## 3 Results and Discussion
### Fabrication of Large-Scale Microfluidic Channels in PG  
In this section, we report on the possibility to generate complex structures on PG surfaces for fabricating open, large-scale area microfluidic devices with central cell culture chambers (Figure 3a,b). Final microfluidic chip was composed of two parts: (1) an open structure fabricated on PG surface by VIS-PLAE and (2) a PDMS sheet that sealed the fabricated structures on PG using the bounding process by activating their surfaces in oxygen plasma. The fabricated structure on PG consisted of cross-shaped microfluidic channels with microreservoirs at each end and a microchannel at the crossed region. In our design, the widths of all microfluidic channels were set to 0.3 mm while the central microchannel was \(2\times 2\) mm\({}^{2}\) (Figure 3a, corresponding to a laser-irradiated sample followed by annealing treatment). It is worth to mention that the entire laser scanning process took 25 min only, at 1-mm/s scanning speed and 10-\(\upmu\)m pitch between each scanning line. The open microstructures resulted from the selective glass removal induced by chemical etching are shown in Figure 3b. Magnified images of the microchannel parts from Figure 3a,b are presented in Figure 3d,e, respectively. Due to the limited etching selectivity in HF between the laser-irradiated and nonirradiated regions, the actual lateral dimensions of obtained structures were larger than the initially set ones, i.e., the laser-irradiated regions. The widening depended on the etching time due to the etching rate of 1-\(\upmu\)m/minute for the nonirradiated regions. Similarly, the depth of the channel can be controlled by the etching time. For example, one can get a 10-\(\upmu\)m-deep channel in only 1 min or a 300-\(\upmu\)m-deep channel in half an hour by taking account of the depth of modified region and the etching selectivity between nonirradiated and irradiated regions. Even though the roughness of the channel surface was significant after the etching, the second annealing treatment can well work to reduce the roughness to nanometer scale [25]. In our design, three of the microreservoirs acted as lateral inlets and the other one, as lateral outlet, while the microchamber was used for cell culture (Figure 3c,f).  
A perfusion system (Elveflow, France) was coupled to the fabricated chip using dedicated metallic connectors glued with the lateral inlets and outlets by SU-8 photoresist. The setup consisted of several elements: a pulsation-free pressure controller, flow sensors, tubing, and reservoirs dedicated to cell cultures. A detailed image of the microfluidic chip during flow monitoring is presented in Figure 3f. Thus, a 3D biomimetic microenvironment could be created in the central chamber for cell growth and visualization, which can be associated with controlled flow aspects allowing a scale up-scale down approach of the device. Indeed, the inlets were easily connected to the perfusing system while channel dimensions could be downsized to hundreds of microns (Figure 3d,e) or smaller, for specific cases. In addition, independent control can be achieved for inlet channels from 1 to 3 to mix any combination of fluids that also allows perfusion switches between different culture media or even drugs by a computer-controlled valve with sequential injection (Figure 3c,f). In a preliminary experiment we combined complete cell media (with serum), incomplete cell media (without serum), and cell media supplemented with growth factors by serial perfusing through the three inlets intermittently and monitored the response of mesenchymal stem cells (MSCs) over time.