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.

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

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

Our research really focuses on flexible and stretchable electronics for sensing applications, while trying to develop innovative materials and manufacturing precisely for flexible electronic systems. 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, high-viscosity super-elastic materials.In this protocol, we have demonstrated a novel microfluidic channel-based printing method for stretchable electrodes. The conducting material of the electrode, the ECPC flooring, can be scraped into the microchannel, thus forming a conductive polymer that exhibits a stretchability as high as the PDMS substrate. Compared with the actions produced by existing fabrication methods, such as inkjet printing, scrape printing, spray printing, and transfer printing, the proposed microfluidic channel-based soft electrons have the advantages of a high printing resolution and high stretchability, with the strong binding to the substrate.The protocol presented in this research combines marriage of stretchable materials and microfluidic channels, enabling a low-cost and rapid application method for producing high-resolution, stretchable electrons for 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

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.

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

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Research

JoVE Journal

Engineering

Elastic Conductive Polymer Composite (ECPCs) Slurry Synthesis and Fabrication of Stretchable Electrodes

To begin, disperse the carbon nanotubes, or CNTs, into a toluene solvent at a one-to-30 weight ratio, and dilute the PDMS base with toluene at an equal weight ratio. Magnetically stir the CNT toluene suspension and PDMS toluene solution at room temperature for one hour. Mix the prepared suspension and solution to form a liquid CNT PDMS toluene mixture, and magnetically stir on a hot plate at 80 degrees Celsius to evaporate the toluene.Then add the PDMS curing agent into the mixture at a 10-to-one weight ratio to complete the synthesis of the ECPCs slurry. To fabricate the microfluidic channel-based stretchable electrodes, mix the PDMS-based solution and the curing agent at a 10-to-one weight ratio, and place the uncured PDMS mixture in a vacuum desiccater until all air bubbles disappear. Prepare the SU8-based mold with different microfluidic channel patterns using the conventional lithography technique on a silicone wafer.Then pour the degassed mixture onto the fabricated mold, and place the mold on a hot plate at 85 degrees Celsius for one hour to cure the PDMS and transfer the SU8 mold pattern on the cured PDMS film. Then peel off the PDMS layer. Next, cast a small amount of the synthesized ECPCs slurry onto the PDMS surface, and using a razor blade, carefully scrape the slurry along the embossed microfluidic channel.Then heat the slurry at 70 degrees Celsius for two hours. Finally, connect a copper wire to each end of the prepared electrodes, using conductive silver paste, and seal the connection with an adhesive rubber sealant. Soft electrodes with different trace designs and printing resolutions can be fabricated through this protocol.The resistance of the electrodes increased with decreasing line width, and the serpentine electrodes exhibited higher resistance than the line structure electrodes, due to their longer effective length. The resistance of the electrodes increased linearly, with the tensile strain caused by the geometric effect. The sensitivity of the serpentine electrodes was lower than the line structure electrode, due to the strained releasing effect.

Fabrication of the Capacitive Pressure Sensor

1.3K 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.

Microfluidic Channel-Based Soft Electrodes for Capacitive Pressure Sensing

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