Flexible Ecoflex®/Graphene Nanoplatelet Foams for Highly Sensitive Low-Pressure Sensors

Abstract

The high demand for multifunctional devices for smart clothing applications, human motion detection, soft robotics, and artificial electronic skins has encouraged researchers to develop new high-performance flexible sensors. In this work, we fabricated and tested new 3D squeezable Ecoflex^® open cell foams loaded with different concentrations of graphene nanoplatelets (GNPs) in order to obtain lightweight, soft, and cost-effective piezoresistive sensors with high sensitivity in a low-pressure regime. We analyzed the morphology of the produced materials and characterized both the mechanical and piezoresistive response of samples through quasi-static cyclic compression tests. Results indicated that sensors infiltrated with 1 mg of ethanol/GNP solution with a GNP concentration of 3 mg/mL were more sensitive and stable compared to those infiltrated with the same amount of ethanol/GNP solution but with a lower GNP concentration. The electromechanical response of the sensors showed a negative piezoresistive behavior up to ~10 kPa and an opposite trend for the 10-40 kPa range. The sensors were particularly sensitive at very low deformations, thus obtaining a maximum sensitivity of 0.28 kPa^-1 for pressures lower than 10 kPa.

Keywords: Ecoflex®; foam; graphene; low-pressure sensor; nanoplatelets; negative piezoresistivity; positive piezoresistivity; wearable devices.

Figure 1

Density (a) and porosity (b) of the produced bulk and foam samples.

Figure 2

Schematic representation of the foam infiltration process.

Figure 3

Ecoflex^®-based samples: bulk sample (a), open cell foam (b), GNP-loaded foam (c,d), and GNP-loaded foam with the conductive plates and wires (e).

Figure 4

Cross section of an Ecoflex^® foam (EF) sample (a) and an EF-3% (b). Magnification of the EF-3% foam (c) and the detail of some embedded and partially emerging GNPs (d).

Figure 5

Stress–strain curves (a), magnification of the linear region of the stress–strain curves (b), and deformation as a function of the cyclic compressive loads (c,d) for different Ecoflex^® samples during cyclic compression tests.

Figure 5

Stress–strain curves (a), magnification of the linear region of the stress–strain curves (b), and deformation as a function of the cyclic compressive loads (c,d) for different Ecoflex^® samples during cyclic compression tests.

Figure 6

Compressive Young’s modulus of the Ecoflex^® bulk (EB) and EF samples.

Figure 7

Differences in the mechanical behavior of the foam with and without the electrical contacts.

Figure 8

Image of the electromechanical setup.

Figure 9

Normalized conductance vs. pressure obtained during three consecutive quasi-static monotonic loading tests at a speed of 0.1 mm/min: response of EF-3% (a) and EFT-3% (b) samples.

Figure 10

Normalized conductance vs. pressure and corresponding polynomial fitting curves obtained at a speed of 0.1 (a) and 1 (b) mm/min for the EF-3% sample.

Figure 11

Normalized conductance vs. pressure and corresponding polynomial fitting curves obtained at a speed of 0.1 (a) and 1 (b) mm/min for the EFT-3% sample.

Figure 12

Normalized conductance measured during 10 consecutive loading/unloading tests of the EF (a) and EFT (b) pressure sensors.

Figure 13

Sensitivity of samples EF-3% (a) and the EFT-3% (b) at two different crosshead speeds.

Figure 14

GNP coating modification during compression and its effect on the overall normalized conductance G/G₀ of the foam. (a) Normalized conductance vs. pressure for the EFT-3% sample. (b–d) Zoom of the death-band zone (1^st zone) and the low- (2^nd zone) and medium- (3^rd zone) pressure ranges of the sensor, respectively. (e–g) Sketch of the effects of different pressure values on the EFT-3% pore and on the induced modification of GNP percolation paths. In particular, in the sketch, the light blue GNPs represent the completely embedded flakes, while the grey ones represent the partially embedded or surface-lying GNPs.

Figure 14

GNP coating modification during compression and its effect on the overall normalized conductance G/G₀ of the foam. (a) Normalized conductance vs. pressure for the EFT-3% sample. (b–d) Zoom of the death-band zone (1^st zone) and the low- (2^nd zone) and medium- (3^rd zone) pressure ranges of the sensor, respectively. (e–g) Sketch of the effects of different pressure values on the EFT-3% pore and on the induced modification of GNP percolation paths. In particular, in the sketch, the light blue GNPs represent the completely embedded flakes, while the grey ones represent the partially embedded or surface-lying GNPs.