## 4 Conclusions  
This work provides a detailed investigation of NO microvalve development for medical applications, where many intricate requirements in flow performance, biocompatibility, hermetic sealing, and device reliability call for sophisticated solutions. We introduce a piezoelectric microvalve with multiple trenches serving as a valve seat and omit any movable soft sealing components in the valve chamber as an important step towards the reliable operation of the microfluidic device. As alternatives for a soft sealing valve seat, we discuss different measures for optimized fluidic performance of the microvalve in closed state, provide insight into important influencing parameters of leakage rates, and propose three design variants of NO valves for fabrication and experimental characterization. Moreover, we derive an analytical model of the leakage rates and perform a dedicated FEA parameter study of the geometric dimensions of the trench design, enabling further NO valve optimization with a view to sealing performance as well as fluidic resistance. The fabricated and tested microvalve variants with concentric trenches exhibit good leakage rates in closed state as low as (19.8 \(\pm\) 4.9) \(\upmu\)L/min. This is comparable to former investigations of NO valves with soft sealing components but with better controllability of geometric parameters, hence, a low sample-to-sample variation, which is substantiated by small standard deviations of the measured leakage. We show that flow rates in open state are as high as (30.1 \(\pm\) 3.4) mL/min. Therefore, the presented sealing concept does not impede fluid flow, and malfunctions of the microvalve in open state are successfully avoided by the omittance of any additional movable components in the valve chamber. Fatigue testing of the microvalves reveals beneficial effects of Parylene-C coating or increased piezoelectric actuator force on leakage rate stability and indicates increased piezoceramic material stability with increased thickness. Due to its simplistic design, energy-efficient piezoelectric actuation, and beneficial material properties, the microvalve with multiple valve seat trenches lays a valuable basis for microfluidic medical device development. The comprehensive investigation of microvalve design provided in this work enables well-founded further development of the microfluidic device. Potential future work on the development of microvalves for medical applications may include optimization of the piezoceramic disk actuator design for increased contact pressure, extended fatigue testing, as well as fluidic characterizations at human body temperature. Conceptualization, C.P.D. and G.S. ; methodology, C.P.D. ; software, W.H., P.K.M. and C.P.D. ; validation, C.P.D., S.T.G., P.K.M., W.H. and G.S. ; formal analysis, C.P.D. and W.H. ; investigation, C.P.D. and S.T.G. ; resources, C.P.D., S.T.G. and W.H. ; data curation, C.P.D., S.T.G. and W.H. ; writing--original draft preparation, C.P.D., P.K.M. and W.H. ; writing--review and editing, C.P.D. and G.S. ; visualization, C.P.D. and W.H. ; supervision, G.S. ; project administration, C.P.D. All authors have read and agreed to the published version of the manuscript. This research was funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy, within the Bavarian funding program for research and development \"Electronic Systems\" under the grant number ESB071/002. The experimental, analytical modeling, and simulation data that support the findings of this study are available in Fordatis--ResearchData Repository of Fraunhofer-Gesellschaft with the identifier ([[https://fordatis.fraunhofer.de/handle/fordatis/218](https://fordatis.fraunhofer.de/handle/fordatis/218)]([https://fordatis.fraunhofer.de/handle/fordatis/218](https://fordatis.fraunhofer.de/handle/fordatis/218)) (accessed on 4 August 2021)). **Acknowledgments:** The authors would like to thank [PERSON] for funding acquisition as well as supervision of the research activity and [PERSON] for assisting in our laboratory work.