## 3 Results and Discussion  
### UV Rheological Characterization  
To characterize the solidification behaviors of the adhesive under UV exposure, rheological measurements were evaluated by a rheometer (Discovery HR-2, TA Instruments, New Castle, PA, USA) equipped with UV curing accessories. Omnicure S2000 model (Excelitas Technologies Corp., Waltham, MA, USA) was used for UV light source with a wavelength of 365 nm. A plate-to-plate geometry with a diameter of 20 mm was selected to place the liquid sample in the gap between two plates. The gap was set to 800 \(\upmu\)m in all measurements. With torques applied, the rheological properties of samples could be measured. The initial viscosity of LOCTITE 3491 was measured under variable shear rates. It was observed that LOCTITE 3491 can be categorized into the Newtonian liquid with shear rate independent behavior. The value of the initial viscosity was 0.95 Pa-s. The initial storage modulus and loss modulus were 1.50 and 9.70 Pa, respectively. Thus, in all rheological measurements, the angular frequency and strain were set to 10 rad/s and 1% constantly.  
Figure 4a describes the UV rheological results of material viscosity under UV exposure of 1 W/cm\({}^{2}\) at 365 nm. As UV exposure time increased, the material was cured rapidly whereas the viscosity grew sharply until it reached several orders larger than the initial value after 15 s. Figure 4b illustrates the moduli variation under the same UV exposure. In rheology, shear modulus is used to describe the viscoelastic behavior of samples, in which the storage modulus G' represents the elastic  
Figure 3: Fabrication process of a typical PDMS–Glass microfluidic device with printed connectors.  
portion (solid-state behavior) and the loss modulus G\" characterizes the viscous portion (liquid-state behavior) [34]. In the modulus measurements, initially, G' was lower than G\", indicating the liquid-like behavior. As UV irradiation continued, G' and G\" were both overgrowing. G' crossed over G\" and became larger than G\", representing the solid-state behavior. The gelation point is defined as the cross over point of the storage modulus and loss modulus where the material is transferred to a gelation-like structure. Furthermore, gelation time (GT) is defined as the time material reaches its gelation point.  
To further characterize the effect of UV intensity (I) on rheological results, the UV curable adhesive was exposed to different UV irradiation with varying intensities. Generally speaking, for all intensities investigated, they showed similarities in the material solution/gelation transition. However, higher UV intensity represented the tendency to accelerate the UV curing reaction. Table 3 summarizes the value of gelation time under different UV intensity from 0.5 to 5.0 W/cm\({}^{2}\). As shown in Figure 5, the relationship between the gelation time and UV intensity was data fitted (\(GT=7.9983\times I^{-0.322}\)) and showed a power-law dependence, which is consistent with previous works [34; 35]. Moreover, a sensitivity analysis on the effect of the data scattering on curve fitting has been performed to find the upper/lower bound for gelation time predication with an assumption of \(\pm 15\%\) in data scattering. Gelation time could be employed to predicate the cross-linking status of materials in UV-assisted coaxial 3D printing. According to Tracepro ray-tracing, the UV intensity for the pre-curing of UV adhesives is simulated to an average of 1.45 W/cm\({}^{2}\). Based on the fitted relationship between gelation time and UV intensity (GT\(\sim\)\(I^{-0.322}\)), the calculated gelation time for 1.45 W/cm\({}^{2}\) intensity is 7 s. Furthermore, with the assumption of \(\pm 15\%\) in data scattering, the upper/lower bound of the estimated gelation time for 1.45 W/cm\({}^{2}\) intensity is 7 s \(\pm\) 1 s.