Near-Infrared-Assisted Direct Ink Writing of Multicolor Conductive Polydimethylsiloxane for Customizable Strain Sensors

3D printing provides a strategy for the fabrication of complex polydimethylsiloxane (PDMS) structures, among which ultraviolet-assisted direct ink writing (UV-DIW) has become a research hotspot due to its advantages of high precision and high efficiency. However, owing to the insufficient penetration depth of ultraviolet light, UV-DIW suffers from critical drawbacks, including restricted printable filament size, poor forming capability for large-scale and unsupported structures, and restriction to colorless and transparent ink systems, which limits its application in customized flexible electronic sensors. In this work, a high-penetration near-infrared-assisted DIW (NIR-DIW) method for PDMS 3D printing was proposed. Upconversion nanoparticles (UCNPs), which could convert 980 nm near-infrared light into ultraviolet–visible light, were introduced into the ink matrix consisting of mercapto-functionalized PDMS and vinyl-functionalized PDMS to construct a NIR-responsive curing system. Benefiting from the high penetration depth of NIR light, multilayer grid structures with line diameters ranging from 0.5 to 1.8 mm were successfully printed, and the support-free bridging molding of multicolor PDMS structures with a span of 1.25 mm as well as the fabrication of integrated multicolor grid structures with a line diameter of 0.5 mm were achieved. Furthermore, ionic liquids were incorporated into the photocurable PDMS matrix to endow the material with electrical conductivity, and piezoresistive sensors with high sensitivity (−0.062 kPa–1) and a wide pressure response range (0–60 kPa), as well as a customized 3D-printed finger sensor were successfully fabricated and operated. This strategy provides a feasible pathway for high-efficiency printing, multicolor integrated fabrication, and functional modification of PDMS materials and holds important reference value for the development and practical application of 3D printing technology in the field of flexible electronic sensing.