High-performance flexible piezoresistive strain sensor based on biaxially stretched conductive polymer composite films with reduced graphene oxide-carbon nanotubes

In recent years, flexible piezoresistive sensors have shown great application potential in human health monitoring, smart robots, wearable electronic devices, and human-computer interaction, while it is challenging to efficiently fabricate highly sensitive and low-cost flexible piezoresistive sensor for detecting micro strains. In this work, reduced graphene oxide (rGO) was prepared by Hummer's method, then carbon nanotubes (CNTs) were immobilized on the surface of rGO by electrostatic assembly. Subsequently, the hybrid nanofillers were introduced into thermoplastic polyurethane (TPU) matrix to prepare conductive polymer composite. The further dispersion and parallel orientation of nanofillers in the matrix were achieved by the sequential biaxial stretching process. It is shown that the sensor prepared by biaxially stretched conductive polymer composite exhibits higher sensing performance compared to the sensor without experiencing biaxial stretching. The rGO-CNT/TPU_4×4 sensor (with a stretching ratio of 4×4) shows high sensitivity (G_F=46.7 at 1.5% strain), high linearity (R²=0.98), responsive capability to different strains and frequencies, excellent stability and repeatability in cyclic loading tests. The flexible piezoresistive sensor can be used to identify subtle human physiological activities, including pulse and exhalation. In addition, a compressible sensor array was fabricated to achieve accurate identification of weight distribution. This study provides an important scientific guidance for the rapid large-scale fabrication and structure and property tuning of high-performance flexible piezoresistive strain sensors.