Abstract:
A novel surface treatment is provided for portions of filter media coming in contact with the filter media holder, such as a filter housing. In certain applications, the treatment is also applied to the filter media holder, depending on the application. Filter media having at least two distinct surface property modifications are provided in liquid filtration applications to enhance the performance of a filtration system, reduce the cost of the system, provide a visual means of detecting fluid bypass, and minimize fluid hold-up volume within the filter media, all with substantially no loss of performance performances parameters, even in steam sterilization applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is claiming the benefit, Under 35 U.S.C. 119(e), of the provisional application filed on Nov. 30, 2001, under 35 U.S.C. 111(b), which was granted Serial No. 60/334,256. The provisional application 60/334,256 is hereby incorporated by reference. The provisional application 60/334,256 is co-pending as of the date of filing of the present application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a filter media with at least two regions with distinct media properties that result in an enhanced sealing of filter media between individual layers or between the filter media and a sealing means. More specifically, the present invention relates to a surface property modification of filter media and/or a sealing surface to enhance the sealing of the filter media within various filter housings. More particularly, the present invention relates to a surface property modification which repels liquid at the points of contact between the porous filter material and the sealing surfaces of the filter housing. Most particularly, the present invention relates to an improved filter medium that results in an improved sealing of filters and provides means for fluid-gas transfer  
           [0004]    2. Discussion of the Related Art  
           [0005]    It is well known in the art that one of the most important aspects of a filter design, especially in critical fluid filter applications such as medical products and semiconductor applications, is the prevention of filter failure.  
           [0006]    In critical applications, even minor amounts of fluid bypass result in a nonconforming fluid for the application at hand. Therefore, providing good sealing between various components of a filter or filtration system is critical.  
           [0007]    One of the most common, but not the most reliable, sealing mechanisms to date is a pinch seal. In a typical pinch seal the filter media is squeezed between two sealing surfaces. In a typical seal, special attention must be focused upon the amount of pressure imposed on the filter media to assure proper sealing and to minimize the possibility of fluid bypass. Special attention must be focused upon the structural properties of the media and its capability to handle the stress caused by the pinch seal. Typically, in a pinch seal, a designer would like to provide a maximum pressure at the seal without compromising the stability of the filter structure. The higher the pressure at the sealing surface, the higher the compression of the media at point of contact. Typically, this can result in a reduced pore size in the affected area, and also reduce the gap or pore size between the filter media and the sealing surface. This prevents fluid flow past the pinch seal as long as the pore size is small enough to block flow. Another way to stop flow past the seal is to block the pores proximate the seal with potting compound.  
           [0008]    The present invention provides a better seal without the aforementioned problems. This is accomplished by making the seal material non-wetting to the fluid being filtered and, thereby, providing a liquid repellent seal. More specifically, a liquid repellant porous seal with a liquid wetting filter area. The wetability (fluid wetting properties) of the seal may be adjusted by methods known in the art. The present invention provides a differential wetability at the sealing surface.  
           [0009]    The present invention teaches that the wetability of the seal may be decreased to prevent fluid bypass past the seal. At the same time, the main filter area is desired to be fluid wettable, especially for fluid filter applications, to provide fast priming times. In gravity feed systems or low pressure applications, the wetability of the filter medium is important as there is a minimum breakthrough pressure required to start fluid flow through a filter. The breakthrough pressure decreases with increased wetability. Therefore, the present invention provides a differential wetability of the filter media at the sealing surface. Also, it is noted that the surface property of the sealing surfaces, such as the housing, may be changed to enhance the seal.  
           [0010]    In general, the wetability of a porous material depends on a number of parameters, including pore size and surface properties of the material involved. In addition, there will be a number of other advantages to the present invention that will be apparent from the descriptions provided and the applications shown.  
           [0011]    Present day filter media, whether in the form of flat sheets of various shapes, such as circles, ovals, or other desired shapes, or in tubular form, are generally homogeneous throughout, and consist entirely of filter media without any treatment to the sealing regions to enhance sealing. This produces problems in the art. With filter media sheets, there tends to be leakage past the commonly used pinch seal, unless this seal is carefully designed. Such attention to the seal increases the cost of the filter housing holding the media, may result in additional components, and an increase in final product weight, to effect a proper seal. Sometimes, adhesives or other agents are needed to effect any seal at all. Also, since the pinch seal is typically wetted by the fluid, there is no possibility of a visual aide in discerning if the seal is adequate and the fluid being filtered is not bypassing the seal. A visual aide in detecting fluid bypass may provide an opportunity to save the fluid such that it may be processed in a single pass.  
           [0012]    The same problems occur with tubular filters when attempting to provide effective seals at or near the ends of the filter tubes. The prior art is replete with special types of end caps and housing modifications designed to effectuate the seal between the tubular filter media, which is usually made of randomly oriented fibers (particles), and an end cap and/or housing. Again, these measures increase costs for the filter products. Thus, those skilled in the filter art continued to search for a better method of sealing filter media within filter housings.  
           [0013]    In critical applications adhesive may be used to increase the reliability of the filter seal by typically filing the gaps or pores between the sealing surface and the media. The adhesive may be surface loaded in the area of the seal, or the adhesive may be present throughout the depth of the filter media for added protection against fluid bypass.  
           [0014]    The addition of adhesive, typically provides a barrier to fluid bypass past the sealing surface. But in addition, it may provide added complications in certain applications. These include separation of various components of the adhesive, which may result in physical property changes in the seal such as bio-compatibility, thermal stability and chemical compatibility, decreased process time as the adhesive is typically thickened to control wicking of the adhesive into the filter media, and reduced versatility of the adhesive and/or the media as each application needs to be considered on a case by case basis to ensure a proper seal. Also, it is important for the adhesive to bond properly to the housing to provide an adequate seal. Delamination of the adhesive between the various components may result in fluid bypass.  
           [0015]    If the adhesive is a multi-component, the various components of the adhesive may separate due to the capillary forces present in the filter media and may compromise the properties of the adhesive and the seal. Also, in certain instances this may result in bio-compatibility and/or leachability issues.  
           [0016]    In addition, adhesives may have different physical properties than the filter media and the housing. Certain environmental conditions may result in a compromise of the seal. An example of such instance may be when the thermal expansion property of adhesives may not match that of the housing. Therefore, certain thermal fluctuations (such as those seen in typical medical applications requiring autoclaving) may result in a compromising of the seal. This is especially important in medical applications requiring autoclaved parts or heat or steam sterilization.  
           [0017]    The present invention provides novel solutions and enhancements to these problems in the prior art. The present invention will describe a hydrophobic (hydrophobic in the context of this application is synonymous with liquid repellent) barrier that may be used in conjunction with the use of adhesives. The hydrophobic barrier will provide a restriction to the wicking of adhesive past a desired point. In addition, it provides a general means of treating media that may be used with various adhesives. Currently, the properties of the filter media and the adhesive are chosen such as to control the wicking of the adhesive. The present invention provides a novel concept, a barrier, such that these factors are not as important. The media may be used with a various adhesives, and one adhesive may be used with a variety of media. The barrier also provides a clear and distinct area where the adhesive will be present. Advantages include reduced variation in filter properties, such as flow rate, efficiency and capacity, as the usable filter area is clearly defined by the barrier.  
           [0018]    The present invention also provides novel means to provide an enhanced sealing mechanism for filtration and separation applications. One novel concept provides a porous seal with enhanced seals to decrease possibility of bypass. The novel concept minimizes fluid hold-up volume within a filter and enables greater fluid recovery. It also decreases cost of the filtration system by the possibility of eliminating components such as adhesive, and also reduces the weight of the filter by providing a porous structure seal through elimination or minimization of adhesive usage. In special configurations to be described, the present invention provides an integral venting means within the filter that is an integral part of the sealing means.  
           [0019]    Further, the present invention describes a novel concept that provides enhanced sealing of filter by providing a barrier to fluids. The barrier provides a means to prevent fluid contact across a pinch seal. In its&#39; simplest form, the barrier provides a definite and clear barrier against fluid wicking or fluid migration. One of the advantages of this novel concept is that it prevents separation of multi-component adhesives. In addition, it broadens the selection criteria for filter media and/or adhesive. This is due to the fact that the barrier provides a broader range of adhesives that may be used with a given filter media structure. The influence of adhesive properties such as surface tension, viscosity, and gel time are minimized and, as such, provides a greater flexibility in the production of various filters. Also, the barrier enables combining a number of filters into a single filter housing. The single housing concept reduces the number of components and provides overall cost savings.  
           [0020]    Providing multiple filters in a single housing with integral non-wetting barrier enables a number of novel concepts for filtration and separation systems. Fluids may be processed side by side. The barrier may be penetrated at elevated pressures providing unique separation and mixing applications. For example, the filter may be subjected to sufficient positive and negative pressure to drive fluid across the barrier. The pressure may be applied by various means known in the art such as centrifugation, infusion pumps, syringe, etc. A number of applications will be described in this application to demonstrate the novelty of this concept.  
           [0021]    Other advantages of the present invention will be apparent from the description hereby provided.  
         SUMMARY OF THE INVENTION  
         [0022]    The problems in the art are solved by the present invention by providing a novel surface treatment for portions of the filter media coming in contact with the filter media holder, such as a filter housing.  
           [0023]    There are various methods known in the art for producing surface (material) property modifications. These include chemical, mechanical, plasma, corona and heat (flame) treatment. Preferred chemical treatments for hydrophobic applications are fluorinated, siliconized, fluorosilicon, polyolefin polymers which result in reducing the critical wetting surface tension of the material.  
           [0024]    In general, most polymers have a low surface tension and may be used as coatings to modify the CWST of a material. Due to their low surface tension they are typically used to provide a liquid repellant region. Preferred chemical treatments for hydrophilic applications include polymers with a hydroxyl functional group such as Poly-vinyl Alcohol and cellulose, and a functionalized cellulose group such as cellulose acetate and ethyl cellulose, carboxylic acid functional group, amine functional group, sulfonic acid functional group. It should be noted that chemical treatments in varying quantity may be applied to various regions to provide distinct wetting properties. For example, the pinch seal region may be made more hydrophobic than the filtering region by applying a larger quantity of the hydrophobic binder to the pinch region. In general the regions may have varying compositions to provide for the desired functions pointed out in the invention.  
           [0025]    In certain applications, an internal vent is provided by means of a novel surface treatment to provide for faster priming of the filter without entrapping gas upstream of the filter media, which may adversely effect the filtration process (increase filtration time, decrease efficiency).  
           [0026]    The present invention provides for a porous media with at least two distinct surface property modifications in liquid filtration applications to enhance the performance of the filtration system, reduce the cost of the system, and provide a visual means of detecting possible fluid bypass.  
           [0027]    The present invention is related to, but not limited to, filter media, fluid processing, gas venting, gas transfer, and fluid transfer. A means of providing a preferential fluid flow within a filter by liquiphobic (liquid repellant) or liquiphilic (liquid wetting) treatment of a porous medium is disclosed.  
           [0028]    The present invention is most suited for, and related to, pinch seals where a porous media is sealed within a housing by pinching or flat gasket sealing of media within the housing. This method is commonly used in price/cost sensitive applications. One of the drawbacks of pinch sealing has been the possibility of fluid bypass.  
           [0029]    One of the possibilities of bypass is due to preferential permeability or wetability of the media in the cross-flow direction. This situation is especially important with thick filter media over sealing surface length seals. The present invention reduces the risk of such bypass.  
           [0030]    These benefits are achieved through the aforementioned surface property modification of the filter media, which is based on the relationship between the critical wetting surface tension (CWST) of the filter media being used, and the surface tension of the fluid being filtered.  
           [0031]    For the purposes of the present application, the definition of the CWST of the media is not the one generally known in the art from U.S. Pat. No. 4,880,548 (&#39;548), the specification of which is incorporated herein by reference. The (&#39;548) patent defines a media to be wetting if it absorbs at least 9 out of the 10 test drops after 10 to 11 minutes at atmospheric pressure. Instead, it is a use based CWST.  
           [0032]    In the present application the CWST of the filter media will be determined based on the particular application or intended use. As in the (&#39;548) patent, a CWST for a series of materials can be determined, but they will all relate to a particular use or application, and may, or may not be relevant to any other use or application.  
           [0033]    For example, if a quick priming filter element is desired, one which would not entrap gas in the upstream chamber for two minutes while the upstream chamber fills, and the filter inlet pressure is 40″ of fluid column, a satisfactory filter media would be one in which at least a portion of the filter media remains unwetted and capable of venting entrapped gas until the upstream chamber is filled. This requires testing of the filter media with a series of standard liquids with varying surface tensions in a sequential manner under test conditions which simulate actual desired operating conditions. This may be done by placing a number of drops (for example 10) or columns of test fluid (if under differential pressure) on representative portions of porous media, and allowing these to stand for a desired time (in this example two minutes). Observation is made after a desired time (two minutes). Wetting is defined as absorption or wetting of the porous media by at least nine of the ten drops or a reduction in the volume of nine out of ten columns by the equivalent of one drop per column within the desired time. Non-wetting is defined as the retention of a negative angle of contact (for drops), or a substantial retention of volume (for columns). For drops, nine out of ten drops must retain a negative angle of contact. For columns there should be substantially no loss of volume due to absorption by the media (less than one drop).  
           [0034]    Testing is continued using liquids of successively higher or lower surface tension, until a pair has been identified, one wetting and one non-wetting, which are the most closely spaced in surface tension. The CWST according to the present application is then in that range. For convenience, the average of the two surface tensions is used as a single number to identify the CWST for the particular application.  
           [0035]    Furthermore, the media in the above example does not need to be non-wetting for any more than the two minutes the fluid takes to fill the chamber and the gas to vent (perform its intended function). The media may be wetting after the time it takes to perform its intended function.  
           [0036]    Any further references to CWST in the present application refer to the CWST as defined above.  
           [0037]    From this definition, and knowledge in the art, it can be seen that if the surface tension (ST) of the fluid is less than the CWST of the filter media, then the fluid will wet the filter media, and the fluid being filtered will flow through the media. The greater the difference between the ST and the CWST, the faster the wetting or priming of the filter media, and the faster the fluid will begin to flow through the filter. Conversely, the closer the ST of the fluid being filtered is to the CWST of the filter media, the longer the priming time of the filter media. If the ST of the fluid being filtered is greater than the CWST of the filter media, the fluid being filtered will simply bead up on the filter media, and no flow will take place.  
           [0038]    Using the above relationship between the ST of the fluid being filtered, and the CWST of the media being used, a surface property modification may be performed on the filter media by treatments known in the art such that liquids are repelled at the points of contact between the porous material and the sealing surfaces. For example, if the filter media is liquid wetting (ST of fluid&lt;CWST of media) a portion of the filter media, most preferably the portion which will be in contact with the sealing surface, is treated such that it becomes liquid repellent (ST of fluid&gt;CWST of media). It is also advantageous if the entire, or at least a portion of, the sealing surface of the housing which will be in contact with the treated porous media would also be liquid repellent. This may occur because of the material of which the housing is made, or by treating the surface(s) of the housing which are to contact the filter media.  
           [0039]    The invention is not limited to surface treatment modification as the filter media may be a composite with appropriate wetting characteristics provided by methods well known in the art.  
           [0040]    The liquid repellent porous media will result in a higher than normal liquid breakthrough pressure compared to a non-treated media of the same structure. Also if the porous media is liquid repellent throughout its depth, the fluid will not advance into the sealed portion, thereby reducing possible liquid holdup within the liquid repellent section of the porous media.  
           [0041]    The present invention is well suited for low pressure applications, such as gravity feed systems. An important feature of the present invention is the creation of a fluid barrier that prevents liquid breakthrough past the sealing surface. Due to the low fluid pressure of such systems, this may be accomplished by surface modification of the porous material or media. For a liquid wetting filter media, the CWST of the media at the seal is reduced, so that the CWST of the media is less than the ST of the fluid being filtered, and therefore, the media will be liquid repellant at the seal.  
           [0042]    For a liquid repellent material, the surface tension of the material at the seal may be increased further, to provide enhanced sealing.  
           [0043]    Likewise, the central portions of the filter media may be treated to increase or decrease how fast wetting of the media takes place, i.e., how fast priming occurs. If a liquid wetting filter media is being used, the central portion may be treated to increase the CWST thereof, and make the media more liquid wetting than otherwise. This may be advantageous where it is desired to use a less costly media which is not particularly fast priming, but treatment in the above manner can make it prime as fast as a more costly material. Conversely, one media may be desirable for a particular application, but primes too fast. Treatment in the above manner can slow the priming time. Other advantageous applications are well within the scope of the present invention.  
           [0044]    In one embodiment of the present invention, a flat sheet of a filter media made of a liquid wetting material is treated about the edge regions thereof so that the CWST of the material at the seal is reduced to a value less than the surface tension of the fluid being filtered, so that the material at the sealing surface is, in effect, liquid repellant.  
           [0045]    In another embodiment of the present invention, a flat sheet of filter media made of a fluid repellent material is treated about its interior regions such that the CWST of the material at the interior regions is increased.  
           [0046]    In a further embodiment of the present invention, a tubular filter made of a liquid wetting material or media is treated at its&#39; sealing surfaces such that the CWST of the material at the sealing surfaces is reduced to a value less than the ST of the fluid being filtered, and thus, is liquid repellant at the sealing surfaces.  
           [0047]    In yet another embodiment of the present invention, a filter media in a tubular configuration is made of a fluid repellent material and treated at its&#39; interior such that it is liquid wetting in the interior regions.  
           [0048]    In a still further embodiment of the present invention, a filter media holder is provided for use with the improved filter media of the present invention such that a visual indication of fluid bypass of the filter media is easily provided.  
           [0049]    In another modification of the present invention, a flat sheet of filter media (single or multi-layer composition) made of a liquid wetting material has an annular portion near the edge of the filter material treated to reduce the CWST of the material at the sealing region of the filter sufficiently such that the media is liquid repellant.  
           [0050]    In a still further embodiment of the present invention, a flat sheet of filter media of a predetermined shape, and made of a fluid repellent material, has an annular portion proximate the sealing region of the filter material treated such that the CWST of the material is decreased in the annular region.  
           [0051]    Thus, it an object of the present invention to provide a surface treatment modification for a filter media which will enhance the sealing of the filter media within a filter holder or housing, or between filter media layers.  
           [0052]    Another object of the present invention is to provide enhanced sealing between a porous media and a filter housing without the use of adhesives.  
           [0053]    Another object of the present invention is to provide a filter media which has been surface treated and mounted in a filter housing to provide an effective seal with a reduced number of components.  
           [0054]    A still further object of the present invention is to provide an improved surface treated filter media which achieves an effective seal, and is lighter in weight for the same application, than the known filter media.  
           [0055]    Another object of the present invention is to provide a filter media having a liquid repellant porous sealing region and a liquid wetting filter area with minimum fluid hold-up volume within the filter media.  
           [0056]    Another object of the present invention is to provide an improved filter housing for use with the surface treated filter media which provides a visible means of detecting fluid bypass, thus providing a visual means of detecting filter or housing failure.  
           [0057]    Another object of the present invention is to provide a novel filter housing which, when used in combination with the surface treated filter media of the present invention, increases fluid recovery.  
           [0058]    Another object of the present invention is to provide a filter media that has at least two distinct surface properties proximate to the sealing surface.  
           [0059]    Another object of the present invention is to provide a filter media that provides a barrier to fluid migration past a sealing surface.  
           [0060]    Another object of the present invention is to provide an improved adhesive bounded seal with a precise adhesive/non-adhesive boundary.  
           [0061]    Another objective of the present invention is to provide a filter media (single or multi-layer) that prevents separation of multi-component fluid mixtures.  
           [0062]    Another object of the present invention is to provide a filter media (single or multi-layer) that prevents the separation of multi-component adhesive at the sealing surface.  
           [0063]    Another objective of the present invention is provide a non-wetting region proximate to the sealing surface which may be breached under elevated differential pressure across the seal.  
           [0064]    Another object of the present invention is to provide a consolidated filter element with porous separating barriers that do not have a liquid communication means.  
           [0065]    Another object of the present invention is to provide a liquid barrier across a sealing means.  
           [0066]    Another object of the present invention is to decrease liquid hold-up volume within a sealing region.  
           [0067]    Another object of the present invention is to provide an integral venting means integral within the filter media.  
           [0068]    Another object of the present invention is to provide a venting means integral with an enhanced sealing means.  
           [0069]    Another object of the present invention is to provide a filter with multiple separated compartments.  
           [0070]    Another object of the present invention is to provide a platform for mixing, transporting, separating fluid(s) across a liquid repellent porous partition.  
           [0071]    Further objects and advantages of the present invention will be apparent from the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification wherein like reference characters designate corresponding parts in the several views. It should be noted that although two-dimensional filter media configurations are discussed within this application, other filter media configurations, such as corrugations, domed, tubular or, in general, three-dimensional configurations, are within the scope of this invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0072]    [0072]FIG. 1 is a perspective view of a known filter media of circular shape.  
         [0073]    [0073]FIG. 2 is a diagrammatic sectional view, showing a representative filter holder holding the filter media of FIG. 1.  
         [0074]    [0074]FIG. 3 is a perspective view of a known filter media in tubular form.  
         [0075]    [0075]FIG. 4 is a diagrammatic representation of one common way of sealing the filter media shown in FIG. 3 in a filter housing.  
         [0076]    [0076]FIG. 5 is a perspective view of a construction embodying the present invention.  
         [0077]    [0077]FIG. 6 is a sectional view, taken in the direction of the arrows, along the section line  6 - 6  of FIG. 5.  
         [0078]    [0078]FIG. 7 is a sectional view, in large part similar to FIG. 6, but showing a modification of the present invention.  
         [0079]    [0079]FIG. 7A is a sectional, elevational, view showing a modification of the construction shown in FIG. 7.  
         [0080]    [0080]FIG. 7B is a sectional, elevational, view showing a further modification of the construction shown in FIG. 7.  
         [0081]    [0081]FIG. 7C is a sectional, elevational, view showing a further modification of the construction shown in FIG. 7.  
         [0082]    [0082]FIG. 8 is a diagrammatic sectional view, showing the construction of FIG. 5 mounted in a novel filter housing.  
         [0083]    [0083]FIG. 9 is a perspective view showing a filter media construction embodying a further modification of the present invention.  
         [0084]    [0084]FIG. 10 is a sectional view, taken in the direction of the arrows, along the section line  10 - 10  of FIG. 9.  
         [0085]    [0085]FIG. 11 is a sectional view, in large part similar to FIG. 10, showing a further modification of the present invention.  
         [0086]    [0086]FIG. 11A is a sectional, elevational, view showing a modification of the construction shown in FIG. 11.  
         [0087]    [0087]FIG. 11B is a sectional, elevational, view showing a further modification of the construction shown in FIG. 11.  
         [0088]    [0088]FIG. 11C is a sectional, elevational, view showing a further modification of the construction shown in FIG. 11.  
         [0089]    [0089]FIG. 12 is a diagrammatic sectional view, showing the construction of FIG. 9 mounted in a novel filter housing.  
         [0090]    [0090]FIG. 13 is a perspective view of a further modification of the present invention wherein a vent is added to the construction of FIG. 9.  
         [0091]    [0091]FIG. 14 is a sectional view, taken in the direction of the arrows, along the section line  14 - 14  of FIG. 13.  
         [0092]    [0092]FIG. 15 is a sectional view, in large part similar to FIG. 14, showing a further modification of the present invention.  
         [0093]    [0093]FIG. 15A is a sectional, elevational, view showing a modification of the construction shown in FIG. 15.  
         [0094]    [0094]FIG. 15B is a sectional, elevational, view showing a further modification of the construction shown in FIG. 15.  
         [0095]    [0095]FIG. 16 is a diagrammatic sectional view, showing the construction of FIG. 13 mounted within a novel filter housing.  
         [0096]    [0096]FIG. 17 is an exploded perspective view of the construction shown in FIG. 16.  
         [0097]    [0097]FIG. 18A is a perspective view, partly fragmented, showing a further modification of the present invention.  
         [0098]    [0098]FIG. 18B is a perspective view, showing a further modification of the present invention.  
         [0099]    [0099]FIG. 18C is a diagrammatic view of an improved filter housing embodying the present invention which can utilize the constructions shown in FIGS. 18A and 18B.  
         [0100]    [0100]FIG. 18D is a modification of the construction shown in FIG. C.  
         [0101]    [0101]FIG. 18E is a view similar in part to FIG. 18D, but showing tapered walls on the end cap illustrated which are designed to slightly “crush” the end of the filter tube when it is sealed in the filter housing of FIG. 18C.  
         [0102]    [0102]FIG. 18F is a view similar to FIG. 18E, but showing the filter tube of FIG. 18E fully installed in the end cap, with the end of the filter tube slightly “crushed”.  
         [0103]    [0103]FIG. 18G is a view similar in part to FIG. 18 e,  but showing the surface treatment only at the outer peripheral wall of the filter tube, and showing only a tapered, outer peripheral wall on the end cap.  
         [0104]    [0104]FIG. 18H is a view similar in part to FIG. 18E, but showing the surface treatment only at the inner peripheral wall of the filter tube, and showing only a tapered, inner peripheral wall on the end cap.  
         [0105]    [0105]FIG. 19 is a perspective view of a fluid filter construction embodying the present invention.  
         [0106]    [0106]FIG. 20 is a front elevational view of the construction shown in FIG. 19.  
         [0107]    [0107]FIG. 21 is a sectional view, taken in the direction of the arrows, along the section line  21 - 21  of FIG. 20.  
         [0108]    [0108]FIG. 22 is a perspective view, similar in part to FIG. 13, but showing the addition of a top or upper vent.  
         [0109]    [0109]FIG. 23 is a perspective view, similar in part to FIG. 22, but showing the upper vent overlapping the surface treatment modification and being constructed of the same material.  
         [0110]    [0110]FIG. 24 is a perspective view, similar in part to FIG. 22, but showing the upper vent overlapping the surface treatment modification and being constructed of a different material.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0111]    Referring to FIGS.  1 - 2 , a typical filter media  30  in a flat sheet form is shown. While the media is shown in a circular shape, the media may be in an oval, square, diamond or other desired shape. The filter media  30  may be made of more than one layer, if desired.  
         [0112]    A section taken through the filter media  30 , such as shown in FIG. 2, reveals no distinct surface property variation proximal to the sealing surface. Thus, nothing is found in the prior art filter media  30  itself to aid in sealing it in a typical filter housing  31 , such as is illustrated in FIG. 2.  
         [0113]    The typical filter housing  31  will normally be of a shape complimentary to the filter media  30 , and designed to pinch seal the filter media  30  between two halves of the filter housing. The filter housing  31  has a first half or portion  32  having a first circular side wall  33 , a first upstanding peripheral wall  34  at the periphery thereof, and a second, inwardly spaced, peripheral upstanding wall  35  spaced inwardly a predetermined desired distance from the upstanding outer peripheral wall  34 . A first annular space  39  is formed between the sidewalls ( 34 ,  35 ). An outlet  36  is formed on the circular sidewall  33  for purposes to be described.  
         [0114]    A mating second half or portion  40  has a second circular sidewall  41 , a second upstanding peripheral wall  42 , and a second, inwardly spaced, upstanding peripheral wall  43 . The second upstanding peripheral wall  43  is evenly spaced a predetermined distance from the second upstanding peripheral wall  42  to provide a second annular space  44 .  
         [0115]    The dimensions of the first filter portion  32  and the second filter portion  40  are chosen such that the inside diameter of the first upstanding peripheral wall  34  is related in a predetermined, desired, manner to the outside diameter of the second upstanding peripheral wall  42 . Depending on how it is desired to fasten the first portion  32  and the second filter portion  40 , these dimensions may be chosen to provide a loose fit, an adjacent fit, or an interference fit between the first half  32  and the second half  40  of the filter housing  31 .  
         [0116]    To assemble the filter housing, the filter media  30  will be placed in the filter housing  31 . The filter media  30  preferably has a diameter substantially equal to the inside diameter of the second upstanding peripheral wall  42 . The filter media may be laid in the second filter portion  40  and, because the second inwardly spaced upstanding peripheral wall  43  is of a height less than the second upstanding peripheral wall  42 , the filter will lay on top of the second inwardly spaced peripheral wall  43 .  
         [0117]    The first or cover portion  32  of the filter housing  31  is then placed over the second filter portion  40 . Since the diameters of the first inwardly spaced peripheral upstanding wall  35  of the first filter portion  32 , and the second inwardly spaced peripheral wall portion  43  of the second filter portion  40 , have been chosen so that when the two halves of the filter  31  are assembled they are substantially directly opposite each other, and the height of the two walls have been carefully chosen, the filter media  30  will be “pinched” between the first inwardly spaced peripheral upstanding wall  35  and the second inwardly spaced upstanding peripheral wall  43  The first portion  32  and the second portion  40  of the filter housing  31  may then be bonded, sonic welded, adhesively or otherwise joined to each other. Fluid will come in the inlet  46 , go through filter media  30 , and exit out the outlet  36 .  
         [0118]    In some cases, to increase the reliability of the pinch seal, and reduce filter failure, adhesive is introduced to the first annular space  39  and the second annular space  44  when the filter housing  31  is assembled. However, because of lack of surface treatment modification of the filter media  30 , all of the aforementioned problems present in sealing flat sheet filter media may occur in one form or another in all the known prior art filter housings.  
         [0119]    Referring to FIGS. 3 and 4, a typical prior art tubular filter  50  is shown. The end  51  of the filter  50  shows no surface treatment modification, and typically none is found at either end of the tube  50 . Therefore, the sealing problems discussed above in regard to flat filter media  30  are also present with tubular filters  50 .  
         [0120]    As shown in FIG. 4, tubular filter  50  is generally sealed in a filter housing  47  between the filter head  48  and an end cap  49 . Filter bowl  52  seals the filter tube  50  within the filter housing  47 .  
         [0121]    Referring now FIGS.  5 - 8 , there is shown a flat sheet of filter media  55  embodying the construction of the present invention. The flat sheet of media is shown in a circular shape, but may be of an oval, diamond, square or any other practical shape, and may be made of any media material. An annular shaped portion  56 , proximate the edge  57  of the filter media  55 , has been treated such that the CWST of the media is less that the ST of the fluid being filtered, and therefore, the annular portion  56  of the filter media  55  is liquid repellant with respect to the fluid being filtered. The interior  54  of the flat sheet  55  is less liquid repellent and the annular shaped portion or perimeter  56  of the filter media  55  is more liquid repellent.  
         [0122]    As shown in FIG. 6, it is preferred that the liquid repellent portion  56  extends entirely through the filter media  55 . However, in some applications, such as shown in FIG. 7, it may be desired that only the surface  53  of each side of the flat sheet of media  55  be treated with liquid repellent to produce a first annular liquid repellent portion  58  and a second annular liquid repellent portion  59 . While this version may be useful in some applications, it is not the most preferred embodiment, because the fluid being filtered may wick to the edge  57 , or beyond, resulting in possible fluid loss. However, it may be useful if fluid flow past the seal is desired without bypass at the sealing surface.  
         [0123]    Modifications which prevent wicking to the edge are shown in FIGS.  7 A- 7 C. In FIG. 7A the filter media  55  has the annular or ring shaped treated portion  56  in the form of an inwardly radially extending U-shaped channel  56 C. In FIG. 7B, it is shown as an outwardly radially extending U-shaped channel  56 D. In FIG. 7C, the annular or ring shaped treated portion  56  is in the form of a box channel  56 E.  
         [0124]    With reference to FIG. 8, there is shown a diagrammatic view of a novel combination of a filter housing  60 , and the flat sheet media  55 , which together increase efficiency, reduce hold-up volume, and achieve the aforementioned advantages. The filter media  55  is shown in a pinch seal arrangement between two halves ( 61 ,  62 ) of the filter housing  60 . The filter  60  consists of an inlet section  61  and an outlet section  62 . The inlet section  61  of filter  60  has an inlet  63  including port  63 A communicating with a first passage  64 , which is in fluid communication with a first or inlet chamber  65  through first port or outlet  64 A.  
         [0125]    A more detailed embodiment of a construction embodying the present invention is shown in FIGS.  19 - 21 .  
         [0126]    The filter  60 , as aforementioned, includes an inlet section  61  which is bonded to an outlet section  62  by a seal  80 . The seal  80  is preferably an ultrasonic seal, and may be full, or partial. It can be understood by those skilled in the art that other seals, such as heat seals, adhesive seals, or any other air tight seal may be used.  
         [0127]    Inlet section  61  includes a recessed top wall  91  and a downstanding side wall  92  extending around the periphery of the top wall  91 . A first downstanding peripheral ridge  93  extends around the periphery of the downstanding sidewall  92  and forms a part of the mechanism which holds the filter element  55  in place, as will be more fully explained hereinafter.  
         [0128]    A first protuberance  95  extends from the recessed top wall  91  and carries the inlet  63  and first passage  64  as previously described. A recess  96  provided by the combination of the top surface of the top wall  91 , and peripheral side walls  97 , almost completely surround the protuberance  95 .  
         [0129]    A peripheral flange  98  extends from the peripheral side wall  97  and forms a groove  79  extending around the periphery of the inlet section  61  of the filter  60 . The groove  79  forms a portion of the construction by which the seal  80  between inlet section  61  and outlet section  62  of the filter  60  is formed.  
         [0130]    The shape of the outlet section  62  of the filter  60  is complimentary in shape to the inlet section  61  so that the inlet section  61  may act as a closure to the outlet section  62 , or vice versa. It can be easily understood by those skilled in the art that the fluid filter  60  may be of any desired shape, such as the generally circular shape described, an oval shape, a diamond or any other desired shape. A filter media of any desired shape may be placed in a housing of any desired shape and still be well within the scope of the present invention.  
         [0131]    Similar to the inlet section  61 , the outlet section  62  of the filter  60  has a bottom wall  110  and an upstanding side wall  111 . The top of the upstanding side wall  111  fits into the groove  79  in the inlet portion  61 , and is preferably sonically welded to form the seal  80 . A second protuberance  114  is provided on the exterior portion of the bottom wall  110  and carries the outlet  72 .  
         [0132]    A second downstanding peripheral ridge  115  complimentary in shape to first downstanding peripheral ridge  93  is provided. First downstanding peripheral ridge  93  and/or second downstanding peripheral ridge  115  may be treated to increase or decrease their surface tension, if desired. It should be understood that the terms “upstanding peripheral ridge” and “down standing peripheral ridge” are used in the sense of describing a pair of substantially opposed peripheral ridges which provide for a pinch seal of a filter media. Other terms may be used to describe these ridges, such as “first” and “second”, or “left laterally extending” and “right laterally extending”, without departing from the scope and spirit of the present invention.  
         [0133]    If desired, a plurality of ribs (not shown because they are well known in the art) is provided on the interior surface(s) of the bottom wall  110 , and/or top wall  96 , to help support the filter media  55 , and provide flow in the second or outlet chamber  68  of the filter  60 . In placing such ribs, one needs to be concerned with the volume occupied by the ribs.  
         [0134]    The volume of the ribs [also] controls the amount of holdup volume of the filter. Typically, for many reasons, there are more ribs on the downstream side of the filter. The main reasons include the fact that the upstream chamber may be typically drained, therefore, the hold up volume on the downstream side becomes important. In addition, the downstream side is typically cleaner, and particulate contamination and blockage of the ribs are not as important. Also, the higher rib count downstream of the filter provides a better support. As fluid flows from the upstream side through the media, the fluid exerts a force on the media. The media, if not well supported, may collapse within the ribs. This could adversely affect the filtration/separation process, including important parameters such as process time, efficiency, and capacity.  
         [0135]    When the outlet portion  62  and the inlet portion  61  are in mating relationships, the first down standing ridge  93  and the second downstanding ridge  115  may be in a 180□ opposed relationship. These ridges will provide the “pinch seals” indicated by the numeral  120 . Since the media which has been treated extends radially inwardly of the pinch seal  120 , a continuous vented area is provided in the filter chamber.  
         [0136]    Returning now to the diagrammatic view of the filter shown in FIG. 8, the outlet section  62  of filter housing  60  has a second or outlet chamber  68  which communicates with outlet  72  including port  72 A through second passageway  69 . The filter element  55  separates the first or inlet chamber  65  from the second or outlet chamber  68 .  
         [0137]    The flat sheet of media or filter element  55  may consist of one or more layers, and be made of a wide variety of filter materials. Filter element  55  is held in place in housing  60  between first annular ridge  66  provided about the perimeter of the first or inlet chamber  65  formed in the inlet section  61  of the filter housing  60 , and the second annular ridge  70  formed in the outlet section  62 . Annular ridge  66  is provided to contact the media  55 , and the second annular ridge  70  is chosen to be in a predetermined desired position opposite the first annular ridge  66 , and pinch media  55  therebetween.  
         [0138]    The first annular ridge  66  and the second annular ridge  70  are preferably positioned so that they contact the treated or annular or ring portion  56  of the filter media  55 , which did not become wetted out, while the interior portion  54  did become wetted out. Any gas entering the filter housing through any means may pass through the non-wetted portion of the filter media, extending inwardly beyond ridges  66  and  70 , into the downstream chamber  68 , allowing fluid to drain from the downstream lines (not shown), resulting in increased fluid recovery. It is desirable that at least the portion of the filter housing adjacent the perimeter or edge  57  of the media  55  be transparent or translucent, so that any fluid bypass past the first annular ridge and the second annular ridge,  66  and  70  respectively, can easily be observed.  
         [0139]    In the embodiment illustrated, the filter element  55  has a liquiphilic (liquid wetting) center  54 , and a liquiphobic (liquid repellant) perimeter or edge  57  as previously described. In use, a biological fluid container (not shown), such as a blood container, is placed in fluid communication with inlet port  63 A. Similarly a biological fluid receiving bag (not shown) is placed in fluid communication by means well known in the art with outlet port  72 A. Fluid flow is initiated, and biological fluid flows in the inlet port  63 A through the first passage  64 , and through first port  64 A into inlet chamber  65 . In operation, as the biological fluid enters the inlet chamber  65 , the fluid may wick into the filter element  55 . The rate at which the biological fluid wicks into the filter element  55  will depend on the properties of the filter media being chosen, and the biological fluid being filtered. These properties include the pore size of the medium, the viscosity of the biological fluid, the surface tension of the biological fluid and the contact angle of the solid-liquid-gas interface. While the fluid level is rising in the inlet chamber  65 , any air entrapped in the inlet chamber  65  is passing through a portion of the filter media  55  which has not yet wetted. The treated perimeter or edge  57  (liquid repellent) assures this possibility.  
         [0140]    As the fluid level continues to rise in inlet chamber  65 , at some point the biological filter element  55  will be sufficiently “wetted”, and the biological fluid being filtered will “breakthrough” the filter element  55 , and start flowing into the outlet chamber  68 . The fluid “breakthrough” depends on the pore size of the material, the surface tension and the contact angle, as well as the pressure differential across the filter element  55 .  
         [0141]    The biological fluid, which has now started flowing though the filter element  55 , will first fill up outlet chamber  68 , and when outlet chamber  68  is sufficiently full, the biological fluid being filtered will enter second passageway  69  and pass into the biological fluid receiving container (not shown) through outlet  72 .  
         [0142]    Due to the pressure differential across the filter element  55 , the biological fluid continues to flow up into second passage  69 . Eventually all of the biological fluid will be drained from the biological fluid container and will have flowed through passage  64 , in the presence of excess gas intended to maximize fluid recovery. It is at this point that one of the advantages of the liquid repellent portion being added to the filter media  55  clearly shows.  
         [0143]    In a prior art construction, while all of the fluid flows through the filter media  55  into the downstream or outlet chamber  68 , the downstream chamber  68  and the downstream line (not shown) would remain full in typical low pressure applications, such as gravity feed systems, because the media remains saturated with fluid, due to inadequate pressure differential across the filter media to allow air to breakthrough the media. Thus, residual fluid will remain in the filter media, downstream chamber, and downstream lines resulting in a substantial hold-up volume.  
         [0144]    The present invention, in addition to providing an enhanced sealing mechanism, provides a novel means to increase fluid recovery, and provides a means to vent gas from the upstream chamber. This is accomplished through providing a differential wetability of the filter media at or about the sealing interface between the filter media and its&#39; sealing means. The liquid repellant section, which provided a means to expel gas from the upstream chamber  65  to the downstream chamber  68  at the onset of filter priming, will at the end of the filtration process, as gas enters the upstream chamber  65 , provides a means for gas to travel across the media through the liquid repellant portion to the downstream chamber to recover fluid in the downstream line. The integral gas vent and enhanced sealing means provides fast priming of the filter, as the filter media may be liquid wetting. This integral sealing and vent prevents gas entrapment in the upstream chamber as the liquid repellant portion provides a barrier to fluid and therefore providing a venting means as the upstream chamber is being filled. This is very important since, in typical filter applications, as the fluid enters the filter and is in contact with a filter media that is easily wetted by the fluid, due to capillary forces, the fluid typically wicks in advance of the upstream chamber fluid gas interface. Therefore, if there is no venting means, this will result in gas entrapment in the upstream chamber. Therefore, in the prior art, most media are chosen such that the filter media is not easily wetted by the fluid such that air is not trapped in the upstream chamber but at the same time not to be liquid repellant to such an extent that no fluid passes through the filter media. Therefore, the present invention provides many benefits that enhances the overall performance of the filter media. These include but are not limited to, the venting capability that prevents gas entrapment in the upstream chamber, provides increased fluid recovery, and provides fast priming of the filter. Also it prevents reduced filter media performance if a bubble of air is inadvertently introduced into the filter. In addition, the present invention provides the filter designer with a wide latitude in choosing the material for the filter media based on the critical wetting surface tension (CWST) of the filter media when used in comparison to the ST of the fluid to be filtered as shown by the following examples.  
       EXAMPLE 1  
       [0145]    In a design where the liquid repellent region extends beyond the pinch seal, such as shown for example in FIG. 8, a filter designer can choose the CWST of the filter media in a very broad range. In the case where the designer selects the CWST of the filter media to be much greater than the ST of the fluid being filtered, the filter media will be very hydrophilic (liquid wetting). The present example includes cases in which the liquid repellent region extends to cover the entire pinch seal, goes beyond the pinch seal towards the filter media edge, or fully extends to the edge of the filter media.  
         [0146]    Using the housing of FIG. 8, for example, as a fluid, preferably a biological fluid, enters the housing through the inlet section  61 , it will begin to fill the upstream chamber  65 . As fluid fills the upstream chamber  65 , gas exits the housing through the portions of the filter media  55  that has not been wetted by the fluid, i.e., the annular portion  56 . Since the filter media  55  has been chosen to be very hydrophilic, the filter  55  is fast priming, while at the same time gas freely passes through the annular portion  56 , and vents out of the gas chamber continuously. Since the media and fluid properties are such that no gas entrapment occurs, there is no need for a separate venting means.  
         [0147]    The fluid will then continue to flow through the filter media  55  until the fluid entering the upstream chamber  65  is exhausted. Because of the vent provided by the treated annular portion  56  that is not wetted by the fluid, and which extends inwardly beyond ridge  70 , as gas enters the upstream chamber it will pass through non-wetted portion  56  and, therefore, in most cases fluid holdback will occur in the upstream and downstream chambers ( 65 ,  68 ), while the inlet  64  and outlet  69  will be clear of fluid.  
       EXAMPLE 2  
       [0148]    In a design where the liquid repellent region does not extend beyond the pinch seal, such as shown, for example, in FIG. 12, the filter designer&#39;s choices for the CWST of the filter media is limited in comparison to Example 1.  
         [0149]    Since the non-wetting region does not extend inwardly beyond the pinch seal, it is preferable for the filter media to have a CWST such that there is no air entrapment in the upstream chamber as fluid first enters the upstream chamber. [This limits the selection of the filter media as compared to example 1.] Under optimal conditions for each example, filter media in example 1 will wet the surface faster than Example 2 under similar conditions. The liquid repellant region, which provides an improved seal, is not wetted by the fluid throughout the filtration process.  
         [0150]    As noted previously, in this example, the liquid repellent portion  56  of the filter medium  55  does not extend past the pinch seal. Therefore there is no gas venting means after gas enters the filter housing at the end of the filtration process. After the filtration process, fluid remains in the downstream chamber and downstream lines. The fluid retained in the downstream lines may be used for post evaluation purposes. For example, in the blood banking industry typically the downstream line is segmented, and the segments are used for various purposes, including quality assurance.  
         [0151]    In this example, fluid enters the upstream chamber  65  and fills the upstream chamber. No air entrapment will occur in the upstream chamber  65  as the filter media  55  is slow priming (not immediately wetted by the fluid). Once the fluid wets the filter media  55 , and fills the downstream chamber  68 , fluid will enter the downstream line (not shown). At the end of the filtration process it is typically desired to filter substantially all of the fluid. Gas typically follows the fluid at the end of the filtration process. When the differential pressure across the filter media  55  is lower than the pressure required to push air through the wetted filter media air does not pass through the filter media. Fluid drains from the upstream chamber  65  under differential pressure and substantially all the fluid is filtered. Gas is trapped in the upstream chamber and the downstream chamber, outlet  69 , and the downstream lines are filled with fluid. The fluid in the downstream chamber  68  and line (not shown) may be used for post filtration samples. It is to be noted that Example 2 is most likely slower priming than Example 1.  
       EXAMPLE 3  
       [0152]    In this example the designer has again chosen a media wherein the CWST of the media is greater or equal to the ST of the fluid being filtered such that no significant air entrapment would occur in absence of differential surface tension property proximal to the sealing means. In this example, An annular portion  56 , which includes a dome shaped, or other shaped, vent such as  76  shown in FIG. 13, has been treated to be more liquid repellent than the filter media  55 . Vent portion  76  acts as a vent after substantially all of the fluid is filtered and gas substantially fills the upstream chamber  65 . The liquid repellent region, which is not wetted by the fluid, allows gas passage from the upstream chamber  65  into the downstream chamber  68  after substantially all the fluid is filtered. In order to recover as much of the fluid as possible, the fluid repellent region  56  extends further inwardly at the bottom of the filter to form a gas vent  76 . The gas passage through this inwardly extending section allows the downstream lines to be drained. By providing a narrower gap and providing ribs in the downstream chamber it is possible to drain the downstream chamber completely.  
       EXAMPLE 4  
       [0153]    Example 4 envisions the same choice by the designer as Example 3, with an additional liquid repellent region or top or upper vent  130  at the top portion of the media, such as shown in FIG. 22. The media shown in FIG. 22 may be identical to the media shown in FIG. 13, except for the addition of the additional liquid repellent region or upper vent  130 . The upper vent  130  may be extending inwardly of the pinch seal to allow air to vent from the upstream chamber into the downstream chamber at the onset of the filtration process. This extra liquid repellent section or upper vent  130  is treated such that is not immediately wetted by the fluid. However, it is wetted during the filtration process. This additional liquid repellent section  130  will provide faster priming of the filter housing. It will also provide a means for preventing gas entrapment in the upstream chamber  65  as the upstream chamber is filled at the start of the process. In addition, it will prevent gas passage through the section at the end of the process such that all fluid in the upstream chamber  65  may empty from the upstream chamber. Further preferred embodiments of the present invention using these design considerations are discussed below.  
         [0154]    Referring now to FIGS.  9 - 12 , there is shown a modification of the invention described in FIGS.  8 - 11  where the liquid repellant region does not extend inwardly past the sealing means. In this embodiment, an integral gas vent is not present. A benefit of the present invention, as described previously, is the enhanced sealing mechanism.  
         [0155]    In this embodiment, in absence of a venting means, the filter media is typically chosen such that there is no significant gas entrapment present in the upstream chamber at the onset of filtration. Due to a lack of a venting means, at the end of the filtration process, as gas enters the upstream chamber, gas typically can not pass through the filter, due to fact that the pressure differential required to pass gas through the wetted filter media exceeds that present as gas enters the upstream chamber. Therefore, gas fills the upstream chamber and substantially all fluid is filtered.  
         [0156]    For ease in illustrating the various surface treatment modifications of the filter construction, the diagrammatic view of FIGS. 8, 12 and  16 , rather than the more detailed filter housing construction views shown in FIGS.  19 - 21 , will be used in the remainder of the application.  
         [0157]    Referring to FIG. 9 there is shown a first modified sheet of flat media  55 A having a first modified or annular treated ring portion  56 A spaced a distance “C” from the edge  57 A of first modified filter element or sheet of media  55 A. Used in conjunction with modified filter media  55 A is first modified filter housing  60 A, shown in FIG. 12. The construction of filter housing  60  and first modified filter housing  60 A is substantially identical except for the placement and dimensions of the second upstanding ridge  115 A formed on the outlet section  62 A and the first upstanding ridge  93 A formed on the inlet section  61 .  
         [0158]    While first upstanding ridge  93 A and second upstanding ridge  115 A are still in an opposed relationship, their width has been increased to dimension D, which is wider than the width E of the first modified annular or ring portion  56 A, and may begin at the outer periphery of the first modified annular or ring portion  56 A and extend beyond the inner diameter of the modified annular or ring portion  56 A. Accordingly, dimension “D” may be greater than dimension “E”, and the first modified annular or ring portion  56 A may be co-extensive with the outer diameter of the annular upstanding ridges ( 93 A,  115 A).  
         [0159]    Referring to FIG. 11, it can be seen that in some instances the treated annular portion  56 A may not extend through the entire depth of the filter media  55 A but may instead have surface treated portions  56 A on both sides of the sheet of media  55 A.  
         [0160]    While this version may be useful in some applications, it is not the most preferred embodiment, because the fluid being filtered may wick to the edge  57 A, or beyond, resulting in possible fluid loss.  
         [0161]    Modifications which prevent this are shown in FIGS.  11 A- 11 C. In FIG. 11A the first modified filter media  55 A has the first modified annular or ring shaped treated portion  56 A in the form of an inwardly radially extending U-shaped channel  56 F. In FIG. 11B, it is shown as an outwardly radially extending U-shaped channel  56 G. In FIG. 11C the first modified annular or ring shaped treated portion  56 A is in the form of a box channel  56 H.  
         [0162]    A still further modification of the invention may be seen by referring to FIGS.  13 - 16 . FIGS. 13 and 14 show a second modified filter media  55 B mounted in a second modified filter housing  60 B (FIG. 16), which may be similar to the first modified filter housing  60 A shown in FIG. 12. In this instance, the dimensions C, D, and E may be equal and uniform around the second modified media  55 B, and may be identical to those of the first modified filter media  55 A except where the filter vent  76  is provided. The filter vent  76  is shown as a semi-circular shape, but may be of any desired shape, and instead of being a width of dimension D spaced a distance C from the edge, the filter vent is of a dimension F which begins at the inner periphery of the second modified annular or ring portion  56 B and extends for a distance F, which brings a portion of the filter vent  76  above the first and second downstanding peripheral ridges  93 B and  115 B respectively, which are pinch sealing the second modified filter element  55 B in modified filter housing  60 B.  
         [0163]    In this modification of the invention, there is an extra passageway for air which extends above the ridges  93 B,  115 B pinching the second modified filter media  55 B. As the fluid being processed passes through the inlet chamber  65 B and through second modified media  55 B, any air entrapped in the inlet chamber  65 B will rise to the top of the inlet chamber  65 B. As the fluid continues to be filtered, the fluid level will drop down to the bottom of the inlet chamber  65 B, and any trapped air can now pass through the filter vent  76 , and up to the top of the inlet chamber  65 B. If a plurality of parallel ribs (not shown) are carefully placed downstream in the outlet chamber  68 B this air will carry any fluid remaining in the outlet chamber up and out through the outlet  72 . Thus, in this modification of the invention, not only the inlet chamber  65 B, but the outlet chamber  68 B, and the downstream line (not shown) will be empty, thus reducing hold back volume to a minimum.  
         [0164]    Referring now to FIG. 15, a modification of the second modified filter element  55 B is shown where the second modified annular or ring portion  56 B does not extend for the entire depth of the second modified filter media  55 B.  
         [0165]    In FIG. 15, there is shown filter vent  76  having a front surface portion  77 , a rear surface portion  78 , and a leg  83  connecting the front surface portion  77  and rear surface portion  78  proximate the middle thereof. The leg  83  is needed for the air to pass between the front surface portion  77  and rear surface portion  78 , and may be placed in any desired position between the two. Even if placed below the fluid level in the filter, the suction pressure is believed to be sufficient to cause the air remaining upstream after the filtering operation to pass through. While this is not a preferred embodiment because the fluid being filtered may wick to, and possibly past, the edge  57 B, it may be useful for some applications.  
         [0166]    Modifications which prevent this are shown in FIGS.  15 A- 15 B. In FIG. 15A the second modified filter media  55 B has the second modified annular or ring shaped treated portion  56 B in the form of an inwardly radially extending U-shaped channel  56 J. In FIG. 15B, it is shown as an outwardly radially extending U-shaped channel  56 K.  
         [0167]    In FIG. 15A the leg  83  is shown connecting front surface portion  77  of the filter vent  76  and the rear surface portion  78  thereof at their outer extremities in the form of a radially inwardly extending channel. In FIG. 15B the leg  83  is placed at the innermost possible position to connect front surface portion  77  and rear surface portion  78  of filter vent  76 . As explained hereinabove the leg  83  can be at the position shown in FIG. 15A, the position shown in FIG. 15B, or any place in between and still perform satisfactorily.  
         [0168]    With the foregoing explanation, additional benefits of the present construction may be seen. When the flat sheet of media has the annular or ring portion  56  treated or present for the entire depth of the flat sheet of media as shown in FIG. 10, any fluid passing past the downstanding peripheral ridges ( 93 ,  115 ) is an indication of fluid bypass or filter failure, since a higher pressure is needed to bypass the pinch seals than if the treated annular or ring portion  56  were absent. Thus, if the filter housing ( 60 ,  60 A,  60 B) were transparent or translucent, at least around the periphery thereof, any fluid which might flow past the annular or ring portion ( 56 ,  56 A,  56 B) could be easily observed by the user of the filter housing ( 60 ,  60 A,  60 B) and the filter process could be stopped, and the fluid being filtered could be saved.  
         [0169]    Referring now to FIGS.  18 A- 18 H, it can be seen that the surface treatment modification of the present invention is not just useful with flat filter media or discs, but can also improve the sealing capabilities of tubular filters as well. In FIG. 18A there is a perspective view of tubular filter  120  having an upper edge region  121 A proximate the upper end  121  of filter  120 , and lower edge region  122 A proximate the lower end  122  of the filter  120  treated with the surface treatment modification of the present invention. The treated regions ( 121 A,  122 A) will preferably be of annular shape and may extend for the entire thickness of the filter tube, but may be of other shapes and cross sections if desired. For example, annular shaped treated regions may extend for a finite depth on the inside or outside of the tubular filter  120 .  
         [0170]    By making the ends of the tubular filter  120  more liquid repellent, it is harder for liquid to bypass the ends of the filter tube when the filter tube is held between a pair of end caps, as is typical in prior art filter housings. Thus, a tubular filter  120  having its ends  121 ,  122 , respectively treated with a liquid repellent is well within the scope of the present invention.  
         [0171]    Referring now to FIG. 18B, there is shown a modification of the construction shown in FIG. 18A wherein the filter tube  120  has annular portions thereof ( 121 A,  122 B) treated with a surface treatment modification to be more liquid repellent than the remainder of the filter tube  120 . However, instead of being proximate the ends of the filter tube ( 121 ,  122 ), they are spaced a short, predetermined distance X therefrom. This will provide a mechanism for sealing the filter tube  120  on its&#39; outer and/or inner surface ( 120 A,  120 B) instead of, or in addition to, its&#39; ends ( 121 ,  122 ).  
         [0172]    Referring now to FIG. 18C, there is shown a filter construction capable of accomplishing this. The filter construction shown in FIG. 18C may be identical to the filter construction shown in FIG. 4, except that the end cap, now identified by the numeral  49 A for the purpose of clarity, has been provided with a first or outer upstanding peripheral wall  49 B, and a second or inner upstanding peripheral wall  49 C spaced inwardly a predetermined distance from first or outer upstanding peripheral wall  49 B to create a pair of spaced walls between which the lower end of the filter tube  120 A can be sealed.  
         [0173]    The spacing between the walls  49 A,  49 B should be such as to put sufficient pressure on the treated annular portion  122 A to avoid fluid bypass.  
         [0174]    The other end of the filter tube  120 A may be sealed in a similar manner by providing a modified upper end cap (not shown), or it may be sealed in a conventional manner.  
         [0175]    Instead of a single treated region ( 121   a,    122 A) being provided proximate upper and/or the lower ends ( 121 ,  122 ) of the tubular filter  120  being provided (as shown in FIG. 18A), a pair of treated annular regions ( 122 C,  122 D) extending for a finite depth, less than the thickness of the tubular filter  120 , may be provided proximate the upper and/or lower end of the tubular filter  120 . This is shown on an enlarged scale in FIG. 18D.  
         [0176]    Referring now to FIGS.  18 E- 18 H, There are shown several modifications of the end cap  49 A illustrated in FIGS. 18C and 18D. In FIG. 18E, there is illustrated a further modified end cap, now identified by the numeral  150  for clarity. End cap  150 , as can end cap  49 A, may have an outer upstanding peripheral wall  150 A (FIG. 18G), an inner upstanding peripheral wall  150 B (FIG. 18H), or both FIGS. 18E, 18F). Also walls  150 A and  150 B, as can walls  49 B,  49 C, can be in a concentric or non-concentric orientation with each other, can be of any desired height, and can be placed anywhere on the end cap ( 150 ,  49 A) depending on the application.  
         [0177]    Outer peripheral wall  150 A may have a first slanted surface  160  provided on its inner portion  162 . The angle which the slanted surface  160  makes with the top surface  164  may vary depending on the application.  
         [0178]    Likewise, the inner peripheral upstanding wall  150 B may have a second slanted surface  168  provided on its outer portion  170 . A flat top surface  175  may be provided as part of the outer peripheral wall  150 B and/or the inner peripheral wall  150 B if desired.  
         [0179]    The slanted portions ( 160 ,  168 ) are designed to push inwardly in on, and slightly crush the surface treated portions ( 122 C,  122 D) as pressure is applied to the end cap  150  to seal the tubular filter  120  in a filter housing, as shown in FIG. 18F.  
         [0180]    The height of the walls ( 49 B,  49 C) ( 150 A,  150 B) should be sufficient so that the inner wall ( 49 C,  150 B) and/or the outer wall ( 49 B,  150 A) contact at least a portion of the treated annular portion  122 C and/or  122 D.  
         [0181]    [0181]FIG. 18G shows a modification of the construction shown in FIG. 18E wherein the end cap  150  has only an outer, upstanding, peripheral wall  150 A sealing against an outer surface treated portion  122 C.  
         [0182]    [0182]FIG. 18H shows a modification of the construction shown in FIG. 18E wherein the end cap  150  has only an inner, upstanding, peripheral wall  150 B sealing against an inner surface treated portion  122 C.  
         [0183]    Referring to FIG. 22, there is shown a construction embodying the present invention, wherein a top or upper vent  130  is added to permit to allow air to vent from the upstream chamber into the downstream chamber at the onset of the filtration process for the purposes described above. The top or upper vent or upper surface treated area  130  can be treated with the same or different treatment as the vent  76 .  
         [0184]    [0184]FIG. 23 shows a construction similar to that shown in FIG. 22 wherein the top or upper vent or upper surface treated area  130  overlaps the annular surface treated portion  56 B, and is treated with the same surface treatment modification.  
         [0185]    [0185]FIG. 24 shows a construction similar to that shown in FIG. 22 wherein the top or upper vent or upper surface treated area  130  overlaps the annular surface treated portion  56 B, but is treated with a different surface treatment modification.  
         [0186]    It may be desirable for some applications to have a top vent  130  as shown in FIGS.  22 - 24 , without the bottom vent  76 , and this is well within the scope of the present invention.  
         [0187]    The present invention is not limited to the circular discs or tubular filters previously illustrated but may also be applied, as will be apparent, to other tubular or cylindrical elements that are pleated, formed and/or rolled. The shape of the treatment may vary to suit the applicability of the filter design. For example as shown in FIGS. 5, 9 and  13 , concentric treatments are shown, but the present invention should be understood not to be limited as such.  
         [0188]    Further, there are various chemical treatments known in the art for producing the surface treatment modifications. The preferred treatments for such applications are fluorinated or siliconized polymers for liquid repellent applications, and polyvinyl alcohol and cellulose acetate for liquid wetting applications. Other treatments will be apparent to those skilled in the art.  
         [0189]    Many preferred embodiments of the present invention have been described herein. The scope of the present invention is broad, and many more embodiments of the invention can be developed using the teachings herein, and these are well within the scope of the present invention. For example, with reference to FIGS. 8, 12,  16  and  21 , the dimensions shown are limited to the example in regard to which they discussed. As long as the hydrophobically treated portion of the media being held in a pinch seal extends radially inwardly of the opposed upstanding walls forming the pinch seal, a vented area will be formed. Therefore, for example, in FIGS. 12 and 16, dimension D may be larger or smaller than dimension E, depending on the application, and be well within the scope of the present invention.  
         [0190]    Thus, by carefully considering the problems present with filtering fluids, a novel surface treatment has been developed which reduces hold-up volume and produces numerous other advantages when compared with prior art devices.