A liquid metal/CNTs/natural rubber composite ink for 3D printing highly conductive, stretchable, and durable wearable sensors

The rapid advancement of flexible electronics requires materials that combine high electrical conductivity, good stretchability, and the capability to form complex three-dimensional structures. Direct ink writing (DIW) 3D printing has emerged as a promising manufacturing strategy due to its flexibility and programmability. However, the development of high-performance conductive inks remains a pivotal challenge. In this study, we report a novel ternary composite ink composed of liquid metal (LM), carbon nanotubes (CNTs), and prevulcanized natural rubber latex (NRL). This formulation effectively suppresses aggregation while establishing additional conductive pathways. Meanwhile, renewable and highly elastic NRL serves as the matrix, and the incorporation of superabsorbent polymer (SAP) beads enables physical concentration, significantly enhancing solid content and yield stress to overcome the inherent rheological limitations of pristine NRL. Systematic characterization reveals that the composite achieves balanced performance for target applications at 30 wt% filler loading, exhibiting a tensile elongation at break of up to 660% and an electrical conductivity of 16.7 S·m⁻¹ , thereby achieving an effective balance between electrical and mechanical properties. Rheological analysis confirms pronounced shear-thinning behavior and rapid structural recovery, ensuring excellent printability. Flexible strain sensors fabricated via DIW using this ink demonstrate high sensitivity, with a maximum gauge factor of 28 achieved at 440–500% strain, which is comparable to or higher than those of previously reported LM-based sensors, as well as a broad detection range of 0–500% strain. This work provides a sustainable and scalable material-process-application framework for high-performance flexible sensors for wearable motion monitoring, while offering a high-value utilization pathway for natural rubber resources. • A LM/CNTs/NRL conductive ink was developed for DIW 3D printing. • SAP-assisted concentration tailors rheology for printable NRL inks. • Printed elastomers show 16.7 S m - ¹ conductivity and 660% stretchability. • Strain sensors achieve a gauge factor up to 28 with wide strain sensing. • Reliable human motion monitoring demonstrated for wearable sensing.