Multiscale Design of Waterborne MWCNT-Polyurethane Inks for Direct Ink Writing: Highly Conductive and Air-Dried...

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.