Method of manufacturing a precision micro-filter
FIELD OF THE INVENTION 
     The present invention relates to precision micro-filters and more particularly to a method and a type of mold for making such micro-filters. 
     BACKGROUND OF THE INVENTION 
     Micro-filters are used, particularly in medicine and biotechnology, to separate and collect very small particles, such as cells, suspended or dissolved in a fluid. For such purposes, micro-filters are often needed with passageways of specific shapes and with an internal diameter on the order of 1 micron and larger. 
     A variety of methods are presently known for making micro-filters. One such method is to make a mold using integrated circuits (IC) processing and lithographic processes. This mold can be formed by using well known &#34;mask and etch technology&#34; to build up a plurality of layers which are subsequently separated from the base substrate. The resultant mold may then be filled with plastic to form a precision micro-filter. The resultant micro-filter, however, tends to have high internal strain. Furthermore, this manufacturing technique for making a micro-filter tends to be expensive and thus non-economical. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a micro-filter mold which includes a base and a plurality of rod-like protrusions is formed by micro-machining. A liquid molding material is then introduced into the resulting micro-filter mold. This liquid, material is allowed to solidify to form a precision micro-filter. The resulting precision micro-filter is then separated from the micro-filter mold. Preferably, a precisely controlled micro electrode discharge machine (EDM) is used to make the micro-filter mold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1(a) is a top view showing the structure of a precision micro-filter. 
     FIG. 1(b) is a side, cross-sectional view of the precision micro-filter of FIG. 1(a) taken in the plane A--A&#39; shown in FIG. 1(a). 
     FIG. 2(a) is a top view of a micro-mold suitable for forming the precision micro-filter of FIG. 1(a). 
     FIG. 2(b) is a cross-sectional, side view of the micro-mold of FIG. 2(a), taken in the plane B--B&#39; of FIG. 2(a). 
     FIG. 3(a) is a top view showing a cover which is suitable for use with the micro-mold of FIG. 2(a). 
     FIG. 3(b) is a side, cross-sectional view showing the cover of FIG. 3(a), in the plane C--C&#39; shown in FIG. 3(a). 
     FIG. 4 is a side view of the micro-mold shown in FIG. 2(a) and the cover shown in FIG. 3(a), prior to assembly of the micro-mold and the cover. 
     FIG. 5(a) is a top view of the micro-mold shown in FIG. 2(a) and the cover shown in FIG. 3(a), after assembly of the micro-mold and the cover to form a micro-mold/cover assembly. 
     FIG. 5(b) is a side, cross-sectional view of the micro-mold/cover assembly of FIG. 5(a) in the plane D--D&#39; shown in FIG. 5(a), including a molding composition in the interior space of the mold. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, an exemplary embodiment of the present invention will be described. 
     A top view of a precision micro-filter 1, constructed in accordance with an exemplary embodiment of the present invention is shown in FIG. 1(a). A side, cross-sectional view of the precision micro-filter of FIG. 1(a), taken in the plane A--A&#39; shown in FIG. 1(a) is shown in FIG. 1(b). The micro-filter may be formed of any suitable molding composition which can be converted, such as by a change in temperature (solidification of a melt for example) from a fluid state to a solid state. Exemplary materials which may be used for this purpose include essentially any conventional thermoplastic resin, such as a polyester resin. 
     The micro-filter includes a plurality of microholes 2. Microholes 2 are shown in FIG. 1(a) and FIG. 1(b) having an exemplary size and shape for illustrative purposes. However, it is understood that the microholes may be of essentially any pre-selected cross-sectional shape and dimension, the possible shapes including round, square, triangular, etc. The size of the micro-holes may be any dimension larger than 1 micrometer. The spacing between the holes may be of any dimension larger than 4 micrometers. Although a predetermined number of micro-holes are shown in FIG. 1(a) and FIG. 1(b), it is understood that any number of microholes may be included in micro-filter 1 in any desired arrangement. 
     The micro-filter of FIG. 1(a) and FIG. 1(b) may be manufactured in accordance with the procedure described below, with reference to FIGS. 2 thru 5. 
     In FIG. 2(a), a top view of mold 10 used for producing precision micro-filter 1 is shown. In FIG. 2(b) a cross-sectional view of mold 10, taken in the plane B--B&#39; of FIG. 2(a), is shown. Mold 10 may be constructed of a variety of materials including stainless steel, aluminum, copper, iron, tungsten, and their alloys. The only requirements are that the mold material must be micro-machinable, as discussed below, and must remain solid under the conditions which produce a change of state (from fluid to solid) in the molding composition to be used. Mold 10 comprises a base 11 and four side walls 6a-d, thus forming an open ended container. In this manner, mold 10 may be used to contain the liquid to be added later. 
     Mold 10 further comprises a plurality of rod-like protrusions 4, extending from base 11 and arranged in N rows and M columns (where N and M are integers). This plurality of rod-like protrusions are of preselected size, shape and spacing to conform to a negative &#34;image&#34;, namely the micro-filter which is produced from this mold as described hereinbelow. 
     In order to form a micro-filter, adapted to selectively permit the passage and reject the passage of various sized particles, the micro-filter is desirably formed having passageways of spacing, shape and dimension which are precisely known. In accordance with the present invention, protrusions 4 of mold 10 are precisely constructed using micro-machining technology. Examples of suitable micro-machining techniques for manufacturing mold 10 include micro-electro-discharge machining, laser-beam micro-machining, or electron-beam micro-machining. The use of micro-machining technology is particularly desirable for the formation of mold 10, due to the highly precise structures which may be formed thereby. 
     In addition, the use of micro-machining technology allows for the formation of rod-like protrusions 4 within mold 10 with a tolerance of approximately 0.1 microns. This tolerance allows for the production of a micro-filter which is suitable for a variety of applications, including the separation of different sized cells. However, it is understood that larger tolerances are acceptable depending on the application for which the micro-filter is being produced. Thus, the acceptable tolerance with which rod-like protrusions are formed is largely application dependent. 
     Apparatus suitable for performing micromachining with high dimensional tolerance is described in a variety of references, including Micro-EDM, Li, H. and Masaki, T., Society of Manufacturing Engineers Technical Paper, MS91-485; Micro Electro-Discharge Machining, T. Masaki, K. Kawata, T. Sato, T. Mizutani, K. Yonemoti, A. Shibuya and T. Masuzawa, Proceedings of International Symposium for Electro-Machining, p, 26-29, 1989; Micro EDMing Excites the High-Tech Community, EDM Today, pp. 32, 34, 46, March/April 1991; Micro Electro-Discharge Machine Brochure, Matsushita Research Institute, Tokyo Inc. May 1990, herein incorporated by reference for their teachings in the field of micro-machining. 
     In FIGS. 3(a) and 3(b), a cover 7 for mold 10 is shown. FIG. 3(a) is a top view of cover 7. FIG. 3(b) is a cross-sectional side view of cover 7, in the plane C--C&#39; of FIG. 3(a). Cover 7 includes a plurality of openings 8. 
     In order to form precision micro-filter 1, cover 7 is assembled with mold 10. FIG. 4 is a side view of micro-mold 10 and cover 7, prior to assembly of micro-mold 10 and cover 7. After micro-mold 10 is assembled with cover 7 (as shown in FIGS. 5(a) and 5(b)), thus forming a micro-mold/cover assembly, micro-mold 10 is filled with a fluid molding composition 9. Composition 9 may be a polymer (such as a polyester) or a ceramic. Composition 9 is added into the mold 10 through openings 8 until it fills the void spaces therein, within walls 6a-d and around protrusions 4. Composition 9 is then allowed to solidify. 
     After molding composition 9 has solidified, cover 7 is removed and micro-filter 1 is separated from mold 10. 
     Because the level of composition 9 may reach the top surface of cover 7, a plurality of protrusions 12 with the shape of cover openings 8, may be formed on the top surface of precision micro-filter 1. This is shown in FIGS. 1(a) and 1(b). However, the existence of protrusions 12, or the lack thereof, is immaterial to the proper operation of precision micro-filter 1. 
     By forming precision micro-filter 1 in the manner described above, a plurality of microopenings 2 are formed, wherein each microopening 2 corresponds precisely, without internal strain, to a respective rod 4 of mold 10. Thus each microopening 2 has a precisely known, preselected, shape and size. In addition, the spacing between each microopening 2 is preselected and precisely known. 
     In an alternative embodiment of the present invention, molding composition 9 is poured into mold 10 without the use of cover 7. In this manner, the top surface of micro-filter 1 is formed by horizontally disposed-gravity settling. However, without using cover 7, the top surface of micro-filter 1 thus produced may not be adequately smooth. 
     In a further alternative embodiment of the present invention, the inner surfaces of mold 10 are coated with a mold release agent and/or polished prior to pouring liquid material 9 into mold 10. In this manner, removal of completed micro-filter 1 from micro-mold 10 is eased. An exemplary mold release agent is silicone. 
     Thus, by forming a mold with high dimensional tolerances using a micro-machining apparatus, a finely detailed, micro-filter may be produced in which the spacing, shape and dimensions of the passageways are precisely controlled, with dimensions on the order of 1 micron and with dimensional tolerances on the order of 0.1 micron. Such a filter may find particular application in the selection, collection or purification of organic cells and other very small biological materials. 
     Furthermore, a micromold which is accurately manufactured in accordance with the present invention can be used repeatedly. The cost of manufacturing micromolds in accordance with the present invention is thus extremely low. As a result, high-precision and low-cost precision micro-filters may be made for a variety of applications, including medical, biotechnical and industrial uses. 
     While the invention has been described in terms of an exemplary embodiment, it is contemplated that it may be practiced as outlined above, with modifications within the spirit and scope of the appended claims.