This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) of Korea, funded by the Ministry of Education (2016R1D1A1B01006801), and partially supported by the Soonchunhyang University Research Fund.

For health and safety purposes, prior to using any ink and cleaning solution, refer to the material safety datasheet (MSDS).
1. Drop-on-demand Electrohydrodynamic Jet Printing Using Silver Nanoparticle Ink
<ol>
	<li>Fill the filtered silver nanoparticle (AgNP) ink in the ink reservoir of the EHD printing system. 
	Note: Commercially available AgNP ink can be used for the inkjet purpose. The ink should have a viscosity of around 10 cP and a surface tension of 20 ~ 40 mN/m to obtain drop-on-demand jetting.</li>
	<li>Make a nozzle for the DOD EHD printing by using a thermal puller.
	<ol>
		<li>Place a glass capillary [inner diameter (ID) of 1 mm] in the thermal puller.</li>
		<li>Set the parameters of the thermal puller; for example, the heating temperature in the range of 580 - 590 &#176;C and a pulling speed of around 18 mm/s. 
		Note: The parameters for the thermal puller should differ according to the target nozzle ID and ambient conditions.</li>
		<li>Operate the thermal puller with the set parameters to apply heat at the center of the capillary and pull at both of its ends to make a nozzle with an ID of 5 &#181;m. 
		Note: Determine the size of the nozzle ID based on the target dot size on the substrate. For reference, the nozzle ID of 5 &#181;m could print 5 &#181;m-sized dots.</li>
		<li>Adjust the glass nozzle length by cutting the glass nozzle via a glass cutter.</li>
	</ol>
	</li>
	<li>Assemble the nozzle to the nozzle holder and the connector, which are connected to the ink reservoir via polytetrafluoroethylene (PTFE) tubing.</li>
	<li>Apply the air pressure to supply ink to the nozzle tip.
	<ol>
		<li>Turn on the air-pressure controller and apply air pressure of 15 ~ 20 kPa to the ink reservoir to supply ink to the nozzle tip. Monitor the ink flow through the transparent glass nozzle and tubing to ensure that no air is trapped inside the tube and the nozzle when supplying the ink. Keep applying air pressure to the ink reservoir until ink appears at the nozzle tip. 
		Note: Do not reduce the pressure before the ink appears at the nozzle tip because that could cause air bubble entrapment at the nozzle tip.</li>
		<li>Reduce the pressure to around 12 kPa to maintain the extruded meniscus without any ink dripping from the nozzle tip. 
		Note: The proper air pressure depends on the nozzle size and ink viscosity. Do not increase the air pressure to more than 30 kPa to avoid excessive air compression, which is undesirable for maintaining the meniscus in a stable condition.</li>
	</ol>
	</li>
	<li>Fix the assembled nozzle head in the printing system.</li>
	<li>Place a glass substrate on the vacuum chuck of the substrate holder and turn on the vacuum pump to hold the substrate.</li>
	<li>Move the Z-axis stage to adjust the stand-off distance (H)&#8212;the gap between the nozzle tip and the substrate position&#8212;to approximately 100 &#181;m. Use the side-view image acquired by the monitoring camera to estimate the stand-off distance by using the distance from the nozzle tip to its reflection, as shown in Figure 2. 
	Note: A smaller stand-off distance leads to a higher electrical field, which could facilitate printing with a lower DC and pulse voltages for jetting. However, a lower stand-off distance could lead to larger drops. Therefore, the magnitude of voltages should be reduced accordingly to obtain the desired dot size. In general, the use of a lower voltage is recommended to obtain smaller printed dots with less spraying. However, a careful operation is required if the stand-off distance becomes less than 50 &#181;m, due to the higher chance of nozzle breakage by collision with the substrate. Considering the trade-off relationship between jetting ability and reliability, we recommend the use of a stand-off distance of 100 &#181;m.</li>
</ol>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57846/57846fig2.jpg" /> 
Figure 2: Stand-off distance adjustment by using side view camera image. The nozzle image from a side-view camera can be used to estimate the stand-off distance. The stand-off distance (H) from the nozzle tip to the substrate can be easily estimated as half the distance from the nozzle tip to its shadow. <a href="https://www.jove.com/files/ftp_upload/57846/57846fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="8">
	<li>Apply DC and pulse voltages 
	Note: The DC and pulse voltages can be controlled via the printing software.
	<ol>
		<li>Increase the DC voltage gradually until the ink drips out of the nozzle tip. 
		Note: Do not apply the target voltage at once. The incremental voltage should be less than 100 V at a time. Overall, do not apply a DC voltage of more than 600 V.</li>
		<li>Reduce the DC voltage slightly from the onset DC voltage until no further ink dripping from the nozzle is observed. 
		Note: After the adjustment of the pneumatic pressure and DC voltage, the meniscus should be in an appropriate shape for jetting as shown in Supplementary Figure S1.</li>
		<li>Set a negative pulse voltage with the parameters of trise = 0 ~ 100 &#181;s, tdwell = 300 &#181;s, and tfall = 0 &#181;s7 (Figure 3) in the software menu.</li>
		<li>Apply the negative pulse voltage at the substrate holder. Then, adjust the magnitude of the pulse voltage, Vpulse, to produce one droplet per single pulse voltage. 
		Note: The magnitude of the negative pulse voltage, Vpulse, should be lower than 600 V.</li>
		<li>Adjust the DC background and 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. 
		Note: To produce small dots with less spraying on the substrate, the magnitude of the pulse voltage, Vpulse, should be as low as possible.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57846/57846fig3.jpg" /> 
Figure 3: Pulse voltage for DOD EHD jetting. The use of trapezoid waveform voltage is recommended to produce DOD EHD jetting7. <a href="https://www.jove.com/files/ftp_upload/57846/57846fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<ol start="9">
	<li>Print patterns 
	Note: Two different kinds of patterns can be used for DOD EHD printing: bitmap image and (CAD)-based vector information. Bitmap image has been widely used in DOD-based inkjet printing. However, in the case of printed electronics applications, CAD-based vector information has advantages over DOD-based inkjet printing, because it is efficient in line-based printing using a single EHD head. At the same time, the vector information can be converted into a bitmap image for bitmap image printing.
	<ol>
		<li>Bitmap image printing
		<ol>
			<li>Load a bitmap image in the printing tab of the printing software and convert it into a binary image.</li>
			<li>Set the parameters for the binary image printing. For instance, set the drop interval (i.e., the distance between 2 consecutive pixels) at 10 &#181;m. 
			Note: The bitmap image does not have any physical dimensions. The physical dimensions of the printed image will be related to the drop interval. For example, the printed image becomes larger if a larger drop interval is used. In conventional inkjet printing, dot per inch (DPI) has been commonly used for this purpose. However, it should be noted that a smaller DPI means a larger drop interval. In order to determine the drop interval, the printed dot size should be considered. In general, the drop interval for EHD DOD printing is significantly smaller than that of conventional inkjet printing.</li>
			<li>Start printing using the selected bitmap on the target location in the substrate.</li>
		</ol>
		</li>
		<li>Vector printing based on CAD information
		<ol>
			<li>Load the CAD information for printing. 
			Note: The file format DXF, which is text-based CAD information, can be used for the printing information.</li>
			<li>Set the parameters for vector printing; for example, set the drop interval at 3 &#181;m and the jetting frequency at 10 Hz. 
			Note: To print the connected line patterns, the drop interval should be chosen so that adjacently deposited drops slightly overlap. However, too much overlapping may result in a larger line width. An overlapping of about 30% is recommended for any practical line printing. In the case of vector printing, the motion speed (v) the following equation. 
			v = d &#215; f 
			Here, 
			d = the drop interval, and 
			f = the jetting frequency.</li>
			<li>Print the loaded patterns on the substrate using pre-determined printing parameters, such as the drop interval, the printing speed, the voltage, etc. 
			Note: After printing, a sintering process might be required to obtain the desired conductivity of the printed patterns, which is beyond the scope of this paper.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Fine Conductive Line Patterning Using Near-field Electrospinning
<ol>
	<li>Make near-field electrospinning (NFES) ink for conductive line printing.
	<ol>
		<li>Mix ethanol and deionized (DI) water with a volume ratio of 3 (ethanol) to 1 (DI water) to prepare solvent 1. For example, mix 9 ml of ethanol and 3 ml of DI water to make 12 ml of solvent 1.</li>
		<li>Mix 0.3 g of poly (ethylene oxide) (PEO, Mwt = 400,000) and 9.7 g of the prepared solvent 1 to make a polymer solution with 3 wt% of PEO by stirring, using a magnetic stirrer for more than 6 h at room temperature (25 &#176;C).</li>
		<li>Mix Ag nano paste ink, which has a viscosity of about 11,000 cP, and the prepared polymer solution, with a weight ratio of 5 (Ag nano paste ink) to 1 (polymer solution) by using a vortex mixer for 10 min to obtain the ink for NFES. For example, 10 g of Ag nano paste ink and 2 g of polymer solution can be mixed to obtain the NFES ink. 
		Note: In this protocol, the ratio of the mixing materials is generally more important than the specific amount of the materials. Commercially available Ag nano paste ink for screen printing purposes, which has Ag solid contents of about 85.5 wt%, can be used for this purpose. Note that the selection of the solvent and the polymer could differ based on the composition of the ink that is used.</li>
	</ol>
	</li>
	<li>Fill the prepared NFES ink in the syringe.</li>
	<li>Connect the syringe with a nozzle via the connecting tube. 
	Note: A commercially available syringe needle with an ID of 100 &#181;m can be used for the nozzle.</li>
	<li>Supply the ink to the nozzle by pushing the syringe manually.</li>
	<li>Install the syringe in the syringe motor, which is attached to the printing system.</li>
	<li>Place a substrate on the vacuum chuck and turn on the vacuum pump to hold the substrate during the printing.</li>
	<li>Control the Z-position (stage) to adjust the stand-off distance. 
	Note: The recommended stand-off distance should be around 2 mm, which is a significantly smaller stand-off distance compared to the one used with conventional electrospinning.</li>
	<li>Adjust the flow rate 
	<ol>
		<li>Operate the syringe pump to fill the NFES ink in the nozzle assembly and generate an ink flow with an initial flow rate of 50 &#181;L/min, which is higher than the target flow rate.</li>
		<li>Set a target flow rate of 1 &#181;L/min when the ink flows out of the nozzle tip. 
		Note: A smaller flow rate can result in a smaller pattern width. However, it could cause line breakage. The trade-off between the line width and the continuation of the line should be considered when the target flow rate is determined.</li>
	</ol>
	</li>
	<li>Apply voltage
	<ol>
		<li>Connect the DC voltage source to the nozzle connector and connect the ground voltage to the substrate holder.</li>
		<li>Increase the DC voltage gradually to 1.5 kV. 
		Note: Since the stand-off distance is in the range of a few millimeters, the DC voltage could be increased up to 2 kV, which is higher than that of DOD EHD jet printing. However, a DC voltage higher than 2 kV should be avoided since it might damage the functional material, especially Ag paste ink, added to the polymer solution. In general, a lower DC voltage is recommended when a thinner printed line is required. However, the printed lines could be easily disconnected when a low voltage is used, because the pulling force for continuous ink printing is related to the DC voltage. Considering the trade-offs, we recommend the use of a DC voltage ranging from 1 kV to 2 kV.</li>
	</ol>
	</li>
	<li>Start the idle printing with a printing speed of 300 mm/s for more than 10 min to obtain a steady-state flow. Adjust the printing parameters such as the DC voltage and flow rate during the idle printing to obtain the desired printing results. 
	Note: Idle printing of more than 10 min is required to obtain a steady-state flow because the viscous ink can be compressed in the lengthy tubing during the ink delivery to the nozzle tip. Without idle printing, the printed line width may change with time. The idle printing speed should, thus, be the same as the actual printing speed so that the jetting parameters can be adjusted during the printing. In this way, the DC voltage is adjusted during the idle printing to obtain the target line width. Note that the flow rate and DC voltage should be balanced, so that the amount of ink pushed by the syringe pump can be equal to the amount of ink pulled down by the electrical field.</li>
	<li>Choose the printing pattern, such as a continuous line and grid patterns. 
	Note: Since the produced fiber can easily be deflected and can be randomly deposited because of the charge repulsion force generated by the printed lines, the printing speed should be greater than 300 mm/s to align the pattern with the printing directions, and the spacing between the printed lines is recommended to be more than 100 &#181;m in order to print the gird or line patterns.</li>
	<li>Print the selected pattern on the substrate using pre-determined printing parameters. 
	Note: A sintering process might be required to obtain the target functionalities of the printed patterns, which is beyond the scope of this paper.</li>
</ol>

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The authors have nothing to disclose.

In this protocol, we focus on printing fine patterns using AgNP ink with two modes: DOD EHD printing and NFES. However, the EHD jet printing application is not limited to the conductive ink using AgNP. Here, we will discuss the general guidelines for the selection of ink, the system configuration, and other printing parameters needed to use EHD jet printing for various fine-pattern applications.
The first and most important step for EHD printing is ink selection and preparation. The ink used i...

EHD jet printing has been widely used in various areas, such as printed electronics, biotechnology, and advanced material applications, because it is capable of high-resolution and low-cost direct patterning1. The printed line width or printed dot size could be reduced to 1 &#181;m, which is significantly smaller than that of conventional piezo-based inkjet printing1.
In EHD printing, a small portion of ink (or meniscus) is pushed out of the nozzle tip and maintained by controlling the flow rate1,2,3,4,5 or the positive air pressure1,6,7. The extruded meniscus is charged and can easily be pulled down from the nozzle tip to the substrate by an electric field, as shown in Figure 1. The conical meniscus is formed during the jetting, producing an ink stream much thinner than the nozzle size.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57846/57846fig1.jpg" /> 
Figure 1: EHD printing. The figure shows the principle of EHD jet printing. Ink is pushed via pressure and pulled via an electric field to form an extruded meniscus from the nozzle. Then, the charged ink can be easily jetted to the substrate via a DC or pulse voltage. <a href="https://www.jove.com/files/ftp_upload/57846/57846fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Even though a single EHD printer can be used for the two different modes, near-field electrospinning (NFES) and drop-on-demand (DOD) EHD jet printing, the realization methods significantly differ in terms of ink, fluidic system, and driving voltage1,2,3. For example, NFES4,5 uses a relatively high-viscous ink [more than 1,000 centipoises (cP)] to form continuous micro-line patterns with high-speed printing up to 1 m/s. On the other hand, DOD EHD jet printing6,7,8 uses low-viscous ink with a viscosity of around 10 cP to print dot-based complex patterns with a low printing speed less than 10 mm/s.
Since the requirement for each mode is significantly different, it may be challenging for inexperienced researchers to achieve the desired results. The empirical &#34;know-how&#34; might be important in practice. To help researchers get used to the printing methods, we present EHD printing protocols for fine conductive patterning using Ag nanoparticle ink. However, we added comments to the protocols so that they are not limited to a conductive patterning using Ag nanoparticle ink. Finally, printing and preparation guidelines are presented in the discussion section.

<table><tbody><tr><td>EHD integrated printing system</td><td>Psolution Ltd., South Korea</td><td>PS300</td><td></td></tr><tr><td>Harima Ag Nanoparticle ink</td><td>Harima Inc., Japan</td><td>Harima NPS-JL</td><td>Ag solid content: ~ 53 wt%, Viscosity: ~10 cP, Surface tension: ~30 mN/m</td></tr><tr><td>Glass capillary</td><td>Narishige Scientific Instrument Lab</td><td>G-1</td><td>Inner diameter: 1 mm; Used to make nozzle for DOD EHD jet printing using thermal puller</td></tr><tr><td>Nozzle thermal puller</td><td>Sutter Instrument, USA</td><td>Sutter P-1000</td><td></td></tr><tr><td>Microscope Slides (Glass subtrate)</td><td>Paul-Marienfeld &amp;amp; Co.KG, Germany</td><td>10 006 12</td><td>Dimension (L x W x T): 76 mm x 26 mm x 1 mm</td></tr><tr><td>Magnetic Stirrer</td><td>Barnstead Thermolyne Corp., USA</td><td>Cimarec SP131635</td><td></td></tr><tr><td>Vortex Stirrer</td><td>Jeiotech, South Korea</td><td>Lab Companion VM-96T</td><td></td></tr><tr><td>Ag nanopaste&amp;nbsp;</td><td>NPK, South Korea</td><td>ES-R001</td><td>Ag solid content: ~85.5 wt%, Viscosity: ~11000 cP</td></tr><tr><td>Poly ethylene oxide (PEO)</td><td>Sigma-Aldrich, USA</td><td>372773-500G</td><td>Mw = 400000</td></tr><tr><td>Ethanol</td><td>Sigma-Aldrich, USA</td><td>459836-500ML</td><td></td></tr></tbody></table>

high-resolution patterning using two modes of electrohydrodynamic jet: drop on demand and near-field electrospinning

Electrohydrodynamic (EHD) jet printing has drawn attention in various fields because it can be used as a high-resolution and low-cost direct patterning tool. EHD printing uses a fluidic supplier to maintain the extruded meniscus by pushing the ink out of the nozzle tip. The electric field is then used to pull the meniscus down to the substrate to produce high-resolution patterns. Two modes of EHD printing have been used for fine patterning: continuous near-field electrospinning (NFES) and dot-based drop-on-demand (DOD) EHD printing. According to the printing modes, the requirements for the printing equipment and ink viscosity will differ. Even though two different modes can be implemented with a single EHD printer, the realization methods significantly differ in terms of ink, fluidic system, and driving voltage. Consequently, without a proper understanding of the jetting requirements and limitations, it is difficult to obtain the desired results. The purpose of this paper is to present a guideline so that inexperienced researchers can reduce the trial and error efforts to use the EHD jet for their specific research and development purposes. To demonstrate the fine-patterning implementation, we use Ag nanoparticle ink for the conductive patterning in the protocol. In addition, we also present the generalized printing guidelines that can be used for other types of ink for various fine-patterning applications.

Dot-based drop-on-demand printing: 
DOD printing is based on one droplet jetting per one jetting trigger. To produce DOD jetting, low-viscous ink with a viscosity of approximately 10 cP should be used. The ink requirement for EHD DOD printing is similar to that of the conventional DOD inkjet, as is the EHD printing method to that of the conventional DOD inkjet. In the case of conventional inkjet printing, the raster printing technique has been widely used, because it...

Watch this Scientific Journal Video about High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning at JoVE.com

High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning

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.

Electrohydrodynamic (EHD) jet printing has drawn attention in various fields because it can be used as a high-resolution and low-cost direct patterning ...

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JoVE Journal

Bioengineering

9.7K Views.  Soonchunhyang University. Hier presenteren we een protocol voor de productie van hoge-resolutie geleidende patronen, gemaakt van electrohydrodynamic (EHD) jet afdrukken. Het protocol bevat twee modi van EHD jet afdrukken: het continu in de buurt van-veld-electrospinning (NFES) en de dot-gebaseerde drop-on-demand (DOD) EHD afdrukken.

Video: High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning

Electrospinning Fundamentals: Optimizing Solution and Apparatus Parameters

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. Electrospinning is a process in which a charged polymer jet is collected on a grounded collector; a rapidly rotating collector results in aligned nanofibers while stationary collectors result in randomly oriented fiber mats. The polymer jet is formed when an applied electrostatic charge overcomes the surface tension of the solution. There is a minimum concentration for a given polymer, termed the critical entanglement concentration, below which a stable jet cannot be achieved and no nanofibers will form - although nanoparticles may be achieved (electrospray). A stable jet has two domains, a streaming segment and a whipping segment. While the whipping jet is usually invisible to the naked eye, the streaming segment is often visible under appropriate lighting conditions. Observing the length, thickness, consistency and movement of the stream is useful to predict the alignment and morphology of the nanofibers being formed. A short, non-uniform, inconsistent, and/or oscillating stream is indicative of a variety of problems, including poor fiber alignment, beading, splattering, and curlicue or wavy patterns. The stream can be optimized by adjusting the composition of the solution and the configuration of the electrospinning apparatus, thus optimizing the alignment and morphology of the fibers being produced. In this protocol, we present a procedure for setting up a basic electrospinning apparatus, empirically approximating the critical entanglement concentration of a polymer solution and optimizing the electrospinning process. In addition, we discuss some common problems and troubleshooting techniques.

Electrospinning techniques can create a variety of nanofibrous scaffolds for tissue engineering or other applications. We describe here a procedure to optimize the parameters of the electrospinning solution and apparatus to obtain fibers with the desired morphology and alignment. Common problems and troubleshooting techniques are also presented.

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. ...

Planar and Three-Dimensional Printing of Conductive Inks

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical devices1-3. In this paper, we focus on one- (1D), two- (2D), and three-dimensional (3D) printing of conductive metallic inks in the form of flexible, stretchable, and spanning microelectrodes.
Direct-write assembly4,5 is a 1-to-3D printing technique that enables the fabrication of features ranging from simple lines to complex structures by the deposition of concentrated inks through fine nozzles (~0.1 - 250 &mu;m). This printing method consists of a computer-controlled 3-axis translation stage, an ink reservoir and nozzle, and 10x telescopic lens for visualization. Unlike inkjet printing, a droplet-based process, direct-write assembly involves the extrusion of ink filaments either in- or out-of-plane. The printed filaments typically conform to the nozzle size. Hence, microscale features (&lt; 1 &mu;m) can be patterned and assembled into larger arrays and multidimensional architectures.
In this paper, we first synthesize a highly concentrated silver nanoparticle ink for planar and 3D printing via direct-write assembly. Next, a standard protocol for printing microelectrodes in multidimensional motifs is demonstrated. Finally, applications of printed microelectrodes for electrically small antennas, solar cells, and light-emitting diodes are highlighted.

Planar and three-dimensional printing of conductive metallic inks is described. Our approach provides new avenues for fabricating printed electronic, optoelectronic, and biomedical devices in unusual layouts at the microscale.

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical ...

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and polar dielectric liquids. These bridges are unique from capillary bridges in that they exhibit extensibility beyond a few millimeters, have complex bi-directional mass transfer patterns, and emit non-Planck infrared radiation. A number of common solvents can form such bridges as well as low conductivity solutions and colloidal suspensions. The macroscopic behavior is governed by electrohydrodynamics and provides a means of studying fluid flow phenomena without the presence of rigid walls. Prior to the onset of a liquid bridge several important phenomena can be observed including advancing meniscus height (electrowetting), bulk fluid circulation (the Sumoto effect), and the ejection of charged droplets (electrospray). The interaction between surface, polarization, and displacement forces can be directly examined by varying applied voltage and bridge length. The electric field, assisted by gravity, stabilizes the liquid bridge against Rayleigh-Plateau instabilities. Construction of basic apparatus for both vertical and horizontal orientation along with operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical electrohydrodynamic liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields and polar dielectric liquids. The construction of basic apparatus and operational examples, including thermographic images, for three liquids (e.g., water, DMSO, and glycerol) is presented.

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Engineering

Prescribed 3-D Direct Writing of Suspended Micron/Sub-micron Scale Fiber Structures via a Robotic Dispensing System

A 3-axis dispensing system is utilized to control the initiating and terminating fiber positions and trajectory via the dispensing software. The polymer fiber length and orientation is defined by the spatial positioning of the dispensing system 3-axis stages. The fiber diameter is defined by the prescribed dispense time of the dispensing system valve, the feed rate (the speed at which the stage traverses from an initiating to a terminating position), the gauge diameter of the dispensing tip, the viscosity and surface tension of the polymer solution, and the programmed drawing length. The stage feed rate affects the polymer solution&#8217;s evaporation rate and capillary breakup of the filaments. The dispensing system consists of a pneumatic valve controller, a droplet-dispensing valve and a dispensing tip. Characterization of the direct write process to determine the optimum combination of factors leads to repeatedly acquiring the desired range of fiber diameters. The advantage of this robotic dispensing system is the ease of obtaining a precise range of micron/sub-micron fibers onto a desired, programmed location via automated process control. Here, the discussed self-assembled micron/sub-micron scale 3D structures have been employed to fabricate suspended structures to create micron/sub-micron fluidic devices and bioengineered scaffolds.

Here, we present a protocol to fabricate freely-suspended, micron/sub-micron scale polymer fibers and &#8220;web-like&#8221; structures generated via automated direct writing procedure by means of a 3-axis dispensing system.

A 3-axis dispensing system is utilized to control the initiating and terminating fiber positions and trajectory via the dispensing software. The polymer ...

Fabricating Superhydrophobic Polymeric Materials for Biomedical Applications

Superhydrophobic materials, with surfaces possessing permanent or metastable non-wetted states, are of interest for a number of biomedical and industrial applications. Here we describe how electrospinning or electrospraying a polymer mixture containing a biodegradable, biocompatible aliphatic polyester (e.g., polycaprolactone and poly(lactide-co-glycolide)), as the major component, doped with a hydrophobic copolymer composed of the polyester and a stearate-modified poly(glycerol carbonate) affords a superhydrophobic biomaterial. The fabrication techniques of electrospinning or electrospraying provide the enhanced surface roughness and porosity on and within the fibers or the particles, respectively. The use of a low surface energy copolymer dopant that blends with the polyester and can be stably electrospun or electrosprayed affords these superhydrophobic materials. Important parameters such as fiber size, copolymer dopant composition and/or concentration, and their effects on wettability are discussed. This combination of polymer chemistry and process engineering affords a versatile approach to develop application-specific materials using scalable techniques, which are likely generalizable to a wider class of polymers for a variety of applications.

Two- and three-dimensional superhydrophobic polymeric materials are prepared by electrospinning or electrospraying biodegradable polymers blended with a lower surface energy polymer of similar composition.

Superhydrophobic materials, with surfaces possessing permanent or metastable non-wetted states, are of interest for a number of biomedical and industrial ...

Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments

Polydimethylsiloxane (PDMS) silicone is a versatile polymer that cannot readily be formed into long filaments. Traditional spinning methods fail because PDMS does not exhibit long-range fluidity at melting. We introduce an improved method to produce filaments of PDMS by a stepped temperature profile of the polymer as it cross-links from a fluid to an elastomer. By monitoring its warm-temperature viscosity, we estimate a window of time when its material properties are amendable to drawing into long filaments. The filaments pass through a high-temperature tube oven, curing them sufficiently to be harvested. These filaments are on the order of hundreds of micrometers in diameter and tens of centimeters in length, and even longer and thinner filaments are possible. These filaments retain many of the material properties of bulk PDMS, including switchable hydrophobicity. We demonstrate this capability with an automated corona-discharge patterning method. These patternable PDMS silicone filaments have applications in silicone weavings, gas-permeable sensor components, and model microscale foldamers.

Here, we present a protocol to produce long filaments of polydimethylsiloxane (PDMS) silicone by gravity-drawing through a furnace. Filaments are on the order of hundreds of micrometers in diameter and tens of centimeters in length and are hydrophobically patternable via an Arduino-controlled corona discharge system.

Polydimethylsiloxane (PDMS) silicone is a versatile polymer that cannot readily be formed into long filaments. Traditional spinning methods fail because ...

Electrospinning Fibrous Polymer Scaffolds for Tissue Engineering and Cell Culture

As the field of tissue engineering evolves, there is a tremendous demand to produce more suitable materials and processing techniques in order to address the requirements (e.g., mechanics and vascularity) of more intricate organs and tissues. Electrospinning is a popular technique to create fibrous scaffolds that mimic the architecture and size scale of the native extracellular matrix. These fibrous scaffolds are also useful as cell culture substrates since the fibers can be used to direct cellular behavior, including stem cell differentiation (see extensive reviews by Mauck et al. and Sill et al. for more information). In this article, we describe the general process of electrospinning polymers and as an example, electrospin a reactive hyaluronic acid capable of crosslinking with light exposure (see Ifkovits et al. for a review on photocrosslinkable materials). We also introduce further processing capabilities such as photopatterning and multi-polymer scaffold formation. Photopatterning can be used to create scaffolds with channels and multi-scale porosity to increase cellular infiltration and tissue distribution. Multi-polymer scaffolds are useful to better tune the properties (mechanics and degradation) of a scaffold, including tailored porosity for cellular infiltration. Furthermore, these techniques can be extended to include a wide array of polymers and reactive macromers to create complex scaffolds that provide the cues necessary for the development of successful tissue engineered constructs.

The process of electrospinning polymers for tissue engineering and cell culture is addressed in this article. Specifically, the electrospinning of photoreactive macromers with additional processing capabilities of photopatterning and multi-polymer electrospinning is described.

As the field of tissue engineering evolves, there is a tremendous demand to produce more suitable materials and processing techniques in order to address ...

Biology

Expanding Nanopatterned Substrates Using Stitch Technique for Nanotopographical Modulation of Cell Behavior

Substrate nanotopography has been shown to be a potent modulator of cell phenotype and function. To dissect nanotopography modulation of cell behavior, a large area of nanopatterned substrate is desirable so that enough cells can be cultured on the nanotopography for subsequent biochemical and molecular biology analyses. However, current nanofabrication techniques have limitations to generate highly defined nanopatterns over a large area. Herein, we present a method to expand nanopatterned substrates from a small, highly defined nanopattern to a large area using stitch technique. The method combines multiple techniques, involving soft lithography to replicate poly(dimethylsiloxane) (PDMS) molds from a well-defined mold, stitch technique to assemble multiple PDMS molds to a single large mold, and nanoimprinting to generate a master mold on polystyrene (PS) substrates. With the PS master mold, we produce PDMS working substrates and demonstrate nanotopographical modulation of cell spreading. This method provides a simple, affordable yet versatile avenue to generate well-defined nanopatterns over large areas, and is potentially extended to create micro-/nanoscale devices with hybrid components.

A protocol for producing a large area of nanopatterned substrate from small nanopatterned molds for study of nanotopographical modulation of cell behavior is presented.

Substrate nanotopography has been shown to be a potent modulator of cell phenotype and function. To dissect nanotopography modulation of cell behavior, a ...

Melt Electrospinning Writing of Three-dimensional Poly(&#949;-caprolactone) Scaffolds with Controllable Morphologies for Tissue Engineering Applications

This tutorial reflects on the fundamental principles and guidelines for electrospinning writing with polymer melts, an additive manufacturing technology with great potential for biomedical applications. The technique facilitates the direct deposition of biocompatible polymer fibers to fabricate well-ordered scaffolds in the sub-micron to micro scale range. The establishment of a stable, viscoelastic, polymer jet between a spinneret and a collector is achieved using an applied voltage and can be direct-written. A significant benefit of a typical porous scaffold is a high surface-to-volume ratio which provides increased effective adhesion sites for cell attachment and growth. Controlling the printing process by fine-tuning the system parameters enables high reproducibility in the quality of the printed scaffolds. It also provides a flexible manufacturing platform for users to tailor the morphological structures of the scaffolds to their specific requirements. For this purpose, we present a protocol to obtain different fiber diameters using melt electrospinning writing (MEW) with a guided amendment of the parameters, including flow rate, voltage and collection speed. Furthermore, we demonstrate how to optimize the jet, discuss often experienced technical challenges, explain troubleshooting techniques and showcase a wide range of printable scaffold architectures.

This protocol serves as a comprehensive guideline to fabricate scaffolds via electrospinning with polymer melts in a direct writing mode. We systematically outline the process and define the appropriate parameter settings for achieving targeted scaffold architectures.

This tutorial reflects on the fundamental principles and guidelines for electrospinning writing with polymer melts, an additive manufacturing technology ...

Vapor Phase Deposition of Electroactive Poly(3,4-ethylenedioxythiophene) onto Electrospun Commodity Polymer Nanofibers

This study investigates the preparation of polyacrylonitrile (PAN) nanofibers through electrospinning to create highly porous and strong materials for applications in water purification, electrocatalysis, and biomedicine. The uniformly white PAN nanofiber mats were cut into 2 cm x 2 cm coupons to ensure consistency. After electrospinning, these nanofibers were coated with an electroactive polymer (EAP) using chemical vapor deposition, with iron (III) chloride (FeCl3) serving as an oxidant for polymerizing 3,4-ethylenedioxythiophene (EDOT) into poly(3,4-ethylenedioxythiophene) (PEDOT). The study examined the impact of different FeCl3 concentrations on PEDOT deposition on the PAN coupons. PEDOT deposition led to an increase in coupon weight. Scanning electron microscopy (SEM) revealed increases in the diameter of the nanofibers treated with increasing FeCl3 oxidant concentration, although higher FeCl3&#160;concentrations caused inter-fiber bridging, implying a concomitant decrease in inter-fiber spacing. Energy dispersive X-ray spectroscopy (EDS) was used to confirm the presence of Fe, Cl, and S in the nanofibers, with sulfur content rising with FeCl3&#160;concentration used, suggesting increased PEDOT deposition efficiency with increasing oxidant concentration. Mechanical testing showed that PEDOT-coated PAN fibers had improved tensile strength and toughness in the hydrated state compared to pure PAN nanofibers. These results highlight the crucial role of FeCl3&#160;concentration in influencing the morphology and properties of PAN-PEDOT composites, enhancing their suitability for applications such as water purification, tissue engineering, biosensing, catalysis, and energy storage.

This paper describes a simple technique for coating electrospun PAN nanofibers with electroactive polymers using vapor deposition. The effects of the oxidant FeCl&#8323; on the deposition of poly(3,4-ethylenedioxythiophene) coatings on PAN were investigated to understand and optimize polymer growth, fiber diameter, and overall mechanical strength of the nanofiber mats.

This study investigates the preparation of polyacrylonitrile (PAN) nanofibers through electrospinning to create highly porous and strong materials for ...

High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning

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