Name: microfluidic preparation of liquid crystalline elastomer actuators
Uploaded: 2018-05-20T13:00:00.000+00:00
Duration: 12 min 4 s
Description: Watch this Scientific Journal Video about Microfluidic Preparation of Liquid Crystalline Elastomer Actuators at JoVE.com

The authors thank the German Science Foundation for funding this work (Ze 230/24-1).

1. Synthesis of Single Actuating LCE Particles
<ol>
	<li>Mounting the device 
	NOTE: All materials used for the microfluidic setup are HPLC supply and commercially available.
	<ol>
		<li>Equip a glass water bath dish [diameter (D): 190 mm, connections: two 29/24 ground glass joints flange-mounted] with two septa. Broach both septa with an awl to fit a tube with an outer diameter (OD) of 1/16 inch through the opening hole.</li>
		<li>Attach a fitting for 1/16 inch OD tubing and the corresponding ferrule to the end of a PTFE tube (tube 1.1; OD: 1/16 inch, inner diameter (ID): 0.17 mm, length (L): 5 cm) and stick the tip (ca. 1 cm) of a polyimide-coated silica capillary (ID: 100 &#181;m, OD: 165 &#181;m, L: 7 cm) into it.</li>
		<li>Screw the tube onto one of the opposing arms of a polyether ether ketone (PEEK) T-junction for 1/16 inch OD tubes, which is mounted on a small metal table. Now, the capillary should protrude a few centemeters out of the T-junction. 
		NOTE: PTFE tubes are best cut with the aid of a tube cutter. For the capillaries, a cleaving stone is best to use.</li>
		<li>Attach a suitable fitting and ferrule to the end of a second PTFE tube (tube 1.2; OD: 1/16 inch, ID: 0.75 mm), which is long enough to reach a syringe pump outside the water bath, and screw it onto the lateral arm of the T-junction.</li>
		<li>Stick a third PTFE tube (tube 1.3; OD: 1/16 inch, ID: 0.17 mm) through one of the septa. Tube 1.3 should be long enough to connect a second syringe pump with tube 1.1 inside the water bath. Add two female luer locks for 1/16 inch OD tubing to the spare end of tubes 1.1 and 1.3, respectively.</li>
		<li>Prepare a fourth PTFE tube (polymerization tube 1.4; OD: 1/16 inch, ID: 0.75 mm) with a fitting plus ferrule and stick it through the second septum. Tube 1.4 should be long enough to leave the water bath and pass a precision heating plate. Connect tube 1.4 via its fitting to the remaining arm of the T-junction and place the end of the glass capillary inside the tube.</li>
		<li>Put the water bath on a hot plate equipped with a thermometer, use adhesive tape to fix tube 1.4 on top of the precision heating plate and attach a 5 mL glass vial to the end of tube 1.4. Connect the end of tube 1.2 to a syringe filled with the continuous phase (silicone oil; viscosity: 1.000 m2/s), connect tube 1.3 to a syringe filled with the hydraulic oil for the monomer phase (silicone oil; viscosity: 100 m2/s) and plug both syringes in a syringe pump. 
		NOTE: In order to connect the tubes to the syringes, barb-to-female-luer-lock connectors for use with 3/32 inch ID tubes are best to use.</li>
		<li>Install a stereomicroscope with the focus being set on the capillary&#39;s tip to enable the observation of the droplet formation and mount a UV-light source (e.g., a 500 W-mercury vapor lamp) with the light cone focused on tube 1.4.</li>
	</ol>
	</li>
	<li>Preparation of the monomer mixture
	<ol>
		<li>To prepare the monomer mixture40, add 200 mg of (4-acryloyloxybutyl)-2,5-di(4-butyloxybenzoyloxy)benzoate to a 50 mL pear-shaped flask.</li>
		<li>Add 7.2 mg of 1,6-hexanediol dimethacrylate (10 mol%) and 6.2 mg of ethyl(2,4,6-trimethylbenzoyl) phenylphosphinate (photoinitiator, 3 w%) to the flask. Dissolve the mixture in about 1 mL of dichloromethane. 
		NOTE: Starting from step 1.2.2., all steps should be performed under UV light-free conditions (e.g., under yellow light).</li>
		<li>Remove the solvent completely under vacuum at 313 K and melt the residual solid at 383 K in an oil bath.</li>
		<li>Prepare a syringe with a barb-to-female-luer-lock connector for use with 3/32 inch ID tubing and attach a PTFE tube (tube 1.5; OD: 1/8 inch, ID: 1.65 mm) via a connecting tube (OD: 1/16 inch, ID: 0.75 mm). Draw up the monomer mixture into the tube 1.5 with the aid of the syringe. 
		NOTE: The amount of monomer should not be less than 70 mg. Otherwise, it becomes very difficult to draw enough monomer mixture into tube 1.5. The protocol can be paused here. If so, store the tube in a refrigerator.</li>
	</ol>
	</li>
	<li>Preparation of the particles
	<ol>
		<li>Attach a male luer lock for 1/8 inch OD tubing to both ends of tube 1.5 containing the monomer mixture. Afterwards, connect both ends of tube 1.5 with the female luer locks on the ends of tubes 1.1 and 1.3. 
		NOTE: The tubes should be rinsed with the liquids provided by the syringe pumps prior to the synthesis.</li>
		<li>Set the water bath&#39;s temperature to 363 K and set the precision heating plate&#39;s temperature to 338 K.</li>
		<li>Make sure the capillary&#39;s tip is in the center of the polymerization tube 1.5 and does not touch the wall. 
		NOTE: The temperatures given here are optimized for this monomer mixture. In general, the water bath&#39;s temperature should be high enough to melt the monomer mixture and the heating plate&#39;s temperature should be in the temperature range of the liquid crystalline phase.</li>
		<li>After the monomer mixture is melted, set the flow rate of the continuous phase (Qc) to a value between 1.5 and 2.0 mL/h and choose flow rate ratios of Qc/Qd (Qd = the flow rate of the hydraulic oil/monomer phase) between 20 and 200. 
		NOTE: For flow rates of Qc = 1.75 mL/h and Qd = 0.35 mL/h, well-actuating particles with a D of 270 &#181;m are observed, for example.</li>
		<li>After the droplet formation begins, wait until the droplets are all the same size before switching on the UV-light. For the described monomer mixture, position the UV source 1 cm above polymerization tube 1.4 at the right end of the precision heating plate. Collect the different fractions of the polymerized particles in the 5-mL glass vial at the end of tube 1.4. While flowing under the UV-light, the droplets&#39; color should change from transparent to white. 
		Caution: Wear UV-protection goggles to protect eyes.</li>
		<li>Put a shield (e.g., a paper box) between the light source and the water bath, in order to prevent any clogging of the capillary. 
		NOTE: In case of a clogging polymerization tube, it might help to heat the clogged part with a heat gun.</li>
		<li>After all the monomer is consumed, clean the setup by injecting acetone into tube 1.3.</li>
	</ol>
	</li>
</ol>
2. Synthesis of Core-shell LCE Particles
<ol>
	<li>Mounting of the device
	<ol>
		<li>Follow step 1.1.1. but use a water bath dish with a D of 190 mm instead.</li>
		<li>Attach a fitting and ferrule to both ends of a fluorinated ethylene propylene (FEP) tubing sleeve (ID: 395 &#181;m, OD: 1/16 inch, L: 1.55 inch), respectively. First, stick a fused silica capillary (ID: 280 &#181;m, OD: 360 &#181;m, L: 8 cm) through the sleeve, in such a way that it protrudes about 3 mm out of one side. Then stick a thinner capillary (ID: 100 &#181;m, OD: 165 &#181;m, L: 11 cm) through the bigger one, so that it protrudes a few millimeters out of its longer side.</li>
		<li>Screw the sleeve onto one of the opposing arms of a PEEK T-junction for 1/16 inch OD tubes (T-junction 1) which is mounted on a small metal table, with the shorter end of the bigger capillary reaching into the T-junction.</li>
		<li>Stick a PTFE tube (tube 2.1; OD: 1/16 inch, ID: 0.17 mm) that is long enough to connect a syringe pump with T-junction 1 through one of the water bath&#39;s septa. Attach a fitting and ferrule to the tube&#39;s end inside the water bath, connect it to the free lateral arm of T-junction 1, and stick the thinner capillary inside tube 2.1.</li>
		<li>Prepare a second PTFE tube (tube 2.2; OD: 1/16 inch, ID: 0.5 mm) with a fitting and a ferrule and connect it to the spare arm of T-junction 1. Stick another PTFE tube (tube 2.3; OD: 1/16 inch, ID: 0.5 mm) through a second hole in the septum next to tube 2.1. Tube 2.3 should be long enough to connect another syringe pump with tube 2.2.</li>
		<li>Add two female luer locks for 1/16 inch OD tubing to the free ends of tubes 2.2 and 2.3 inside the water bath, respectively.</li>
		<li>Connect the free end of the sleeve to one of the opposing arms of a second PEEK T-junction (T-junction 2) which is also mounted on the small metal table. Prepare a fourth PTFE tube (tube 2.4; OD: 1/16 inch, ID: 0.75 mm) with a fitting plus ferrule. Tube 2.4 is long enough to reach a third syringe pump outside the water bath and connect it to the lateral arm of T-junction 2.</li>
		<li>Prepare a fifth PTFE tube (polymerization tube 2.5; OD: 1/16 inch, ID: 0.75 mm) with a fitting plus ferrule and stick it through the other septum. Tube 2.5 should be long enough to leave the water bath and pass a high precision heating plate. Connect the fitting of tube 2.5 with the remaining arm of the T-junction. Now the glass capillaries&#39; tips should be located inside tube 2.5.</li>
		<li>Put the water bath on a hot plate equipped with a thermometer, use adhesive tape to fix tube 2.5 on top of a precision heating plate and attach a 5 mL glass vial to the tube&#39;s end. Connect the end of tube 2.1 to a syringe filled with glycerol (inner phase), connect tube 2.3 to a syringe filled with the hydraulic oil for the monomer phase (silicone oil; viscosity: 100 m2/s), connect tube 2.4 to a syringe filled with the continuous phase (silicone oil; viscosity: 1.000 m2/s) and plug all syringes in syringe pumps.</li>
		<li>Follow step 1.1.7., but read tube 2.5 instead of tube 1.4.</li>
	</ol>
	</li>
	<li>Preparation of the monomer mixture
	<ol>
		<li>Follow all steps of 1.2.</li>
	</ol>
	</li>
	<li>Preparation of the core-shell particles
	<ol>
		<li>Attach a male luer lock for 1/8 inch OD tubes to both ends of the tube containing the monomer mixture, respectively. Afterwards, connect both ends of this tube with the female luer locks on the ends of the tubes 2.2 and 2.3.</li>
		<li>Follow steps 1.3.2-1.3.4.</li>
		<li>Observe the droplet formation via a stereo microscope.</li>
	</ol>
	</li>
</ol>
3. Synthesis of Janus LCE Particles
<ol>
	<li>Mounting of the device
	<ol>
		<li>Follow step 1.1.1.</li>
		<li>Attach a fitting and ferrule to both ends of an FEP tubing sleeve (ID: 395 &#181;m, OD: 1/16 inch, L: 1.55 inch), respectively. Stick two parallelly aligned fused silica capillaries (ID: 100 &#181;m, OD: 165 &#181;m, L1: 8 cm, L2: 11 cm) through the sleeve. The short capillary protrudes about 3 mm out of one side of the sleeve. On the other side of the sleeve, both capillaries have the same length.</li>
		<li>Super-glue the capillaries by putting some glue on one end of the sleeve and wait until it is cured.</li>
		<li>Connect two PEEK T-junctions by screwing the sleeve onto one of the opposing arms, respectively, and mount both on a small metal table.</li>
		<li>Follow steps 2.1.4-2.1.7.</li>
		<li>Prepare a fifth PTFE tube (tube 3.5; OD: 1/16 inch, ID: 0.75 mm, L: 5 cm) with a fitting plus ferrule and connect it with the remaining arm of T-junction 2. Both tips of the glass capillaries are located inside tube 3.5.</li>
		<li>Stick another PTFE tube (tube 3.6; OD: 1/16 inch, ID: 0.5 mm) through the other septum. Tube 2.6 should be long enough to leave the water bath and pass a precision heating plate. Connect tubes 3.5 and 3.6 via fitting systems for 1/16 inch OD tubing.</li>
		<li>Put the water bath on a hot plate equipped with a thermometer, use adhesive tape to fix tube 3.6 on top of a precision heating plate and attach a 5 mL glass vial to the tube&#39;s end. Connect the end of tube 3.1 to a syringe filled with an aqueous monomer mixture (aq. monomer phase), connect tube 3.3 to a syringe filled with the hydraulic oil for the LC-monomer phase (silicone oil; viscosity: 100 m2/s), connect tube 3.4 to a syringe filled with the continuous phase (silicone oil; viscosity: 1.000 m2/s) and plug all syringes in syringe pumps.</li>
		<li>Follow step 1.1.8, but read tube 3.6 instead of tube 1.4.</li>
	</ol>
	</li>
	<li>Preparation of the liquid crystalline (LC) monomer mixture
	<ol>
		<li>Follow all steps of 1.2.</li>
	</ol>
	</li>
	<li>Preparation of the aqueous monomer mixture
	<ol>
		<li>Prepare a solution of 40 wt% acrylamide in distilled water. Add 10 mol% of the crosslinking agent N,N&#39;-methylenebis(acrylamide) and 2 wt% of the initiator 2-hydroxy-2-methylpropiophenone to the solution. (Both amounts are with respect to the acrylamide.) 
		NOTE: In order to raise the viscosity of the aqueous monomer mixture, polyacrylamide can be added.</li>
		<li>Stir the mixture for 24 h at RT and fill it into a 1 mL syringe, afterward.</li>
	</ol>
	</li>
	<li>Preparation of the Janus particles
	<ol>
		<li>Attach a male luer lock for 1/8 inch OD tubes to both ends of the tube containing the LC monomer mixture, respectively. Afterwards, connect both ends of this tube with the female luer locks on the ends of tubes 3.2 and 3.3.</li>
		<li>Follow steps 1.3.2-1.3.4.</li>
		<li>Observe the droplet formation via a stereo microscope.</li>
	</ol>
	</li>
</ol>
4. Analysis of the Particles
<ol>
	<li>Put the particles on a hot-stage under an optical microscope connected to a computer with imaging software. To analyze the particles&#39; actuation, take pictures at temperatures above and below their phase transition temperature, and measure their D. 
	NOTE: A drop of silicon oil prevents the particles from sticking on the object slide.</li>
	<li>To estimate the particles&#39; clearing temperature, determine the temperature at which the particles lose their birefringence under a polarized optical microscope (POM).</li>
</ol>

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J., S&#225;nchez-Somolinos, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=4D+printed+actuators+with+soft-robotic+functions.">4D printed actuators with soft-robotic functions.</a> Macromolecular Rapid Communications. 1700710, 3-9 (2017).</li><li>Ohm, C., Kapernaum, N., Nonnenmacher, D., Giesselmann, F., Serra, C., Zentel, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic+synthesis+of+highly+shape-anisotropic+particles+from+liquid+crystalline+elastomers+with+defined+director+field+configurations.">Microfluidic synthesis of highly shape-anisotropic particles from liquid crystalline elastomers with defined director field configurations.</a> Journal of the American Chemical Society. 133 (14), 5305-5311 (2011).</li><li>Ohm, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+actuating+fibres+of+oriented+main-chain+liquid+crystalline+elastomers+by+a+wetspinning+process.">Preparation of actuating fibres of oriented main-chain liquid crystalline elastomers by a wetspinning process.</a> Soft Matter. 7, 3730 (2011).</li><li>Hessberger, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Co-flow+microfluidic+synthesis+of+liquid+crystalline+actuating+Janus+particles.">Co-flow microfluidic synthesis of liquid crystalline actuating Janus particles.</a> Journal of Materials Chemistry C. 4, 8778-8786 (2016).</li><li>Fleischmann, E. -. K., Liang, H. -. L., Kapernaum, N., Giesselmann, F., Lagerwall, J., Zentel, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=One-piece+micropumps+from+liquid+crystalline+core-shell+particles.">One-piece micropumps from liquid crystalline core-shell particles.</a> Nature Communications. 3, 1178 (2012).</li><li>Khan, I. U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidic+conceived+drug+loaded+Janus+particles+in+side-by-side+capillaries+device.">Microfluidic conceived drug loaded Janus particles in side-by-side capillaries device.</a> International Journal of Pharmaceutics. 473 (1-2), 239-249 (2014).</li><li>Vennes, M., Martin, S., Gisler, T., Zentel, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anisotropic+particles+from+LC+polymers+for+optical+manipulation.">Anisotropic particles from LC polymers for optical manipulation.</a> Macromolecules. 39 (24), 8326-8333 (2006).</li><li>Serra, C. a., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Engineering+polymer+microparticles+by+droplet+microfluidics.">Engineering polymer microparticles by droplet microfluidics.</a> Journal of Flow Chemistry. 3 (3), 66-75 (2013).</li><li>Seo, M., Nie, Z., Xu, S., Lewis, P. C., Kumacheva, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microfluidics:+from+dynamic+lattices+to+periodic+arrays+of+polymer+disks.">Microfluidics: from dynamic lattices to periodic arrays of polymer disks.</a> Langmuir. 21 (11), 4773-4775 (2005).</li><li>Kim, K., Pack, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microspheres+for+drug+delivery.">Microspheres for drug delivery.</a> BioMEMS and Biomedical Nanotechnology. 2, 19-50 (2006).</li><li>Kim, J. -. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Titanium+dioxide/poly(methyl+methacrylate)+composite+microspheres+prepared+by+in+situ+suspension+polymerization+and+their+ability+to+protect+against+UV+rays.">Titanium dioxide/poly(methyl methacrylate) composite microspheres prepared by in situ suspension polymerization and their ability to protect against UV rays.</a> Colloid and Polymer Science. 280 (6), 584-588 (2002).</li><li>Serra, C., Berton, N., Bouquey, M., Prat, L., Hadziioannou, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+predictive+approach+of+the+influence+of+the+operating+parameters+on+the+size+of+polymer+particles+synthesized+in+a+simplified+microfluidic+system.">A predictive approach of the influence of the operating parameters on the size of polymer particles synthesized in a simplified microfluidic system.</a> Langmuir. 23 (14), 7745-7750 (2007).</li><li>Chang, Z., Serra, C. a., Bouquey, M., Prat, L., Hadziioannou, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Co-axial+capillaries+microfluidic+device+for+synthesizing+size-+and+morphology-controlled+polymer+core-polymer+shell+particles.">Co-axial capillaries microfluidic device for synthesizing size- and morphology-controlled polymer core-polymer shell particles.</a> Lab on a Chip. 9, 3007-3011 (2009).</li><li>Braun, L. B., Hessberger, T., Serra, C. A., Zentel, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=UV-free+microfluidic+particle+fabrication+at+low+temperature+using+ARGET-ATRP+as+the+initiator+system.">UV-free microfluidic particle fabrication at low temperature using ARGET-ATRP as the initiator system.</a> Macromolecular Reaction Engineering. 10 (6), 611-617 (2016).</li><li>Thomsen, D. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Liquid+crystal+elastomers+with+mechanical+properties+of+a+muscle.">Liquid crystal elastomers with mechanical properties of a muscle.</a> Macromolecules. 34, 5868-5875 (2001).</li><li>Ohm, C., Serra, C., Zentel, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+continuous+flow+synthesis+of+micrometer-sized+actuators+from+liquid+crystalline+elastomers.">A continuous flow synthesis of micrometer-sized actuators from liquid crystalline elastomers.</a> Advanced Materials. 21 (47), 4859-4862 (2009).</li></ol>

The authors have nothing to disclose.

We have described the fabrication of particles with different morphologies via a microfluidic approach to produce LCE microactuators. For this purpose, capillary-based microfluidic setups were built which allow droplet formation followed by photopolymerization at defined temperatures.
Here, one critical aspect of a successful synthesis is the correct mounting of the setup. All connections between the single parts must be fixed properly to prevent any leaking of the liquids, and the device must be clean before every synthesis to prevent clogging. It is also crucial that the experiment is performed under UV-free conditions, since, otherwise, premature polymerization of the monomer mixture and thus again clogging of the setup would be the result.
To this day, the microfluidic approach described here is the only method able to produce actuating LCE particles. Hereby, the microfluidic process fulfills two requirements at the same time. Besides the fabrication of a multitude of equally sized micro-objects, an orientation of the liquid crystalline director is induced in these particles. In addition, it is a quite simple procedure since a large number of actuators can be synthesized in a single step. Applying other methods, the orientation of the mesogens usually requires an additional step like the stretching of the sample or the application of photo-alignment layers. Additionally, these processes are manual, which means the production of many actuators is very time-consuming. Furthermore, the LCE morphology is-in most cases-limited to polymer films. Drawbacks of the microfluidic approach are the limitation of the particle size (as the diameter is restricted to values between 200 and 400 &#181;m), the vulnerability to clogging the capillary, and the necessity of UV-free conditions during the particle preparation in the setup.
On-chip systems are often used for microfluidic particle fabrications since they can easily be produced and are made of just one piece. These setups, however, not only lack the necessary adjustability of different temperatures during the flow but also are not flexible enough to easily exchange clogged or broken parts of the microreactor. Hence, the capillary-based setups we use are more suitable for the synthesis of LCE actuators, as they fulfill the crucial requirements.
Aside from our presented results of actuating Janus-particles and core-shell micropumps, more complex actuating particles featuring new properties could be synthesized in the future and open new possibilities for soft actuator applications. The further modification of Janus particles to multi-responsive particles is already in progress. Therefore, we are aiming for the introduction of a second temperature responsive polymer besides the actuating LCE. Further possibilities for new particle designs can also arise from the use of liquid crystalline azo-monomers, which results in the light-driven actuation of LCE-particles17,18. In that case, we can think of Janus particles containing both a temperature-responsive as well as a photo-actuating part. The synthesis of light-driven core-shell particles or pipe-like structures offers another possible particle design, which would lead to photo-responsive micropumps. The modification of the principle microfluidic procedures which we presented above should allow a variety of new actuators.

Microfluidic syntheses have become a well-known method for the fabrication of liquid crystalline elastomer (LCE) actuators in the last few years1,2,3. This approach not only enables the production of a large number of well-actuating particles but also allows the fabrication of shapes and morphologies which are not accessible by other methods. Since LCE actuators are promising candidates for an application as artificial muscles in micro-robotics, new methods to synthesize such particles are of great importance for this future technology4.
In LCEs, the mesogens of a liquid crystal (LC) are attached to the polymer chains of an elastomeric network5,6,7,8. The linkage of the mesogens to the polymer chain can thereby happen in the form of a side-chain, a main-chain, or a combined LC-polymer9,10,11. The distance between the crosslinking points should be far enough to allow a free reorientation of the polymer chain in between (in fact, this is true for any elastomer, which differentiates them from &#34;thermosets&#34;). Thereby, crosslinking can be permanent or reversible due to strong non-covalent interactions12,13,14. This kind of material combines the properties of both, the anisotropic behavior of a liquid crystal with the entropic elasticity of an elastomer. In the temperature range of its liquid crystalline phase, the polymer chains adopt a (more or less) stretched conformation caused by the anisotropy of the liquid crystalline phase, which is quantified by the nematic order parameter. When the sample is brought above the nematic-to-isotropic phase transition temperature, the anisotropy disappears, and the network relaxes to the energetically favored random coil conformation. This leads to a macroscopic deformation and thus actuation5,15. Besides the heating of the sample, this phase transition can also be induced by other stimuli such as light or solvent diffusion in the LCEs16,17,18,19.
In order to obtain a strong deformation, it is necessary that the sample either forms a monodomain or features at least a preferred orientation of the single domain&#39;s directors during the crosslinking step20. For the production of LCE films, this is often achieved by the stretching of a pre-polymerized sample, via the orientation of the domains in an electric or magnetic field, with the aid of photo-alignment layers or via 3D-printing21,22,23,24,25,26.
A different approach is the continuous preparation of LCE particles with capillary-based microfluidic droplet generators. Liquid crystalline monomer droplets are dispersed in a highly viscous continuous phase, which flows around the droplets and applies a shear rate on the droplets&#39; surface. Therefore, a circulation inside the monomer droplet is observed, which causes an overall alignment of the liquid crystalline phase27. Thereby, the magnitude of the shear rates acting on the droplets has a strong influence on both the droplet&#39;s shape and the size, as well as on the orientation of the liquid crystalline director field. These well-oriented droplets can then be polymerized further downstream in the microfluidic setup. Thus, the preparation of actuators with varying shapes (e.g., particles and fibers) and more complex morphologies like core-shell and Janus particles are possible28,29,30,31. It is even possible to prepare oblate particles, which extend along their symmetry axis and highly prolate, fiber-like particles, which shrink at the phase transition. Both types of particles can be made with the same kind of microfluidic setup, just by varying the shear rate27. Here, we present the protocol of how to produce such LCE actuators of different morphologies in self-manufactured capillary-based microfluidic devices.
Besides the effect of mesogen alignment in LCE droplets and the accessibility of polymers with varying shapes, microfluidic approaches have further advantages. Compared to other particle fabrication methods like precipitation in a non-solvent or suspension polymerization32 (which leads to particles with a broad size-distribution), monodisperse particles (the coefficient of variation of the particle size is &#60;5%) can be synthesized using microfluidics33,34. In addition, it is easy to break the sphere symmetry of the droplets by a flow. Thus, large particles with a cylindrical symmetry are accessible, which is needed for actuators. This is different from LC-particles made by suspension polymerization32. Furthermore, the particle size is well adjustable by microfluidics in a range from several micrometers to hundreds of microns, and additives can easily be brought into the particles or at their surface. This is why microfluidic particle preparation is often used in topics like drug delivery35 or the manufacturing of cosmetics36.
The microfluidic setups used in this article were introduced by Serra et al.33,37,38. These are self-manufactured and consist of high-performance liquid chromatography (HPLC) polytetrafluorethylene (PTFE) tubes and T-junctions, as well as fused silica capillaries that provide the single phases. Thus, the setup can easily be modified, and single parts can simply be exchanged as they are commercially available. A photoinitiator is added to the monomer mixtures, which enables the use of an appropriate light source to induce the polymerization of the droplets on-the-fly, after they left the capillary. Irradiation aside from the capillaries is necessary to prevent a clogging of the setup. Other types of the polymerization only start the polymerization after the droplet has left the capillary (e.g., with initiators based on redox processes)39. However, due to the quickness of the photo-induced crosslinking polymerization and the ability to be remotely controlled, photoinitiation is the most advantageous one.
Since the LCE&#39;s monomer mixture is crystalline at room temperature, a careful temperature control of the whole microfluidic setup is necessary. Therefore, the part of the setup in which the droplet formation occurs is placed in a water bath. Here, the droplets are formed at high temperatures in the isotropic melt of the mixture. For the orientation, the droplets must be cooled into the liquid crystalline phase. Therefore, the polymerization tube is placed on a hot plate which is set to the lower temperature range of the LC-phase (Figure 1).
Here, we describe a flexible and straightforward method for the fabrication of LCE actuators in a flow. This protocol provides the steps required to build the microfluidic setup for the synthesis of single particles as well as Janus and core-shell particles in a few minutes. Next, we describe how to run a synthesis and show the typical outcome as well as the properties of the actuating particles. Finally, we discuss the advantages of this method and why we think it might bring progress to the field of LCE actuators.

<table><tbody><tr><td>NanoTight fitting for 1/16&#039;&#039; OD tubings</td><td>Postnova_IDEX</td><td>F-333N</td><td></td></tr><tr><td>NanoTight ferrule for 1/16&#039;&#039; OD tubings</td><td>Postnova_IDEX</td><td>F-142N</td><td></td></tr><tr><td>PEEK Tee for 1/16&amp;rdquo; OD Tubing</td><td>Postnova_IDEX</td><td>P-728</td><td>T-junction</td></tr><tr><td>Female Fitting for 1/16&amp;rdquo; OD Tubing</td><td>Postnova_IDEX</td><td>P-835</td><td>female luer-lock</td></tr><tr><td>Male Fitting for 1/8&amp;rdquo; OD Tubing</td><td>Postnova_IDEX</td><td>P-831</td><td>male luer-lock</td></tr><tr><td>Female Luer Connectors for use with 3/32&amp;rdquo; ID tubings</td><td>Postnova_IDEX</td><td>P-858</td><td>for the syrringe&#039;s tip</td></tr><tr><td>NanoTight FEP tubing sleeve ID: 395 &amp;micro;m OD: 1/16&#039;&#039;</td><td>Postnova_IDEX</td><td>F-185</td><td></td></tr><tr><td>Fused Silica Capillary Tubing ID: 100 &amp;micro;m OD: 165 &amp;micro;m</td><td>Postnova</td><td>Z-FSS-100165</td><td>glass capillary</td></tr><tr><td>Fused Silica Capillary Tubing ID: 280 &amp;micro;m OD: 360 &amp;micro;m</td><td>Postnova</td><td>Z-FSS-280360</td><td>glass capillary</td></tr><tr><td>&amp;lsquo;&amp;lsquo;Pump 33&amp;rsquo;&amp;rsquo; DDS</td><td>Harvard Apparatus</td><td>70-3333</td><td>syringe pump</td></tr><tr><td>Precision hot plate</td><td>Harry Gestigkeit GmbH</td><td>PZ 28-2</td><td></td></tr><tr><td>Stereomicroscope stemi 2000-C</td><td>Carl Zeiss Microscopy GmbH</td><td>455106-9010-000</td><td></td></tr><tr><td>Mercury vapor lamp Oriel LSH302</td><td>LOT</td><td></td><td>Intensity: 500 W</td></tr><tr><td>Teflon Kapillare, 1/16&#039;&#039; x 0,75mm</td><td>WICOM</td><td>WIC 33104</td><td>teflon tube</td></tr><tr><td>Teflon Kapillare, 1/16&#039;&#039; x 0,50mm</td><td>WICOM</td><td>WIC 33102</td><td>teflon tube</td></tr><tr><td>Teflon Kapillare, 1/16&#039;&#039; x 0,17mm</td><td>WICOM</td><td>WIC 33101</td><td>teflon tube</td></tr><tr><td>Silicion oil 1.000 cSt</td><td>Sigma Aldrich</td><td>378399</td><td></td></tr><tr><td>Silicion oil 100 cSt</td><td>Sigma Aldrich</td><td>378364</td><td></td></tr><tr><td>1,6-hexanediol dimethacrylate</td><td>Sigma Aldrich</td><td>246816</td><td>Crosslinker</td></tr><tr><td>Lucirin TPO</td><td>Sigma Aldrich</td><td>415952</td><td>Initiator</td></tr><tr><td>Polarized optical microscope BX51</td><td>Olympus</td><td></td><td>For analysis</td></tr><tr><td>Hotstage TMS 94</td><td>Linkam</td><td></td><td>For analysis</td></tr><tr><td>Imaging software Cell^D</td><td>Olympus</td><td></td><td>For analysis</td></tr></tbody></table>

microfluidic preparation of liquid crystalline elastomer actuators

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

In this protocol, we present the synthesis of LCE particles with different morphologies via a microfluidic approach. The microfluidic setups for the fabrication of single, core-shell, and Janus particles are shown in Figure 129,38,41. One advantage of the continuous flow production is the very good control over both size and shape of the particles. Figure 2a illustrates the advantage of the single droplet setup: a very narrow size distribution with all particles having the same shape41. Hereby, the size of the spheres can easily be adjusted by changing the ratio of flow rates of the different phases. Following the protocol, particle diameters between 200 and 400 &#181;m can be produced in a well-controlled manner by choosing the flow rate ratios, as shown in Figure 2b1. The best results are obtained for flow rates of the continuous phase (Qc) between 1.5 and 2.0 mL/h and for flow rate ratios of QC/Qd (Qd = the flow rate of the monomer phase) between 20 and 200. For the flow rates of Qc = 1.75 mL/h and Qd = 0.35 mL/h, well-actuating particles with a diameter of 270 &#181;m are observed, for example. If higher ratios Qc/Qd are selected, the droplet formation is less controlled and the particles&#39; size distribution becomes much broader. For lower ratios, the particles are not spherical anymore. In addition to the flow rate adjustments, the distance of the UV-lamp to the polymerization tube as well as the position between the left and the right end of the precision hot plate can change the actuation properties of LCE particles, which happens, for example, if the polymerization kinetics change by reason of choosing monomer mixture compositions or applied polymerization temperatures different from the values described here.
Figure 3a shows an actuating particle which elongates up to 70% when it is heated above its phase transition temperature, which proves that the requirement of inducing an orientation of the liquid crystalline director before polymerization is fulfilled. This alignment of the mesogens results from the shear between the highly viscous continuous phase and the monomer droplets&#39; surface. If silicon oils of lower viscosity are used, the particle&#39;s actuation is reduced.&#160;
Furthermore, the microfluidic device allows the control over different kinds of actuation patterns, such as elongation or contraction during the phase transition, by varying the shear rate acting on the droplets during the polymerization. This can be processed easily at constant flow rates of the continuous phase by using different inner diameters of the polymerization tube. Figure 3a shows a prolate shaped particle, which elongates along its rotational axis and was synthesized at lower shear rates in a broader polymerization tube (ID: 0.75 mm). The liquid crystalline molecules (mesogens) are aligned along a concentric director field in this case. On the other side, rod-like particles (as illustrated in Figure 3b) feature a contraction during the phase transition and a bipolar alignment of the mesogens&#39; director field. This particle was produced at higher shear rates in a thinner polymerization tube (ID: 0.5 mm).
The protocol describes another advantage of the microfluidic process. Besides single particles, samples of more complex morphologies can also be synthesized. Figure 3c shows an actuating core-shell particle and Figure 3d a Janus particle which both were produced following part 2 and 3 of the protocol29,30.
If all steps of the protocol are done correctly, particles having the properties shown in Figure 4 should be obtained3,41. In Figure 4a, the heating and cooling curves are plotted for single particles synthesized at different flow rates. By heating the particle from room temperature, the liquid crystalline order is - at first - reduced for a little bit, resulting in a small deformation of the particle. However, close to the phase transition temperature, all orientation is suddenly lost and the particle shows a strong elongation just by heating it up for a few degrees. By cooling the particle down, a hysteresis can be observed, and the original shape is obtained. This process is reversible over many actuation cycles, as shown in Figure 4b.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57715/57715fig1.jpg" /> 
Figure 1: Microfluidic setups. (a) The general setup includes three syringes, which contain the hydraulic silicone oil (1), the aqueous monomer mixture (3), and the continuous phase silicone oil (4). The liquid crystalline monomer mixture (2) is placed in the water bath (5) at 363 K, which heats up the liquid crystal to the isotropic state. The droplet&#39;s polymerization is initiated on the hot plate (6) at 338 K in the nematic state of the liquid crystal by UV-irradiation (7). (The single particle setup equals the general setup, but lacks the second capillary, syringe (3) and the second T-junction). (b) This panel shows a setup containing two capillaries side by side to each other, which allows the Janus droplet formation. (c) The core-shell setup is composed of a capillary which is telescoped into a broader second capillary. <a href="/files/ftp_upload/57715/57715fig1large.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/57715/57715fig2.jpg" /> 
Figure 2:&#160;Representative particles obtained in the microfluidic single particle setup. (a) This panel shows a microscopy image of monodisperse LCE particles prepared in the microfluidic single particle setup. Scale bar = 200 &#181;m. (b) This panel shows the dependence of the particles&#39; diameter with respect to the ratio of the oil&#39;s flow rate (Qc) to the monomer mixture&#39;s flow rate (Qd). The size of the obtained particles is only dependent on the velocity ratio of both phases and not on their absolute values. (This figure has been modified from Ohm, Fleischmann, Kraus,Serra, and Zentel1 and Ohm, Serra, and Zentel41.) <a href="/files/ftp_upload/57715/57715fig2large.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/57715/57715fig3.jpg" /> 
Figure 3:&#160;Optical microscopy images of four different particle morphologies in the nematic state (at 353 K) and after phase transition in the isotropic state (at 413 K). These panels show (a) the elongation of an oblate-shaped LCE particle (concentric director field), (b) the contraction of a rod-like shaped LCE-particle (bipolar director field), (c) the elongation of an oblate-shaped core-shell particle, and (d) the contraction of a prolate-shaped Janus particle (left part: LCE, right part: acrylamide hydrogel). Scale bars = 100 &#181;m. <a href="/files/ftp_upload/57715/57715fig3large.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/57715/57715fig4.jpg" /> 
Figure 4:&#160;Actuation properties of representative single particles. (a) This panel shows the heating and cooling curves of LCE particles being prepared in the single particle microfluidic setup at different flow rates for the continuous phase. The particles prepared at the highest flow rate show the strongest actuation (about 70%) and both curves form a hysteresis, respectively. (b) This is a plot of 10 actuation cycles of LCE particles showing no decrease of their actuation over the cycle number. This proves that the particles are crosslinked, and the actuation is completely reversible. Note: This graph was drawn for a particle made from a main-chain LCE system but looks the same for the LCE system used in this article. (This figure has been modified from Ohm, Serra, and Zentel41.) <a href="/files/ftp_upload/57715/57715fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Microfluidic Preparation of Liquid Crystalline Elastomer Actuators at JoVE.com

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation ...

microfluidic-preparation-of-liquid-crystalline-elastomer-actuators

Nanyang Technological University

Research

JoVE Journal

Chemistry

8.9K Views.  Johannes Gutenberg University. This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

Video: Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

Microfluidic Chips Controlled with Elastomeric Microvalve Arrays

Miniaturized microfluidic systems provide simple and effective solutions for low-cost point-of-care diagnostics and high-throughput biomedical assays. Robust flow control and precise fluidic volumes are two critical requirements for these applications. We have developed microfluidic chips featuring elastomeric polydimethylsiloxane (PDMS) microvalve arrays that: 1) need no extra energy source to close the fluidic path, hence the loaded device is highly portable; and 2) allow for microfabricating deep (up to 1 mm) channels with vertical sidewalls and resulting in very precise features.

The PDMS microvalves-based devices consist of three layers: a fluidic layer containing fluidic paths and microchambers of various sizes, a control layer containing the microchannels necessary to actuate the fluidic path with microvalves, and a middle thin PDMS membrane that is bound to the control layer. Fluidic layer and control layers are made by replica molding of PDMS from SU-8 photoresist masters, and the thin PDMS membrane is made by spinning PDMS at specified heights. The control layer is bonded to the thin PDMS membrane after oxygen activation of both, and then assembled with the fluidic layer. The microvalves are closed at rest and can be opened by applying negative pressure (e.g., house vacuum). Microvalve closure and opening are automated via solenoid valves controlled by computer software.

Here, we demonstrate two microvalve-based microfluidic chips for two different applications. The first chip allows for storing and mixing precise sub-nanoliter volumes of aqueous solutions at various mixing ratios. The second chip allows for computer-controlled perfusion of microfluidic cell cultures.

The devices are easy to fabricate and simple to control. Due to the biocompatibility of PDMS, these microchips could have broad applications in miniaturized diagnostic assays as well as basic cell biology studies.

We demonstrate protocols for manufacturing and automating elastomeric polydimethylsiloxane (PDMS)-based microvalve arrays that need no extra energy to close and feature photolithographically defined precise volumes. A parallel subnanoliter-volume mixer and an integrated microfluidic perfusion system are presented.

Miniaturized microfluidic systems provide simple and effective solutions for low-cost point-of-care diagnostics and high-throughput biomedical assays.  ...

Biology

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline ...

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tunable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied.

This manuscript shows the fabrication process for the manufacture of dielectric elastomer soft actuators based on silicone membranes. The three key stages of production are presented in detail: blade casting of thin silicone membranes; pad printing of compliant electrodes; and the assembly of all the components.

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an ...

Engineering

Microfluidic Pneumatic Cages: A Novel Approach for In-chip Crystal Trapping, Manipulation and Controlled Chemical Treatment

The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. Herein, we introduce a microfluidic-based technology, employing a double-layer microfluidic device, which can trap and localize in situ and ex situ synthesized structures on microfluidic channel surfaces. Crucially, we show how such a device can be used to conduct controlled chemical reactions onto on-chip trapped structures and we demonstrate how the synthetic pathway of a crystalline molecular material and its positioning inside a microfluidic channel can be precisely modified with this technology. This approach provides new opportunities for the controlled assembly of structures on surface and for their subsequent treatment.

Herein, we describe the fabrication and operation of a double-layer microfluidic system made of polydimethylsiloxane (PDMS). We demonstrate the potential of this device for trapping, directing the coordination pathway of a crystalline molecular material and controlling chemical reactions onto on-chip trapped structures.

The precise localization and controlled chemical treatment of structures on a surface are significant challenges for common laboratory technologies. ...

Free-form Light Actuators &#8212; Fabrication and Control of Actuation in Microscopic Scale

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers&#39; attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.
In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others.

Here, we fabricate 3D polymeric micro/nano structures in which both the shape and the molecular alignment can be engineered with nanometer scale accuracy by the use of direct laser writing. Light induced deformation of several types of liquid crystalline elastomer microstructures can be controlled in the microscopic scale.

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted ...

A Microfluidic Platform to Study Bioclogging in Porous Media

Bacterial biofilms are found in several environmental and industrial porous media, including soils and filtration membranes. Biofilms grow under certain flow conditions and can clog pores, thereby redirecting the local fluid flow. The ability of biofilms to clog pores, the so-called bioclogging, can have a tremendous effect on the local permeability of the porous medium, creating a pressure buildup in the system, and impacting the mass flow through it. To understand the interplay between biofilm growth and fluid flow under different physical conditions (e.g., at different flow velocities and pore sizes), in the present study, a microfluidic platform is developed to visualize biofilm development using a microscope under externally-imposed, controlled physical conditions. The biofilm-induced pressure buildup in the porous medium can be measured simultaneously using pressure sensors and, later, correlated with the surface coverage of the biofilm. The presented platform provides a baseline for a systematic approach to investigate bioclogging caused by biofilms in porous media under flow conditions and can be adapted to studying environmental isolates or multispecies biofilms.

The present protocol describes a microfluidic platform to study biofilm development in quasi-2D porous media by combining high-resolution microscopy imaging with simultaneous pressure difference measurements. The platform quantifies the influence of pore size and fluid flow rates in porous media on bioclogging.

Bacterial biofilms are found in several environmental and industrial porous media, including soils and filtration membranes. Biofilms grow under certain ...

Environment

Double Emulsion Generation Using a Polydimethylsiloxane (PDMS) Co-axial Flow Focus Device

Double emulsions are useful in a number of biological and industrial applications in which it is important to have an aqueous carrier fluid. This paper presents a polydimethylsiloxane (PDMS) microfluidic device capable of generating water/oil/water double emulsions using a coaxial flow focusing geometry that can be fabricated entirely using soft lithography. Similar to emulsion devices using glass capillaries, double emulsions can be formed in channels with uniform wettability and with dimensions much smaller than the channel sizes. Three dimensional flow focusing geometry is achieved by casting a pair of PDMS slabs using two layer soft lithography, then mating the slabs together in a clamshell configuration. Complementary locking features molded into the PDMS slabs enable the accurate registration of features on each of the slab surfaces. Device testing demonstrates formation of double emulsions from 14 &#181;m to 50 &#181;m in diameter while using large channels that are robust against fouling and clogging.

Double Emulsion Generation Using a Polydimethylsiloxane PDMS Co-axial Flow Focus Device

Microfluidic double emulsions generation typically involves devices with patterned wettability or custom-fabricated glass components. Here we describe the fabrication and testing of an all polydimethylsiloxane (PDMS) double emulsion generator that does not require surface treatment or complicated fabrication processes, and is capable of producing double emulsions down to 14 &#181;m.

Double emulsions are useful in a number of biological and industrial applications in which it is important to have an aqueous carrier fluid. This paper ...

Bioengineering

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and as soft robots. Here, we demonstrate the preparation of shape-responsive liquid crystal elastomers (LCEs) and LCE nanocomposites along with characterization of their shape-responsiveness, mechanical properties, and microstructure. Two types of LCEs &#8212; polysiloxane-based and epoxy-based &#8212; are synthesized, aligned, and characterized. Polysiloxane-based LCEs are prepared through two crosslinking steps, the second under an applied load, resulting in monodomain LCEs. Polysiloxane LCE nanocomposites are prepared through the addition of conductive carbon black nanoparticles, both throughout the bulk of the LCE and to the LCE surface. Epoxy-based LCEs are prepared through a reversible esterification reaction. Epoxy-based LCEs are aligned through the application of a uniaxial load at elevated (160 &#176;C) temperatures. Aligned LCEs and LCE nanocomposites are characterized with respect to reversible strain, mechanical stiffness, and liquid crystal ordering using a combination of imaging, two-dimensional X-ray diffraction measurements, differential scanning calorimetry, and dynamic mechanical analysis. LCEs and LCE nanocomposites can be stimulated with heat and/or electrical potential to controllably generate strains in cell culture media, and we demonstrate the application of LCEs as shape-responsive substrates for cell culture using a custom-made apparatus.

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and ...

Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, compliant and biomimetic nature of this deformation, actuators made of the laminate have received increasing interest in soft robotics and (bio)medical applications. However, methods to easily fabricate the active material in large (even industrial) quantities and with a high batch-to-batch and within-batch repeatability are needed to transfer the knowledge from laboratory to industry. This protocol describes a simple, industrially scalable and reproducible method for the fabrication of ionic carbon-based electromechanically active capacitive laminates and the preparation of actuators made thereof. The inclusion of a passive and chemically inert (insoluble) middle layer (e.g., a textile-reinforced polymer network or microporous Teflon) distinguishes the method from others. The protocol is divided into five steps: membrane preparation, electrode preparation, current collector attachment, cutting and shaping, and actuation. Following the protocol results in an active material that can, for example, compliantly grasp and hold a randomly shaped object as demonstrated in the article.

This article describes a fast and simple manufacturing process of ionic electromechanically active composite materials for actuators in biomedical, biomimetic and soft robotics applications. The key fabrication steps, their importance for the actuators&#39; final properties, and some of the main characterization techniques are described in detail.

Ionic electromechanically active capacitive laminates are a type of smart material that move in response to electrical stimulation. Due to the soft, ...

Microfluidic Synthesis of Microgel Building Blocks for Microporous Annealed Particle Scaffold

The microporous annealed particle (MAP) scaffold platform is a subclass of granular hydrogels. It is composed of an injectable slurry of microgels that can form a structurally stable scaffold with cell-scale porosity in situ following a secondary light-based chemical crosslinking step (i.e.,&#160;annealing). MAP scaffold has shown success in a variety of regenerative medicine applications, including dermal wound healing, vocal fold augmentation, and stem cell delivery. This paper describes the methods for synthesis and characterization of poly(ethylene glycol) (PEG) microgels as the building blocks to form a MAP scaffold. These methods include the synthesis of a custom annealing macromer (MethMAL), determination of microgel precursor gelation kinetics, microfluidic device fabrication, microfluidic generation of microgels, microgel purification, and basic scaffold characterization, including microgel sizing and scaffold annealing. Specifically, the high-throughput microfluidic methods described herein can produce large volumes of microgels that can be used to generate MAP scaffolds for any desired application, especially in the field of regenerative medicine.

This protocol describes a set of methods for synthesizing the microgel building blocks for microporous annealed particle scaffold, which can be used for a variety of regenerative medicine applications.

The microporous annealed particle (MAP) scaffold platform is a subclass of granular hydrogels. It is composed of an injectable slurry of microgels that ...