Patent Publication Number: US-2004050701-A1

Title: Electrostatically guiding ionized droplets in chemical array fabrication

Description:
TECHNICAL FIELD  
       [0001] This invention relates to chemical arrays. In particular, the invention relates to chemical array manufacture using electrostatically guided droplets.  
       BACKGROUND ART  
       [0002] Droplet generators are used to deposit small amounts of different materials to deposition surfaces. The droplet generator can be a spraying apparatus, a fogger, or a precision applicator, such as an ink-jet print head.  
       [0003] An ink-jet print head is an element of an ink-jet printing device, which has the ability to project droplets of liquid material through nozzles and onto a surface, such that the generated droplets are launched in a trajectory to the surface. These devices are commonly used for printing ink onto paper, but are also used to dispense many chemical and biological materials, especially in chemical array manufacture. There are two common types of ink-jet heads, differing primarily in the mechanism used to produce the mechanical energy necessary to project the droplets. The first type of print head is sometimes called bubble-jet print head. It relies on a propellant in the liquid or ink that can be vaporized by a small electric resistive heater. When circuitry at the nozzle is activated, the microscopic heater creates a small vapor bubble, which displaces the liquid, thereby forcing the liquid through the nozzle where it is accelerated and projected as a small droplet onto the deposition surface. The bubble in the ink-jet print head rapidly collapses when the vapor suddenly cools and condenses. The second type of ink-jet print head uses a piezoelectric device to displace the liquid. A piezoelectric material in the nozzle changes shape when an electric field is applied thereto. When the electric field is applied, the piezoelectric material deforms to push the liquid in the form of a droplet out of the nozzle. As mentioned above, the generated droplets are launched or projected in a trajectory to a surface. The trajectory of a generated droplet controls the final position of the droplet on the surface. For conventional ink-jet equipment, the final position of a droplet on a surface is controlled only by the trajectory that the droplet takes from the ink-jet nozzle to the surface. Unfortunately, due to the manufacturing variations of the ink-jet nozzle and the buildup of deposits on the nozzle over time, variations in the final droplet positions are inevitable.  
       [0004] A spraying apparatus forces a liquid under pressure through a fine nozzle or an array of nozzles. The forced liquid generates liquid droplets from the nozzle. The droplets may move to their target in an unaided ballistic path, or they may be entrained in an airflow. The most common spraying apparatus is of the aspirator type. An aspirator apparatus produces small droplets by guiding an air stream across an orifice of a liquid inlet, thereby producing a vacuum and thus, drawing the liquid into the air stream. The liquid is broken into a spray of small droplets by the air stream. Another type of spraying apparatus accelerates a liquid stream through a small orifice with short bursts of pressure. These and other spraying apparatuses are well known in the art.  
       [0005] A fogger or aerosol generator is used to generate small liquid droplets (i.e., microdroplets) in a cloud. Fogs can be created by heating a liquid to the vapor phase and then cooling the vapor until condensation occurs. Other methods of generating a fog include atomizing the liquid into small droplets by launching high frequency sound into the liquid, such as by a piezoelectric device. These and other aerosol generators also are well known in the art.  
       [0006] Each of these droplet generators may be used to produce chemical arrays. Chemical arrays are comprised of a plurality of probes applied in a regular or spatially addressable pattern on a substrate. The probes are small spots or features of reagent, genetic, chemical or biological materials, which typically are used to test chemical or biological reactions. The precision with which the features correspond to a desired pattern can be important to the successful function of the array in an assay or test. Smaller features and closer packing of the features can allow a single array to contain more tests in a given area. This is desirable because smaller arrays utilize fewer materials in their manufacture and can react to smaller test samples sizes in their use. However, smaller features and closer packing of the features require higher precision to produce a spatially addressable pattern.  
       [0007] The droplets projected from an ink-jet nozzle are launched with the intent that they arrive at a predetermined deposition position. However, the precision of the ink-jet technology is based on how much a droplet actually overlaps the predetermined position. The same is true for the other droplet generation equipment. The distance between the intended deposition position and the final droplet position defines the placement error. Placement errors using ink-jet technology can be the result of a number of factors, such as vibration, variations in nozzle precision, air turbulence in the gap between the printhead and the deposition surface, and residues built up at the nozzle exit. Placement errors are a source of errors in final assay results. Assay errors are quite costly, since the expensive materials, scarce test samples, time and labor have already been expended. Further, placement errors significantly impact the ability to make the desirable smaller arrays that have more closely packed probe features.  
       [0008] Thus, it would be advantageous to have a technique that reduces the uncertainty in the placement of droplets using conventional droplet generator equipment. Such a technique would solve a long-standing problem in the art of making chemical arrays.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention electrostatically guides a droplet of a material to a location on an array of deposition sites. The present invention addresses droplet placement errors in the art when conventional equipment is used to generate droplets for chemical array fabrication. The droplet is electrostatically guided to a specific location on the array using both droplet ionization and a charge-influenced deposition site on the array.  
       [0010] An apparatus that electrostatically guides a chemical or biochemical material to be deposited in chemical array fabrication is provided. The apparatus comprises a substrate that comprises a photoconductive layer overlying an electrically conductive layer, and an electric charge differential generated on the surface of the photoconductive layer by selectively illuminating the surface with light. The electric charge differential induces or guides a droplet of the chemical or biochemical material having a charge to preferentially deposit on a location of the surface due to an electrostatic force.  
       [0011] Another apparatus that electrostatically guides a chemical or biochemical material to be deposited in chemical array fabrication is provided. The apparatus comprises a substrate that comprises a plurality of deposition sites spatially arranged in an array pattern on an array surface, and circuitry defined in the substrate that electrically influences the plurality of deposition sites. When a charge is supplied to the circuitry, a deposition site is influenced by the charge and an electric charge or field differential is created at the array surface. The electric charge or field differential guides an ionized droplet of the material to preferentially deposit on the electrically influenced deposition site or another location on the array surface due to an electrostatic force.  
       [0012] A deposition mask having an electrostatically changeable mask pattern is provided. In an embodiment, the deposition mask comprises the apparatus having a substrate including a photoconductive layer overlying a conductive layer. In this embodiment, the changeable mask pattern comprises a pattern of different electric charges for each selectively illuminated set of locations, a selected set of illuminated locations being provided for each chemical or biochemical material to be deposited and for each incidence that a same chemical or biochemical material is deposited during the chemical array fabrication. Each selected set has at least one illuminated location, wherein the at least one illuminated location is a same location or a different location relative to other selected sets. In another embodiment, the deposition mask comprises the apparatus having a substrate including a plurality of deposition sites and circuitry that electrically influences the plurality of sites. Each selected set has at least one deposition site, wherein the at least one deposition site of each selected set is one or both of a same deposition site and a different deposition site of the plurality relative to other selected sets.  
       [0013] A system for electrostatically guiding a chemical or biochemical material in chemical array fabrication is provided. In an embodiment, the system comprises the apparatus having a substrate including a photoconductive layer overlying a conductive layer. In this embodiment, the system further comprises one or more of a light source that provides the selective illumination to the surface; a source of charge or a sink for charge that provides an electric charge of the differential; and a droplet generator that provides the charged droplet of the chemical or biochemical material to the apparatus. In another embodiment, the system comprises the apparatus having a substrate including a plurality of deposition sites and circuitry that electrically influences the deposition sites. In this embodiment, the system further comprises a source of ionized droplets of the material; and control electronics comprising the source of charge. The control electronics one or more of supplies, removes and varies the charge applied to the circuitry. The control electronics optionally comprises charge-sensing circuitry that monitors a volume of the material that is accreted on the substrate surface.  
       [0014] A method of electrostatically guiding a chemical or biochemical material to be deposited in chemical array fabrication is provided. The method comprises generating an electric charge or field differential on an array; and exposing the array to an ionized droplet of the chemical or biochemical material. The array comprises a plurality of deposition sites on a surface of the array. The plurality of deposition sites is in a spatially addressable array pattern. The electric charge or field differential comprises a first charge that electrically influences a selected set of deposition sites of the plurality, and a second charge on an area surrounding the selected set. The first charge is different from the second charge. The ionized droplet has a charge, such that the electric charge or field differential guides the material to preferentially deposit on either the selected set of influenced deposition sites or the surrounding area due to an electrostatic force.  
       [0015] One or more of the following advantages may be realized by using the present invention. Higher precision deposition and volume control of droplets are possible from conventional ink-jet print heads, aerosol generators and sprayers than that provided without the present invention. The charged, or otherwise charged-influenced, surface patterns on the substrate essentially force the droplets into predefined regions, making the final position of the material more precise. When deposition is more precise, higher density array patterns that are denser than that conventionally used can be designed. When a higher density array pattern is used, the deposited materials, such as expensive biological probe materials, can be made smaller and more closely spaced, since placement error is reduced. Since the droplets can be smaller, less material is used for each chemical array, and less material is wasted. Moreover, the ‘soft’ deposition mask created by the changeable mask pattern of electric charges can be used for the mass deposition of materials according to the present invention. Separate physical masking steps using traditional masking materials are not necessary. After a deposition process, the charge applied to the pattern mask of the present invention can be removed and reapplied without the application of physical materials or requiring a chemical process. Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016] The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, where like reference numerals designate like structural elements, and in which:  
     [0017]FIG. 1 illustrates a cross sectional side view of an apparatus that electrostatically guides a material in chemical array fabrication according to an embodiment of the present invention.  
     [0018]FIG. 2A illustrates a cross sectional side view of the apparatus embodiment of FIG. 1 when illuminated with light.  
     [0019]FIG. 2B illustrates a cross sectional view of the apparatus embodiment of FIG. 1 after the illumination in FIG. 2A.  
     [0020]FIG. 2C illustrates a perspective view of the apparatus embodiment of FIGS. 2A and 2B.  
     [0021]FIG. 3 illustrates a perspective view of the apparatus embodiment of FIG. 2B after illumination is complete.  
     [0022]FIG. 4A illustrates a cross sectional side view of a portion of the apparatus embodiment of FIG. 3 with an ionized droplet deposited on an illuminated region.  
     [0023]FIG. 4B illustrates a perspective view of ionized droplets placed on the illuminated regions of the apparatus embodiment of FIG. 3.  
     [0024]FIG. 5A illustrates a perspective view of an apparatus that electrostatically guides a material in chemical array fabrication according to another embodiment of the present invention.  
     [0025]FIG. 5B illustrates a perspective view of an apparatus that electrostatically guides a material in chemical array fabrication according to another embodiment of the present invention.  
     [0026]FIG. 6 illustrates a cross sectional side view of an apparatus that electrostatically guides a material in chemical array fabrication according to another embodiment of the present invention.  
     [0027]FIG. 7 illustrates a block diagram of a method of electrostatically guiding a material in chemical array fabrication according to an embodiment of the present invention.  
     [0028]FIG. 8 illustrates a block diagram of a system for electrostatically guiding a material in chemical array fabrication according some embodiments of the present invention.  
     [0029]FIG. 9 illustrates a block diagram of an embodiment of the system of FIG. 8 including the apparatus embodiment of FIG. 1.  
     [0030]FIG. 10A illustrates a perspective view of an embodiment of the system illustrated FIG. 8 including the apparatus embodiment of either FIGS. 5A or  5 B.  
     [0031]FIG. 10B illustrates a cross sectional side view of an embodiment of the system including the apparatus embodiment of FIG. 6.  
     [0032]FIG. 10C illustrates a magnified view of a section labeled  10 C of the embodiment of the system illustrated in FIG. 10B. 
    
    
     DETAILED DESCRIPTION  
     Definitions  
     [0033] In the present application, unless a contrary intention appears, the following terms refer to the indicated characteristics. A “biopolymer” is a polymer of one or more types of repeating units. Biopolymers are typically found in biological systems and particularly include polysaccharides (such as carbohydrates), and peptides (which term is used to include polypeptides and proteins) and polynucleotides as well as their analogs such as those compounds composed of or containing amino acid analogs or non-amino acid groups, or nucleotide analogs or non-nucleotide groups. This includes polynucleotides in which the conventional backbone has been replaced with a non-naturally occurring or synthetic backbone, and nucleic acids (or synthetic or naturally occurring analogs) in which one or more of the conventional bases has been replaced with a group (natural or synthetic) capable of participating in Watson-Crick type hydrogen bonding interactions. Polynucleotides include single or multiple stranded configurations, where one or more of the strands may or may not be completely aligned with another. A “nucleotide” refers to a sub-unit of a nucleic acid and has a phosphate group, a 5 carbon sugar and a nitrogen containing base, as well as functional analogs (whether synthetic or naturally occurring) of such sub-units which in the polymer form (as a polynucleotide) can hybridize with naturally occurring polynucleotides in a sequence specific manner analogous to that of two naturally occurring polynucleotides. For example, a “biopolymer” includes DNA (including cDNA), RNA, oligonucleotides, and PNA and other polynucleotides as described in U.S. Pat. No. 5,948,902 and the references cited therein (all of which are incorporated herein by reference), regardless of the source. An “oligonucleotide” generally refers to a nucleotide multimer of about 10 to 100 nucleotides in length, while a “polynucleotide” includes a nucleotide multimer having any number of nucleotides. A “biomonomer” references a single unit, which can be linked with the same or other biomonomers to form a biopolymer (for example, a single amino acid or nucleotide with two linking groups one or both of which may have removable protecting groups). A biomonomer fluid or a biopolymer fluid reference a liquid containing either a biomonomer or biopolymer, respectively (typically in solution). A “biochemical” refers to a biomonomer, a biomonomer fluid, an oligonucleotide, an oligonucleotide fluid, a biopolymer, a biopolymer fluid, or any reagent used in the fabrication of a biological array. A “chemical” refers to any and all chemical substances used in the fabrication of a chemical array, including biochemicals used in the fabrication of a biological array.  
     [0034] An “array”, unless a contrary intention appears, includes any one-, two- or three-dimensional arrangement of addressable regions bearing a particular chemical moiety or moieties (for example, biopolymers such as polynucleotide sequences) associated with that region. An array is “addressable” in that it has multiple regions of different moieties (for example, different polynucleotide sequences) such that a region (a “feature” or “spot” of the array) at a particular predetermined location (an “address”) on the array will detect a particular target or class of targets (although a feature may incidentally detect non-targets of that feature). Array features are typically, but need not be, separated by intervening spaces. In the case of an array, the “target” will be referenced as a moiety in a mobile phase (typically fluid), to be detected by probes (“target probes”) which are bound to the substrate at the various regions. However, either of the “target” or “target probes” may be the one that is to be evaluated by the other (thus, either one could be an unknown mixture of polynucleotides to be evaluated by binding with the other). An “array layout” refers to one or more characteristics of the features, such as feature positioning on the substrate, one or more feature dimensions, and an indication of a moiety at a given location. “Hybridizing” and “binding”, with respect to polynucleotides, are used interchangeably. A “region” refers to any finite small area on the array that can be illuminated and any resulting fluorescence therefrom simultaneously (or shortly thereafter) detected, for example a pixel.  
     [0035] When one item is indicated as being “remote” from another, this is referenced that the two items are at least in different buildings, and may be at least one mile, ten miles, or at least one hundred miles apart. “Communicating” information references transmitting the data representing that information as electrical signals over a suitable communication channel (for example, a private or public network). “Forwarding” an item refers to any means of getting that item from one location to the next, whether by physically transporting that item or otherwise (where that is possible) and includes, at least in the case of data, physically transporting a medium carrying the data or communicating the data.  
     [0036] Reference to a singular item, includes the possibility that there are plural of the same items present. “May” means optionally. Methods recited herein may be carried out in any order of the recited events, which is logically possible, as well as the recited order of events. All patents and other references cited in this application are incorporated into this application by reference except insofar as they may conflict with those of the present application (in which case the present application prevails).  
     Array Description  
     [0037] Any given substrate may carry one, two, three, four or more arrays disposed on a front surface of the substrate. Depending upon the use, any or all of the arrays may be the same or different from one another and each may contain multiple spots or features. A typical array may contain more than ten, more than one hundred, more than one thousand, more ten thousand features, or even more than one hundred thousand features, in an area of less than 20 cm 2  or even less than 10 cm 2 . For example, features may have widths (that is, diameter, for a round spot) in the range from a 10 μm to 1.0 cm. In other embodiments, each feature may have a width in the range of 1.0 μm to 1.0 mm, usually 5.0 μm to 500 μm, and more usually 10 μm to 200 μm. Non-round features may have area ranges equivalent to that of circular features with the foregoing width (diameter) ranges. At least some, or all, of the features may be of different compositions (for example, when any repeats of each feature composition are excluded the remaining features may account for at least 5%, 10%, or 20% of the total number of features). Interfeature areas will typically (but not essentially) be present which do not carry any polynucleotide (or other biopolymer or chemical moiety of a type of which the features are composed). Such interfeature areas typically will be present where the arrays are formed by processes involving drop deposition of reagents but may not be present when, for example, photolithographic array fabrication processes are used. It will be appreciated though, that the interfeature areas, when present, could be of various sizes and configurations.  
     [0038] Each array may cover an area of less than 100 cm 2 , or even less than 50 cm 2 , 10 cm 2  or 1 cm 2 . In many embodiments, the substrate carrying the one or more arrays will be shaped generally as a rectangular solid (although other shapes are possible), having a length of more than 4 mm and less than 1 m, usually more than 4 mm and less than 600 mm, more usually less than 400 mm; a width of more than 4 mm and less than 1 m, usually less than 500 mm and more usually less than 400 mm; and a thickness of more than 0.01 mm and less than 5.0 mm, usually more than 0.1 mm and less than 2 mm and more usually more than 0.2 and less than 1 mm. With arrays that are read by detecting fluorescence, the substrate may be of a material that emits low fluorescence upon illumination with the excitation light. Additionally in this situation, the substrate may be relatively transparent to reduce the absorption of the incident illuminating laser light and subsequent heating if the focused laser beam travels too slowly over a region. For example, the substrate may transmit at least 20%, or 50% (or even at least 70%, 90%, or 95%), of the illuminating light incident on the front as may be measured across the entire integrated spectrum of such illuminating light or alternatively at 532 nm or 633 nm.  
     [0039] Arrays can be fabricated using drop deposition from pulse jets of either polynucleotide precursor units (such as monomers) in the case of in situ fabrication, or the previously obtained polynucleotide. Such methods are described in detail in, for example, the references including U.S. Pat. Nos. 6,242,266; 6,232,072; 6,180,351; 6,171,797; and 6,323,043; U.S. patent application Ser. No. 09/302,898, filed Apr. 30, 1999, by Caren et al.; and the references cited therein. As already mentioned, these references are incorporated herein by reference. Other drop deposition methods can be used for fabrication, as previously described herein. Also, instead of drop deposition methods, photolithographic array fabrication methods may be used, such as described in U.S. Pat. Nos. 5,599,695; 5,753,788; and 6,329,143. Interfeature areas need not be present particularly when the arrays are made by photolithographic methods as described in those patents.  
     Reading Array Material  
     [0040] Following receipt by a user, an array made by an apparatus or a method of the present invention will typically be exposed to a sample (for example, a fluorescently labeled polynucleotide or protein containing sample) and the array then read. Reading of the array may be accomplished by illuminating the array and reading the location and intensity of resulting fluorescence at multiple regions on each feature of the array. For example, a scanner may be used for this purpose, which is similar to the AGILENT MICROARRAY SCANNER manufactured by Agilent Technologies, Palo Alto, Calif. Other suitable apparatus and methods are described in U.S. patent applications Ser. No. 10/087,447, “Reading Dry Chemical Arrays Through The Substrate” by Corson et al.; and Ser. No. 09/846,125, “Reading Multi-Featured Arrays” by Dorsel et al. However, arrays may be read by any other method or apparatus than the foregoing, with other reading methods including other optical techniques (for example, detecting chemiluminescent or electroluminescent labels) or electrical techniques (where each feature is provided with an electrode to detect hybridization at that feature in a manner disclosed in U.S. Pat. Nos. 6,251,685 and 6,221,583, and elsewhere). A result obtained from the reading may be used in that form or may be further processed to generate a result such as that obtained by forming conclusions based on the pattern read from the array (such as whether or not a particular target sequence may have been present in the sample, or whether or not a pattern indicates a particular condition of an organism from which the sample came). A result of the reading (whether further processed or not) may be forwarded (such as by communication) to a remote location if desired, and received there for further use (such as further processing).  
     Modes For Carrying Out The Invention  
     [0041] The present invention provides for electrostatically guiding droplet placement onto an array surface in chemical or biological array fabrication. The present invention creates differences in electric charge and/or electric field at the array surface on which the droplets are deposited, so that the droplets favor desired positions based on the charge or the electric field. The droplets can comprise one or more of a liquid medium and an aerosol medium, for example. The droplets are ionized, such that they have an overall charge polarity. The composition of the droplets is dependent on the intended use of the droplets and the array and is not a limitation to the invention. For example, the droplets can be any one or more of a variety of chemicals and/or biochemicals that are used for a variety of purposes. In particular, the material of the droplets may be a reagent, a chemical composition, and/or a biochemical composition used in the fabrication or manufacture of chemical or biological probes arrays. Further, the material droplet may be used in the subsequent processing of the chemical or biological probe arrays for at least one or more of analytical, diagnostic, and therapeutic purposes.  
     [0042] Accordingly, an apparatus that electrostatically guides ionized droplets of a material to be deposited in chemical array fabrication is provided. In some embodiments, the apparatus comprises a substrate that comprises a photoconductive layer overlying a conductive layer. The conductive layer is electrically conductive, while the photoconductive layer is an electrical insulator or is electrically nonconductive in the absence of light.  
     [0043] The insulative or nonconductive photoconductive layer is chargeable with an electrostatic charge on an array surface of the photoconductive layer. The array surface is an exposed surface of the photoconductive layer, which is opposite to a surface of the photoconductive layer that is adjacent to the underlying conductive layer. Moreover, the photoconductive layer can be made electrically conductive in a location by illuminating the location with light, thereby allowing the electrostatic charge on the array surface to pass or dissipate through the photoconductive layer to the conductive layer at the illuminated location. The charge on the illuminated location is therefore different from the electrostatic surface charge on a remaining non-illuminated surface area. An electric charge differential and pattern thereof are created on the array surface when a location is illuminated with light. The electric charge differential and pattern thereof can be effectively erased and reapplied as the same or a different pattern by discharging the charged surface, reapplying an electrostatic surface charge on the array surface of the photoconductive layer and illuminating the same or a different location on the photoconductive layer with light. Any photoconductor material may be useful for these embodiments of the present invention including, but not limited to, a calcognide class of materials; organic photoconductors (OPC); amorphous silicon; selenium alloys, such as selenium tellurium (SeTe); zinc oxide; calcium sulfide; and pure sulfur, for example, provided that the particular photoconductive material used does not interfere with the chemistry of chemical or biological array fabrication, including in situ oligonucleotide synthesis and other microarray chemistry. Other photoconductive materials not listed herein, which are known to those skilled in the art, also may be within the scope of the present invention.  
     [0044] For the purposes of the present invention, a photoconductive material differs from other photosensitive materials in that other photosensitive materials use the photoelectric effect to charge a surface of the material, as described by Loewy et al. in U.S. Pat. No. 6,004,752. In contrast, a photoconductive material uses a photoconductive effect, wherein photoconductive material resists the passage of electric current unless exposed to light. In the photoelectric effect, free electrons are stripped or emitted from a surface of the photosensitive material when subjected to light, giving a positive charge to the surface of the photosensitive material. However in the photoconductive effect, the light changes the electrical conductivity of the photoconductive material from being electrically insulating to being electrically conductive in the illuminated location. When the conductivity is changed during illumination, a charged surface of the photoconductive layer is discharged to a conductive layer through the electrically conductive illuminated location of the photoconductive layer. The photoconductive effect is used in Xerography, for example. This electrically conductive feature of the photoconductive material allows the surface at the illuminated location to acquire the charge present on the conductive layer.  
     [0045] An array pattern of locations can be defined on the surface of the photoconductive layer with light. The array pattern of locations comprises a plurality of deposition sites on the array surface of the photoconductive layer. The plurality of deposition sites is spatially arranged in an array pattern that is spatially addressable on the array surface. Either a deposition site or an area surrounding the deposition site corresponds to or is an illuminated location, whether the location is a presently illuminated location, a previously illuminated location or a subsequently illuminated location (i.e., ‘subsequently’ as in ‘to be’ illuminated for fabrication of a chemical array). The light illuminates the deposition site or the surrounding area until the electrical conductivity of the illuminated location changes and an electric charge changes on the array surface of the photoconductive layer at the illuminated location. The period of time for the illumination to effect a change in conductivity and in electric charge will depend on the photoconductive material chosen and is not intended to limit the scope of the present invention. The period of time may be referred to herein as ‘momentary’ with the understanding that this ‘momentary’ period of time is preferably. The conductivity of the material at the location returns to being electrically nonconductive when the illumination is removed. The different charge on the array surface at the illuminated location will eventually discharge but will remain for a limited time, depending on the photoconductive material. The ‘limited time’ is preferably at least an amount of time sufficient to electrostatically guide ionized droplets in accordance with the invention. Either a deposition site or a surrounding area is distinguished or delineated for each electric charge differential pattern to be created. For the purposes of simplicity only, and not by limitation, hereinafter the illuminated location is generally referred to as the deposition site or the plurality of deposition sites. While the deposition site or the plurality of deposition sites is referred to as being the illuminated location(s) to change the electric charge, it should be understood that either a deposition site or the area surrounding the deposition site can be illuminated to achieve the electric charge differential or a charge differential pattern according to the invention.  
     [0046] Each deposition site is rendered electrically conductive and is effectively electrically connected to the conductive layer when illuminated with light, such that the illuminated deposition site can acquire the electric charge of the underlying conductive layer. The conductive layer is chargeable with a first electric charge that electrically influences the illuminated or distinguished deposition sites of the plurality. The conductive layer may be attached to ground, such that first electric charge is of zero potential or a neutral charge. Alternatively, the conductive layer may have a charge with a positive polarity or a negative polarity and a potential greater than zero applied thereto. The electric charge on the conductive layer is preferably an electrostatic charge, although a non-static or variable electric charge on the conductive layer is within the scope of the present invention. The insulative surface of the photoconductive layer is chargeable with a second electric or electrostatic charge, which is different from the first charge. For example, the insulative surface may have no charge provided thereto, such that the second charge is of zero potential or a neutral charge. Alternatively, the insulative surface may have a charge with a potential greater than zero and a positive or a negative polarity. The electric charge on the insulative surface is an electrostatic charge. The difference between the first charge and the second charge may be a difference in charge polarity and/or a difference in charge potential. Preferably, the charge polarities are different.  
     [0047] During chemical or biological array fabrication, the chemical or biochemical material is deposited as an ionized droplet that also has a charge. The charge on the ionized droplets facilitates electrostatic guidance according to the invention. When the electric charge differential is generated or created on the surface of the photoconductive layer by selective illumination, as described above, the electric charge differential guides the ionized droplet of the material to preferentially deposit on an illuminated location or a non-illuminated location of the surface, such as either a deposition site or the non-illuminated remainder of the photoconductive layer surface, due to an electrostatic force. Consequently, the deposited material chemically bonds to the photoconductive layer surface at the preferred location.  
     [0048] In other embodiments, an apparatus comprises an electrically insulative substrate comprising a spatially arranged and/or spatially addressable array pattern of feature locations or deposition sites on an array surface of the apparatus. An area of the array surface that essentially surrounds the spatially arranged deposition sites is relatively nonconductive. The substrate further comprises electrically conductive circuitry associated with the plurality of deposition sites. The circuitry is defined in the substrate. By ‘defined in the substrate’, it is meant that the circuitry, or a portion thereof, is embedded in the substrate or provided adjacent to a surface of the substrate. By ‘adjacent’ it is meant that the circuitry, or a portion thereof is on or just under the surface. The circuitry is electrically connectable to a source of charge and is electrically chargeable. Generally, the source of charge is external to the substrate. The circuitry is ‘associated with’ the plurality of deposition sites in that the circuitry either directly or indirectly electrically influences a deposition site of the plurality with an electric charge from the source of charge. By ‘electrically influence’ it is meant that a deposition site acquires the electric charge of the circuitry or the electric charge applied to the circuitry generates an electric field at a deposition site.  
     [0049] Hereinafter, the electric charge provided to the circuitry to directly or indirectly influence a deposition site is a first electric charge. The surrounding area of the array surface may or may not be charged with an electrostatic surface charge. Hereinafter, the electrostatic surface charge on the surrounding area is called a second charge, which is different from the first charge. The second charge may be of zero potential (or have a neutral charge), such as when the surrounding surface is not charged, or have a positive or negative polarity and a potential greater than zero, when the surrounding surface is charged. Moreover, the first charge may be zero potential (or have a neutral charge), such as when the circuitry is attached to ground, or have a positive or negative polarity and a potential greater than zero. Preferably, the first charge is a variable or non-static charge as compared to an electrostatic charge, but an electrostatic first charge is within the scope of these embodiments. The first charge differs from the second charge in the charge polarity and/or the charge potential. Preferably, the first charge has different charge polarity from the second charge.  
     [0050] During chemical array fabrication using these embodiments of the apparatus, a chemical or biochemical material is deposited on the array surface as an ionized droplet of the material having a charge. The charge on the ionized droplet facilitates electrostatic guidance according to the invention. When one or both of the first charge and the second charge is applied to the substrate, an electric charge or field differential is created on the array surface. The electric charge or field differential guides the ionized droplet of the material to preferentially deposit on either a deposition site or the surrounding area due to an electrostatic force. Consequently, the deposited material chemically bonds to the array surface at the preferred location.  
     [0051] In either apparatus embodiments of the present invention, the first charge and the second charge preferably are of opposite polarity to each other. In some embodiments of the apparatus, either the first charge or the second charge is zero or neutral, when the other of the first charge and the second charge is not. The ionized droplets of the material to be deposited are generated from any one or more of the conventional droplet generators discussed herein, but the droplets are also ionized before, during or after droplet generation. The charge of the ionized droplets facilitates electrostatic guidance of the ionized droplet to a desired location on the apparatus. For example, the charge of the ionized droplet is made sufficiently different from the first charge or the electric field created by the first charge, such that the ionized droplet is electrostatically attracted to a deposition site influenced by the first charge rather than a surrounding area. Moreover or alternatively, the charge of the ionized droplets is made sufficiently similar to the second charge, such that the ionized droplet is electrostatically repelled by the surrounding area into a deposition site. As such, the electrostatic force guiding the ionized droplet to a particular location on the array surface is an attractive force, a repulsive force, or both an attractive force and a repulsive force.  
     [0052]FIG. 1 illustrates a cross sectional view of an apparatus  100  according to some embodiments. The apparatus  100  comprises a substrate  110  that comprises a photoconductive layer  130  overlying a conductive layer  120 . The substrate  110  may further comprise additional layers (not shown) for chemical, biological, mechanical, structural or other purposes. As mentioned above, the conductive layer  120  is electrically conductive, and the photoconductive layer  130  is essentially electrically nonconductive or an insulator in the absence of light. In a location on the photoconductive layer  130  that is illuminated with light, the photoconductive layer  130  becomes momentarily electrically conductive, thereby allowing electric charge to pass through the photoconductive layer  130  at the illuminated location during the illumination, as illustrated in FIG. 2A. While momentarily illuminated, the electrically conductive location is effectively connected to the conductive layer  120  to change the charge of the illuminated location. The conductive layer  120  is selectively connectable to ground and/or a source of charge. The illuminated location acquires the charge of the conductive layer during illumination.  
     [0053] In some embodiments, the conductive layer  120  is a solid, continuous conductive plate and the photoconductive layer  130  is an insulative layer on the conductive plate  120 . In some of these embodiments, once a chemical array of biopolymers is fabricated on the photoconductive layer  130  surface, the photoconductive layer  130  is separable from the conductive plate  120  for use in an subsequent assay, for example a hybridization assay. In others of these embodiments, the conductive layer  120  is a solid, continuous layer that remains with the photoconductive layer  130 . Once the chemical array of biopolymers is fabricated on the photoconductive layer surface, the conductive layer  120  provides support to the photoconductive layer  130  during a subsequent assay.  
     [0054] However, it is within the scope of this embodiment of the apparatus  100  for the conductive layer  120  to be a circuit layer that comprises a pattern of circuitry on an electrically insulative support. The circuitry pattern comprises conductive circuit pads at least underlying each of the illuminated locations and conductive circuit paths that ultimately and selectively connect the circuit pads to ground and/or a source of charge (not shown). The conductive layer  120  (in particular, the circuit layer embodiment) provides for selective application of differing first charges to selected sites of the illuminated deposition sites. Therefore for the purposes of the present invention, reference to the ‘conductive layer  120 ’ includes the conductive plate and the circuit layer described above and is not intended to limit the scope of the present invention to only the solid, continuous conductive layer embodiments.  
     [0055] As mentioned above, the conductive layer  120  is chargeable with a first charge. The insulative photoconductive layer  130  is chargeable on the surface with a second charge that is different from, and preferably has an opposite polarity to the first charge. Also as mentioned above, the photoconductive effect permits a statically charged surface on the non-illuminated photoconductive layer  130  to be discharged or dissipated to the conductive layer  120  through the photoconductive layer  130  at an illuminated location  134  during illumination. The conductive illuminated location  134  acquires the charge of the underlying conductive layer  120 , as illustrated in FIG. 2A.  
     [0056] For example, consider that the photoconductive layer  130  has second charge of a negative polarity applied to a surface  132  thereof, while the underlying conductive layer  120  has a first charge of a positive polarity applied thereto. Depending on the application, the charge polarities on the conductive layer  120  and the photoconductive layer  130  could be reversed, or the conductive layer  120  could be connected to ground, without altering the concept of the present invention. FIGS. 1 and 2A illustrate a positive first charge applied to the conductive layer  120  and a negative second charge applied to the surface  132  for the purpose of this example only. The first charge can be applied to the conductive layer  120  through a direct electrical connection to a voltage source, a current source or other charge generating means or source of charge known in the art. The voltage or current source preferably has a zero volts or zero amps selection to optionally and effectively connect the conductive layer  120  to ground, when desired. Alternatively, the first charge may be applied to the conductive layer  120  via any indirect means for generating a charge known in the art, without limiting the scope of the present invention.  
     [0057] To make the apparatus  100 , the photoconductive layer  130  is protected from ambient light during the charging processes. A negative or positive second charge is applied to the surface  132  of the photoconductor layer  130  in the absence of light (i.e., while in the insulating state), for example using a corona wire or other technique known to one skilled in the art. For applying an electrostatic charge to a material, see for example, the techniques described in U.S. Pat. No. 6,004,752, incorporated by reference in its entirety herein.  
     [0058] Moreover, the conductive layer  120  is electrically connected to ground or a source of charge to produce a first charge that is different from the second charge in the conductive layer  120  while the photoconductive layer  130  is protected from ambient light. As mentioned above, a positive first charge and a negative second charge are illustrated in FIGS. 1 and 2A, by way of example. Then, a light  150  is applied to a location  134  on the surface  132  of the insulating photoconductive layer  130 , while the remainder of the photoconductive surface  132  is protected from the light  150 . When the light  150  is applied to the location  134 , as shown in FIG. 2A, the illuminated location  134  becomes electrically conductive during illumination and the second charge on the photoconductive layer surface  132  will be discharged or dissipated through the photoconductive layer  130  to the underlying conductive layer  120  at that location  134 . The arrows pointing to the conductive layer  120  in FIG. 2A illustrate the discharging of the negative second charge into the conductive layer  120 . The second charge on the remainder of the surface  132  remains unchanged. The charge of the illuminated location  134  of the photoconductive layer  130  changes to or acquires the first charge of the underlying conductive layer  120  during illumination. The arrows pointing to the photoconductive layer  130  within the dashed lines in FIG. 2A illustrate the acquisition of the positive first charge at the surface where illuminated. In this embodiment, the light  150  is collimated or focused light beam.  
     [0059]FIG. 2A illustrates a cross sectional side view of the apparatus  100  when illuminated in a location  134 . In effect, the illuminated location  134  is electrically connected to the underlying conductive layer  120  during illumination. The conductive layer  120  electrically influences the illuminated location  134  by providing the first charge that the illuminated location  134  ultimately acquires during illumination. This first charge is different from the charge before illumination and therefore, is different from the second charge on a non-illuminated remainder of the surface  132  surrounding the illuminated location  134 .  
     [0060] Once the negative second charge is conducted or discharged through the photoconductive layer  130  to the conductive layer  120 , such that the illuminated location  134  acquires the positive first charge at the surface, the light  150  is removed or terminated. The electrical conductivity of the illuminated location  134  returns to being electrically nonconductive when the illumination is terminated. Further, the positive first charge on the conductive layer  120  preferably is removed and/or terminated after or when the illumination is terminated. FIG. 2B illustrates the cross sectional view of the apparatus  100  after illumination of the location  134  from FIG. 2A. After illumination, the photoconductive layer  130  comprises a negative second charge surrounding a positive first charge at the surface  132 , and the conductive layer  120  no longer has a positive first charge.  
     [0061] The light  150  can illuminate a plurality of locations either by moving the light to predetermined locations for illumination, or by moving the substrate  110  relative to the light  150  to illuminate the predetermined locations, such as with a computer controlled movable stage. Alternatively, a mask can be applied to the surface  132  of the photoconductive layer  130 . The mask protects the surface from the light  150  except for a predetermined pattern of locations formed in the mask. In this alternative embodiment, the substrate and/or the light need not be moved relative to the other to create a plurality of illuminated locations  134 . Moreover, the light  150  illuminates all or a subset of the predetermined locations at one time and need not be a focused or collimated beam.  
     [0062] The beam of light  150  is generated by a light source. There are many light sources and illumination techniques that can be used for the invention ranging from those light sources that produce the focused or collimated light beam, such as a laser, to other light sources that produce non-collimated light. The intensity of the light need not be more than is necessary to effect a change in the conductivity of (i.e., activate) the photoconductive layer. Moreover, the wavelength of the light is that which would not be detrimental to the chemical or biochemical materials to be deposited. As mentioned above, when using a non-collimated light source, a negative or a mask layer is used to produce the illuminated image(s). Moreover, a light source having an addressable array of small light-emitting devices (e.g., light emitting diodes (LEDs)) may be utilized to supply the light needed to stimulate the photoconductive effect on the substrate  110 . When the array light source is used, it is held with a light-emitting surface thereof in close proximity to the surface  132  of the photoconductive layer  130 . The array light source illuminates the photoconductive layer  130  at the predetermined locations  133  corresponding to the desired deposition sites  134 . In a preferred embodiment, the array light source has 1 to 500 micron spacing between centers and during illumination, there is 0 to 200 microns or more distance between the light emitting surface and the surface  132  of the photoconductive layer  130 . Any of the illumination processes are repeatable until all of the desired deposition sites  134  have had a change in electric charge at the surface  132  for each chemical or biochemical material to be deposited.  
     [0063]FIGS. 2A and 2C illustrate a point or collimated beam of light  150  on the illuminated location  134  with the rest of the surface otherwise being protected from the beam of light  150 . A scanning laser can provide a focused and precisely located beam of light  150  to each of the illumination locations  133 . A mask or other protective means may be used to block the light from the surrounding or non-illuminated area  136 . Typically, a photomask that defines the pattern of predetermined locations is used to block the light from the surface of the photoconductive layer  130  except at locations where a change in charge is desired.  
     [0064]FIG. 2C illustrates a perspective view of the apparatus  100  being illuminated with a collimated beam of light  150  wherein a plurality of locations  133 ,  134 ,  135  are in different stages of the process of being created. The conductive layer  120  and the photoconductive layer  130  overlying the conductive layer  120  are illustrated with an exemplary array pattern of illumination locations  133 , some of which have been illuminated  134  and others, illustrated as dashed circles  135 , are to be illuminated by the light  150 . The illuminated locations  134  have the changed or first charge and the non-illuminated surrounding area  136  on the surface  132  has the original second charge (not shown in FIG. 2C). For the example above, the illuminated locations  134  have the positive first charge of the conductive layer  120  and the non-illuminated surface  136  has the negative second charge.  
     [0065] The array pattern illustrated in FIG. 2C exemplifies one of many different regular, spatially addressable patterns that can be formed according to the invention. One skilled in the art is familiar with array patterns, especially those used for biological array fabrication. All such patterns are within the scope of the present invention. The pattern depicted in FIG. 2C is neither to scale in size of the substrate  110  nor in number of the illumination locations  133 . Further, the shape of the apparatus  100  illustrated in the Figures can range anywhere from round to square and rectangular to elliptical, for example. Therefore, the illustrated sizes, shapes and numbers are not intended to limit the scope of the invention in any way. In some embodiments, the pattern created on the surface of the apparatus  100  can be created by a pulsed, scanning laser. As stated before, this effect could be produced by any method capable of selectively illuminating regions  133  on the apparatus surface  132 .  
     [0066]FIG. 3 illustrates a perspective view of the apparatus  100  with its exemplary array of illumination locations  133 , all of which being distinguished or delineated by illumination (locations  134 ) on the photoconductive layer  130 . The illuminated locations  134 , also referred to herein as deposition sites or regions, have the first charge that is different from the second charge of the surrounding non-illuminated surface area  136 . The apparatus  100  effectively comprises an electric charge differential on the surface  132  of the photoconductive layer  130 . In accordance with the invention, the apparatus  100  electrostatically guides ionized droplets of the material to be deposited to either the deposition site  134  or the surrounding non-illuminated remainder of the surface  136  using an electrostatic force. The ionized droplets have a charge that facilitates the electrostatic guidance of the droplets. One or both of an attractive force between opposite or different charges and a repulsive forces between the same or less different charges may be employed to guide the ionized droplets during chemical array fabrication according to the invention. For example, the ionized droplets are ionized with a charge that is opposite to or different from the first charge on the deposition sites  134 , when the material of the ionized droplets is to be deposited on the deposition sites  134 . Alternatively or additionally, the charge of the ionized droplets is the same as or similar to the second charge on the non-illuminated surrounding area  136  when the material is to be deposited on the deposition sites  134 . Since opposite charges attract and same charges repel, the ionized droplets will more likely, and advantageously, more accurately be placed on the deposition sites  134  of different charge instead of the non-illuminated surrounding area  136  of similar charge on the surface  132 .  
     [0067] The ionized droplets essentially are guided into position by their inherent attraction to the differently charged deposition sites  134  and their inherent repulsion from the similarly charged non-illuminated surrounding surface  136 . FIG. 4A illustrates a cross sectional side view of the apparatus  100  having an ionized droplet  144  with a negative charge on the surface  132  of the photoconductive layer  130 , wherein the ionized droplet  144  is more accurately positioned over the differently charged deposition site  134  than if no charges existed. By more accurate, it is meant that the ionized droplet  144  overlaps the illuminated location  134  consistently more than the overlap attained without the present invention. The dashed outline of the ionized droplet  144  is provided in FIG. 4A to exemplify a less accurate position and overlap of the droplet  144  without the benefit of the present invention. The directional arrow emphasizes that the present invention improves position accuracy.  
     [0068] The present invention works particularly well using both attractive and repulsive forces together to guide the ionized droplets into charged desired locations on the surface  132  when the undesired locations are also charged.  
     [0069] It should be noted that the present invention works well for using a repulsive electrostatic force alone to guide the ionized droplets into uncharged or neutrally charged desired locations on the surface  132  when undesired locations are charged. As such, using just a repulsive electrostatic force, the second charge alone on the non-illuminated area  136  can guide similarly charged ionized droplets into deposition sites  134  having a zero potential or neutral charge, such as when the conductive layer  120  is attached to ground during the illumination process. Likewise, the first charge alone on the deposition sites  134  can guide similarly charged ionized droplets into the non-illuminated area  136  having no charge, such as when an electrostatic charge is not applied to the surface  132 .  
     [0070] Moreover, the present invention also works well using an attractive electrostatic force alone to guide the ionized droplets into charged desired locations on the surface  132  when the undesired locations on the surface  132  are not charged or are neutrally charged. As such, the second charge alone on the non-illuminated area  136  can guide differently charged ionized droplets to the non-illuminated area  136 , when the deposition sites are neutrally charged. Likewise, the first charge alone on the deposition sites  134  can guide differently charged ionized droplets into the deposition sites  134 , when the non-illuminated area  136  is neutrally charged.  
     [0071] The electric charge differential created on the surface  132  by the apparatus  100  effectively acts as a deposition or pattern mask for the application of different materials or the same material at different times to the substrate  110 . Therefore, after a first material is applied to a first created electric charge differential mask pattern, the charge pattern can be erased, changed or reapplied advantageously without the application of physical materials or requiring a chemical process to do so. For example, a second material is deposited on a second selected set of deposition sites  134  that comprises fewer or more deposition sites  134 , having the same and/or different deposition sites than those that received the first material. A second charge differential pattern mask is created by reapplying the second charge to the entire surface  132 , illuminating the deposition sites  134  of the second selected set with light, as described above, to change the charge at the selected illuminated locations  134  to the first charge, and depositing ionized droplets of the second material on the substrate  110  having the second electric charge differential pattern mask on the surface  132 . This procedure is repeated for each material to be deposited and each incidence that a same material is deposited.  
     [0072] Each selected set of deposition sites  134  comprises a presently illuminated region  134 . A selected set may further comprise a previously illuminated region  134  that is presently illuminated and/or a newly illuminated region  135  that was not previously illuminated. As such, each selected set may comprise the same or different sites  134  from other selected sets and may differ by one or more deposition sites  134 . FIG. 4B illustrates a perspective view of the apparatus  100  having a plurality of ionized droplets  144  of a material deposited on the illuminated regions  134  of the photoconductive layer  130  using the electrostatic guidance of the invention according to any one or more of the embodiments of the apparatus  100  described above.  
     [0073] As mentioned above, the apparatus  100  of the present invention can also electrostatically guide a material to the surrounding surface  136  instead of the deposition site  135 . In some embodiments, a layer of material on the surrounding surface area  136  may be desired. Droplets of the material can be ionized and applied to the surface  132  to electrostatically cover or coat the surface area  136  surrounding the deposition sites  135 . This material may provide a reagent or another chemical to the surrounding surface  136  or provide an optical surface, for example. In some embodiments, the material may be coated over the surrounding area  136  to block light therefrom. For an optical surface, the surrounding area  136  may be coated with an opaque or black material layer, for example, to create greater optical contrast that may be useful for subsequent processing of the apparatus  100 . As mentioned above, a layer of material deposited on the surrounding surface  136  may be achieved by illuminating the surrounding surface  136  instead of the plurality of deposition sites  135  to change the charge on the surrounding surface  136  for such a deposition.  
     [0074] According to other embodiments of the invention, an apparatus  200  comprises a substrate  210  made of a relatively electrically nonconductive or electrically insulative material. The substrate  210  comprises a plurality of deposition sites in a spatially addressable array pattern on an array surface of the apparatus  200 , and electrical circuitry  233  including a plurality of electrode pads  234  in an array pattern similar to the pattern of deposition sites, and a plurality of circuit paths  236 . The circuitry  233  is made of an electrically conductive material. The conductivity of the circuitry  233  and the substrate is relative, such that semiconductor materials may be used to fabricate the apparatus  200 . The plurality of electrode pads  234  either is or corresponds to the plurality of deposition sites. FIG. 5A illustrates an example of the plurality of electrode pads  234  on or just under a surface  212  of the substrate  210 . The electrode pads  234  are electrically isolated from one another by the nonconductive substrate material. The plurality of electrically conductive circuit paths  236  is connected to the electrode pads  234  to provide controlled electrical access to the electrode pads  234 . The circuit paths  236  permit controlled charges to be established separately or independently on each electrode pad  234 . The circuit paths  236  are embedded in the substrate  210 . FIG. 5A illustrates only some circuit paths  236  that are possible by dashed-lines through the substrate  210  in FIG. 5A.  
     [0075]FIG. 5B illustrates another example of more complex electrode design for the apparatus  200 . In this example, each deposition site  235  either comprises or corresponds to at least two individually controlled electrode pads  234   a  and  234   b  that are electrically isolated from one another by the relatively nonconductive substrate material, as illustrated at the substrate surface  218  within each site  235 . The electrode pads  234   a ,  234   b  at each site  235  are individually electrically connected to respective circuit paths  236   a ,  236   b , such that individual control of the deposition of the ionized droplets or aerosol particles of a material at each site  235  is achieved. FIG. 5B illustrates the circuit paths  236   a ,  236   b  as dashed lines through the substrate  210  by way of example. The electrode shading is removed from one of the sites  235  to better illustrate the embedded circuit paths  236   a ,  236   b . It may be advantageous and is within the scope of the present invention to use more than two electrode pads per deposition site  235 , for control of droplet trajectories as the droplets or particles approach the electrodes.  
     [0076] The number and shape of the electrode pads  234 ,  234   a ,  234   b  are not restricted to the examples shown in FIGS. 5A and 5B. Some or all of the electrical circuitry  233  could be located on the substrate surface  212  and be optionally covered with an insulating material (not shown in FIGS. 5A and 5B). In addition or alternatively, some or all of the electrical circuitry  233  may be embedded in the substrate  210  (the circuit paths  236 ,  236   a ,  236   b  are shown embedded in FIGS. 5A and 5B by way of example). The electrode pads  234 ,  234   a ,  234   b  may be one or more of on the surface  212  and in or just underneath the surface  212  (i.e., ‘at’ or ‘adjacent to’ the surface  212 ). A variety of technologies and materials that are well known in the art of thick film, thin film and semiconductor circuit fabrication may be used to fabricate the electrical circuitry  233  and substrate  210  for the apparatus  200 . As such, the apparatus  200  may comprise a multilayer substrate  210  depending on the electrical circuitry pattern and fabrication implementation used. The materials used for the apparatus  200  also include those materials classified as semi-conductive materials.  
     [0077] Suitable circuitry terminations are provided at a convenient location on the substrate  210  to connect to off-substrate electronics or electronic circuitry (not shown). FIG. 5A illustrates one example of a termination  232  provided along an edge  216  of the substrate  210 . In other embodiments, the terminations (not illustrated) may be on an opposite surface  214  from the surface  212 , as illustrated in by the dashed lines in FIGS. 5A and 5B. The apparatus  200  embodiments illustrated in FIGS. 5A and 5B are not to scale and are exemplary only. It is within the scope of the invention for the shape of the substrate  210  to be any shape from round to square and rectangular to elliptical. Further, the pattern and number of deposition electrode pads  234 ,  234   a ,  234   b  illustrated are exemplary only. For example, the array pattern could be annular, having a circular pattern or spiral pattern, for example, instead of a series of rows and columns. The shape of the electrode pads  234 ,  234   a ,  234   b  can be any shape ranging from square to rectangular, to elliptical or to circular. Also, the substrate  210  may be part of a larger wafer having a plurality of arrays thereon, or may be the wafer itself (not shown). The wafer is diced into individual substrates  210 , each having an array of the plurality, according to techniques that are well known in the art. The number, size and shape of the deposition electrode pads  234 ,  234   a ,  234   b  are limited by the fabrication technology used, and therefore, are not intended as a limitation to the present invention.  
     [0078] The electrical circuitry  233  connects to off-substrate control electronics or electronic circuitry, such as a voltage or current source, to provide a charge and/or a ground connection to the electrode pads  234 ,  234   a ,  234   b . The circuit paths  236 ,  236   a ,  236   b  supply the first charge to the electrode pads to accrete the material to, or to repel the material from, the deposition sites electrically influenced by the charged electrode pads. When a set or selection of pads of the plurality of electrode pads  234 ,  234   a ,  234   b  are charged with the first charge, ionized droplets can be deposited onto substrate surface  212  at each deposition site either directly or indirectly corresponding to the charged set of electrode pads.  
     [0079] For the purposes of the invention, a deposition site directly corresponds to an electrode pad when the electrode pad functions as, or essentially is, the deposition site, such that the first charge on the electrode pad is a charge on the deposition site. A deposition site indirectly corresponds to an electrode pad when there is some degree of separation between the electrode pad and the respective deposition site and the first charge on the electrode pad electrically influences the respective deposition pad with a corresponding electric field, as is further described below with respect to apparatus  300 . For example, using different materials for the electrode pad and the deposition site and/or providing a physical space between the electrode pad and the deposition site may achieve the degree of separation.  
     [0080] Each selected set of electrode pads comprises either one particular electrode pad, all of the electrode pads of the plurality, or any number of the same and/or different electrode pads therebetween. The ionized droplets of a first material have a charge that facilitates the electrostatic guidance of the first material for chemical array fabrication. The ionized droplets will be electrostatically guided to the deposition sites corresponding to or electrically influenced by a first set of charged electrode pads, while no preferential guidance is provided to any uncharged electrode pads or other surfaces. After the first material is deposited on respective deposition sites corresponding to the first selected set of charged electrode pads, the charge is removed. The deposited material chemically bonds to the deposition site surface, so that the removal of the charge does not impact the deposited material.  
     [0081] A second material is deposited by applying another first charge to a second selected set of pads  234 ,  234   a ,  234   b . Some of the electrode pads of this second set may be the same as the previously charged electrode pads of the first set for the deposition of the first material. Other electrode pads of the second set may not have been previously charged for receiving the first material. In this way, a variety of different materials can be deposited on deposition sites corresponding to a variety of different sets of charged electrode pads sequentially, until each electrode pad  234 ,  234   a ,  234   b  or corresponding deposition site comprises a desired chemical composition of one or more deposited materials.  
     [0082] Further, the apparatus  200  can control the volume of each deposited material on each deposition site with the charge that is applied (or the length of time that the charge is applied) to each electrode pad  234 ,  234   a ,  234   b  of a set for each material. Accordingly, array fabrication materials are more precisely placed in desired locations in more precise volumes using the present invention than for an array without the present invention. The apparatus  200  effectively creates a ‘soft’ deposition mask having a changeable pattern mask with different electric charge or field differentials created at the array surface. The deposition mask can be changed with each different application of charge to different sets of electrode pads  234 ,  234   a ,  234   b.    
     [0083] In some cases, it may be desirable for the substrate  210  and/or at least some of the electrical circuitry  233  to be transparent, when the apparatus  200  is used for a DNA/RNA biological microarray assay, for example. Transparent substrate materials used for biological array manufacture are well known in the art. A common transparent substrate is transparent to optical scanning, such as with the laser. Optically transparent materials used for the electrical circuitry  233  also are known in the art, such as the materials used in display technology, for example. Accordingly, the apparatus  200  can be manufactured using well-known optical transparent substrate materials and using optical transparent electrically conducting materials.  
     [0084] According to still other embodiments, an apparatus  300  can be used in the fabrication of biochemical arrays, such as DNA/RNA microarrays, without regard to transparent circuitry. FIG. 6 illustrates a cross sectional side view of an embodiment of the apparatus  300  comprising a substrate  310  that comprises a membrane sheet  330 , a plurality of deposition sites on an array surface  332  of the membrane sheet  330 , a plate  320  underlying the membrane sheet  330 , and electrical circuitry  333 . In these embodiments, the electrical circuitry  333  comprises electrically conductive electrode pads  324  and electrically conductive circuit paths  326  that are embedded in and/or provided on or adjacent to a surface  322  of the plate  320 , in much the same way as described above for apparatus  200 . Preferably, the plate  320  is similar or equivalent to the substrate  210  described above. The electrode pads  324  are arranged in an array pattern on or in the surface  322  of the plate  320 . A circuit path  326  provides a first charge to a respective electrode pad  324 . The membrane sheet  330  is adjacent to the surface  322  of the plate  320  where the electrode pads  324  are located. The charge on the electrode pads  324  of the underlying plate  320  generates an electric field through the membrane sheet  330  at deposition locations or sites  334  on the array surface  332  of the membrane sheet  330  corresponding to the locations of the underlying electrode pads  324 . Therefore, the deposition sites  334  indirectly correspond to the electrode pads  324  and the circuitry  333  of the apparatus  300  electrically influences the deposition sites  334  on the membrane sheet  330  with the generated electric field.  
     [0085] According to the invention, a material can be deposited on a selected set of the deposition locations or sites  334  of the membrane sheet  330  by charging a corresponding set of the underlying electrode pads  324  and introducing ionized droplets of the material to the array surface of the membrane  330 . The ionized droplets of the material have a charge that facilitates electrostatic guidance of the material for chemical array fabrication. An electrostatic force between the charge of the ionized droplets and the electric field of the corresponding electrically influenced deposition sites  334  guide the ionized droplets of the material to the influenced deposition sites  334  or a remaining array surface  332  surrounding the influenced deposition sites  334  depending on whether the electrostatic force is one or both of an attractive force and a repulsive force.  
     [0086] Advantageously, the material is deposited more precisely on the deposition sites  334  of apparatus  300 , for example, than without the present invention. Other materials may be deposited on the same set or other sets of deposition sites  334  or remaining surrounding surface to a final composition at each deposition site  334  and/or on the array surface in much the same way as described above for the apparatus  200 . Moreover, electronic control circuitry (not illustrated in FIG. 6) can vary the charge provided to one or more charged electrode pads  324  to increase or decrease ongoing accretion of ionized droplets to the deposition site  334  on membrane sheet  330  corresponding to the charged electrode pad  324  during a deposition process. Advantageously, the apparatus  300  overcomes any obstacles to providing transparent circuitry to the substrate, as might be considered when using some embodiments of the apparatus  200 . Further, the apparatus  300  essentially eliminates any compatibility issues between the material to be deposited and the material of the electrode pads, also as might be considered for some embodiments of the apparatus  200 .  
     [0087] The membrane sheet  330  of the apparatus  300  is made from flexible membrane substrate materials that are commonly used for biological array manufacture and that are well known in the art. Due to the flexibility of the membrane material, the membrane sheet  330  may be mounted to a membrane support  360 , such as a support frame, to provide mechanical stability to the membrane sheet  330 . After the array of materials are deposited on the membrane  330 , the membrane  330  can be removed from the plate  320  for further processing, such as a hybridization assay of the deposited material with a target sample, for example, or other assay, and a subsequent optical interrogation process. Both of the apparatus embodiments  100  and  200  also can be further processed for hybridization or other assays, and subsequently interrogated by a variety of means known in the art.  
     [0088] The present invention also provides a method of electrostatically guiding a chemical or biochemical material to be deposited in chemical array fabrication. FIG. 7 illustrates a method  400  that comprises generating  410  an electric charge or field differential on an array; and exposing  420  the array to an ionized droplet of the chemical or biochemical material to be deposited. The ionized droplet has a charge that facilitates electrostatic guidance according to the method  400 . As described above, the charge of the ionized droplets is different from the charge or field influencing a surface location on which the ionized droplet is to be deposited, such that the material preferentially deposits at the influenced surface location. Otherwise, the charge of the ionized droplets is similar to the charge or field influencing a surface location, such that the material preferentially deposits at locations other than the influenced surface location.  
     [0089] The array comprises a plurality of deposition sites in a spatially addressable array pattern on an array surface. The electric charge or field differential comprises a first charge that electrically influences a selected set of the deposition sites of the plurality, and a second charge on an area surrounding the selected set. The first charge is different from the second charge. Preferably, the array is any embodiment of the apparatus  100 ,  200 ,  300  described above.  
     [0090] The electric charge or field differential may be generated  410  on the array in several ways. The electric charge or field differential is generated  410  by applying or supplying the first charge to regions on the array that either directly or indirectly correspond to the selected set of deposition sites. According to some embodiments, as described above for the apparatus  100 , generating the electric charge or field differential comprises providing an array substrate that comprises a layer of an electrically nonconductive photoconductive material over an underlying electrically conductive layer. In these embodiments, generating an electric charge or field differential further comprises applying the second charge to a surface of the photoconductive layer, while the photoconductive layer is protected from exposure to light. Generating an electric charge or field differential further comprises connecting the conductive layer to a first charge source or sink; and illuminating unprotected regions of the electrostatically charged surface of the photoconductive layer with light. The unprotected regions correspond to the selected set of deposition sites or to the surrounding area. The illuminated regions become electrically conductive while being illuminated and dissipate or discharge the second charge on the photoconductive layer surface at the illuminated region to the conductive layer. The illuminated region acquires the first charge of the underlying conductive layer also while being illuminated. The illumination is momentary, such that the charge at the illuminated location is changed from the second charge to the first charge, while the second charge at a non-illuminated remaining portion of the photoconductive layer surface is unchanged. As a result, an electric charge differential is generated at the array surface.  
     [0091] According to other embodiments of the method, generating an electric charge or field differential comprises providing an array substrate, such as that described above for the apparatus  200 . The array substrate is made of an electrically nonconductive material that comprises the plurality of deposition sites and further comprises electrical circuitry. The electrical circuitry comprises a plurality of electrically conductive electrode pads connected to electrically conductive circuit paths that are embedded in the substrate. The plurality of electrode pads either directly or indirectly corresponds to the plurality of deposition sites. Preferably, the circuit paths are embedded in the substrate and the electrode pads are on or in the substrate adjacent to the array surface. Generating an electric charge or field differential further comprises applying the first charge to the circuit paths that interconnect to a selected set of the electrode pads to charge the selected set of electrode pads. The selected set of electrode pads correspond to the selected set of influenced deposition sites. The surrounding area is a portion of the array surface not influenced by the first charge.  
     [0092] According to still other embodiments of the method  400 , generating an electric charge or field differential comprises providing an array substrate, such as the apparatus  300  described above. The array substrate comprises an electrically nonconductive membrane sheet overlying a plate of an electrically nonconductive material having electrically conductive circuitry embedded therein and/or disposed thereon. The plurality of deposition sites are on the membrane sheet, such that the membrane sheet provides the array surface. The circuitry comprises electrically conductive circuit paths connected to electrically conductive electrode pads. Each deposition site on the membrane sheet has an indirectly corresponding electrode pad in the plate. The electric charge or field differential is generated according to the other embodiments of the method  400  by applying the first charge to a selected set of electrode pads of the plate corresponding to the selected set of deposition sites on the membrane sheet. The first charge on the selected set of electrode pads creates an electric field through the membrane sheet at each deposition site of the selected set of deposition sites indirectly corresponding to the charged electrode pads.  
     [0093] For the purposes of the method  400 , the selected set of deposition sites may be any desired deposition sites ranging from one particular site to all of the plurality of sites. Each selected set may be the same or different from previously selected sets. According to some of the embodiments using the apparatus  200  or  300 , either same or different sets of electrode pads may be charged to influence respective same or different sets of deposition sites during chemical array fabrication to deposit a plurality of chemical or biochemical materials on the array surface until desired compositions of materials are achieved at each deposition site.  
     [0094] Further according to the method  400 , the array is exposed  420  to ionized droplets of the material to be deposited in several ways according to the invention. Exposing  420  the array to ionized droplets comprises forming ionized droplets of the material and dispensing the ionized droplets to the array. Droplets of the material are formed using droplet generators. A droplet generator is any piece of equipment that produces droplets of the material composition. In some embodiments, the droplet generator is any one of a variety of print heads, including, but not limited to ink-jet print heads. Ink-jet deposition equipment has characteristics, such as defined or controlled droplet positioning and defined droplet volume, which makes this technology quite attractive for chemical array manufacture. In other embodiments, the equipment includes, but is not limited to, sprayers and aerosol generators (also known as fog generators or foggers) that generate microdroplets and that are designed to provide a blanket of coverage to a surface rather than defined positioning of droplets. All of such equipment for generating droplets of a material composition are well known in the art and are within the scope of the present invention.  
     [0095] The material composition may be charged or ionized prior to loading the composition into the droplet generation equipment, charged by or in the equipment, charged as the equipment generates the droplets, or otherwise, using well-known methods. Any method or technique used to charge or ionize droplets is within the scope of the present invention.  
     [0096] Ink-jet technology can be used to deposit a plurality of different material compositions selectively on the array surface in a single layer during the step of exposing  420  the array to ionized droplets. The ionized droplets are fired from an ink-jet print head to the surface of the array at predetermined locations. The ink-jet print head directs individual ionized droplets to electrically influenced deposition sites. While the ink-jet technology has relatively precise positioning compared to other droplet generators, array manufacturers strive to improve the precision. Improved precision allows for more densely packed array patterns to be manufactured on a single substrate. Further, improved precision minimizes errors in droplet placement and improves results of biological array assays of a target material under test. Target samples typically are available in only very minute quantities and are expensive to obtain. More densely packed arrays use less of the target sample in an assay. Further, more accurate assay results minimize target sample waste. Advantageously, the present invention enhances the precision of ink-jet deposition equipment by rendering the predetermined locations with a different charge than the rest of the surface and electrostatically guiding the fired ionized droplets having a charge to the differently charged predetermined locations using one or both of attractive and repulsive forces, for example.  
     [0097] Aerosol generators or foggers also can be used to form ionized droplets of, and to separately dispense, a variety of different chemical compositions on the differently charged surface of an array according to the invention. Chemical species to be deposited are first converted into aerosols of electrically charged microdroplets. Preparation of aerosols is a well-known process; see for example Hinds, William C.,  Aerosol Technolog,  Wiley Inc., 2 nd  Edition, January 1999, which is incorporated herein by reference. An aerosol is generally taken to mean a suspension of solid or liquid particles in a continuous gas phase. However, the present invention is useful also where the continuous phase is a rarified gas, a dense gas, an immiscible liquid, or a partially miscible liquid. The immiscible liquid and partially miscible liquid are usually known as emulsions. All suitable forms of these mixtures are within the scope of the present invention. “Suitable” refers to the continuous phase being an insulator, and the dispersed phase being ionized.  
     [0098] The microdroplets from the aerosol generator are dispensed as a blanket or a fog over the differently charged array surface. When a sprayer or aerosol generator is used to generate droplets of a material, exposing  420  the array to ionized droplets comprises either moving the array through the fog of charged microdroplets or moving the fog of microdroplets over the array. The charged microdroplets in the fog are either attracted to or repelled by the electrically influenced deposition sites of the array due to electrostatic attractive or repulsive forces, and in some embodiments, the charged microdroplets are guided by both attractive and repulsive forces. As such, charged microdroplets from the fog can be deposited to synthesize polymers or biopolymers on the influenced deposition sites. Further, a fog of a material can be deposited to buildup into a continuous layer on the area surrounding the deposition sites. To build-up a continuous layer on the surrounding area, the charge of ionized droplets and on the electrically influenced deposition sites are rendered the same or similar so that the microdroplets are repelled therefrom to the surrounding area. Alternatively or additionally, an electrostatic charge can be applied to the surface of the surrounding area that is different from the electrically influenced deposition sites, such that the charged ionized droplets are further attracted to the surrounding area.  
     [0099] In accordance with the method  400 , both generating  410  an electric charge or field differential on an array and exposing  420  the array to ionized droplets of a material are repeated for each different set of deposition sites, each continuous layer applied to the surrounding area, each different material to be deposited, and for each different application of the same material. In this way, material layers can be built up on the surface of the array by creating a series of electric charge or field differential deposition masks and coating selected regions on each successive layer. Over a number of cycles, numerous small areas on the substrate (e.g., ultimately probes) can be built up in many layers where the position and size of the application area is controlled by light-produced different charges (as in apparatus  100 ). Moreover, the charge provided to the electrode pads can be varied to increase or decrease ongoing accretion of ionized droplets to the corresponding deposition sites during a deposition process (as in apparatus  200  and  300 ). As such, the volume of material deposited is controlled by the electric charge that is applied to the circuitry with the control electronics. For the invention, it is possible to vary accretion of the material on a number of deposition sites ranging from one to all sites, depending on the complexity of the circuitry and the control electronics employed. As such, the present method  400  optionally further comprises varying the first charge that influences a deposition site of the selected set of sites on the array to control accretion of the material on the selected deposition sites.  
     [0100] A system  500  for electrostatically guiding ionized droplets of a chemical or biochemical material in chemical array fabrication is provided. The system  500  comprises an array apparatus  510 , and a source of ionized droplets  550 . The array apparatus can be any embodiment of the apparatus  100 ,  200 ,  300 . The source of ionized droplets  550  can be any of the droplet generating means described above. FIG. 8 illustrates an embodiment of the system  500 . The system  500  may further comprise control electronics  580  that one or more of supplies, removes and varies a charge to the array apparatus  510 . Depending on the embodiment, the control electronics  580  comprise one or both of means for generating a controllable charge (i.e., a source of charge) to electrically conductive material, and means for generating an electrostatic charge to a surface, such as to the surface of a nonconductive material. The controllable charge generating means is preferably controllable by being adjustable. In some embodiments, the control electronics  580  further comprises charge-sensing circuitry that controls accretion volumes of the ionized material on the array apparatus, as is further described below.  
     [0101] When the apparatus  100  is employed as the array apparatus  510 , the system  500 ′ further comprises a light source  570 . The control electronics  580  comprise both the means  540  for electrostatic charge generation and the means  560  for controllable charge generation. FIG. 9 illustrates a block diagram of the system  500 ′ according to some embodiments. As mentioned above with respect to apparatus  100 , the electrostatic charge generating means  540  can be a corona wire, for example. The controllable charge generating means can be a ground connection and/or a voltage source or a current source, as are known in the art. The voltage source and the current source each provides a selection of positive and negative charges, as well as a zero volts or zero amps selection, respectively. The light source  570  can be a laser, such as a scanning laser, or other collimated light source, or a non-collimated light source, for example. In some embodiments of the system  500 ′, the system  500 ′ optionally further comprises means  590  for blocking ambient light from the photoconductive layer during chemical array fabrication. The means  590  for blocking ambient light protects the surface from the light, and can range from a light-blocking mask positioned over the photoconductive layer to a light blocking enclosure to house the apparatus  100 , and further to a dark room, such as that used in photography. Not illustrated in FIG. 9 is that the system  500 ′ optionally further comprises a movable stage that one or more of moves the apparatus  510  to align with the light source  570  and/or the ionized droplet generator  550  and moves the light source  570  and/or the droplet generator  550  to align with the apparatus  510 . The movable stage facilitates automation of chemical array fabrication using the system  500 ′.  
     [0102] When apparatus  200  or  300  is employed as the apparatus  510 , the system  500 ″ further comprises a cell or chamber  520  that encloses the array  510 . FIG. 10A illustrates a perspective view of the system  500 ″, according to some embodiments. Not illustrated in FIG. 10A are the ionized droplet source  550  and the control electronics  580  for simplicity purposes only. The cell  520  comprises a volume space  526  adjacent to deposition sites  534  of the array apparatus  510 , and a port  528 . In some embodiments, the cell  520  encloses the surface  512  of the array  510 . In other embodiments, the cell encloses at least the surface  512  and preferably, more of the array apparatus  510 , such as enclosing all of the apparatus  510 .  
     [0103]FIG. 10B illustrates a cross sectional view of an embodiment of the system  500 ″ using an embodiment of the apparatus  300  as the array apparatus  510 . In this embodiment of the system  500 ″, the plate  320  of apparatus  300  functions as a part (e.g., substrate portion  520   a ) of the cell  520 . The membrane sheet  330  of the apparatus  300  is the apparatus  510 , as described herein. The cell  520  comprises a substrate portion  520   a  underlying the membrane sheet  510 . The substrate portion  520   a  comprises circuitry  523  (not illustrated in FIG. 10B) that electrically influences deposition sites  534  on the membrane sheet  510 . Also, the array membrane  510  is supported by the frame support  360 , which was described above for the apparatus  300 . FIG. 10C is a magnified view of a section labeled  10 C of the embodiment illustrated in FIG. 10B. The magnified view in FIG. 10C illustrates the cell circuitry  523  having electrodes  524 . The electrodes  524  are electrically connected to circuit paths (not illustrated) that ultimately electrically connect to the control electronics  580  (also not illustrated in FIG. 10C).  
     [0104]FIG. 10C further illustrates that the cell  520  may further comprise an array pattern of electrodes  524 ′ on an interior surface  522  of a cell portion  520   b  spaced from the array membrane  510  and facing the deposition sites  534  of the array membrane  510 , according to some embodiments. The electrodes  524 ′ are offset from the array membrane  510  surface  512  by a short distance defined by the volume  526  of the cell  520 . Charges on the electrodes  524 ′ also influence the motion of microdroplets, thereby assisting in control of droplet trajectories. Therefore the electrodes  524 ′ of the cell portion  520   b  may be considered to be a part of the cell circuitry  523 .  
     [0105] An aerosol of a chemical composition to be deposited on the array membrane  510  flows from the ionized droplet source  550 , which is an aerosol generator for this embodiment, into the cell  520  through port  528  to fill the volume  526 , as illustrated with bold arrows in FIG. 10B, for example. The deposition sites  534  (equivalent to sites  334  of the apparatus  300 ) on the array membrane  510  are electrically influenced by a first charge on the electrode pads  524  of the cell  520  that underlie the corresponding deposition sites  534 . Intervening areas  536  of the array membrane  510  surface  512  have an electrostatic charge that is different from the first charge, which may be a neutral charge or a non-neutral charge, depending on the embodiment. The aerosol is ionized with a charge that different from the first charge influencing the array deposition sites  534  so that the ionized droplets are guided to and deposit on the charge-influenced deposition sites  534 . The microdroplets of the aerosol are typically carried in the gas phase. The choice of material for the gas phase of the aerosol includes, but is not limited to, air, an inert gas, and a chemically active gas. The gas phase may be at typical atmospheric pressure, rarefied pressure, or dense pressure. An immiscible or partially miscible liquid may also be used as the carrier phase, as mentioned above.  
     [0106] An aspect of the invention is the ability to control accretion at each site by monitoring and controlling charge, in particular with respect to apparatus  200 ,  300  and system  500 ,  500 ″. Thus, the control electronics  580  of the system  500 ″ optionally further comprise the charge sensing and controlling circuitry, as mentioned above with respect to FIG. 8. As cumulative droplet accretion at a deposition site  534  builds up, control circuitry adjusts or reverses the charge at the electrode pad(s)  524 ,  524 ′, thereby adjusting or reversing the electric field influencing the corresponding deposition sites  534 . As such, the ionized droplets are repelled and prevented from further accretion at the corresponding deposition site  534 . The charge sensing and controlling circuitry controls the potential at each deposition site  524  to allow buildup of the desired deposition volume of each chemical species or material to be deposited. The end result effectively is a ‘soft’ deposition mask, having a changeable electric charge or field differential pattern that also has a changeable charge potential/polarity pattern. Advantageously, the same droplet generator equipment  550 , deposition cell  520  and array  510  are capable of generating any desired patterns of microarrays using the charge sensing and controlling circuitry. The charge sensing and controlling circuitry that may be used for the present invention is similar to the charge coupling circuitry commonly used in charge-coupled device (CCD) cameras and is known to one skilled in the art.  
     [0107] The system  500 ″ for depositing chemical species eliminates using moving print heads for deposition, and eliminates using robots or movable stages for physical transfer of array substrates between process stations. As such, the system  500 ″ may have no mechanical moving parts. However, in some embodiments of the system  500 ″, it may be advantageous to move the deposition cell  520  between successive aerosol sources  550  for chemical array fabrication. The system  500 ″ is amenable to automation such that one or several cells  520  may be connected with multiple aerosol sources  550  and other reagents or process sources, either simultaneously or sequentially.  
     [0108] In situ synthesis of DNA and RNA microarrays is one example of a process where an array substrate is exposed to multiple chemical species sequentially. The embodiments  500 ′,  500 ″ of the system  500  are adapted to flood the array  510  (e.g., in aqueous washing, gas-drying, common chemical steps, etc.) when all deposition sites  534  must be treated in the same manner. While reagents and washes typically are flooded over the substrate surface during in situ synthesis, the different nucleotide species are precisely deposited in desired locations to achieve a variety of different nucleotide sequences (e.g. as probes) on the array. The system  500  is amenable to many known automation techniques that would efficiently connect the fabrication steps for simultaneous or sequential processing, and further connect the fabricated chemical array to an assay process, such that the apparatus  510  is not manually handled between fabrication and assay processes.  
     [0109] The system  500 ″ controls the volume  526  of the cell  520  occupied by the aerosol as it moves over the array surface  512 . For example, the volume  526  can be made very small, and environmental factors, such as pressure, temperature and other factors can be controlled and manipulated. Furthermore, in some embodiments, the port  528  is a narrow slot in the cell  520  in communication with the volume space  526 . Advantageously, the narrow slot  526  constitutes a Hele Shaw cell, in which aerosol flow is even and free of turbulence, as is well known to one skilled in the art. See for example, Horace Lamb,  Hydrodynamics,  Sixth Ed., Dover Publications, N.Y., 1936, p. 86 and p. 582, which is incorporated by reference herein. Turbulence has an attendant risk of causing uncontrolled deposition. Therefore, the system  500 ″ advantageously can control turbulence in the volume space  526  also.  
     [0110] Thus, there have been described embodiments of a novel apparatus, a method and a system for electrostatically guiding a material in chemical array manufacture. It should be understood that the above-described embodiments are merely illustrative of the some of the many specific embodiments that represent the principles of the present invention. Clearly, those skilled in the art can readily devise numerous other arrangements without departing from the scope of the present invention.