High‑Sensitivity Flexible Strain Sensor Using Poly(vinylidene fluoride) Piezoelectric Film

Abstract

We demonstrate a 4 × 4 flexible sensor array comprising 16 micro‑capacitive units fabricated from a 50 µm thick poly(vinylidene fluoride) (PVDF) piezoelectric film. Optical imaging and piezoresponse force microscopy (PFM) confirm a sharp PVDF/electrode interface and robust piezoelectric domains. The array delivers an ultra‑high signal‑to‑noise ratio, with an output voltage linearly proportional to applied pressure at 12 mV kPa⁻¹. The transient response recovers in under 2.5 µs, underscoring its fast electromechanical performance. Theoretical simulations show that signal cross‑talk between adjacent cells is negligible below 178 kPa, confirming the scalability of the design.

Background

Poly(vinylidene fluoride) (PVDF) is a chemically stable piezoelectric polymer renowned for its pyroelectric, piezoelectric, and ferroelectric properties [1, 2]. Its impressive mechanical robustness—Young’s modulus of 2,500 MPa and tensile strength near 50 MPa—makes PVDF‑based pressure sensors highly flexible and fatigue‑resistant [3, 4]. Compared with conventional ferroelectric PZT sensors, PVDF offers a nontoxic, biocompatible alternative [5, 6] and superior softness, allowing it to conform to complex geometries essential for advanced strain‑sensing applications [7, 8]. Recent work has demonstrated PVDF sensors integrated into smart catheters for real‑time pressure monitoring [11], driver‑hand pulse‑wave monitoring for safety applications [12], and artificial skin that senses both pressure and temperature [13]. However, most reported devices provide single‑point sensing with limited spatial resolution.

In real‑world scenarios—such as wearable biosensors that map pressure across the human body—there is a clear demand for multipoint, highly flexible, and ultra‑sensitive sensing arrays [14–16]. Here we present a 4 × 4 flexible sensor array based on PVDF film, achieving a sensitivity of 12 mV kPa⁻¹ and a response time of 2.5 µs. We also map the pressure distribution of a human finger during common movements, showcasing the array’s practical potential.

Design and Experimental

Design and Fabrication of the Sensor Array

The sensor adopts a sandwich architecture: a 50 µm PVDF film (Jinzhou Kexin Inc., China) is coated on both sides with 20 µm aluminum electrode layers. Figure 1a illustrates the design, where 16 micro‑capacitor units are arranged in a 4 × 4 grid, each group of four units sharing a common output wire to minimize routing complexity.

a Schematic diagram of the sensor array. b Physical picture of the completed device

Fabrication proceeds on a PDMS‑coated glass slide to provide a stiff backing. The PVDF film, already metallized, is positioned on the substrate, then spin‑coated with a 3000 rpm photoresist layer for 40 s. Photolithography followed by wet etching of aluminum via a mask aligner (ABM, Inc., USA) yields the 16‑unit pattern. The finished array is released from the slide, and each capacitor is wired to conductive traces using silver glue. To ensure biocompatibility, the entire assembly is encapsulated in PDMS and cured at 60 °C for 12 h. Figure 1b shows the finished, flexible sensor bent around a cylinder.

Piezoelectric Property of the Sensor Array Based on the PVDF Film

PFM measurements (Seiko, Inc., Japan) were performed on the PVDF surface under a 2 V AC bias over a 2 × 2 µm² area. The resulting phase contrast confirms well‑defined piezoelectric domains and a clear interface with the underlying electrodes.

Calibration for the Sensor Array

Calibration involved applying controlled pressures to the array using an electro‑mechanical platform linked to a National Instruments DAQ‑USB6008. The sensor’s output voltage was recorded while varying pressure, enabling the construction of a voltage‑pressure calibration curve.

Results and Discussion

Figure 2a displays the post‑etching surface morphology under an optical microscope, revealing a distinct contrast that indicates a clean PVDF/aluminum interface. Figures 2b and 2c show the PVDF surface and its PFM phase image, respectively; the smooth morphology and strong phase signal attest to robust piezoelectric behavior.

a Surface morphology after etching. b PVDF surface. c Phase PFM image.

Under a constant 98.1 kPa pressure applied to a single electrode, the sensor produces a 123.1 mV output with negligible noise, as shown in Figure 3a. When identical pressure is simultaneously applied to four electrodes, each outputs ~190 mV, demonstrating excellent synchronization across the array (Figure 3b). A linear voltage‑pressure calibration from 60–150 kPa yields a slope of 12 mV kPa⁻¹ with an offset of –159.2 mV (Figure 3c).

Filtered output voltages: a single electrode, b four electrodes, c calibration curve.

Transient response tests involved applying impulse pressures (≈75.1 kPa) at 90 Hz. The sensor’s voltage rises and falls within 2 ms, indicating a rapid electro‑mechanical response (Figure 4a). Across the 60–150 kPa range, the response time remains consistently under 2 ms, and the output amplitudes align with the linear calibration.

Hold‑and‑release responses for pressures: a 75.1 kPa, b 58.2 kPa, c 67.8 kPa, d 81.9 kPa, e 98.1 kPa, f 153.6 kPa. Inset: response of bare PVDF.

Signal cross‑talk between adjacent electrodes was examined through COMSOL Multiphysics simulations. Each electrode spans 1.4 mm². Figure 5b shows the strain distribution when pressure is applied to electrode A, with increased strain observed at neighboring sites. The simulated interference voltage varies linearly with applied pressure, with a slope of 0.028 mV kPa⁻¹ and an intercept of 5 × 10⁻⁴ mV (Figure 5c). Interference drops below 5 mV for pressures under 178 kPa, rendering it practically insignificant. Reducing electrode area further mitigates interference, as confirmed by simulations with 1.2, 1.0, and 0.8 mm² electrodes (Figure 5d).

a Simulation geometry. b Displacement map. c Interference voltage vs. pressure for 1.4 mm². d Results for 0.8, 1.0, 1.2 mm² electrodes.

To demonstrate practical utility, we mapped pressure distribution across a thumb finger during three common motions: shiatsu, kneading, and rub. Figure 6 shows that during shiatsu, a concentrated 76 kPa spot appears at the finger’s center, while kneading yields higher pressure on the front surface, and rub produces a more uniform ~68 kPa distribution. These findings align with prior clinical observations and confirm the sensor’s ability to capture nuanced finger‑movement pressures.

Finger‑pressure maps: a shiatsu, b kneading, c rub.

In summary, the fabricated 4 × 4 PVDF‑based sensor array offers flexible, high‑sensitivity strain detection with rapid response times. Its demonstrated pressure‑mapping capability on human fingers indicates strong potential for wearable biosensing and robotic haptic applications.

Abbreviations

PFM:

Piezoresponse force microscopy

PVDF:

Poly(vinylidene fluoride)