The development and characterization of carbon nanofiber/polylactic acid filament for additively manufactured piezoresistive sensors

In this work, an investigation into the effects of manufacturing parameters on carbon nanofiber (CNF)/polylactic acid (PLA) filament for fused filament fabrication (FFF) additive manufacturing (AM)-produced strain sensors was conducted. A design of experiments (DOE) approach was undertaken to explore the impact of CNF weight fraction, extrusion temperature, and number of extrusions on sensor performance. The initial extrusion combined pulverized PLA material and CNFs via a dry melt-mixing process. Through the DOE process, it was found that extruding CNF/PLA material at 185 °C for two total extrusions yielded material with dramatic improvements in electrical properties compared to unmodified material. Using the optimal manufacturing procedure, piezoresistive dog-bone shaped sensors were manufactured via FFF with 5.0, 7.5, and 10.0 wt % CNF/PLA filament and three different sizes. Piezoresistive testing under monotonic and cyclic loading conditions revealed strong and repeatable behavior from the piezoresistive AM sensors produced with 7.5 and 10.0 wt % CNFs. In general, 7.5 wt % CNF/PLA sensors showed greater strain sensitivity than 10.0 wt % CNF/PLA sensors for monotonic loading. Additionally, sensor size appeared to contribute significantly to the performance of 7.5 wt % CNF/PLA sensors. However, the 10.0 wt % CNF/PLA sensors exhibited greater repeatability in their piezoresistive response, and sensor size had much less impact on strain sensing performance. The printed sensors also exhibited a strong response under cyclic loading, however, variability in their piezoresistive response was significantly higher than under monotonic loading conditions. The results of this study are an important step towards the realization of low-cost, easily produced, and highly customizable strain sensors produced via FFF technology.