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

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.