Investigation of Piezoresistive Behaviors in TPMS Lattice Structures Produced with MWCNT-Reinforced PLA Filament

Although the piezoresistive behavior of conductive polymer composites has been widely studied, the geometry-dependent piezoresistive response of MWCNT/PLA-based TPMS lattice structures produced by fused filament fabrication (FFF) remains insufficiently explored. The primary objective of this study is to comparatively investigate the mechanical, thermal, electrical, and piezoresistive behaviors of Gyroid (G), Diamond (D), and Schwarz Primitive (P) TPMS lattice structures produced using the same material system, constant relative density, and XY printing orientation. For this purpose, a 2 wt.% multi-walled carbon nanotube (MWCNT)-reinforced polylactic acid (PLA) composite filament was fabricated via solvent mixing and used to produce the three TPMS topologies through FFF. Mechanical and electromechanical properties were assessed via Shore D hardness, monotonic compression tests, and in-situ electrical measurements using three independent replicate specimens for each topology (n = 3). The highest hardness was recorded for Diamond (75.6 Shore D) and the lowest for Schwarz Primitive (71.2 Shore D). At 50% compressive strain, compressive stresses were 14.164, 8.777, and 3.152 MPa for Diamond, Gyroid, and Schwarz Primitive, respectively. Specific energy absorption (SEA) values followed the same order at 9.161, 6.149, and 2.917 J g−1. Electrical conductivity and resistivity variations under loading were characterized using the four-probe method, with Diamond exhibiting the highest and Schwarz Primitive the lowest conductivity. Piezoresistive analysis showed that Diamond achieved the highest positive compressive gauge factor (GF) values of 4.74 and 2.74 at 10% and 25% strains, respectively. At 50% strain, Gyroid demonstrated superior sensitivity (GF = 1.66) over Diamond (1.57) and Schwarz Primitive (1.49). Electrical resistance exhibited a monotonic decrease with advancing compressive strain across all topologies, validating a coherent and continuous piezoresistive response under uniaxial compression. These results indicate that Diamond offers the best overall performance in mechanical strength, SEA, and low-to-medium strain sensitivity, while Gyroid is preferable for high-strain sensing. Overall, 2 wt.% MWCNT/PLA-based TPMS structures demonstrate considerable potential for piezoresistive sensor applications.