Conductive polymer composites (CPCs) of carbon nanotubes (CNTs) and graphite nanosheet (GNP)-filled thermoplastic polyurethane (TPU) are 3D-printed into flexible piezoresistive sensors via fused filament fabrication. The sensor, with a customized lever-cross structure, allows detection of stretching and out-of-plane forces of different magnitudes and frequencies. The out-of-plane force direction is obtained by combing the relative electrical resistance change in the cross section of the sensor with a force analysis. The 75-CNT/25-GNP sensor (CNT-to-GNP mass ratio of 75%-to-25%) demonstrates excellent sensing performance at a total nanoparticle loading of 3 wt%. The linearity of the 75-CNT/25-GNP sensor is 0.98, while those of the 100-CNT and 50-CNT/50-GNP sensors are 0.93 and 0.86, respectively. The gauge factor of the 75-CNT/25-GNP sensor is 52% higher than that of the 100-CNT sensor, and its sensing strain range is 79% above that of the 50-CNT/50-GNP sensor. Excellent sensing stability is demonstrated for the 75-CNT/25-GNP sensor after 1500 stretching (out-of-plane force) cycles. The synergistic effect of CNTs and GNPs on sensing performance of piezoresistive sensors is clearly shown in this study.
- 3D printing
- conductive polymer composites
- out-of-plane force sensors
- piezoresistive sensors
- synergistic effect