Bioengineering
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High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning
Chapters
Summary July 10th, 2018
Here, we present a protocol to produce high-resolution conductive patterns using electrohydrodynamic (EHD) jet printing. The protocol includes two modes of EHD jet printing: the continuous near-field electrospinning (NFES) and the dot-based drop-on-demand (DOD) EHD printing.
Transcript
Electrohydrodynamic jet printing is a non-contacting, direct patterning method that can be used in various fields, such as printed electronics, advanced materials, biotechnology, and so on. The electrohydrodynamic jet printing method uses a high electric field to pull down charged ink to the substrate. For this purpose, a fluidic system is used to push ink to the nozzle, and a high voltage power supply is used to produce the electrical field.
The main advantage of this technique is that it can be used to print very small dots or patterns, compared to conventional inkjet printing method. Based on the electrical and fluidic configurations, three different modes of drop-on-demand printing, electrospinning, and electrospray, can be implemented. For fine patterning, we will focus on DOD and near-field electropsinning.
DOD uses both DC voltage and pulse voltage for jetting, whereas near-field electrospinning uses only DC voltage for jetting. Individuals knew that this method would have a hard time attaining proper jetting because it more requires specific things and different printing pen methods, such as voltage, the nozzle, printing speed, and stand-up distance. Courageous student Mr.Oh will demonstrate both drop-on-demand and near-field electrospinning using silver nanoparticle ink, in order to help individuals to understand the printing processors.
For drop-on-demand printing, first fill the ink reservoir of the electrohydrodynamic jet printer with filtered silver nanoparticle ink. Then, prepare the nozzle from a glass pipette, as described in the accompanying text protocol. Assemble the nozzle holder by connecting the nozzle to the ink reservoir via Teflon tubing.
Next, turn on the air pressure controller, and apply an air pressure of 15 to 20 kilopascal to the ink reservoir. Monitor the ink's flow through the glass nozzle and tubing to ensure that no air is trapped when supplying the ink. Keep applying air pressure to the reservoir until ink appears at the nozzle's tip.
Do not reduce the pressure before the ink appears at the nozzle tip because that could cause air bubble entrapment at the tip. Once ink appears at the nozzle's tip, reduce the pressure to around 12 kilopascal. This will maintain the extruded meniscus without any ink dripping from the nozzle's tip.
Then, fix the assembled nozzle head in the printing system. Using a side-view camera to visualize the gap between the nozzle tip and the substrate, move the Z-axis of the stage to adjust the gap to approximately 100 micrometers. A smaller gap leads to a higher electric field, which could facilitate printing with a lower and pulse voltages for jetting.
However, a lower gap could also lead to larger drops if the voltage is not adjusted properly. At this point, monitor the ink at the nozzle while beginning to apply DC and pulse voltages. Gradually increase the DC voltage at increments less than 100 volts at a time.
Once the ink begins to drip from the nozzle, reduce the DC voltage slightly until the ink stops dripping from the nozzle. Next, set a negative pulse voltage with the rise time equal to zero to 100 microseconds, the dwell time equal to 300 microseconds, and the fall time equal to zero microseconds. Then, apply the negative pulse voltage at the substrate holder.
Now, adjust the magnitude of the pulse voltage to produce one droplet per single pulse. Then, adjust the DC background and the pulse voltages to obtain the target droplet size on the substrate, while observing the jetted dots on the substrate in the side-view camera image. First, load a bitmap image in the printing tab of the printing software, and convert it into a binary image.
Then, set the parameters for the binary image printing. For example, set the distance between two drops, or drop interval, to be ten micrometers. Once set up, start printing using the selected bitmap on the target location of the substrate.
To prepare for vector printing, load the pattern's CAD information into the printing software. Then, set the parameters for the print, such as the printing speed and the dot spacing. With the parameters now set, begin printing.
To perform near-field electrospinning, first prepare the specially formulated silver nanopaste ink. To accomplish this, mix three parts ethanol and one part deionized water to make 12 milliliters of solvent. Then, mix 0.3 grams of polyethylene oxide and 9.7 grams of the prepared solvent to make a 3%by-weight polymer solution.
Using a magnetic stirrer, thoroughly mix the solution for more than six hours by stirring at room temperature. With the solvent now prepared, mix five parts of silver nanopaste ink with one part of the prepared polymer solution. Combine the two using a vortex mixer, mixing for ten minutes to properly suspend the ink.
Next, fill the prepared ink into a syringe, and connect the syringe to a nozzle via the Teflon connecting tube. Supply the ink to the nozzle by pushing the syringe manually. When the ink reaches the nozzle, install the syringe into the syringe pump attached to the printing system.
Operate the syringe pump to generate an ink flow with an initial flow rate of 50 microliters per minute. When the ink flows out of the nozzle's tip, lower the flow rate to one microliter per minute. Next, apply the DC voltage source to the nozzle connector while the ground voltage is connected to the substrate holder.
Increase the DC voltage gradually to 1.5 kilovolts. The DC voltage could be increased up to two kilovolts. However, a DC voltage higher than two kilovolts should be avoided since it might damage the ink.
Once set up, start the idle printing with a printing speed of 300 millimeters per second for at least 10 minutes to obtain steady-state flow. This is needed because the viscous ink may be compressed in the lengthy tubing. Adjust the printing parameters, such as the DC voltage and flow rate during idle printing, to obtain the desired printing results.
Finally, print your selected pattern on the substrate using the now-defined printing parameters. Dot-based drop-on-demand printing and raster printing uses a single axis to print dots in the main direction and then move to the next swath in the sub-direction. This raster image has a drop size of around four microns.
In contrast, dot-based drop-on-demand printing in vector mode performs simultaneous movements in the X and Y directions and is used to print the lines. The resulting image has line widths of four microns. Near-field electrospinning uses highly viscous ink to print patterns continuously.
As a result, this method is suitable for printing straight lines using a high printing speed, and is sensitive to changes in print speed. Discardable slow regions should be included in the design to ensure a consistent line size in the desired region. In some cases, a low jetting speed can be used to generate a wave pattern by using a low printing speed of less than 100 millimeters per second.
The patterns can become wavy, as shown here. This type of wavy pattern might be useful in stretchable electronics applications. After its development, this technique paved the way for researchers to produce fine patterns for their specific applications.
Please note that this printing method is not limited to only silver nanoparticle ink, but can also be used in other applications with various ink. While you're attempting this procedure, it is important to remember to use the appropriate ink for printing. Refer to the general guidelines for ink selection provided in the text of this article.
Be sure to adjust the printing parameters according to your ink selection and applications. Don't forget that working with chemicals, high voltage, and high pressure can be hazardous, and precautions should always be taken while performing this procedure.
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