Highly Stretchable, Electrically Conductive TPU–PANI Nanofibrous Membrane for Advanced Flexible Strain Sensors

Abstract

We have engineered a thermoplastic polyurethane (TPU) nanofibrous composite, functionalized in situ with polyaniline (PANI) via electrospinning, to create a flexible strain sensor that can endure strains up to 165% with rapid response and long‑term stability. The composite maintains consistent conductivity across a broad temperature range and conforms to non‑planar surfaces, offering a low‑cost, scalable route to high‑performance wearable sensing and conductive substrates.

Background

Nanofibrous membranes are prized for their large specific surface area, high porosity, elasticity, and superior mechanical properties, making them attractive for tissue scaffolds, protective apparel, drug delivery systems, and electronic devices [1–10]. Among fabrication techniques, electrospinning stands out for its simplicity, affordability, and versatility in producing non‑woven mats [11–15]. However, achieving electrical conductivity in such membranes typically requires incorporating conductive polymers or carbon‑based semiconductors. Polyaniline (PANI) offers high conductivity but suffers from limited elasticity due to its polar backbone [16]. Thermoplastic polyurethane (TPU) provides excellent stretchability and abrasion resistance [17]. By combining TPU with PANI through in‑situ polymerization, we aim to overcome PANI’s brittleness while preserving the high elasticity of TPU, thereby creating a composite suitable for wearable strain sensors and stretchable conductors.

Experimental

Preparation of PANI/TPU Nanofibrous Membrane

The fabrication proceeds in three stages. First, a TPU solution is prepared by dissolving 2.4 g TPU in 8.8 g N,N‑dimethylformamide (DMF) and 8.8 g tetrahydrofuran (THF), then stirred for 5 h until homogeneous. Electrospinning is performed at a 10–12 cm needle‑collector distance, 12 kV, and 15 µL min⁻¹ feed rate, with a roller collector to achieve uniform thickness. The resulting TPU mat is then subjected to PANI polymerization: 4.6 g ammonium persulfate (APS) is dissolved in 50 mL deionized water (solution A); 1.875 g aniline and 2.54 g sulfosalicylic acid (SSA) are dissolved in 50 mL DI water (solution B). The TPU membrane (10 cm × 10 cm) is immersed in solution B, followed by slow addition of solution A while stirring at room temperature for 30 min. After 12 h at 275 K, the membrane is washed, dried for 48 h at room temperature, and becomes the green PANI/TPU composite.

Sensor Assembly

The sensor is fabricated by sandwiching a 1 cm × 2 cm × 0.05 cm composite membrane between two PDMS films (1.5 cm × 3 cm × 0.05 cm) to protect the fibers. Copper leads are bonded with silver paste, and the active width is 15 mm with a 1.5 mm electrode spacing. See the schematic below.

Figure 1. Schematic of the sensor assembly process.

Characterization is performed using optical microscopy (Olympus BX51), SEM (FEI DB235), FTIR (Thermo Nicolet iN10), a dynamical mechanical analyzer (TA Q‑800), a Keithley 6485 high‑resistance meter, and a PPMS (Quantum Design) for temperature‑dependent studies.

Results and Discussion

Morphology and Composition

The pristine TPU mat displays a uniform, high‑elastic fibrous network. Post‑polymerization, the membrane turns deep green (Figure 2a,b) and SEM reveals PANI particles decorating the fiber surfaces (Figure 2d). FTIR spectra confirm the presence of PANI: new bands at 3250 cm⁻¹ (N–H stretching) and 1514 cm⁻¹ (C=C aromatic) appear in the composite, absent in pure TPU (Figure 3).

Figure 2. (a) Optical image of TPU membrane. (b) Optical image of PANI/TPU membrane. (c) SEM of TPU. (d) SEM of PANI/TPU.

Figure 3. FTIR spectra of TPU and PANI/TPU membranes.

Stretchability and Sensitivity

The PANI/TPU sensor exhibits excellent elasticity, tolerating strains up to 165% while maintaining linear I‑V characteristics (Figure 4a). Current decreases gradually with increasing strain, indicating a tunable resistance suitable for strain sensing (Figure 4b). Mechanical testing shows the composite’s stress–strain curve comprises an elastic region (0–19 %), a plastic plateau (19–140 %), and an ultimate break at ~165 % (Figure 5). The tensile strength rises to 1.93 MPa relative to TPU alone, though strain at break slightly decreases due to the brittle nature of PANI.

Figure 4. (a) I‑V curves at various strains. (b) Current response vs. strain (5 V bias). (c‑d) SEM images of fibers unstrained and strained.

Figure 5. Stress–strain curves of TPU and PANI/TPU membranes.

The gauge factor (GF) is calculated as GF = (ΔR/R₀)/ε. The composite shows a GF of 6.73 in the elastic regime (0–120 %) and 49.51 in the plastic regime (120–150 %), indicating high sensitivity, especially compared to PANI/PVDF sensors (GF ≈ 1) [21] and superior to many polymeric sensors, though lower than ultrathin silicon or PEDOT:PSS/PVA films.

Figure 6. (a) Relative resistance change vs. strain. (b) Stability test at 30.7 % strain. (c) I‑V before and after 100 cycles. (d) I‑V after 1000 cycles.

Stability tests under constant 30.7 % strain demonstrate near‑perfect recovery of current over time, confirming excellent repeatability. Endurance tests over 1000 cycles show negligible change in I‑V characteristics, attesting to durability.

Bendability and Temperature Stability

When bent on substrates with curvature from 0 to 0.4 mm⁻¹, the I‑V response varies minimally (Figure 7a), evidencing suitability for non‑planar applications. Temperature cycling from 240 K to 360 K results in only modest resistance changes (2.97 kΩ to 1.60 kΩ), indicating robust performance across typical ambient ranges (Figure 8).

Figure 7. (a) I‑V curves under different curvatures. (b) Optical images during curvature testing.

Figure 8. I‑V curves at various temperatures.

Finger‑Motion Detection

The sensor’s high sensitivity enables detection of finger bending and release. In a 2000‑cycle test, the current peaks sharply upon bending and returns to baseline upon release (Figure 9a). A simple LED circuit demonstrates real‑time visual feedback: the LED maintains brightness under mild bending and dims with increasing tensile strain, offering a low‑complexity display of strain state (Figure 10).

Figure 9. (a) Current vs. time during finger bending. (b) Photograph of the wearable sensor.

Figure 10. (a) LED brightness under various curvatures. (b) LED brightness under 0–60 % tensile strain.

Conclusions

We have demonstrated a facile, low‑cost fabrication route for a highly stretchable, electrically conductive TPU–PANI nanofibrous composite. The resulting strain sensor reliably detects strains up to 165% with fast, repeatable response, maintains conductivity across a wide temperature window, and can be integrated into wearable or flexible electronic systems. Its light weight, low cost, and robust performance make it an attractive platform for next‑generation wearable biosensing and flexible circuitry.

Abbreviations

DI water

Deionized water

PANI

Polyaniline

TPU

Thermoplastic polyurethane