Electrically Conductive Nanocarbon/Elastomer Composite Inks for Flexible and Wearable Strain Sensing

Flexible strain sensors are essential components of wearable devices for health monitoring, motion tracking, human-machine interaction, and rehabilitation. Here, we report an eco-friendly, all-carbon conductive ink composed of carbon nano-onions (CNOs) and carbon nanotubes (CNTs) dispersed in a poly(styrene-ethylene-butylene-styrene) elastomeric matrix. The ink is formulated using biomass-derived 2-methyltetrahydrofuran to ensure environmental compatibility. The combination of 0D CNOs and 1D CNTs provides high electrical conductivity, mechanical robustness, and tunable viscosity. Compressive strain sensors prepared by dip coating polyurethane sponges show a modulus of ≈460 kPa, a gauge factor of ≈1.1, and electrical hysteresis of 11.3% under 75% compression. Integrated into a football, the sensors detect contact, rotation, and rebound. Tensile strain sensors made by blade coating on stretchable textiles achieve gauge factors of 10-12 at 0.6% strain, a tensile modulus ≈3.2 MPa, and hysteresis of 7.7%. When positioned around the chest, the sensors monitored breathing in real time. Overall, the optimized interplay between ink rheology and conductive network morphology enables the fabrication of strain sensors with high performance and excellent cycling stability (>10 000 compression and 7000 tension cycles). This study establishes a scalable route to solvent-safe, carbon-based inks for the sustainable production of flexible and wearable electronics.