A Flexible and Highly Sensitive Pressure Sensor Based on a PDMS Foam Coated with Graphene Nanoplatelets

Abstract

The demand for high performance multifunctional wearable devices is more and more pushing towards the development of novel low-cost, soft and flexible sensors with high sensitivity. In the present work, we describe the fabrication process and the properties of new polydimethylsiloxane (PDMS) foams loaded with multilayer graphene nanoplatelets (MLGs) for application as high sensitive piezoresistive pressure sensors. The effective DC conductivity of the produced foams is measured as a function of MLG loading. The piezoresistive response of the MLG-PDMS foam-based sensor at different strain rates is assessed through quasi-static pressure tests. The results of the experimental investigations demonstrated that sensor loaded with 0.96 wt.% of MLGs is characterized by a highly repeatable pressure-dependent conductance after a few stabilization cycles and it is suitable for detecting compressive stresses as low as 10 kPa, with a sensitivity of 0.23 kPa^-1, corresponding to an applied pressure of 70 kPa. Moreover, it is estimated that the sensor is able to detect pressure variations of ~1 Pa. Therefore, the new graphene-PDMS composite foam is a lightweight cost-effective material, suitable for sensing applications in the subtle or low and medium pressure ranges.

Keywords: PDMS; foam; graphene; nanoplatelets; piezoresistivity; pressure sensor.

Figure 1

The main steps to obtain piezoresistive MLG/PDMS foams.

Figure 2

MLG suspension after: (a) ultrasonication, (b) solvent evaporation and (c) re-dispersion step.

Figure 3

Products at the end of the distinct production steps: (a) sugar lump extracted from molds; (b) sample after PDMS infiltration; (c) PDMS foam after leaching; (d) PDMS foam after MLGs infiltration.

Figure 4

Optical image of a porogen (a); and porogen size distribution (b).

Figure 5

(a) SEM image of the cross-section of a PDMS foam after leaching of a preformed sugar based template; (b) Higher magnification view of the pore marked in (a).

Figure 6

AFM images of a MLG flake and corresponding height profiles, before (a,b) and after (c,d) solvent evaporation and re-dispersion.

Figure 7

(a) SEM image of a MLGs coated PDMS foam; (b) Higher magnification view of the area marked in (a); (c) Higher magnification of flakes marked in (b).

Figure 8

Stress-strain response of two bulk PDMS samples produced with a stoichiometric ratio 10:1 and 20:1 of PDMS prepolymer and curing agent for a loading-unloading compression test.

Figure 9

Strain rate dependent stress-strain characteristics of the (a) F10; and (b) F20 foams (v₁ = 1 mm/min, v₈ = 8 mm/min, v₃₂ = 32 mm/min).

Figure 10

First stress derivative of a F10 (a); and F20 (b) foams pointing out the three foam transition regimes.

Figure 11

Effective DC conductivity of samples of the (a) MLG-F10; and (b) MLG-F20 foam types, for different weight concentration of MLGs.

Figure 12

Adopted parallel compression platen system for the pressure tests and corresponding sketch of the electromechanical measurements.

Figure 13

Mechanical stabilization of a MLG-F20 foam based pressure sensor: conductance G measured as function of applied pressure p during consecutive quasi-static monotonic loading tests.

Figure 14

Relative percentage conductance variation as function of the applied pressure p at different crosshead speed (v₁ = 1 mm/min, v₂ = 2 mm/min, v₈ = 8 mm/min, v₃₂ = 32 mm/min).

Figure 15

Conductance measured at the beginning (0 kPa) and at the end (70 kPa) of six consecutive monotonic loading tests repeated at different crosshead speeds.

Figure 16

Conductance measured at the beginning (0 kPa) and at the end (70 kPa) of one hundred consecutive monotonic loading tests carried out at the crosshead speed v₃₂.

Figure 17

MLG coating modification during compression and its effect on the overall conductance G of the foam. (a) Before loading, MLGs adhere to the PDMS foam walls and create the conducting paths that affect the value of the foam initial conductance G₀; (b) Applying a pressure in the linear-plateau region of the foam response, the MLG coating undergoes modification due to the sliding between flakes and due to the conducting path degradation occurring especially at higher crosshead speeds; (c) In proximity of the densification region of the foam, at higher pressures, the collapsing cells cause the formation of additional parallel conducting paths and the increase of G.

Figure 18

Sensitivity ranges of the fabricated sensor as function of the applied pressure and strain rate.

Figure 19

Calculated output signal (b) of a sensor pre-compressed at 60 kPa and subjected to a pressure variation of ±δp which is applied with a deformation speed of v₁ or v₃₂ (a).