Influence of the Porosity of Polymer Foams on the Performances of Capacitive Flexible Pressure Sensors

Abstract

This paper reports on the study of microporous polydimethylsiloxane (PDMS) foams as a highly deformable dielectric material used in the composition of flexible capacitive pressure sensors dedicated to wearable use. A fabrication process allowing the porosity of the foams to be adjusted was proposed and the fabricated foams were characterized. Then, elementary capacitive pressure sensors (15 × 15 mm² square shaped electrodes) were elaborated with fabricated foams (5 mm or 10 mm thick) and were electromechanically characterized. Since the sensor responses under load are strongly non-linear, a behavioral non-linear model (first order exponential) was proposed, adjusted to the experimental data, and used to objectively estimate the sensor performances in terms of sensitivity and measurement range. The main conclusions of this study are that the porosity of the PDMS foams can be adjusted through the sugar:PDMS volume ratio and the size of sugar crystals used to fabricate the foams. Additionally, the porosity of the foams significantly modified the sensor performances. Indeed, compared to bulk PDMS sensors of the same size, the sensitivity of porous PDMS sensors could be multiplied by a factor up to 100 (the sensitivity is 0.14 %.kPa^-1 for a bulk PDMS sensor and up to 13.7 %.kPa^-1 for a porous PDMS sensor of the same dimensions), while the measurement range was reduced from a factor of 2 to 3 (from 594 kPa for a bulk PDMS sensor down to between 255 and 177 kPa for a PDMS foam sensor of the same dimensions, according to the porosity). This study opens the way to the design and fabrication of wearable flexible pressure sensors with adjustable performances through the control of the porosity of the fabricated PDMS foams.

Keywords: electromechanical characterizations; microporous PDMS foam; polymer-based flexible pressure sensors; sensor behavioral modeling; sensor sensitivity and measurement range.

Figure 1

Principle of operation of the flexible normal pressure capacitive sensor.

Figure 2

(a) Fabrication process of the microporous PDMS foams; (b) example of 4:1 small pore PDMS foam sample; (c) compressibility of the of 4:1 small pore PDMS foam sample.

Figure 3

(a) Small sugar particles observed with an optical microscope; (b) Size distribution of sugar particles after the sieving procedure; (c–e) SEM images of the cross-section of microporous PDMS foams featuring small, medium, and large pores, respectively.

Figure 4

(a) Electromechanical test bench used to measure the sensor response curve (capacitance change versus applied pressure); (b) Typical sensor response curve (blue dots) for a sensor featuring a 6:1 small pore foam and first order exponential behavior model (black line) allowing sensor sensitivity (S) and pressure range (PR) to be defined and estimated from the characteristic pressure P_C and the maximum relative capacitance change ΔC_max/C₀. (c) Capacitance changes (black line) and foam permittivity changes (blue line), versus applied pressure, for a sensor featuring a 6:1 small pore foam layer; (d) Changes of the PDMS foam thickness d with applied pressure for a sensor featuring a 6:1 small pore foam layer.

Figure 4

(a) Electromechanical test bench used to measure the sensor response curve (capacitance change versus applied pressure); (b) Typical sensor response curve (blue dots) for a sensor featuring a 6:1 small pore foam and first order exponential behavior model (black line) allowing sensor sensitivity (S) and pressure range (PR) to be defined and estimated from the characteristic pressure P_C and the maximum relative capacitance change ΔC_max/C₀. (c) Capacitance changes (black line) and foam permittivity changes (blue line), versus applied pressure, for a sensor featuring a 6:1 small pore foam layer; (d) Changes of the PDMS foam thickness d with applied pressure for a sensor featuring a 6:1 small pore foam layer.

Figure 5

Typical response curve for a sensor featuring 4:1 small pore PDMS foam. Experimental data obtained during a load cycle (blue dots) and an adjusted load model (blue line) and a release cycle (red cross) with an adjusted release model (red line). Average load and release model is represented by the black line.

Figure 6

Variation of the capacitance according to the applied pressure, depending on the sugar:PDMS ratio (small pores PDMS foam, (d₀ = 5 mm ± 0.5 mm).

Figure 7

Variation of the capacitance according to the applied pressure, depending on the pore size for (a) 4:1 and (b) 6:1 PDMS foams (d₀ = 5 mm ± 0.5 mm).

Figure 8

Sensitivity and pressure range of the fabricated sensors according the used bulk PDMS or PDMS foams. Sugar:PDMS ratios were 0:1, 4:1, and 6:1, and the pore sizes were small (S), medium (M), and large (L).

Figure 9

Variation of the capacitance according to the applied pressure depending on the thickness of the porous PDMS medium pores with sugar:PDMS ratios of 4:1 and 6:1.

Figure 10

Variation of the capacitance according to the applied pressure, depending on the dynamics of the applied load (continuously or discontinuously with 10 pauses at each measurement point) for a 6:1 medium pore, porous PDMS that was 5 mm thick.