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.