Multiscale Design of Waterborne MWCNT–Polyurethane Inks for Direct Ink Writing: Highly Conductive and Air-Dried Structures for Absorption-Dominant EMI Shielding, Sensing, and Thermal Management

Achieving stable, printable aqueous dispersions of pristine multiwalled carbon nanotubes (MWCNTs) remains challenging due to their hydrophobic surfaces and strong bundling tendency. Here, we present a hybrid multiscale design strategy that enables MWCNTs to form conductive, waterborne polyurethane (WPU) inks suitable for direct ink writing (DIW). The approach employs a sustainable two-step assembly: ultrasound-assisted templating of isophorone diisocyanate hard segments onto MWCNTs to promote interfacial wetting, followed by emulsification that organizes WPU nanoparticles into "swimming-ring" hybrid structures, forming a polymer-mediated aqueous network. This process converts unstable MWCNT suspensions into uniform, highly conductive dispersions. Application-specific functionality is achieved by integrating magnetic graphene oxide and sodium alginate to produce rheology-tunable inks for high-fidelity 3D printing and post-deposition crosslinking, enabling room-temperature dried, dense, and flexible architectures. The resulting hybrids exhibit multifunctional performance, including absorption-dominant electromagnetic interference shielding, with a total shielding effectiveness of 42 dB at 1 mm and an absorption-to-reflection coefficient ratio (A/R) of 1.27, rapid piezoresistive sensing with a response time of ≈111 ms, and efficient Joule heating reaching 40 °C at 0.5 V and above 100 °C at 9 V.