## 2 Materials and Methods
### Materials and Preparation of Films
Anycubic Basic resin cures under the same conditions and is made up of epoxy and acrylate monomers, along with a photoinitiator. After printing, millireactors were removed from the build platform, and the support structures were detached. Both the test plates and millireactors were then rinsed in isopropanol to eliminate excess resin. The channels of the millireactors were also rinsed with isopropanol and subsequently dried with an air compressor. The pH sensor films were prepared using the sol-gel method, resulting in organic-inorganic hybrid materials combining litmus and TiO\({}_{2}\) for enhanced optical and chemical stability. Tetraethoxysilane and phenyltrimethoxysilane were utilized as precursors, with ethanol serving as the solvent and hydrochloric acid as the reaction catalyst. Two distinct pH-sensing films were created based on different indicators. Prior to the preparation of the pH-sensing films, a litmus solution was prepared. The concentration of the prepared litmus solution was 5 mg/mL. The solution was prepared on a magnetic stirrer in a water bath by dissolving 250 mg of powdered litmus in 50 mL of distilled water. This process was conducted for one hour under continuous stirring at 200 rpm and a temperature of 80 \({}^{\circ}\)C. The preparation conditions were adapted from the literature [32]. After preparation, the solution was filtered and subsequently concentrated to half its initial volume by water evaporation under the same conditions as those used for the initial preparation. The pH sensor film was based on litmus and TiO\({}_{2}\). Quantities of precursors, solvent, indicator, and TiO\({}_{2}\) to increase the color intensity, as predetermined in preliminary experiments, were added to the glass vial, and the vial was placed in a water bath on a magnetic stirrer with stirring at 300 rpm and a temperature of 60 \({}^{\circ}\)C. At 60 \({}^{\circ}\)C, the catalyst was added (Table 1). The mixing/reaction lasted for 1 h and 30 min. \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **Quantity** \\ \hline TEOS & 1.7 mL \\ PTMS & 0.3 mL \\ Ethanol & 0.3 mL \\ HCl & 2.4 mL \\ Litmus solution & 1.6 mL \\ TiO\({}_{2}\) & 10 mg \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Quantities of components of the pH sensor film based on litmus. TiO\({}_{2}\) enhances brightness, opacity, and UV resistance through strong light scattering. Optimized particle size (\(\sim\)200-300 nm) maximizes reflectance in the visible spectrum, increasing opacity and pigment contrast, resulting in more intense and vivid colors [33]. The pH sensor films were applied to the test plates using the drop-coating method. The plates were immersed in glass bottles containing the film mixture for 10 s. Afterward, they were removed, and any excess film was removed mechanically. The coated plates were then left to dry for 7 days at room temperature. For the milliractor channels, the film was applied using a syringe. The film solution was injected into the channels until the entire volume was filled, left in the channels for 10 s, and then blown out with an air compressor to ensure that the film layer was as thin and uniform as possible. The film was also allowed to dry for 7 days at room temperature.