Synergistic conducting-polymer-coated silicon nanowires for breath waveform sensing

Silicon-nanowire (SiNW) resistive breath sensors coated with polypyrrole (PPy), polyaniline (PANI), and PPy/PANI composites were fabricated and systematically evaluated for respiratory waveform sensing. Scanning electron microscopy revealed distinct morphologies, including globular PPy networks, fibrous PANI networks, and interpenetrating hybrid architectures in the composite coating. Among the individual polymers, PANI-based sensors exhibited the highest normalized response (ΔR/R₀ = 0.108-0.162) but suffered from baseline drift after prolonged storage, whereas PPy-based sensors demonstrated fast kinetics with excellent stability although with lower sensitivity (ΔR/R₀ = 0.045-0.060). The PPy/PANI composite combined intermediate sensitivity (ΔR/R₀ = 0.096-0.144) with the fastest recovery time (0.2-1.8 s), highest response slope, and superior long-term stability, indicating a synergistic effect between the two conducting polymers. Benchmarking against representative humidity and respiration sensors reported in the literature showed that the PPy/PANI-coated SiNWs outperform most previously reported PANI- and PPy-based platforms, highlighting the synergistic effect of the PPy/PANI heterointerface and the high-surface-area SiNW scaffold. Statistical analysis (one-way ANOVA and Bonferroni post-hoc test, n = 6, p < 0.01) confirmed the statistical superiority of the composite sensor in terms of dynamic response parameters and figure of merit. The composite sensor successfully detected tracked multiple breathing regimes and detected irregular respiratory events such as coughing, as well as localized humidity sources, demonstrating strong potential for wearable and practical respiratory monitoring applications such as sleep apnea detection and breath pattern analysis.