Patent Publication Number: US-8993348-B2

Title: Exchangeable carriers pre-loaded with reagent depots for digital microfluidics

Description:
This patent application is a National Phase application claiming the benefit of PCT/EP2009/062657 filed on Sep. 30, 2009, in English, entitled EXCHANGEABLE CARRIERS PRE-LOADED WITH REAGENT DEPOTS FOR DIGITAL MICROFLUIDICS; which further claims priority of the U.S. patent application Ser. No. 12/285,326 filed on Oct. 1, 2008 now U.S. Pat. No. 8,187,864, the whole content of which is incorporated herein by explicit reference for all intents and purposes. 
     FIELD OF THE INVENTION 
     The present invention relates to exchangeable, reagent pre-loaded carriers for digital microfluidics, and more particularly the present invention relates to removable plastic sheets on which reagents are strategically located in pre-selected positions as exchangeable carriers for digital microfluidic (DMF) devices. 
     BACKGROUND TO THE INVENTION 
     Microfluidics deals with precise control and manipulation of fluids that are geometrically constrained to small, typically microliter, volumes. Because of the rapid kinetics and the potential for automation, microfluidics can potentially transform routine bioassays into rapid and reliable tests for use outside of the laboratory. Recently, a new paradigm for miniaturized bioassays has been emerged called “digital” (or droplet based) microfluidics. Digital microfluidics (DMF) relies on manipulating discrete droplet of fluids across a surface of patterned electrodes, see e.g. U.S. Pat. No. 7,147,763; U.S. Pat. No. 4,636,785; U.S. Pat. No. 5,486,337; U.S. Pat. No. 6,911,132; U.S. Pat. No. 6,565,727; U.S. Pat. No. 7,255,780; JP 10-267801; or Lee et al. 2002 “Electrowetting and electrowetting-on-dielectric for microscale liquid handling”  Sensors  &amp;  Actuators  95: 259-268; Pollack et al. 2000 “Electrowetting-based actuation of liquid droplets for microfluidic applications”  Applied Physics Letters  77: 1725-1726; and Washizu, M. 1998 “Electrostatic actuation of liquid droplets for microreactor applications”  IEEE Transactions on Industry Applications  34: 732-737. This technique is analogous to sample processing in test tubes, and is well suited for array-based bioassays in which one can perform various biochemical reactions by merging and mixing those droplets. More importantly, the array based geometry of DMF seems to be a natural fit for large, parallel scaled, multiplexed analyses. In fact, the power of this new technique has been demonstrated in a wide variety of applications including cell-based assays, enzyme assays, protein profiling, and the polymerase chain reaction. 
     Unfortunately, there are two critical limitations on the scope of applications compatible with DMF—biofouling and interfacing. The former limitation, biofouling, is a pernicious one in all micro-scale analyses—a negative side-effect of high surface area to volume ratios is the increased rate of adsorption of analytes from solution onto solid surfaces. We and others have developed strategies to limit the extent of biofouling in digital microfluidics, but the problem persists as a road-block, preventing wide adoption of the technique. 
     The second limitation for DMF (and for all microfluidic systems) is the “world-to-chip” interface—it is notoriously difficult to deliver reagents and samples to such systems without compromising the oft-hyped advantages of rapid analyses and reduced reagent consumption. A solution to this problem for microchannel-based methods is the use of pre-loaded reagents. Such methods typically comprise two steps:
     (1) reagents are stored in microchannels (or in replaceable cartridges), and   (2) at a later time, the reagents are rapidly accessed to carry out the desired assay/experiment.   

     Two strategies have emerged for microchannel systems—in the first, reagents are stored as solutions in droplets isolated from each other by plugs of air (see Linder et al. 2005 “Reagent-loaded cartridges for valveless and automated fluid delivery in microfluidic devices”  Analytical Chemistry  77: 64-71) or an immiscible fluid (see Hatakeyama et al. 2006 “Microgram-scale testing of reaction conditions in solution using nanoliter plugs in microfluidics with detection by MALDI-MS”  Journal of the American Chemical Society  128: 2518-2519 and Zheng et al. 2005 “A microfluidic approach for screening submicroliter volumes against multiple reagents by using preformed arrays of nanoliter plugs in a three-phase liquid/liquid/gas flow”  Angewandte Chemie—International Edition  44: 2520-2523) until use. In a second, reagents are stored in solid phase in channels, and are then reconstituted in solution when the assay is performed (Furuberg et al. 2007 “The micro active project: Automatic detection of disease-related molecular cell activity”  Proceedings of SPIE - Int. Soc. Opt. Eng .; Garcia et al. 2004 “Controlled microfluidic reconstitution of functional protein from an anhydrous storage depot”  Lab on a Chip  4: 78-82; and Zimmermann et al. 2008 “Autonomous capillary system for one-step immunoassays”  Biomedical Microdevices ). Pre-loaded reagents in microfluidic devices is a strategy that will be useful for a wide range of applications. Until now, however, there has been no analogous technique for digital microfluidics. 
     In response to the twin challenges of non-specific adsorption and world-to-chip interfacing in digital microfluidics, we have developed a new strategy relying on removable polymer coverings (see Abdelgawad and Wheeler 2008 “Low-cost, rapid-prototyping of digital microfluidics devices”  Microfluidics and Nanofluidics  4: 349-355; Chuang and Fan 2006 “Direct handwriting manipulation of droplets by self-aligned mirror-EWOD across a dielectric sheet”  Proceedings of Mems:  19 th IEEE International Conference on Micro Electro Mechanical Systems, Technical Digest:  538-541; and Lebrasseur et al. 2007 “Two-dimensional electrostatic actuation of droplets using a single electrode panel and development of disposable plastic film card”  Sensors and Actuators a - Physical  136: 358-366). After each experiment, a thin film is replaced, but the central infrastructure of the device is reused. This effectively prevents cross-contamination between repeated analyses, and perhaps more importantly, serves as a useful medium for reagent introduction onto DMF devices. 
     SUMMARY AND OBJECTIVES OF THE INVENTION 
     To demonstrate this principle of using a single electrode panel and of disposable plastic coverings, we pre-loaded dried spots of enzymes to the plastic coverings for subsequent use in proteolytic digestion assays. The loaded reagents were found to be active after &gt;1 month of storage in a freezer. As the first technology of its kind, we propose that this innovation may represent an important step forward for digital microfluidics, making it an attractive fluid-handling platform for a wide range of applications. Even using a two-plate design (with or without double electrode panel) turned out to be applicable to reagent pre-loaded carriers according to the present invention. 
     The present invention provides removable, disposable carriers, e.g. plastic sheets which are be pre-loaded with reagents. The new method involves manipulating reagent and sample droplets on DMF devices that have been attached with pre-loaded carriers. When an assay is complete, the sheet can be removed, analyzed, if desired, and the original device can be reused by reattaching a fresh pre-loaded sheet to start another assay. 
     These removable, disposable plastic films, pre-loaded with reagents, facilitate rapid, batch scale assays using DMF devices with no problems of cross-contamination between assays. In addition, the reagent cartridge devices and method disclosed herein facilitate the use of reagent storage depots. For example, the inventors have fabricated sheets with pre-loaded dried spots containing enzymes commonly used in proteomic assays, such as trypsin or α-chymotrypsin. After digestion of the model substrate ubiquitin, the product-containing sheets were evaluated by matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). The present invention very advantageously elevates DMF to compatibility with diverse applications ranging from laboratory analyses to point-of-care diagnostics. 
     Thus, an embodiment of the present invention includes a carrier (preferably in the form of a sheet or film) that is pre-loaded with reagents for use with a digital microfluidic device, the digital microfluidic device including an electrode array, said electrode array including an array of discrete electrodes, the digital microfluidic device including an electrode controller, the pre-loaded carrier comprising:
         an electrically insulating sheet having a back surface and a front hydrophobic surface, said electrically insulating sheet being removably attachable to said electrode array of the digital microfluidic device with said back surface being adhered to a surface of said electrode array, said electrically insulating sheet covering said discrete electrodes for insulating the discrete electrodes from each other and from liquid droplets on the front hydrophobic surface,       wherein said electrically insulating sheet has one or more reagent depots located in one or more pre-selected positions on the front hydrophobic surface of the electrically insulating sheet;   wherein in operation the electrode controller being capable of selectively actuating and de-actuating said discrete electrodes for translating liquid droplets over the front hydrophobic surface of the electrically insulating sheet; and   wherein said one or more pre-selected positions on said front working surface of said electrically insulating sheet are positioned to be accessible to droplets actuated over the front hydrophobic surface of the electrically insulating sheet.   

     In another embodiment of the present invention there is provided a digital microfluidic device, comprising:
         a first substrate having mounted on a surface thereof an electrode array, said electrode array including an array of discrete electrodes, the digital microfluidic device including an electrode controller capable of selectively actuating and de-actuating said discrete electrodes;   an electrically insulating sheet having a back surface and a front hydrophobic surface, said electrically insulating sheet being removably attached to said electrode array of the digital microfluidic device (preferably with said back surface being adhered to said array of discrete electrodes), said electrically insulating sheet electrically insulating said discrete electrodes from each other in said electrode array and from liquid droplets on the front hydrophobic surface, said electrically insulating sheet having one or more reagent depots located in one or more pre-selected positions on the front hydrophobic surface of the electrically insulating sheet, said one or more pre-selected positions on said front hydrophobic surface being positioned to be accessible to the liquid droplets actuated over the front hydrophobic surface of the electrically insulating sheet;       wherein liquid droplets are translatable across said front hydrophobic surface to said one or more reagent depots by selectively actuating and de-actuating said discrete electrodes under control of said electrode controller.   

     In an embodiment of the apparatus there may be included a second substrate having a front surface which is optionally a hydrophobic surface, wherein the second substrate is in a spaced relationship to the first substrate thus defining a space between the first and second substrates capable of containing droplets between the front surface of the second substrate and the front hydrophobic surface of the electrically insulating sheet on said electrode array on said the substrate. An embodiment of the device may include an electrode array on the second substrate, covered by a dielectric sheet. In this case the electrode array on the first substrate may be optional and hence may be omitted. There may also be insulating sheets pre-loaded with reagent depots on one or both of the substrates. 
     The present invention also provides a digital microfluidic method, comprising the steps of:
         preparing a digital microfluidic device having an electrode array including an array of discrete electrodes, the digital microfluidic device including an electrode controller connected to said array of discrete electrodes for applying a selected pattern of voltages to said discrete electrodes for selectively actuating and de-actuating said discrete electrodes in order to move liquid sample drops across said electrode array in a desired pathway over said discrete electrodes;   providing a removably attachable electrically insulating sheet having a back surface and a front working surface;   removably attaching said electrically insulating sheet to said electrode array of the digital microfluidic device (preferably with said back surface being adhered thereto), said electrically insulating sheet having hydrophobic front surface and one or more reagent depots located in one or more pre-selected positions on the front working surface of the electrically insulating sheet, said one or more pre-selected positions on said front working surface of said electrically insulating sheet are positioned to be accessible to droplets actuated over the front working surface of the electrically insulating sheet;   conducting an assay by directing one or more sample droplets over said front working surface to said one or more reagent depots whereby the one or more sample droplets is delivered to said one or more reagent depots which is reconstituted by the one or more sample droplets and mixed with at least one selected reagent contained in the one or more reagent depots;   isolating any (or at least one) resulting reaction product formed between said mixed sample droplet and said at least one selected reagent in each (or at least one) of said one or more reagent depots; and optionally   removing said removably attachable electrically insulating sheet from the surface of the electrode array of the digital microfluidic device and preparing the digital microfluidic device for a new assay.       

     A further understanding of the functional and advantageous aspects of the invention can be realized by reference to the following detailed description and drawings. Additional elements of the present invention and additional preferred embodiments arise from the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention are described in greater detail with reference to the accompanying drawings that shall not limit the scope of the present invention. There is shown in: 
         FIG. 1A  protein adsorption from an aqueous droplet onto a DMF device in which the upper image shows a device prior to droplet actuation, paired with a corresponding confocal image of a central electrode, the lower image shows the same device after a droplet containing FITC-BSA (7 μg/ml) has been cycled over the electrode  4  times, paired with a confocal image collected after droplet movement. The two images were processed identically to illustrate that confocal microscopy can be used to detect the non-specific protein adsorption on device surfaces as a result of digital actuation. 
         FIG. 1B  mass spectrum of 10 μM angiotensin I (MW 1296); 
         FIG. 1C  cross-contamination on a digital microfluidic device: mass spectrum of 1 μM angiotensin II (MW 1046). The droplet was actuated over the same surface as the former on the same device, resulting in cross-contamination from angiotensin I; 
         FIG. 2  a schematic depicting the removable pre-loaded carrier strategy where in step:
         (1) a fresh piece of a carrier in the form of a plastic sheet with a dry reagent is affixed to a DMF device;   (2) reagents in droplets are actuated over on top of the carrier, exposed to the preloaded dry reagent, merged, mixed and incubated to result in a chemical reaction product;   (3) residue is left behind as a consequence of non-specific adsorption of analytes;   (4) the carrier with a product droplet or dried product is peeled off; and   (5) the product is analyzed if desired;       

         FIG. 3  MALDI-MS analysis of different analytes processed on different carriers using a single DMF device:
         a) 35 μM Insulin   b) 10 μM Bradykinin   c) 10 μM 20 mer DNA Oligonucleotide   d) 0.01% ultramarker;       

         FIG. 4  pre-loaded carrier analysis. MALDI peptide mass spectra from pre-spotted (Top) trypsin and (Bottom) α-chymotrypsin digest of ubiquitin were shown, peptide peaks were identified through database search in MASCOT, and the sequence coverage was calculated to be over 50%; 
         FIG. 5  a bar graph showing percent activity versus time showing the pre-loaded carrier stability assay in which the fluorescence of protease substrate (BODIPY-casein) and an internal standard were evaluated after storing carriers for 1, 2, 3, 10, 20, and 30 days, the carriers were stored at −20° C. or −80° C. as indicated on the bar graph, and the mean response and standard deviations were calculated for each condition from 5 replicate carriers; 
         FIG. 6  different embodiments of DMF devices according to the present invention, wherein: 
         FIG. 6A  shows a one-sided open DMF device with one carrier pre-loaded with reagents attached to a first substrate; 
         FIG. 6B  shows a one-sided open DMF device with one carrier pre-loaded with reagents and a dielectric layer below the carrier; 
         FIG. 6C  shows a one-sided closed DMF device with a second substrate defining a space or gap between the first and second substrates; 
         FIG. 6D  shows a two-sided closed DMF device with a second substrate defining a space or gap between the first and second substrates. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally speaking, the systems described herein are directed to exchangeable, reagent pre-loaded carriers for digital microfluidic devices, particularly suitable for high throughput assay procedures. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms. The figures are not to scale and some features may be exaggerated or minimized to show details of particular elements while related elements may have been eliminated to prevent obscuring novel aspects. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. For purposes of teaching and not limitation, the illustrated embodiments are directed to exchangeable, reagent pre-loaded carriers for digital microfluidic devices. 
     As used herein, the term “about”, when used in conjunction with ranges of dimensions of particles or other physical or chemical properties or characteristics, is meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present invention. 
     The basic problem to be solved by the present invention is to provide a means of adapting digital microfluidic devices so that they can be used for high throughput batch processing while at the same time avoiding bio-fouling of the DMF devices as discussed above in the Background. To illustrate how problematic bio-fouling is, studies have been carried out by the inventors to ascertain the scope of this problem. 
     Protein Adsorption on DMF and Cross Contamination Analysis 
     Confocal microscopy was used to evaluate protein adsorption on surfaces. In general, a droplet containing 7 μg/ml FITC-BSA is translated on a DMF device. Two images were taken on a spot before and after droplet actuation. A residue is left on the surface as a consequence of non-specific protein adsorption during droplet actuation in which it can be detected by confocal microscopy. Such residues can cause two types of problems for DMF:
     (1) the surface may become sticky, which impedes droplet movement, and   (2) if multiple experiments are to be performed, cross-contamination may be a problem.   

     A FluoView 300 scanning confocal microscope (OLYMPUS, Markam, ON) equipped with an Ar +  (488 nm) laser was used, in conjunction with a 100× objective (N.A. 0.95) for analysis of proteins adsorbed to DMF device surfaces ( FIG. 1A ). Fluorescence from adsorbed labeled proteins was passed through a 510-525 nm band-pass filter, and each digital image was formed from the average of four frames using FluoView image acquisition software (OLYMPUS). 
     MALDI-MS was used to evaluate the amount of cross contamination of two different peptide samples actuated across the same path on the same device. Specifically, 2 μl droplet of 10 μM angiotensin I in the first run, and 2 μl droplet of 1 μM angiotensin II in the second. As shown in  FIG. 1B , the spectrum of angiotensin I generated after the first run is relatively clean; however, as shown in  FIG. 1C , the spectrum of angiotensin II generated is contaminated with residue from the previous run. In these tests, after actuation by DMF, the sample droplets were transferred to a MALDI target for crystallization and analysis, meaning that the cross-contamination comprised both (a) an adsorption step in the first run, and (b) a desorption step in the second run. The intensity from the Angiotensin I contaminant was estimated to be around 10% of most intense Angiotensin II peak (MW 1046). This corresponds to roughly about 1% or 0.1 μM of Angiotensin I fouling non-specifically on the DMF device. Even though the tested peptides are less sticky compare to proteins, this result is in agreement with Luk&#39;s reported value, which is less than 8% of FITC-BSA adsorbing to DMF device (see Luk et al. 2008 “Pluronic additives: A solution to sticky problems in digital microfluidics,”  Langmuir  24: 6382-6389). In addition to contamination, smooth droplet movement, especially during the run of angiotensin II sample, was obstructed due to non-specific adsorption of previous run. Thus, a higher actuation voltage was required to force the droplet to move over to the next set of electrodes. This however does not always work if the droplet becomes stuck permanently due to high adhesion to the fouled surfaces, increasing actuation voltage will not help in this case, not to mention potential dielectric breakdown and ruin the device if the voltage is too high. 
     Exchangeable, Pre-Loaded, Disposable Carriers 
     The present invention provides exchangeable, pre-loaded, disposable carriers on which reagents are strategically located in pre-selected positions on the upper surface. These carriers can be used as exchangeable carriers for use with digital microfluidic devices where the carrier is applied to the electrode array of the digital microfluidic device. 
     Referring to  FIG. 2 , a pre-loaded, electrically insulating disposable sheet shown generally at  10  according to the present invention has one pre-loaded reagent depot  12  mounted on a hydrophobic front surface of electrically insulating sheet  10 . This disposable carrier  10  may be any thin dielectric sheet or film so long as it is chemically stable toward the reagents pre-loaded thereon. For example, any polymer based plastic may be used, such as for example saran wrap. In addition to plastic food-wrap, other carriers, including generic/clerical adhesive tapes and stretched sheets of paraffin, were also evaluated for use as replaceable DMF carriers. 
     The disposable carrier  10  is affixed to the electrode array  16  of the DMF device  14  with a back surface of the carrier  10  adhered to the electrode array  16  in which the reagent depot  12  deposited on the surface of the carrier  10  (across which the reagent droplets are translated) is aligned with pre-selected individual electrode  18  of the electrode array  16  as shown in steps ( 1 ) and ( 2 ) of  FIG. 2 . Two reagents droplets  20  and  22  are deposited onto the device prior to an assay. This depositing of the droplets  20  and  22  is preferably done utilizing dispenser tips  36  that are connected to a sample reservoir  32  or to solvent reservoir  34  (see  FIG. 2 ). Alternatively, reservoirs  32  and  34  can be in connections with a device or are integral parts of a device whereby droplet  20  and  22  are dispensed from the reservoirs using DMF actuation. 
     As can be seen from step ( 3 ) of  FIG. 2 , during the assay reagent droplets  20  and  22  are actuated over the top of disposable sheet or carrier  10  to facilitate mixing and merging of the assay reagent droplets  20  and  22  with the desired reagent depot  12  over electrode  18 . After the reaction has been completed, the disposable carrier  10  may then be peeled off as shown in step ( 4 ) and the resultant reaction products  26  analyzed if desired as shown in step ( 5 ). Afresh disposable carrier  10  is then attached to the DMF device  14  for next round of analysis. The product  26  can be also analyzed while the removable carrier is still attached to the DMF device  14 . This process can be recycled by using additional pre-loaded carriers. In addition, the droplets containing reaction product(s) may be split, mixed with additional droplets, and/or incubated for cell culture if they contain cells. 
     As a consequence, cross contamination is avoided as residues  28  and  30  from assays conducted on a previous disposable sheet or carrier  10  will be removed along with the disposable carrier  10 . The assay described above was done using one preloaded reagent  12  but it will be appreciated that the pre-loaded carrier  10  can be loaded with multiple reagents assayed in series or in parallel with multiple droplet reagents  20  and  22 . 
     In an embodiment of the present invention the pre-loaded electrically insulating sheet  11  and the electrode array  16  may each include alignment marks for aligning the electrically insulating sheet  11  with the electrode array when affixing the electrically insulating sheet to the electrode array such that one or more pre-selected positions  13  on front working surface  11   a  of the electrically insulating sheet  11  are selected to be in registration with one or more pre-selected discrete actuating electrodes  18  of the electrode array. When the reagent depots  12  are in registration with pre-selected electrodes  18  they may be located over top of a selected electrode or next to it laterally so that it is above a gap between adjacent electrodes. 
       FIG. 6A  shows a one-sided open DMF device with a carrier  10  that is pre-loaded with reagents  12  for use with a digital microfluidic device  14  and that is attached to a first substrate  24 . The digital microfluidic device includes an array  16  of discrete electrodes  17  and an electrode controller  19 . The pre-loaded carrier  10  comprises an electrically insulating sheet  11  having a front hydrophobic surface  11   a  and a back surface  11   b . This electrically insulating sheet  11  is removably attachable to a surface  16 ′ of the electrode array  16  of the digital microfluidic device  14 . When positioned on the electrode array  16  of the digital microfluidic device  14 , said electrically insulating sheet  11  covers said discrete electrodes  17  and provides electrical insulation to the discrete electrodes  17  from each other and from liquid droplets  20 , 22 , 33  present on the front hydrophobic surface  11   a . The electrically insulating sheet  11  according to a first embodiment of the present invention has one or more reagent depots  12  located in one or more pre-selected positions  13  on its front hydrophobic surface  11   a . In operation, the electrode controller  19  of the digital microfluidic device  14  is capable of selectively actuating and de-actuating said discrete electrodes  17  for translating liquid droplets  20 , 22 , 33  over the front hydrophobic surface  11   a  of the electrically insulating sheet  11  and said one or more pre-selected positions  13  on the front working surface  11   a  of said electrically insulating sheet  11  are positioned to be accessible to droplets  20 , 22 , 33  actuated over the front hydrophobic surface  11   a  of the electrically insulating sheet  11 . 
     Preferably, said electrically insulating sheet  11  is attachable or attached to the surface  16 ′ of said electrode array  16  by an adhesive  15  that contacts the back surface  11   b  of the electrically insulating sheet  11  with the surface  16 ′ of the electrode array  16  and/or the surface  24 ′ of the first substrate  24 . It is even more preferred that said electrically insulating sheet  11  includes an adhesive  15  on said back surface  11   b  thereof which is able to contact said electrode array for adhering said electrically insulating sheet to said first substrate  24 . 
       FIG. 6B  shows a one-sided open DMF device with one carrier pre-loaded with reagents and a dielectric layer below the carrier. The digital microfluidic device  14  (as depicted similarly in  FIG. 6A ) includes important features such as an electrode controller  19 ; in addition, liquid droplets  20 , 22 , 33  to be translated are presented here. However, in the embodiment as shown in  FIG. 6B , the adhesive  15  only contacts the back surface  11   b  of the electrically insulating sheet  11  with the surface  24 ′ of the first substrate  24 ; alternately, the adhesive  15  could be present on the entire back surface  11   b  of the electrically insulating sheet  11  (not shown). In this embodiment, the digital microfluidic device  14  preferably includes a dielectric layer  25  applied directly to said surface  16 ′ of said electrode array  16  so that it is sandwiched between said electrode array  16  and said electrically insulating sheet  11 . 
       FIG. 6C  shows a one-sided closed DMF device with a second substrate defining a space or gap between the first and second substrates. The digital microfluidic device  14  (as depicted similarly in  FIG. 6B ) includes important features such as an electrode controller  19 ; in addition, liquid droplets  20 , 22 , 33  to be translated are present. In this embodiment, the digital microfluidic device  14  preferably further includes a second substrate  27  having a front surface  27 ′ which is optionally a hydrophobic surface. The second substrate  27  is in a spaced relationship to the first substrate  24  thus defining a space or gap  29  between the first and second substrates  24 , 27  capable of containing droplets  20 , 22 , 33  between the front surface  27 ′ of the second substrate  27  and the front hydrophobic surface  11   a  of the electrically insulating sheet  11  on said electrode array  16  on said first substrate  24 . Preferably, the electrode controller  19  also controls an electrostatic charge of the second substrate surface  27 ′. In contrast to  FIG. 6B , the adhesive  15  here only contacts the back surface  11   b  of the electrically insulating sheet  11  with the dielectric layer  25  that is positioned on the surface  16 ′ of the electrode array  16  of the first substrate  24 . Alternately, the adhesive  15  could be present on the entire back surface  11   b  of the electrically insulating sheet  11  (not shown). 
       FIG. 6D  shows a two-sided closed DMF device with a second substrate defining a space or gap between the first and second substrates. The digital microfluidic device  14  (as depicted similarly in the  FIGS. 6A-6C ) includes an array  16  of discrete electrodes  17  and an electrode controller  19 . The pre-loaded carrier  10  comprises a first electrically insulating sheet  11  having a front hydrophobic surface  11   a  and a back surface  11   b . This first electrically insulating sheet  11  is removably attachable to a surface  16 ′ of a first electrode array  16  of the digital microfluidic device  14 . In this embodiment, the digital microfluidic device  14  preferably further includes a second substrate  27  having a front surface  27 ′. The front surface  27 ′ of the second substrate  27  according to a preferred embodiment is not hydrophobic and it includes an additional, second electrically insulating sheet  31  having a back surface  31   b  and a front hydrophobic surface  31   a . This additional electrically insulating sheet  31  is removably attached to said front surface  27 ′ of the second substrate  27  with the back surface  31   b  adhered to said front surface  27 ′. Said additional electrically insulating sheet  31  has none, one or more reagent depots  12  located in one or more pre-selected positions  13  on the front hydrophobic surface  31   a  of the additional electrically insulating sheet  31 . 
     In contrast to  FIG. 6B , the adhesive  15  here only contacts the back surface  11   b  of the electrically insulating sheet  11  with the surface  16 ′ of the electrode array  16  of the first substrate  24 . On the opposite side, the adhesive  15  is present on the entire back surface  31   b  of the additional electrically insulating sheet  31 . Alternately, the adhesive  15  could be present on the entire back surface  11   b  of the electrically insulating sheet  11  (not shown). Preferably (as shown in  FIG. 6D ), the digital microfluidic device  14  includes an additional electrode array  35  mounted on the front surface  27 ′ of the second substrate  27 , the additional electrode array  35  being covered by the additional electrically insulating sheet  31  having said front hydrophobic surface  31   a . As shown in  FIGS. 6B and 6C , also this digital microfluidic device  14  of  FIG. 6D  preferably includes a dielectric layer  25  applied directly to said surface  27 ′ of said second electrode array  35  so that it is sandwiched between said electrode array  35  and said second electrically insulating sheet  31 . Another dielectric layer  25  may be positioned between the electrically insulating sheet  11  and the surface  16 ′ of the electrode array  16  (not shown). In an alternate embodiment (not shown), said additional electrode array  35  on the second substrate  27  is coated with a hydrophobic coating and the second insulating layer  31  is not present. 
     The disposable carriers  10  may be packaged with a plurality of other carriers and sold with the reagent depots containing one or more reagents selected for specific assay types. Thus the carriers  10  in the package may have an identical number of preloaded reagent depots  12  with each depot including an identical reagent composition. The reagent depots preferably include dried reagent but they could also include a viscous gelled reagent. 
     One potential application of the present invention may be culturing and assaying cells on regent depots. In such applications the reagent depots can include bio-substrate with attachment factors for adherent cells, such as fibronectin, collagen, laminin, polylysine, etc. and any combination thereof. Droplets with cells can be directed to the bio-substrate depots to allow cell attachment thereto in the case of adherent cells. After attachment, cells can be cultured or analyzed in the DMF device. 
     While the DMF device  14  has been shown in  FIG. 2  to have a single substrate  24  with an electrode array  16  formed thereon, it will be appreciated by those skilled in the art that the DMF device may include a second substrate  27  having a front surface  27 ′ which is optionally a hydrophobic surface, wherein the second substrate is in a spaced relationship to the first substrate thus defining a space between the first and second substrates capable of containing droplets between the front surface of the second substrate and the front hydrophobic surface of the electrically insulating sheet on said electrode array on the first substrate (see  FIG. 6C ). The second substrate may be substantially transparent. Departing from the embodiment as depicted in  FIG. 6C , the pre-loaded carrier  10  (comprising a first electrically insulating sheet  11  and having a front hydrophobic surface  11   a  and a back surface  11   b ) may be removably attached to the surface  27 ′ of the second substrate  27  of the digital microfluidic device  14 . The same time, the electrode array  16  may be coated with a non-removable electrical insulator (not shown). 
     When the front surface of the second substrate is not hydrophobic, the device may include an additional electrically insulating sheet having a back surface and a front hydrophobic surface being removably attachable to the front surface of the second substrate with the back surface adhered to the front surface and additional electrically insulating sheet has one or more reagent depots located in one or more pre-selected positions on the front hydrophobic surface of the electrically insulating sheet. 
     Additionally, there may be included an additional electrode array  35  mounted on the front surface  27 ′ of the second substrate  27 , and including a layer applied onto the additional electrode array  35  having a front hydrophobic surface. The layer applied onto the additional electrode array has a front hydrophobic surface  31   a  which may be an additional electrically insulating sheet  31  having one or more reagent depots  12  located in one or more pre-selected positions  13  on the front hydrophobic surface. In this two plate design as depicted in  FIG. 6D , the first substrate  24  may optionally not have the pre-loaded insulating sheet or carrier  11  with reagent depots  12  mounted thereon. 
     The present invention and its efficacy for high throughput assaying will be illustrated with the following studies and examples, which are meant to be illustrative only and non-limiting. 
     EXPERIMENTAL DETAILS 
     Reagents and Materials 
     Working solutions of all matrixes (α-CHCA, DHB, HPA, and SA) were prepared at 10 mg/ml in 50% analytical grade acetonitrile/deionized (DI) water (v/v) and 0.1% TFA (v/v) and were stored at 4° C. away from light. Stock solutions (10 μM) of angiotensin I, II and bradykinin were prepared in DI water, while stock solutions (100 μM) of ubiquitin and myoglobin were prepared in working buffer (10 mM Tris-HCl, 1 mM CaCl 2  0.0005% w/v Pluronic F68, pH 8). All stock solutions of standards were stored at 4° C. Stock solutions (100 μM) of digestive enzymes (bovine trypsin and α-chymotrypsin) were prepared in working buffer and were stored as aliquots at −80° C. until use. Immediately preceding assays, standards and enzymes were warmed to room temperature and diluted in DI water (peptides) and working buffer (proteins, enzymes, and fluorescent reagents). Flourescent assay solution (3.3 μM quenched, bodipy-casein and 2 μM rhodamine B in working buffer) was prepared immediately prior to use. 
     Device Fabrication and Operation 
     Digital microfluidic devices with 200 nm thick chromium electrodes patterned on glass substrates were fabricated using standard microfabrication techniques. Prior to experiments, devices were fitted with (a) un-modified carriers, or (b) reagent-loaded carriers. When using un-modified carriers (a), a few drops of silicone oil were dispensed onto the electrode array, followed by the plastic covering. The surface was then spin-coated with Teflon-AF (1% w/w in Fluorinert FC-40, 1000 RPM, 60 s) and annealed on a hot plate (75° C., 30 min). When using pre-loaded carriers (b), plastic coverings were modified prior to application to devices. Modification comprised three steps: adhesion of coverings to unpatterned glass substrates, coating with Teflon-AF (as above), and application of reagent depots. The latter step was achieved by pipetting 2 μl droplet(s) of enzyme (6.5 μM trypsin or 10 μM α-chymotrypsin) onto the surface, and allowing it to dry. The pre-loaded carrier was either used immediately, or sealed in a sterilized plastic Petri-dish and stored at −20° C. Prior to use, pre-loaded carriers were allowed to warm to room temperature (if necessary), peeled off of the unpatterned substrate, and applied to a silicone-oil coated electrode array, and annealed on a hot plate (75° C., 2 min). In addition to food wraps, plastic tapes and paraffin have also been used to fit onto the device. Tapes were attached to the device by gentle finger press, whereas paraffin are stretched to about 10 mm thickness and then wrap around the device to make a tight seal free of air bubbles. 
     Devices had a “Y” shape design of 1 mm×1 mm electrodes with inter-electrode gaps of 10 μm. 2 μl droplets were moved and merged on devices operating in open-plate mode (i.e., with no top cover) by applying driving potentials (400-500 V RMS ) to sequential pairs of electrodes. The driving potentials were generated by amplifying the output of a function generator operating at 18 kHz, and were applied manually to exposed contact pads. Droplet actuation was monitored and recorded by a CCD camera. 
     Analysis by MALDI-MS 
     Matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) was used to evaluate samples actuated on DMF devices. Matrix/sample spots were prepared in two modes: conventional and in situ. In conventional mode, samples were manipulated on a device, collected with a pipette and dispensed onto a stainless steel target. A matrix solution was added, and the combined droplet was allowed to dry. In in situ mode, separate droplets containing sample and matrix were moved, merged, and actively mixed by DMF, and then allowed to dry onto the surface. In in situ experiments involving pre-loaded carriers, matrix/crystallization was preceded by an on-chip reaction: droplets containing sample proteins were driven to dried spots containing digestive enzyme (trypsin or α-chymotrypsin). After incubation with the enzyme (room temp., 15 min), a droplet of matrix was driven to the spot to quench the reaction and the combined droplet was allowed to dry. After co-crystallization, carriers were carefully peeled off of the device, and then affixed onto a stainless steel target using double-sided tape. Different matrixes were used for different analytes: α-CHCA for peptide standards and digests, DHB for ultramarker, HPA for oligonucleotides and SA for proteins. At least three replicate spots were evaluated for each sample. 
     Samples were analyzed using a MALDI-TOF Micro-MX MS (Waters, Milford, Mass.) operating in positive mode. Peptide standards and digests were evaluated in reflectron mode over a mass to charge ratio (m/z) range from 500-2′000. Proteins were evaluated in linear mode over a m/z range from 5′000-30′000. At least one hundred shots were collected per spectrum, with laser power tuned to optimize the signal to noise ratio (S/N). Data were then processed by normalization to the largest analyte peak, baseline subtraction, and smoothed with a 15-point running average. Spectra of enzyme digests were analyzed with the Mascot protein identification package searching the SwissProt database. The database was searched with 1 allowed missed cleavage, a mass accuracy of +/−1.2 Da, and no further modifications. 
     Peptide/Protein MS Analysis on Exchangeable Carriers 
     To illustrate the new strategy, four different types of analytes were processed using a single DMF device, using afresh removable carrier for each run. As shown in  FIG. 3 , the four analytes included insulin (MW 5733), bradykinin (MW 1060), a 20-mer oligonucleotide (MW 6135), and the synthetic polymer, Ultramark 1621 (MW 900-2200). Each removable carrier was analyzed by MALDI-MS in-situ, and no evidence for cross-contamination was observed. In our lab, conventional devices are typically disposable (used once and then discarded); however, in experiments with removable carriers, we regularly used devices for 9-10 assays with no drop-off in performance. Thus, in addition to eliminating cross-contamination, the removable carrier strategy significantly reduces the fabrication load required to support DMF. 
     In addition to plastic food-wrap, other carriers, including clerical adhesive tape and stretched sheets of wax film, were also evaluated for use as replaceable carriers. As was the case for food wrap, carriers formed from tape and wax film were found to support droplet movement and facilitate device re-use (data not shown). In addition, carriers formed from these materials were advantageous in that they did not require an annealing step prior to use. Other concerns, however, made these materials less attractive. Coverings formed from adhesive tape tended to damage the actuation electrodes after repeated applications (although presumably, this would not be a problem for low-tack tapes). In addition, as the tape carriers tested were relatively thick (˜45 μm), larger driving potentials (˜900 V RMS ) were required for droplet manipulation. In contrast, the thickness of stretched wax was ˜10 μm, resulting in driving potentials similar to those used for carriers formed from food wrap. However, the thickness of carriers formed in this manner was observed to be non-uniform, making them less reliable for droplet movement. In summary, it is likely that a variety of different carriers are compatible with the removable covering concept, but because those formed from food-wrap performed best in our hands, we used this material for the experiments reported here. 
     Two drawbacks to the removable carrier strategy are trapped bubbles and material incompatibility. In initial experiments, bubbles were occasionally observed to become trapped between the carrier and the device surface during application. When a driving potential was applied to an electrode near a trapped bubble, arcing was observed, which damaged the device. We found that this problem could be overcome by moistening the device surface with a few drops of silicone oil prior to application of the plastic film. Upon annealing, the oil evaporates, leaving a bubble-free seal. The latter problem, material incompatibility, is more of a concern. If aggressive solvents are used, materials in the carrier might leach into solution, which could interfere with assays. In our experiments, no contaminant peaks were observed in any MALDI-MS spectra (including in control spectra generated from bare carrier surfaces, not shown), but we cannot rule out the possibility of this being a problem in other settings. Given the apparent wide range of materials that can be used to form carriers (see above), we are confident that alternatives could be used in cases in which Teflon-coated food wrap is not tenable. 
     Preloaded Carriers and its Stability Analysis 
     In exploring exchangeable carrier strategy to overcome fouling and cross-contamination, we realized that the technology could, in addition, serve as the basis for an exciting new innovation for digital microfluidics. By pre-depositing reagents onto carriers (and by having several such carriers available), this strategy transformed DMF techniques into a convenient new platform for rapid introduction of reagents to a device, and can be a solution to the well-known world-to-chip interface problem for microfluidics (see Fang et al. 2002 “A high-throughput continuous sample introduction interface for microfluidic chip-based capillary electrophoresis systems”  Analytical Chemistry  74: 1223-1231 and Liu et al. 2003 “Solving the “World-to-chip” Interface problem with a microfluidic matrix”  Analytical Chemistry  75: 4718-4723). 
     To illustrate the new strategy, we prepared food wraps pre-spotted with dry digestive enzymes, and then used DMF to deliver droplets containing the model substrate, ubiquitin, to the spots. After a suitable incubation period, droplets containing MALDI matrix were delivered to the spot, which was dried and then analyzed. As shown in  FIG. 4 , MALDI mass spectra were consistent with what is expected of peptide mass fingerprints for the analyte. In fact, when evaluated using the proteomic search engine, MASCOT, the performance was excellent, with sequence identification of 50% or above for all trials. 
     In optimizing the pre-loaded carrier strategy for protease assays, we observed the method to be quite robust. First, pluronic F68 was used as a solution additive to facilitate movement of the analyte droplet (in this case, ubiquitin); this reagent has been shown to reduce ionization efficiencies for MALDI-MS (see Boernsen et al. 1997 “Influence of solvents and detergents on matrix-assisted laser desorption/ionization mass spectrometry measurements of proteins and oligonucleotides”  Rapid Communications in Mass Spectrometry  11: 603-609). Fortunately, the amount used here (0.0005% w/v) was low enough such that this effect was not observed. Second, trypsin and α-chymotrypsin autolysis peaks were only rarely observed, which we attribute to the low enzyme-to-substrate ratio and the short reaction time. Third, in preliminary tests, we determined that the annealing step (75° C., 2 min) did not affect the activity of dried enzymes. In the future, if reagents sensitive to these conditions are used, we plan to evaluate carriers formed from materials that do not require annealing (such as low-tack tape). Regardless, the robust performance of these first assays suggests that the strategy may eventually be useful for a wide range of applications, such as immunoassays or microarray analysis. 
     As described, the preloaded carrier strategy is similar to the concept of pre-loaded reagents stored in microchannels (see Linder et al. 2005; Hatakeyama et al. 2006; Zheng et al. 2005; Furuberg et al. 2007; Garcia et al. 2004; Zimmermann et al. 2008; and Chen et al. 2006 “Microfluidic cartridges pre-loaded with nanoliter plugs of reagents: An alternative to 96-well plates for screening”  Current Opinion in Chemical Biology  10: 226-231). Unlike these previous methods, in which devices are typically disposed of after use, in the present preloaded carrier strategy, the fundamental device architecture can be reused for any number of assays. Additionally, because the reagents (and the resulting products) are not enclosed in channels, they are in an intrinsically convenient format for analysis. For example, in this work, the format was convenient for MALDI-MS detection, but we speculate that a wide range of detectors could be employed in the future, such as optical readers or acoustic sensors. Finally, although this proof-of-principle work made use of food wrap carrier carrying a single reagent spot, we speculate that in the future, a microarray spotter could be used to fabricate preloaded carriers carrying many different reagents for multiplexed analysis. 
     To be useful for practical applications, pre-loaded carriers must be able to retain their activity during storage. To evaluate the shelf-life of these reagent spots, we implemented a quantitative protein digest assay. The reporter in this assay, quenched bodipy-labeled casein, has low fluorescence when intact, but becomes highly fluorescent when digested. In this preloaded reagent stability assays, a droplet containing the reporter was driven to a pre-loaded spot of trypsin, and after incubation the fluorescent signal in the droplet was measured in a plate reader (as described previously, see Luk et al. 2008 “Pluronic additives: A solution to sticky problems in digital microfluidics,”  Langmuir  24: 6382-6389; Barbulovic-Nad et al. 2008 “Digital microfluidics for cell-based assays”  Lab on a Chip  8: 519-526; Miller and Wheeler 2008 “A digital microfluidic approach to homogeneous enzyme assays”  Analytical Chemistry  80: 1614-1619). In preliminary experiments with freshly prepared preloaded carriers, it was determined that at the concentrations used, the reaction was complete within 30 minutes. An internal standard (IS), rhodamine B, was used to correct for alignment errors, evaporation effects, and instrument drift over time. 
     In shelf-life experiments, preloaded carriers were stored for different periods of time (1, 2, 3, 10, 20, or 30 days) at −20° C. or −80° C. In each experiment, after thawing the carrier, positioning it on the device, driving the droplet to the trypsin, and incubating for 30 minutes, the reporter/IS signal ratio was recorded. At least five different carriers were evaluated for each condition. As shown in  FIG. 5 , shelf-life performance was excellent—carriers stored at −80° C. retained &gt;75% of the original activity for periods as long as 30 days. Carriers stored at −20° C. retained &gt;50% of the original activity over the same period. The difference might simply be the result of different average storage temperature, or might reflect the fact that the −20° C. freezer was used in auto-defrost mode (with regular temperature fluctuations), while the temperature in the −80° C. freezer was constant. Regardless, the performance of these carriers was excellent for a first test, and we anticipate that the shelf-life might be extended in the future by adjusting the enzyme suspension buffer pH or ionic strength or by adding stabilizers such as such as trehalose, a disaccharide that have been used widely in the industry to preserve proteins in the dry state (see Draber et al. 1995 “Stability of monoclonaligm antibodies freeze-dried in the presence of trehalose” Journal of Immunological Methods  181: 37-43). 
     In summary, the inventors have developed a new strategy for digital microfluidics, which facilitates virtually un-limited re-use of devices without concern for cross-contamination, as well as enabling rapid exchange of pre-loaded reagents. The present invention allows for the transformation of DMF into a versatile platform for lab-on-a-chip applications. 
     As used herein, the terms “comprises”, “comprising”, “including” and “includes” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in this specification including claims, the terms “comprises”, “comprising”, “including” and “includes” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components. 
     The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents. 
     The same reference numbers relate to the same features, even when these reference numbers are only displayed in the Figures and not particularly referred to in the specification. 
     REFERENCE NUMBERS 
       10  Disposable, preloaded carrier 
       11  Electrically insulating sheet 
       11   a  Front hydrophobic surface of  11 ; front working surface 
       11   b  Back surface of  11   
       12  Pre-loaded reagent depot 
       13  Pre-selected position 
       14  Digital microfluidic (DMF) device 
       15  Adhesive 
       16 , 16 ′ Electrode array; surface of  16   
       17  Discrete electrodes 
       18  Pre-selected individual electrode 
       19  Electrode controller 
       20  Reagent droplet 
       21  Alignment marks 
       22  Reagent droplet 
       23  Patterned conductive coating 
       24 , 24 ′ First substrate; surface of  24   
       25  Dielectric layer 
       26  Resultant reaction product 
       27 , 27 ′ Second substrate; front surface of  27   
       28  Previous assay residue 
       29  Space 
       30  Previous assay residue 
       31  Additional electrically insulating sheet 
       31   a , 31   b  Front hydrophobic surface of  31 ; back surface of  31   
       32  Sample reservoir 
       33  Solvent droplet 
       34  Solvent reservoir 
       35 , 35 ′ Additional electrode array; surface of  35   
       36  Dispenser tip