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

Summary

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.

Abstract

Soft pressure sensors play a significant role in developing "man-machine" 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.

Introduction

In recent years, flexible pressure sensors have been drawing attention due to their indispensable application in soft robotics^1,2,3, "man-machine" haptic interfaces^4,5, and health monitoring^6,7,8. Generally, the mechanisms for pressure sensing include piezoresistive^1,4,7, piezoelectric^2,6, capacitive^{2,3,9,10,11,12,13}, 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-pyramids^2,9,14, micro-pillars¹⁵, and micro-pores^{9,10,11,12,13,16,17} 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 environment^9,14. 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 recently^{9,10,11,12,13,16,17}. 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⁻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.

Protocol

1. Fabrication of the soft capacitive pressure sensor with a porous PDMS dielectric layer

1. Manufacturing of the porous PDMS dielectric layer
   1. Prepare the sugar/erythritol porous template following the steps below.
      1. Filter the sugar with sample sieves with apertures of 270 µm and 500 µm. Choose sugar with a particle diameter in the range of 270-500 µm.
         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.
      2. Grind erythritol (see Table of Materials) into powder to ensure a more uniform mixing with the sugar.
      3. Weigh a certain amount of filtered sugar and erythritol powder with a mass ratio of 20:1. Shake to mix them evenly.
      4. 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.
      5. Heat the mold with the sugar/erythritol mixture in a convection oven at 135 °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).
   2. Fabricate the porosity-controllable PDMS dielectric layer.
      1. 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.
      2. 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.
      3. 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.
      4. 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.
      5. Place the Petri dish with the PDMS/toluene solution-filled porous template in a vacuum desiccator, and degas for 20 min.
      6. Transfer the Petri dish from the vacuum desiccator into the oven at 90 °C for 45 min to evaporate the toluene and cure the liquid PDMS.
      7. 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 °C until the sugar template completely dissolves. Clean the porous PDMS with DI water.
2. Fabrication of the flexible electrode layers based on ECPCs
   1. Synthesize the ECPCs ink.
      1. Weigh 0.16 g of CNTs (diameter: 10-20 nm, length: 10-30 µm, 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.
      2. 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.
      3. Add 0.2 g of PDMS curing agent into the mixed solution. Magnetically stir at 75 °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.
   2. Scrape-coat the electrodes following the steps below.
      1. Weigh toluene, PDMS base, and PDMS curing agent in a centrifuge tube with a mass ratio of 2:10:1. Stir the solution evenly.
      2. Centrifuge the solution at 875 x g for 30 s at room temperature to remove air bubbles.
      3. 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.
      4. Place the mold in a vacuum desiccator, and degas for 10 min.
      5. Cure the PDMS in the mold on a hot plate at 90 °C for 15 min. Peel off the patterned PDMS film after cooling at room temperature.
      6. 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.
      7. 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.
      8. Cure the ECPCs ink on a hot plate at 90 °C for 15 min.
      9. Repeat steps 1.2.2.3-1.2.2.8 to manufacture both the upper and lower electrode layers.
3. Bonding and packaging of the soft capacitive sensors
   1. 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.
   2. 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 °C for 15 min.
   3. Repeat steps 1.3.1-1.3.2 to connect the wire for both the upper and lower electrode layers.
   4. 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.
   5. Place the porous PDMS dielectric layer fabricated in step 1.1.2 onto the electrode layer.
   6. Cure the PDMS glue on a hot plate at 95 °C for 10 min. Place a glass Petri dish onto the porous PDMS to ensure good contact between the two layers during heating.
   7. 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.
   8. Repeat step 1.3.6 to finish the bonding between the porous PDMS layer and the other electrode layer.
      ​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.

2. Experimental process of sensor performance characterization

1. Stepping pressure loading setup and data acquisition system
   1. 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.
   2. Fix the indenter on a vertical linear moving stage controlled by a stepping motor (see Table of Materials) through a standard pull-pressure sensor.
   3. 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.
2. Testing the sensing performance
   1. 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).
   2. 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.
   3. 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.
   4. 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.

Results

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...

Discussion

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 PFA^11,12,13,18,

Materials

List of materials used in this article

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