Abstract:
An electrophoresis cassette is made from two substrates separated by spacers, and a one-piece molded plastic edge adapter. The edge adapter provides the contoured edge to facilitate sample loading, and because it is made from molded plastic is easier to make. Furthermore, the edge adapter has vertically extending arms which define the sides of the loading region and which, when glued to the back substrate, prevent leakage of buffer solution from the region surrounding the electrophoresis origin. In one variation, a groove in the bottom edge of the edge adapter receives the top edge of the front substrate to further define the position of the edge adapter in relation to the substrate. Vertical divider fins may optionally be formed on the contoured surface of the edge adapter. These divider fins define loading wells for the introduction of sample into the lanes of the electrophoresis gel.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to an improved design for an electrophoresis gel cassette. 
     Gel electrophoresis is a powerful analytic tool, which is used routinely in the analysis of nucleic acid sequences. Such analyses are now shifting from the research environment to become a part of the battery of tests performed by diagnostic laboratories. With this shift has come a need for simplified and less expensive gel cassettes. 
     The basic gel cassette is formed from two parallel plates held apart by spacers to define a gel cavity. One edge of the front plate may be lower than the back plate, and this edge of the front plate may be beveled or otherwise contoured to facilitate loading. (see, for example, U.S. Pat. No. 5,993,628 and 5,627,022). Because the plates are generally made from glass (to minimize interference with fluorescent detection and/or gel polymerization that could occur if plastics were used, the formation of this beveled or contoured edge during manufacturing can be expensive and time-consuming. Furthermore, the edge is prone to breakage in shipment and subsequent handling. Defects in the edge, such as chips or ripples, or a rough surface at the beveled edge can be translated into defects in the gel. Thus, quality control on this edge is a significant element in the overall cost of the gel cassette. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an electrophoresis cassette is made from two substrates separated by spacers, and a one-piece molded plastic edge adapter. The edge adapter provides the contoured region to facilitate sample loading, and because it is made from molded plastic is easier to make. Furthermore, the edge adapter has vertically extending arms which define the sides of the loading region and which, when affixed to the back substrate, prevent leakage of buffer solution from the region surrounding the electrophoresis origin. In a preferred embodiment, a groove in the bottom edge of the edge adapter receives the top edge of the front substrate to further define the position of the edge adapter in relation to the substrate. In an alternative embodiment of the invention, vertical divider fins are formed on the contoured surface of the edge adapter. These divider fins define loading wells for the introduction of sample into the lanes of the electrophoresis gel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a part of an electrophoresis cassette in accordance with the invention; 
     FIG. 2 shows a plan view of an electrophoresis cassette in accordance with the invention; 
     FIG. 3 shows an exterior side view of an electrophoresis cassette in accordance with the invention; 
     FIG. 4 shows a cross section along line A—A of an electrophoresis cassette in accordance with the invention; 
     FIG. 5 shows a cross section an alternative embodiment of an electrophoresis cassette in accordance with the invention; 
     FIG. 6 shows a perspective view of an embodiment of the edge adapter with molded loading wells; and 
     FIG. 7 shows a cross-section of another embodiment of an electrophoresis cassette in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides electrophoresis cassettes which are formed in two parts. As shown in FIG. 1, the first part of the cassette is an assembled cassette formed from a front substrate  10 , a back substrate  11 , and spacers  12 . The spacers  12  are disposed between the front substrate  10  and the back substrate  11 . The front substrate  10  and the back substrate  11  and the spacers are held together in an assembled cassette to define a gel cavity  13 . As shown, the length of the front substrate  10  is less than the length back substrate  11  along a direction of migration, thereby leaving a top portion  14  of the back substrate  11  exposed in the assembled cassette. The upper edge  15  of the front substrate  10  preferably is blunt, and is thus more easily manufactured and less susceptible to damage than contoured edges. 
     The second part of the electrophoresis cassette of the invention is an edge adapter which is placed over the upper edge  15  of the substrate  10  to define a loading trough. FIG. 2 shows an exterior plan view of the assembled cassette with an embodiment of the edge adapter in place. The edge adapter has a generally U-shaped profile. While the edge adapter is a single piece, it is convenient to describe the adapter in terms of portions or regions. It will be understood that the separate description of these portions or regions does not mean that they are separable without destruction of the edge adapter. 
     The, edge adapter has a U shaped main body portion, which is itself made up of a rectangular transverse portion  21  extending from one side edge to the other side edge of the cassette, and two arms  22  extending vertically from opposing ends of the transverse portion  21  towards the top of the edge adapter. The width of the U-shaped main body member is substantially equal to the width of the front and back substrates. The U-shaped main body member has an overall height from the top edge to the bottom edge that is substantially equal to difference between the length of the front and back substrates. Thus, the U-shaped main body member extends upwards from a line parallel to the top edge  15  (shown as a dashed line in FIG. 2) of the front substrate  10  to the tops of the arms which are flush with the top edge of the back substrate. 
     The edge adapter also includes a contoured region  23  extending along the upper edge of the transverse portion  21  of the U-shaped body member between the arms  22 . The contoured.region  23  has a fixed edge which is contiguous with the upper edge of the transverse portion  21  of the U-shaped body member and a free edge  24  disposed opposite to the fixed edge. The edge adapter shown in FIG. 2 also comprises an interlocking portion  25  extending along and being contiguous with the lower edge of the transverse portion  21  of the U-shaped main body member. 
     FIG. 3 shows a side end view of the assembled cassette with the edge adapter of FIG. 2 in place. As shown, the interlocking portion  25  and the transverse portion  11  of the U-shaped main body member in concert form a groove that extends transversely along the interior surface of the edge adapter. This groove is sized to receive the upper edge  15  of the front substrate  10 , such that the transverse portion  21  and the arms  22  of the U-shaped main body member form an upward extension of the front substrate. The arms  22  are joined to the back substrate  11  by glue  31  which fills the gap having the same width as the gel cavity. This allows the edge adapter to define a space over the top edge of the gel and to prevent buffer leakage from this space in use. 
     The contoured portion  23  has a contoured surface on the interior surface as indicated by the dashed line in FIG.  3 . This is more clearly shown in FIG. 4 which shows a cross section view along line A—A in FIG.  2 . As can be seen, the contoured region  23  at the fixed edge which is contiguous with the transverse portion  21  has a thickness which is greater than the thickness of the contoured region  23  at the free edge  24 , and the contoured region  23  has a height which is less than the height of the arms  22 . The shape of the contoured region may be flat, as depicted in FIG. 5, or it may be curved. When flat, the angle formed between the contoured region and the plane of the front substrate (and also the U-shaped main body member) is suitably 30 to 60°, preferably about 45°. 
     FIGS. 3 and 4 also illustrate the position of the interlocking portion  25 , which extends along and is contiguous with the lower edge of the transverse portion  21 . The, interlocking portion  25  has a width which is substantially the same as the width of the front and back substrate  10 ,  11  and a thickness which less than the thickness of the transverse portion  21 . The interlocking portion  25  and the lower edge of the transverse portion  21  in concert form a groove extending transversely along the interior surface of the edge adapter. The groove has a thickness which is substantially equal to the thickness of the front substrate  10 . Thus, when the edge adapter is disposed with the top edge  15  of the front substrate  10  within the groove and the arms  22  extending upwards over the top portion  14  of the back substrate  11 , the contoured edge  23  provides a sample application region for a gel formed within the gel cavity  13 . 
     To use the embodiment of the invention depicted in FIGS. 2-4, a starting cassette structure of the type shown in FIG. 1 is prepared, and the edge adapter is then glued into position to form the finished cassette. Glue  31  is used to fill the gap space between the arms  22  and the back substrate  11 , and to affix the top  1  of the front substrate  10  to the abutting portion of the U-shaped main body member, and optionally to the interlocking portion. A suitable adhesive is a UV activated acrylate adhesive such as MINICO® M07950-R (Emerson &amp; Cuming, Inc, Woburn, MA.), which is the same type of adhesive used in the construction of the MICROCEL™ cassettes. As noted below, this glue can contain beads which are used as spacers to define the separation between the substrates. Use of the same glue, with the beads, to affix the arms to the back substrate is desirable as it ensures the same spacing. 
     The edge adapter also may be releasably affixed to the substrates in a manner which permits it to be removed without destruction of the edge adapter. For example, the edge adapter might be releasably affixed to the substrate using a clamp mechanism, or with a non-hardening adhesive. The use a releasably-affixed edge adapter allows reuse of the edge adapter, if desired. Thus, the term “affixed” encompassed joining the edge adapter to the substrate in both a releasable and a non-releasable (with destruction of the edge adapter) manner. 
     Gel is then introduced into the gel cassette and polymerized. The cassette is filled to a point where the top of the gel is approximately even with the top edge of the transverse portion  21 . As a result, a portion of the edge adapter corresponding to the height of the transverse portion is in contact with the gel. This does not suffer from the difficulties normally associated with the use of plastic because (1) the plastic is remote from the detection sites and thus is unlikely to create spurious fluorescence; (2) the extent of contact with the plastic is small (for example, the height of the transverse portion will generally be less than 10 mm, and preferably less than 5 mm), such that the interference with gel polymerization is minimal; and (3) if necessary, the surface of the edge adapter that will come in contact with the gel can be treated with a surface coating, for example of bindsilane, which will further reduce any interference with polymerization. If desired, a comb may be used to define sample loading wells when the gel is being formed. 
     FIG. 5 shows an alternative embodiment of the gel cassette of the invention in the same view as FIG.  4 . In this case, the height of the transverse portion is substantially reduced (for example to less than 0.5 mm, preferably less than 0.1 mm), such that the length of the effective extension of the front substrate  10  by the edge adapter is much smaller. This further minimizes the risk of significant interaction of the plastic with the polymerizing gel. Furthermore, because the height of the transverse portion is small, the gel can be filled only to the top of the top substrate, without impairing the ability to load samples. In this case, the edge adapter maybe attached to the starting cassette structure before or after filling with the gel. 
     FIG. 6 shows a perspective view of an embodiment of the edge adapter with molded loading wells. Thus, in this case dividers  60  are formed on interior of the contoured region  23 , extending from the upper edge of the transverse portion  21  to the top of the contoured region  23 . Arms  22  are disposed at each end of the contoured region  23 . The wells facilitate the introduction of sample into the lanes of the gel cassette. The transverse portion in this embodiment is preferably short, for example having a height of less than 0.5 mm, and more preferably less than 0.1 mm. 
     FIG. 7 shows a cross-sectional view of another embodiment of the invention. This embodiment differs from the previous embodiments in that it lacks the interlocking portion. Thus, the transverse portion  71  of the edge adapter abuts against the entire top edge  75  of the front substrate  10 . Other portions of the electrophoresis cassette are the same. 
     The edge adapter in each of the foregoing embodiments is a single piece structure which is preferably made from plastic. Suitable plastics are rigid moldable plastics which will have minimal interaction with the chemical reagents used in preparation of polyacrylamide gels. Specific plastics which may be used include acrylics, polycarbonates and the like. Preferably, the edge adapter is formed as a single integral piece by a molding process. However, it is not outside the scope of the invention to fabricate several separate molded plastic pieces for assembly into an edge adapter having a structure as described above. Thus, as used in the specification and claims of this application, the term “single piece of molded plastic” refers to edge adapters which are integrally formed as a single piece, and edge adapters which are assembled from two or more pre-formed pieces into an assembled whole~which cannot be non-destructively disassembled. 
     The foregoing discussion describes various embodiments of an edge adapter and gel cassette in accordance with the invention. In doing so, little attention was given to the specific assembly of the starting gel cassette structure of FIG.  1 . This structure may be prepared in any of the numerous ways known in the art, including but not limited to glue line spacers as described in commonly, assigned U.S. Pat. No. 5, 627,022 and as used in commercially available MICROCEL™ cassettes, glued fiber spacers as described in commonly assigned U.S. Pat. Nos. 5,618,398 and 5,599,434, and polyester or MYLAR® spacer strips or gaskets, as disclosed in U.S. Pat. No. 5,993,628. When a U-shaped spacer (for example of polyester is used), only one spacer is required. When glue line spacers are used, the glue contains inert beads, for example glass beads, whose size determines the separation between the glass plates. Typically, the size of these beads is 20 μm or 50 μm. The starting gel cassette may be constructed with or without interior lane dividers.