Ultra‑Sensitive Paper‑Based Capacitive Flexible Pressure Sensor for Wearable and Artificial‑Skin Applications

Introduction

Flexibility and seamless integration with curved surfaces have positioned FPS at the forefront of wearable electronics, robotic skins, and biomedical monitoring (refs 1–9). While field‑effect transistors, piezoelectric, and piezoresistive mechanisms exist, capacitive FPS have emerged as the most attractive due to their straightforward architecture, wide dynamic range, and excellent long‑term stability (refs 20–22). PDMS, known for its flexibility, biocompatibility, and dielectric strength, frequently serves as both the dielectric and a structural layer in capacitive FPS (refs 20,26,23–25). Silver nanowires (AgNWs) combine outstanding electrical conductivity, optical transparency, and mechanical robustness, making them ideal for flexible electrodes in various devices such as solar cells and heaters (refs 27–34). Previous works have integrated AgNWs with PDMS, achieving enhanced sensitivity by embedding AgNWs within the polymer matrix (refs 35–2). However, many high‑performance FPS still require intricate micro‑patterning or molding of dielectric layers (refs 2,35,36,7,21,26). Our approach bypasses these complexities by using standard printing paper coated with AgNWs as electrodes and laminating PDMS on both sides, resulting in a sensor with high sensitivity, large dynamic range, and robust mechanical resilience.

Methods

Preparation of AgNWs, AgNW Films, PDMS Films, and Capacitive FPS

AgNWs were synthesized via a hydrothermal route. A 0.3 M PVP/ethylene glycol (EG) solution (0.2 g PVP in 6 mL EG, stirred 20 min) was mixed with 0.1 M AgNO₃/EG and 0.01 M NaCl/EG. The combined solution was transferred to a Teflon‑lined reactor, heated to 140 °C for 2 h and then to 160 °C for 30 min, yielding white AgNW powder after repeated washing and centrifugation with acetone and deionized water. The final product was dispersed in 100 mL ethanol for film fabrication.

AgNW films were produced by airbrush spraying onto a 20 mm × 20 mm cleaned paper substrate heated to 100 °C. A 0.5 mm nozzle at 150 mm distance and 0.1 MPa pressure was used; varying spray duration adjusted film thickness and resistance. Post‑deposition, the substrate remained on the hotplate for 1 h to remove residual PVP. PDMS (Sylgard 184) was mixed with a 10:1 curing agent ratio, stirred 20 min, degassed 10 min, spin‑coated onto a cleaned glass slide, and cured at 65 °C for 2 h. The resulting freestanding PDMS film was peeled off.

The sandwich‑type capacitive FPS (Fig. 1) was assembled by placing two AgNW‑paper electrodes on either side of the PDMS dielectric. Copper leads were affixed with conductive silver paint, and the sensor was sealed with transparent tape.

a Structure of AgNW‑paper‑based capacitive FPS and simplified mechanism. b Test platform for capacitive FPS

Characterization and Test

Surface morphologies were examined with a FEI Inspect F50 SEM. UV‑Vis spectra (Shimadzu 1700) assessed AgNW purity. The sensor was evaluated on a custom pressure platform equipped with a force gauge (HP‑5) and a homemade LM555‑based oscillator that translated capacitance changes into frequency. Data acquisition employed a Keithley 2700 multimeter.

Results and Discussions

SEM images (Fig. 2a) reveal uniform, long, thin AgNWs (~100 nm diameter) with minimal impurities, confirming high‑quality nanowires. UV‑Vis spectra (Fig. 3) show characteristic plasmon peaks at 355 nm and 380 nm, further validating purity.

a–d SEM photos of AgNW film and paper

UV‑Vis spectrum of the AgNWs

Pressure‑capacitance measurements (Fig. 4a) exhibit two linear regimes: a highly sensitive low‑pressure zone and a lower‑sensitivity high‑pressure zone, with a transition at ~2 kPa. The sensor’s peak sensitivity of 1.05 kPa⁻¹ surpasses most reported devices (refs 12,26,37–39) while maintaining a simple fabrication process. The underlying mechanism stems from the paper’s micro‑groove structure; applied pressure forces air into the grooves, inducing indentations in the AgNW network that amplify the effective electrode area and reduce inter‑electrode spacing—both enhancing capacitance change.

Low‑pressure tests (Fig. 4b) confirm detection down to 1 Pa with full recovery, indicating excellent stability and rapid response. Short‑term repeatability over 500 cycles at 81 Pa (Fig. 4c) shows negligible drift, and long‑term aging after one month (Fig. 4d) demonstrates unchanged low‑pressure response, with only modest decline at high pressure that does not affect overall performance.

Response test of AgNW‑paper‑based capacitive FPS: a pressure‑capacitance relations; b low‑pressure response; c short‑term repeatability; d one‑month aging.

Practical demonstrations highlight the sensor’s versatility: (i) bending tests (Fig. 5a) reveal a near‑linear capacitance‑angle curve, enabling precise joint‑angle monitoring; (ii) finger‑tap detection (Fig. 5b) registers ~700 pF spikes per click; (iii) voice‑induced vibration sensing (Fig. 5c) distinguishes individual syllables with high repeatability. An 8 × 8 pixel array (Fig. 5d) fabricated with a 2 mm × 2 mm pixel size successfully maps a pencil tip and a plasticine bullet, showing negligible crosstalk and clear shape recognition.

Applications of AgNW‑paper‑based capacitive FPS: a bending test, b finger tap test, c voice test, d 8 × 8 array, e pencil tip detection, f bullet shape detection.

Conclusion

By leveraging hydrothermally grown AgNWs on ordinary paper and PDMS dielectric layers, we produced a capacitive FPS that combines high sensitivity (1.05 kPa⁻¹), a wide dynamic range (1 Pa–2 kPa), and remarkable stability. The sensor reliably detects human motions—bending, tapping, and speech—and the 8 × 8 array demonstrates low crosstalk and object‑shape recognition. These attributes underscore the platform’s suitability for artificial skins, motion monitoring, wearables, and tactile object identification.

Availability of Data and Materials

The datasets used and/or analyzed during this study are available from the corresponding author upon reasonable request.

Abbreviations

FPS

Flexible pressure sensors

AgNWs

Silver nanowires

M

Mole per liter

PVP

Polyvinyl pyrrolidone

PS

Polystyrene

EG

Ethylene glycol

NaCl

Sodium chloride

SEM

Scanning electron microscope

UV-Vis

Ultraviolet‑visible

ΔC

Capacitance variation

pF

Picofarad