Three-dimensional printed re-entrant strain sensor using various carbon types/thermoplastic polyurethane filaments

In this study, to fabricate a soft conductive re-entrant (RE) strain sensor using a fused filament fabrication three-dimensional (3D) printer, carbon/thermoplastic polyurethane (TPU) filaments with three carbon materials—graphene (GR), carbon fiber (CF), and carbon black (CB)—were analyzed. The 3D printing performance of carbon/TPU was evaluated to confirm the applicability to 3D printing. Firstly, the results of the carbon/TPU filament, morphology, and X-ray diffraction analysis confirmed GR-TPU exhibited crystallinity, and the size of the CB particles was the smallest at <100 nm. For the thermal property, thermogravimetric analysis indicated that the carbon content of CB-TPU was the highest. In addition, differential scanning calorimetry showed that, depending on the carbon material content, the T g of CB-TPU was the highest. However, since the particles were the smallest, they were easily dispersed and the T m was the lowest. An analysis of the tensile properties indicated that GR-TPU was the weakest, CF-TPU was hard and brittle, and CB-TPU was the softest and toughest. The conductivity of CB-TPU was 36.12 mA, so it was 25 times higher than the other filaments. The results of the 3D printed RE pattern using various types of carbon/TPU and CB-TPU exhibited the best output performance. The results of the tensile property of the 3D printed RE pattern were similar to those of the filaments. In addition, the conductivities of the 3D printed RE patterns were improved more than the filament status, as ordered during 3D printing. Therefore, 3D printed carbon/TPU can be used as a basis for manufacturing soft sensors.