<p> Electronic supporting information</p><p>Direct, one-step molding of 3D printed structures for convenient fabrication of truly 3D PDMS microfluidic chips</p><p>Ho Nam Chan, Yangfan Chen, Yiwei Shu, Yin Chen, Qian Tian and Hongkai Wu*</p><p>Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China</p><p>*Corresponding author E-mail: [email protected] Tel: +852-2358-7246</p><p>Supporting video 1. Demonstration of the peeling process. In this video, a PDMS was peeled off against a 3D printed chip (basket-weaving configuration with design 2) diagonally with bare hands. As shown in the video, 36 crossover features are replicated directly in a single step and no PDMS was torn off from the bulk to the 3D printed master.</p><p>Supporting video 2. Operating video of peristaltic valve. 1 µm diameter fluorescent microbeads solution was flowed through the channel continuously with a rate of 0.4µL/min. 0.5 Hz pressure pulse (140kPa) was sent to the valve repeatedly. When the pressure is applied, the microbeads stop in position. The exposure time of each frame is 300 ms.</p><p>Supporting video 3. Pulse injection video demonstrated by the “Injection on- demand” 3D microfluidic device. A 5.8 kPa gas pressure pulse that last for 0.04 s was sent to the blue channel. The bottom channel was flowed with water with a rate of 4µL/min by a syringe pump. Pressure driven parabolic flow pattern was observed after the injection of blue dye solution. Supporting Fig. 1 (a) The cross-section image of rectangular crossover features (red dashed square) peeled with perpendicular direction. The red arrows depict the cracks generated during the peeling process. The crack starts at the edge of the rectangular features. (b) The cross-section image of rectangular crossover features peeled in parallel direction. Noted that 2 cracks (red arrow) were generated simultaneously during the peeling process, thus, a piece of PDMS was torn off from the bulk body. (c) The top view image of the 3D printed master with a piece of PDMS stuck under the crossover feature. (d) The bottom view image of a peeled PDMS body with a torn off failure.</p>