Patent Publication Number: US-9415590-B2

Title: Electrohydrodynamic jet printing device with extractor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/816,423, filed Apr. 26, 2013, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to structures and methods for printing and, more specifically, for electrohydrodynamic jet printing. 
     BACKGROUND 
     Processes such as ink jet printing and lithography are used to fabricate a wide range of electronics and bio-sensors at the micro- and nano-scale (μ/n-scale). Despite advancements in these processes over time, they are not always able to meet certain performance requirements (e.g. resolution, material diversity, or process flexibility) and/or cost requirements (e.g. material use or cycle time), particularly in emerging applications in biotechnology and flexible electronics. 
     Ink jet printing has seen rapid development in the past few decades. Some ink jet systems are able to provide high throughput (e.g., 50-175 kHz jetting frequency) at low system costs. Printing processes are generally considered to be environmentally friendly processes as applied to device fabrication since they are additive processes that produce minimal waste. But the nozzle diameter of a typical ink jet print head, such as a thermally actuated bubble jet system print head, is about 20-30 μm, resulting in a print resolution that is too coarse for micro-scale applications. Some ink jet print systems use piezoelectric materials to generate mechanical waves that eject ink droplets from the nozzle. But the mechanical vibration of the nozzle can limit the accuracy of such systems, rendering them unable to meet the tight resolution requirements of μ/n-scale applications. The types of ink materials that can be used in thermal- and piezoelectric-actuated ink jet systems is also somewhat limited by nozzle clogging issues and the need to withstand exposure to the temperatures used in thermal actuation. 
     Lithography processes have proven capable in mass manufacturing and in some μ/n-scale manufacturing applications, but are not able to provide the flexibility, material diversity, and cost effectiveness required for all μ/n-scale manufacturing applications, particularly in new and emerging applications. Optical lithography employs an etching process to produce a specific pattern determined by a pre-designed mask. While highly accurate, this process is not suitable for biological materials or electro-optical components that are susceptible to the aggressive materials used in the etching process. The masking requirement also makes photolithography less flexible than printing processes, not to mention more time consuming and costly. Nanoimprint lithography can achieve high resolution and accuracy, demonstrates high throughput at a low cost for mass production, is compatible with many materials, and can create 3D structures at the nano-scale. However, nanolithography also requires a mask and is thus less flexible than printing processes. Lithography processes can also be relatively complex, often including multiple steps for fabrication, and is generally not considered environmental friendly, as etching away material necessarily includes material waste. 
     Electrohydrodynamic jet printing, also known as e-jet or EHD printing, is a type of printing that has shown promise for use in printed electronics and bio-sensor applications. A typical e-jet printing process relies on an electrostatic field between a conductive nozzle and a conductive substrate to extract a printing fluid from the nozzle without the increased temperatures or mechanical vibrations associated with thermal- and piezoelectric-actuated ink jet printing. E-jet printing has been somewhat limited by low product throughput and the requirement for a conductive substrate to generate the necessary electrostatic field. Process sensitivity has also plagued e-jet printing. For example, nozzle-to-substrate distance can be a critical parameter affecting the generated ink-extraction field, thus generally limiting the process to flat substrates. Additionally, once a layer of non-conductive ink is deposited onto the conductive substrate, the character of the generated electrostatic field is changed, greatly limiting the use of e-jet printing in 3D-printing applications. 
     SUMMARY 
     An embodiment of a printing device includes a nozzle and an extractor. The nozzle has a discharge opening and is configured to provide polarized printing fluid at the discharge opening. The extractor is configured to provide an electrostatic field at the discharge opening that extracts the polarized printing fluid from the nozzle through the discharge opening for deposition on a printing substrate in response to an applied voltage. The nozzle and the extractor are configured to move together with respect to the printing substrate. 
     An embodiment of a method of printing comprises the steps of: (a) applying a voltage across two components of a print head to generate an electrostatic extraction field between the two components sufficient to extract polarized printing fluid from a nozzle of the print head; and (b) providing a directionality field to propel the extracted printing fluid toward a printing substrate. 
     An embodiment of the printing device includes a first nozzle and a second nozzle. The first nozzle has a discharge opening and is configured to provide polarized printing fluid at the discharge opening. The second nozzle has a gas discharge port in fluid communication with a pressurized fluid flow passage to provide a gas flow field at the gas discharge port. The nozzles are arranged to provide an electrostatic field at the discharge opening that extracts the polarized printing fluid from the first nozzle and into the gas flow field for deposition on a printing substrate when a voltage potential is applied across the nozzles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a schematic view of an e-jet printing system that requires a conductive substrate; 
         FIG. 2  is a schematic view of an embodiment of a printing device that includes an extractor with an electrically conductive layer that generates an electrostatic extraction field; 
         FIG. 3  is a computer simulation of printing fluid trajectory through the extractor of  FIG. 2 ; 
         FIG. 4  is a schematic view of an embodiment of the printing device that includes an extractor with multiple electrically conductive layers; 
         FIGS. 5A-5B  are side views of a nozzle of the printing device, illustrating electrostatic field lines with a single conductive layer and with two conductive layers, respectively; 
         FIG. 6  is a computer simulation of printing fluid trajectory through the extractor of  FIG. 4 ; 
         FIG. 7  is a schematic view of an embodiment of the printing device that includes a directionality unit; 
         FIG. 8  is an isometric top view of an embodiment of the printing device that includes a rotational plate and translational stages; 
         FIG. 9  is an enlarged view of a portion of the printing device of  FIG. 8  showing the extractor and the nozzle; 
         FIG. 10  is an enlarged bottom view of a portion of the printing device of  FIG. 8  showing the extractor plate and XY-translational stages; 
         FIG. 11  is an isometric top view of the device of  FIG. 8  showing the rotational plate in a rotated position; 
         FIG. 12  is a side elevation view of an embodiment of the printing device that includes the components of  FIGS. 8-11  as part of an e-jet printing system; 
         FIG. 13  is a top plan view of the device of  FIG. 12 ; 
         FIG. 14  illustrates a misaligned nozzle and extractor plate opening each rotating about a rotational axis; 
         FIG. 15  illustrates an aligned nozzle and extractor plate opening rotating about the rotational axis; 
         FIG. 16  is a photomicrograph of a patterned “M” logo printed on a non-conductive substrate with an embodiment of the printing device; 
         FIG. 17  is a photomicrograph of another pattern printed on a non-conductive substrate with an embodiment of the printing device, illustrating variable and controllable printed feature sizes; 
         FIG. 18  is a schematic side view of an embodiment of the printing device that provides a gas flow field that directs printing fluid toward the printing substrate in a different direction than the extraction direction; 
         FIG. 19  is a schematic side view of an embodiment of the printing device that provides a gas flow field that directs printing fluid toward the printing substrate in the same direction as the extraction direction; and 
         FIG. 20  is a schematic side view of an embodiment of the printing device that includes a plurality of nozzles and gas discharge ports. 
     
    
    
     DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
       FIG. 1  illustrates an example of an e-jet printing system  100 . The illustrated system includes an ink chamber  102 , a pressure source to maintain the ink chamber at a positive pressure P, a conductive nozzle  104 , a conductive substrate  106 , x-, y-, and z-translational stages  108 - 112  for moving the substrate and nozzle relative to each other, a high voltage power supply  114 , and a computer controller  116 . As used herein, the term “ink” refers to any printing fluid intended for deposition on a printing substrate in some desired pattern. Some non-limiting examples of suitable printing fluids include aqueous and/or organic solvent-borne pigments or dyes, metals, molten metals, polymers, biological materials, and particles suspended in aqueous and/or organic solvents. Other materials that can be transformed to a polarized fluid (e.g. liquid) form are also suitable printing fluids. 
     E-jet printing process parameters include the amount of backpressure P on the ink, the applied voltage potential V across the nozzle  104  and substrate  106 , and the standoff height H—i.e., the distance between the nozzle and the substrate. Other factors that can affect the ink dynamics during printing include the nozzle diameter, the type of ink material, and the type of substrate material. In operation, the pressure source ensures that ink remains present at the nozzle tip. The ink is electrically charged by the voltage applied at the nozzle. In this case, the conductive substrate is grounded, and an electrostatic field is generated between the conductive nozzle and the conductive substrate. The electrostatic forces interact with the surface tension of the ink, deforming the meniscus of the ink material into a shape known as a Taylor cone at the nozzle tip. When the electrostatic force is sufficient to overcome the surface tension of the ink, a droplet of ink is released from the nozzle and deposited onto the substrate. 
     The example of  FIG. 1  is limited to an electrically conductive substrate, which is required to generate the electrostatic extraction field. This constraint, along with the requirement for substrate flatness (to maintain a consistent electrostatic field by maintaining a constant stand-off height H), prevents the use of a more diverse range of substrate materials, such as glass, non-conformal biological materials, and flexible materials such as polymers (e.g. for flexible electronics applications). E-jet printing systems such as that depicted in  FIG. 1  can have relatively low throughput due to the single nozzle. The complexity involved with employing multiple nozzles—i.e., crosstalk of the electrostatic fields between multiple e-jet nozzles—can be problematic. Adjacent electrostatic fields generated between adjacent nozzles and the conductive substrate would overlap and change the dynamics of the ink flow when compared to a single nozzle. In addition, once a layer of non-conductive ink is deposited on the conductive substrate, the electrostatic field generated in the vicinity of the deposited ink is changed due to the changed surface conductivity of the printed substrate, thereby affecting the deposition of subsequent ink drops at or near the same location. This makes overlapping printed layers, as is required for 3-D printing, difficult to achieve. 
     The e-jet printing device described below does not rely on substrate conductivity and addresses many problems associated with traditional e-jet printing. The device makes it possible to print on a variety of non-conductive, flexible and/or contoured substrates and to employ multiple nozzles for higher throughput, making e-jet printing competitive with lithography techniques in some applications. The device also enables the use of a wider range of ink and substrate materials than is currently possible in ink jet printing processes. 
     With reference to  FIG. 2 , an e-jet printing device  10  is illustrated schematically. In this example, the device  10  is an e-jet print head, but it should be understood that the device  10  may include other components not shown here and that the illustrated print head may be only one component of the overall device. The illustrated print head  10  includes an extractor  12 , in the form of an extractor plate having an opening  14  formed through the plate, and an ink nozzle  16  arranged so that an electrostatic field is generated between the nozzle and the extractor plate when a voltage potential is applied across the nozzle and the plate. A flowable ink material or printing fluid  18  is extracted from the nozzle  16  by the electrostatic field and passes through the opening  14  to be deposited on a printing substrate  20 . Thus, the electrostatic field is generated between components of the print head  10  without reliance on substrate conductivity to help generate the field. 
     Such a print head may be referred to as an integrated print head and reduces the effect of certain substrate characteristics (e.g., conductivity, flexibility, contour) on the ink-releasing dynamics when compared to the system of  FIG. 1 . For example, a non-conductive substrate  20  may be used with the e-jet print head  10  of  FIG. 2 . In addition, the distance or stand-off height between the nozzle  16  and the substrate  20  does not play a critical role in the printing dynamics with the e-jet print head  10  of  FIG. 2 , thus allowing greater flexibility in the stand-off height and accommodating e-jet printing on non-flat surfaces. The electrostatic field characteristics are unaffected by already-deposited ink, as well, accommodating 3D-e-jet. 
     The extractor  12  of  FIG. 2  includes an electrically conductive portion or layer  22 , such as copper, aluminum, gold, or indium tin oxide (ITO). The conductive layer  22  may be relatively thin, and in one example is between 25-35 μm thick. In other examples, the layer  22  may be as small as 500 nm. The opening  14  may be circular as shown with a central axis  24  oriented generally perpendicular with the extractor plate  12 . In one example, the opening  14  has a diameter in a range from 30-150 μm. The nozzle  16  is electrically conductive, or has an electrically conductive portion or layer, and includes a discharge opening  26  aligned along the central axis  24  of the opening  14 . The nozzle  16  may be made from an electrically conductive material, may be at least partially coated with a conductive material, and/or may include a conductive portion at the discharge opening  26 . A voltage potential V can be applied across the nozzle  16  and the extractor plate  12  as shown. In one example, the conductive layer  22  is electrically grounded. With the nozzle  16  aligned along the central axis  24  of the circular opening  14  as shown, the applied voltage potential generates an electrostatic field that is symmetric about the central axis  24 . At least a portion of the generated field is located between an edge  28  of the opening  14  and the nozzle  16 , and some of the polarized or charged ink  18  from the nozzle is drawn into a Taylor cone  30  at the discharge opening  26 . Attractive forces F act on the ink meniscus from 360 degrees with respect to the central axis  24 . A majority of the radial or horizontal components F H  of the attractive forces cancel each other while the axial or vertical components F v  combine, resulting in a strong force in the axial direction with respect to the central axis  24 . When the resulting axial force is sufficient to overcome the surface tension of the ink  18  with the nozzle  16 , a droplet of ink is extracted and released from the nozzle. The axial force propels the extracted droplet through the opening  14  and onto the substrate  20   
     The cancellation of the radial components F H  of the forces F only occurs when the nozzle  16  is aligned along the central axis  24 , in this example. Misalignment of the nozzle  16  and the opening  14  results in non-symmetric radial components that will change the direction of droplet projection from axial and may result in poor printing results where axial droplet projection is relied upon. Of course, skilled artisans in possession of the present disclosure, where the electrostatic field is generated between print head components without reliance on a conductive substrate, may devise ways to use a non-symmetric field to control ink flow and/or directionality via non-concentric nozzle-opening arrangements and/or non-circular opening shapes. The opening  14  is sized so that ink droplets pass through the opening without hitting the plate  12 . It may also be desirable to reduce the amount of scattering of the ink droplets that pass through the opening  14  before they reach the substrate  20 , as discussed further below. 
       FIG. 3  represents a computer simulation of ink flow from the nozzle  16 , through the opening  14  of the extractor plate  12 , and toward the printing substrate. In this simulation, the extractor plate  12  is modeled as a layer of copper material with a thickness of T=80 μm. The voltage potential across the nozzle  16  and extractor plate  12  is 400V, with the extractor plate electrically grounded. The offset height is H=15 μm, and the diameter of the opening  14  is D=60 μm. The distal end of the nozzle  16  has an outer diameter of 3.6 μm, and the discharge opening  26  has an inner diameter of 2 μm. The overall outer shape of the trajectory region, within which each of the simulated ink drops travels toward the printing substrate, is outlined by dashed lines. As shown, the majority of the ink droplets pass through the opening  14  without striking the plate  12  under the modeled conditions. This is particularly true if the generated electrostatic field results in an initial velocity of at least 10 m/s for extracted ink droplets. This simulated result has been confirmed experimentally. In the experiments, a scattering effect was observed and is indicated in the simulation by the increase in the diameter of the trajectory region below the extractor plate  12  after the ink has passed through the opening  14 . 
       FIG. 4  schematically illustrates another embodiment of the printing device  10 . In this example, the extractor  12  is in the form of an extractor plate that includes first and second electrically conductive portions or layers  22 ,  32 . The layers  22 ,  32  are generally parallel and are spaced apart in the axial direction with respect to the central axis  24 . An electrically insulating or dielectric layer (not shown in  FIG. 4 ) may be disposed between the first and second layers  22 ,  32 . The extractor plate opening  14  extends through all of the layers of the plate  12 , and the nozzle  16  extends partially through the opening. In this example, the first or bottom layer  22  is electrically grounded, and the nozzle  16  and the second or top layer  32  each have a positive voltage applied. In this case, a common positive voltage is applied to the nozzle  16  and the second layer  32 , but the applied voltages could be different. 
     As shown in  FIGS. 5A and 5B , the second conductive layer  32  reconfigures the electrostatic field generated between the nozzle  16  and the extractor plate  12  when compared to the configuration of  FIG. 2 .  FIG. 5A  illustrates exemplary electrostatic field lines surrounding the end of the nozzle without the second conductive layer, and  FIG. 5B  illustrates exemplary field lines in the presence of the second conductive layer. In  FIG. 5B , the outer surface of the nozzle is non-conductive. The effect of the second layer  32  is that the generated electrostatic field exists primarily between the two layers  22 ,  32 , which are the same shape, making the field generally unidirectional as shown and without the contoured field lines of  FIG. 5A . The configuration of  FIG. 4  thus not only eliminates the need for a conductive substrate, but also eliminates the need for a conductive nozzle. For example, only the inner surface of the nozzle  16  may be electrically conductive to charge the ink flowing therethrough. In another embodiment, the ink may be charged before it reaches the nozzle. This configuration may be particularly useful in e-jet printing with a plurality of adjacent nozzles with potential for the reduction or elimination of crosstalk between adjacent nozzles and their associated fields. This is only one example of an extractor  12  having more than one conductive layer. The extractor  12  may have any number of conductive layers, and skilled artisans may employ the concept of using multiple extractor plates and/or multiple conductive layers to manipulate the electrostatic field(s) generated in the vicinity of an e-jet or other ink jet printing nozzle in other ways. 
       FIG. 6  represents a computer simulation of ink flow from the nozzle  16 , through the extractor plate  12  of  FIG. 4 , and toward the printing substrate. In this simulation, the extractor plate  12  is modeled to include first and second electrically conductive layers  22 ,  32  made of copper, each having a thickness of T 1 =T 2 =35 μm. The conductive layers  22 ,  32  are separated by an electrically floating insulating layer having a thickness of T 3 =38.1 μm. The voltage potential between the bottom layer  22  and each of the nozzle  16  and the top layer  32  is 400V, with the bottom layer at electrical ground. The stand-off height is about 45 μm≦H≦50 μm, and the diameter of the opening  14  is D=60 μm. The distal end of the nozzle  16  has an outer diameter of 6.65 μm, and the discharge opening  26  has an inner diameter of 5 μm. The overall outer shape of the trajectory region, within which each of the simulated ink drops travels toward the printing substrate, is outlined by dashed lines. As shown, the scattering effect indicated in the simulation of  FIG. 3  is reduced by the presence of the second layer  32  and the orientation of the field lines, providing improved droplet registration and more accurate drop-on-drop printing. 
     The printing device  10  of  FIG. 7  includes a directionality unit  34 . In this example, the directionality unit  34  is integrated with the electrically conductive layer  22  as part of the extraction plate  12 . The directionality unit  34  may be an electrically conductive plate or layer configured to receive an applied voltage V 2 , which may be the same voltage or a different voltage than the voltage V 1  applied at the nozzle  16 . In one embodiment, the voltage V 2  applied to the directionality unit  34  is variable. Unit  34  may function to further control the trajectory of ink droplets on the way from the nozzle  16  to the substrate  20  and may help reduce deflection errors on the printed substrate. Stated differently, the illustrated directionality unit  34  may help control or stabilize the radial position of the ink droplets passing therethrough. The thickness (i.e., the axial dimension) of the illustrated directionality unit  34  may be greater than the thickness of the conductive layer  22  to allow the electric directionality field associated with the unit to have sufficient time and distance to affect the ink trajectory. In another embodiment, the directionality unit includes a permanent magnet and the directionality field includes a magnetic field. 
     In the example of  FIG. 7 , the directionality unit  34  has an opening  36  of a different size than the opening  14  of the conductive layer  22 . In the illustrated example, the opening  34  is larger than opening  14 . This may help prevent scattered ink droplets entering the opening  36  from being deposited within the opening  36 . The directionality unit  34  is not limited to a conductive layer or ring of material with a single applied voltage. For example, the unit  34  may include portions or segments arranged about its perimeter or about the central axis of the nozzle, with each portion adapted to receive a voltage that may be different from the voltage received by another portion. The unit  34  could include left and right halves with a voltage potential applied thereacross, or it could include a plurality of segments with alternating amounts of voltage applied to adjacent segments. Other configurations are possible. The directionality unit  34  may be employed with multiple conductive layer extraction plates as well, such as that of  FIG. 4 . The directionality unit  34  may also be configured to provide a directionality field that includes some other type of field that affects the direction of the printing fluid after it is extracted from the nozzle, such as an electromagnetic field or a gas flow field. Some examples of other types of directionality units and flow fields are subsequently discussed. 
     Using a conductive surface that is part of the print head to generate the electrostatic field required for e-jet printing can presents its own challenges that are not necessarily present when a conductive printing substrate is used to generate the field. Nozzle alignment, for example, is not a factor in the operation of the e-jet printing system of  FIG. 1 . The scattering effect depicted in  FIG. 3  is also at least partially a by-product of the presence of the above-described extractor plates. The printing device depicted in  FIGS. 8-11  includes components to address some of these challenges.  FIG. 8  is an isometric view of the top or ink supply side of the device,  FIG. 9  is an enlarged view of the nozzle and extractor plate of  FIG. 8 ,  FIG. 10  is an enlarged isometric view of the bottom or substrate side of the extractor plate, and  FIG. 11  is the isometric view of  FIG. 8  with a portion of the device rotated. 
     The printing device  10  of  FIGS. 8-11  includes an ink source  31  fluidly connected with the nozzle  16 , a nozzle mount  33  that supports the ink source and the nozzle, XYZ-translational stages  35  adapted to support the nozzle mount and adjust the location of the nozzle in the x-, y-, and z-directions, an extractor mount  38  that supports the extractor  12 , XY-translational stages  40  adapted to support the extractor mount and adjust the location of the extractor plate in the x- and y-directions, a support base  42 , and a rotational plate  44  configured to rotate with respect to the support base about a rotational axis  46 . In this particular example, the nozzle mount  33  is affixed to the rotational plate  44  via the XYZ-translational stages  35 , and the extractor  12  is affixed to the rotational plate via the XY-translational stages  40  through an opening in the support base  42  so that the extractor plate and nozzle rotate together about the rotational axis  46  when the rotational plate is turned. The particular extractor  12  shown here is an extractor plate with a single conductive layer similar to that of  FIG. 2 , but the extractor plate(s) could include one or more additional conductive layers to affect the generated electrostatic field and/or the directionality of the ink being deposited. Other non-plate type extractors are possible, as well. 
       FIG. 12  is a side elevation view of the printing device  10  in the form of an e-jet printing system that includes all of the components shown in  FIGS. 8-11  together with additional components. Among the additional components is a high-resolution camera  50  and a light source  52  arranged to illuminate the printing substrate and/or the nozzle and extractor.  FIG. 13  is a top plan view of the printing device  10  of  FIG. 12 . Other device components are labeled in  FIGS. 12 and 13  consistent with the previous figures for context. The illustrated system includes other components not described in detail here, such as support frames and structures, devices for moving the substrate and print head relative to one another, electrical and plumbing connections, etc. 
     A working model of the printing device of  FIGS. 12-13 , including the components of  FIGS. 8-11 , has been constructed using the commercial-off-the-shelf (COTS) components listed in TABLE I. The list is non-limiting, and some of the listed components may be modified prior to use in the printing device  10 . For instance, the listed micropipettes, which may be used as the nozzle  16 , are glass and may be coated with a conductive material such as gold (e.g., via a sputtering process) and may also be coated with a hydrophobic coating prior to use. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 COTS Component 
                 Supplier 
                 Part No. 
               
               
                   
                   
               
             
            
               
                   
                 Syringe 
                 Nordson EFD 
                 7012072 
               
               
                   
                 Micropipette 
                 World Precision 
                 TIP2TW1-L 
               
               
                   
                 XYZ Stages 
                 Newport 
                 M-MT-XYZ 
               
               
                   
                 Rotational Plate 
                 McMaster-Carr 
                 6031K17 
               
               
                   
                 Support Base 
                 McMaster-Carr 
                 9057K13 
               
               
                   
                   
               
            
           
         
       
     
     The COTS components of TABLE I were assembled as follows. The nozzle was formed from a glass micropipette ranging in diameter from 300 nm to 10 μm. The nozzle was connected to the syringe with a Luer lock. The syringe acted as the ink source or reservoir to supply the nozzle with ink. The extractor was a 30 μm copper foil with a 120 μm opening located at its center. The syringe was held in place by the nozzle mount, which was attached to the XYZ-translational stages. The XYZ-stages were attached at the top of the rotational plate, thereby enabling a user to both rotate and translate the nozzle with respect to the support base. The extractor was attached to the extractor mount with adhesive tape, and a ground wire (element  48  of  FIG. 9 ) electrically connected the extractor with ground. The extractor mount was attached to the XY-stages via a flexible fixture. The four corners of the extractor mount were held in place with set screws which could be adjusted to change the angle of the extractor plate with respect to the central axis of the opening and/or ensure parallel alignment between the extractor plate and the printing substrate. This adjustment could also be useful for controlling directionality of the ink. The fixture was mounted to the XY-translational stages, which in turn was affixed to the rotational plate, thereby enabling the user to both rotate and translate the extractor plate with respect to the support base. With the XYZ-translational stages for the nozzle mount and the XY-translational stages for the extractor plate mounted to the same rotational plate, the nozzle and extractor plate could rotate together in the same direction with the same rotational velocity. The relative positions of the nozzle and the opening in the extractor plate could thereby be viewed from virtually all directions from a single viewpoint when the rotational plate was turned, enabling a method of using the printing device that includes the step of aligning the opening of the extractor plate with the nozzle, described further below. 
     For purposes of the working model, a symmetric electrostatic field as described in conjunction with  FIG. 2  was desired, which necessitated proper nozzle-to-opening alignment. Given the micro-scale of both the nozzle tip and the extractor opening, the high-resolution camera was provided to visually align the two components. Aligning the nozzle and the extractor opening in both the x- and y-directions required a clear view from at least two directions. The above-described rotational plate was employed for this purpose, eliminating the need for multiple cameras and/or a camera that is moved around the print head for multiple views. A movable camera would require refocusing at each position, leading to long-set-up times and potential inaccuracies, not to mention large space requirements around the print head. The fixture required to mount a camera in more than one location could also be complex and costly, especially if more than one camera is used. Even then, the number of views is finite. The rotating feature of the working model provided an infinite number of views around the print head from a single stationary camera. 
     As shown in  FIG. 14 , when a misaligned nozzle  16  and extractor plate opening  14  are rotated about the rotational axis  46  in view of the high-resolution camera, the nozzle and opening appear to rotate about the rotational axis. With the device shown in  FIGS. 8-13 , the nozzle and extractor plate can be adjusted independently in the x- and y-directions until they both appear stationary when rotated via the rotation plate, as shown in  FIG. 15 . At this stage, the nozzle and extractor plate opening are aligned with the rotational axis of the rotational plate, ensuring that the nozzle tip is aligned with the center of the opening in the extractor plate. The vertical offset between the nozzle and the extractor plate can then be adjusted using the translational z-axis on the nozzle mount. Other rotational configurations are possible. For example, in another embodiment, the device is constructed so that only the extractor plate rotates about the rotational axis, and the camera is used to observe the extractor plate opening rotating about the rotational axis. The x- and y-position of the extractor plate is adjusted until the opening appears stationary, and the nozzle is brought into alignment with the rotational axis by other means. 
     In order to provide consistent and steady ink-releasing dynamics in an e-jet printing system, a stable Taylor cone is maintained at the nozzle tip with a constant voltage baseline voltage. The Taylor cone formed in the presence of the electrostatic field generated with the extractor is more sensitive to the baseline voltage than when formed with a conductive substrate only. For example, a considerably higher baseline voltage may be necessary to reduce the scattering effect when an extractor is used to generate the field. But the baseline voltage must also be kept sufficiently low that the Taylor cone does not release ink droplets at the baseline voltage. Thus, unlike with conductive substrate e-jet systems, experimental optimization may be necessary to determine the maximum baseline voltage that will not cause ink droplets to be released from the Taylor cone. 
     With respect to the above-described working model of the printing device in operation, the deposited feature size on the substrate, in drop-on-demand printing mode, was controlled by charging the nozzle with a pulsed signal using different pulse widths. The amount of ink released, and therefore the feature size of the deposited material on the substrate, is proportional to the pulse width of the pulse signal. It was observed that the printing process demonstrated scattering behavior as the pulse width was increased beyond a threshold value. This threshold value somewhat limits the feasible feature size that can be achieved with a given nozzle diameter for a single conductive layer extractor plate. The multiple conductive layer extractor plate and/or the directionality unit described above may be effective to increase the threshold value by reducing scattering. 
     To print a particular pattern with the working model, a computer program (Matlab) was used to convert JPEG images into G-code files. A customized control program (LabView) then executed the G-code file and controlled both the pulsing voltage signal and the translational positioning stages. After aligning the nozzle with the central axis of the extractor plate opening as described above, the distance between the nozzle tip and the extractor plate was determined and optimized experimentally. In this case, the extractor plate was positioned approximately 30 μm above the substrate. The baseline voltage was also optimized experimentally, as noted above, to minimize scattering and to ensure sufficient Taylor cone stability. 
       FIG. 16  is a photomicrograph of a University of Michigan logo printed using the above-described e-jet printing device to evaluate and demonstrate the accuracy and registration capability of the device. Table II, below, lists the printing parameters used to print the logo in  FIG. 16 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Pulsed High Voltage 
                 500 
                 V 
               
               
                   
                 Baseline Voltage 
                 290 
                 V 
               
               
                   
                 Pulse Width 
                 10 
                 ms 
               
               
                   
                 Extractor Plate-to-Substrate Distance 
                 ~30 
                 μm 
               
               
                   
                 Extractor Plate-to-Nozzle Distance 
                 ~20 
                 μm 
               
            
           
           
               
               
               
            
               
                   
                 Ink Material 
                 NOA 73 
               
            
           
           
               
               
               
               
            
               
                   
                 Nozzle Diameter 
                 2 
                 μm 
               
            
           
           
               
               
               
            
               
                   
                 Substrate Material 
                 Glass 
               
            
           
           
               
               
               
               
            
               
                   
                 Ink Chamber Pressure 
                 5 
                 psi 
               
               
                   
                 Droplet Diameter 
                 ~7 
                 μm 
               
               
                   
                   
               
            
           
         
       
     
     In summary, optical adhesive NOA73 (Norland Products, Cranbury, N.J.) was successfully printed onto a non-conductive glass substrate in 7 μm droplets with a 2 μm pipette. The printing device was capable of printing smaller ink droplets, but in this case a pulse width of 10 ms was selected to create the 7 μm diameter droplets by fusing some ink drops together in order to make the printed features more visible in the relatively large logo pattern. The consistency of the droplet diameters, as well as the placement of the droplets, demonstrates the printing capability of the printing device described herein. As is apparent in  FIG. 16 , scattering effects were successfully mitigated through proper selection and experimental determination of the printing parameters as described above. 
       FIG. 17  is a photomicrograph of another pattern printed using the described printing device. The illustrated pattern demonstrates controlled droplet resolution. The scattering effect was again minimized by optimizing the pulsed high voltage, the baseline voltage, and the offset height between the nozzle and the extractor plate. In this pattern, a matrix of dots with varying diameters was printed. The parameters used for printing the illustrated pattern are listed in Table III. 
     
       
         
           
               
               
             
               
                 TABLE III 
               
               
                   
               
               
                 Parameter 
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Pulsed High Voltage 
                 400 
                 V 
               
               
                 Baseline Voltage 
                 240 
                 V 
               
               
                 Pulse Width (top-to-bottom in FIG. 19) 
                 (20, 10, 5, 2, 1, and 0.5) 
                 ms 
               
               
                 Extractor Plate-to-Substrate Distance 
                 ~30 
                 μm 
               
               
                 Extractor Plate-to-Nozzle Distance 
                 ~20 
                 μm 
               
            
           
           
               
               
            
               
                 Ink Material 
                 NOA 73 
               
            
           
           
               
               
               
            
               
                 Nozzle Diameter 
                 2 
                 μm 
               
            
           
           
               
               
            
               
                 Substrate Material 
                 Glass 
               
            
           
           
               
               
               
            
               
                 Ink Chamber Pressure 
                 5 
                 psi 
               
            
           
           
               
               
            
               
                 Droplet Diameter 
                 ~7 μm to ~1 μm 
               
               
                   
               
            
           
         
       
     
     In the printed pattern of  FIG. 17 , the pulsed high voltage and the baseline voltage are the same for all printed features, and the pulse width was varied to control the printed feature size. As indicated in Table III, the pulse width was varied from 20 ms (top row of pattern) to 0.5 ms (bottom row of pattern). The individual (vertical) columns of printed features in  FIG. 17  are spaced about 30 μm apart. The droplet diameters ranged respectively from about 7 μm to about 1 μm. As shown in  FIG. 17 , the above-described printing device demonstrates high accuracy and feature size consistency when e-jet printing on a non-conductive substrate. 
       FIG. 18  schematically illustrates an embodiment of the printing device  10  in which the extractor  12  includes a gas discharge port  54  arranged to discharge pressurized gas from a fluid flow passage  56  in a direction toward the printing substrate  20  to provide a gas flow field  58  between the nozzle  16  and the printing substrate in which the extracted printing fluid travels for deposition on the printing substrate. The gas discharge port  54  and fluid flow passage  56  together at least partially define the directionality unit  34  in this embodiment. In this case, the gas flow field  58  is the directionality field that affects the direction of the printing fluid after it is extracted from the nozzle  16 . 
     As in previous examples, the nozzle  16  of  FIG. 18  is configured to provide polarized printing fluid  18  at the discharge opening  26 , and the extractor  12  is configured to provide the electrostatic field at the discharge opening of the nozzle to extract the polarized printing fluid from the nozzle for deposition on the printing substrate  20  in response to an applied voltage V. The nozzle  16  and the extractor  12  are configured to move together as part of the print head  10  with respect to the printing substrate  20 , and generation of the electrostatic field does not rely on substrate conductivity. Both the nozzle  16  and the extractor  12  may include an electrically conductive portion, and the voltage V is applied across the conductive portions to provide the electrostatic field. For instance, the extractor  12  and/or the nozzle  16  may be constructed from a pipette with an electrically conductive coating layer (e.g., a sputtered layer of gold) at the inner surface and/or outer surface. In some embodiments, the nozzle  16  and/or the extractor  12  are formed from an electrically conductive material. In this case, the applied voltage polarizes the printing fluid  18 , but it is possible to polarize the printing fluid by other means. 
     The extractor  12  of the printing device  10  depicted in  FIG. 18  is in the form of a nozzle. In some embodiments, the printing device  10  thus includes a plurality of nozzles  12 ,  16 , one of the nozzles having the discharge opening  26  from which the printing fluid  18  is extracted, and another of the nozzles having the gas discharge port  54  that provides the gas flow field  58  to direct the extracted printing fluid toward the substrate  20 . In the illustrated example, the extraction field is an electrostatic field generated between the conductive portions of the two nozzles  12 ,  16 . The central or longitudinal axis  24  of the nozzle  16  from which the printing fluid is extracted is arranged at an obtuse angle with the printing substrate  20  and with the desired direction of travel of the printing fluid between the discharge opening  26  and the substrate. Also, the configuration of the electrostatic field in this example is such that the direction in which the printing fluid is extracted from the nozzle  16  has a component in the opposite direction (vertically upward in  FIG. 18 ) from the substrate  20 , because the extractor nozzle  12  is located on the opposite side of the ink nozzle from the substrate. Thus, for the resultant force acting on the extracted printing fluid to have a direction toward the substrate, the strength of the gas flow field must be sufficient to overcome—and be greater than—the net electrostatic force acting on the printing fluid in the opposite direction. 
     The gas discharged from the gas discharge port  54  can be air or any other suitable fluid, such as an inert gas or a gas selected to react with the printing fluid, for example. The gas may be temperature controlled to affect the printing fluid at the ink nozzle opening, or for process consistency and/or variability (i.e., viscosity control). The gas may be air with moisture control for similar reasons, or for purposes of reacting with the printing fluid. In another example, the fluid flowing through the fluid flow passage may be a volatile liquid that vaporizes upon exiting the discharge port to become the gas. 
     Other multi-nozzle configurations are possible, and the gas flow field  58  and/or the extraction field could be provided with other configurations. For instance, a separate nozzle could be provided to generate the gas flow field—i.e., the gas discharge nozzle does not necessarily participate in generating the electrostatic extraction field. In some of the subsequent examples, the gas discharge opening is embodied by the opening formed through the electrically conductive layer of the extractor, for example. Or one of the above-described extraction plates could be combined with a separately provided fluid flow passage and/or gas discharge port. Also, multiple printing fluid nozzles  16  could be arranged so that their respective discharge openings are located in the same gas flow field provided by a single gas discharge port. 
       FIG. 19  schematically illustrates an embodiment of the printing device  10  in which the extraction field is provided by the extractor  12  in the form of a cap or closure that partially defines the fluid flow passage  56 . A portion of the directionality field is provided by the electrostatic field generated between the extractor and the nozzle, and another portion of the directionality field is provided by the gas flow field generated between the gas discharge port  54  and the substrate  20 . The net electrostatic forces acting on the extracted printing fluid are in a direction toward the printing substrate  20 , which is the same direction as the net forces of the gas flow field  58 . In this particular example, the fluid flow passage  56  is defined at least in part by surfaces of a chamber housing  60 , the nozzle  16 , and the extractor  12 . A fluid inlet port  62  is provided to receive fluid for the fluid flow passage. The extractor  12  has a conductive portion or layer  22 , or is made from a conductive material, and operates similar to the extractor plate of  FIG. 2  via the opening  14  formed through the extractor. In this embodiment, the opening  14  of the extractor  12  and the gas discharge port  54  of the directionality unit  34  are one and the same. The extractor  12  in this example may be considered to include an extractor plate with a flange at its perimeter to form the cap shape, and the flange does not necessarily include a conductive portion. 
       FIG. 20  schematically illustrates an embodiment of the printing device  10  including a plurality of nozzles  16  with an extractor  12  that has a corresponding plurality of openings  14  through which the printing fluid travels from the discharge opening  26  of each nozzle to the substrate  20 . A plurality of extraction fields are provided by the extractor  12 , which also partially defines the fluid flow passage  56  between a pair of fluid inlet ports  62  and the gas discharge ports  54 . Here again, the openings  14  of the extractor  12  serve as the gas discharge ports  54  at which the gas flow fields  58  are provided as directionality fields for the extracted printing fluid. A portion of each directionality field is provided by the electrostatic field generated between the extractor and each nozzle, and another portion of each directionality field is provided by the gas flow field generated between each gas discharge port  54  and the substrate  20 . 
     In this particular example, the fluid flow passage  56  is defined at least in part by surfaces of the extractor  12 , the nozzles  16 , and a housing  64  of the printing fluid reservoir  31 . The extractor  12  has a conductive portion or layer  22 , or is made from a conductive material, and operates similar to the extractor plate of  FIG. 19  via the openings  14  formed through the extractor. The extractor  12  in this example may be considered to include an extractor plate with a flange at its perimeter to form the cap shape, and the flange does not necessarily include a conductive portion. The extractor  12  of  FIG. 20  also includes shields  66  located between adjacent nozzles  16  and extractor openings  14 . The shields  66  are electrically conductive or have an electrically conductive portion or layer to help isolate the adjacently generated electrostatic fields at each nozzle  16  and corresponding opening  14 . 
     Another feature of the example of  FIG. 20  that can help reduce the presence of extraneous electric fields is the configuration of the electrically conductive portion  68  of each nozzle  16 . In this example, only a ring-shaped portion of each nozzle  16  is electrically conductive, with the remainder of each nozzle and the housing  64  of the printing fluid reservoir  31  being formed from an electrically insulating material. The discharge opening  26  of each nozzle  16  is formed at the conductive portion  68  of each nozzle. The applied voltage V in this example is applied commonly across the plurality of conductive rings  68  and the extractor  12 , which is grounded in this example. It is also possible to apply a different voltage to each nozzle  16  in this embodiment. 
     Consistent with the above-described embodiments of the printing device, a method of printing may include the step of extracting polarized printing fluid from the nozzle of the print head and the step of providing a directionality field to propel the extracted printing fluid toward the printing substrate. Using the above-described printing device, the extraction step can be performed by applying a voltage across two different and/or electrically isolated components of the print head to generate an electrostatic extraction field between the two components sufficient to extract the polarized fluid. Multiple combinations of the printing device features described above can be employed to practice this method, including but not limited to single conductive layer extraction plates, multi-layer extraction plates, multiple nozzle configurations (e.g., multiple ink nozzles and/or multiple gas discharge nozzles), directionality units and/or fields (e.g. gas flow fields, electric fields, magnetic fields, etc.), shielding between adjacent nozzle, etc. The method can be performed where the printing substrate is non-conductive, contoured, flexible, or any combination thereof. The directionality field may be no more than the electrostatic field generated between the isolated components of the print head, or it may include that electrostatic field in addition to one or more other fields that affect droplet trajectory. The directionality field may include a gas flow field, a separately provided electric field and/or magnetic field, or the resultant combination of any of these types of fields. 
     It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims. 
     As used in this specification and claims, the terms “e.g.,” “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.