Patent Publication Number: US-9415591-B2

Title: Apparatuses and methods for electrohydrodynamic printing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/948,851 filed Mar. 6, 2014, which is incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under N00014-08-C-0390 and N00014-11-C-0391 awarded by the Office of Naval Research. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The present invention relates generally to electrohydrodynamic printing and more specifically, but not by way of limitation, to nozzles for electrohydrodynamic printers. 
     2. Description of Related Art 
     Examples of electrohydrodynamic printer nozzles are disclosed in U.S. patent application Ser. No. 12/713,886 and U.S. patent application Ser. No. 12/669,287. 
     Electrohydrodynamic (EHD) printing is a highly versatile printing technology that can provide printing resolutions in the micron to submicron range. EHD printing generally uses a strong electric field to eject printing media onto a substrate. Typically, a large bias voltage is applied to a nozzle that is in fluid communication with a printing media reservoir. The electric field generated by the bias voltage draws the printing media through the nozzle and ejects it towards a substrate. Such printers are capable of printing high resolution features that are orders of magnitude smaller than printer nozzle size (e.g., inner diameter) [1]. Thus, EHD printers can be used during creation of a variety of devices, including, but not limited to, electronics (e.g., printed circuit boards), sensors (e.g., transmission fluid temperature sensors, and gas sensors), power modules, interconnects, biomedical devices (e.g., templates for cell growth), displays, actuators, energy harvesters, transistors, and organic light-emitting diodes (LEDs), just to name a few. The range of potential applications illustrate the usefulness of EHD printers in direct printing (e.g., sensors), front-end and back-end fabrication (e.g., transistors and PCBs, respectively), and packaging (e.g., interconnects). 
     EHD printing technology can also reduce cost and waste present in traditional microfabrication. For example, mask-based lithography, in general, is a microfabrication process used to create micro- or nano-scale patterns on a substrate and is commonly used to create integrated circuits. Typically, a light-sensitive chemical, also known as a photoresist, is deposited onto a substrate. An optical mask comprising a pattern can then be used to mask desired portions of the substrate. For example, in simpler proximity or contact systems, the optical mask is placed in close proximity to or in direct contact with the substrate. A specialized light source can then be used to expose the unmasked portions of the substrate, thus transferring the desired pattern to the substrate (e.g., by exposing unmasked portions of the light-sensitive photoresist). Traditional mask-based lithography can involve highly specialized equipment. For example, optical masks typically are constructed out of a fused quartz substrate layered with chromium, where the chromium layer is etched with a laser to create the desired masking pattern. Additionally, photoresists can comprise relatively expensive chemicals that are usually wasted (e.g., removed from the substrate and discarded) during the masked based lithography process. Current alternative methods for achieving similar results are electron beam lithography, which is time consuming and expensive, nano-imprint technology, which generally involves expensive molds made of specialized materials, and piezo-driven printing, which is typically limited to low viscosity printing materials (e.g., with a viscosity less than 50 centipoise (cP)) and thus can require multiple superimposed printing runs when printing thicker structures and offers a relatively low printed feature resolution. 
     SUMMARY 
     The present EHD printers, components, and methods are capable of directly printing micro- or nano-scale patterns onto a substrate without the need for the specialized equipment or substantial amounts of chemicals (which may be harmful to the environment). Additionally, the present EHD printers, components, and methods are not limited to light-sensitive printing materials, and thus printed patterns may not require additional developing steps before use. Therefore, the present EHD printers, components, and methods can accomplish direct pattern printing in both an economical and time-efficient fashion. For optimal direct pattern transfer, the printing media can be optimized for viscosity, surface tension, electrical conductivity, solvent content, and/or evaporation rate. For example, for maskless lithography, it may be desirable that printing media be highly viscous to create thick structures, contain little solvent, adhere to the substrate, and/or resist any subsequent post-processing steps that may be used after direct pattern transfer. Embodiments of the present printing media are so modified and, in some embodiments, comprise a modified commercially available photoresist. 
     Damage can frequently occur to an EHD printer nozzle. Printer nozzle tips are typically small and potentially fragile. Additionally, due to the high bias voltages involved, arcing can occur and burn the nozzle, which may necessitate nozzle replacement or repair. Embodiments of the present EHD nozzles, however, can be constructed from relatively inexpensive components, without the need for specialized fabrication equipment, and can include robust, reliable, and reusable electrical connections to the printer nozzle and/or the printer head that make nozzle assembly and disassembly relatively quick, thus facilitating replacement of the EHD nozzle assembly or EHD nozzle tip in the event of damage (e.g., due to arcing). 
     Embodiments of the present apparatus and methods can be configured to provide an easily repairable and/or replaceable EHD nozzle and/or nozzle tip through depressible electrical connectors configured to allow for both releasable coupling and electrical communication between the EHD nozzle, EHD nozzle tip, EHD printer head and/or EHD printer. 
     Some embodiments of the present EHD printer nozzles comprise: a circuit having at least one depressible electrical connector; and a housing having a first end, a second end, and a channel extending from the first end to the second end, the housing configured to be releasably coupled to a printer head, and the channel configured to removably receive a dispensing device with a conductive tip such that electrical communication is permitted between the conductive tip and the at least one depressible electrical connector; where the circuit is configured to apply a voltage across the conductive tip; and where the EHD printer nozzle is configured to be removably coupled to an EHD printer head. In some embodiments, the circuit comprises two depressible electrical connectors, the depressible electrical connectors configured to contact substantially opposite sides of the conductive tip. In some embodiments, at least one depressible electrical connector comprises a spring-loaded electrical connector. In some embodiments, the spring-loaded electrical connector comprises a pogo-pin. In some embodiments, the circuit comprises at least one header pin configured to be in electrical communication with the printer head when the first end is coupled to the printer head. In some embodiments, the circuit comprises at least one contact printed circuit board (PCB). In some embodiments, the nozzle further comprises an electrode disposed proximate the second end of the housing. In some embodiments, the circuit is configured to apply a voltage across the electrode. In some embodiments, the circuit further comprises first and second parallel portions, the first parallel portion configured to be in electrical communication with the conductive tip and the second parallel portion configured to be in electrical communication with the electrode. Some embodiments further comprise a second circuit configured to apply a voltage across the electrode. In some embodiments, the circuit is configured to apply a first voltage across the conductive tip and the second circuit is configured to apply a second voltage across the electrode, where the second voltage is different than the first voltage. In some embodiments, the electrode comprises an opening having a transverse dimension. In some embodiments, the opening is substantially centered on a longitudinal axis of the conductive tip. 
     Some embodiments of the present EHD printer heads comprise: an embodiment of the present nozzles; and a reservoir in fluid communication with the nozzle, the reservoir configured to contain printing media; where the reservoir is configured to be coupled to a fluid source such that the fluid source can deliver fluid to or remove fluid from the reservoir to adjust an internal pressure of the reservoir. Some embodiments comprise a power source configured to electrically communicate with the circuit to apply a voltage across the conductive tip. In some embodiments, the power source is configured to electrically communicate with the circuit to apply a voltage across the electrode. In some embodiments, the power source is configured to electrically communicate with the second circuit to apply a voltage across the electrode. Some embodiments further comprise a second power source configured to electrically communicate with the second circuit to apply a voltage across the electrode. 
     Some embodiments of the present EHD printers comprise: an embodiment of the present printer heads and a power source configured to supply a voltage to the conductive tip. In some embodiments, the power source is further configured to supply a voltage to the electrode. Some embodiments further comprise a second power source configured to supply a voltage to the electrode. Some embodiments comprise a fluid source configured to deliver fluid to or remove fluid from the reservoir to adjust an internal pressure of the reservoir. Some embodiments further comprise a working surface. Some embodiments further comprise at least one orientation actuator configured to adjust an orientation of the working surface relative to the printer head. Some embodiments further comprise at least one sensor configured to capture data indicative of the orientation of the working surface relative to the printer head. Some embodiments further comprise a processor configured to adjust the orientation of the working surface relative to the printer head based on the data captured by the at least one sensor. 
     Some embodiments of the present methods comprise: inserting a dispensing device with a conductive tip into an EHD nozzle, the nozzle having a housing with at least one depressible electrical connector, where the dispensing device is inserted such that the depressible electrical connector contacts the conductive tip; and applying a voltage across the conductive tip by enabling electrical communication between the depressible electrical connector and a power source. In some embodiments, the nozzle further has an electrode and the present methods further comprise applying a voltage across the electrode by enabling electrical communication between the electrode and a second power source. In some embodiments, the power source and the second power source comprise the same power source. 
     Some of the present direct printing methods for maskless lithography comprise: generating an electric field around an EHD printer nozzle, the nozzle having a housing with at least one depressible electrical connector and a dispensing device with a conductive tip disposed in the housing such that electrical communication is permitted between the conductive tip and the depressible electrical connector, where the electric field is generated by enabling electrical communication between the depressible electrical connector and a power source to apply a voltage across the conductive tip; and ejecting viscous fluid from the nozzle onto a substrate. In some embodiments, the nozzle further has an electrode and the generating an electric field further comprises enabling electrical communication between the electrode and a power source. In some embodiments, the power source and the second power source comprise the same power source. Some embodiments further comprise adjusting a distance between the electrode and the conductive tip. Some embodiments further comprise maintaining a constant hydrostatic pressure at an exit of the nozzle by adjusting an internal pressure of a fluid reservoir that is in fluid communication with the nozzle. Some embodiments further comprise adjusting the electric field. Some embodiments further comprise adjusting a distance between the nozzle and the substrate. Some embodiments further comprise moving the nozzle relative to the substrate. 
     Some embodiments of the present methods further comprise curing the viscous fluid. In some embodiments, the curing comprises ultraviolet (UV) curing. In some embodiments, the UV curing comprises exposing the viscous fluid to ultraviolet light having a power of approximately 500 watt (W) for a time of approximately 1 minute. In some embodiments, the curing comprises baking. In some embodiments, the baking comprises heating the viscous fluid at a temperature within the range of approximately 100 degrees Celsius (° C.) to approximately 110° C. for a time of approximately 1 minute. In some embodiments, the curing comprises sintering. In some embodiments, the sintering comprises heating the viscous fluid at a temperature greater than or equal to approximately 400° C. for a time of approximately 45 minutes. 
     In some embodiments of the present methods, the substrate comprises a silicon wafer. In some embodiments, the substrate comprises glass. In some embodiments, the substrate comprises polymer. In some embodiments, the substrate comprises ceramic. 
     In some embodiments of the present methods, the viscous fluid comprises a negative epoxy resist modified with at least one of a surfactant and a solvent such that the viscous fluid has a viscosity and a surface tension suitable for maskless lithography. In some embodiments, the viscous fluid comprises an ionic metal salt. In some embodiments, the ionic metal salt comprises at least one of zinc nitrate, zinc acetate, and tin nitrate. In some embodiments, the viscous fluid comprises poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate). In some embodiments, the viscous fluid comprises from 1-10% poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate). In some embodiments, the viscous fluid comprises a matrix material. In some embodiments, the viscous fluid comprises from 1-20% of the matrix material. In some embodiments, the matrix material comprises at least one of polyethylene glycol, polyvinylpyrrolidone, and polyvinyl alcohol. In some embodiments, the viscous fluid comprises a solvent. In some embodiments, the viscous fluid comprises from 10-90% of the solvent. In some embodiments, the solvent comprises at least one of ethylene glycol, N-Methyl-2-pyrrolidone (NMP), N-methylpyrrolidone, dimethyl sulfoxide, ethanol, and methanol. In some embodiments, the viscous fluid comprises a surfactant. In some embodiments, the surfactant comprises anionic fluorinated polyether di(ammonium sulfate) salt. 
     The term “coupled” is defined as connected, although not necessarily directly, and not necessarily mechanically; two items that are “coupled” may be unitary with each other. The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise. The term “substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” and “about” may be substituted with “within [a percentage] of” what is specified, where the percentage includes 0.1, 1, 5, and 10 percent. 
     Further, a device or system that is configured in a certain way is configured in at least that way, but it can also be configured in other ways than those specifically described. 
     The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, an apparatus that “comprises,” “has,” “includes,” or “contains” one or more elements possesses those one or more elements, but is not limited to possessing only those elements. Likewise, a method that “comprises,” “has,” “includes,” or “contains” one or more steps possesses those one or more steps, but is not limited to possessing only those one or more steps. 
     Any embodiment of any of the apparatuses, systems, and methods can consist of or consist essentially of—rather than comprise/include/contain/have—any of the described steps, elements, and/or features. Thus, in any of the claims, the term “consisting of” or “consisting essentially of” can be substituted for any of the open-ended linking verbs recited above, in order to change the scope of a given claim from what it would otherwise be using the open-ended linking verb. 
     The feature or features of one embodiment may be applied to other embodiments, even though not described or illustrated, unless expressly prohibited by this disclosure or the nature of the embodiments. 
     Some details associated with the embodiments are described above and others are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings illustrate by way of example and not limitation. For the sake of brevity and clarity, every feature of a given structure is not always labeled in every figure in which that structure appears. Identical reference numbers do not necessarily indicate an identical structure. Rather, the same reference number may be used to indicate a similar feature or a feature with similar functionality, as may non-identical reference numbers. The figures are drawn to scale (unless otherwise noted), meaning the sizes of the depicted elements are accurate relative to each other for at least the embodiment depicted in the figures. 
         FIG. 1A  depicts certain components of one example of an EHD printer. 
         FIG. 1B  depicts a cutaway cross-sectional side view of the nozzle of the printer of  FIG. 1A  during two operating modes. 
         FIG. 2A  depicts a cutaway partially cross-sectional side view of a first embodiment of the present nozzles. 
         FIG. 2B  depicts an exploded side view of the first embodiment of the present nozzles. 
         FIG. 2C  depict a cutaway partially cross-sectional side view of the nozzle of  FIG. 2A  further comprising an additional electrode. 
         FIGS. 2D and 2E  depict cross-sectional top views of additional electrodes that are suitable for use in at least the nozzle of  FIG. 2C . 
         FIG. 3  depicts a partially cross-sectional side view of a depressible electrical connector that is suitable for use in the first embodiment of the present nozzles. 
         FIG. 4  depicts a top view of a PCB suitable for use in at least the first embodiment of the present nozzles. 
         FIG. 5  depicts a perspective view of a first embodiment of the present printer heads. 
         FIG. 6  depicts a perspective view of a first embodiment of the present printers. 
         FIG. 7  depicts an enlarged side view of certain components of a nozzle of the first embodiment of the present printers during a printing operation mode. 
         FIG. 8  depicts a flow chart of some of the present methods for performing maskless lithography. 
         FIGS. 9A and 9B  depict features printed using some embodiments of the present viscous fluids. 
         FIGS. 10A and 10B  are graphs representing some aspects of the relationship between printed feature line width, printing speed, printing media viscosity, and applied bias voltage. 
         FIG. 11  depicts printed features at various printing speeds. 
         FIG. 12  is a graphical representation of printed feature characteristics before and after sintering. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Referring now to the drawings, and more particularly to  FIGS. 1A-1B ,  FIG. 1A  depicts certain components of an illustrative example of an EHD printer  10 , which is generally representative of some of the present apparatuses and methods; and  FIG. 1B  depicts cutaway cross-sectional side views of a nozzle of printer  10  in two operating modes. Typically, EHD printers work by using a strong electric field to cause the ejection of printing media onto a substrate. For example, as shown, printer  10  comprises a reservoir  14  which can contain printing media  18 . EHD printing is desirable, in part, due to its ability to print micro- and nano-scale features with various materials [ 2 ], and printing media  18  can therefore comprise a variety of mediums, for example, mediums with viscosities within the range of about 1 cP to about 1000 cP and electrical conductivities within the range of about 10 −13  millisiemens per cm (mS/cm) to 10 −3  mS/cm, including, for example, metals, semiconductors, polymers, and living cells. The capability to print at such high viscosities, for example, can allow printing of thicker microstructures. Pressure (e.g., indicated by arrows  22 ) can be internally applied to reservoir  14  to create a meniscus  26  at the exit of nozzle  30  (e.g., in printer  10 , comprising a gold coated glass capillary with a 10 micrometer (μm) inner diameter), which is in fluid communication with reservoir  14  (e.g., as shown in operating mode I of  FIG. 1B ). A large bias voltage, which can be supplied by power supply  34  (which can include and/or be controlled by a function generator  38 ), can be applied to nozzle  30 . Through application of bias voltage, meniscus  26  can form into cone  42  and printing media  18  can be ejected as jet  46  (e.g., a continuous jet during printing operation) onto substrate  50 . For example, as bias voltage is applied to nozzle  30 , a voltage difference between substrate  50  and nozzle  30  can be realized. Mobile ions in printing media  18  can accumulate at the surface of meniscus  26  where mutual Coulombic repulsion and electrostatic attraction to substrate  50  can create tangential stress on meniscus  26 , resulting in the formation of cone  42  (also known as a Taylor cone) (e.g., as shown in operating mode II). When the bias voltage is sufficiently high, the tangential stress can overcome the surface tension of printing media  18  at the surface of cone  42 , and printing media can be ejected towards substrate  50 . By controlling printing media characteristics (e.g., viscosity, surface tension, conductivity and/or the like), stand-off distance  62  (e.g., the distance between nozzle  30  and substrate  50 ), pressure  22  (e.g., back pressure), bias voltage, nozzle characteristics (e.g., inner diameter  58 , shape, and/or the like) and/or the like, ejection characteristics can be adjusted. For example, as shown in operating mode II, printing media  18  is ejected as stable jet  54 . However, ejection characteristics (e.g., flow rate, jet diameter, stability, and/or the like) can vary, to include, without limitation, droplets, whipping (e.g., unstable) jets, and/or the like. As shown, during EHD printing, diameter  54  of jet  46  can be significantly smaller (e.g., up to two orders of magnitude) than nozzle  30  exit diameter  58 . 
       FIGS. 2A and 2B  depict different views of a first embodiment  66  of the present nozzles. In the embodiment shown, nozzle  66  comprises a circuit  70  (e.g., indicated by dashed lines) which has at least one depressible electrical connector  74  (e.g., two depressible electrical connectors  74 , as in the depicted embodiment). As used in this disclosure, the term depressible means deformable and/or compressible, regardless of an ability to completely return to a pre-depressed state. Referring additionally to  FIG. 3 , in the embodiment shown, depressible electrical connector  74  is a spring-loaded electrical connector (e.g., a pogo-pin connector). In this embodiments, connector  74  comprises a spring  76  that allows the connector to depress (e.g., a first portion  82  of connector  74  can move inwardly relative to a second portion  86  substantially along longitudinal axis  90  of the connector and through compression of spring  76 ) while permitting continuous electrical communication through the connector (e.g., through spring  76  and/or contact between first portion  82  and second portion  86 ). In the embodiment shown, connector  74  additionally comprises coupling features (e.g., ridge  94 ) to facilitate secure coupling of connector  74  with other components (as described in more detail below). In other embodiments, depressible electrical connectors  74  can comprise any connectors that permit the functionality described in this disclosure (e.g., elastomeric connectors, fuzz button connectors, spring probe connectors, and/or the like). 
     In the embodiment shown, depressible electrical connectors  74  may additionally provide and/or improve structural stability by applying a restraining force (e.g., via spring and/or spring-like compression of the connectors) to conductive tip  138  when dispensing device  126  is received by nozzle housing  98  and the connectors are in electrical communication (and contact) with the conductive tip (as described in more detail below). In this embodiment, nozzle  66  comprises a housing  98  having a first end  102 , a second end  106 , and a channel  110  extending from the first end to the second end (e.g., the housing may be fabricated using a stereolithography (SLA) three-dimensional (3D) printer). In the embodiment shown, housing  98  (e.g., first end  102 ) is configured to be releasably coupled to a printer head  114 . Such releasable coupling can be accomplished through a friction fit between nozzle housing  98  and printer head  114  and/or with interlocking features  118  configured to securely and precisely locate the nozzle housing relative to the printer head (e.g., nozzle housing  98  is physically restrained from moving past and rests against interlocking features  118  when fully inserted into printer head  114 ). In other embodiments, such releasable coupling may be accomplished through different and/or additional features such as fasteners (e.g., screws, pins, and/or the like) removably inserted into and/or through printer head  114  and into and/or through nozzle housing  98 , other interlocking features (e.g., tabs), a threaded connection between printer head  114  and nozzle housing  98 , latches, and/or the like. Additionally and/or alternatively, such releasable coupling can be achieved and/or facilitated through coupling of header pins  122  of circuit  70  to printer head  114  (described in more detail below). 
     In the embodiment shown, circuit  70  comprises at least one header pin  122  (e.g., two header pins, as shown) configured to be in electrical communication with printer head  114  when first end  102  of nozzle housing  98  is releasably coupled to printer head  114 . For example, in the embodiment shown, header pins  122  are constructed from a conductive material and protrude past first end  102  of nozzle housing  98  where such protruding sections can be received with conductive receptacles (e.g., sockets) on and/or within printer head  114  (e.g., such that printer head  114  and circuit  70  are in electrical communication). In the embodiment shown, channel  110  is configured to removably receive a dispensing device  126 . For example, channel  110  can comprise interlocking features that substantially correspond to an outside surface of dispensing device  126  such that dispensing device  126  can be received by channel  110  through first end  102  and be engaged (e.g., removably received) by such interlocking features. In other embodiments, channel  110  can be substantially hollow and can receive dispensing device  126  through first end  102  and/or second end  106 . In such embodiments, securing of dispensing device  126  relative to nozzle housing  98  can be accomplished through releasable attachment between dispensing device  126  and printer head  114  and/or fluid reservoir  134  (e.g., through surfaces configured for a friction fit, fasteners, interlocking features, a threaded connection, latches, and/or the like). 
     In the embodiment shown, dispensing device  126  comprises a threaded portion  130  for releasable coupling with printer head  114  and/or reservoir  134  (shown in  FIG. 2B ). In this embodiment, the reservoir may be inserted into and/or through printer head  114  and twisted to engage threaded portion  130  of the dispensing device, thus securing and/or sealing reservoir  134  and/or dispensing device  126  relative to printer head  114 . However, in other embodiments, dispensing device  126  can comprise any releasable coupling structure which permits the functionality described in this disclosure, including, but not limited to, surfaces configured for a friction fit, fasteners, interlocking features, latches, and/or the like disposed on dispensing device  126 , printer head  114 , and/or reservoir  134 . Additionally, sealing between dispensing device  126  and printer head  114  and/or reservoir  134  can be accomplished through any structure which permits the functionality described in this disclosure, including, but not limited to, O-rings, sealant, sealing tape, compression fittings, and/or the like. 
     In the embodiment shown, dispensing device  126  comprises a conductive tip  138  such that electrical communication is permitted between circuit  70  and the conductive tip. For example, in the embodiment shown, when dispensing device  126  is received by nozzle housing  98 , conductive tip  138  can be in electrical communication with depressible electrical connectors  74  (e.g., in contact) such that electricity can flow through circuit  70  and into conductive tip  138 . To illustrate, circuit  70  can apply a bias voltage across conductive tip  138 , for example, supplied by power source  142 ). In the embodiment shown, nozzle  66  comprises two depressible electrical connectors  74 , where the connectors are configured to contact substantially opposite sides of the conductive tip (e.g., to facilitate circuit  70  in applying a bias voltage across the conductive tip). However, in other embodiments, the present nozzles may comprise any number of depressible electrical connectors which permits the functionality described in this disclosure (e.g., 1, 2, 3, 4, or more depressible electrical connectors). In the embodiment shown, conductive tip  138  comprises stainless steel, however, in other embodiments, conductive tip  138  can comprise any material which permits the functionality described in this disclosure, including, but not limited to, silver, gold, copper, aluminum, graphite, conductive polymers, and/or the like. In this embodiment, conductive tip  138  has an outer diameter  146  of 0.24 millimeters (mm) and an inner diameter of 0.1 mm (e.g., conductive tip is 38 gauge (ga)). In the embodiment shown, circuit  70  comprises at least one PCB  150  (e.g., two PCBs). Referring additionally to  FIG. 4 , PCB  150  comprises an electrically insulative substrate  154 . In this embodiment, PCB  150  additionally comprises at least two holes  162   a  and  162   b , and a conductive trace  158  between the holes (e.g., to permit electrical communication between hole  162   a  and hole  162   b ). PCBs  150  can be used to facilitate assembly and/or disassembly of nozzle housing  66  (e.g., for initial assembly or for repair in the event of damage, for example, due to arcing), as described in more detail below with reference to  FIG. 2B . 
       FIG. 2B  depicts an exploded side view of nozzle  66 . To assemble nozzle  66 , dispensing device  126  can be inserted through first end  102  of nozzle housing  98  where it can be engaged (e.g., with interlocking features, as described above) within channel  110 . Depressible connectors  74  can then be inserted into receptacles  106  of nozzle housing  98  where the depressible connectors can depress and contact nozzle tip  138  (e.g., as described above). PCBs  150  can be inserted into slots  170  along and/or within the sides of nozzle housing  98 . In the embodiment shown, holes  162   a  and  162   b  of PCB  150  have different sizes (e.g., hole  162   a  has a larger diameter than hole  162   b ) configured to facilitate correct assembly of nozzle  66 . For example, smaller hole  162   b  can be configured to receive depressible electrical connector  74  (e.g., sized to securely receive the connector such that ridge  94  rests against an upper surface of PCB  150 ), and larger hole  162   a  can be configured to receive header pin  122  (e.g., to dictate a desired orientation of PCBs  150  relative to nozzle housing  66  during assembly, for example, to prevent user assembly error). In this embodiment, PCBs  150  can be inserted into nozzle housing  98  until depressible electrical connectors  74  securely lock into place (e.g., such that some spring or spring-like tension within the connector is released) into a hole (e.g.,  162   b ) on the PCBs (e.g., thus securing PCBs  150  and depressible electrical connectors  74  relative both to each other and to the nozzle housing). Solder can be applied to the connection to strengthen the connection and/or enhance electrical communication between PCBs  150  and depressible electrical connectors  74 . Header pins  122  may be inserted into holes on PCBs  150  (e.g., holes not occupied by electrical connectors  74 , for example, in this embodiment, holes  162   a ) and optionally soldered into place (e.g., similar to as described above). In the embodiment shown, header pins  122  can additionally be secured to nozzle housing  98  through insertion of locating pins  170  into respective holes and/or slots in nozzle housing  98  (e.g., as shown in  FIGS. 2A and 2B ). 
       FIG. 2C  depicts a side view of nozzle  66 , further comprising an additional electrode  167  (e.g., disposed proximate second end  106  of housing  98 ). In the embodiment shown electrode  167  comprises a conductive material (e.g., stainless steel, silver, gold, copper, aluminum, graphite, conductive polymers, and/or the like). In the embodiment shown, a bias voltage can be applied to the electrode (e.g., via electrical communication, for example, through circuit  70 , with power source  142 , similar to as described above). For example, circuit  70  can comprise two parallel portions (both in electrical communication with power source  142 ) (e.g., portion  70   a  in electrical communication with conductive tip  138  and portion  70   b  in electrical communication with electrode  167 ). As shown, portion  70   b  can be disposed within channels  172  in housing  98  (e.g., extending from header pins  122  to electrode  167 ). However, in other embodiments, portion  70   b , housing  98  and/or nozzle  66  can comprise any suitable structure that permits the functionality described in this disclosure, including, but not limited to, wires and/or traces disposed within and/or on housing  98 , similar structure as described above (e.g., with PCBs, depressible electrical connectors, header pins, and/or the like, which may be the same and/or different than (e.g., additional to) those described with reference to  FIGS. 2A and 2B ), and/or the like. Some embodiments are configured such that a first bias voltage can be applied to conductive tip  138  and a second bias voltage, different than the first, can be applied to electrode  167  (e.g., via a dedicated circuit for the electrode and a dedicated (e.g., a second) circuit for the conductive tip, a voltage divider configured to split bias voltage between conductive tip  138  and electrode  167 , which may be adjustable, and/or the like). Furthermore, in these embodiments, power source  142  can be configured to provide two distinct (e.g., different) voltages and/or two power sources can be provided (e.g., one power source in electrical communication with the conductive tip through a first circuit and a different power source connected to the electrode through a second circuit) which may be individually adjustable. Power sources may form part of the present printer heads and/or printers (described in more detail below), and/or may be provided separately. As shown, electrode  167  has an opening  168  (e.g., with a transverse dimension  169 ) that is substantially centered on a longitudinal axis  175  of the conductive tip (e.g., if opening  168  is circular, as shown, the opening and the conductive tip are substantially concentric). In the embodiment shown, opening  168  is larger than the inner diameter of the conductive tip  138 . In the embodiment shown, electrode  167  is placed in proximity to conductive tip  138  (e.g., electrode  167  is placed within a distance  171  to conductive tip  138  such that an electrostatic field generated by the electrode under an applied bias voltage can affect an EHD printing jet during printing operation). In the embodiment shown, distance  171  is substantially fixed (e.g., and substantially defined by the configuration of nozzle housing  98 ), however, in other embodiments, the distance between the electrode and the conductive tip may be adjustable (e.g., through a slidable coupling, threaded connection, and/or the like between housing  98  and electrode  167 ). Additionally, in some embodiments, housing  98  may be slidably and/or rotatably (e.g., threadably) coupled to printer head  114  independently of dispensing device  126  and/or conductive tip  138  (e.g., to allow for an adjustable distance  171 , alone or in addition to the above).  FIGS. 2D and 2E  depict cross-sectional views of electrodes suitable for use in the present nozzles. For example, such electrodes can comprise any cross-sectional shape which permits the functionality described in this disclosure, including, but not limited to, square (e.g.,  FIG. 2D , or a “plate”), polygonal, and/or the like (e.g., and may have rounded corners), circular (e.g., electrode  167   a  of  FIG. 2E , or a “ring”), elliptical, and/or otherwise rounded, and/or the like. Electrode  167  can be configured to provide desirable EHD printing properties. For example, transverse dimension  169 , distance  171 , an applied bias voltage to electrode  167  and/or the conductive tip  138 , and/or the like can be adjusted to control EHD printing properties (e.g., to focus the EHD printing jet during printing operation). The addition of electrode  167  can also enhance electrostatic force generation (e.g., to facilitate and/or enhance printing on a substrate with a low conductivity). 
     The assembled nozzle  66  can then be coupled to (e.g., inserted into) printer head  114  (e.g., and secured as described above). In the embodiment shown, reservoir  134  can be coupled to (e.g., inserted into) printer head  114  and turned to engage threaded portion  130  of dispensing device  126  in order to securely fasten and/or seal dispensing device  126  to reservoir  134  and/or printer head  114  (e.g., for printing operation). Through such features, the present dispensing devices (e.g.,  126 ) can be quickly and easily replaced within the nozzle (e.g., in the event of damage due to arcing, for example, to conductive tip  138 ). For example, nozzle housing  98  can be removed from printer head  114  and depressible electrical connectors can allow dispensing device  126  to be removed from nozzle housing  98  with minimal effort. Additionally, in the event of more extensive damage, the entire nozzle assembly can be easily be replaced, if needed. 
       FIG. 5  depicts a perspective view of a first embodiment  114  of the present printer heads. In the embodiment shown, printer head  114  housing may be fabricated using a SLA printer. In the present embodiments, printer head  114  can comprise any of the present printer nozzles (e.g., nozzle  66 , as described above). In the embodiment shown, printer head  114  comprises a reservoir  134  in fluid communication with nozzle  66 , and reservoir  134  is configured to contain printing media (e.g., reservoir  134  is substantially hollow and sealable, for example, comprising a syringe). In this embodiment, reservoir  134  has an internal volume of about 3 milliliters (mL) and a cap  174  (e.g., an engineered fluid dispensing (EFD) cap) that can be removed to permit filling reservoir  134  with printing media (e.g., cap  174  is connected and/or sealed to reservoir  134  via a screw, compression and/or the like connection). In the embodiment shown, reservoir  134  is configured to be coupled to a fluid source  178  (e.g., a precision pressure regulator) such that fluid source  178  can deliver fluid (e.g., air) to or remove fluid from reservoir  134  to adjust an internal pressure (e.g., back pressure) of the reservoir (e.g., to vary and/or control the hydrostatic pressure at the exit of conductive tip  138  of nozzle  66  during printing operation). 
     Unless otherwise indicated by the context of its use, the term “pressure” includes, but is not limited to, positive pressures, negative (vacuum) pressures, and zero (ambient) pressures, all relative to an ambient (e.g., atmospheric) pressure. For example, in the embodiment shown, cap  174  comprises a nipple  182  configured to accept a fluid line  186  from fluid source  178 . Nipple  182  can be and/or can be configured to be connected to fluid line  186  through any structure that permits the functionality of this disclosure, including, but not limited to, barbed, compression, push lock, and/or like fittings and/or the like. Some embodiments of the present printer heads comprise a fluid source (e.g., fluid source  178  coupled to printer head  114  and forming part of printer head  114 ). In the embodiment shown, printer head  114  further comprises a power source  142  (e.g., a Trek 615-10 high voltage generator, available from TREK, Inc.) configured to electrically communicate with nozzle  66  (e.g., through circuit  70  and to apply a bias voltage and/or ejection voltage across conductive tip  138 ), such as, for example, through wired connections within printer head  114  comprising conductive receptacles and/or sockets connected to header pins  122  of circuit  70 . Generally, in a voltage pulse train (e.g., which can be supplied by the power source(s) of the present disclosure), a bias voltage can correspond to a base voltage of the pulse train, and an ejection voltage can correspond to a peak voltage of the pulse train). 
     In the embodiment shown, printer head  114  further comprises a processor  144  (e.g., a microprocessor). Unless otherwise indicated by the context of its use, the terms “a processor” or “the processor” mean one or more processors and may include multiple processors configured to work together to perform a function. Processor  144  can be configured to control any fluid source (e.g.,  178 ) and/or power supply (e.g.,  142 ) of the present printer heads and/or printers (e.g., based on data captured by sensors, described in more detail below). In those of the present embodiments that include a processor, the present printer heads and/or printers can also comprise at least one sensor (e.g., a pressure sensor) configured to capture data indicative of the internal pressure (e.g., back pressure) within reservoir  134  (e.g., a sensor disposed within reservoir  134 ). Processor  144  can, for example, receive data from the sensor and control fluid source  178  based on the data (e.g., to correspond the internal pressure of reservoir  134  to a desired pressure value). In some embodiments of the present printer heads, fluid source  178 , power source  142 , and/or processor  144  may form part of the present printers, and in such embodiments, may not form part of the present printer heads. In the embodiment shown, printer head  114  comprises a mount  190  configured to securely locate printer head  114  (e.g., relative to a printer and/or a working surface). In the embodiment shown, mount  190  comprises mounting holes  194  configured to accept fasteners (e.g., screws, pins, and/or the like) to secure printer head  114  to a printer; however, in other embodiments, printer head  114  can be mounted with any structure which permits the functionality described in this disclosure. Components of the present nozzles and/or printers (e.g., printer head  114  housing and nozzle housing  98 , reservoir  134 , cap  174 , PCBs  150 , dispensing device  126 , depressible electrical connectors  74 , header pins  122 , wiring, and/or the like) can be commercially available, and may comprise a combined cost of about $50 United States dollars. 
       FIG. 6  depicts an embodiment  194  of the present printers. In the present embodiments, printer  194  can comprise any of the present printer heads (e.g., printer head  114 ). In the embodiment shown, printer  194  comprises printer head  114  and a working surface  198  (e.g., a 300 by 300 mm working surface). Working surface  198  can be configured to secure a substrate  202  for printing operation. For example, the working surface can comprise a vacuum surface such that substrate  202  is securely held in place by vacuum (e.g., negative pressure, supplied through vacuum lines  206  by a pump or fluid source (not expressly shown)). In other embodiments, working surface  198  can secure substrate  202  through any alternative and/or additional structure(s) that permits the functionality described in this disclosure, including, but not limited to, clamps, fasteners, interlocking features, latches, adhesive, and/or the like. 
     In the embodiment shown, printer  194  comprises at least one orientation actuator  210  (e.g., stage(s)) configured to adjust an orientation of working surface  198  relative to printer head  114 . In the embodiment shown, orientation actuator  210  comprises three stages (e.g., an x-stage, a y-stage, and a z-stage) configured to move working surface  198  relative to printer head  114  (e.g., in directions along transverse axes  214 ,  218 , and  222 , respectively). In other embodiments, orientation actuator  210  can comprise (e.g., additionally) a theta stage configured to move working surface  198  relative to printer head  114  in a rotational direction, as indicated by arrow  226 . 
     In the embodiment shown, printer  194  comprises a processor  144  configured to adjust the orientation of working surface  198  relative to printer head  114  (e.g., through control of orientation actuator  210 ). However, in other embodiments, orientation actuator(s) (e.g.,  210 ) may be coupled to the printer head (e.g., as opposed to or in addition to, the working surface, and be configured to move the printer head (e.g.,  114 ) relative to the working surface. In the embodiment shown, printer  194  comprises at least one sensor  230  configured to capture data indicative of the orientation of working surface  198  relative to printer head  114  (e.g., a high-speed camera, such as a Phantom V-130, available from Vision Research, Inc., configured to capture image data). In the embodiment shown, processor  144  can be further configured to adjust the orientation of the working surface relative to the printer head based on the data captured by the at least one sensor (e.g., by receiving data from sensor  230  and calculating the location of the printer head and/or nozzle relative to the working surface and/or substrate  202 ). For example, working surface  198  and/or substrate  202  may comprise fiducials which can be recognized by processor  144  in data captured by sensor  230  (e.g., by analyzing the pixels in images captured by sensor  230  to determine fiducial locations). The location of printer head  114  and/or nozzle  66  can be determined (e.g., through calibration and/or information provided by orientation actuator  210 ) and/or acquired through locating fiducials disposed on printer head  114  and/or nozzle  66 . By comparing the relative locations of substrate  202  and/or working surface  198  with printer head  114  and/or nozzle  66 , processor  144  can precisely actuate any required adjustments (e.g., by communicating with orientation actuator  210 ) (e.g., a machine vision system). 
     In the embodiment shown, printer  194  comprises a fluid source  178  and a power source  142 , the operation of each substantially similar to as described above with reference to  FIG. 5 . In this embodiment, printer  194  comprises a user interface  234  configured to allow user monitoring and/or control of printer  194  (e.g., starting and stopping of printing operations, manual printing operations, and/or the like) as well as to display information to a user (e.g., information regarding printing operations, such as, for example, data from sensor  230 , orientation actuator  210 , and/or the like, as well as regarding printer operation, such as, for example, hardware failures, software failures, and/or the like). Such configuration of user interface  234  may be accomplished through a graphical user interface (GUI) (e.g., provided by software) on a computer which may be connected to and/or in control of printer  194 , sensor  230 , orientation actuator  210 , power source  142 , fluid source  178  and/or the like. In some embodiments, processor  144  can perform many or all of the monitoring and/or control functions and be configured to relay information to and/or from user interface  234  (e.g., as opposed to the user interface being provided by a separate computer and/or processor). In the embodiment shown, printer  194  further comprises a memory  238  configured to store information regarding printing operations, for example, desired printing patterns. For example, processor  144  can read information from memory  238  and communicate with power source  142 , fluid source  178 , user interface  234 , orientation actuator  210 , and/or sensor  230  to effectuate the desired patterns (e.g., to print the desired patterns onto substrate  202 ). 
       FIG. 7  depicts an enlarged side view of the nozzle exit (e.g., end of conductive tip  138 ) of an EHD printer (e.g.,  194 ) of the present disclosure during printing operation. As shown, printing media  18  (e.g., in this example, ethylene glycol) at the nozzle forms a cone  42  (e.g., due to repulsive Coulombic and electrostatic forces, as described above with reference to  FIGS. 1A and 1B ) and a stable jet  46  of printing media is ejected towards substrate  50  (e.g., a gold coated glass slide). In this example, the internal pressure (e.g., back pressure, indicated by arrows  22 ) of the printing media reservoir is 0.5 kilopascals (kPa), the bias voltage applied to the conductive tip is 2 kilovolts (kV), the stand-off distance  62  is within the range of about 0.8 to about 1.0 mm, and the conductive tip has an outer diameter of 0.24 mm and an inner diameter  58  of 0.1 mm. 
     Some of the present methods include inserting a dispensing device (e.g.,  126 ) with a conductive tip (e.g.,  138 ) into an EHD nozzle (e.g., into nozzle housing  98 ), where the nozzle has at least one depressible electrical connector (e.g.,  74 ) and the inserting is such that the depressible electrical connector contacts the conductive tip (e.g., as shown in  FIG. 2A ), and applying a bias voltage across the conductive tip by enabling electrical communication between the depressible electrical connector and a power source (e.g.,  142 ) (e.g., through circuit  70 ). In others of the present methods, the nozzles can include an electrode (e.g.,  167 ) and the methods can further comprise applying a voltage (e.g., by enabling electrical communication between the power source, for example, power source  142  and/or a second power source). 
     Referring to  FIG. 8 , others of the present methods comprise performing ( 242 ) maskless lithography by generating an electric field around an EHD printer nozzle (e.g.,  66 ) ( 246 ), the nozzle having a housing (e.g.,  98 ) with at least one depressible electrical connector (e.g.,  74 ) and a dispensing device (e.g.,  126 ) with a conductive tip (e.g.,  138 ) disposed in the housing such that electrical communication is permitted between the conductive tip and the depressible electrical connector, where the electric field is generated by enabling electrical communication between the depressible electrical connector and a power source (e.g.,  142 ) to apply a bias voltage across the conductive tip. Though not required in all embodiments, a constant hydrostatic pressure (e.g.,  22 ) can be maintained  250  at the exit of the nozzle (e.g., at the end of conductive tip  138 ), for example, by processor (e.g.,  144 ) control of a fluid source (e.g.,  178 ) coupled to the reservoir (e.g.,  134 ) in fluid communication (and thus pressure communication) with the dispensing device (e.g.,  126 ) and therefore the conductive tip (e.g.,  138 ). In others of the present methods, the nozzles can include an electrode (e.g.,  167 ) and the methods can further comprise adjusting a distance (e.g.,  171 ) between the electrode and the conductive tip (e.g.,  138 ). 
     Embodiments of the present methods can further comprise ejecting  254  viscous fluid (e.g., printing media  18 ) from the nozzle (e.g., through application of pressure  22  and bias voltage from power source  142 ). Viscous fluid (e.g., printing media  18 ) can comprise a variety of materials, as described above. In some embodiments, the viscous fluid comprises a negative epoxy resist (e.g., KMPR photoresists and/or SU-8 photoresists, available from MicroChem Corp.) modified with at least one of a surfactant and/or a solvent such that the viscous fluid has a viscosity and a surface tension suitable for maskless lithography (e.g., a high viscosity, for example, from about 300 cP to about 1000 cP to print relatively thick microstructures, for example features having a width on the order of a few hundreds of micrometers and a height on the order of tens of micrometers). In some embodiments, the viscous fluid comprises an ionic metal salt (e.g., zinc nitrate, zinc acetate, tin nitrate, and/or the like). In some embodiments, the viscous fluid comprises a matrix material (e.g., polyethylene glycol, polyvinylpyrrolidone, and/or the like). Additionally, a solvent (e.g., ethylene glycol, N-Methyl-2-pyrrolidone (NMP), methanol, and/or the like) or surfactant (e.g., a material which can reduce the surface tension of the viscous fluid) can be included within any of the viscous fluids used in the methods explicitly described above. 
     In some embodiments, the viscous fluid comprises poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (“PEDOT:PSS”). PEDOT:PSS is a generally transparent and conducting polymer that is ductile, elastic, and stable, having a gauge factor of 5-20 (as compared to a conventional metal film having a gauge factor of 2). Thus, PEDOT:PSS may be particularly suited for strain-based sensor applications, such as, for example, pressure, strain, and touch sensors (e.g., for use in touch screen technologies). Such viscous fluids comprising PEDOT:PSS (e.g., comprising from 1-10% by weight PEDOT:PSS) may comprise a matrix material (e.g., comprising from 1-20% by weight of the matrix material) (e.g., a dissolvable polymeric material such as polyvinylpyrrolidone, polyvinyl-alcohol, mixtures thereof, and/or the like), a solvent (e.g., from 10-90% by weight of the solvent) (e.g., N-methylpyrrolidone, dimethyl sulfoxide, methanol, ethanol, mixtures thereof, and/or the like), and/or the like. 
       FIGS. 9A and 9B  depict interconnected pads  278  printed on a gold-coated glass substrate as printed ( FIG. 9A ) and after drying (e.g., at room temperature for approximately 3 hours) ( FIG. 9B ). As shown, after drying, pads  278  have a length  282  of approximately 2220 μm, a width  286  of approximately 2160 μm, with interconnecting features  290  between pads having a transverse dimension  294  of approximately 150 μm. 
     Some embodiments of the present methods include directing  258  the fluid to a substrate (e.g.,  202 ). Substrates of the present methods can comprise a variety of materials; however, it can be desirable that substrates be electrically conductive and/or coated with a thin electrically conductive material to facilitate generation of electrostatic forces between conductive tip (e.g.,  138 ) and the substrate. For example, the substrate (e.g.,  202 ) can comprise, but is not limited to comprising, silicon (e.g., a wafer), glass, polymer, ceramic, and/or the like. While not required in all embodiments, the electric field between the conductive tip (e.g.,  138 ) and the substrate (e.g.,  202 ) can be adjusted  266  (e.g., by processor  144  control of power source  142 ). Also, while not required in all embodiments, the distance between the nozzle (e.g., end of conductive tip  138 ) and the substrate (e.g.,  202 ) can be adjusted  270  and/or the nozzle can be moved relative to the substrate (e.g., by processor  144  monitoring of sensor  230  and/or control of orientation actuator  210 ). Fluid (e.g., printing media  18 ) selection, bias voltage applied to the nozzle (e.g., conductive tip  138 ), printing speed (e.g., speed at which printer head  114  moves relative to substrate  202 , for example, during actuation of orientation actuator  210 ), and stand-off distance (e.g.,  62 ) can have an effect on the characteristics of printed features. 
     While studies have been conducted that can predict jet characteristics, printed feature characteristics (e.g., shape, line width, thickness, and/or the like) can sometimes be difficult to predict. Line width and thickness can be described in terms of flow rate and jetting diameter in conjunction with post deposition spreading. Flow rate can be approximated as: 
     
       
         
           
             
               
                 
                   Q 
                   ≈ 
                   
                     
                       
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           d 
                           N 
                           4 
                         
                       
                       
                         128 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         µL 
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                         + 
                         
                           
                             1 
                             2 
                           
                           ⁢ 
                           
                             ɛ 
                             0 
                           
                           ⁢ 
                           
                             E 
                             2 
                           
                         
                         - 
                         
                           
                             4 
                             ⁢ 
                             γ 
                           
                           
                             d 
                             N 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where Q represents flow rate, d N  and L represent the diameter and length of the nozzle, respectively, ΔP represents the hydrostatic pressure with respect to the nozzle exit, Σ 0  represents the permittivity of free space, γ represents the surface tension of the air-fluid interface, and E represents the magnitude of the electric field [1, 3]. Jetting diameter can be approximated as: 
                   d   ∝         γ     ɛ   0         ⁢         d   N       E               (   2   )               
[1, 3]. While EQS. (1) and (2) can predict flow rate and jetting diameter with relative accuracy, predicting the geometry of a printed feature (e.g., shape, line width, thickness, and/or the like) can be difficult to the complex nature of the factors involved. For example, flow rate is directly proportional to applied bias voltage (e.g., as applied bias voltage is directly proportional to the magnitude of the electric field); however, jetting diameter is inversely proportional to applied bias voltage [3]. Therefore, for a given fluid (e.g., with given characteristics), an increase of applied bias voltage can increase flow rate while decreasing jetting diameter (which constitute counteracting values with respect to printed feature geometry). To illustrate, smaller jetting diameters could be expected to create printed features with smaller line widths, however, more fluid is typically ejected with increased flow rate, which can result in more post deposition spreading (and potentially features with larger line widths). Additionally, post deposition spreading and/or printed feature characteristics can be a function of volume of fluid deposited per unit area, solvent evaporation rate, fluid viscosity, fluid surface tension, substrate properties, and/or the like. For example, fluids with a high surface tension may hold together after printing, resulting in minimal post deposition spreading, and fluids with a low surface tension may spread out after printing, resulting in a larger post deposition spreading (e.g., and thus an increase in line width). Table 1 provides an example of such effects.
 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Fluid Viscosity and Applied Bias Voltage versus Printed Line 
               
               
                 Width and Thickness for a Printing Speed of 1000 mm/minute 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Applied Bias 
                 Line width 
                 Thickness 
               
               
                   
                 Ink 
                 Voltage (V) 
                 (μm) 
                 (nm) 
               
               
                   
                   
               
               
                   
                 I-455 
                 850 
                 43 (42*) 
                 607 (48*) 
               
               
                   
                 I-455 
                 750 
                 39 (38*) 
                 416 (33*) 
               
               
                   
                 I-312 
                 850 
                 47 (46*) 
                 476 (38*) 
               
               
                   
                 I-312 
                 750 
                 37 (36*) 
               
               
                   
                   
               
            
           
         
       
     
     Values marked with an asterisk (*) indicate measured values after sintering (described in more detail below). Measurements were performed using a scanning electron microscope (SEM) and surface profile meter. I-312 and I-455 represent Zinc-containing fluids with viscosities of 312 and 455 cP, respectively, which can otherwise be similar to the viscous fluids described above. Both fluids contain the same Zinc concentration, solid loading, solvent percentage, surface tension, and conductivity values. As shown, for a given applied bias voltage, in general, more viscous fluids produce printed structures with smaller widths, but larger thicknesses, at least in part due to viscous effects on post deposition spreading (e.g., more viscous inks may more resistant to post deposition spreading than less viscous inks). For example, at a bias voltage of 850 volts (V), features printed with I-455 fluid have a line width of 43 μm, and features printed with I-312 fluid have a line width of 47 μm (e.g., more viscous fluid I-455 generally prints features with smaller line widths than less viscous fluid I-312). Also at 850 V, I-455 fluid prints features with a thickness of 607 nanometers (nm), and I-312 fluid prints features with a thickness of 476 nm (e.g., more viscous fluid I-455 generally prints features with larger thicknesses than less viscous fluid I-312). Using I-455 and/or I-312 fluid in the apparatuses of the present disclosure, ZnO macrostructures with line widths ranging from about 18 to about 65 μm and thicknesses ranging from about 33 to 62 nm can be printed. As shown in Table 1, the present fluids are suitable for maskless lithography applications (e.g., for fabricating TFT and/or gas sensors, and/or the like). 
     Some aspects of the relationship between printed feature line width, fluid viscosity, applied bias voltage, and printing speed are shown in  FIGS. 10A and 10B .  FIG. 10A  graphs data for both fluids (I-312 and I-455) at an applied bias voltage of 850 V. As shown, for both fluids, line width decreases with printing speed, and such decreases are more pronounced for the lower viscosity fluid I-312. This may be explained, in part, due to post deposition spreading. Dispensed fluid volume per unit area decreases with increases in printing speed. At low volumes per unit area, solvent may evaporate before the printed features can undergo substantial post deposition spreading. Referring now to  FIG. 10B , graphed is data for I-312 fluid printed feature line width at applied bias voltages of 750 V and 850 V at various printing speeds. As shown, the effect of bias voltage on printed feature characteristics (e.g., line width) dominates over effects due to viscosity (e.g., compare  FIG. 10A  with  FIG. 10B ). Therefore, flow rate, as opposed to jetting diameter, may be a dominant factor in determining printed feature characteristics. 
       FIG. 11  shows some aspects of the relationship between printing speed and printed feature characteristics. The depicted features were printed at an applied bias voltage of 450 V (e.g., an ejection voltage of 650 V), an ejection frequency of 600 kilohertz (kHz), and a back pressure of 10 kilopascals (kPa). As shown, generally, at low printing speeds, printed features resemble lines (e.g., from about 200 mm/min to about 600 mm/min, for this particular bias voltage, back pressure, and fluid). Also, as printing speeds increase, line width tends to decrease. At sufficiently high printing speeds (e.g., above about 800 mm/min) printed features resemble dots. Therefore, by controlling printing speed, printed feature characteristics can be controlled. For example, for drop on demand dot printing (e.g., where dot features are desirable), higher printing speeds may be advantageous. 
     Referring back to  FIG. 8 , in the embodiment shown, the present methods for performing maskless lithography can comprise curing  274  the viscous fluid. Curing can comprise heating the printed features after printing by baking (e.g., at a temperature within the range of approximately 100 degrees Celsius (° C.) to approximately 110° C., for a time of approximately one minute), sintering (e.g., at a temperature greater than or equal to approximately 400° C. for a time of approximately 45 minutes), and/or the like, as well as by exposure to light (e.g., by ultraviolet (UV) curing, for example, under approximately 500 watts (W) of power, for a time of about one minute). Specific curing parameters (e.g., power, temperature, time, and/or the like) can vary based on the materials in the printing media and/or the substrate. In some embodiments curing can comprise etching.  FIG. 12  is a graphical representation of printed feature characteristics before and after sintering [4]. In the example depicted, features printed in I-455 fluid at a printing speed of 1000 mm/min and an applied bias voltage of 850 V were sintered (e.g., heated above approximately 400° C. for approximately 45 minutes). At these temperatures, similar to other fluids with metallic elements or metal oxide nanoparticles, I-455 fluid produces polycrystalline ZnO (e.g., a metal oxide printed microstructure). No detectable cracks or pinholes were observed, and as shown, sintering resulted in a lateral shrinkage (e.g., a line width decrease) of approximately 3% and a thickness shrinkage (e.g., a thickness decrease) of approximately 92%. The relatively large thickness shrinkage may be attributed, in part, to the removal of matrix materials that may occur during sintering, and the relatively small lateral shrinkage may be attributed, in part, to a strong adhesion of the fluid to the substrate. 
     The present masked based lithography methods (e.g.,  242 ) can offer lower manufacturing costs, less use of chemicals (and thus a lower environmental impact), and faster production cycles, as well as flexibility in substrate size and shape, and fluid selection. 
     The above specification and examples provide a complete description of the structure and use of illustrative embodiments. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the various illustrative embodiments of the methods and systems are not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, elements may be omitted or combined as a unitary structure, and/or connections may be substituted. Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and/or functions, and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. 
     The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively. 
     REFERENCES 
     These references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
     1. U. Chen. et al., “A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production,”  Phy. Fluids.,  2002; 14: L1-L4.   2. J.-U Park et al., “High-resolution electrohydrodynamic jet printing,”  Nat. Matter.,  2007; 6: 782-789.   3. K. Choi et al., “Scaling laws for jet pulsations associated with high-resolution electrohydrodynamic printing,”  Appl. Phy. Lett.,  2008; 92: 123109 (1-3).   4. B. S. Barros et al., “Synthesis and X-ray Diffraction Characterization of Nanocrystalline ZnO Obtained by Pechini Method,”  Inorganic Matter.,  2006, 42(12):1348-1351.