several strategies can be used: a metallic adhesion layer that could be chromium or titanium can be added, or a self assembled monolayer silane or any chemicals that is susceptible to increase metallic adhesion could also be used. Once the electrode is patterned, the substance and the base are oxygen plasma treated and bonded together (step 5c). The bonding strength is increased with a 30 to 60 min cured in an oven at 70° C. During step 5d, the plastic film 85 is peeled off easily since the bonding strength is higher at the membrane-base interface, thus leaving the dielectric substance 13 and the second electrode 18 attached underneath.
For the embodiment where the second electrode is within the dielectric substance 13, the manufacturing process requires an additional step after patterning the second electrode: another elastic substance that could be PDMS is poured and spin coated on the electrode 18 and cured at 70° C. for 30 to 60 minutes. The process then continues with step 5c.
In the context of the prototype, hereinafter described in greater details, a prototype of the electrical microvalve has been manufactured by transferring 10-micrometer-thick membranes spanning a channel that is ˜12 μm deep and 100 μm wide (shown on
An important challenge is the fabrication of thin PDMS membranes with the high yield necessary for large scale integration. The current fabrication method for making thin valves is based on spin-coating 3-5% PDMS diluted in toluene and patterning Au by wet etching on top of it to define the electrodes. There are many parameters that may be changed, such as the solvents used for spinning or even the supporting polymer. Indeed, for a valve with a low aspect ratio of 1:50 (current design) the strain produced by closing the valve is very small (only 0.12 percent assuming a circular cross section for the channel), which can be achieved using a wide range of materials, including polymers such as SU-8 or PMMA (both of which can be coated down to nanometer thicknesses), or dielectric films coated by evaporation or sputtering. With this geometry, the electrical material alone may form the membrane 14, whereas the dielectric substance 13 is attached to the first electrode.
Reference is now made to
Another way to reduce stress is to provide second electrodes that are longer than the width of the microchannel. The electrodes may take the shape that is rectangular, spiral, sinusoidal or saw-tooth shaped. This improvement reduces the stress on the electrode and reduces chances of tearing the membrane 14.
Particularities of the Second Electrode
Reference is now made to
Microvalve with Recesses
To prevent the membrane 14 from deflecting upwards under pressure caused by fluid flowing in the microchannel 10, or inwards under its own weight, the lid 22 may be provided with posts or walls 32, as shown in
In prior art MSL chips, the pressure in a pneumatic or hydraulic control line deflects a thin elastomeric membrane—serving as a valve—into a sample channel and closes and opens it. Such microfluidic architecture and variants of it have been successfully used for a variety of applications including pumping, protein crystallization, immunoassays, quantitative PCR, bacterial culture, etc. The success of this approach is rooted in the versatility of the technology, in the low cost of the chips made out of polydimethylsiloxane (PDMS), and in the ease with which it can be fabricated and operated using a computer. Large-scale parallelization is accessible with MSL using a dual control layer (a pneumatic multiplexer controls the pressure in pneumatic lines which deflect membranes into samples channels and thereby control sample flow) similar to RAM architecture. Thus, n chambers can be addressed using 2 log 2 n pneumatic or hydraulic control lines only, e.g. 1024 chambers using 20 control lines. One drawback is that this architecture is organized around few inlets and outlets, and large volumes of samples are expended in the maze of channels. But significantly, the control depends on macroscopic solenoid valves that need to be connected with macroscopic pins to the chip. Only a single MSL chip can be operated at one time.
Reference is now made to
In this particular application, a base 212 may contain many microchannels 210 (of which only one is shown for clarity purposes). The microchannel 210 is adapted to receive fluid. The microchannel 210 is covered with a flexible membrane 255, which is adapted to obstruct/close the microchannel 210 upon pressure actuation 249 in a partially superposed channel 250. The flexible membrane 255 can be with or without resilient force, depending on the application to be implemented. As in some instances, the microchannel 210 may be used to carry sample fluids with electrical conductivity; it is preferable to use the electrical microvalve of the present invention so as to indirectly actuate the flexible valve so as to not affect the operation of the electrical valve by the conductive fluids.
For such applications, the present invention, shown on
More particularly, in the case of
In the embodiment shown on
The approach of the present invention is thus compatible with large scale MIP and with cell culture, is low cost, and can regulate pressures of at least 50 kPa, and thus overcomes all of the above mentioned shortcomings. A single, unregulated pressure line connected to the micro-electro pneumatic chip is sufficient because an air manifold distributes the gas within the chip, and directs it to different branches connected to a disposable MSL chip. The pressure in each branch acts on a pneumatic valve in an MSL chip, but is regulated with an electrical microvalve 201 under the control of the HV ICs. Electrical microvalves 201 in the micro-electro pneumatic chip operate independently of the sample fluid composition, and small electrical microvalves can be used to actuate much larger pneumatic valves, or even multiple valves connected together. The electrical microvalve 201 exploits the elastic properties of ultra thin films on the dielectric membrane, such as ultra thin Au films which can be strained up to 20% without rupture, well beyond the current requirements.
Using this indirect actuation scheme, low aspect ratio channels with thin, membranes that collapse (and thus eliminate the mechanical resistance opposing the closing of the valve) can be used. These pneumatic valves are functional because in use the fluid pressure opens them up. Lids covering the electrical microvalve 201 (
a shows a microvalve that features two second electrodes 18a and 18b and that is preferentially used for hydraulic applications. The microvalve is used to actuate a flexible valve located downstream of opening 301. Microvalve 18a is actuated first and closes channel 10. Microvalve 18b is actuated thereafter and displaces the fluid between electrodes 18a and 18b, which creates a pressure in the channel 10 and in the opening 301 and thus displaces the flexible valve to close a microchannel containing sample fluid.
b shows another embodiment of a hydraulic microvalve. Here the geometry of channel 10 is such as the width is wider on the edge 305 of the electrode 18 and narrower on the edge 304. Thus, upon application of electrical force, the valve initially closes on the edge 305. Once it is closed on the edge 305, the closure of the other areas of the valve will contribute to increase the pressure downstream of the valve and in the opening 301 and on the flexible membrane that interrupts a flow of sample fluid. It will be apparent to the skilled in the art, that instead of a V-shaped width, a channel with variable depth may be used or an electrode with areas without electrode material (The larger the area without electrode material, the smaller the electric force and the later the electrode will close). For example if the non-electrode are is higher on the edge 306, the edge 305 will close faster as described above. Different driving voltages may be used to increase the time lag between the closure between the edge 305 and the edge 306 of the electrode.
Although throughout the present specification, the expression microfluidic circuit is being used, it is meant to also include microfluidic chips, and all other similar expressions commonly used in the field.
Another example of application of the microfluidic circuit with the microvalve of the present invention is to realize an architecture that can be interfaced directly with microelectronic chips and that is therefore scalable. As the microvalve of the present invention can be closed by applying a voltage, it can therefore be directly controlled using electronic chips. Thus, using a computer, complex fluidic operations can be programmed and using a microelectronic chip the microvalves in the microfluidic circuit actuated accordingly. This concept hinges on the large-scale integration of microelectronic chips and allows accelerating the integration and parallelization of microfluidics.
Using the microvalve of the present invention renders microfluidic circuits extraordinarily versatile and ideally suited for performing complex experimental protocols in parallel with high throughput while economizing reagents and reducing costs. Such microfluidic circuits could transform high cell biology—specifically high throughput cell assay—and medicine akin to the way that DNA chips and high throughput sequencers have transformed, and are still transforming, them.
An additional aspect of the present invention lies in the overall concept and architecture for integrated electronic microfluidic systems with two fluidic chips—a disposable MSL chip reversibly connected to an micro-electro pneumatic or a micro-electro-hydraulic chip with electrical microvalve—controlled using HV ICs, and in the technical details supporting their realization. More particularly, the following aspects are of interest: the indirect valving concept using a micropneumatic circuit (with a gas manifold) or micro hydraulic circuit (with a liquid manifold) controlled by electrical microvalve which is a significant advance because it acts as a bridge between microelectronic ICs and microfluidics, thus paving the way for large scale MIP of microfluidics; the concept and design of the flimsy, non-self-supporting electrical microvalve formed across low aspect ratio conduits; and finally, the simplified fabrication process to make these valves.
Reference is now made to
On
In the context of the present invention, a prototype electronic microfluidic system using the teachings of the present invention has been built and used for automated cell culture and assays. The following phases were followed in the development of the prototype:
Phase 1: Develop electrostatic elastomeric valves (electrical microvalve), also called microvalve, for regulating the pressure in a manifold embedded in a micro-electro pneumatic (MEP) chip; an electrical interface and connections to a sample chip;
Phase 2: Design and microfabricate a multilayer soft lithography (MSL) sample chip with pneumatic membrane valves suitable for cell culture. The MSL chip was disposable and could be connected to the MEP chip;
Phase 3: Integration of high voltage (HV) ICs, and the above mentioned MEP and MSL chips on a custom designed PCB connected to a computer with a control program that is used to program the HV ICs. The HV ICs control the electrical microvalves which control the membrane valves in the MSL chip; and
Phase 4: Testing of the electronic microfluidic chips and demonstration of complex fluidic operations with the delivery of cells to the microfluidic compartments, and assess the merit of electronic microfluidics for HCS.
Phase 1 consisted of building MEP chips with electrical microvalve. The major task was the fabrication of a MEP chip with at least 40 independent electrical microvalves that could regulate at least 0.5 bar using 300 V. The current test chips microfabricated were 25×25 mm2 in size, feature sets of 32 electrical microvalves with variable dimensions and a standardized interface with 168 electrical connections by patterning Au on PDMS (which forms excellent electrical contacts 13). The thickness of the membrane and the depth of the pneumatic channel were adjusted during fabrication. Molding processes that were developed previously were used to define (vertical) via in both the MEP and MSL chips and which served as pneumatic connections between the electrical microvalves and the membrane valves of the MSL chip. The design of the electrical microvalve and pressure manifold were optimized for efficiency and the fabrication process for higher yield, which necessitated continuous efforts and careful processing of the chips in the clean rooms. Next, the 2nd generation MEP chip were designed and microfabricated. The current processes was further refined for increasing the yield and different surface chemical treatments based on silanes and thiols were used to control adhesion depending on the requirements.
Phase 2 consisted of building an MSL chip suitable for cell culture, and interconnection via matching the ones of the MEP chip. MSL is a well-established technology, and published design rules were followed to make an MSL chip. Synthesis of a set of universal requirements of HCS and cellular assays were elaborated to guide the design of the MSL chip. The fluidic network architecture was defined by improving on the functions and features of published MSL chips and applications. Channel dimensions of ˜50 μm width, ˜20 μm depth, and cell culture micro compartments ˜400 μm wide (with support posts) are foreseen. The MEP chip and MSL chips were aligned using a homemade alignment tool and reversibly clamped together on the PCB. For small chips, mechanical clamping was used, whereas for larger chips a vacuum-based clamping using a manifold is being foreseen.
Phase 3 consisted of building an electronic microfluidic system comprising a custom-designed PCB, the MEP and MSL chips developed in phase 1 and 2, five HV ICs bonded to a glass carrier and connected to the PCB, and a computer connection. Programming in Labview™ was developed for controlling the 5 HV IC chips each featuring 10 programmable HV control lines operating at up to 300 V and supporting a load of 2 mA.
Phase 4 more particularly consisted of testing the electronic microfluidic system and demonstrating complex fluidic operations and delivery of cells to the micro compartments. Using a pressure regulator, the manifold was pressurized, as well as the sample containers. Although a single pressure line would be sufficient, additional pressure lines were used in this prototype to simplify operation. The PCB was mounted on an inverted microscope equipped with an incubation chamber enabling both observation and cultivation of cells in the electronic microfluidic system. The system was qualified, the merit of the technology assessed and an analysis of the shortcomings provide and improvements proposed. This evaluation was performed with respect to live cell assays and HCS.
The present invention has been described with regard to preferred embodiments. The description as much as the drawings were intended to help the understanding of the invention, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications may be made to the invention without departing from the scope of the invention as described herein, and such modifications are intended to be covered by the present description.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2007/001997 | 11/5/2007 | WO | 00 | 9/21/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60864268 | Nov 2006 | US |
