Name: sensitivity enhancement of soft capacitive pressure sensors using a solvent evaporation-based porosity control technique
Uploaded: 2023-03-24T14:40:00.000+00:00
Duration: 10 min 28 s
Description: Watch this Scientific Journal Video about Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique at JoVE.com

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

1. Fabrication of the soft capacitive pressure sensor with a porous PDMS dielectric layer
<ol>
	<li>Manufacturing of the porous PDMS dielectric layer
	<ol>
		<li>Prepare the sugar/erythritol porous template following the steps below.
		<ol>
			<li>Filter the sugar with sample sieves with apertures of 270 &#181;m and 500 &#181;m. Choose sugar with a particle diameter in the range of 270-500 &#181;m. 
			NOTE: A larger or smaller sugar particle size is also acceptable as long as the uniformity is within the tolerance limits. The diameter of the sugar particle will affect the pore size of the porous PDMS layer manufactured in a later step but will not determine the pore size completely.</li>
			<li>Grind erythritol (see Table of Materials) into powder to ensure a more uniform mixing with the sugar.</li>
			<li>Weigh a certain amount of filtered sugar and erythritol powder with a mass ratio of 20:1. Shake to mix them evenly.</li>
			<li>Fill the sugar/erythritol mixture into a commercially obtained sugar/erythritol metal mold (see Table of Materials). Press the surface to make the filler compact. 
			NOTE: To ensure easy demolding in the next step, a layer of Al foil can be placed in the mold before the sugar/erythritol.</li>
			<li>Heat the mold with the sugar/erythritol mixture in a convection oven at 135 &#176;C for 2 h, as shown in Figure 1A. After cooling at room temperature, take out the lump plate sugar (i.e., the porous template).</li>
		</ol>
		</li>
		<li>Fabricate the porosity-controllable PDMS dielectric layer.
		<ol>
			<li>Weigh 5 g of toluene, 5 g of PDMS base, and 0.5 g of PDMS curing agent (see Table of Materials) in a centrifuge tube (i.e., the mass ratio of PDMS base/toluene/curing agent is 10:10:1). Stir the solution evenly. 
			NOTE: The mass ratio of PDMS base solution to curing agent is fixed at 10:1, while the mass ratio of PDMS to toluene is used to control the porosity of the PDMS dielectric layer. The porosity decreases when increasing the PDMS fraction. The minimum porosity is obtained when no toluene is added.</li>
			<li>Centrifuge the solution at 875 x g for 30 s at room temperature to remove air bubbles. 
			NOTE: If the solution volume is large, the solution can be prepared in a beaker. The centrifugal treatment is replaced by vacuum degassing for 15 min.</li>
			<li>Place the square sugar/erythritol porous template obtained in step 1.1.1 in a Petri dish. Insert double-sided tape as spacers beneath the four corners to lift the template from the Petri dish surface. 
			NOTE: The template can also be placed on a Si wafer, but this method will lead to a thicker layer of PDMS on the interface between the template and the Si wafer, which may affect the sensor performance.</li>
			<li>Pour the PDMS/toluene solution onto the template, and slightly incline the Petri dish so that the solution can completely fill all the gaps among the sugar particles, as shown in Figure 1B.</li>
			<li>Place the Petri dish with the PDMS/toluene solution-filled porous template in a vacuum desiccator, and degas for 20 min.</li>
			<li>Transfer the Petri dish from the vacuum desiccator into the oven at 90 &#176;C for 45 min to evaporate the toluene and cure the liquid PDMS.</li>
			<li>Immerse the cured PDMS embedded in the porous template in deionized water (DI water), as shown in Figure 1C. Heat on a hot plate at 140 &#176;C until the sugar template completely dissolves. Clean the porous PDMS with DI water.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Fabrication of the flexible electrode layers based on ECPCs
	<ol>
		<li>Synthesize the ECPCs ink.
		<ol>
			<li>Weigh 0.16 g of CNTs (diameter: 10-20 nm, length: 10-30 &#181;m, see Table of Materials) and 4 g of toluene in a beaker, and magnetically stir at 250 rpm for 1.5 h. Meanwhile, weigh 2 g of PDMS base and 2 g of toluene into a beaker, and magnetically stir at 200 rpm for 1 h. Cover the beaker with sealing film while stirring to prevent solvent evaporation.</li>
			<li>Mix the CNTs/toluene suspension with the PDMS base/toluene solution, and cover the beaker with a sealing film. Magnetically stir at 250 rpm for 2 h.</li>
			<li>Add 0.2 g of PDMS curing agent into the mixed solution. Magnetically stir at 75 &#176;C and 250 rpm for 1 h. Uncover the beaker for solvent evaporation and suspension concentration when stirring, as shown in Figure 1D, E. 
			NOTE: The duration of stirring and heating is adjustable. The viscosity of the mixture increases with stirring time, which facilitates the following scrape coating operation. However, the duration should not be too long to prevent the PDMS solution from curing. When the mixture is concentrated to a viscosity convenient for scrape coating, the ECPCs ink synthesis process is finished.</li>
		</ol>
		</li>
		<li>Scrape-coat the electrodes following the steps below.
		<ol>
			<li>Weigh toluene, PDMS base, and PDMS curing agent in a centrifuge tube with a mass ratio of 2:10:1. Stir the solution evenly.</li>
			<li>Centrifuge the solution at 875 x g for 30 s at room temperature to remove air bubbles.</li>
			<li>Pour 1.3 g of PDMS/toluene solution into a commercially obtained electrode metal mold (see Table of Materials) with an embossed electrode pattern, as shown in Figure 1F. 
			NOTE: The embossed pattern at the bottom of the mold is 0.2 mm thick.</li>
			<li>Place the mold in a vacuum desiccator, and degas for 10 min.</li>
			<li>Cure the PDMS in the mold on a hot plate at 90 &#176;C for 15 min. Peel off the patterned PDMS film after cooling at room temperature.</li>
			<li>Attach the flat side of the PDMS film onto a Si wafer (i.e., expose the side with the electrode pattern). Ensure no air bubbles exist between the PDMS film and the Si wafer.</li>
			<li>Scrape-coat the ECPCs ink prepared in step 1.2.1 into the electrode pattern, as Figure 1G shows. Clean the excessive ink with an isopropyl alcohol (IPA)-dipped dust-free wipe.</li>
			<li>Cure the ECPCs ink on a hot plate at 90 &#176;C for 15 min.</li>
			<li>Repeat steps 1.2.2.3-1.2.2.8 to manufacture both the upper and lower electrode layers.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Bonding and packaging of the soft capacitive sensors
	<ol>
		<li>Attach the metal wire (see Table of Materials) to the electrode. Drop silver conductive paint (see Table of Materials) at the connection location to ensure good conductivity, as shown in Figure 1H. Wait until the silver conductive paint dries at room temperature.</li>
		<li>Drop the liquid PDMS solution prepared in step 1.2.2.1 onto the connection to seal the dried silver conductive paint completely. Cure the PDMS on a hot plate at 90 &#176;C for 15 min.</li>
		<li>Repeat steps 1.3.1-1.3.2 to connect the wire for both the upper and lower electrode layers.</li>
		<li>Apply a thin layer of liquid PDMS prepared in step 1.2.2.1 evenly onto the electrode film as an adhesion layer for bonding between the electrode layer and the dielectric layer.</li>
		<li>Place the porous PDMS dielectric layer fabricated in step 1.1.2 onto the electrode layer.</li>
		<li>Cure the PDMS glue on a hot plate at 95 &#176;C for 10 min. Place a glass Petri dish onto the porous PDMS to ensure good contact between the two layers during heating.</li>
		<li>Repeat step 1.3.4 for the other electrode layer. Reverse the bonded electrode-dielectric layer obtained in step 1.3.6, and place it on the other single electrode layer (i.e., to have the porous PDMS layer in direct contact with the electrode layer). Ensure that the two electrodes are strictly aligned opposite to each other.</li>
		<li>Repeat step 1.3.6 to finish the bonding between the porous PDMS layer and the other electrode layer. 
		&#8203;NOTE: An illustration of the final sensor is shown in Figure 1I. Illustrations of the structure and materials of the sensor are shown in Figure 1J.</li>
	</ol>
	</li>
</ol>
2. Experimental process of sensor performance characterization
<ol>
	<li>Stepping pressure loading setup and data acquisition system
	<ol>
		<li>Use a 3D-printed indenter with a loading area that is a circle of 2.5 cm in diameter for the pressure loading (see Table of Materials) of the sensor under test.</li>
		<li>Fix the indenter on a vertical linear moving stage controlled by a stepping motor (see Table of Materials) through a standard pull-pressure sensor.</li>
		<li>Measure the capacitance of the soft capacitive pressure sensor with an LCR meter while recording the standard pressure data using a data acquisition (DAQ) device. Connect both the LCR meter and the DAQ to a computer running the LabVIEW data-logging program (see Table of Materials). 
		NOTE: Illustrations of the experimental setup are shown in Figure 2. A spring is applied between the indenter and the standard pull-pressure sensor, which converts the vertical displacement of the linear moving stage to loading pressure.</li>
	</ol>
	</li>
	<li>Testing the sensing performance
	<ol>
		<li>Control the stepping motor to drive the indenter to move down vertically by a programmed distance. Record the capacitance and the standard pressure data by increasing the loading force with the same interval in each consecutive loading cycle until the loading pressure reaches 40 N (~80 kPa).</li>
		<li>Control the stepping motor to drive the indenter to move up vertically by the same distance as in the last step. Record the capacitance and the standard pressure data after the indenter has stabilized. Repeat the operation by decreasing the loading force with the same interval; in each consecutive loading cycle, the loading pressure drops to 0 N.</li>
		<li>Control the stepping motor to drive the indenter to move down vertically by a programmed distance. Record the capacitance and the standard pressure data. Repeat the loading and unloading tests for 2,500 cycles while recording the capacitance of the device under test (DUT) as a function of the standard pressure reading.</li>
		<li>Control the indenter to press down rapidly and remain steady for a few seconds before returning to 0 N loading. Repeat this five times, and record the capacitance as a function of time.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Ozioko, O., 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=SensAct:+The+soft+and+squishy+tactile+Sensor+with+integrated+flexible+actuator.">SensAct: The soft and squishy tactile Sensor with integrated flexible actuator.</a> Advanced Intelligent Systems. 3 (3), 1900145 (2021).</li><li>Qiu, 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=A+biomimetic+drosera+capensis+with+adaptive+decision-predation+behavior+based+on+multifunctional+sensing+and+fast+actuating+capability.">A biomimetic drosera capensis with adaptive decision-predation behavior based on multifunctional sensing and fast actuating capability.</a> Advanced Functional Materials. 32 (13), 2110296 (2021).</li><li>Ntagios, M., Nassar, H., Pullanchiyodan, A., Navaraj, W. T., Dahiya, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robotic+hands+with+intrinsic+tactile+sensing+via+3D+printed+soft+pressure+sensors.">Robotic hands with intrinsic tactile sensing via 3D printed soft pressure sensors.</a> Advanced Intelligent Systems. 2 (6), 1900080 (2019).</li><li>Tang, Z., Jia, S., Zhou, C., Li, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+Printing+of+highly+sensitive+and+large-measurement-range+flexible+pressure+sensors+with+a+positive+piezoresistive+effect.">3D Printing of highly sensitive and large-measurement-range flexible pressure sensors with a positive piezoresistive effect.</a> ACS Applied Materials &amp; Interfaces. 12 (25), 28669-28680 (2020).</li><li>Dai, Y., Chen, J., Tian, W., Xu, L., Gao, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+PVDF/Au/PEN+multifunctional+flexible+human-machine+interface+for+multidimensional+sensing+and+energy+harvesting+for+the+internet+of+things.">A PVDF/Au/PEN multifunctional flexible human-machine interface for multidimensional sensing and energy harvesting for the internet of things.</a> IEEE Sensors Journal. 20 (14), 7556-7568 (2020).</li><li>Yang, 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=Flexible+piezoelectric+pressure+sensor+based+on+polydopamine-modified+BaTiO3/PVDF+composite+film+for+human+motion+monitoring.">Flexible piezoelectric pressure sensor based on polydopamine-modified BaTiO3/PVDF composite film for human motion monitoring.</a> Sensors and Actuators A: Physical. 301, 111789 (2020).</li><li>Gao, Y. 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=Wearable+microfluidic+diaphragm+pressure+sensor+for+health+and+tactile+touch+monitoring.">Wearable microfluidic diaphragm pressure sensor for health and tactile touch monitoring.</a> Advanced Materials. 29 (39), 1701985 (2017).</li><li>Meng, K., 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=Flexible+weaving+constructed+self-powered+pressure+sensor+enabling+continuous+diagnosis+of+cardiovascular+disease+and+measurement+of+cuffless+blood+pressure.">Flexible weaving constructed self-powered pressure sensor enabling continuous diagnosis of cardiovascular disease and measurement of cuffless blood pressure.</a> Advanced Functional Materials. 29 (5), 180688 (2019).</li><li>Yang, J. 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=Microstructured+porous+pyramid-based+ultrahigh+sensitive+pressure+sensor+insensitive+to+strain+and+temperature.">Microstructured porous pyramid-based ultrahigh sensitive pressure sensor insensitive to strain and temperature.</a> ACS Applied Materials &amp; Interfaces. 11 (21), 19472-19480 (2019).</li><li>Chen, S., Zhuo, B., Guo, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large+area+one-step+facile+processing+of+microstructured+elastomeric+dielectric+film+for+high+sensitivity+and+durable+sensing+over+wide+pressure+range.">Large area one-step facile processing of microstructured elastomeric dielectric film for high sensitivity and durable sensing over wide pressure range.</a> ACS Applied Materials &amp; Interfaces. 8 (31), 20364-20370 (2016).</li><li>Ding, 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=Influence+of+the+pore+size+on+the+sensitivity+of+flexible+and+wearable+pressure+sensors+based+on+porous+Ecoflex+dielectric+layers.">Influence of the pore size on the sensitivity of flexible and wearable pressure sensors based on porous Ecoflex dielectric layers.</a> Materials Research Express. 6 (6), 066304 (2019).</li><li>Yoon, J. I., Choi, K. S., Chang, S. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+means+of+fabricating+microporous+structures+for+the+dielectric+layers+of+capacitive+pressure+sensor.">A novel means of fabricating microporous structures for the dielectric layers of capacitive pressure sensor.</a> Microelectronic Engineering. 179, 60-66 (2017).</li><li>Wang, J., Li, L., Zhang, L., Zhang, P., Pu, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexible+capacitive+pressure+sensors+with+micro-patterned+porous+dielectric+layer+for+wearable+electronics.">Flexible capacitive pressure sensors with micro-patterned porous dielectric layer for wearable electronics.</a> Journal of Micromechanics and Microengineering. 32 (3), 034003 (2022).</li><li>Mannsfeld, S. C. B., 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+sensitive+flexible+pressure+sensors+with+microstructured+rubber+dielectric+layers.">Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers.</a> Nature Materials. 9 (10), 859-864 (2010).</li><li>Wan, 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=A+highly+sensitive+flexible+capacitive+tactile+sensor+with+sparse+and+high-aspect-ratio+microstructures.">A highly sensitive flexible capacitive tactile sensor with sparse and high-aspect-ratio microstructures.</a> Advanced Electronic Materials. 4 (4), 1700586 (2018).</li><li>Kwon, 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=Highly+sensitive,+flexible,+and+wearable+pressure+sensor+based+on+a+giant+piezocapacitive+effect+of+three-dimensional+microporous+elastomeric+dielectric+layer.">Highly sensitive, flexible, and wearable pressure sensor based on a giant piezocapacitive effect of three-dimensional microporous elastomeric dielectric layer.</a> ACS Applied Materials &amp; Interfaces. 8 (26), 16922-16931 (2016).</li><li>Li, 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=A+porous+and+air+gap+elastomeric+dielectric+layer+for+wearable+capacitive+pressure+sensor+with+high+sensitivity+and+a+wide+detection+range.">A porous and air gap elastomeric dielectric layer for wearable capacitive pressure sensor with high sensitivity and a wide detection range.</a> Journal of Materials Chemistry C. 8 (33), 11468-11476 (2020).</li><li>Kim, J. O., 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+ordered+3D+microstructure-based+electronic+skin+capable+of+differentiating+pressure,+temperature,+and+proximity.">Highly ordered 3D microstructure-based electronic skin capable of differentiating pressure, temperature, and proximity.</a> ACS Applied Materials &amp; Interfaces. 11 (1), 1503-1511 (2019).</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=A+soft+sponge+sensor+for+multimodal+sensing+and+distinguishing+of+pressure,+strain,+and+temperature.">A soft sponge sensor for multimodal sensing and distinguishing of pressure, strain, and temperature.</a> ACS Applied Materials &amp; Interfaces. 14 (7), 9570-9578 (2022).</li><li>Hwang, J., Kim, Y., Yang, H., Oh, J. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+hierarchically+porous+structured+PDMS+composites+and+their+application+as+a+flexible+capacitive+pressure+sensor.">Fabrication of hierarchically porous structured PDMS composites and their application as a flexible capacitive pressure sensor.</a> Composites Part B: Engineering. 211, 108607 (2021).</li><li>Jung, 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=Linearly+sensitive+pressure+sensor+based+on+a+porous+multistacked+composite+structure+with+controlled+mechanical+and+electrical+properties.">Linearly sensitive pressure sensor based on a porous multistacked composite structure with controlled mechanical and electrical properties.</a> ACS Applied Materials &amp; Interfaces. 13 (24), 28975-28984 (2021).</li><li>Choi, 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=Synergetic+effect+of+porous+elastomer+and+percolation+of+carbon+nanotube+filler+toward+high+performance+capacitive+pressure+sensors.">Synergetic effect of porous elastomer and percolation of carbon nanotube filler toward high performance capacitive pressure sensors.</a> ACS Applied Materials &amp; Interfaces. 12 (1), 1698-1706 (2020).</li><li>Choi, S. 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=A+polydimethylsiloxane+(PDMS)+sponge+for+the+selective+absorption+of+oil+from+water.">A polydimethylsiloxane (PDMS) sponge for the selective absorption of oil from water.</a> ACS Applied Materials &amp; Interfaces. 3 (12), 4552-4556 (2011).</li><li>Rinaldi, A., Tamburrano, A., Fortunato, M., Sarto, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+flexible+and+highly+sensitive+pressure+sensor+based+on+a+PDMS+foam+coated+with+graphene+nanoplatelets.">A flexible and highly sensitive pressure sensor based on a PDMS foam coated with graphene nanoplatelets.</a> Sensors. 16 (12), 2148 (2016).</li></ol>

The authors have nothing to disclose.

This work proposes a simple method based on solvent evaporation to control the porosity, and a series of experimental results have proved its feasibility. Although the porous structure has been widely used in the flexible capacitive pressure sensor, the porosity control still needs further optimization. Unlike existing methods for changing the particle size of the PFA11,12,13,18,<sup...

In recent years, flexible pressure sensors have been drawing attention due to their indispensable application in soft robotics1,2,3, &#34;man-machine&#34; haptic interfaces4,5, and health monitoring6,7,8. Generally, the mechanisms for pressure sensing include piezoresistive1,4,7, piezoelectric2,6, capacitive2,3,9,10,11,12,13, and triboelectric8 sensors. Among them, capacitive pressure sensors stand out as one of the most promising methods in tactile sensing due to their high sensitivity, low limit of detection (LOD), etc.
For better sensing performance, various microstructures such as micro-pyramids2,9,14, micro-pillars15, and micro-pores9,10,11,12,13,16,17 have been introduced to flexible capacitive pressure sensors, and the fabrication methods have also been optimized to further improve the sensing performances of such structures. However, most of these structures require sophisticated microfabrication facilities, which significantly increases the manufacturing costs and operational difficulties. For example, as the most commonly used microstructure in soft pressure sensors, micro-pyramids rely on lithographically defined and wet-etched Si wafers as the molding template, which requires precision equipment and a strict cleanroom environment9,14. Therefore, micropore structures (porous structures) that can be made by simple fabrication processes and with low-cost raw materials while maintaining high sensing performances have drawn increasing attention recently9,10,11,12,13,16,17. This will be discussed, alongside the disadvantages of changing the PFA and its amount, as the motivation for using our fraction control method.
Herein, this work proposes a simple and low-cost method based on the solvent-evaporation technique to fabricate a porous flexible capacitive pressure sensor with controllable porosity. The complete manufacturing process includes the fabrication of the porous PDMS dielectric layer, the scrape coating of the electrodes, and the bonding of three functional layers. Specifically, this work innovatively uses a PDMS/toluene mixed solution with a certain mass ratio to fabricate the porous PDMS dielectric layer based on the sugar/erythritol mixture template. Meanwhile, a uniform PFA particle size ensures uniform pore morphology and distribution; thus, the porosity can be controlled by changing the PDMS/toluene mass ratio. The experimental results show that the sensitivity of the proposed pressure sensor can be enhanced more than two-fold by increasing the PDMS/toluene mass ratio from 1:8 to 1:1. The variation in the micropore wall thickness due to different PDMS/toluene mass ratios is also confirmed by optical microscope images. The optimized soft capacitive pressure sensor shows a high sensing performance with a sensitivity and response time of 3.47% kPa&#8722;1 and 0.2 s, respectively. This method achieves the fast, low-cost, and easy-operation fabrication of a porous dielectric layer with controllable porosity.

<table><tbody><tr><td>3D printer</td><td>Zhejiang Qidi Technology Co., Ltd</td><td>X-MAX</td><td></td></tr><tr><td>3D printing metarials</td><td>Zhejiang Qidi Technology Co., Ltd</td><td>3D Printing Filament PLA 1.75 mm</td><td></td></tr><tr><td>Carbon nanotubes (CNTs)</td><td>XFNANO</td><td>XFM13</td><td></td></tr><tr><td>Data acquisition (DAQ)</td><td>National Instruments</td><td>USB6002</td><td></td></tr><tr><td>Double side tape</td><td>Minnesota Mining and Manufacturing (3M)</td><td>3M VHB 4910</td><td>1 mm thick</td></tr><tr><td>Electrode metal mold</td><td>Guangdong Shunde Molarobot Co., Ltd</td><td></td><td>This metal mold is a round metal plate with a flat bottom round groove and an embossed electrode pattern of 0.2 mm thick in the middle of the groove.</td></tr><tr><td>Erythritol</td><td>Shandong Sanyuan Biotechnology Co.,Ltd.</td><td></td><td></td></tr><tr><td>Isopropyl Alcohol (IPA)</td><td>Sinopharm chemical reagent Co., Ltd</td><td>80109218</td><td></td></tr><tr><td>LabVIEW</td><td>National Instruments</td><td>LabVIEW 2019</td><td></td></tr><tr><td>LCR meter</td><td>Keysight</td><td>EA4980AL</td><td></td></tr><tr><td>Metal wire</td><td>Hangzhou Hongtong WIRE&amp;amp;CABLE Co., Ltd.</td><td>2UEW/155</td><td></td></tr><tr><td>Microscope</td><td>Aosvi</td><td>T2-3M180</td><td></td></tr><tr><td>Numerical modeling software</td><td>COMSOL</td><td>COMSOL Multiphysics 5.6</td><td></td></tr><tr><td>Polydimethylsiloxane (PDMS)</td><td>Dow Chemical Company</td><td>SYLGAR 184 Silicone Elastomer Kit</td><td>Two parts (base and curing agent)</td></tr><tr><td>Sealing film</td><td>Corning</td><td>PM-996</td><td>parafilm</td></tr><tr><td>Si wafer</td><td>Suzhou Crystal Silicon Electronic &amp;amp; Technology Co.,Ltd</td><td>ZK20220416-03</td><td>Diameter (mm): 50.8 +/- 0.3&lt;br/&gt; Type/Orientation: P/100&lt;br/&gt; Thickness (&amp;micro;m): 525 +/- 25</td></tr><tr><td>Silver conductive paint</td><td>Electron Microscopy Sciences</td><td>12686-15</td><td></td></tr><tr><td>Stepping motor</td><td>BEIJING HAI JIE JIA CHUANG Technology Co., Ltd</td><td>57H B56L4-30DB</td><td></td></tr><tr><td>Sugar/erythritol template metal mold</td><td>Guangdong Shunde Molarobot Co., Ltd</td><td></td><td>This metal mold is a 5 mm thick square metal plate with a flat bottom square groove of 2.5 mm deep.</td></tr><tr><td>Toluene</td><td>Sinopharm chemical reagent Co., Ltd</td><td>10022819</td><td></td></tr></tbody></table>

sensitivity enhancement of soft capacitive pressure sensors using a solvent evaporation-based porosity control technique

Soft pressure sensors play a significant role in developing &#34;man-machine&#34; tactile sensation in soft robotics and haptic interfaces. Specifically, capacitive sensors with micro-structured polymer matrices have been explored with considerable effort because of their high sensitivity, wide linearity range, and fast response time. However, the improvement of the sensing performance often relies on the structural design of the dielectric layer, which requires sophisticated microfabrication facilities. This article reports a simple and low-cost method to fabricate porous capacitive pressure sensors with improved sensitivity using the solvent evaporation-based method to tune the porosity. The sensor consists of a porous polydimethylsiloxane (PDMS) dielectric layer bonded with top and bottom electrodes made of elastic conductive polymer composites (ECPCs). The electrodes were prepared by scrape-coating carbon nanotubes (CNTs)-doped PDMS conductive slurry into mold-patterned PDMS films. To optimize the porosity of the dielectric layer for enhanced sensing performance, the PDMS solution was diluted with toluene of different mass fractions instead of filtering or grinding the sugar pore-forming agent (PFA) into different sizes. The evaporation of the toluene solvent allowed the fast fabrication of a porous dielectric layer with controllable porosities. It was confirmed that the sensitivity could be enhanced more two-fold when the toluene to PDMS ratio was increased from 1:8 to 1:1. The research proposed in this work enables a low-cost method of fabricating fully integrated bionic soft robotic grippers with soft sensory mechanoreceptors of tunable sensor parameters.

The photograph of the lumped sugar/erythritol porous template is shown in Figure 3A. Figure 3B shows the flexible electrode layer with a scrape-coated ECPCs pattern. Figure 3C shows the soft capacitive pressure sensor with a porous dielectric layer fabricated with the proposed method. Four porous PDMS dielectric layers were fabricated based on PDMS/toluene solutions with different mass ratios of 1:1, 3:1, 5:1, and 8...

Watch this Scientific Journal Video about Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique at JoVE.com

Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique

A simple and cost-efficient fabrication method based on the solvent evaporation technique is presented to optimize the performance of a soft capacitive pressure sensor, which is enabled by porosity control in the dielectric layer using different mass ratios of the molding PDMS/toluene solution.

Soft pressure sensors play a significant role in developing &#34;man-machine&#34; tactile sensation in soft robotics and haptic interfaces. Specifically, ...

sensitivity-enhancement-soft-capacitive-pressure-sensors-using

Research

JoVE Journal

Engineering

972 Views.  Zhejiang University. We stelden een nieuwe methode voor om een flexibele concurrentiedruksensor met controleerbare prestaties te fabriceren. Het wordt bereikt door de oplosmiddelmassafractie aan te passen om de porositeit van de diëlektrische laag te regelen. Het optimaliseren van de concurrerende druksensor wordt bereikt door een kostenefficiënte en eenvoudige bedieningsmethode die het gebruik van geavanceerde microfabricagefaciliteiten vermijdt.Om de poreuze PDMS-diëlektrische laag te vervaardigen, we...

Video: Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique

Fine-tuning the Size and Minimizing the Noise of Solid-state Nanopores

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the creation of a nanopore in a thin insulating membrane remains challenging. Fabrication methods involving specialized focused electron beam systems can produce well-defined nanopores, but yield of reliable and low-noise nanopores in commercially available membranes remains low2,3 and size control is nontrivial4,5. Here, the application of high electric fields to fine-tune the size of the nanopore while ensuring optimal low-noise performance is demonstrated. These short pulses of high electric field are used to produce a pristine electrical signal and allow for enlarging of nanopores with subnanometer precision upon prolonged exposure. This method is performed in situ in an aqueous environment using standard laboratory equipment, improving the yield and reproducibility of solid-state nanopore fabrication.

A methodology for preparing solid-state nanopores in solution for biomolecular translocation experiments is presented. By applying short pulses of high electric fields, the nanopore diameter can be controllably enlarged with subnanometer precision and its electrical noise characteristics significantly improved. This procedure is performed in situ using standard laboratory equipment under experimental conditions.

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the ...

Microbubble Fabrication of Concave-porosity PDMS Beads

Microbubble fabrication (by use of a fine emulsion) provides a means of increasing the surface-area-to-volume (SAV) ratio of polymer materials, which is particularly useful for separations applications. Porous polydimethylsiloxane (PDMS) beads can be produced by heat-curing such an emulsion, allowing the interface between the aqueous and aliphatic phases to mold the morphology of the polymer. In the procedures described here, both polymer and crosslinker (triethoxysilane) are sonicated together in a cold-bath sonicator. Following a period of cross-linking, emulsions are added dropwise to a hot surfactant solution, allowing the aqueous phase of the emulsion to separate, and forming porous polymer beads. We demonstrate that this method can be tuned, and the SAV ratio optimized, by adjusting the electrolyte content of the aqueous phase in the emulsion. Beads produced in this way are imaged with scanning electron microscopy, and representative SAV ratios are determined using Brunauer&#8211;Emmett&#8211;Teller (BET) analysis. Considerable variability with the electrolyte identity is observed, but the general trend is consistent: there is a maximum in SAV obtained at a specific concentration, after which porosity decreases markedly.

Procedures used to generate microstructured concave-porosity polydimethylsiloxane beads are presented. Effects of electrolyte concentration and identity within the aqueous phase are particularly emphasized.

Microbubble fabrication (by use of a fine emulsion) provides a means of increasing the surface-area-to-volume (SAV) ratio of polymer materials, which is ...

Chemistry

Microfluidic Devices for Characterizing Pore-scale Event Processes in Porous Media for Oil Recovery Applications

Microfluidic devices are versatile tools for studying transport processes at a microscopic scale. A demand exists for microfluidic devices that are resistant to low molecular-weight oil components, unlike traditional polydimethylsiloxane (PDMS) devices. Here, we demonstrate a facile method for making a device with this property, and we use the product of this protocol for examining the pore-scale mechanisms by which foam recovers crude oil. A pattern is first designed using computer-aided design (CAD) software and printed on a transparency with a high-resolution printer. This pattern is then transferred to a photoresist via a lithography procedure. PDMS is cast on the pattern, cured in an oven, and removed to obtain a mold. A thiol-ene crosslinking polymer, commonly used as an optical adhesive (OA), is then poured onto the mold and cured under UV light. The PDMS mold is peeled away from the optical adhesive cast. A glass substrate is then prepared, and the two halves of the device are bonded together. Optical adhesive-based devices are more robust than traditional PDMS microfluidic devices. The epoxy structure is resistant to swelling by many organic solvents, which opens new possibilities for experiments involving light organic liquids. Additionally, the surface wettability behavior of these devices is more stable than that of PDMS. The construction of optical adhesive microfluidic devices is simple, yet requires incrementally more effort than the making of PDMS-based devices. Also, though optical adhesive devices are stable in organic liquids, they may exhibit reduced bond-strength after a long time. Optical adhesive microfluidic devices can be made in geometries that act as 2-D micromodels for porous media. These devices are applied in the study of oil displacement to improve our understanding of the pore-scale mechanisms involved in enhanced oil recovery and aquifer remediation.

The goal of this procedure is to easily and rapidly produce a microfluidic device with customizable geometry and resistance to swelling by organic fluids for oil recovery studies. A polydimethylsiloxane mold is first generated, and then used to cast the epoxy-based device. A representative displacement study is reported.

Microfluidic devices are versatile tools for studying transport processes at a microscopic scale. A demand exists for microfluidic devices that are ...

Environment

Sensing of Barrier Tissue Disruption with an Organic Electrochemical Transistor

The gastrointestinal tract is an example of barrier tissue that provides a physical barrier against entry of pathogens and toxins, while allowing the passage of necessary ions and molecules. A breach in this barrier can be caused by a reduction in the extracellular calcium concentration. This reduction in calcium concentration causes a conformational change in proteins involved in the sealing of the barrier, leading to an increase of the paracellular flux. To mimic this effect the calcium chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N&#39;,N&#39;-tetra acetic acid (EGTA) was used on a monolayer of cells known to be representative of the gastrointestinal tract. Different methods to detect the disruption of the barrier tissue already exist, such as immunofluorescence and&#160;permeability assays. However, these methods are time-consuming and costly and not suited to dynamic or high-throughput measurements. Electronic methods for measuring barrier tissue integrity also exist for measurement of the transepithelial resistance (TER), however these are often costly and complex. The development of rapid, cheap, and sensitive methods is urgently needed as the integrity of barrier tissue is a key parameter in drug discovery and pathogen/toxin diagnostics. The organic electrochemical transistor (OECT) integrated with barrier tissue forming cells has been shown as a new device capable of dynamically monitoring barrier tissue integrity. The device is able to measure minute variations in ionic flux with unprecedented temporal resolution and sensitivity, in real time, as an indicator of barrier tissue integrity. This new method is based on a simple device that can be compatible with high throughput screening applications and fabricated at low cost.

The Organic Electrochemical Transistor is integrated with live cells and used to monitor ion flux across the gastrointestinal epithelial barrier. In this study, an increase in ion flux, related to disruption of tight junctions, induced by the presence of the calcium chelator EGTA (ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetra acetic acid), is measured.

The gastrointestinal tract is an example of barrier tissue that provides a physical barrier against entry of pathogens and toxins, while allowing the ...

Bioengineering

Deposition of Porous Sorbents on Fabric Supports

A microwave deposition technique for silanes, previously described for production of oleophobic fabrics, is adapted to provide a fabric support material that can be subsequently treated by dip coating. Dip coating with a sol preparation provides a supported porous layer on the fabric. In this case, the porous layer is a porphyrin functionalized sorbent system based on a powdered material that has been demonstrated previously for the capture and conversion of phosgene. A representative coating is applied to cotton fabric at a loading level of 10 mg/g. This coating has minimal impact on water vapor transport through the fabric (93% of the support fabric rate) while significantly reducing transport of 2-chloroethyl ethyl sulfide (CEES) through the material (7% of support fabric rate). The described approaches are suitable for use with other fabrics providing amine and hydroxyl groups for modification and can be used in combination with other sol preparations to produce varying functionality.

This report details a microwave-initiated approach for deposition of porphyrin functionalized porous organosilicate sorbents on a cotton fabric and demonstrates reduction in 2-chloroethyl ethyl sulfide (CEES) transport through the fabric resulting from this treatment.

A microwave deposition technique for silanes, previously described for production of oleophobic fabrics, is adapted to provide a fabric support material ...

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, ...

A Simple and Scalable Fabrication Method for Organic Electronic Devices on Textiles

Today, wearable electronics devices combine a large variety of functional, stretchable, and flexible technologies. However, in many cases, these devices cannot be worn under everyday conditions. Therefore, textiles are commonly considered the best substrate to accommodate electronic devices in wearable use. In this paper, we describe how to selectively pattern organic electroactive materials on textiles from a solution in an easy and scalable manner. This versatile deposition technique enables the fabrication of wearable organic electronic devices on clothes.

In this paper, we present a protocol to selectively deposit organic materials on textiles, which allows for the direct integration of organic electronic devices with wearables. The fabricated devices can be fully integrated in textiles, respecting their mechanical appearance and enabling sensing capabilities.

Today, wearable electronics devices combine a large variety of functional, stretchable, and flexible technologies. However, in many cases, these devices ...

Microfluidic Channel-Based Soft Electrodes for Capacitive Pressure Sensing

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  

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 ...

Ultrasensitive Detection of Biomarkers by Using a Molecular Imprinting Based Capacitive Biosensor

The ability to detect and quantitate biomolecules in complex solutions has always been highly sought-after within natural science; being used for the detection of biomarkers, contaminants, and other molecules of interest. A commonly used technique for this purpose is the Enzyme-linked Immunosorbent Assay (ELISA), where often one antibody is directed towards a specific target molecule, and a second labeled antibody is used for the detection of the primary antibody, allowing for the absolute quantification of the biomolecule under study. However, the usage of antibodies as recognition elements limits the robustness of the method; as does the need of using labeled molecules. To overcome these limitations, molecular imprinting has been implemented, creating artificial recognition sites complementary to the template molecule, and obsoleting the necessity of using antibodies for initial binding. Further, for even higher sensitivity, the secondary labeled antibody can be replaced by biosensors relying on the capacitance for the quantification of the target molecule. In this protocol, we describe a method to rapidly and label-free detect and quantitate low-abundant biomolecules (proteins and viruses) in complex samples, with a sensitivity that is significantly better than commonly used detection systems such as the ELISA. This is all mediated by molecular imprinting in combination with a capacitance biosensor.

Here, we present a protocol for the detection and quantification of low abundant molecules in complex solutions using molecular imprinting in combination with a capacitance biosensor.

The ability to detect and quantitate biomolecules in complex solutions has always been highly sought-after within natural science; being used for the ...

Immunology and Infection

In Vitro Multiparametric Cellular Analysis by Micro Organic Charge-modulated Field-effect Transistor Arrays

Modern electrophysiology has been constantly fueled by the parallel development of increasingly sophisticated tools and materials. In turn, discoveries in this field have driven technological progress in a back-and-forth process that ultimately determined the impressive achievements of the past 50 years. However, the most employed devices used for cellular interfacing (namely, the microelectrode arrays and microelectronic devices based on transistors) still present several limitations such as high cost, the rigidity of the materials, and the presence of an external reference electrode. To partially overcome these issues, there have been developments in a new scientific field called organic bioelectronics, resulting in advantages such as lower cost, more convenient materials, and innovative fabrication techniques.
Several interesting new organic devices have been proposed during the past decade to conveniently interface with cell cultures.&#160;This paper presents the protocol for the fabrication of devices for cellular interfacing based on the organic charge-modulated field-effect transistor (OCMFET). These devices, called micro OCMFET arrays (MOAs), combine the advantages of organic electronics and the peculiar features of the OCMFET to prepare transparent, flexible, and reference-less tools with which it is possible to monitor both the electrical and the metabolic activities of cardiomyocytes and neurons in vitro, thus allowing a multiparametric evaluation of electrogenic cell models.

In Vitro Multiparametric Cellular Analysis by Micro Organic Charge-modulated Field-effect Transistor Arrays

Here, we present the fabrication protocol of an organic charge-modulated field-effect transistor (OCMFET)-based device for in vitro cellular interfacing. The device, called a micro OCMFET array, is a flexible, low-cost, and reference-less device, which will enable the monitoring of the electrical and metabolic activities of electroactive cell cultures.

Modern electrophysiology has been constantly fueled by the parallel development of increasingly sophisticated tools and materials. In turn, discoveries in ...

Sensitivity Enhancement of Soft Capacitive Pressure Sensors Using a Solvent Evaporation-Based Porosity Control Technique

Samenvatting

Meer video's verkennen

Hoofdstukken in deze video