Magnetic Field-Directed Self-Assembly of CNT-Fe₃O₄ Hybrids for Tunable Soft Piezoresistive Sensors

Carbon nanotubes (CNTs), typically magnetically inert, can be effectively aligned in polydimethylsiloxane (PDMS) using moderate magnetic fields when hybridized with Fe3O4 nanoparticles. We demonstrate that these CNT-Fe3O4 hybrids rapidly self-organize into aligned, bundled structures under an applied magnetic field (∼120 mT), driven by local magnetic gradients originating from Fe3O4 nanoparticles interacting with trace magnetic impurities within the CNTs. This field-induced anisotropy yields composites with significantly enhanced electrical conductivity, lower percolation thresholds, and superior piezoresistive sensitivity compared to non-aligned samples. Furthermore, we observe evidence of spontaneous local organization improving conductivity even without an external field. This simple, functionalization-free hybridization strategy offers a powerful route to control microstructure in soft composites, enabling tunable electromechanical properties for applications like soft sensors and providing insights into magnetically assisted self-assembly in hybrid materials.