3D printing by direct ink writing (DIW) of UV-curable elastomers with embedded sensors for soft robotic and flexible electronic applications

Abstract

Abstract The study focused on the direct ink writing (DIW) three-dimensional printing and characterization of UV-curable elastomers embedded with strain sensors for soft robotic applications. DIW was chosen due to its ability to precisely deposit multiple composite compositions in order to fabricate complex structures with varied spatial functionality. By leveraging a thiol–ene click chemistry, a range of elastomer compositions were developed using poly(mercaptopropylmethylsiloxane-co-dimethylsiloxane) and vinyl-terminated polydimethylsiloxane with varying molecular weights and photoinitiator concentrations. Rheological analysis demonstrated that photoinitiator concentration directly influenced viscosity, with a controlled range of 10–150 cP being optimal for DIW-based patterning. Upon UV exposure using a 5 W and 365 nm UV source, the elastomers exhibited rapid curing, with viscosity increasing significantly within ∼1 s, demonstrating high polymerization efficiency. A processing map was constructed, revealing the optimized printing speeds and UV source positioning to reduce excessive print spreading, thereby enhancing structural fidelity. Mechanical testing of printed dog-bone specimens printed with the optimal parameters showed a time-dependent increase in elastic modulus and tensile strength over seven days due to prolonged crosslinking. Additionally, silver-filled conductive polymer composites were embedded within the elastomer matrix to create strain sensors, exhibiting linearity (linear sensor output resistance to strain level) and demonstrating a stable cyclic response under 5% strain for 100 cycles. These results suggest that thiol–ene UV-curable silicone elastomers are promising materials for applications in the development of soft structures, particularly in the development of complex smart and multi-functional materials required for soft robotics and flexible electronics.