Piezoresistive Behaviour of Carbon-Functionalised Knitted Structures: The Influence of Nanomaterial Type and Application Method

This study investigates the interplay between carbon nanofiller morphology and functionalisation methods in textile piezoresistive sensors. We compare two distinct functionalisation strategies (knife coating and foulard impregnation) for incorporating graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) into polyester knits. Thermal and structural analyses confirmed that both methods preserve nanofiller integrity, though impregnation better maintains the fabric’s native thermal stability by avoiding the formation of continuous surface films. Electrical analysis revealed that the two-dimensional GNPs required the continuous film of knife coating to percolate, whereas the one-dimensional CNTs are inferred to physically entangle to establish effective conductive pathways in both methods. However, it was the resulting structural architectures that influenced the final piezoresistive performance. The best-performing knife-coated GNP formulation (5 wt.%) achieved a low-force sensitivity of S2–20N = 0.032 N−1 but exhibited dynamic instability under cyclic compression (maximum coefficient of variation, CV ~28%), likely due to poor mechanical interlocking. Impregnation preserved the knit’s porosity and seems to have shifted the sensing mechanism to inter-yarn contact resistance, an advantage CNTs could leverage. The best-performing impregnated 2 wt.% CNT formulation combined high cyclic dynamic stability (CV < 10%) with broad-range sensitivity across low-force (S2–20N = 0.027 N−1) and high-force (S20–80N = 0.0024 N−1) regimes. This piezoresistive response and dynamic signal stability suggested a behaviour comparable to the commercial Velostat benchmark, highlighting the potential of this scalable fabrication route for wearable piezoresistive sensors.