Porous Polydimethylsiloxane Elastomer Hybrid with Zinc Oxide Nanowire for Wearable, Wide-Range, and Low Detection Limit Capacitive Pressure Sensor

Abstract

We propose a flexible capacitive pressure sensor that utilizes porous polydimethylsiloxane elastomer with zinc oxide nanowire as nanocomposite dielectric layer via a simple porogen-assisted process. With the incorporation of nanowires into the porous elastomer, our capacitive pressure sensor is not only highly responsive to subtle stimuli but vigorously so to gentle touch and verbal stimulation from 0 to 50 kPa. The fabricated zinc oxide nanowire-porous polydimethylsiloxane sensor exhibits superior sensitivity of 0.717 kPa^-1, 0.360 kPa^-1, and 0.200 kPa^-1 at the pressure regimes of 0-50 Pa, 50-1000 Pa, and 1000-3000 Pa, respectively, presenting an approximate enhancement by 21-100 times when compared to that of a flat polydimethylsiloxane device. The nanocomposite dielectric layer also reveals an ultralow detection limit of 1.0 Pa, good stability, and durability after 4000 loading-unloading cycles, making it capable of perception of various human motions, such as finger bending, calligraphy writing, throat vibration, and airflow blowing. A proof-of-concept trial in hydrostatic water pressure sensing has been demonstrated with the proposed sensors, which can detect tiny changes in water pressure and may be helpful for underwater sensing research. This work brings out the efficacy of constructing wearable capacitive pressure sensors based on a porous dielectric hybrid with stress-sensitive nanostructures, providing wide prospective applications in wearable electronics, health monitoring, and smart artificial robotics/prosthetics.

Keywords: capacitive pressure sensor; porous polydimethylsiloxane; stress-sensitive; wearable electronic; zinc oxide nanowire.

Figure 1

Schematic illustration for fabricating a porous polydimethylsiloxane-based nanocomposite dielectric film hybrid with zinc oxide nanowire for the formation of flexible capacitive pressure sensors.

Figure 2

(a) SEM image of as-grown ZnO nanowires with ~30−70 nm diameter and ~5−7 μm length. Inset: a selected area electron diffraction pattern of a single-crystalline nanowire. (b) HRTEM images of a ZnO nanowire indicating the spacing of ~2.6 Å between two crystalline planes along [0001] growth direction. Cross-sectional SEM images of a portion of (c) a porous PDMS film and (d) a nanocomposite porous PDMS film with randomly distributed ZnO nanowires. Note that the arrow marks indicate the existence of nanowires. (e) Photo images of fabricated flexible ZnO nanowire–porous PDMS capacitive pressure sensors: a single cell (top) and a 4 × 4 multipixel array (bottom).

Figure 3

(a) Measured relative capacitance change (ΔC/C₀) as a function of the applied pressure (P) for the capacitive pressure sensors with different types of dielectric layers: flat PDMS, porous PDMS, and ZnO nanowire–porous PDMS (sensing area: 10 mm × 10 mm). (b) Pressure-response plots for these nanocomposite sensors with varying ZnO nanowire loading. (c) Comparison of the sensitivity of capacitive pressure sensors at different applied pressure ranges. (d) Proposed sensing mechanisms with graphical capacitance change for the capacitors containing (i) flat PDMS, (ii) porous PDMS, and (iii) ZnO nanowire–porous PDMS.

Figure 4

(a) Time-resolved capacitive response of nanocomposite capacitive pressure sensors based on flat PDMS, porous PDMS, and ZnO nanowire–porous PDMS, respectively. (b) Limit of detection test by means of the sequential detection of water droplets. (c) Stability test of a ZnO nanowire–porous PDMS device for 4000 cycles at 300 Pa; magnified curves of (i) 100−110, (ii) 1900−2000, and (iii) 3890−3900 cycles, respectively.

Figure 5

Real-time capacitance variations of the sensor in response to (a) calligraphy writing, (b) index finger straightening and bending, (c) air streaming, and (d) vocal speaking. (e) Photographs of a working pressure sensor array and their spatial pressure responses to the letters N, C, T, and U.