Name: drawing and hydrophobicity-patterning long polydimethylsiloxane silicone filaments
Uploaded: 2019-01-07T14:00:00.000+00:00
Duration: 7 min 56 s
Description: Watch this Scientific Journal Video about Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments at JoVE.com

The authors gratefully acknowledge insight and assistance from W. Cook, S. J .S. Rubin, J. Zehner, C. Barraugh, C. Fukushima, M. Mulligan, M. Keckley, and A. Bosshardt, and financial support from The Rose Hills Foundation and The Johnson Summer Student Research Grant.&#160; The authors also acknowledge preliminary work on viscosity as a means of tracking silicone polymerization by the students of Advanced Laboratory in Chemistry (Fall 2017).

1. PDMS Filaments
<ol>
	<li>Furnace and extrusion assembly
	<ol>
		<li>Attach 1.59 mm inner-diameter high temperature silicone rubber tubing to house air supply through a metering valve (see Figure 1). Connect the other end of the tubing to an extrusion adaptor consisting of a sheath around a 1.08 mm radius opening, which will produce a steady downward airflow around the filament (e.g., a custom machined brass sheath at an airflow of approximately 1.4 L/min; see Supplemental Figure 1 for specifications). 
		NOTE: Preferably produce filaments in a reduced air-flow environment with a fire extinguisher nearby.</li>
		<li>Cover the surface below a cylindrical ceramic tube furnace (e.g., 17 mm inner diameter, 107.7 mm length) with foil to capture any excess PDMS.</li>
		<li>Mount the extrusion adaptor above the vertical tube furnace, centering the opening to the furnace and leveling the furnace so the filament will pass through.</li>
		<li>Connect the extruder to the extrusion adaptor via high temperature silicone rubber semi-clear tubing.</li>
		<li>Heat the furnace until the inner temperature is approximately 250 &#176;C as measured by an infrared thermometer, using a variable transformer to regulate temperature.</li>
		<li>Move the furnace out from under the extrusion adaptor so that the extrusion adapter does not heat up prior to filament production.</li>
	</ol>
	</li>
	<li>Partially pre-curing PDMS
	<ol>
		<li>Preheat a disposable sample tube to 65.0 &#176;C in a viscometer with temperature control. Use an appropriate viscometer and spindle, such that one can measure viscosities in the range of 200-10000 mPa&#183;s.</li>
		<li>Thoroughly mix 18.0 g of PDMS base with 1.8 g of its curing agent in a weigh boat and place the mixture in a room temperature (RT) vacuum desiccator for 15 min or until no bubbles remain. Vent the desiccator periodically to pop the bubbles near the surface. 
		NOTE: The following timing assumes use of the PDMS base and curing agent specified in the Table of Materials.</li>
		<li>Pour 17.7 g of the mixture into the preheated sample tube on the countertop (some is lost to adhesion to the weigh boat). Re-insert the sample tube into the viscometer.</li>
		<li>Take viscosity measurements once per minute, keeping the viscometer spinning slowly (5 rpm).</li>
		<li>When the viscosity reaches 4000 mPa&#183;s, remove the sample tube with pliers and immediately pour into the room temperature extruder. Produce filaments in a time window that starts approximately 4.5 min from heat removal and lasts for a subsequent 4 min.</li>
	</ol>
	</li>
	<li>Extruding PDMS to form filaments
	<ol>
		<li>During the delay time, ensure the furnace is at 250 &#176;C using an infrared thermometer.</li>
		<li>Approximately 4 min after removal of the sample tube from the viscometer, move the furnace back underneath the extrusion adapter and align the inner needle of the extrusion adapter with the tube furnace using a heat-safe glove.</li>
		<li>Periodically twist the screw on the extruder, allowing a steady stream of PDMS to drip through the furnace. 
		NOTE: If droplets form instead of filaments, wait 30 s for further RT curing and try again. Constant twisting pushes the PDMS through the furnace too quickly. Generally, small flare-ups can be handled by turning off the furnace and using a non-conductive stick to dislodge any built up PDMS from the furnace. 
		CAUTION: If PDMS drips onto the walls or top of the furnace, there is the possibility of a flare-up.</li>
		<li>Once the PDMS stream begins to thin, twist the extruder an additional half-revolution. Each trial has about 16 revolutions using the given extruder.</li>
		<li>Gather the filaments on wooden sticks beneath the furnace after each twist and lay them across wooden racks to finish curing for approximately 12 h.</li>
	</ol>
	</li>
</ol>
2. Patterning Surfaces of PDMS Filaments with Computer-controlled Corona Discharge
<ol>
	<li>Arduino-controlled filament-positioning and corona discharge control system: assemble the open-source Arduino microprocessor-controlled stepper motor that pulls the filament under a corona discharge device per the wiring diagram in the Supplemental Materials. Upload the custom Arduino code software in the supplemental materials to the system.</li>
	<li>Patterning filament surface with the device
	<ol>
		<li>Enter desired pattern into code (&#8220;Pattern&#8221; array) and upload it to the Arduino via USB.</li>
		<li>Wash a cured filament with 1% sodium dodecyl sulfate and rinse with excessive pure water. Dry the filament with air.</li>
		<li>Place filament on an electrically non-conducting slab with a cut-out (e.g., laser-cut acrylic, see Supplemental Figure 3A) that allows the filament to be suspended in air. Use double-sided tape to secure the ends of the filament to the slab.</li>
		<li>Place the slab in a ventilated corona discharge box in between acrylic tracks and ensure that it is level. Place an immobile metal slab under the acrylic, aligning the filament along the edge of the metal slab.</li>
		<li>Place the corona discharge electrode (e.g., a spring electrode tip) approximately 3 mm above the filament and plug the corona discharger into the Arduino-controlled outlet.</li>
		<li>Tie a non-conductive line to the circular cut-out on the acrylic slab. Adhere the other end of the line to the Arduino-controlled stepper motor (see Supplemental Figure 3BC).</li>
		<li>Press the button in the Arduino circuit to begin patterning with the programmed code. Corona discharge is a produced by a low current 4.5 MHz electric field with an output voltage of 10-40 kV that modifies the surface of the PDMS to render it hydrophilic. 
		CAUTION: The current is low, but the device produces ozone and UV radiation in air. Preferably use it in a fume hood, behind a vented acrylic shield.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

The basis of this method is manipulating the material properties of curing PDMS so they are suitable for gravity drawing. Gravity drawing droplets into stable filaments is governed by three dimensionless parameters16. They relate the droplet&#8217;s inertia relative to gravity (Froude), surface-tension (Weber), and viscosity (Reynolds). The critical step of crosslinking the PDMS until it experimentally evidences stable extension per our method most dramatically alters its Reynolds number, reducing it by more than an order of magnitude, from 0.83 to 0.07. In contrast, the next largest change in one of the other dimensionless parameters is the Weber number, which merely doubles. This supports the use of viscosity as a sensitive proxy for tracking PDMS spinnability.
A critical improvement over our earlier filament production method is that the viscosity profile during the protocol is used to determine experimental working times. To determine limitations of the technique, we pre-cured a batch of PDMS per the protocol, removed it from heat, and took viscosity measurements at room temperature as the PDMS continued cross-linking. The resulting viscosity profile (Figure 4) suggests that the window for spinnability is extended dramatically by removing the PDMS from the 65 &#176;C heated viscometer. Our protocol involves removal of the PDMS prior to entering the spinnability window, then allowing of the sample to continue crosslinking for about 4.5 min as it cools to room temperature. Subsequently, the experimenter has approximately 4 min to draw it before the ongoing crosslinking renders the PDMS no longer drawable.
The method as shown readily produces filaments with diameters on the order of 100 s of &#181;m and lengths on the order of 0.5 m. The filament length is limited by accessible space beneath the extrusion and tube furnace. A reasonable modification of the technique would be to install it in a longer chimney to produce longer filaments. A modification that we have not yet explored is mechanically pulling the filaments instead of relying on a gravity drop, which may yield thinner filaments.
A critical step to hydrophobically patterning the filament is exposure to corona discharge in ambient conditions. This introduces some ambiguity, as the shape/intensity of the discharge is affected by ambient conditions and local conductivity. It can be tuned by placing grounded conductors under the filament as well as adjusting the voltage of the corona device (10-40 kV). The mechanism of the corona surface-modification is likely electron energy transfer lysing the PDMS side chains and backbone. To break these bonds, electrons would need less energy than the average energy required to produce a dielectric-barrier discharge17. Thus, an observable discharge that envelops the filament is likely to produce the surface modification and can be readily tested via water droplet contact angle measurements.
This method enables relatively facile production of silicone PDMS filaments and subsequent complex hydrophobic patterning. The initial aim is to produce a model foldamer system in which hydrophobic patterns can be designed to produce observable filament folding pathways and folded structures. This testbed may provide generalizable design rules for engineering folding pathways. These filaments may also have material applications as part of hydrophobic or chemically reactive weaves via solvent swelling, or in the use of reactive compounds suspended in the gas-permeable PDMS.

Polydimethylsiloxane (PDMS) silicone is a broadly used material with many manufacturing and research applications. It is heat and water resistant, electrically insulating, hydrophobic, gas permeable, food-safe, biocompatible, and flexible with an almost ideal Poisson ratio. Additionally, it can readily serve as a host for various functional molecules, added either before or after curing1,2. Its surface is readily modifiable by UVO, oxygen plasma, or Corona discharge to switch its hydrophobicity and induce short-term self-adhesion3,4,5. In particular, it has also been used in microfluidics6.
Filaments of PDMS are particularly useful in producing high surface-area silicone weaves, silicone fiber sensors7, and silicone-based additive manufacturing materials (3D printing). In our labs, we use hydrophobically patterned filaments of PDMS as a platform to study folding. The team studies the filament&#8217;s conformational statistics in an aqueous environment via an athermal acoustic excitation and imaging system previously reported8.
Forming high aspect-ratio filaments from PDMS via traditional form-casting is challenging. Filaments have large surface area-to-volume ratios, which complicates release from the molds9. Researchers have had success sheathing PDMS with carrier polymers for continuous electrospinning into nanoscale filaments10,11,12, though the resulting filaments are not pure PDMS.
The predominant manufacturing method to produce macroscale filaments out of other materials involves drawing out a viscous liquid from a reservoir through a pore. Typically, the viscous liquid is a thermoplastic or glass that is fluid at high temperatures in the reservoir and cools into an (often amorphous) solid filament as it is drawn out through a chimney. This process is sometimes called melt spinning, and it is incompatible with PDMS because PDMS does not exhibit long-range fluidity at melting. Block co-polymers of silicone and alpha-methyl styrene have been shown to produce filaments via melt spinning, but again, the resulting filaments are not pure PDMS13.
The method we outline here is akin to melt-spinning, except the relative temperature of the reservoir and chimney are switched. The PDMS is fluid in a room temperature reservoir, as it has not yet completed cross-linking. The viscosity of the PDMS varies as the silicone oil crosslinks with a curing agent, a process that can be thermally accelerated. Before placing it in the reservoir, we heat the curing PDMS until it reaches a viscosity suitable for long gravity-drips, then cure it post-drip via a hot tube furnace in the chimney. The approach is somewhat comparable to &#8220;dry-spinning&#8221;, in which polymers are dissolved in volatile solvents that evaporate during drawing.
To our knowledge, the only reported method of producing long filaments of pure PDMS is our previous publication8 . The method introduced here is a significant improvement upon the original approach, with the intent of minimizing the art of the process. Most notably, by measuring the viscosity during the pre-curing stage and timing cool-down periods, we are able to report an experimentally accessible window of filament spinnability. We also introduce a means of producing repeatable, localized surface modifications on the filament via an Arduino-controlled corona patterning system, enabling longitudinal hydrophobic patterning along the filament.

<table><tbody><tr><td>2 part PDMS Silicone</td><td>Dow Corning Sylgard 184</td><td>4019862</td><td></td></tr><tr><td>Thermosel</td><td>Brookfield</td><td>HT-110 115, HT-115A DP</td><td></td></tr><tr><td>viscometer</td><td>Brookfield</td><td>RVT115</td><td></td></tr><tr><td>Disposible sample chamber</td><td>Brookfield</td><td>HT-2DB-100</td><td></td></tr><tr><td>Disposible spindle</td><td>Brookfield</td><td>SC4-27D-100, SC4-DSY</td><td></td></tr><tr><td>Extruder</td><td>Makin&#039;s</td><td>35055</td><td></td></tr><tr><td>High-temperature silicone tubing</td><td>McMaster-Carr</td><td>51135K16</td><td></td></tr><tr><td>Cylindrical Tube heater (Ceramic)</td><td></td><td></td><td>Ours is a custom: 17.0 mm inner diameter, 38.7 mm outer diameter, 107.7 mm length, 150 Ohm. Companies include Watlow and Omega. Critical design considerations: smaller inner diameters will require better furnace-filament alignment, longer tubes should also be sufficient.</td></tr><tr><td>Variable Transformer for heater</td><td>Variac</td><td>3PN1010</td><td></td></tr><tr><td>Metering valve</td><td>Swagelok</td><td>SS-2MA1</td><td></td></tr><tr><td>Corona Discharge Device</td><td>Electro-Technic</td><td>BD20A</td><td></td></tr><tr><td>Arduino Kit</td><td>Elegoo</td><td>EL-KIT-003</td><td></td></tr><tr><td>Nylon Fishing Line</td><td>EoongSng</td><td>B075DYVC3F</td><td></td></tr><tr><td>Pasta Drying Rack</td><td>Norpro</td><td>B00004UE7U</td><td></td></tr><tr><td>Infrared thermometer</td><td>Nubee</td><td>81175535214</td><td></td></tr><tr><td>Flatbed scanner</td><td>Canon</td><td>CanoScan 9000F MKII</td><td></td></tr></tbody></table>

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.

The method (Figure 1) successfully produces filaments of approximately 200 &#181;m in diameter (Figure 2) and of varying lengths on the order of 10 cm. Longer filaments could be produced by raising the drawing apparatus, and average filament diameters can vary between 50-300 &#181;m per run. Filaments are flexible and robust, and their modulus of elasticity is comparable to bulk PDMS8. In this protocol, the working time between pre-curing the PDMS and drawing the filaments is conveniently long enough for transporting the material (Figure 3).
Hydrophobicity patterning via the corona discharge can be verified via droplet contact angles. The contact angle analysis is complicated by the cylindrical shape of the filament; droplets can form either a symmetrical barrel shape or asymmetrical shell shape14. For our filaments, using 1 &#181;L water droplets, these two shapes correspond to corona-treated hydrophilic surfaces and untreated hydrophobic PDMS, respectively (Figure 4).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58826/58826fig1.jpg" /> 
Figure 1: Schematic of the filament production method. Degassed, cross-linking PDMS is heated at 65 &#176;C until its viscosity reaches 4000 mPa&#183;s. It is then cooled and transferred to an extruder that pushes the material through a downward air-flow-sheathed opening, then through a tube furnace before being harvested as filaments. <a href="https://www.jove.com/files/ftp_upload/58826/58826fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58826/58826fig2.jpg" /> 
Figure 2: Histogram of filament widths from a demonstration of the method. Widths are per cm of filament over 6 filaments, whose total length was approximately 80 cm. Filaments were scanned by a flatbed scanner and analyzed by custom Octave scripts15. Inset: a representative image of an extended filament next to a ruler.<a href="https://www.jove.com/files/ftp_upload/58826/58826fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58826/58826fig3.jpg" /> 
Figure 3: PDMS viscosity vs. time. Viscosity measurements (at 5 RPM) of degassed PDMS base and curing agent as a function of time, with different temperature profiles. Continuous heating (gray diamonds) is at 65 &#176;C, demonstrating a rapid increase in viscosity. Black data points are at 65 &#176;C, prior to removal to room temperature when viscosity reaches 4000 mPa&#183;s per the method above. Red data points represent the same sample&#39;s viscosity after cooling to 25 &#176;C. Gap shown in the data occurs while the viscometer is being cooled to 25 &#176;C with ice-water flow. Cyan window is time when filament production occurs as per the method. <a href="https://www.jove.com/files/ftp_upload/58826/58826fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/58826/58826fig4.jpg" /> 
Figure 4: Hydrophobically patterned filament. (A) Filament was patterned with an alternating hydrophobicity pattern, every 2 cm, using the Arduino-controlled-Corona method described above, and visualized with 1 &#181;L droplets of water. (B) Droplets adopt either a shell (left) or barrel (right) conformation depending on the filament&#8217;s local hydrophobicity14. Shell contact angles are estimated directly, whereas planar-equivalent contact angles on barrel-droplets are determined by fitting to an analytical solution8. <a href="https://www.jove.com/files/ftp_upload/58826/58826fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 1" src="/files/ftp_upload/58826/58826figS1.jpg" /> 
Supplemental Figure 1: Dimensions of extrusion adapter with airflow sheath. <a href="https://www.jove.com/files/ftp_upload/58826/58826figS1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 2" src="/files/ftp_upload/58826/58826figS2.jpg" /> 
Supplemental Figure 2: Wiring diagram for Corona patterner. Corona discharge device is plugged into the relayed power source. The filament is loaded on a tray attached by a non-conducting wire to the stepper motor via a spindle (Supplemental Figure 3). The pattern is uploaded onto the Arduino microprocessor on the bottom left. <a href="https://www.jove.com/files/ftp_upload/58826/58826figS2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 3" src="/files/ftp_upload/58826/58826figS3.jpg" /> 
Supplemental Figure 3: Schematic of parts for filament patterner. (A) Tray with an opening where the filament is suspended. (B) Spindle for the nonconducting wire connecting the stepper motor to the tray via the 0.8 cm hole. (C) Bottom view of the spindle, showing the adapter to the stepper motor shaft. We fabricated all parts from laser-cut 3.2 mm acrylic, which was glued together when necessary. <a href="https://www.jove.com/files/ftp_upload/58826/58826figS3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Supplemental Figure 4" src="/files/ftp_upload/58826/58826figS4.jpg" /> 
Supplemental Figure 4: Image of the filament patterner. <a href="https://www.jove.com/files/ftp_upload/58826/58826figS4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments at JoVE.com

Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments

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 ...

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JoVE Journal

Engineering

8.8K Views.  Claremont McKenna College. 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.

Video: Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments

Micropunching Lithography for Generating Micro- and Submicron-patterns on Polymer Substrates

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of low weight, easy tailoring of properties and a wide spectrum of applications2,3. Due to sensitivity of conducting polymers to environmental conditions (e.g., air, oxygen, moisture, high temperature and chemical solutions), lithographic techniques present significant technical challenges when working with these materials4. For example, current photolithographic methods, such as ultra-violet (UV), are unsuitable for patterning the conducting polymers due to the involvement of wet and/or dry etching processes in these methods. In addition, current micro/nanosystems mainly have a planar form5,6. One layer of structures is built on the top surfaces of another layer of fabricated features. Multiple layers of these structures are stacked together to form numerous devices on a common substrate. The sidewall surfaces of the microstructures have not been used in constructing devices. On the other hand, sidewall patterns could be used, for example, to build 3-D circuits, modify fluidic channels and direct horizontal growth of nanowires and nanotubes.
A macropunching method has been applied in the manufacturing industry to create macropatterns in a sheet metal for over a hundred years. Motivated by this approach, we have developed a micropunching lithography method (MPL) to overcome the obstacles of patterning conducting polymers and generating sidewall patterns. Like the macropunching method, the MPL also includes two operations (Fig. 1): (i) cutting; and (ii) drawing. The "cutting" operation was applied to pattern three conducting polymers4, polypyrrole (PPy), Poly(3,4-ethylenedioxythiophen)-poly(4-styrenesulphonate) (PEDOT) and polyaniline (PANI). It was also employed to create Al microstructures7. The fabricated microstructures of conducting polymers have been used as humidity8, chemical8, and glucose sensors9. Combined microstructures of Al and conducting polymers have been employed to fabricate capacitors and various heterojunctions9,10,11. The "cutting" operation was also applied to generate submicron-patterns, such as 100- and 500-nm-wide PPy lines as well as 100-nm-wide Au wires. The "drawing" operation was employed for two applications: (i) produce Au sidewall patterns on high density polyethylene (HDPE) channels which could be used for building 3D microsystems12,13,14, and (ii) fabricate polydimethylsiloxane (PDMS) micropillars on HDPE substrates to increase the contact angle of the channel15.

A micropunching lithography approach is developed to generate micro- and submicron-patterns on top, sidewall and bottom surfaces of polymer substrates. It overcomes the obstacles of patterning conducting polymers and generating sidewall patterns. This method allows rapid fabrication of multiple features and is free of aggressive chemistry.

Conducting polymers have attracted great attention since the discovery of high conductivity in doped polyacetylene in 19771. They offer the advantages of ...

Synthesis of Poly(N-isopropylacrylamide) Janus Microhydrogels for Anisotropic Thermo-responsiveness and Organophilic/Hydrophilic Loading Capability

Janus microparticles are compartmentalized particles with differing molecular structures and/or functionality on each of their two sides. Because of this unique property, Janus microparticles have been recognized as a new class of materials, thereby attracting a great deal of attention from various research fields. The versatility of these microparticles has been exemplified through their uses as building blocks for self-assembly, electrically responsive actuators, emulsifiers for painting and cosmetics, and carriers for drug delivery. This study introduces a detailed protocol that explicitly describes a synthetic method for designing novel Janus microhydrogels composed of a single base material, poly(N-isopropylacrylamide) (PNIPAAm). Janus microdroplets are firstly generated via a hydrodynamic focusing microfluidic device (HFMD) based on the separation of a supersaturated aqueous NIPAAm monomer solution and subsequently polymerized through exposure to UV irradiation. The resulting Janus microhydrogels were found to be entirely composed of the same base material, featured an easily identifiable compartmentalized morphology, and exhibited anisotropic thermo-responsiveness and organophilic/hydrophilic loading capability. We believe that the proposed method introduces a novel hydrogel platform with the potential for advanced synthesis of multi-functional Janus microhydrogels.

Synthesis of PolyN-isopropylacrylamide Janus Microhydrogels for Anisotropic Thermo-responsiveness and Organophilic/Hydrophilic Loading Capability

We present a protocol to synthesize Janus microhydrogels composed entirely of the same base material, poly(N-isopropylacrylamide) (PNIPAAm), with a clearly compartmentalized structure base on the phase separation of a supersaturated NIPAAm monomer solution. The synthesized Janus microhydrogels show unique properties such as anisotropic thermo-responsiveness and organophilic/hydrophilic loading capability.

Janus microparticles are compartmentalized particles with differing molecular structures and/or functionality on each of their two sides. Because of this ...

Chemistry

Microfluidic Fabrication of Polymeric and Biohybrid Fibers with Predesigned Size and Shape

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and used to dictate the shape as well as the diameter of a core stream.&#160;Grooves in the top and bottom of a microfluidic channel were designed to direct the sheath fluid and shape the core fluid.&#160;By matching the viscosity and hydrophilicity of the sheath and core fluids, the interfacial effects are minimized and complex fluid shapes can be formed.&#160;Controlling the relative flow rates of the sheath and core fluids determines the cross-sectional area of the core fluid.&#160;Fibers have been produced with sizes ranging from 300 nm to ~1 mm, and fiber cross-sections can be round, flat, square, or complex as in the case with double anchor fibers. Polymerization of the core fluid downstream from the shaping region solidifies the fibers.&#160;Photoinitiated click chemistries are well suited for rapid polymerization of the core fluid by irradiation with ultraviolet light.&#160;Fibers with a wide variety of shapes have been produced from a list of polymers including liquid crystals, poly(methylmethacrylate), thiol-ene and thiol-yne resins, polyethylene glycol, and hydrogel derivatives. Minimal shear during the shaping process and mild polymerization conditions also makes the fabrication process well suited for encapsulation of cells and other biological components.

Two adjacent fluids passing through a grooved microfluidic channel can be directed to form a sheath around a prepolymer core; thereby&#160;determining both shape and cross-section. Photoinitiated polymerization, such as thiol click chemistry, is well suited for rapidly solidifying the core fluid into a microfiber with predetermined size and shape.

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and ...

Bioengineering

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 &#956;m). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Surface fabrication methods for patterned deposition of nanometer thick brushes or micron thick, crosslinked films of an azlactone block co-polymer are reported. Critical experimental steps, representative results, and limitations of each method are discussed. These methods are useful for creating functional interfaces with tailored physical features and tunable surface reactivity.

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl ...

One-Step Approach to Fabricating Polydimethylsiloxane Microfluidic Channels of Different Geometric Sections by Sequential Wet Etching Processes

Polydimethylsiloxane (PDMS) materials are substantially exploited to fabricate microfluidic devices by using soft lithography replica molding techniques. Customized channel layout designs are necessary for specific functions and integrated performance of microfluidic devices in numerous biomedical and chemical applications (e.g., cell culture, biosensing, chemical synthesis, and liquid handling). Owing to the nature of molding approaches using silicon wafers with photoresist layers patterned by photolithography as master molds, the microfluidic channels commonly have regular cross sections of rectangular shapes with identical heights. Typically, channels with multiple heights or different geometric sections are designed to possess particular functions and to perform in various microfluidic applications (e.g., hydrophoresis is used for sorting particles and in continuous flows for separating blood cells6,7,8,9). Therefore, a great deal of effort has been made in constructing channels with various sections through multiple-step approaches like photolithography using several photoresist layers and assembly of different PDMS thin sheets. Nevertheless, such multiple-step approaches usually involve tedious procedures and extensive instrumentation. Furthermore, the fabricated devices may not perform consistently and the resulted experimental data may be unpredictable. Here, a one-step approach is developed for the straightforward fabrication of microfluidic channels with different geometric cross sections through PDMS sequential wet etching processes, that introduces etchant into channels of planned single-layer layouts embedded in PDMS materials. Compared to the existing methods for manufacturing PDMS microfluidic channels with different geometries, the developed one-step approach can significantly simplify the process to fabricate channels with non-rectangular sections or various heights. Consequently, the technique is a way of constructing complex microfluidic channels, which provides a fabrication solution for the advancement of innovative microfluidic systems.

Several methods are available for the fabrication of channels of non-rectangular sections embedded in polydimethylsiloxane microfluidic devices. Most of them involve multistep manufacturing and extensive alignment. In this paper, a one-step approach is reported for fabricating microfluidic channels of different geometric cross sections by polydimethylsiloxane sequential wet etching.

Polydimethylsiloxane (PDMS) materials are substantially exploited to fabricate microfluidic devices by using soft lithography replica molding techniques. ...

Construction of Modular Hydrogel Sheets for Micropatterned Macro-scaled 3D Cellular Architecture

Hydrogels can be patterned at the micro-scale using microfluidic or micropatterning technologies to provide an in vivo-like three-dimensional (3D) tissue geometry. The resulting 3D hydrogel-based cellular constructs have been introduced as an alternative to animal experiments for advanced biological studies, pharmacological assays and organ transplant applications. Although hydrogel-based particles and fibers can be easily fabricated, it is difficult to manipulate them for tissue reconstruction. In this video, we describe a fabrication method for micropatterned alginate hydrogel sheets, together with their assembly to form a macro-scale 3D cell culture system with a controlled cellular microenvironment. Using a mist form of the calcium gelling agent, thin hydrogel sheets are easily generated with a thickness in the range of 100 - 200 &#181;m, and with precise micropatterns. Cells can then be cultured with the geometric guidance of the hydrogel sheets in freestanding conditions. Furthermore, the hydrogel sheets can be readily manipulated using a micropipette with an end-cut tip, and can be assembled into multi-layered structures by stacking them using a patterned polydimethylsiloxane (PDMS) frame. These modular hydrogel sheets, which can be fabricated using a facile process, have potential applications of in vitro drug assays and biological studies, including functional studies of micro- and macrostructure and tissue reconstruction.

We describe the fabrication of micropatterned hydrogel sheets using a simple process, which can be assembled and manipulated in a freestanding form. Using these modular hydrogel sheets, a simple macro-scaled 3D cell culture system can be generated with a controlled cellular microenvironment.

Hydrogels can be patterned at the micro-scale using microfluidic or micropatterning technologies to provide an in vivo-like three-dimensional (3D) tissue ...

Fabricating Degradable Thermoresponsive Hydrogels on Multiple Length Scales via Reactive Extrusion, Microfluidics, Self-assembly, and Electrospinning

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), their ultimate clinical use has been hampered by the lack of biologically-relevant degradation observed for most smart materials. This is particularly true for temperature-responsive hydrogels, which are almost uniformly based on polymers that are functionally non-degradable (e.g., poly(N-isopropylacrylamide) (PNIPAM) or poly(oligoethylene glycol methacrylate) (POEGMA)). As such, to effectively translate the potential of thermoresponsive hydrogels to the challenges of remote-controlled or metabolism-regulated drug delivery, cell scaffolds with tunable cell-material interactions, theranostic materials with the potential for both imaging and drug delivery, and other such applications, a method is required to render the hydrogels (if not fully degradable) at least capable of renal clearance following the required lifetime of the material. To that end, this protocol describes the preparation of hydrolytically-degradable hydrazone-crosslinked hydrogels on multiple length scales based on the reaction between hydrazide and aldehyde-functionalized PNIPAM or POEGMA oligomers with molecular weights below the renal filtration limit. Specifically, methods to fabricate degradable thermoresponsive bulk hydrogels (using a double barrel syringe technique), hydrogel particles (on both the microscale through the use of a microfluidics platform facilitating simultaneous mixing and emulsification of the precursor polymers and the nanoscale through the use of a thermally-driven self-assembly and cross-linking method), and hydrogel nanofibers (using a reactive electrospinning strategy) are described. In each case, hydrogels with temperature-responsive properties similar to those achieved via conventional free radical cross-linking processes can be achieved, but the hydrazone cross-linked network can be degraded over time to re-form the oligomeric precursor polymers and enable clearance. As such, we anticipate these methods (which may be generically applied to any synthetic water-soluble polymer, not just smart materials) will enable easier translation of synthetic smart materials to clinical applications.

Protocols are described for the fabrication of degradable thermoresponsive hydrogels based on hydrazone cross-linking of polymeric oligomers on the bulk scale, microscale, and nanoscale, the latter for preparation of both gel nanoparticles and nanofibers.

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), ...

Polydimethylsiloxane-polycarbonate Microfluidic Devices for Cell Migration Studies Under Perpendicular Chemical and Oxygen Gradients

This paper reports a microfluidic device made of polydimethylsiloxane (PDMS) with an embedded polycarbonate (PC) thin film to study cell migration under combinations of chemical and oxygen gradients. Both chemical and oxygen gradients can greatly affect cell migration in vivo; however, due to technical limitations, very little research has been performed to investigate their effects in vitro. The device developed in this research takes advantage of a series of serpentine-shaped channels to generate the desired chemical gradients and exploits a spatially confined chemical reaction method for oxygen gradient generation. The directions of the chemical and oxygen gradients are perpendicular to each other to enable straightforward migration result interpretation. In order to efficiently generate the oxygen gradients with minimal chemical consumption, the embedded PC thin film is utilized as a gas diffusion barrier. The developed microfluidic device can be actuated by syringe pumps and placed into a conventional cell incubator during cell migration experiments to allow for setup simplification and optimized cell culture conditions. In cell experiments, we used the device to study migrations of adenocarcinomic human alveolar basal epithelial cells, A549, under combinations of chemokine (stromal cell-derived factor, SDF-1&#945;) and oxygen gradients. The experimental results show that the device can stably generate perpendicular chemokine and oxygen gradients and is compatible with cells. The migration study results indicate that oxygen gradients may play an essential role in guiding cell migration, and cellular behavior under combinations of gradients cannot be predicted from those under single gradients. The device provides a powerful and practical tool for researchers to study interactions between chemical and oxygen gradients in cell culture, which can promote better cell migration studies in more in vivo-like microenvironments.

The control of chemical and oxygen gradients is essential for cell cultures. This paper reports a polydimethylsiloxane-polycarbonate (PDMS-PC) microfluidic device capable of reliably generating combinations of chemical and oxygen gradients for cell migration studies, which can be practically utilized in biological labs without sophisticated instrumentation.

This paper reports a microfluidic device made of polydimethylsiloxane (PDMS) with an embedded polycarbonate (PC) thin film to study cell migration under ...

A Versatile Method of Patterning Proteins and Cells

Substrate and cell patterning techniques are widely used in cell biology to study cell-to-cell and cell-to-substrate interactions. Conventional patterning techniques work well only with simple shapes, small areas and selected bio-materials. This article describes a method to distribute cell suspensions as well as substrate solutions into complex, long, closed (dead-end) polydimethylsiloxane (PDMS) microchannels using negative pressure. This method enables researchers to pattern multiple substrates including fibronectin, collagen, antibodies (Sal-1), poly-D-lysine (PDL), and laminin. Patterning of substrates allows one to indirectly pattern a variety of cells. We have tested C2C12 myoblasts, the PC12 neuronal cell line, embryonic rat cortical neurons, and amphibian retinal neurons. In addition, we demonstrate that this technique can directly pattern fibroblasts in microfluidic channels via brief application of a low vacuum on cell suspensions. The low vacuum does not significantly decrease cell viability as shown by cell viability assays. Modifications are discussed for application of the method to different cell and substrate types. This technique allows researchers to pattern cells and proteins in specific patterns without the need for exotic materials or equipment and can be done in any laboratory with a vacuum.

This report describes a simple, easy to perform technique, using low pressure vacuum, to fill microfluidic channels with cells and substrates for biological research.

Substrate and cell patterning techniques are widely used in cell biology to study cell-to-cell and cell-to-substrate interactions. Conventional patterning ...

Pattern Generation for Micropattern Traction Microscopy

Micropattern traction microscopy allows control of the shape of single cells and cell clusters. Furthermore, the ability to pattern at the micrometer length scale allows the use of these patterned contact zones for the measurement of traction forces, as each micropatterned dot allows for the formation of a single focal adhesion that then deforms the soft, underlying hydrogel. This approach has been used for a wide range of cell types, including endothelial cells, smooth muscle cells, fibroblasts, platelets, and epithelial cells. 
This review describes the evolution of techniques that allow the printing of extracellular matrix proteins onto polyacrylamide hydrogels in a regular array of dots of prespecified size and spacing. As micrometer-scale patterns are difficult to directly print onto soft substrates, patterns are first generated on rigid glass coverslips that are then used to transfer the pattern to the hydrogel during gelation. First, the original microcontact printing approach to generate arrays of small dots on the coverslip is described. A second step that removes most of the pattern to leave islands of small dots is required to control the shapes of cells and cell clusters on such arrays of patterned dots. 
Next, an evolution of this approach that allows for the generation of islands of dots using a single subtractive patterning step is described. This approach is greatly simplified for the user but has the disadvantage of a decreased lifetime for the master mold needed to make the patterns. Finally, the computational approaches that have been developed for the analysis of images of displaced dots and subsequent cell-generated traction fields are described, and updated versions of these analysis packages are provided.

We describe improvements to a standard method for measuring cellular traction forces, based on microcontact printing with a single subtractive patterning step of dot arrays of extracellular matrix proteins on soft hydrogels. This method allows for simpler and more consistent fabrication of island patterns, essential for controlling cell cluster shape.

Micropattern traction microscopy allows control of the shape of single cells and cell clusters. Furthermore, the ability to pattern at the micrometer ...