This work was supported by the National Natural Science Foundation of China under Grant 62273304.

1. Synthesis of the ECPCs slurry
<ol>
	<li>Disperse the CNTs into a toluene solvent at a weight ratio of 1:30 and dilute the PDMS base with toluene at a weight ratio of 1:1. 
	NOTE: The whole experimental procedure, which is shown in Figure 1, should be carried out in a well-ventilated fume hood.</li>
	<li>Magnetically stir the CNTs/toluene suspension and the PDMS/toluene solution at room temperature for 1 h. 
	NOTE: This step allows the CNTs to be well-dispersed into the PDMS matrix in the following step.</li>
	<li>Mix the CNTs/toluene suspension and the PDMS/toluene solution to form a liquid CNTs/PDMS/toluene mixture, and magnetically stir this blend on a hotplate at 80 &#176;C to evaporate the solvent (toluene). 
	NOTE: The evaporation of the solvent increases the solution viscosity, which needs to be precisely controlled to facilitate the mixing process in the next step. The time required for complete solvent evaporation is 2 h.</li>
	<li>Add PDMS curing agent into the CNTs/PDMS/toluene mixture at a weight ratio of 10:1. 
	NOTE: At this stage, the synthesis of the ECPCs slurry is complete.</li>
</ol>
2. Fabrication of the microfluidic channel-based stretchable electrodes
<ol>
	<li>Prepare the SU-8-based mold with different patterns of microfluidic channels using the conventional lithography technique on a Si wafer. 
	NOTE: The lithography process of the mold follows the standard method suggested in the data sheet of the photoresist used; the thickness of molds is about 100 &#181;m, while three different line widths of 50 &#181;m, 100 &#181;m, and 200 &#181;m are used for all the trace structures.</li>
	<li>Perform a silanization process on the SU-8 mold by immersing the mold into the (3-aminopropyl) triethoxysilane solution. 
	NOTE: This step facilitates peeling off the PDMS.</li>
	<li>Mix the PDMS base solution and the curing agent with a weight ratio of 10:1, and place the uncured PDMS mixture in a vacuum desiccator until all the air bubbles disappear.</li>
	<li>Pour the degassed mixture onto the mold fabricated in step 2.1, and place the mold with the uncured PDMS solution on a hotplate at 85 &#176;C for 1 h to completely cure the PDMS and transfer the pattern of the mold onto the cured PDMS film. Peel off the PDMS layer with the help of a blade.</li>
	<li>Cast a small amount of the ECPCs prepared in step 1 onto the PDMS surface. Carefully scrape the ECPCs slurry along the embossed microfluidic channel with the help of a razor blade. 
	NOTE: During this scrape-coating process, the highly viscous ECPCs slurry is effectively trapped in the microchannel pattern, and residues left on the PDMS surface can be removed by the blade simultaneously. If it is hard to scrape the ECPCs slurry into the microchannel, it is recommended to heat the sample to increase its viscosity. This coating step may be repeated multiple times until the microchannel is filled and continuous conducting electrodes are formed.</li>
	<li>Heat the sample at 70 &#176;C for 2 h.</li>
	<li>Connect copper wires to the two ends of the electrodes fabricated in the last step using conductive silver paste. The connection spot is further sealed and protected by the adhesive rubber sealant. 
	&#8203;NOTE: At this stage, the fabrication of the ECPCs-based stretchable electrodes is complete, as shown in Figure 2.</li>
</ol>
3. Fabrication of the capacitive pressure sensor
<ol>
	<li>Fabricate the soft electrode with an interdigitated fringe effect design using the proposed method (steps 2.1-2.7). 
	NOTE: The interelectrode gap and line width of the interdigitated fringe effect design are set to be the same, and two configurations are fabricated: 200 &#181;m and 300 &#181;m. Before the heating procedure (step 2.6), which may cure the electrode, it is recommended to clean the electrode surface with adhesive tape to avoid a potential short-circuit between the two electrode traces in the interdigitated structure, since the scotch tape can selectively stick to the excessive uncured ECPCs slurry remaining on the PDMS surface, and the ECPCs filled in the microchannel can be retained.</li>
	<li>Prepare a 3D-printed mold. 
	NOTE: The mold is designed to have a cavity (3 cm wide, 4 cm long, and with a height of 10 mm) with an opening into which the liquid silicone can be poured.</li>
	<li>Mix the two components of the platinum silicone flexible foam thoroughly with weight ratios for Part A:Part B of 1:1 and 6:1 to prepare dielectric layers of soft silicone foams with two pore sizes. Stir quickly. 
	NOTE: The porosity can be controlled by adjusting the mixing ratio of Part A and Part B.</li>
	<li>Pour the mixture from the last step into the mold made in step 3.2.</li>
	<li>Use a board with several holes to cover the mold opening.</li>
	<li>Cure the mixture at room temperature for 1 h. 
	NOTE: Since the silicone foam expands to two to three times its original volume after curing, the foam will grow out of the holes, meaning that the thickness of the foam in the cavity will be equal to the height of the mold cavity.</li>
	<li>Cut the excess silicone foam that comes through holes and remove the board.</li>
	<li>Place the prepared dielectric foam on top of the interdigitated soft electrode layer to finalize the pressure sensor fabrication. 
	&#8203;NOTE: The thickness of the cured silicone foam is 10 mm.</li>
</ol>
4. Strain characterization for the electrode
<ol>
	<li>Clamp the electrode fabricated in step 2 between the moving stages of a modified stepper motor.</li>
	<li>Apply uniaxial strain to the electrode by controlling the moving stage to stretch the electrode. 
	NOTE: The applied stretchability can be calculated from the displacement of the moving stage.</li>
	<li>Use a multimeter to record the resistance measurement.</li>
</ol>
5. Pressure characterization for the electrode
<ol>
	<li>Fabricate a zig-zag electrode with an equivalent design to the interdigitated electrode (steps 2.1-2.7). 
	NOTE: Considering that the comb electrodes of the interdigitated electrode have multiple fingers, the zig-zag electrode is designed to assemble the fingers in a single conducting pathway to evaluate the electrical properties of the interdigitated electrode. The tested electrode includes six fingers with a width of 300 &#181;m, and the spacing between the fingers is 2 mm.</li>
	<li>Assemble the pressure loading platform by connecting a 3D-printed loading rod (2.5 cm in diameter), a standard pressure sensor, and the moving stage of a stepper motor.</li>
	<li>Place the fabricated electrode beneath the 3D-printed loading rod.</li>
	<li>Apply pressure to the electrode by controlling the moving stage to drive the loading rod moving vertically toward the electrode by a programmed distance. 
	NOTE: The pressure can be controlled by setting the displacement of the moving stage, and the standard pressure is calculated by the force measurement from the standard force sensor.</li>
	<li>Use a multimeter to record the resistance measurement.</li>
</ol>
6. Pressure characterization for the capacitive pressure sensor
<ol>
	<li>Use the same platform as in step 5 to apply pressure to the capacitive pressure sensor fabricated in step 3.</li>
	<li>Use an LCR meter to record the capacitance measurement. 
	NOTE: The capacitance is measured at a testing frequency of 1 kHz.</li>
</ol>

Microfluidic Channel-Based Soft Electrodes for Capacitive Pressure Sensing

<ol>
	<li>Sun, Z. D., 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=Artificial+intelligence+of+things+(AIoT)+enabled+virtual+shop+applications+using+self-powered+sensor+enhanced+soft+robotic+manipulator.">Artificial intelligence of things (AIoT) enabled virtual shop applications using self-powered sensor enhanced soft robotic manipulator.</a> Advanced Science. 8 (14), 2100230 (2021).</li><li>Lo, L. -. 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=Inkjet-printed+soft+resistive+pressure+sensor+patch+for+wearable+electronics+applications.">Inkjet-printed soft resistive pressure sensor patch for wearable electronics applications.</a> Advanced Materials Technology. 5 (1), 1900717 (2020).</li><li>Zhu, M. 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=Haptic-feedback+smart+glove+as+a+creative+human-machine+interface+(HMI)+for+virtual/augmented+reality+applications.">Haptic-feedback smart glove as a creative human-machine interface (HMI) for virtual/augmented reality applications.</a> Science Advances. 6 (19), (2020).</li><li>Woo, S. -. J., Kong, J. -. H., Kim, D. -. G., Kim, J. -. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+thin+all-elastomeric+capacitive+pressure+sensor+array+based+on+micro+contact+printed+elastic+conductors.">A thin all-elastomeric capacitive pressure sensor array based on micro contact printed elastic conductors.</a> Journal of Materials Chemistry C. 2 (22), 4415-4422 (2012).</li><li>Tang, J., 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=Highly+stretchable+electrodes+on+wrinkled+polydimethylsiloxane+substrates.">Highly stretchable electrodes on wrinkled polydimethylsiloxane substrates.</a> Scientific Reports. 5, 16527 (2015).</li><li>Lo, L. -. 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=An+inkjet-printed+PEDOT:PSS-based+stretchable+conductor+for+wearable+health+monitoring+device+applications.">An inkjet-printed PEDOT:PSS-based stretchable conductor for wearable health monitoring device applications.</a> ACS Applied Materials &amp; Interfaces. 13 (18), 21693-21702 (2021).</li><li>Luo, R. -. B., Li, H. -. B., Du, B., Zhou, S. -. S., Zhu, Y. -. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+strategy+for+high+stretchable,+flexible+and+conductive+polymer+films+based+on+PEDOT:PSS-PDMS+blends.">A simple strategy for high stretchable, flexible and conductive polymer films based on PEDOT:PSS-PDMS blends.</a> Organic Electronics. 76, 105451 (2020).</li><li>Zhang, Y., 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=Highly+stable+flexible+pressure+sensors+with+a+quasi-homogeneous+composition+and+interlinked+interfaces.">Highly stable flexible pressure sensors with a quasi-homogeneous composition and interlinked interfaces.</a> Nature Communications. 13, 1317 (2022).</li><li>Hong, S., Lee, S., Kim, D. -. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Materials+and+design+strategies+of+stretchable+electrodes+for+electronic+skin+and+its+applications.">Materials and design strategies of stretchable electrodes for electronic skin and its applications.</a> Proceedings of the IEEE. 107 (10), 2185-2197 (2019).</li><li>Shi, H., 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=Screen-printed+soft+capacitive+sensors+for+spatial+mapping+of+both+positive+and+negative+pressures.">Screen-printed soft capacitive sensors for spatial mapping of both positive and negative pressures.</a> Advanced Functional Materials. 29 (23), 1809116 (2019).</li><li>Mahmoudinezhad, M. H., Anderson, I., Rosset, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interdigitated+sensor+based+on+a+silicone+foam+for+subtle+robotic+manipulation.">Interdigitated sensor based on a silicone foam for subtle robotic manipulation.</a> Macromolecular Rapid Communications. 42 (5), 2000560 (2019).</li></ol>

The authors have nothing to disclose.

In this protocol, we have demonstrated a novel microfluidic channel-based printing method for stretchable electrodes. The conductive material of the electrode, the ECPCs slurry, can be prepared by the solvent evaporation method, which allows the CNTs to be well-dispersed into the PDMS matrix, thus forming a conductive polymer that exhibits a stretchability as high as the PDMS substrate.
In the scraping process, the ECPCs slurry is rapidly filled into the PDMS microfluidic channel with the help...

Soft pressure sensors have been widely explored in applications such as pneumatic robotic grippers1, wearable electronics2, human-machine interface systems3, etc. In such applications, the sensory system requires flexibility and stretchability to ensure conformal contact with arbitrary curvilinear surfaces. Therefore, it requires all the essential components, including the substrate, the transducing element, and the electrode, to provide consistent functionality under extreme deformation conditions4. Moreover, to maintain high sensing performance, it is essential to keep the changes in the soft electrodes to the minimum level to avoid interference in the electrical sensing signals5.
As one of the core components in soft pressure sensors, stretchable electrodes capable of sustaining high stress and strain levels are crucial for the device to preserve stable conductive pathways and impedance characteristics6,7. Soft electrodes with excellent performance usually possess 1) high spatial resolution at the micrometer scale and 2) high stretchability with strong bonding to the substrate, and these are indispensable characteristics to enable highly integrated soft electronics in a wearable size8. Therefore, various strategies have been proposed recently to develop soft electrodes with the above properties, such as ink-jet printing, screen printing, spray printing, and transfer printing, etc.9. The ink-jet printing method6&#160;has been widely used due to its advantages of simple fabrication, no masking requirement, and a low amount of material waste, but it is hard to achieve high-resolution patterning due to limitations in terms of the ink viscosity. Screen printing10&#160;and spray printing11&#160;are simple and cost-effective patterning methods that require a shadow mask on the substrate. However, the operation of placing or removing the mask can reduce the clarity of the patterning. Although transfer printing4 has been reported to be a promising way to achieve high-resolution printing, this method suffers from a complicated procedure and a time-consuming printing process. Furthermore, most of the soft electrodes produced by these patterning methods have other disadvantages, such as delamination from the substrate.
Herein, we present a novel printing method for the rapid fabrication of cost-effective and high-resolution soft electrodes based on microfluidic channel configurations. Compared to other conventional fabrication methods, the proposed strategy utilizes elastic conductive polymer composites (ECPCs) as the conductive material and lithographically embossed microfluidic channels to pattern the electrode traces. The ECPCs slurry is prepared by the solvent evaporation method and consists of 7 wt.% carbon nanotubes (CNTs) well-dispersed in a polydimethylsiloxane (PDMS) matrix. By scraping the ECPCs slurry into the microfluidic channel, high-resolution electrodes defined by lithographic patterning can be produced. In addition, since the electrode is mainly based on PDMS, strong bonding is created at the interface between the ECPCs-based electrode and the PDMS substrate. Thus, the electrode can sustain a stretch level as high as the PDMS substrate. The experimental results confirm that the proposed stretchable electrode can respond linearly to axial strains up to 30% and exhibit excellent stability in a high-pressure range of 0-400 kPa, indicating the great potential of this method for fabricating soft electrodes in capacitive pressure sensors, which is also demonstrated in this work.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Camera</td>
			<td>OPLENIC DIGITAL CAMERA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Carbon nanotubes (CNTs)</td>
			<td>Nanjing Xianfeng Nano-technology</td>
			<td></td>
			<td>Diameter:10-20 nm,Length:10-30 &#956;m</td>
		</tr>
		<tr>
			<td>Hotplate Stirrer</td>
			<td>Thermo Scientific</td>
			<td>Super-Nuova+</td>
			<td>Stirring and Heating Equipment</td>
		</tr>
		<tr>
			<td>LCR meter</td>
			<td>Keysight</td>
			<td>E4980AL</td>
			<td>Capacitance Measurment Equipment</td>
		</tr>
		<tr>
			<td>Microscope</td>
			<td>SDPTOP</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Multimeter</td>
			<td>Fluke</td>
			<td></td>
			<td>Resistance measurment Equipment</td>
		</tr>
		<tr>
			<td>Oven</td>
			<td>Yamoto</td>
			<td>DX412C</td>
			<td>Heating equipment</td>
		</tr>
		<tr>
			<td>Photo mask</td>
			<td>Shenzhen Weina Electronic Technology</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Photoresist</td>
			<td>Microchem</td>
			<td>SU-8 3050</td>
			<td></td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (PDMS)</td>
			<td>Dow Corning</td>
			<td>Sylgard 184</td>
			<td>Silicone Elastomer</td>
		</tr>
		<tr>
			<td>Silicone Foam</td>
			<td>Smooth on</td>
			<td>Soma Foama 25</td>
			<td>Two-component Platinum Silicone Flexible Foam</td>
		</tr>
		<tr>
			<td>Silicone wafer</td>
			<td>Suzhou Crystal Silicon Electronic &#38; Technology</td>
			<td></td>
			<td>Diameter:2inch</td>
		</tr>
		<tr>
			<td>Stirrer</td>
			<td>IKA</td>
			<td>Color Squid</td>
			<td>Stirring Equipment</td>
		</tr>
		<tr>
			<td>Toluene</td>
			<td>Sinopharm Chemical Reagent</td>
			<td></td>
			<td>Solvent for the Preparation of ECPCs</td>
		</tr>
		<tr>
			<td>Triethoxysilane</td>
			<td>Macklin</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

microfluidic channel-based soft electrodes and their application in capacitive pressure sensing

Flexible and stretchable electrodes are essential components in soft artificial sensory systems. Despite recent advances in flexible electronics, most electrodes are either restricted by the patterning resolution or the capability of inkjet printing with high-viscosity super-elastic materials. In this paper, we present a simple strategy to fabricate microchannel-based stretchable composite electrodes, which can be achieved by scraping elastic conductive polymer composites (ECPCs) into lithographically embossed microfluidic channels. The ECPCs were prepared by a volatile solvent evaporation method, which achieves a uniform dispersion of carbon nanotubes (CNTs) in a polydimethylsiloxane (PDMS) matrix. Compared to conventional fabrication methods, the proposed technique can facilitate the rapid fabrication of well-defined stretchable electrodes with high-viscosity slurry. Since the electrodes in this work were made up of all-elastomeric materials, strong interlinks can be formed between the ECPCs-based electrodes and the PDMS-based substrate at the interfaces of the microchannel walls, which allows the electrodes to exhibit mechanical robustness under high tensile strains. In addition, the mechanical-electric response of the electrodes was also systematically studied. Finally, a soft pressure sensor was developed by combining a dielectric silicone foam and an interdigitated electrodes (IDE) layer, and this demonstrated great potential for pressure sensors in soft robotic tactile sensing applications.

Following the protocol, ECPCs can be patterned via&#160;the&#160;microfluidic channel, which leads to the formation of stretchable electrodes with a high resolution. Figures 3A, B shows photographs of soft electrodes with different trace designs and printing resolutions. Figure 3C shows the different line widths of the fabricated electrodes, including 50 &#181;m, 100 &#181;m, and 200 &#181;m. The resistance of each electrode is presented in <str...

Watch this Scientific Journal Video about Microfluidic Channel-Based Soft Electrodes and Their Application in Capacitive Pressure Sensing at JoVE.com

Author Spotlight: Microfluidic Channel-Based Soft Electrodes and Their Application in Capacitive Pressure Sensing  

Flexible electrodes have a wide range of applications in soft robotics and wearable electronics. The present protocol demonstrates a new strategy to fabricate highly stretchable electrodes with high resolution via lithographically defined microfluidic channels, which paves the way for future high-performance soft pressure sensors.

Microfluidic Channel-Based Soft Electrodes and Their Application in Capacitive Pressure Sensing

Flexible and stretchable electrodes are essential components in soft artificial sensory systems. Despite recent advances in flexible electronics, most ...

microfluidic-channel-based-soft-electrodes-their-application

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1.9K Views.  Zhejiang University. Flexible electrodes have a wide range of applications in soft robotics and wearable electronics. The present protocol demonstrates a new strategy to fabricate highly stretchable electrodes with high resolution via lithographically defined microfluidic channels, which paves the way for future high-performance soft pressure sensors.

Video: Author Spotlight: Microfluidic Channel-Based Soft Electrodes and Their Application in Capacitive Pressure Sensing  

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators 1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.
Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides 3, asymmetric split-ring resonators 4, and photonic band gap structures integrated into parallel-plate waveguides 5. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.
The sensor design we use here is based on a simple parallel-plate waveguide 6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove 6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index 9.
Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves 10. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic ...

Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new attention as a unique mechanism for pattern transformation. Nature is full of such examples where a wealth of exotic patterns are formed through mechanical instability2-5. Inspired by this elegant mechanism, many studies have demonstrated creation and transformation of patterns using soft materials such as elastomers and hydrogels6-11. Swelling gels are of particular interest because they can spontaneously trigger mechanical instability to create various patterns without the need of external force6-10. Recently, we have reported demonstration of full control over buckling pattern of micro-scaled tubular gels using projection micro-stereolithography (P&mu;SL), a three-dimensional (3D) manufacturing technology capable of rapidly converting computer generated 3D models into physical objects at high resolution12,13. Here we present a simple method to build up a simplified P&mu;SL system using a commercially available digital data projector to study swelling-induced buckling instability for controlled pattern transformation.
A simple desktop 3D printer is built using an off-the-shelf digital data projector and simple optical components such as a convex lens and a mirror14. Cross-sectional images extracted from a 3D solid model is projected on the photosensitive resin surface in sequence, polymerizing liquid resin into a desired 3D solid structure in a layer-by-layer fashion. Even with this simple configuration and easy process, arbitrary 3D objects can be readily fabricated with sub-100 &mu;m resolution.
This desktop 3D printer holds potential in the study of soft material mechanics by offering a great opportunity to explore various 3D geometries. We use this system to fabricate tubular shaped hydrogel structure with different dimensions. Fixed on the bottom to the substrate, the tubular gel develops inhomogeneous stress during swelling, which gives rise to buckling instability. Various wavy patterns appear along the circumference of the tube when the gel structures undergo buckling. Experiment shows that circumferential buckling of desired mode can be created in a controlled manner. Pattern transformation of three-dimensionally structured tubular gels has significant implication not only in mechanics and material science, but also in many other emerging fields such as tunable matamaterials.

We demonstrate controlled pattern transformation of swelling gel tubes by elastic instability. A simple projection micro stereo-lithography setup is built using an off-the-shelf digital data projector to fabricate three-dimensional polymeric structures in a layer-by-layer fashion. Swelling hydrogel tubes under mechanical constraint display various circumferential buckling modes depending on dimension.

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new ...

Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

This paper discusses fluorescent core microcavity-based sensors that can operate in a microfluidic analysis setup. These structures are based on the formation of a fluorescent quantum-dot (QD) coating on the channel surface of a conventional microcapillary. Silicon QDs are especially attractive for this application, owing in part to their negligible toxicity compared to the II-VI and II-VI compound QDs, which are legislatively controlled substances in many countries. While the ensemble emission spectrum is broad and featureless, an Si-QD film on the channel wall of a capillary features a set of sharp, narrow peaks in the fluorescence spectrum, corresponding to the electromagnetic resonances for light trapped within the film. The peak wavelength of these resonances is sensitive to the external medium, thus permitting the device to function as a refractometric sensor in which the QDs never come into physical contact with the analyte. The experimental methods associated with the fabrication of the fluorescent-core microcapillaries are discussed in detail, as well as the analysis methods. Finally, a comparison is made between these structures and the more widely investigated liquid-core optical ring resonators, in terms of microfluidic sensing capabilities.

Fluorescent-core microcavity sensors employ a high-index quantum-dot coating in the channel of silica microcapillaries. Changes in the refractive index of fluids pumped into the capillary channel cause shifts in the microcavity fluorescence spectrum that can be used to analyze the channel medium.

This paper discusses  fluorescent core microcavity-based sensors that can operate in a microfluidic  analysis setup. These structures are based on the ...

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 1 will be shown.

The laser heated diamond anvil cell combined with synchrotron micro-diffraction techniques allows researchers to explore the nature and properties of new phases of matter at extreme pressure and temperature (PT) conditions. Heterogeneous samples can be characterized in situ under high pressure by 2D mapping and combined powder, single-crystal and multigrain diffraction approaches.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary ...

Picoinjection of Microfluidic Drops Without Metal Electrodes

Existing methods for picoinjecting reagents into microfluidic drops require metal electrodes integrated into the microfluidic chip. The integration of these electrodes adds cumbersome and error-prone steps to the device fabrication process. We have developed a technique that obviates the needs for metal electrodes during picoinjection. Instead, it uses the injection fluid itself as an electrode, since most biological reagents contain dissolved electrolytes and are conductive. By eliminating the electrodes, we reduce device fabrication time and complexity, and make the devices more robust. In addition, with our approach, the injection volume depends on the voltage applied to the picoinjection solution; this allows us to rapidly adjust the volume injected by modulating the applied voltage. We demonstrate that our technique is compatible with reagents incorporating common biological compounds, including buffers, enzymes, and nucleic acids.

We have developed a technique for picoinjecting microfluidic drops that does not require metal electrodes. As such, devices incorporating our technique are simpler to fabricate and to use.

Existing methods for picoinjecting reagents into microfluidic drops require metal electrodes integrated into the microfluidic chip. The integration of ...

Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensor for resource-limited settings. Taking advantages from both pH meters and strips, the pH sensing platform is composed of hydrophobic barrier-patterned paper micropad (&#181;PAD) using polydimethylsiloxane (PDMS), screen-printed electrode (SPE) modified with IrO2-graphene films and molded acrylonitrile butadiene styrene (ABS) plastic holder. Repetitive cathodic potential cycling was employed for graphene oxide (GO) reduction which can completely remove electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene thin film with excellent stability and electronic properties. A uniform and smooth IrO2 film in nanoscale grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO2-RGO electrode showed slightly super-Nernstian responses from pH 2-12 in Britton-Robinson (B-R) buffers with good linearity, small hysteresis, low response time and reproducibility in different buffers, as well as low sensitivities to different interfering ionic species and dissolved oxygen. A simple portable digital pH meter is fabricated, whose signal is measured with a multimeter, using high input-impedance operational amplifier and consumer batteries. The pH values measured with the portable electrochemical paper-microfluidic pH sensors were consistent with those measured using a commercial laboratory pH meter with a glass electrode.

The study demonstrates the growth of iridium oxide-reduced graphene oxide (IrO2-RGO) nanohybrid thin films on irregular and rough screen-printed carbon substrate through a green electrochemical synthesis, and their implementation as a pH sensor with a patterned paper-fluidic platform.

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Robotic Sensing and Stimuli Provision for Guided Plant Growth

Robot systems are actively researched for manipulation of natural plants, typically restricted to agricultural automation activities such as harvest, irrigation, and mechanical weed control. Extending this research, we introduce here a novel methodology to manipulate the directional growth of plants via their natural mechanisms for signaling and hormone distribution. An effective methodology of robotic stimuli provision can open up possibilities for new experimentation with later developmental phases in plants, or for new biotechnology applications such as shaping plants for green walls. Interaction with plants presents several robotic challenges, including short-range sensing of small and variable plant organs, and the controlled actuation of plant responses that are impacted by the environment in addition to the provided stimuli. In order to steer plant growth, we develop a group of immobile robots with sensors to detect the proximity of growing tips, and with diodes to provide light stimuli that actuate phototropism. The robots are tested with the climbing common bean, Phaseolus vulgaris, in experiments having durations up to five weeks in a controlled environment. With robots sequentially emitting blue light-peak emission at wavelength 465 nm-plant growth is successfully steered through successive binary decisions along mechanical supports to reach target positions. Growth patterns are tested in a setup up to 180 cm in height, with plant stems grown up to roughly 250 cm in cumulative length over a period of approximately seven weeks. The robots coordinate themselves and operate fully autonomously. They detect approaching plant tips by infrared proximity sensors and communicate via radio to switch between blue light stimuli and dormant status, as required. Overall, the obtained results support the effectiveness of combining robot and plant experiment methodologies, for the study of potentially complex interactions between natural and engineered autonomous systems.

Distributed robot nodes provide sequences of blue light stimuli to steer the growth trajectories of climbing plants. By activating natural phototropism, the robots guide the plants through binary left-right decisions, growing them into predefined patterns that by contrast are not possible when the robots are dormant.

Robot systems are actively researched for manipulation of natural plants, typically restricted to agricultural automation activities such as harvest, ...

Rapid Manufacturing of Thin Soft Pneumatic Actuators and Robots

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with an ultrathin form factor using a heat press and a laser cutter machine. The method starts with the lamination of thermoplastic polyurethane (TPU) sheets using a heat press for 10 min at the temperature of ~93 &#176;C. Next, the parameters of the laser cutter machine are optimized to produce a rectangular balloon with maximum burst pressure. Using the optimized parameters, the soft actuators are laser cut/welded three times sequentially. Next, a dispensing needle is attached to the actuator, allowing it to be inflated. The effect of geometrical parameters on the deflection of the actuator are studied systematically by varying the channel width and length. Finally, the performance of the actuator is characterized using an optical camera and a fluid dispenser. Conventional fabrication methods of soft pneumatic actuators based on silicone molding are time consuming (several hours). They also result in strong but bulky actuators, which limits the actuator&#8217;s applications. Moreover, microfabrication of thin pneumatic actuators is both time-consuming and expensive. The proposed manufacturing method in the current work resolves these issues by introducing a fast, simple, and cost-effective fabrication method of ultrathin pneumatic actuators.

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with a thin form factor. The fabrication method starts with lamination of thermoplastic polyurethane (TPU) sheets followed by laser cutting/welding of a two-dimensional pattern to form actuators and robots.

This protocol describes a method for rapid manufacturing of soft pneumatic actuators and robots with an ultrathin form factor using a heat press and a ...

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a result, these platforms have not been fully exploited for direct observation of high-pressure transport in fractures. This work introduces microfluidic platforms that enable direct observation of multiphase flow in devices featuring surrogate permeable media and fractured systems. Such platforms provide a pathway to address important and timely questions such as those related to CO2 capture, utilization and storage. This work provides a detailed description of the fabrication techniques and an experimental setup that may serve to analyze the behavior of supercritical CO2 (scCO2) foam, its structure and stability. Such studies provide important insights regarding enhanced oil recovery processes and the role of hydraulic fractures in resource recovery from unconventional reservoirs. This work presents a comparative study of microfluidic devices developed using two different techniques: photolithography/wet-etching/thermal-bonding versus Selective Laser-induced Etching. Both techniques result in devices that are chemically and physically resistant and tolerant of high pressure and temperature conditions that correspond to subsurface systems of interest. Both techniques provide pathways to high-precision etched microchannels and capable lab-on-chip devices. Photolithography/wet-etching, however, enables fabrication of complex channel networks with complex geometries, which would be a challenging task for laser etching techniques. This work summarizes a step-by-step photolithography, wet-etching and glass thermal-bonding protocol and, presents representative observations of foam transport with relevance to oil recovery from unconventional tight and shale formations. Finally, this work describes the use of a high-resolution monochromatic sensor to observe scCO2 foam behavior where the entirety of the permeable medium is observed simultaneously while preserving the resolution needed to resolve features as small as 10 &#181;m.

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

This paper describes a protocol along with a comparative study of two microfluidic fabrication techniques, namely photolithography/wet-etching/thermal-bonding and Selective Laser-induced Etching (SLE), that are suitable for high-pressure conditions. These techniques constitute enabling platforms for direct observation of fluid flow in surrogate permeable media and fractured systems under reservoir conditions.

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a ...