## 2 Materials and Methods
### UV-Assisted Coaxial 3D Printing System  
A UV-assisted coaxial 3D printing system is shown in Figure 1a. A commercial dispenser (PACHDA, Shenzhen, China) served for the 3D movement stage with an \(x\)-\(y\) axis moving plate and one robot arm at the \(z\)-axis. The extruder part of the original dispenser was replaced by an assembled coaxial nozzle with two syringe drivers. The assembled coaxial nozzle consisted of a 28 G (inner diameter, ID, 0.17 mm; outer diameter, OD, 0.35 mm) inner steel needle for water, an 18 G (ID 0.91 mm, OD 1.26 mm) outer steel needle for UV adhesive and other accessories. A transparent jacket \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Solutions**} & **Mechanical Property** & **Interconnect Density** & **Clogging Issue** & **Target Material** & **Fabrication Efficiency** & **Cost** \\ \hline DRIE interlock structures [7; 8] & Poor & High & No & Silicon & Low & High \\ \hline Tubings or needles & & & & & & \\ through punched & Fair & High & No & PDMS & High & Low \\ holes [11; 12] & & & & & & \\ \hline Ultrasonic welding [16; 17] & Good & Moderate & Yes & Thermoplastic & High & Moderate \\ \hline Adhesive gluing [18; 19] & Good & High & Yes & All & High & Low \\ \hline Fixing accessories [20; 21; 22; 23; 24; 25; 26] & Good & \begin{tabular}{c} Moderate \\ /Low \\ \end{tabular} & No & All & \
\begin{tabular}{c} Moderate \\ /Low \\ \end{tabular} & Moderate \\ \hline Monolithic 3D printed & Fair & Moderate & No &
\begin{tabular}{c} Photosensitive resin \\ /thermoplastic \\ \end{tabular} & High & Moderate \\ \hline This work & Good & Moderate & No & All & High & Moderate \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of microfluidic interface solutions. (ID 1 mm, OD 2 mm) with a length of 25 mm was attached to the outer needle for the pre-curing of materials. Figure 1b represents the bottom view of the assembled coaxial nozzle for clarification. Fluids were fed through syringes to the coaxial nozzle by Luer tapers. Syringe drivers with stepper motor could quantitatively control the flow rates of both fluids. Moreover, a converged UV-LED light source attached to the coaxial nozzle was designed for UV curing. Two UV LEDs with a power of 3 W at 365 nm were placed on each side for more uniform radiation. With the assistance of the transparent jacket attached to the coaxial nozzle, UV adhesive was able to be pre-cured before its deposition to stabilize the structure. During the printing process, the inner flow (water) was selected as the sacrificial layer, while the outer flow (UV adhesive) was cured for shell formation under UV radiation. As the coaxial nozzle moved upwards, vertical hollow connectors could be directly printed on the substrate with matching holes. Finally, the printed connectors on the substrate was illuminated by a UV mercury lamp with high power to fully cure. According to preliminary experiments conducted, a UV light source with the requirements of a large radiation area with uniform flux was suitable for the direct hollow connector printing process. Thus, a converged UV-LED light source was designed. Figure 2a schematically describes its design. UV light emitted from the chips at 365 nm on each side could be converged through packaged aluminum reflectors and glass lenses. Dimensions of the converged UV LED were clarified in Figure 2b. The UV-LED chip of purchased products (UV Z5 series, Seoul Viosys, Korea) had an irradiation area of 1.4 \(\times\) 1.4 mm\({}^{2}\) with the maximum radiant flux of 1 W. The chip was encapsulated by silicone resin with a hemispherical shape (radius, 1.45 mm). For electrical connection, the chip was bonded to an aluminum substrate (thickness, 1 mm) with solder pads. The aluminum reflector was in a cone shape (\(\mathcal{O}_{1}\) 2.9 and \(\mathcal{O}_{2}\) 8 mm) with a height of 7.9 mm. The diameter and height of the glass lens were 15 and 6.5 mm, respectively. A cylindrical plastic frame (\(\mathcal{O}_{3}\) 15 mm, height of 9 mm, and thickness of 3 mm) was used for packaging the converged UV-LED.