## 2 Materials and Methods  
### Materials and Preparation of Films  
Formlabs High Temp, an acrylate-based resin, and Anycubic Basic, a resin based on acrylate and epoxy, were utilized for the fabrication of test plates and millireactors. For film preparation, tetraethoxysilane (TEOS) (98%, Alpha Aesar, Ward Hill, MA, USA), phenyltrimethoxysilane (PTMS) (97%, Sigma Aldrich, St. Louis, MO, USA), ethanol (97%, GRAM-MOL, Zagreb, Croatia), hydrochloric acid (HCI) (0.1 M, BDH Prolabo, Bengaluru, India), litmus (98%, Acros Organics, Geel, Belgium), and titanium dioxide (trade name: AEROXIDE TiO\({}_{2}\) P 25, Evonik Industries, Essen, Germany) were used. Furthermore, the chemicals used for the preparation of solutions with varying pH values included hydrochloric acid (0.1 M, BDH Prolabo) and sodium hydroxide (0.1 M, Kemika, Zagreb, Croatia). Test plates and millireactors were designed in Autodesk Fusion with a focus on design for additive manufacturing, ensuring compatibility with SLA and DLP technologies for optimized performance and precision. To test the behavior of the films in solutions with different pH values, test plates with dimensions of 20 mm \(\times\) 10 mm \(\times\) 1.5 mm were designed. For contact angle measurement and FTIR analysis, test plates with dimensions of 50 mm \(\times\) 10 mm \(\times\) 1.5 mm were designed. Two types of millireactors were designed in the same program: one tubular (Figure 1) millireactor as a reference and one with static mixers (Figure 2). The channel diameters were 2.00 mm, and the inlet and outlet diameters were 1.75 mm. The diameters of the inlet and outlet was set in such a way as to correspond to the diameter of the Teflon tubes used for the supply of liquids to the millireactor. After designing, all models were saved in STL file format. The prepared STL files for the designed millireactors were imported into the Preform slicer from Formlabs and the Chitubox slicer. Preform is primarily designed for the Formlabs Form 2 3D printer, which uses stereolithography (SLA). In contrast, Chitubox is compatible with the Anycubic Photon M3 printer, which uses digital light processing (DLP) technology. When configuring the print settings for millireactors in the Preform slicer, a layer thickness of 0.050 mm was set. Such process enhancements improve resolution and  
Figure 1: (**a**) Design of a tubular millireactor, (**b**) sketch of the tubular millireactor with dimensions in millimeters. Figure 2: (**a**) Design of a millireactor with static mixers, (**b**) sketch of the millireactor with static mixers with dimensions in millimeters. reduce residual resin, ensuring high-quality additive manufacturing. The millireactors were oriented at a 45\({}^{\circ}\) inclination along the X and Y axes and at a 0\({}^{\circ}\) angle along the Z axis to reduce the residual resin within the channels, which could lead to printing issues or clogging. Notably, the millireactor with static mixers was oriented at a 90\({}^{\circ}\) angle along the Z axis due to previous blockages in the channels in other orientations. Support structures were utilized during the printing of the models, excluding internal supports, with a density of 1.45 and a contact point size of 0.60 mm. The same orientation and support configurations were applied for the millireactor models in the Chittobox slicer, where the support density was set to 80.00% with a 90\({}^{\circ}\) angle, cross width of 2.00 mm, and a starting height of 3.00 mm, also excluding internal support structures. In the preliminary experiments, the sensor films composed of litmus and litmus with TiO\({}_{2}\) were investigated to develop an effective pH-sensing mechanism. For the printing of millireactors on the Form 2 3D printer, High Temp V2 resin was used, while Anycubic Basic resin was utilized on the Anycubic Photon M3 printer. High Temp resin, composed of acrylate monomers, oligomers, and a photoinitiator, is initially in a liquid state and cures under the influence of light with a wavelength of 405 nm in 3D printers.