## 4 Conclusions  
Test plates and millireactors with 2 mm channel diameters, both with and without static mixers, were successfully fabricated using Anycubic Basic and High Temp resins via DLP and SLA technologies. A pH-sensitive film based on litmus and TiO\({}_{2}\) was prepared using the sol-gel method and applied to the test plates, demonstrating uniform surface coverage and consistent color changes in response to solutions with varying pH values. Contact angle measurements confirmed that the test plates, with and without the pH sensor film, were hydrophilic, a property that facilitates ion diffusion and enhances film performance. This study showed the integration of pH-sensitive films into 3D printed millireactors, underscoring their potential for application in continuous industrial processes, lab-on-chip systems, and advanced diagnostics. The pH-sensitive film remained stable throughout the tests, maintaining its responsiveness and suggesting its potential for long-term use in continuous pH monitoring applications. Sensor films exhibited excellent stability and reversibility, calculated as RSD = 2.4-3.3%. The findings also highlight the advantages of combining innovative sensor films with advanced manufacturing techniques for the development of efficient and durable chemical sensing platforms. They also demonstrate that the pH-sensitive films maintain consistent performance on complex geometries, such as static mixers.  
This study demonstrates that the stable pH-sensitive film enables the continuous monitoring of pH throughout the entire reactor volume, providing valuable insights into the effectiveness of static mixers. By integrating this optical sensing method, one can assess the degree of mixing within the reactor in real time, ensuring more uniform reactant distribution. Improved mixing not only enhances reaction control but also contributes to process stability, reproducibility, and efficiency. The ability to visualize and quantify pH variations across different regions of the reactor confirms that static mixers play a key role in optimizing reaction conditions. This approach presents a significant step forward in refining the design and functionality of millireactors, particularly in applications requiring precise control over chemical reactions and mixing dynamics.  
M.-P.M.: Conceptualization, Data Curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing--original draft, Writing--review and editing. E.F.: Formal analysis, Methodology, Validation, Visualization, Writing--original draft, Writing--review and editing. P.K.: Methodology, Validation, Visualization, Writing--review and editing, A.N.: Data curation, Formal analysis, Investigation, Validation. D.V.: Conceptualization, Funding acquisition, Methodology, Supervision, Validation, Visualization, Writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Croatian Science Foundation under the projects HRZZ-DOK-NPOO-2023-10-1144, HRZZ-DOK-2021-02-5999, and HRZZ-IP-2022-10-8004.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.