3D Printing of silicone layer

A customized print head system was used for the direct printing of the silicone layer. This system takes advantage of a pneumatic pump (up to 90 psi) with switchable solenoid valves for applying positive pressure to control the flow of the ink through the print head. The speed in x and y directions was adjusted at 500 mm/min, while a commercial extrusion nozzle (25-gauge needle) was connected to the ink reservoir with a blunt tip diameter of 215 μm.

The ink consists of the cross-linkers and receptors indicated in the materials section, combined with TPO-L as a photo-initiator. This silicone layer was printed on top of the PEDOT: PSS and used as the substrate of the snow-TENG. Exposure of this layer to UV light leads to polymer crosslinking and eventually results in a mechanically strong, yet flexible and stretchable, silicone layer. This was achieved using a controllable Omnicure S1000 (Lumen Dynamics) light source that was positioned at a 45° angle to the surface, 4 cm from the extrusion nozzle, producing a light intensity of 2.3 W cm⁻². This light source has a broad UV wavelength spectrum in the range of 275–500 nm, and strong intensity of 100 Watts. The printing setup utilized two of these light sources, which has the potential for shortening the cure time and increasing the production efficiency.