Abstract:
An apparatus for sealing fluid containing vessels comprising a sheet of elastomer forming a sealing member having a top and bottom surface, and disposed on a plate containing a plurality of wells for storing fluids. A product that can seal the wells of a multiwell plate, and has the aspects of being vapor resistant, heat sealable, and able to be manipulated by automated analytical equipment. The sealing material is constituted of an elastomer made of synthetic rubber and a layer, comprising a polymer film or a foil attached to one side of the elastomer to form a vapor barrier. The combination of these elements produces a sealing mat that couples the barrier properties of the polymer film or foil with the sealing properties of the elastomer septum.

Description:
CLAIM OF PRIORITY  
       [0001]    The present Application claims priority to U.S. Provisional Application No. 60/236,512, filed on Sep. 29, 2000, in the names of Deborah C. Audino and Gregory R. Martin. The content of the Provisional Application is incorporated herein in its entirety. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The invention relates to a septum for sealing liquid containing vessels. In particular, the invention relates to an elastomer sheet used to thermally seal wells of a microtiter test plate used in biological or chemical assays.  
         BACKGROUND  
         [0003]    Today, the reacting and testing of large numbers of biological samples constitutes a fundamental technique of analysis in modem biological or medical diagnostic science. Specific examples include polymerase chain reaction (PCR) techniques, radio-immune assay (RIA), enzyme linked immune-sorbent assay (ELISA), enzyme immune assay (EIA), enzyme assays, including receptor binding assays, membrane capture assays, cell washing, and others similar tests. The physical equipment used to perform these large numbers of required tests efficiently, accurately, and safely, has been for many years the microtiter plates or “microplates,” also known as multiwell plates. Multiwell plates come in various sizes, from 6 to over 1536 wells. The most typical multiwell plates contain ninety-six (96) molded plastic wells in an 8×12 array with a typical volume capacity of about 200 microliters.  
           [0004]    In the striving for ever increasing efficiency and to reduce manually repetitive tasks performed by laboratory technicians, many multi-sample plates have been adapted for use in automated handling systems. Such systems employ multiwell plates for storing, reacting and/or analyzing liquid samples, and typically includes a liquid handling device, which transfers fluid between selected containers and/or wells, and an automated plate handling apparatus to manipulate multiwell plates that contain the samples.  
           [0005]    Usually, multiwell plates possess a lid designed to prevent dust or other contaminants from entering the wells, as well as to reduce the rate of evaporation. These lids generally are sturdy enough to withstand handling by the variety of automated scientific instruments and robotic means used to remove and replace the lids on the multiwell plates.  
           [0006]    Nevertheless, in several ways the design of the standard multiwell plate still has some shortcomings. Many multiwell plate lids fit loosely and are not designed to seal the tops of the open wells. Samples of test compounds being stored in plates need to avoid adsorption of water from the surrounding atmosphere. Samples handled in an automated system may need to be heated and/or agitated at various points during the processing cycle. Such operations normally require the wells containing the samples to be sealed. Liquids can spill out of the wells or aerosols can form during fluid transfers. Liquids escaping from the wells, however minute in quantity, can contaminate the analysis being performed, and may also create a hazard if the testing involves infectious materials. Moreover, condensation tends to form from the wells since the lids do not create a tight seal. Over time, this condensation can spread along the lid and drip from one well into neighboring wells, creating cross-contamination between the samples in the wells. Thus, the seals typically need to be fluid-tight to prevent loss of sample fluid, especially when the contents of a well is heated, creating a positive pressure in the well. Often after the heating and/or agitation step, the plates need to be uncovered in order to add other reagents, or to extract reacted samples. A case illustrative from the realm of molecular biology—a typical PCR involves cycling liquid samples, contained within individual wells of a test plate, 30 times between temperatures of about 50° C. to about 94-95° C. During this cycling process, liquid may evaporate, contributing excessively to cross-talk between the wells. Compounds being stored in hygroscopic solvents need to avoid adsorption of water so as avoid unintended dilution or hydrolysis during storage.  
           [0007]    To minimize such evaporation, adsorption, or cross talk, manufactures in the microplate industry have developed a variety of sealing devices for multiwell plates. Products in the form of tapes and sealing mats are available on the market to cover the wells of a multiwell plate. For example, tapes are typically made of polyester, polypropylene, or a fluoropolymer, and sold by a variety of suppliers including Hybaid, Rainin, MJ Research, Techne, Top Flight, and Corning Costar. Sealing mats are typically made from synthetic rubber, polypropylene, or silicone and have a matrix of sealing plugs situated on one surface of the mat for use in plugging individual wells.  
           [0008]    In the past elastomer mats have depended on either a pressure system or a friction fit to mechanically seal the multiwell plates. Examples, such as U.S. Pat. Nos. 5,056,427 (the ‘427 patent), 5,604,130 (the ‘130 patent), and 5,853.586 (the ‘586 patent), describe sealing mechanisms that are held to the plate by either a pressure plate or a pressure differential. The ‘427 patent claims a tight sealing structure for use with a reagent tray that suppresses evaporation of fluids during thermal reactions. A planar, elastic, sealing member is placed on top of the tray to cover the openings, and is secured to the reagent tray by a pressure plate. Likewise, the ‘130 patent discloses a sealing cover for a multiwell, microtitration plate that uses another pressure apparatus. The cover contains a pad, fashioned from a flexible polymer sheet, and a plurality of resiliently compressible ridges formed on the sheet. The ridges deform when pressure is applied to the cover, which effectively forms a fluid-tight seal between the pad and well openings in the plate. When the pressure is released, the ridges rebound sufficiently to their original form to break the seal. In the ‘586 patent, a flexible sealing member collapses, in the direction of a filtration vacuum, into each individual well of a multiwell plate.  
           [0009]    Although useful in preventing liquid content loss by evaporation or cross-talk, the tapes and mats are not designed for used in automated assay processes. The materials from which these products are made are too thin to be efficiently worked by most automated instruments. The polypropylene mats need significant pressure to place and remove them, while the silicone mats are too flimsy for a machine to handle them, requiring a lab worker to manually fit and remove them. Additionally, silicone has too large a free volume and, thus permits free diffusion of water or solvent vapors as well other gases through the seal. Hence, there is a need for a cover capable of effectively sealing the wells of a multiwell plate in an automated system. The seal should prevent the loss of well contents during heating or agitation processes, yet be usable without the need to apply unnecessary force or unduly complex systems.  
         SUMMARY OF INVENTION  
         [0010]    The present invention comprises a product that can seal the wells of a multiwell plate, has the aspects of being vapor resistant, heat sealable, and is able to be manipulated by automated analytical equipment. The sealing material is constituted of an elastomer made of synthetic rubber and a layer, comprising a polymer film or a foil attached to one side of the elastomer to form a vapor barrier. Alternatively, the sealing layer of the invention comprises a cross-linked elastomer that is laminated to a thermoplastic film. The combination of these elements produces a sealing mat that couples the barrier properties of the polymer film or foil with the sealing properties of the elastomer septum.  
           [0011]    The invention comprises also a microtiter well plate having such a sealing material. The bottom surface of the elastomer is sealed thermally to the top surface of the microtiter well plate, in a manner that it covers the entire plate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a perspective view of one embodiment of the present invention showing an elastomer sheet adhered to a multiwell test plate of a standard configuration.  
         [0013]    [0013]FIG. 2 is an embodiment of a septum with a laminated structure in which a metal foil with a polymer coating on both sides of the foil is attached to an elastomer sheet.  
         [0014]    [0014]FIG. 3 is a top-down sequence of the layers of another laminate embodiment comprises: an elastomer sheet, a polymer binder, a metallic foil, and another polymer layer.  
         [0015]    [0015]FIG. 4 is a cross-linked elastomer, having a thickness that is laminated to a thermoplastic film.  
         [0016]    [0016]FIG. 5 is an apparatus that alternatively embodies the invention wherein an elastomer material of a sealing member is set in a lid designed to fittingly engage a multiwell plate.  
         [0017]    [0017]FIGS. 5A and 5B are two variations of a partial cross-sectional view of an elastomer sealing sheet under a micro-titer plate lid placed over the micro-titer well plate.  
         [0018]    [0018]FIG. 6 is an exploded view showing one embodiment of the invention with a micro-titer plate lid having a plurality of holes, which extend through the material of the lid, and an elastomer sealing sheet arranged below it, ready to be welded to the underside or inner surface of the lid.  
         [0019]    [0019]FIG. 6A is a cross-sectional view of part of the lid of a micro-titer plate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The invention comprises certain elastomers used either alone or as part of a laminated structure for sealing liquid containers, in particular, for multiwell plates employed in the biological research or pharmaceutical industries. The invention also includes microtiter plate devices having such an elastomer-sealing structure. As currently conceived in the most simplistic form, an elastomer film  10  welds to the upper surface  12  of a multiwell plate  14  to produce an integral seal of each well  16 , as shown in FIG. 1. This is achieved by selecting a material of a similar chemical composition, or at least of a compatible composition, as that of the plate&#39;s upper surface. Thermoplastic elastomers (TPEs) can be welded to polymers that are similar or compatible to one of the TPE&#39;s phases. The affinity of similar chemical materials increases the strength of the bond between the elastomer septum and the plate to which it is sealed.  
         [0021]    At the present time, a large number of storage or multiwell test plates are molded out of polypropylene (PP). A compatible TPE such as santoprene®, an elastic material that is made from polypropylene blended with ethylene-propylene dienemethylene (EPDM) rubber particles is well suited for sealing polypropylene plates. Thus, a santoprene® elastomer sheet, for example, can be cut to size and welded thermally quite easily and securely to the polypropylene substrate. Similarly, styrene-butadiene rubber could weld to styrene made objects. Other TPEs are poly(ester-block-ether) and nylon-block-polyether. These TPE&#39;s, as well as thermoset elastomers, can be joined or laminated to a film compatible with the polymer of the plate being sealed. Silicone materials, although usable, are not as favored as they are highly permeable for certain applications.  
         [0022]    Elastomers in general have low durometer hardness. By selectively choosing materials with the appropriately low durometer hardness and high elasticity, any perforation that is made in the elastomer film by piercing with a syringe needle, a pipette tip, or any other insertion will close-up again once the insertion is removed. An elastomer material that has a durometer value of about 30-70 shore A, such as santoprene® is illustrative. A more preferred hardness is within the range of between about 35-60 shore A. Available from Advanced Elastomer Systems, santoprene® deforms to accommodate the insertion, then rapidly recovers its original shape to close the puncture, preventing either air from outside from seeping in or vapors from within each well from leaking out. In an embodiment of the invention, a santoprene® sheet having a durometer value of about 55 shore A was used and found to work well. The elastomer sheet in a relaxed, non-compressed state will have on average a thickness of about 0.020 to about 0.130 of an inch. Certain sections of an elastomer sheet, however, may be slightly thinner or thicker, contingent on the type of use and the corresponding part or section of a test plate that the sheet will cover and seal. For instance, those parts of the elastomer sheet which engage the tops of inter-well walls of a multiwell plate may be slightly thicker, so as to better bind with the plate, while the parts of the sheet that are immediately over a well opening may be thinner, to permit ease of penetration by a syringe needle or pipette tip. This description is in no way limiting to the invention, since the reverse may also be applicable and a viable embodiment.  
         [0023]    Another embodiment of the septum includes a laminated structure in which a metal foil  18  with a polymer coating  20   a ,  20   b  on both top and bottom sides of the foil is attached to an elastomer sheet  10 , as illustrated in FIG. 2. Some problems at this time with existing sealing mechanisms have partly motivated our decision to combine a polymer with a metallic foil. Previously, seals such as sealing tapes often contaminated the samples in wells because solvent splashing onto the under surface of pressure sensitive adhesive leached contaminants into samples. A polymeric material combined with a metal foil is better because it is not extractable and can not be affected by solvents leaching. The metal foil also acts as an effective vapor barrier to avoid gas diffusion into or out of the well. For example of the designed function, the properties of the elastomer-foil laminate can keep dimethyl sulfoxide (DMSO) solvent in the wells of a multiwell plate, while keeping water, oxygen or other gases out to prevent hydrolysis or oxidation.  
         [0024]    In another embodiment, a laminate of multiple layers comprises, from top to bottom: an elastomer sheet  10 , a first polymer binder  21 , a metallic foil  18 , and a second polymer layer  22 . This embodiment is illustrated schematically in FIG. 3. The first polymer coating bonds the metal foil to the elastomer, while the second polymer layer is welded to seal the laminated structure to a storage or test plate, which is the final substrate. A heating block, such as a platten, can be used to supply heat through the elastomer to the surface of the well-plate so as to melt the second polymer layer against the interface of the plate. For instance, a santoprene® elastomer will bind to a polypropylene coating on one side of the foil, while the same or another type of polymer applied on the other side of the foil is left free to adhere to the surface of the plate. So as not to deform or damage the underlying test plate, the adhesive polymer that is selected will ideally soften and melt at a temperature lower than that for polypropylene compounds. Typically, a polypropylene test plate melts between about 160° C. to about 189° C. Consequently, the adhesive polymer will ideally soften or melt at a temperature at or below about 157° C. Other adhesive polymers that can be used include any grade of polypropylene with a lower melting temperature. Test plates made from polystyrene, which melts between about 240-250° C., can better tolerate slightly higher temperatures.  
         [0025]    When using a tie layer between an elastomer and test plate surface, most ideally the tie layer will melt first, before the test plate material starts to soften. First, the best seal is achieved when both faces melt just enough to have physical entanglements of the polymers. But desiring to minimize heat exposure heat history to working biological samples contained within, a second type of seal can be achieved when we use materials that melt at a lower temperature to avoid plate deformation. These kinds of materials, nonetheless, still have good sealing properties.  
         [0026]    A co-polymer such as Plexar™ may be employed to make a hot-melt adhesive with good adhesion properties between the metal foil and the plate. Plexar™ is poly(ethylene-co-maleic anhydride), and commercially available from Equistar. The polar maleic anhydride functionalizes the polymer to make it stick to metal. Plexar™ film is mentioned specifically, as it will stick to both metals and santoprene®. Other co-polymers that may work to stick metal foils to the plate include materials that contain vinyl acetate and the like. This composite structure will enhance the sealing properties of the elastomer with the barrier properties of the metal foil. Any monomer that has highly polar groups can produce good adhesion.  
         [0027]    It is envisioned that elastomer materials or polymer films having chemical compositions that are only partially compatible with the material of a multiwell test plate will permit us to create releasable seals. That is, once a elastomer sheet adheres to the test plate material, any incompatibility of the polymer film will allow release when force is applied to pull the sheet off. Generally, the less compatible the sheet is with the plate or lid material the less likely the sheet will bond securely with the plate or lid. Therefore, up to a certain point, by selectively employing partially compatible polymers, we can more likely make peelable seals.  
         [0028]    In an alternative embodiment, the layers of the laminate can comprise: an elastomer sheet, a metal foil and another polymer layer. The polymer coating between the elastomer sheet and metal foil can be omitted when direct lamination of the foil to elastomer is achieved. Also, if a cross-linked elastomer is used, then the lower layer of a laminate would need to be a thermoplastic polymer that is compatible with or the same as the material of the underlying microplate (such as either polypropylene or polystyrene), since the cross-linked elastomer itself can&#39;t be melted to form a seal. Examples of cross-linked elastomer materials suitable for use with the invention include butyl rubbers, that have low permeability to air, neoprene, that imparts good chemical resistance, and other like materials, including Buna-N or better termed as nitrile rubber (i.e., poly(acrylonitrile-co-1,3-butadiene)). As mentioned before, silicone also can be used but it is least desirable. As illustrated in FIG. 4, a cross-linked elastomer  24  having a thickness of about 0.20-0.130 inches is laminated to a thermoplastic film  26  having a thickness that is as thin as possible, possibly about 0.001-0.004 inches. A metalic foil layer  18  could also be sandwiched between the elastomer and thermoplastic layers to provide possibly a better vapor barrier. Another option is to omit the foil layer entirely. It is believed that these cross-linked variations of the present invention can be important since these designs for a sealing septum may be much more commercially practical. Preferred embodiments may include, for example, the following. For sealing polypropylene (PP) plates, a polypropylene film or compatible material is corona treated on one side. This side is coated with a layer of partially epoxydized polybutadiene (PolyBD 605E resin from Sartomer) with a dissolved (1-4% prefered) cationic photoinitiator (SarCat K185 from Sartomer). The polybutadiene then polymerizes to form a cross-linked elastomer (like a thermoset) that is bonded to the thermoplastic film. For sealing polystyrene plates, like that for PP, above, except that the thermoplastic film that is corona treated would be a polystyrene film or compatible material.  
         [0029]    The invention is embodied by an apparatus for sealing fluid containing vessels. In its broadest iteration, the apparatus comprises a sheet of elastomer forming a sealing member having a top and bottom surface, disposed on a plate containing a plurality of wells for storing fluids, and the sealing member is thermo-chemically bonded to the plate, wherein the elastomer has a propensity to reseal after being punctured. The elastomer is made of a thermoplastic polymer material. Alternatively, the elastomer can be made of a thermosetting polymer joined or laminated to a film having a chemical affinity to the plate&#39;s chemical composition.  
         [0030]    An apparatus that alternatively embodies the invention comprises setting an elastomer material of the sealing member in a lid  30  designed to fittingly engage a multiwell plate  14 . FIGS.  5  shows a perspective view of such an apparatus. The lid  30  has several openings  32  that conform to the arrangement of micro-titer wells. FIGS. 5A and 5B show two variations of a partial cross-sectional view of a micro-titer plate with a cover  34  and elastomer seal  36  configured over one well  38 . In FIG. 5A, the lid is open  39  over the well, like as shown in FIGS. 6 and 6A. In FIG. 5B the lid is one solid piece without any holes extending through the lid material. In FIG. 5A, the well underneath the elastomer seal may be accessed through the elastomer with, for instance, a syringe or pipette tip. The elastomer self-seals once the intrusion is removed.  
         [0031]    [0031]FIG. 6 shows an exploded, perspective view of a microtiter plate cover or lid  30  having a top surface  40  and a descending skirt  42  or sidewall with a plurality of holes  44 , extending through the lid material, arranged in the format of a 96-well plate. An elastomer sealing film  10  of an embodiment described herein is located under the lid  30 . FIG. 6A is a partial cut-away view of the lid shown in FIG. 6.  
         [0032]    An elastomer could be either molded as part of a polypropylene lid or welded in a secondary operation to the lid. No glue or other polymer adhesive is employed to engage or adhere the elastomer sheet to the lid. Rather an elastomer, like santoprene®, directly welds with the polypropylene material of the underside of the lid or top surface of the plate. Melting at the interface between the lid or plate and the thermoplastic elastomer achieves the seal. Other ways of attaching the septum can include insert molding the seal by injecting the elastomer material onto the plate for lids that that have access openings. Variations in the type of elastomer used are also possible so long as these materials can directly bond with the material of the plate or lid.  
         [0033]    The lid is in the form of a substantially rectangular rigid frame that has a top and bottom surface and is surrounded by a peripheral skirt. FIG. 6 is an exploded view of another embodiment of the present invention. The bottom of the lid holds an elastomer sheet, which is sized to fit within the confines of the peripheral skirt, flush against the top of a multiwell plate.  
         [0034]    The dimensions of the lid are preferably sized such that the outer skirt section of the frame will fit over an industry standard 96 well plate. The skirt section preferably extends perpendicularly from the outer frame periphery. This skirt section helps to center the frame over a multiwell plate and preferably extends approximately 0.4 cm from the frame&#39;s top surface. Furthermore, the skirt section serves as a suitable region for a clamping device from an automated robotic extension to attach in order to secure the lid to a plate, or conversely, to remove the lid from a plate.  
         [0035]    The size of the lid frame should be compatible with plates that fit into PCR equipment that are currently available such as the GENE AMP 9600 manufactured by Perkin Elmer, or the DNA ENGINE PTC 200 made by MJ Research. Preferably, the outer dimensions of the frame are approximately 8.5 cm×12.5 cm. But, not withstanding this preference, the framed seal does not have to be limited to use for PCR plates, rather its dimensions can be changed to fit any multiwell plate.  
         [0036]    The top surface of the lid has a matrix of holes extending through it. The holes permit access to individual wells through the elastomer sheet by means of a needle, pipette tip, or other penetrating device. The holes in the lid are designed to correspond to the number of wells in a corresponding microplate, i.e., 48, 96, 384, 1536, etc. Preferably the holes are arranged in a matrix of mutually perpendicular 8 and 12 holerows for use with a 96 well plate.  
         [0037]    Therefore to recapitulate, our invention is an apparatus for sealing fluid containing vessels comprising a sheet of elastomer forming a sealing member having a top and bottom surface, and disposed on a plate containing a plurality of wells for storing fluids. The sealing member is thermally bonded to the plate, wherein the elastomer has a propensity to reseal after being punctured. In one variation, the elastomer is made of a thermoplastic polymer material. Another way of characterizing the elastomer is that it is made of a thermosetting polymer laminated to a film having an affinity to the plate&#39;s chemical composition or is laminated to a film that is compatible to said plate&#39;s composition. In one version, the apparatus also has a film of elastomer having a top and bottom surface, bonded to a metallic foil on at least one of said surfaces. The foil is thermally sealable to a multiwell test plate, wherein the elastomer and foil together make a vapor resistant barrier. The elastomer is thermally sealed to said test plate by means of heat bonding a compatible polymer between said metallic foil and said test plate. Another iteration of the invention, is an apparatus comprising a film of elastomer having a top and bottom surface, bonded to a polymer film on at least one of said surfaces, and the polymer film is likewise, thermally sealed to a multiwell test plate. The elastomer and polymer film together also make a vapor resistant barrier.  
         [0038]    In an alternative embodiment, our invention is an apparatus comprising a planar sealing member composed of elastic material and disposed on a test plate having a plurality of fluid receiving wells. A metallic foil or polymer film is adhered to the planar sealing member. A rigid lid defined by a peripheral skirt and a top and bottom surface having a predetermined matrix of openings with extending through, is attached to the test plate and the openings correspond to the wells in the plate. Alternatively, the lid has a single large rectangular opening. The sealing member and metallic foil or polymer film is either molded or welded to said bottom surface, with the sealing member in direct contact with the bottom surface, and the metallic foil or polymer film as a barrier layer exposed toward the open wells in the test plate. The polymer film or metallic foil is heat sealed to the plate. A polymer, that is compatible with the plate&#39;s chemical composition, adheres the metallic foil to the plate by thermally bonding.  
         [0039]    The apparatus is can also be characterized by having an elastomer made of a thermoplastic polymeric compound chosen from the group consisting of: an elastic material that is made from polypropylene blended with ethylene-propylene diene methylene (EPDM) rubber particles, styrene-butadiene, poly(ester-block-ether), or nylon-block-polyether. Alternatively, the elastomer is made of a cross-linked polymeric compound chosen from the group consisting of: polybutadiene, cross-linked epoxidized polybutadiene, ethylene-propylene diene methylene (EPDM), polyisobutylene, polychloroprene (neoprene), cis-1,4-polyisoprene, polyurethane, nitrile rubber (Buna-N), epichlorohydrin rubber, silicone block copolymers or silicone. More particularly, the elastomer can be made of a polypropylene blended with EPDM rubber particles. The elastomer can be hydrophobic. The elastomer&#39;s lower surface has a chemical composition that is totally compatible to form a bond with said plate&#39;s chemical composition, thereby creating a non-releasable seal; or, the elastomer&#39;s lower surface has a chemical composition that is partially compatible to form a bond with said plate&#39;s chemical composition, thereby creating a releasable seal.  
         [0040]    The apparatus can have a base test plate made from polypropylene or polystyrene. The compatible polymer is of a chemically similar composition as that of said test plate. More particularly, a specific embodiment uses a compatible polymer that is poly(ethylene-co-maleic anhydride). The compatible polymer melts at a lower temperature than a temperature at which said test plate begins to deform. Hence, the heating temperature is at or below about 165-160° C., preferably about 157° C. or below.  
         [0041]    Our invention also incorporates a method of sealing a fluid container comprising the following steps:  
         [0042]    a) providing a roll of film made of an elastomer having a top and bottom surface, the bottom surface is bonded to a layer comprising a metallic foil or a thermoplastic film that is compatible with the polymer material of a microtiter test plate, and a test plate having a plurality of wells located in a top surface of said test plate; b) cutting an unrolled roll of said elastomer film, wherein each cut piece of film corresponds with the length and width dimensions of said top surface of said test plate, whereby each well in said test plate is covered by said elastomer film; c) disposing planarly said cut piece of film onto said top surface of said test plate; d) affixing said cut piece of film to said top surface of said test plate, whereby each well in said test plate is covered by said elastomer film; e) applying heat to said cut piece of film to thereby thermo-chemically bond said cut piece of film to said top surface of test plate.  
         [0043]    Although a preferred embodiment of the invention has been disclosed in detail for the purpose of illustration, those skilled in the art can appreciate that variations or modifications may be made thereof and other embodiments may be perceived without departing from the scope of the invention, as defined by the appended claims and their equivalents.