Abstract:
A microneedle array, constructed of silicon and silicon dioxide compounds or of a molded plastic material, is provided to penetrate the stratum corneum and epidermis layers of skin, but not into the dermis. The microneedles can be used to either dispense a liquid drug, or to sample a body fluid. The delivery of drugs and sampling of fluids can be performed by way of passive diffusion (time release), instantaneous injection, or iontophoresis. A complete closed-loop system can be manufactured including active elements, such as micro-machined pumps, as well as passive elements such as sensors. A “smart patch” can thereby be fabricated that samples body fluids, performs chemistry to decide on the appropriate drug dosage, and then administers the corresponding amount of drug. An electric field may be used to increase transdermal flow rate. Such a system can be made disposable, and can be used with medical devices to dispense drugs by iontophoretic/microneedle enhancement, to sample body fluids (while providing an iontophoretically/microneedle-enhanced body-fluid sensor), and as a closed-loop drug delivery system with fluid sampling feedback using a combination of the other two devices. As a drug dispensing system, the microneedle array includes electrodes that apply an electric potential to the skin between the electrode locations. One of the electrode assemblies is filled with an ionized drug, and the charged drug molecules move into the body due to the applied electric potential. As a body-fluid sampling system, the microneedle array also includes electrodes to assist in moving fluid from the body into a receiving chamber, and which further includes a bioelectrochemical sensor to measure the concentration of a particular substance.

Description:
TECHNICAL FIELD 
     The present invention relates generally to medical devices and is particularly directed to a fluid dispensing device and a fluid sampling device of the type which penetrates the stratum corneum and epidermis, but not into the dermis of skin. The invention is specifically disclosed as an array of microneedles which painlessly and with minimal trauma to the skin enable fluid transfer either into a body as a dispensing device, or from the body to sample body fluid. 
     BACKGROUND OF THE INVENTION 
     Topical delivery of drugs is a very useful method for achieving systemic or localized pharmacological effects. The main challenge in transcutaneous drug delivery is providing sufficient drug penetration across the skin. The skin consists of multiple layers starting with a stratum cornuem layer about (for humans) twenty (20) microns in thickness (comprising dead cells), a viable epidermal tissue layer about seventy (70) microns in thickness, and a dermal tissue layer about two (2) mm in thickness. 
     The thin layer of stratum corneum represents a major barrier for chemical penetration through skin. The stratum corneum is responsible for 50% to 90% of the skin barrier property, depending upon the drug material&#39;s water solubility and molecular weight. The epidermis comprises living tissue with a high concentration of water. This layer presents a lesser barrier for drug penetration. The dermis contains a rich capillary network close to the dermal/epidermal junction, and once a drug reaches the dermal depth it diffuses rapidly to deep tissue layers (such as hair follicles, muscles, and internal organs), or systemically via blood circulation. 
     Current topical drug delivery methods are based upon the use of penetration enhancing methods, which often cause skin irritation, and the use of occlusive patches that hydrate the stratum corneum to reduce its barrier properties. Only small fractions of topically applied drug penetrates through skin, with very poor efficiency. 
     Convention methods of biological fluid sampling and non-oral drug delivery are normally invasive. That is, the skin is lanced in order to extract blood and measure various components when performing fluid sampling, or a drug delivery procedure is normally performed by injection, which causes pain and requires special medical training. An alternative to drug delivery by injection has been proposed by Henry, McAllister, Allen, and Prausnitz, of Georgia Institute of Technology (in a paper titled “Micromachined Needles for the Transdermal Delivery of Drugs), in which an array of solid microneedles is used to penetrate through the stratum corneum and into the viable epidermal layer, but not to the dermal layer. In this Georgia Tech design, however, the fluid is prone to leakage around the array of microneedles, since the fluid is on the exterior surface of the structure holding the microneedles. 
     Another alternative to drug delivery by injection is disclosed in U.S. Pat. No. 3,964,482 (by Gerstel), in which an array of either solid or hollow microneedles is used to penetrate through the stratum corneum, into the epidermal layer, but not to the dermal layer. Fluid is to be dispensed either through hollow microneedles, through permeable solid projections, or around non-permeable solid projections that are surrounded by a permeable material or an aperture. A membrane material is used to control the rate of drug release, and the drug transfer mechanism is absorption. The microneedle size is disclosed as having a diameter of 15 gauge through 40 gauge (using standard medical gauge needle dimensions), and a length in the range of 5-100 microns. The permeable material may be filled with a liquid, hydrogel, sol, gel, of the like for transporting a drug through the projections and through the stratum corneum. 
     Another structure is disclosed in WO 98/00193 (by Altea Technologies, Inc.) in the form of a drug delivery system, or analyte monitoring system, that uses pyramidal-shaped projections that have channels along their outer surfaces. These projections have a length in the range of 30-50 microns, and provide a trans-dermal or trans-mucous delivery system, which can be enhanced with ultrasound. 
     Another struture, disclosed in WO 97/48440, WO 97/48441, and WO 97/48442 (by ALZA Corp.) is in the form of a device for enhancing transdermal agent delivery or sampling. It employs a plurality of solid metallic microblades and anchor elements, etched from a metal sheet, with a length of 25-400 mm. WO 96/37256 (by Silicon Microdevices, Inc.) disclosed another silicon microblade structure with blade lengths of 10-20 mm. For enhancing transdermal delivery. 
     Most of the other conventional drug delivery systems involve an invasive needle or plurality of needles. An example of this is U.S. Pat. No. 5,848,991 (by Gross) which uses a hollow needle to penetrate through the epidermis and into the dermis of the subject&#39;s skin when the housing containing an expansible/contractible chamber holding a reservoir of fluidic drug is attached to the skin. Another example of this is U.S. Pat. No. 5,250,023 (by Lee) which administers fluidic drugs using a plurality of solid needles that penetrate into the dermis. The Lee drug delivery system ionizes the drug to help transfer the drug into the skin by an electric charge. The needles are disclosed as being within the range of 200 microns through 2,000 microns. 
     Another example of a needle that penetrates into the dermis is provided in U.S. Pat. No. 5,591,139, WO 99/00155, and U.S. Pat. No. 5,855,801 (by Lin) in which the needle is processed using integrated circuit fabrication techniques. The needles are disclosed as having a length in the range of 1,000 microns through 6,000 microns. 
     The use of microneedles has great advantages in that intracutaneous drug delivery can be accomplished without pain and without bleeding. As used herein, the term “microneedles” refers to a plurality of elongated structures that are sufficiently long to penetrate through the stratum corneum skin layer and into the epidermal layer, yet are also sufficiently short to not penetrate to the dermal layer. Of course, if the dead cells have been completely or mostly removed from a portion of skin, then a very minute length of microneedle could be used to reach the viable epidermal tissue. 
     Since microneedle technology shows much promise for drug delivery, it would be a further advantage if a microneedle apparatus could be provided to sample fluids within skin tissue. Furthermore, it would be a further advantage to provide a microneedle array in which the individual microneedles were of a hollow structure so as to allow fluids to pass from an internal chamber through the hollow microneedles and into the skin, and were of sufficient length to ensure that they will reach into the epidermis, entirely through the stratum corneum. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary advantage of the present invention to provide a microneedle array in the form of a patch which can perform intracutaneous drug delivery. It is another advantage of the present invention to provide a microneedle array in the form of a patch that can perform interstitial body-fluid testing and/or sampling. It is a further advantage of the present invention to provide a microneedle array as part of a closed-loop system to control drug delivery, based on feedback information that analyzes body fluids, which can achieve real time continuous dosing and monitoring of body activity. It is yet another advantage of the present invention to provide an iontophoretically/microneedle-enhanced transdermal drug delivery system in order to achieve high-rate drug delivery and to achieve sampling of body fluids. It is a yet further advantage of the present invention to provide a method for manufacturing an array of microneedles using microfabrication techniques, including standard semiconductor fabrication techniques. It is still another advantage of the present invention to provide a method of manufacturing an array of microneedles comprising a plastic material by a “self-molding” method, a micromolding method, a microembossing method, or a microinjection method. 
     Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. 
     To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a first embodiment of an improved microneedle array is constructed of silicon and silicon dioxide compounds using MEMS (i.e., Micro-Electro-Mechanical-Systems) technology and standard microfabrication techniques. The microneedle array may be fabricated from a silicon die which can be etched in a microfabrication process to create hollow cylindrical individual microneedles. The resulting array of microneedles can penetrate with a small pressure through the stratum corneum of skin (including skin of animals, reptiles, or other creatures—typically skin of a living organism) to either deliver drugs or to facilitate interstitial fluid sampling through the hollow microneedles. The drug reservoir, and/or the chemical analysis components for sampling body fluid, may be fabricated inside the silicon die, or an additional thick film layer can be bonded or otherwise attached over the silicon substrate to create the reservoir. The delivery of drugs and sampling of fluids can be performed by way of passive diffusion (e.g., time release), instantaneous injection, or iontophoresis. A complete closed-loop system can be manufactured including active elements, such as micro-machined pumps, heaters, and mixers, as well as passive elements such as sensors. A “smart patch” can thereby be fabricated that samples body fluids, performs chemistry to decide on the appropriate drug dosage, and then administers the corresponding amount of drug. Such a system can be made disposable, including one with an on-board power supply. 
     In a second embodiment, an array of hollow (or solid) microneedles can be constructed of plastic or some other type of molded or cast material. When using plastic, a micro-machining technique is used to fabricate the molds for a plastic microforming process. The molds are detachable and can be re-used. Since this procedure requires only a one-time investment in the mold micro-machining, the resulting plastic microstructure should be much less expensive than the use of microfabrication techniques to construct microneedle arrays, as well as being able to manufacture plastic microneedle arrays much more quickly. It will be understood that such hollow microneedles may also be referred to herein as “hollow elements,” or “hollow projections,” including in the claims. It will also be understood that such solid microneedles may also be referred to herein as “solid elements,” or “solid projections” (or merely “projections”), including in the claims. 
     Molds used in the second embodiment of the present invention can contain a micropillar array and microhole array (or both), which are fabricated by micro-machining methods. Such micro-machining methods may include micro electrode-discharge machining to make the molds from a variety of metals, including stainless steel, aluminum, copper, iron, tungsten, and their alloys. The molds alternatively can be fabricated by microfabrication techniques, including deep reactive etching to make silicon, silicon dioxide, and silicon carbide molds. Also, LIGA or deep UV processes can be used to make molds and/or electroplated metal molds. 
     The manufacturing procedures for creating plastic (or other moldable material) arrays of microneedles include: “self-molding,” micromolding, microembossing, and microinjection techniques. In the “self-molding” method, a plastic film (such as a polymer) is placed on a micropillar array, the plastic is then heated, and plastic deformation due to gravitational force causes the plastic film to deform and create the microneedle structure. Using this procedure, only a single mold-half is required. When using the micromolding technique, a similar micropillar array is used along with a second mold-half, which is then closed over the plastic film to form the microneedle structure. The micro-embossing method uses a single mold-half that contains an array of micropillars and conical cut-outs (microholes) which is pressed against a flat surface (which essentially acts as the second mold-half) upon which the plastic film is initially placed. In the microinjection method, a melted plastic substance is injected between two micro-machined molds that contain microhole and micropillar arrays. 
     Of course, instead of molding a plastic material, the microneedle arrays of the present invention could also be constructed of a metallic material by a die casting method using some of the same structures as are used in the molding techniques discussed above. Since metal is somewhat more expensive and more difficult to work with, it is probably not the preferred material except for some very stringent requirements involving unusual chemicals or unusual application or placement circumstances. The use of chemical enhancers, ultrasound, or electric fields may also be used to increase transdermal flow rate when used with the microneedle arrays of the present invention. 
     In the dispensing of a liquid drug, the present invention can be effectively combined with the application of an electric field between an anode and cathode attached to the skin which causes a low-level electric current. The present invention combines the microneedle array with iontophoresis enhancement, which provides the necessary means for molecules to travel through the thicker dermis into or from the body, thereby increasing the permeability of both the stratum corneum and deeper layers of skin. While the transport improvement through the stratum corneum is mostly due to microneedle piercing, iontophoresis provides higher transport rates in epidermis and dermis. 
     The present invention can thereby be used with medical devices to dispense drugs by iontophoretic/microneedle enhancement, to sample body fluids (while providing an iontophoretically/microneedle-enhanced body-fluid sensor), and a drug delivery system with fluid sampling feedback using a combination of the other two devices. For example, the body-fluid sensor can be used for a continuous noninvasive measurement of blood glucose level by extracting glucose through the skin by reverse iontophoresis, and measuring its concentration using a bioelectrochemical sensor. The drug delivery portion of this invention uses the microneedle array to provide electrodes that apply an electric potential between the electrodes. One of the electrodes is also filled with an ionized drug, and the charged drug molecules move into the body due to the applied electric potential. 
     Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description and claims serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is an elevational view in partial cross-section of a bottom mold provided at the initial step of a “self-molding” method of manufacturing an array of plastic microneedles, as constructed according to the principles of the present invention. 
     FIG. 2 is an elevational view in partial cross-section of the mold of FIG. 1 in a second step of the self-molding procedure. 
     FIG. 3 is an elevational view in partial cross-section of the mold of FIG. 1 in a third step of the self-molding procedure. 
     FIG. 4 is an elevational view in partial cross-section of the mold of FIG. 1 in a fourth step of the self-molding procedure. 
     FIG. 5 is an elevational view in partial cross-section of the mold of FIG. 1 in a fifth step of the self-molding procedure. 
     FIG. 6 is an elevational view in cross-section of an array of hollow microneedles constructed according to the self-molding procedure depicted in FIGS. 1-5. 
     FIG. 7 is a cross-sectional view of a top mold-half used in a micromolding procedure, according to the principles of the present invention. 
     FIG. 8 is an elevational view of the bottom half of the mold that mates to top mold-half of FIG. 7, and which is used to form plastic microneedles according to the micromolding procedure. 
     FIG. 9 is an elevational view in partial cross-section of one of the method steps in the micromolding procedure using the mold halves of FIGS. 7 and 8. 
     FIG. 10 is an elevational view in partial cross-section of the mold of FIG. 9 depicting the next step in the micromolding procedure. 
     FIG. 11 is a cross-sectional view of an array of plastic microneedles constructed according to the micromolding procedure depicted in FIGS. 7-10. 
     FIG. 12 is an elevational view in partial cross-section of a top mold-half and a bottom planar surface used in creating an array of molded, plastic microneedles by a microembossing procedure, as constructed according to the principles of the present invention. 
     FIG. 13 is an elevational view in partial cross-section of the mold of FIG. 12 in a subsequent process step of the microembossing method. 
     FIG. 14 is an elevational view in partial cross-section of the mold if FIG. 12 showing a later step in the microembossing procedure. 
     FIG. 15 is a cross-sectional view of a microneedle array of hollow microneedles constructed by the mold of FIGS. 12-14. 
     FIG. 15A is a cross-sectional view of an array of microneedles which are not hollow, and are constructed according to the mold of FIGS. 12-14 without the micropillars. 
     FIG. 16 is an elevational view in partial cross-section of a two-piece mold used in a microinjection method of manufacturing plastic microneedles, as constructed according to the principles of the present invention. 
     FIG. 17 is a cross-sectional view of a microneedle array of hollow microneedles constructed by the mold of FIG.  16 . 
     FIG. 18 is a cross-sectional view of the initial semiconductor wafer that will be formed into an array of microneedles by a microfabrication procedure, according to the principles of the present invention. 
     FIG. 19 is a cross-sectional view of the semiconductor wafer of FIG. 18 after a hole pattern has been established, and after a silicon nitride layer has been deposited. 
     FIG. 20 is a cross-sectional view of the wafer of FIG. 18 after a photoresist mask operation, a deep reactive ion etch operation, and an oxidize operation have been performed. 
     FIG. 21 is a cross-sectional view of the wafer of FIG. 20 after the silicon nitride has been removed, and after a deep reactive ion etch has created through holes, thereby resulting in a hollow microneedle. 
     FIG. 22 is a perspective view of a microneedle array on a semiconductor substrate, including a magnified view of individual cylindrical microneedles. 
     FIG. 23 is a cross-sectional view of an iontophoretically enhanced body-fluid sensor, based upon a hollow microneedle array, as constructed according to the principles of the present invention. 
     FIG. 24 is a cross-sectional view of an iontophoretically enhanced body-fluid sensor, based upon a solid microneedle array, as constructed according to the principles of the present invention. 
     FIG. 25 is a cross-sectional view of an electrode, based upon a hollow microneedle array, as constructed according to the principles of the present invention. 
     FIG. 26 is a cross-sectional view of an electrode, based upon a solid microneedle array, as constructed according to the principles of the present invention. 
     FIG. 27 is a perspective view of a sensing system attached to a human hand and forearm, which includes an iontophoretically enhanced body-fluid sensor as per FIG.  23  and an electrode as per FIG.  25 . 
     FIG. 28 is a cross-sectional view of an iontophoretically enhanced drug delivery system, based upon a hollow microneedle array, as constructed according to the principles of the present invention. 
     FIG. 29 is a cross-sectional view of an iontophoretically enhanced drug delivery system, based upon a solid microneedle array, as constructed according to the principles of the present invention. 
     FIG. 30 is a perspective view of a closed-loop drug-delivery system, as viewed from the side of a patch that makes contact with the skin, as constructed according to the principles of the present invention. 
     FIG. 31 is a perspective view of the closed-loop drug-delivery system of FIG. 30, as seen from the opposite side of the patch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings, wherein like numerals indicate the same elements throughout the views. 
     Referring now to the drawings, FIG. 1 shows a mold generally designated by the reference numeral  10  that comprises a plurality of micropillars, including micropillars  12  and  14 , that are mounted to a base  16  having a planar upper surface  18 . Micropillar  12  preferably is cylindrical in shape, and has an outer diameter designated “D 1 ,” whereas micropillar  14  (which also preferably is cylindrical in shape) has a diameter designated “D 2 .” The centerlines of micropillars  12  and  14  are separated by a distance “D 3 ,” and the vertical height of micropillars  12  and  14  is designated by the letter “L 1 .” 
     In a preferred configuration, the diameters D 1  and D 2  are in the range of 1-49 microns, more preferably about ten (10) microns (i.e., 10 microns=10 micrometers), the height L 1  in the range of 50-200 microns, whereas the separation distance D 3  is in the range of 50-1000 microns, more preferably from 50-200 microns. 
     Microelectrode-discharge machining can be used to fabricate the mold  10  from metals, such as stainless steel, aluminum, copper, iron, tungsten, or other metal alloys. Mold  10  could also be fabricated from silicon or silicon carbide using integrated circuit processing, or photolithographic processing. 
     FIG. 2 depicts the mold  10  and a thin layer of plastic, such as a polymer film, designated by the reference numeral  20 , which is placed on the micropillars  12  and  14 , thereby making contact at the reference numerals  22  and  24 , respectively. Once the polymer film is placed on the micropillars, the polymer is heated to just above the melting temperature of the plastic material. Micropillars  12  and  14  are also heated to a certain extent, but are held just below the melting temperature of the plastic material. This establishes a temperature gradient within the plastic film, after which the plastic film is subjected to natural gravitational forces, or placed in a centrifuge. Furthermore, an air-pressure gradient also can be established across the deforming plastic film, by applying pressure from above, or by applying a vacuum from below the film level. The overall effect on the plastic film is that it will undergo a “self-molding” operation, by way of the gravitational force or centrifugal force, and the air-pressure gradient can be used to accelerate the self-molding process. 
     FIG. 3 depicts the mold  10  at a further step in the processing of the plastic film, showing the result of the temperature gradient. This result is that the areas contacting the micropillars (at the reference numerals  22  and  24 ) will have a smaller deformation as compared to the remaining portions of the plastic film  20  that are between the pillars  12  and  14 . Therefore, the portions  30 ,  32 , and  34  of the plastic material will undergo greater deformation, as viewed on FIG.  3 . 
     FIG. 4 depicts the mold  10  at yet a later step in the self-molding process, showing the initial stage in which the mold (including micropillars  12  and  14 ) is heated above the melting temperature of the plastic material  20 . During this latter stage of the self-molding process, the plastic material will continue to melt and to be removed from the tops of the pillars  12  and  14 . As viewed in FIG. 4, the remaining portions not in contact with micropillars  12  and  14  will continue to deform downward (as viewed on FIG. 4) at the reference numerals  30 ,  32 , and  34 . 
     FIG. 5 depicts the mold  10  at the final stage of self-molding, which illustrates the fact that the plastic material has completely melted down and away from the tops  22  and  24  of the micropillars  12  and  14 . At this point the mold and the plastic material are both cooled down, thereby forming the final shape that will become the microneedles. This final shape includes an outer wall  40  and  42  for the microneedle being formed by micropillar  12 , and an outer wall at  44  and  46  for the microneedle being formed at the micropillar  14 . 
     FIG. 6 illustrates the cross-sectional shape of the microneedle array, generally designated by the reference numeral  60 , after it has been detached from the mold  10 . The left hand microneedle  62  has a relatively sharp upper edge, which appears as points  50  and  52 . Its outer wall is illustrated at  40  and  42 , which are sloped with respect to the vertical, as designated by the angles “A 1 ” and “A 2 .” The right-hand side microneedle  64  exhibits a similar sharp top edge, as indicated by the points  54  and  56 , and also exhibits a sloped outer wall at  44  and  46 . The angle of this outer wall is indicated at the angles “A 3 ” and “A 4 .” The preferred value of angles A 1 -A 4  is in the range of zero (0) to forty-five (45) degrees. 
     The inner diameter of the left-hand microneedle  62  is indicated by the distance “D 1 ,” and the inner diameter of the right-hand microneedle  64  is indicated by the distance “D 2 .” These distances D 1  and D 2  are substantially the same distance as the diameter of micropillars  12  and  14 , as indicated in FIG.  1 . Furthermore, the distance D 3  between the centerlines of the microneedles on FIG. 6 is essentially the same as the distance D 3  between the micropillars on FIG.  1 . The length “L 2 ” of the microneedles on FIG. 6 is somewhat less than the length L 1  on FIG. 1, although this length L 2  could theoretically be a maximum distance of L 1 . 
     It will be understood that the plastic material (also referred to herein as the “polymer film”) may consist of any type of permanently deformable material that is capable of undergoing a gradual deformation as its melting point is reached or slightly exceeded. This “plastic material” could even be some type of metallic substance in a situation where the metallic material would deform at a low enough temperature so as to not harm the mold itself. The preferred material is a polyamide such as nylon, although many other types of polymer material certainly could be used to advantage. Other potential materials include: polyesters, vinyl, polysterene, polycarbonate, and acrylonitrilebutadisterene (ABS). Of course, one important criterion is that the material which makes up the microneedles does not chemically react with skin, or with the fluidic substance that is being transported through the hollow interiors of the microneedle array. 
     FIG. 7 depicts a top mold-half, generally designated by the reference numeral  110 , of a second embodiment of the present invention in which the manufacturing method for creating an array of hollow microneedles is performed by a micromolding procedure. The top mold-half  110  includes two “microholes” that have sloped side walls, designated by the reference numerals  112  and  114  for the left-hand microhole  113 , and by the reference numerals  116  and  118  for the right-hand microhole  117 . The microholes  113  and  117  have a vertical (in FIG. 7) dimension referred to herein as a distance “L 11 ”. Microholes  113  and  117  correspond to a pair of micropillars  122  and  124  that are part of a bottom mold-half, generally designated by the reference number  120 , and illustrated in FIG.  8 . 
     Referring back to FIG. 7, the sloped side walls of the microhole  113  are depicted by the angles “A 11 ” and “A 12 ,” with respect to the vertical. The side walls of microhole  117  are also sloped with respect to the vertical, as illustrated by the angles “A 13 ” and “A 14 ” on FIG.  7 . Since microhole  113  preferably is in a conical overall shape, the angle A 11  will be equal to the angle A 12 ; similarly for microhole  117 , the angle A 13  will be equal to the angle A 14 . It is preferred that all microholes in the top mold-half  110  exhibit the same angle with respect to the vertical, which means that angles A 11  and A 13  are also equal to one another. A preferred value for angles A 11 -A 14  is in the range of zero (0) through forty-five (45) degrees. The larger the angle from the vertical, the greater the trauma to the skin tissue when a microneedle is pressed against the skin. On FIG. 7, the illustrated angle A 11  is approximately twelve (12) degrees. 
     Referring now to FIG. 8, the bottom mold-half  120  includes a base  126  having a substantially planar top surface  128 , upon which the two micropillars  122  and  124  are mounted. These micropillars are preferably cylindrical in shape, and have a diameter of D 11  and D 12 , respectively. The distance between the centerlines of these micropillars is designated as D 13 . Diameters D 11  and D 12  preferably are in the range 1-49 microns, more preferably about 10 microns. The distance “D 13 ” represents the separation distance between the center lines of micropillars  122  and  124 , which preferably is in the range 50-1000 microns, more preferably about 200 microns. 
     The two mold-halves  110  and  120  can be fabricated from metals using microelectrode-discharge machining techniques. Alternatively, the molds could be fabricated from silicon or silicon carbide using integrated circuit processing or lithographic processing. 
     On FIG. 8, a thin plastic film, generally designated by the reference numeral  130 , is placed on top of the micropillars and heated above the glass transition temperature of the plastic material while the plastic material  130  rests upon the tops of the pillars at  132  and  134 , thereby causing the plastic material to become sufficient pliable or “soft” for purposes of permanently deforming the material&#39;s shape. Preferably, the temperature of the plastic material will not be raised above its melting temperature, although it would not inhibit the method of the present invention for the plastic material to become molten just before the next step of the procedure. In FIG. 9, the top mold-half  110  is pressed downward and begins to deform the plastic film  130 . While a portion of the plastic material  130  temporarily resides above the micropillars at  132  and  134 , a larger amount of the plastic material is pressed downward directly by the mold top-half  110  at  140 ,  142 , and  144 . As can be seen in FIG. 9, the two mold halves  110  and  120  are aligned so that the microholes  113  and  117  correspond axially to the micropillars  122  and  124 , respectively. The two mold halves now begin to operate as a single mold assembly, generally designated by the reference numeral  100 . 
     In FIG. 10, the two mold halves  110  and  120  have completely closed, thereby squeezing all of the plastic material  130  away from the tops of the micropillars  122  and  124 . At this point, the plastic microneedles are formed, and the mold and the plastic material are both cooled down. 
     The wall  112  and  114  of the first microhole  113  causes a side outer wall to be formed out of the plastic material at  150  and  152 . The corresponding inner wall of the microneedle  182  is depicted at  160  and  162 , which is caused by the shape of the micropillar  122 . Since the outer wall is sloped, it will converge with the inner wall  160  and  162 , near the top points at  170  and  172 . A similar outer wall  154  and  156  is formed by the inner wall  116  and  118  of microhole  117 . The inner wall of the microneedle  184  is depicted at  164  and  166 , and these inner and outer walls converge near points  174  and  176 . 
     FIG. 11 illustrates the microneedle array, generally designated by the reference numeral  180 , after the mold is removed from the plastic material  130 . A lower relatively planar base remains, as illustrated at  140 ,  142 , and  144 . On FIG. 11, two different microneedles are formed at  182  and  184 . The angles formed by the walls are as follows: angle A 11  by walls  150  and  160 , angle A 12  by walls  162  and  152 , angle A 13  by walls  154  and  164 , and angle A 14  by walls  166  and  156 . The points at the top if the microneedles (designated at  170 ,  172 ,  174 , and  176 ) are fairly sharp, and this sharpness can be adjusted by the shape of the mold with respect to the microholes and micropillar orientations. 
     The inner diameter of microneedle  182  is designated by the distance D 11 , and the inner diameter of the microneedle  184  is designated by the distance D 12 . The distance between the centerlines of these microneedles is designated as D 13 . These distances correspond to those illustrated on FIG.  8 . 
     It is preferred that all of the angles A 11 -A 14  are equal to one another, and that the angles fall within the range of zero (0) to forty-five (45) degrees. The preferred angle really depends upon the strength of the material being used to construct the microneedles, in which a greater angle (e.g., angle A 11 ) provides greater strength. However, this angular increase also causes greater trauma to the skin. 
     Microneedle array  180  also includes a relatively flat base structure, as indicated at the reference numerals  140 ,  142 , and  144 . This base structure has a vertical thickness as designated by the dimension L 15  (see FIG.  11 ). The microneedle height is designated by the dimension L 12  on FIG.  11 . The height must be sufficient to penetrate the skin through the stratum corneum and into the epidermis, and a preferred dimension for height L 12  is in the range of 50-200 microns (although, certainly microneedles shorter than 50 microns in length could be constructed in this manner—for use with skin cosmetics, for example). The thickness L 15  can be of any size, however, the important criterion is that it be thick enough to be mechanically sound so as to retain the microneedle structure as it is used to penetrate the skin. 
     Referring now to FIG. 12, a top mold-half  210  is combined with a planar bottom mold-half  240  to create an entire mold, generally designated by the reference numeral  200 . The top mold-half  210  contains an array of microholes with micropillars at the center of each of the microholes. For example, a microhole  213 , having its conical wall at  212  and  214 , is preferably concentric with a micropillar  222 , and a microhole  217 , having its conical wall at  216  and  218 , is preferably concentric with a micropillar  224 . 
     The fabrication method used in conjunction with the mold  200  is referred to herein as “microembossing” for the reason that the bottom mold-half  240  is simply a flat or planar surface. This greatly simplifies the construction of this particular mold. A thin plastic film at  230  is placed upon the top surface  242  of this bottom mold-half  240 . In the later steps, it will be seen that the plastic material  230  is heated while the top mold-half  210  is pressed down against the bottom mold-half  240 . 
     Microhole  213  and micropillar  222  have an angular relationship as illustrated by the angles “A 21 ” and “A 22 .” A similar angular relationship exists for microhole  217  and micropillar  224 , as illustrated by the angles “A 23 ” and “A 24 .” These angles A 21 -A 24  will preferably be in the range of zero (0) to forty-five (45) degrees from the vertical. As noted hereinabove, the greater the angle, the greater the transport rate, however, also the greater trauma to the skin tissue when used. 
     Micropillar  222  preferably has a cylindrical shape with an outer diameter designated at “D 21 ,” and micropillar  224  similarly has a preferred cylindrical shape having a diameter “D 22 .” Diameters D 21  and D 22  preferably are in the range 1-49 microns, more preferably about 10 microns. The distance “D 23 ” represents the separation distance between the center lines of micropillars  222  and  224 , which preferably is in the range 50-1000 microns, more preferably about 200 microns. 
     The length of the micropillars from the bottom surface  228  of the top mold-half  210  to the closed end of the microholes at  215  and  225 , respectively, is designated as the length “L 21 .” The micropillars  222  and  224  are somewhat longer than this length L 21 , since they are to mate against the upper surface  242  of the bottom mold-half  240 , and therefore are longer by a distance designated as “L 25 .” In this manner, the microneedles will be hollow throughout their entire length. The combined length of dimensions L 21  and L 25  preferably will be approximately 150 microns. 
     The molds  210  and  240  will preferably be made from a metal, in which microelectrode-discharge machining can be used to fabricate such metallic molds. Alternatively, the molds could be fabricated from silicon or silicon carbide, for example, using integrated circuit processing or lithographic processing. 
     Referring now to FIG. 13, after the plastic material is heated above its glass transition temperature, thereby causing the plastic material to become sufficient pliable or “soft” for purposes of permanently deforming the material&#39;s shape. Preferably, the temperature of the plastic material will not be raised above its melting temperature, although it would not inhibit the method of the present invention for the plastic material to become molten just before the top mold  210  begins to be pressed down against the plastic material  230 . This top mold movement begins to deform that plastic material  230  such that it begins to fill the microholes, as illustrated at  232  and  234  (for microhole  213 ) and at  236  and  238  (for microhole  217 ). 
     In FIG. 14, the top mold-half  210  has now been completely closed against the bottom planar mold-half  240 , and the plastic material  230  has now completely filled the microholes, as illustrated at  232 ,  234 ,  236 , and  238 . The shape of the plastic material now has a conical outer wall at  250  and  252 , and a corresponding cylindrical inner wall at  260  and  262 , for the left-hand microneedle  282  on FIG.  14 . Correspondingly for the right-hand microneedle  284 , the plastic material shape has an outer conical wall at  254  and  256 , as well as a cylindrical inner wall at  264  and  266 . The conical outer walls and the cylindrical inner walls converge at the top points  270  and  272 , and  274  and  276 . The bottom surface  228  of the top mold-half  210  causes a base to be formed in the plastic material  230  at the locations indicated by the reference numerals  244 ,  246 , and  248 . Once this shape has been formed, the mold and the plastic material are cooled down, and then the molds are separated so that the plastic microneedle array is detached to form the shape as illustrated in FIG.  15 . 
     In FIG. 15, a microneedle array  280  has been formed out of the plastic material  230 , which as viewed on FIG. 15 depicts two microneedles  282  and  284 . The left-hand microneedle  282  comprises an outer conical wall as viewed at  250  and  252 , and a hollow interior cylindrical wall at  260  and  262 . These walls converge at the top points (as viewed on this Figure) at  270  and  272 , and the convergence angle is given as “A 21 ” and “A 22 .” The right-hand microneedle  284  comprises an outer conical wall  254  and  256  and a hollow interior cylindrical wall  262  and  264 . These walls converge at the top points (on this Figure) at  274  and  276 , and the convergence angle is given as “A 23 ” and “A 24 .” Angles A 21 -A 24  are preferably in the range of zero (0) to forty-five (45) degrees. 
     Microneedle array  280  also includes a relatively flat base structure, as indicated at the reference numerals  244 ,  246 , and  248 . This base structure has a vertical thickness as designated by the dimension L 25 . The microneedle height is designated by the dimension L 22 . The height must be sufficient to penetrate the skin through the stratum corneum and into the epidermis, and has a preferred dimension in the range of 50-200 microns (although, as noted above, much shorter microneedles could be constructed in this manner). The thickness L 25  can be of any size, however, the important criterion is that it be thick enough to be mechanically sound so as to retain the microneedle structure as it is used to penetrate the skin. 
     The inside diameter of the hollow microneedles is illustrated as D 21  and D 22 , which correspond to the diameters of a cylindrical hollow opening. The distance D 23  represents the separation distance between the centerlines of the two microneedles  282  and  284 , in this array  280 . 
     FIG. 15A represents an alternative embodiment in which a microneedle array  290  comprises “solid” microneedles  292  and  294 , rather than hollow microneedles as seen at  282  and  284  on FIG.  15 . These solid microneedles  292  and  294  are formed by a similar mold as viewed on FIG. 12, but with the micropillars  222  and  224  removed from this mold, and a change in shape of the microholes  213  and  217 . This simple change allows the solid microneedles to be formed within conical microholes (not shown on FIG.  12 ), and produces a pointed conical shape, as exhibited by the outer conical wall  250  and  252  for microneedle  292 , with a top pointed surface at  296 . Similarly, the microneedle  294  has a conical outer wall  254  and  256 , with a similar top pointed surface at  298 . The other dimensions and features of the solid microneedle array  290  can be exactly the same as those features of the hollow microneedle array  280  of FIG. 15, or the dimensions may be different since this is for a different application. 
     The holes  251 ,  253 ,  255 , can be fabricated during the microstamping or micrembossing procedure via inclusion of appropriate micropillars located adjacent to the microholes  213  and  217  in FIG.  12 . 
     Referring to FIG. 16, a mold  300  consists of two mold-halves  310  and  340 . These mold-halves  310  and  340  are virtually identical in shape, and probably in size, as compared to the mold-halves  210  and  240  of the mold  200  on FIG.  12 . The main difference in FIG. 16 is that these mold-halves are to be used in a microinjection procedure in which molten plastic material is injected from the side at  330  into the opening between the mold-halves formed by the bottom surface  328  of the top mold-half  310  and the top surface  342  of the bottom mold-half  340 . 
     The mold structure  300  is preferably made of a metallic material by a micro-machining process, although it could be made of a semiconductor material such as silicon or silicon carbide, if desired. On FIG. 16, the plastic material  330  is being filled from the left-hand side in this view, and has already filled a first microhole  313  with plastic material. The plastic material is illustrated as it is advancing, and has reached the point at the reference numeral  336 . As time proceeds, the plastic material will reach and fill the second microhole  317 , which has a conical inner wall at  316  and  318 , and a corresponding micropillar  324 . 
     At the first microhole  313 , the plastic material has filled the shape around a micropillar  322  and within the conical walls of this microhole  313 , to form a hollow cone having an outer wall at  332  and  334 . The plastic material will be forced upward until it reaches a top point as seen at the reference numerals  370  and  372 . The outer conical shape at  332  and  334  will converge with the interior shape of the micropillar  322  at an angle designated by the angles “A 31 ” and “A 32 .” Microhole  317  also exhibits a converging angular shape at “A 33 ” and “A 34 ,” which is the convergence angle between the conical walls  316  and  318  and the outer cylindrical shape of the micropillar  324 . 
     The separation between the surfaces  328  and  342  is given by the length dimension “L 35 ,” which will become the thickness of the planar face material that will remain once the mold is opened. The vertical dimension (in FIG. 16) of the microholes is given by the dimension “L 31 ” which preferably will create microneedles long enough to penetrate through the stratum corneum and into the epidermis, but not so long as to penetrate all the way to the dermis. 
     FIG. 17 illustrates the microneedle array, generally designated by the reference numeral  380 . On FIG. 17, two microneedles are illustrated at  382  and  384 . These microneedles have a length “L 32 ,” which in theory should be exactly the same as the dimension L 31  on FIG. 16, assuming the mold was properly filled with material. A preferred distance for L 32  is in the range of 50-200 microns. 
     The plastic material  330  has a planar base structure, as illustrated at  344 ,  346 , and  348 . The thickness of this base structure is the dimension L 35 . The microneedles themselves exhibit a conical outer wall at  350  and  352  for the left-hand microneedle  382 , and at  354  and  356  for the right-hand microneedle at  384 . Each microneedle has a hollow interior, as illustrated by the cylindrical surface  360  and  362  for microneedle  382 , and  364  and  366  for microneedle  384 . These surfaces converge to form points (as illustrated on FIG. 17) at  370  and  372  for microneedle  382 , and at  374  and  376  for microneedle  384 . The convergence angle of these walls is designated by the angles A 31 -A 34 , and preferably will be in the range of zero (0) to forty-five (45) degrees. 
     The inner diameter of microneedle  382  is given by the dimension D 31 , and for microneedle  384  is given by dimension D 32 . These dimensions preferably are in the range 1-49, more preferably about 10 microns. The separation distance between the center lines of the microneedles is given at D 33 , which preferably is in the range 50-1000 microns, more preferably about 200 microns. The height L 32  is preferably in the range of 50-200 microns and, depending upon the convergence angle A 31 -A 34 , the bottom width of the conical microneedles will vary depending upon the exact application for usage. In one preferred embodiment, this bottom dimension, designated by “D 34 ” and “D 35 ,” will be approximately twenty (20) microns. The vertical thickness at L 35  will likely be made as thin as possible, however, the important criterion is that it is sufficiently thick to be mechanically sound to hold the microneedle array  380  together as a single structure during actual usage. It is likely that, for most plastic materials that might be used in this molding procedure, the dimension L 35  will be in the range of ten (10) microns through two (2) mm, or greater. 
     The angular relationship between the microneedles and the corresponding planar base surface is preferably perpendicular, although an exact right angle of 90 degrees is not required. This applies to all microneedle embodiments herein described, including microneedles  62 ,  64  and planar surfaces  30 ,  32 ,  34  of FIG. 6, microneedles  182 ,  184  and planar surfaces  140 ,  142 ,  144  of FIG. 11, microneedles  282 ,  284  and planar surfaces  244 ,  246 ,  248  of FIG. 15, microneedles  292 ,  294  and planar surfaces  244 ,  246 ,  248  of FIG. 15A, microneedles  382 ,  384  and planar surfaces  344 ,  346 ,  348  of FIG. 17, and microneedle  470  and planar surfaces  440 ,  446  of FIG.  21 . 
     It will be understood that other methods of forming plastic microneedles could be utilized to create hollow microneedles in an array, without departing from the principles of the present invention. It will also be understood that various types of materials could be used for such molding procedures, including metallic materials that might be cast using higher temperature dies of a similar shape and size, without departing from the principles of the present invention. 
     It will be further understood that variations in dimensions and angular relationships could be utilized to construct an array of hollow microneedles, without departing from the principles of the present invention. It will be still further understood that the angular relationship between the microneedles and their planar base surface need not be precisely perpendicular (although that configuration is preferred), but could have some variation without departing from the principles of the present invention; the microneedles also need not be exactly parallel with one another, even though that configuration is preferred. 
     It will be yet further understood that other microneedle shapes could be used than a cylindrical shape, if desired, without departing from the principles of the present invention. Moreover, it will be understood that, with only simple modifications to the molds, an array of solid microneedles could be fabricated using the molding techniques described herein, without departing from the principles of the present invention. 
     While there are conventional hollow needles that can be arranged in an array, such conventional needles are all much larger in both length and diameter than those disclosed hereinabove, and therefore, will penetrate all the way into the dermal layer, thereby inflicting a certain amount of pain to the user. Moreover, these larger needles can be made using more conventional manufacturing techniques, since their dimensions will allow for a relaxed standard of manufacture. 
     Referring now to FIG. 18, a procedure for forming dry etched microneedles will be described using an example of microfabrication (e.g., semiconductor fabrication) techniques. Starting with a single crystal silicon wafer at reference numeral  400 , it is preferred to use a double side polish wafer and to grow an oxide layer on the entire outer surface. In FIG. 18, a cross-section of this wafer appears as a substrate  410 , a top oxide layer  412 , and a bottom oxide layer  414 . Any single crystal silicon wafer will suffice, although it is preferred to use a crystal structure 100-type wafer, for reasons that will be explained below. A 110-type wafer could be used, however, it would create different angles at certain etching steps. 
     To create the structure depicted in FIG. 19, certain process steps must first be performed, as described below. The first step is a pattern oxide step which is performed on the top side only to remove much of the top oxide layer  412 . The pattern used will create multiple annular regions comprising two concentric circles each, of which the cross-section will appear as the rectangles  416  and  418  on FIG.  19 . In perspective, these annular-shaped features will have the appearance as illustrated on the perspective view of FIG. 22 at the reference numerals  416  and  418 . These annular oxide patterns are the initial stages of the array locations of the multiple microneedles that will be formed on this substrate  410 . 
     The next step is to deposit a layer of silicon nitride using a low pressure vapor deposition step, which will form a silicon nitride layer on both the top and bottom surfaces of the substrate  410 . This appears as the uppermost layer  420  and the bottommost layer  422  and  424 . It will be understood that the bottommost layer  422  and  424  is one continuous layer at this step, although it is not illustrated as such on FIG. 19, since a later step etches out a portion of the bottom side of the substrate between the layers  422  and  424 . 
     Next in the process is a pattern bottom procedure in which a square hole is patterned beneath the annulus  416 ,  418 , which is not directly visible on FIG.  19 . The square holes placed by the pattern bottom procedure are now used in a KOH etching step that is applied to the bottom side only of the substrate  410 . This KOH etching step creates a window along the bottom of the substrate as viewed along the surfaces  432 ,  430 , and  434  on FIG.  19 . This window interrupts the oxide layer  414  along the bottom of substrate  410 , and divides it (on FIG. 19) into two segments  413  and  415 . This window (or hole) also interrupts the silicon nitride layer into two segments (on FIG. 19)  422  and  424 . 
     The slope angle of the etched window along surfaces  432  and  434  is 54.7 degrees, due to the preferred 100-type silicon material. If type-110 silicon material was used, then this slope would be 90 degrees. That would be fine, however, crystalline silicon 100-type material is less expensive than silicon 110-type material. After the KOH time etching step has been completed, the silicon wafer will have the appearance as depicted in FIG.  19 . 
     The next fabrication operation is to perform a pattern top nitride procedure using a photoresist mask. This removes the entire upper silicon nitride layer  420  except where the photoresist mask was located, which happens to be aligned with the upper oxide annulus at  416  and  418 . The remaining upper silicon nitride is indicated at the reference numeral  426  on FIG. 20, although at this stage in the fabrication procedure, the upper surface will still be a planar surface at the level of the oxide layer  416  and  418 , across the entire horizontal dimension of FIG.  20 . 
     The next fabrication step is to perform a deep reactive ion etch (DRIE) operation on the top surface of the substrate  410 , which will etch away a relatively deep portion of the upper substrate except at locations where the silicon nitride layer still remains, i.e., at  426 . In this DRIE procedure, it is preferred to remove approximately 50-70 microns of material. After that has occurred, the remaining photoresist mask material is removed. This now exposes the top silicon nitride layer  426 . 
     The next fabrication step is to oxidize all of the bare silicon that is now exposed along the outer surfaces. This will form a layer of silicon dioxide at locations on FIG. 20, such as at  440 ,  442 ,  444 ,  446 ,  452 ,  450 , and  454 . The outer silicon nitride layers at  426 ,  423 , and  425  are not oxidized. The outer silicon nitride layers  423  and  425  are essentially the same structures as layers  422  and  424  on FIG. 19, although the silicon dioxide layers  452  and  454  are now formed above these “pads”  423  and  425 . It is preferred that this oxidation be a minimal amount, just enough for a future DRIE masking procedure, and that the oxidized thickness be approximately 5,000 Angstroms. At this point in the fabrication procedure, the silicon wafer has the appearance of that depicted in FIG.  20 . 
     The next step in the fabrication procedure is to remove the silicon nitride layer on the top, which will remove the layer at  426  as seen on FIG.  20 . This will expose a circular region in the very center of the annulus such that pure silicon is now the outermost material on the top side of the wafer. After that has occurred, a deep reactive ion etch operation is performed to create a through-hole at the reference numeral  460  on FIG.  21 . After this step has been performed, there will be pure silicon exposed as the inner wall of the through-hole  460 . Therefore, the next step is to oxidize the entire wafer, which will place a thin cylindrical shell of silicon dioxide around the inner diameter of through-hole  460 , and this oxidized layer is viewed on FIG. 21 at  462  and  464 . 
     After these steps have been performed, a microneedle  465  is the result, having an outer diameter at “D 41 ,” and an inner diameter through-hole at “D 42 .” It is preferred that the inner diameter D 42  have a distance in the range of 5-10 microns. The height of the microneedle is given at the dimension “L 41 ,” which has a preferred dimension in the range of 50-200 microns. On FIG. 21, the substrate  410  has been divided into halves at  410 A and  410 B. In addition, the bottom oxide layer  450  has been divided in halves at  450 A and  450 B. 
     The bottom chamber formed by the sloped surfaces  452  and  454 , in combination with the horizontal surfaces  450 A and  450 B, act as a small, recessed storage tank or chamber generally indicated by the reference numeral  470 . This chamber  470  can be used to store a fluid, such as insulin, that is to be dispensed through the cylindrical opening  460  in the hollow microneedle  465 . At the scale of FIG. 21, this chamber is not very large in overall physical volume, and it normally would be preferred to interconnect all of such chambers for each of the microneedles in the overall array so that a common fluid source could be used to dispense fluid to each of these chambers  470 . Furthermore, there may be a need to dispense a physically much larger volume of fluid, and it also may be desirable to provide a pressure source, such as a pump. In such situations, it may be preferable to have an external storage tank that is in communication with each of the fluid chambers  470  on the wafer that is used to make up the array of microneedles, such as microneedle  465 . 
     FIG. 22 depicts an array of microneedles on substrate  410 , and also illustrates a magnified view of some of these microneedles  465 . Each microneedle  465  exhibits a cylindrical shape in the vertical direction, and has an outer diameter D 41 , an annular shaped upper surface at  416  and  418 , and a through-hole at  460 . Each of the microneedles  465  extends out from the planar surface  440  of the substrate  410 . 
     As can be seen in FIG. 22, substrate  410  can either be made much larger in height so as to have a very large internal volume for holding a fluid substance, or the substrate itself could be mounted onto a different material that has some type of fluidic opening that is in communication with the chambers  470  of the individual microneedles  465 . 
     It will be understood that other semiconductor substances besides silicon could be used for the fabrication of the array of microneedles depicted on FIG. 22, without departing from the principles of the present invention. Moreover, other microneedle shapes could be used than a cylindrical shape with an annular top surface, and in fact, the top surface of such microneedles could be sloped to create a sharper edge, if desired, without departing from the principles of the present invention. 
     It will also be understood that the preferred dimensions discussed hereinabove are only preferred, and any microneedle length or diameter that is appropriate for a particular chemical fluidic compound and for a particular skin structure could be used without departing from the principles of the present invention. As discussed above, it is preferred that the microneedle penetrate through the stratum corneum into the epidermis, but not penetrate into the dermis itself. This means that such microneedles would typically be no longer than two hundred (200) microns, though they must typically be at least fifty (50) microns in length. Of course, if cosmetic applications were desired, then the microneedle could be much shorter in length, even as short as one (1) micron. Finally, it will be understood that any size or shape of fluid-holding chamber could be used in a drug-delivery system, which will be further discussed hereinbelow. In addition, for a body-fluid sampling system, a fluid-holding chamber would also preferably be in communication with the through-holes  460  of each of the microneedles  465 . 
     FIG. 23 depicts an iontophoretically enhanced body-fluid sensor that is based upon a hollow microneedle array, generally designated by the reference numeral  500 . Sensor  500  includes a plurality of microneedles  530 , which are each hollow, having a vertical opening throughout, as indicated at  532 . A fluid chamber  510  is in communication with the hollow portions  532  of the array of microneedles  530 . 
     Fluid chamber  510  is constructed of a bottom (in FIG. 23) planar surface  512 —which has openings that are aligned with the microneedles  530 —a left vertical wall  514 , and a right vertical wall  516 . The top (or ceiling) of the fluid chamber  510  is made up of a planar material which is divided into individual electrodes. The middle electrode  525  is part of the fluid sensor, and makes it possible to measure a current or voltage within the fluid chamber  510 . Electrodes  520  and  522  are electrically connected to one another (and can be of a single structure, such as an annular ring) so as to act as the iontophoretic electrodes (i.e., as either an anode or a cathode) that facilitate the transport of fluid through the hollow microneedles  530  from the skin into the fluid chamber  510 . 
     The height of the fluid chamber structure is designated as “L 50 ,” which could be any reasonable dimension that is large enough to hold a sufficient volume of fluid for a particular application. Of course, if desired, the fluid chamber  510  could be connected to a much larger external reservoir (not shown), and a pump could even be used if pressure or vacuum is desired for a particular application. 
     The layer  540  represents the stratum corneum, the layer  542  represents the viable epidermis, and the largest layer  544  represents the dermis, which contains nerves and capillaries. 
     The application of microneedles  530  into the stratum corneum  540  and epidermis  542  decreases the electrical resistance of the stratum corneum by a factor of approximately fifty (50). The applied voltage, therefore, during iontophoresis can be greatly reduced, thereby resulting in low power consumption and improved safety. lontophoresis provides the necessary means for molecules to travel through the thicker dermis into or from the body. The combination of the microneedles and the electric field that is applied between the electrodes  520  and  522  (acting as an anode, for example) and a remotely placed electrode (e.g., electrode assembly  505 , viewed on FIG. 25, and acting as a cathode, for example) provides for an increase in permeability for both the stratum corneum and the deeper layers of skin. 
     While the transport improvement in stratum corneum is mostly due to microneedle piercing, the iontophoresis provides higher transport rates in the epidermis and dermis. This is not only true for small sized molecules, but also for the larger and more complex useful molecules. 
     The body-fluid sampling sensor  500  can be used for a continuous noninvasive measurement of blood glucose level, for example. Glucose is extracted through the skin by reverse iontophoresis, and its concentration is then characterized by a bioelectrochemical sensor. The sensor comprises the chamber  510  that is filled with hydrogel and glucose oxidase, and the electrode  525 . The glucose molecules are moved from the body by the flow of sodium and chloride ions caused by the applied electric potential. The detection of the glucose concentration in the hydrogel pad is performed by the bioelectrochemical sensor. 
     An alternative embodiment  550  is depicted in FIG. 24, in which the microneedles  580  are solid, rather than hollow. A fluid-filled chamber  560  is provided and also comprises hydrogel filled with glucose oxidase. The chamber  560  is made of a bottom wall  562  that has openings proximal to the individual microneedles  580 , in which these openings are designated by the reference numeral  585 . Chamber  560  also includes side walls  564  and  566 , as well as electrodes  570 ,  572 , and  575 . 
     The electrode  575  is constructed as part of the bioelectrochemical sensor. The electrodes  570  and  572  act as the iontophoretic electrodes, acting either as an anode or cathode to set up an electric current through the skin which flows to a remotely-attached (to the skin) electrode (e.g., electrode assembly  555 , viewed on FIG.  26 ). 
     As in the sensor  500  of FIG. 23, the transport rate of fluids is enhanced by not only the piercing effect of the microneedles  580 , but also the electric field inducing a current through the skin. In the glucose sampling example, glucose is attracted into the chamber  560 , and its concentration is measured by the bioelectrochemical sensor. 
     The height of the fluid chamber structure is designated as “L 55 ,” which could be any reasonable dimension that is large enough to hold a sufficient volume of fluid for a particular application. Of course, if desired, the fluid chamber  560  could be connected to a much larger external reservoir (not shown), and a pump could even be used if pressure or vacuum is desired for a particular application. 
     FIG. 25 depicts an iontophoretic electrode assembly that is based upon a hollow microneedle array, generally designated by the reference numeral  505 . Electrode assembly  505  includes a plurality of microneedles  531 , each being hollow and having a vertical opening throughout, as indicated at  533 . A fluid chamber  511  is in communication with the hollow portions  533  of the array of microneedles  531 . 
     Fluid chamber  511  is constructed of a bottom planar surface  513 —which has openings that are aligned with the microneedles  531 —a left vertical wall  515 , and a right vertical wall  517 . The top (or ceiling) of fluid chamber  511  is made of a planar electrode material  526 . The electrode  526  is to be electrically connected to a low-current voltage source (not shown on FIG.  25 ), either through a substrate pathway (such as a integrated circuit trace or a printed circuit foil path) or a wire (also not shown on FIG.  25 ). 
     The height of the fluid chamber  511  is given by the dimension “L 52 ,” which can be of any practical size to hold a sufficient amount of hydrogel, for example, to aid in the conduction of current while acting as the electrode. In electrode assembly  505 , the fluid within chamber  511  preferably would not be electrically charged. 
     As can be seen in FIG. 25, the hollow microneedles  531  penetrate the stratum corneum  540  and into the viable epidermis  542 . The microneedles  531  preferably will not be sufficiently long to penetrate all the way to the dermis  544 . 
     An alternative embodiment  555  is depicted in FIG. 26, in which the microneedles  581  are solid, rather than hollow. A fluid chamber  561  is provided and preferably is filled with hydrogel (which is not electrically charged). Chamber  561  is made of a bottom wall  563  that has openings proximal to the individual microneedles  581 , in which these openings are designated by the reference numeral  586 . Chamber  561  also includes side walls  565  and  567 , as well as a top (or ceiling) electrode  576 . The electrode  576  may act as a cathode, for example, in a situation where electrode assembly  555  is being used in conjunction with a body-fluid sensor, such as sensor assembly  550  viewed on FIG. 24, in which its electrodes  570  and  572  may act, for example, as an anode. The height “L 57 ” of fluid chamber  561  could be any reasonable dimension that is large enough to hold a sufficient volume of the hydrogel to enhance the fluid flow via the electric field between the respective anode and cathode of the system. 
     FIG. 27 illustrates a portion of a human arm and hand  590 , along with a drug delivery electrode assembly  500  and a second electrode assembly  505 . Both electrodes are attached to the skin of the human user, via their microneedles, such as the hollow microneedles  530  (viewed on FIG. 23) and the hollow microneedles  531  (viewed on FIG.  25 ). 
     Since an electrical voltage is applied between the two electrode assemblies  500  and  505 , it is preferred to use a low current power supply, generally designated by the reference numeral  596 , that is connected to each of the electrodes via a wire  592  or a wire  594 , respectively. It will be understood that any type of physical electrical circuit could be used to provide the electrical conductors and power supply necessary to set up an appropriate electrical potential, without departing from the principles of the present invention. In fact, the electrode assemblies and wiring, along with an associated power supply, could all be contained on a single apparatus within a substrate, such as that viewed on FIGS. 30 and 31 herein, or by use of printed circuit boards. 
     FIG. 28 depicts an iontophoretically enhanced fluidic drug delivery apparatus that is based upon a hollow microneedle array, generally designated by the reference numeral  600 . Drug-delivery apparatus  600  includes a plurality of microneedles  630 , which are each hollow, having a vertical opening throughout, as indicated at  632 . A fluid chamber  610  is in communication with the hollow portions  632  of the array of microneedles  630 . 
     Fluid chamber  610  is constructed of a bottom (in FIG. 28) planar surface  612 —which has openings that are aligned with the microneedles  630 —a left vertical wall  614 , and a right vertical wall  616 . The top (or ceiling) of the fluid chamber  610  is made up of a planar material  620  that acts as an electrode. Electrode  620  is part of the drug delivery apparatus, and makes it possible to induce a current flow through fluid chamber  610 . Electrodes  620  and  622  are connected so as to act as the iontophoretic electrodes (i.e., as either an anode or a cathode) that facilitate the transport of fluid through the hollow microneedles  630  from the fluid chamber  610  into the skin. 
     The height of the fluid chamber structure is designated as “L 60 ,” which could be any reasonable dimension that is large enough to hold a sufficient volume of fluid for a particular drug delivery application. Of course, if desired, the fluid chamber  510  could be connected to a much larger external reservoir (not shown), and a pump could even be used if pressure or vacuum is desired for a particular application. 
     The layer  540  represents the stratum corneum, the layer  542  represents the viable epidermis, and the largest layer  544  represents the dermis, which contains nerves and capillaries. 
     The application of microneedles  630  into the stratum corneum  540  and epidermis  542  decreases the electrical resistance of the stratum corneum by a factor of approximately fifty (50). The applied voltage, therefore, during iontophoresis can be greatly reduced, thereby resulting in low power consumption and improved safety. lontophoresis provides the necessary means for molecules to travel through the thicker dermis into or from the body. The combination of the microneedles and the electric field that is applied between the electrodes  620  and  622  (acting as anodes, for example), and another electrode (e.g., electrode assembly  505 , acting as a cathode) that is attached elsewhere on the skin of the user, provides for an increase in permeability for both the stratum corneum and the deeper layers of skin. While the transport improvement in stratum corneum is mostly due to microneedle piercing, the iontophoresis provides higher transport rates in the epidermis and dermis. This is not only true for small sized molecules, but also for the larger and more complex useful molecules. 
     The drug delivery apparatus  600  can be used for a continuous non-invasive medical device that can continuously deliver a fluidic drug through the skin and into the body. For example, insulin could be delivered to the blood stream via the microneedles  531 , through the stratum corneum  540  and epidermis  542 , and also into the dennis  544  where the insulin would be absorbed into the capillaries (not shown). 
     An alternative embodiment  650  is depicted in FIG. 29, in which the microneedles  680  are solid, rather than hollow. A fluid-filled chamber  660  is provided and also contains hydrogel. Chamber  660  is made of a bottom wall  662  that has openings proximal to the individual microneedles  680 , in which these openings are designated by the reference numeral  685 . Chamber  660  also includes side walls  664  and  666 , as well as electrodes  670 ,  672 , and  675 . 
     The electrode  675  is constructed as part of the bioelectrochemical sensor. The electrodes  670  and  672  act as the iontophoretic electrodes, acting either as the anode or cathode to set up an electric current through the skin, in conjunction with another electrode assembly (such as electrode assembly  655 , viewed on FIG. 26) placed elsewhere on the user&#39;s skin. 
     As in the drug delivery apparatus  600  of FIG. 28, the transport rate of fluids is enhanced by not only the piercing effect of the microneedles  680 , but also the electric field inducing a current through the skin. In the insulin dispensing example, insulin is repelled from the chamber  660 , and therefore, flows out through openings  685  proximal to microneedles  680 , then into the user&#39;s skin. 
     The height of the fluid chamber structure is designated as “L 65 ,” which could be any reasonable dimension that is large enough to hold a sufficient volume of fluid for a particular application. Of course, if desired, the fluid chamber  660  could be connected to a much larger external reservoir (not shown), and a pump could even be used if pressure or vacuum is desired for a particular application. 
     FIG. 30 depicts a closed-loop drug-delivery system generally designated by the reference numeral  700 . This closed-loop system  700  includes a pair of iontophoretic pads, generally designated by the reference numerals  500  and  505 , which each include an array of microneedles for fluid sampling. Pad  500  comprises a sensor assembly (as described hereinabove with respect to FIG.  23 ), and pad  505  comprises an electrode assembly (as described hereinabove with respect to FIG.  25 ). 
     Closed-loop system  700  also includes a pair of iontophoretic pads, generally designated by the reference numerals  600  and  605 , that each include an array of microneedles for drug delivery. Pad  600  comprises a drug delivery apparatus (as described hereinabove with respect to FIG.  28 ), and pad  505  comprises an electrode assembly (as described hereinabove with respect to FIG.  25 ). Of course, iontophoretic pads having solid microneedles could instead be used, such that pads  500  and  600  (with hollow microneedles) could be replaced by pads  550  and  650  (with solid microneedles), and pad  505  (with hollow microneedles) could be replaced by a pad  555  (with solid microneedles). 
     Pads  500  and  600  are mounted to a substrate  710 , which can be made of either a solid or a somewhat flexible material. Within substrate  710  preferably resides a reservoir  712  (within the substrate  710 ) that holds the fluid which is to be dispensed through the microneedles of pads  600 . Reservoir  712  could be made up of individual “small” chambers, such as a large number of chambers  610  that are connected to a source of fluidic drug. 
     It will be understood that the reservoir  712  preferably is completely contained within substrate  710 , and cannot be seen from this view of FIG.  31 . As an alternative, however, a fluid channel (such as a flexible at  730 ) could be connected into substrate  710  and, by use of a pump (not shown), further quantities of the fluid could be provided and dispensed through the microneedles of pads  600 , using fluidic pressure. 
     FIG. 31 illustrates the opposite side of the closed-loop system  700 . A controller  720  is mounted to the upper surface (in this view) of substrate  710 . Controller  720  preferably comprises a type of microchip that contains a central processing unit that can perform numeric calculations and logical operations. A microprocessor that executes software instructions in a sequential (or in a parallel) manner would be sufficient. A microcontroller integrated circuit would also suffice, or an ASIC that contains a microprocessor circuit. 
     Adjacent to controller  720  is an iontophoretic power supply with a battery, the combination being generally designated by the reference numeral  722 . In addition, a visual indicator can be placed on the surface of the substrate, as at  730 . This visual indicator could give a direct reading of the quantity of interest, such as glucose concentration, or some other body-fluid parameter. The visual indicator preferably comprises a liquid crystal display that is capable of displaying alphanumeric characters, including numbers. 
     While a pumping system that creates fluid pressure could be used for dispensing a fluidic drug into a body through hollow microneedles, such as emplaced on pads  600 , it is preferred to use an iontophoresis method to enhance the delivery of the drugs through the microneedles. As discussed hereinabove, application of microneedles can decrease the electrical resistance of the stratum corneum by a factor of fifty (50), and so the voltage necessary to facilitate iontophoresis can be greatly reduced, improving safety and requiring much less power consumption. By use of the iontophoresis, the molecules making up the fluid drug will travel through the thicker dermis into or from the body, and the combination of both transport-enhancing methods provides an increase in permeability for both the stratum corneum and the deeper layers of the skin. The transport improvement in the stratum corneum is mostly due to microneedle piercing, although the iontophoresis provides higher transport rates in the epidermis and dermis. 
     The closed-loop drug-delivery system and fluid-sampling system  700  can be used for continuous noninvasive measurement of blood glucose level by extracting, via reverse iontophoresis, glucose through the skin and measuring its concentration by the bioelectrochemical sensor (such as the sensor constructed of the hydrogel chamber  510  and sensor electrode  525 , along with the controller  720 ). The hydrogel pads containing microneedles (i.e., pads  500 ) enhance the reverse iontophoresis to move glucose molecules from the body by the flow of sodium and chloride ions, which are caused by the applied electric potential via electrodes  520  and  522 . Once the glucose concentration is measured within the hydrogel pads  500 , the proper amount of insulin, for example, can be dispensed through the other pair of pads  600  that make up part of the closed-loop system  700 . 
     As discussed hereinabove, drug delivery is performed by applying an electric potential between two microneedle array electrodes. One of the electrodes is filled with an ionized drug (such as insulin), and the charged drug molecules move into the body due to the electric potential. Controller  720  will determine how much of a drug is to be dispensed through the microneedle array  600  at any particular time, thereby making the closed-loop system  700  a “smart” drug-delivery system. 
     This smart drug-delivery system can be used as an artificial pancreas for diabetes patients, as a portable hormone-therapy device, as a portable system for continuous out-patient chemotherapy, as a site-specific analgesic patch, as a temporary and/or rate-controlled nicotine patch, or for many other types of drugs. Such systems could be made as a disposable design, or as a refillable design. 
     It will be understood that the closed-loop system  700  can be used in many applications, including as a painless and convenient transdermal drug-delivery system for continuous and controlled outpatient therapies, a painless and convenient body-fluid sampling system for continuous and programmed outpatient body-fluid monitoring, as a high-rate transdermal drug delivery system, or as a high-accuracy transdermal body-fluid sampling system. More specifically, the closed-loop system  700  of the present invention can be used as a portable high-accuracy painless sensor for outpatient blood glucose-level monitoring, as a portable system for continuous or rate controlled outpatient chemotherapy, as a temporary and rate controlled nicotine patch, as a site-specific controlled analgesic patch, as an externally attached artificial pancreas, as externally attached artificial endocrine glands, as temperature-controlled fever-reducing patches, as heart rate-controlled nitroglycerin high-rate transdermal patches, as temporarily controlled hormonal high-rate transdermal patches, as erectile dysfunction treatment high-rate transdermal patches, and as a continuous accurate blood-analysis system. Another use of the closed-loop system  700  of the present invention is to form a portable drug delivery system for outpatient delivery of the following drugs and therapeutic agents, for example: central nervous system therapy agents, psychic energizing drugs, tranquilizers, anticonvulsants, muscle relaxants and anti-parkinson agents, smoking cessation agents, analgetics, antipyretics and anti-inflammatory agents, antispasmodics and antiulcer agents, antimicrobials, antimalarias, sympathomimetric patches, antiparasitic agents, neoplastic agents, nutritional agents, and vitamins. 
     It will be understood that various materials other than those disclosed hereinabove can be used for constructing the closed-loop system  700 , and for constructing individual body-fluid sampling sensors and individual drug-delivery systems. Such other materials could include diamond, bio-compatible metals, ceramics, polymers, and polymer composites, including PYREX®. It will yet be further understood that the iontophoretically/microneedle-enhanced transdermal method of transport of the present invention can also be combined with ultrasound and electroporation, in order to achieve high-rate drug delivery into individual cells. 
     It will also be understood that the length of the individual microneedles is by far the most important dimension with regard to providing a painless and bloodless drug-dispensing system, or a painless and bloodless body-fluids sampling system using the opposite direction of fluid flow. While the dimensions discussed hereinabove are preferred, and the ranges discussed are normal for human skin, it will further be understood that the microneedle arrays of the present invention can be used on skin of any other form of living (or even dead) creatures or organisms, and the preferred dimensions may be quite different as compared to those same dimensions for use with human skin, all without departing from the principles of the present invention. 
     It yet will be understood that the chemicals and materials used in the molds and dies can be quite different than those discussed hereinabove, without departing from the principles of the present invention. Further, it will be understood that the chemicals used in etching and layering operations of microfabrication discussed above could be quite different than those discussed hereinabove, without departing from the principles of the present invention. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described in order to best illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.