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 wi...

<ol>
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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. 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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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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&#38;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 &#38; Technology Co.,Ltd</td>
			<td>ZK20220416-03</td>
			<td>Diameter (mm): 50.8 +/- 0.3 
			Type/Orientation: P/100 
			Thickness (&#181;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, ...

We proposed a new method to fabricate flexible competitive pressure sensor with controllable performance. It is accomplished by adjusting the solvent mass fraction to control the porosity of the dielectric layer. Optimizing the competitive pressure sensor is accomplished by a cost-efficient and easy operation method that avoids using sophisticated microfabrication facilities.To manufacture the porous PDMS dielectric layer, weigh filtered sugar and erythritol powder with a mass ratio of 20 to 1 and mix them evenly by shaking. Fill the mixture into a commercially-obtained sugar erythritol metal mold and press the surface to make the filler compact. Heat the mixture in a convection oven at 135 degrees Celsius for two hours.After heating, allow the mixture to cool at room temperature before removing the lump plate sugar. To fabricate the porosity-controllable PDMS dielectric layer, weigh five grams of toluene, five grams of PDMS base, and 0.5 grams of PDMs curing agent in a centrifuge tube and stir the solution evenly. Centrifuge the solution at 875G for 30 seconds at room temperature to remove air bubbles.Place the square sugar-erythritol porous template in a Petri dish. Insert double-sided tape as spacers beneath the four corners to lift the template from the Petri dish surface. Pour the PDMS toluene solution into the template and slightly incline the dish to fill all the gaps among the sugar particles.Then, place the dish in a vacuum desiccator and degas for 20 minutes. After degassing, transfer the dish from the desiccator into the oven at 90 degrees Celsius for 45 minutes to evaporate the toluene and cure the liquid PDMS. Next, immerse the cured PDMS embedded in the porous template in deionized water.Heat on a hot plate at 140 degrees Celsius until the sugar template completely dissolves and clean the porous PDMS with deionized water. For the fabrication of the flexible electrode layers based on ECPCs, first, synthesize ECPCs ink by weighing 0.16 grams of carbon nanotubes, or CNTs, and four grams of toluene in a beaker. Cover the beaker with sealing film to prevent solvent evaporation and magnetically stir at 250 RPM for 90 minutes.Weigh two grams of PDMS base and two grams of toluene in a beaker, and place on a magnetic stirrer at 200 RPM for one hour. After preparing both solutions, 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 two hours.After mixing, uncover the beaker and add 0.2 grams of PDMS curing agent into the mixed solution. Magnetically stir at 75 degrees Celsius and 250 RPM for one hour. To scrape-coat the electrodes, weigh toluene, PDMS base, and PDMS curing agent in a centrifuge tube with a mass ratio of 2 to 10 to 1 and stir the solution evenly.Then, centrifuge the solution at 875G for 30 seconds at room temperature to remove air bubbles. Pour 1.3 grams of PDMS toluene solution into a commercially-obtained electrode metal mold with an embossed electrode pattern. Place the mold in a vacuum desiccator and degas for 10 minutes.Then, cure the PDMS in the mold on a hot plate at 90 degrees Celsius for 15 minutes. After cooling at room temperature, peel off the patterned PDMS film. Attach the flat side of the PDMS film onto a silicon wafer.Scrape-coat the ECPCs ink into the electrode pattern. Cure the ECPC's ink on hot plate at 90 degrees Celsius for 15 minutes. For bonding and packaging of the soft capacitive sensors, attach the metal wire to the electrode.Drop silver conductive paint at the connection location for good conductivity and wait until the silver conductive paint dries. Drop the PDMS solution onto the connection to completely seal the dried silver conductive paint. Cure the PDMS on a hot plate at 90 degrees Celsius for 15 minutes.After curing, repeat the steps to connect the wire for the upper and lower electrode layers. Apply a thin layer of PDMS evenly onto the electrode film as an adhesion layer for bonding between the electrode and dielectric layers. Then, place the porous PDMS dielectric layer fabricated onto the electrode layer.Cure the PDMS glue at 95 degrees Celsius for 10 minutes. Place a glass Petri dish onto the porous PDMS to ensure good contact between the two layers during heating. Apply a thin layer of PDMS evenly onto the other electrode layer.Then, reverse the bonded electrode dielectric layer and place it on the other single electrode layer. After aligning the two electrodes, finish the bonding between the porous PDMS layer and the other electrode layer. For testing the sensing performance, control the stepping motor to drive the indentor 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 Newtons. Again, control the stepping motor and 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 as a function of the standard pressure reading.Control the indentor to press down rapidly and remain steady for a few seconds before returning to zero Newton loading. Repeat this procedure five times and record the capacitance as a function of time. Optical microscope images of the porous PDMS dielectric layers fabricated with different PDMS toluene mass ratios showed that the pore wall thickness decreased with an increasing mass ratio of the PDMS toluene solution.The simulation analysis showed that a higher porosity contributed to a larger compressive strain, with improved linearity under the same applied compression pressure. The capacitance pressure response curve of the sensors with porous PDMS dielectric layers with different PDMS toluene mass ratios exhibited different sensitivity. In the pressure loading range of 0 to 10 kilopascals, the sensor with a one to one PDMS toluene mass ratio exhibited twofold higher sensitivity than that of the sensor with the eight to one PDMS toluene mass ratio.Upon increased pressure, the dielectric layer's pores gradually reduced in size, decreasing sensitivity until it reached the same level for all porosities. The capacitive response to five consecutive loading, unloading tests under the same loading pressure of 10 kilopascals is shown. The response time of loading was found to be 0.2 seconds.The cyclic tests revealed that the as-fabricated soft capacitive sensor had excellent repeatability after 2, 500 cycles. The porosity of the PDMS dielectric layer will decrease as the PDMS toluene mass ratio increases, which will affect the sensor performance.

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

Research

JoVE Journal

Engineering

793 vistas.  Zhejiang University. Propusimos un nuevo método para fabricar sensores de presión competitivos flexibles con un rendimiento controlable. Se logra ajustando la fracción de masa del disolvente para controlar la porosidad de la capa dieléctrica. La optimización del sensor de presión competitivo se logra mediante un método de operación rentable y fácil que evita el uso de sofisticadas instalaciones de microfabricación.Para fabricar la capa dieléctrica PDMS porosa, pesar el azúcar filtrado y el polv...