Rapid Fabrication of Hierarchical Porous Polyaniline/Polyurethane Sponge Composites for Flexible Pressure and Tunable Gas Sensors

Background

Flexible sensors—encompassing pressure, strain, gas, temperature, and displacement—are pivotal for the next generation of wearable and foldable devices. Yet, their fabrication often requires expensive, multi‑step processes. Three‑dimensional (3D) sponges, especially polyurethane variants, are attractive alternatives because of their elasticity, high specific surface area, low density, and inexpensive production. When combined with conductive fillers, sponges have been employed in pressure sensors, supercapacitors, and oil absorbents. Conductive polymers, particularly polyaniline (PANI), are advantageous for sensors due to their intrinsic electrical conductivity, mechanical robustness, and large surface area. PANI can be synthesized via doping or in‑situ polymerization, the latter offering a more scalable and effective route.

Piezoresistive pressure sensors convert mechanical deformation into resistance changes and are prized for their simple design and low cost. The sensing principle hinges on the modulation of contact pathways within the conductive network. For gas sensing, PANI’s conductivity is modulated by protonation/doping changes upon interaction with alkaline gases such as NH₃, which reduces the number of charge carriers and increases resistance.

In this study, we leverage in‑situ polymerization to grow a porous PANI network on a polyurethane sponge, producing a composite that serves simultaneously as a high‑performance pressure sensor and an adjustable‑sensitivity NH₃ gas sensor.

Methods

Materials

Ammonium persulfate (APS, M_w = 228.20 g mol⁻¹), 5‑sulfosalicylic acid (SSA, M_w = 254.22 g mol⁻¹), and aniline (M_w = 93.13 g mol⁻¹) were sourced from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial polyurethane sponge (Domaxe, China) was used as the scaffold.

Preparation of PANI/Sponge Composite

The polymerization mixture was prepared by dissolving 2.5422 g SSA and 1.8626 g aniline in 50 mL deionized water, followed by magnetic stirring for 20 min. The sponge was then submerged, and 4.5640 g APS dissolved in 50 mL deionized water was added slowly to ensure homogeneous mixing. The reaction was carried out at 2 °C for 24 h. After washing with deionized water to remove residual monomers and oxidant, the sample was dried at room temperature for 48 h. The resulting PANI/sponge composite exhibited a color change from yellow to deep green, and the PANI loading was ~35 wt % (determined by weight difference).

Sensor Assembly

A 2 × 2 × 2 cm³ PANI/sponge composite was sandwiched between two copper electrodes (copper sheet). Copper wires were soldered onto the electrodes and connected to a Keithley 6487 high‑resistance meter for electrical measurements.

Characterization

Scanning electron microscopy (SEM, JEOL JSM‑7500F) and Raman spectroscopy (Renishaw inVia Plus, 532 nm laser) were employed to examine morphology and structure. Electrical properties were measured with a Keithley 6487 system.

Results and Discussion

Morphological and Structural Properties

SEM images reveal that the pristine sponge displays a smooth, interconnected porous framework, whereas the PANI‑coated sponge shows a rough surface densely covered with nanobranches. The nanostructured PANI coating increases the composite’s specific surface area and improves electrical conductivity. The composite possesses a multi‑hierarchical pore architecture comprising micropores of the sponge and nanoporosity introduced by the PANI branches.

Raman Spectroscopy

The Raman spectrum of the composite displays characteristic PANI peaks at 1486, 1407, 1216, and 1163 cm⁻¹, corresponding to C=C/C=N stretching, C–N stretching, and C–N bending, respectively. A band at 1588 cm⁻¹ confirms C–C stretching. These features confirm successful polymerization of PANI on the sponge scaffold.

Pressure Sensitivity Test

When subjected to cyclic pressure, the composite’s resistance decreases sharply with applied load and recovers immediately upon release. The relative resistance change (ΔR/R₀) increases from 0 to 13 kPa, with a sensitivity of 8.0 kPa⁻¹ in the 0–8 kPa range and 54.5 kPa⁻¹ in the 8–13 kPa range. The sensing mechanism is attributed to the compression‑induced reduction of contact resistance within the porous network.

Stability and Recoverability

Under continuous loading–unloading cycles up to 12 mm compression, the sensor exhibits linear current–pressure response, rapid recovery within 35 s, and negligible hysteresis, demonstrating excellent durability suitable for wearable applications.

Finger Bending Detection

Mounting a 2 × 1 × 0.5 cm³ sensor on a rubber glove, the device accurately tracks finger flexion and release, with clear, repeatable current spikes correlating to bending events, underscoring its suitability for soft robotics and human‑motion monitoring.

Adjustable Sensitivity Gas Sensor

By varying the pre‑compression of the composite within a sealed chamber, the NH₃ detection sensitivity can be tuned. Higher compression reduces pore volume and diffusion rate, leading to longer response times and lower steady‑state currents. This controllable behavior arises from the adjustable internal contact density, offering a straightforward means to tailor gas‑sensor performance.

Conclusions

We have demonstrated a facile, scalable method to fabricate a hierarchical PANI/PU sponge composite that functions as a high‑sensitivity, low‑cost pressure sensor and a tunable NH₃ gas sensor. The flexible, porous architecture delivers rapid response, excellent recoverability, and adjustable gas‑sensing performance, making it a promising candidate for next‑generation wearable and portable sensing devices.