## 4 Conclusions  
Microfluidic interface with external tubing is significant for microfluidics developments. In this study, an alternative approach for microfluidic connectors direct printing was proposed using a UV-assisted coaxial 3D printing system. An assembled coaxial nozzle was designed for the co-flow extrusion where the water served as the sacrificial layer and the UV adhesive was utilized for shell formation. A converged UV-LED light source was designed and simulated with Tracepro ray tracing. The estimated UV intensity for the pre-curing of adhesives was averaged to 1.45 W/cm\({}^{2}\) with a length of 6 mm. To master the process, UV rheological characterizations were performed to investigate the UV curing kinematics. Gelation time was found to describe the material state transition. Moreover, the relationship between gelation time and UV intensity was investigated and showed a power-law dependence (GT\(\sim I^{-0.322}\)). Thus, gelation time was selected to predicate cross-linking status during the printing process. To explore requirements for hollow connectors successful printing, processing criteria including co-flow criterion and pre-curing criterion were parametrically studied. It was found  
Figure 9: Demonstration of printed connectors for microfluidic interconnections: overview (**a**); top view (**b**); and side view (**c**) of the typical PDMS–Glass microfluidic device with printed hollow connectors.  
that thread jetting co-flow behavior regime and pre-cure time greater or equal to 6 s were the necessary requirements for hollow connectors direct printing. To further demonstrate the integration feasibility, printed hollow connectors were successfully assembled to a typical PDMS-Glass microfluidic device. The printed connector had a height of 5 mm with an inner channel diameter of 150 \(\upmu\)m. Directly 3D-printed connectors can provide a general interface solution to an existing microfluidic device. It presents the advantages of rapid prototyping, material-compatible, fixture-free, easy to plug/unplug, leak-free, and non-clogging. Future studies may exercise the printing technology developed on different substrate materials for their intended test configurations.  
Conceptualization, Q.X., J.C.C.L., and S.-W.R.L.; Data curation, Q.X.; Formal analysis, Q.X., J.C.C.L., and S.-W.R.L.; Investigation, Q.X.; Methodology, Q.X.; Project administration, J.C.C.L.; Supervision, S.-W.R.L.; Writing--original draft, Q.X.; and Writing--review and editing. J.C.C.L. and S.-W.R.L. All authors have read and agreed to the published version of the manuscript. This research was financially supported by the Innovation and Technology Commission (Grant No. ITS/006/18) from the Government of the Hong Kong Special Administrative Region. This study was also supported by the HKUST Special Fund of Foshan (Grant No. FSUST19-FYTRI11). The authors are grateful to the Nanosystem Fabrication Facility (NFF) and Yao Laboratory at HKUST for the assistance of microfluidics fabrication. Additionally, the authors would also like to extend their gratitude to the Materials Characterization and Preparation Facility (MCPF) at HKUST and HKUST LED-FPD Technology R&D Center at Foshan for material characterizations. The authors declare no conflict of interest.