Clay/Conjugated Polymer Functional Coatings for High-Temperature-Operable and Wide-Range Foam Piezoresistive Sensors

The increasing popularity of wearable microelectronics highlights the need for high-temperature-operable piezoresistive sensors. In this study, we present a piezoresistive sensor made from a conjugated polymer, polypyrrole (PPy), combined with a clay-filled multilayer (10 layers of chitosan/montmorillonite–phytic acid, [CH/MMT-PA]₁₀) applied onto polyurethane foam (PUF). The clay-filled multilayer coating enhanced the sensor’s fire safety and expanded its pressure-detection range. The PPy/PUF sensor alone slightly reduced the peak heat release rate (pkHRR), a critical flammability metric, and was completely consumed during open flame and cone calorimetry (CC) tests. In contrast, the PPy/[CH/MMT-PA]₁₀/PUF significantly lowered the pkHRR and demonstrated excellent flame retardancy, self-extinguishing once the flame was removed, and retained a considerable portion of unburned foam following flame exposure, along with substantial char residue after the CC assessment. The PPy/[CH/MMT-PA]₁₀/PUF sensor exhibited a linear piezoresistive response between 1.53 and 17.5 kPa, with a maximum detection threshold of ∼69 kPa, whereas the PPy/PUF sensor exhibited a limited range (0.52 to 4.91 kPa) and a considerably lower maximum sensing threshold of ∼16 kPa. Due to its poor fire safety, the PPy/PUF sensor entirely lost its functionality when exposed to extreme temperatures or during a flame attack; in contrast, the PPy/[CH/MMT-PA]₁₀/PUF sensor maintained outstanding performance at higher temperatures and after flame exposure. Additionally, the PPy/[CH/MMT-PA]₁₀/PUF sensor can be affixed to a chair, functioning as a sensor for detecting back movements and sit–stand actions. This study holds promise for the advancement of various soft electronic sensors.