Process design and direct writing of flexible multilayered ceramic-magneto-polymer composites for humidity-sensitive devices

Direct Writing (DW) has emerged as a versatile additive manufacturing (AM) technique for fabricating multi-material structures in various fields, particularly in sensors and wearable devices. These devices typically consist of multiple functional elements that must be integrated within a single platform. DW enables the fabrication of such devices within one build area in a single process. However, multi-material and composite printing using different inks remains challenging due to difficulties in controlling the ink flow behavior, printing parameters, and ink-substrate compatibility. Especially multi-material DW is limited by mismatched ink rheology, differing drying behaviors, and poor ink-substrate interactions, often resulting in unstable deposition and interfacial defects. Also, the wettability between the polymer substrates and ceramic-filled ink is critical to achieving uniform deposition, defect-free interfaces, and reliable performance in flexible electronic systems. This interfacial spreading and adhesion are especially important for printed devices, such as humidity sensors, where reliable response depends on how the active layer interacts with moisture and transduces changes into measurable electrical signals. As a result, understanding and controlling the ink-substrate wettability are critical for ensuring consisting printing quality and functional reliability in multi-material DW. In this study, we address these challenges by developing a process-design framework for DW of SiC-Fe 3 O 4 -PVA composites, with a focus on substrate wetting behavior, drying kinetics, and ink spreading dynamics. And finally, the mechanical behavior of the printed structure under different humidity conditions were studied to evaluate the stability of the printed structure under different humidity conditions. The influence of deposition speed and build platform temperature on the morphology of printed PVA substrates and SiC-Fe 3 O 4 composite lines were evaluated. Printed lines exhibited defects such as surface cracks, delamination from the substrate, and irregular line edges under non-optimal conditions. It was found that build platform temperature of 70°C created a uniform and defect-free PVA substrates. Surface analysis of printed composite lines demonstrated that dispensing speed and substrate temperature influence the surface roughness and line morphology, with higher substrate temperatures suppressing instabilities and yielding improved surface properties. Contact angle measurements further revealed that substrate type and thermal conditions dictate droplet shape and surface wettability, enabling precise control over adhesion, line resolution, and layer uniformity. Furthermore, mechanical characterization under controlled humidity conditions revealed tunable tensile behavior of the composites, with decreased strength and increased elongation at higher relative humidity due to PVA plasticization. These humidity dependent mechanical responses contribute to the improved structural stability and functional reliability of the printed devices, highlighting the potential of this approach to enhance the performance of DW-fabricated humidity sensors. By enabling improved control over the DW process, the presented process-design framework is expected to support more stable and reliable material deposition for future humidity sensing applications. The findings provide a pathway for scalable fabrication of SiC-based nanocomposites with engineered interfaces and humidity-responsive behavior, paving the way for next-generation electronic skins, biomedical sensors, and adaptive multifunctional platforms. • Process-design framework developed for multi-material DW of SiC-Fe 3 O 4 -PVA inks • Substrate temperature critically controls wetting, spreading, and interfacial stability • Optimized DW conditions yield defect-free lines with improved surface morphology • Ink-substrate wettability enables enhanced adhesion and line resolution • Humidity-dependent mechanical response supports reliable DW-fabricated sensors