Preparation and properties of 3D printed polypyrrole nanotube/polydimethylsiloxane composite strain sensing composites

Flexible resistive strain sensors, as an important category of flexible sensors, have been widely applied across various fields due to their good flexibility, simple structure, and easy data readout. In existing studies, conductive fillers used in composite materials for filled strain sensors mainly consist of metallic conductive powders and carbon-based conductive powders, with few reports on the use of conductive polymers alone. Polypyrrole (PPy) nanotubes were synthesized by chemical oxidation polymerization using methyl orange (MO) as a dopant and anhydrous ferric chloride (FeCl₃) as an oxidant, with a high conductivity of up to 121.70 S·cm^-1. PPy nanotubes were used as conductive fillers and thickeners, and polydimethylsiloxane (PDMS) was used as the matrix to prepare printing ink by mechanical blending. The printability and rheological properties of the ink were characterized using a direct-writing 3D printer and an intelligent rheometer. The 3D printed samples were characterized and tested using scanning electron microscopy (SEM), intelligent tensile tester, digital multimeter, differential scanning calorimeter (DSC) and other instruments to investigate the effect of PPy nanotube concentration on the microstructure, electrical properties, mechanical properties, differential thermal properties, dynamic thermodynamic properties, and strain sensing properties of PPy/PDMS composite materials. The results show that the ink exhibits good printability when the concentration of PPy nanotubes reaches 7~9 wt%. The ink with 7 wt% shows superior performance in 20-layer continuous printing tests, with the printed dumbbell-shaped tensile specimens achieving a tensile strength of 3.02 MPa and an elongation at break of 178.64%, and a high gauge factor (GF) of 36.14. In 100-cycle tensile tests, the ink shows low resistance signal peak stability coefficient (α, 1.714) and shoulder peak proportion (Psp, 9.8%). It also exhibits good durability and stability in 1000-cycle tensile tests. The skin sensor patches made from this ink exhibit good signal stability and repeatability in monitoring finger, wrist, elbow, and knee joint movements, indicating that 3D-printed PPy/PDMS composites have potential applications in flexible electronics, wearable devices, and human motion monitoring.