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>
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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 PFA11,12,13,18,19 and the ratio of polymer substrate to PFA17,20, we change the concentration of the polymer substrate solution while keeping the size of the PFA (i.e., sugar) uniform. As a result, the thickness of the pore wall changes while the pore distribution holds, which means that the porosity can be controlled by the solution concentration.
The most critical step for the porosity control is preparing the PDMS/toluene solution. The mass ratios of the PDMS/toluene solution were chosen as 1:1, 3:1, 5:1, and 8:1, respectively, to fabricate dielectric layers with different porosities. It was experimentally confirmed that the decrease in the mass ratio led to higher porosity and increased sensitivity in the low-pressure range.
Heating the sugar/erythritol mixture to fabricate the porous template is also a critical, innovative step. Different from existing methods of heating pure sugar21,22, adding water23, and applying pressure24, the melting point difference of these two PFA components was utilized to fabricate the porous template. In this protocol, the heating temperature is higher than the melting point of erythritol and lower than the melting point of sugar. Thus, the erythritol powder gradually melts during the heating process and bonds the solid sugar particles into a plate sugar lump. The mass ratio of sugar to erythritol was also found to be essential to the success of this step. A higher fraction of the erythritol will fill the gap among the sugar particles, while a lower fraction will lead to a failure in bonding.
However, some limitations exist in the device fabricated by this method. As the loading pressure increases, the pores in the dielectric layer gradually close, and the pore walls come into contact with each other, resulting in a more solid PDMS-like mechanical property. This phenomenon explains the independence of sensitivity from porosity, which was found in the higher pressure range above 40 kPa for our sensor. It is also worth noting that the sensor made with an 8:1 PDMS base/toluene mass ratio showed a significantly higher sensitivity of 3.78% kPa&#8722;1 compared to other sensors below 5 kPa, which may be attributed to a coupling of the mechanical and electrical properties induced by the porous structure.
The research proposed in this work enables a low-cost and easy-operation fabrication method of porous capacitive pressure sensors with tunable sensor parameters, which has broad application prospects in soft robotics, haptic interfaces, etc. In the future, fully integrated bionic soft robotic grippers with soft sensory mechanoreceptors of tunable sensor parameters can be further researched based on this method.

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 8:1, respectively. Optical microscope images showing the pore morphologies of the different structures are presented in Figure 3D. It was found that the pore wall thickness increased with an increasing mass ratio of the PDMS/toluene solution.
To verify the dependence of the mechanical properties on the porosity, a finite element analysis (FEA) was carried out to simulate the pressure developed in the porous PDMS dielectric layer as a function of the compressive strain using advanced numerical modeling software (see Table of Materials). A 3D model of the porous PDMS with open pores was created, with a length of 2 mm on the z-axis. The location of the pores was fixed, while the diameters were changed to obtain different porosities. An increasing pressure was applied in the z-axis, while periodic and symmetric boundary conditions were applied in the x-axis and y-axis. The simulation result in Figure 4A shows that a higher porosity contributed to a larger compressive strain with improved linearity under the same applied compression pressure. Figure 4B,C shows the capacitance-pressure response curve of the sensors with porous PDMS dielectric layers with different PDMS/toluene mass ratios. In the pressure loading range of 0-10 kPa, the sensor with a 1:1 PDMS/toluene mass ratio exhibited the highest sensitivity of 3.47% kPa&#8722;1, which was more than two-fold higher than that of the sensor with the 8:1 PDMS/toluene mass ratio (1.48% kPa&#8722;1). As the pressure increased, the pores in the dielectric layer gradually reduced in size, leading to a decrease in the sensitivity until it reached the same level of 0.66%-0.89% kPa&#8722;1 for all porosities, as shown in Figure 4C. Figure 4D shows the capacitive response to five consecutive loading-unloading tests under the same loading pressure of approximately 10 kPa. The response time of loading (i.e., the time required for the sensor capacitance to reach 90% of its steady-state value) was determined to be about 0.2 s, as shown in Figure 4E. In addition, as shown in Figure 4F, the cyclic tests also revealed that the as-fabricated soft capacitive sensor had excellent repeatability after 2,500 cycles.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65143/65143fig01.jpg" /> 
Figure 1: Schematic of the fabrication process. (A-C) The fabrication flow of the porous PDMS dielectric layer. (D,E)&#160;The preparation of the ECPCs ink. (F,G) The scrape-coating process of the electrode layer. (H,I) The wire connection and bonding process of the soft capacitive pressure sensor with the electrode-porous dielectric layer-electrode sandwich structure. (J) Illustrations of the structure and materials of the sensor. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/65143/65143fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/65143/65143fig02.jpg" /> 
Figure 2: Experimental setup. (A) Stepping pressure loading setup. (B) Data acquisition system. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/65143/65143fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/65143/65143fig03.jpg" /> 
Figure 3: Sensor structures. (A) Photograph of the sugar/erythritol template. (B) The flexible electrode layer with the scrape-coated ECPCs pattern. (C) Photograph of the soft capacitive pressure sensor with the porous dielectric layer. (D) Optical microscope images of the porous PDMS dielectric layers fabricated with different PDMS/toluene mass ratios (PDMS base:toluene = 1:1, 3:1, 5:1, and 8:1). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/65143/65143fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/65143/65143fig04.jpg" /> 
Figure 4: Simulation and characterization result. (A) Simulated stress-strain curve of the porous PDMS layers with different porosities under low-pressure loadings. (B,C) Pressure-response curve of the capacitive pressure sensors fabricated with PDMS/toluene solutions with different mass ratios (PDMS base:toluene = 1:1, 3:1, 5:1, and 8:1). (D,E) Dynamic response of the sensor (PDMS base:toluene = 1:1). (F) The stability test result of the porous capacitive pressure sensor (2,500 cycles of loading). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/65143/65143fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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

Nanyang Technological University

Research

JoVE Journal

Engineering

943 Views.  Zhejiang University. 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.

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

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

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

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

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

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

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

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

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

Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many cases validated experimentally, for light weight or impact energy absorbing structures, as high surface area heat exchangers or electrodes, as implants to the body, and many more. Although great progress has been made in understanding their structure-properties relationships, the large number of different processing techniques, each producing material with different characteristics and structure, means that understanding of the individual effects of all aspects of structure is not complete. The replication process, where molten metal is infiltrated between grains of a removable preform material, allows a markedly high degree of control and has been used to good effect to elucidate some of these relationships. Nevertheless, the process has many steps that are dependent on individual &#8220;know-how&#8221;, and this paper aims to provide a detailed description of all stages of one embodiment of this processing method, using materials and equipment that would be relatively easy to set up in a research environment. The goal of this protocol and its variants is to produce metal foams in an effective and simple way, giving the possibility to tailor the outcome of the samples by modifying certain steps within the process. By following this, open cell aluminum foams with pore sizes of 1&#8211;2.36 mm diameter and 61% to 77% porosity can be obtained.

Replication is one of the processing techniques used for the production of porous metal sponges. In this paper one implementation of the method for the production of open celled porous aluminum is shown in detail.

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

Blast Quantification Using Hopkinson Pressure Bars

Near-field blast load measurement presents an issue to many sensor types as they must endure very aggressive environments and be able to measure pressures up to many hundreds of megapascals. In this respect the simplicity of the Hopkinson pressure bar has a major advantage in that while the measurement end of the Hopkinson bar can endure and be exposed to harsh conditions, the strain gauge mounted to the bar can be affixed some distance away. This allows protective housings to be utilized which protect the strain gauge but do not interfere with the measurement acquisition. The use of an array of pressure bars allows the pressure-time histories at discrete known points to be measured. This article also describes the interpolation routine used to derive pressure-time histories at un-instrumented locations on the plane of interest. Currently the technique has been used to measure loading from high explosives in free air and buried shallowly in various soils.

This protocol details the use of Hopkinson pressure bars to measure reflected blast loading from near-field explosive events. It is capable of interpolating a pressure-time history at any point on a reflective boundary and as such can be used to fully characterize the spatial and temporal variations in loading produced.

Near-field blast load measurement presents an issue to many sensor types as they must endure very aggressive environments and be able to measure pressures ...

The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various characteristic length scales can result in pressure fluctuations over a wide frequency range. This property of turbulence requires sensing devices to have sufficient sensitivity over a wide range of frequencies. Furthermore, the small characteristic length scales of turbulent structures require small sensing areas and the ability to place the sensors in very close proximity to each other. The complex geometries of the solid bodies, often including large surface curvatures or discontinuities, require the probe to have the ability to be set up in very limited spaces. The development of a remote microphone probe, which is inexpensive, consistent, and repeatable, is described in the present communication. It allows for the measurement of pressure fluctuations with high spatial resolution and dynamic response over a wide range of frequencies. The probe is small enough to be placed within the interior of typical wind tunnel models. The remote microphone probe includes a small, rigid, and hollow tube that penetrates the model surface to form the sensing area. This tube is connected to a standard microphone, at some distance away from the surface, using a "T" junction. An experimental method is introduced to determine the dynamic response of the remote microphone probe. In addition, an analytical method for determining the dynamic response is described. The analytical method can be applied in the design stage to determine the dimensions and properties of the RMP components.

Here, we present a protocol to measure, with high spatial resolution, the unsteady surface pressure in turbulent flows. This method demonstrates the construction of a remote microphone probe (RMP) and the determination of its frequency-dependent, complex transfer function. An analytical determination of the dynamic response is presented and validated.

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Gain-compensation Methodology for a Sinusoidal Scan of a Galvanometer Mirror in Proportional-Integral-Differential Control Using Pre-emphasis Techniques

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. However, the responsiveness of a galvanometer mirror is limited by its inertia; hence, the gain of a galvanometer mirror is reduced when the control path is steep. In this research, we propose a method to extend the corresponding frequency using a pre-emphasis technique to compensate for the gain reduction of galvanometer mirrors in sine-wave path tracking using proportional-integral-differential (PID) control. The pre-emphasis technique obtains an input value for a desired output value in advance. Applying this method to control the galvanometer mirror, the raw gain of a galvanometer mirror in each frequency and amplitude for sine-wave path tracking using a PID controller was calculated. Where PID control is not effective, maintaining a gain of 0 dB to improve the trajectory tracking accuracy, it is possible to expand the speed range in which a gain of 0 dB can be obtained without tuning the PID control parameters. However, if there is only one frequency, amplification is possible with a single pre-emphasis coefficient. Therefore, a sine wave is suitable for this technique, unlike triangular and sawtooth waves. Hence, we can adopt a pre-emphasis technique to configure the parameters in advance, and we need not prepare additional active control models and hardware. The parameters are updated immediately within the next cycle because of the open loop after the pre-emphasis coefficients are set. In other words, to regard the controller as a black box, we need to know only the input-to-output ratio, and detailed modeling is not required. This simplicity allows our system to be easily embedded in applications. Our method using the pre-emphasis technique for a motion-blur compensation system and the experiment conducted to evaluate the method are explained.

We propose a method to extend the corresponding frequency by using a pre-emphasis technique. This method compensates for the gain reduction of a galvanometer mirror in sine-wave path tracking using proportional-integral-differential control.

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. ...

Study of Siphon Breaker Experiment and Simulation for a Research Reactor

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent this outflow, a control device is required. A siphon breaker is a type of safety device that can be utilized to control the loss of coolant water effectively.
To analyze the characteristics of siphon breaking, a real-scale experiment was conducted. From the results of the experiment, it was found that there are several design factors that affect the siphon breaking phenomenon. Therefore, there is a need to develop a theoretical model capable of predicting and analyzing the siphon breaking phenomenon under various design conditions. Using the experimental data, it was possible to formulate a theoretical model that accurately predicts the progress and the result of the siphon breaking phenomenon. The established theoretical model is based on fluid mechanics and incorporates the Chisholm model to analyze two-phase flow. From Bernoulli&#39;s equation, the velocity, quantity, undershooting height, water level, pressure, friction coefficient, and factors related to the two-phase flow could be obtained or calculated. Moreover, to utilize the model established in this study, a siphon breaker analysis and design program was developed. The simulation program operates on the theoretical model basis and returns the result as a graph. The user can confirm the possibility of the siphon breaking by checking the shape of the graph. Furthermore, saving the entire simulation result is possible and it can be used as a resource for analyzing the real siphon breaking system.
In conclusion, the user can confirm the status of the siphon breaking and design the siphon breaker system using the program developed in this study.

The siphon breaking phenomenon was investigated experimentally and a theoretical model was proposed. A simulation program based on the theoretical model was developed and the results of the simulation program were compared with experimental results. It was concluded that the results of the simulation program matched the experimental results well.

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent ...

Studying Large Amplitude Oscillatory Shear Response of Soft Materials

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water &#8212; two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl ...