Abstract
Multifunctional additive manufacturing (AM) has opened the door to exciting new possibilities for highly customizable and on-demand printable sensors and actuators for application in areas such as embedded structural sensing, robotics, and, among other examples, human healthcare and monitoring technology. In particular, strain or deformation sensors printed via fused filament fabrication (FFF) methods typically make use of the piezoresistive effect, which can be achieved by modifying the base polymer with conductive micro-to-nanoscale fillers. However, sensors of this type often suffer from challenges such as limited flexibility due to the use of comparatively hard polymers and additives and highly inconsistent electrical properties. To overcome these challenges, we herein present the preliminary results of a study of carbon nanofiber (CNF)-modified thermoplastic polyurethane (TPU) sensors produced via FFF printing. The CNF/TPU filament used to print these sensors was produced using a novel wet mixing process, wherein the TPU is first dissolved in dimethylformamide (DMF) solvent prior to dispersing the CNFs. To demonstrate the strain sensing properties of this material, simple dog bone-shaped sensors were printed out of TPU modified with 7.5 and 10 wt.% CNFs. And to showcase the potential of this material for large deformation sensing, resistance changes were measured for finite strains up to 50%. The CNF/TPU sensors showed non-linear but monotonically increasing resistance up to approximately 35% strain, beyond which point resistance rapidly became immeasurable. The results of this study are an important step towards the realization of next-generation printable plastics with functional and consistent properties for applications such as strain sensing.
