Microvalves play a crucial role in manipulating fluid states within a microfluidic system and are finding widespread applications in fields such as biology, medicine, and environmental preservation. Leveraging the characteristics and features of microvalves enables the realization of various complicated microfluidic functions. Continuous advancement in the manufacturing process contributes to more flexible control modes for passive microvalves. As a consequence, these valves are progressively shrinking in size while simultaneously improving in precision and stability. Although active microvalves have the benefits of low leakage, rapid response time, and wide adaptability range, the energy supply system limits the size and even their applicability in integration and miniaturization. In comparison, passive microvalves have the advantage of relying solely on the fluid flow or fluid driving pressure to control the open/close of fluid flow over active microvalves, in spite of having slightly reduced control accuracy. Their self-sustaining feature is highly consistent with the need for assembly and miniaturization in the point-of-care testing technology. Hence, these valves have attracted significant interest for research and application purposes. This review focuses on the recent literature on passive microvalves and details existing passive microvalves from three different aspects: operating principle, processing method, and applications. This work aims to increase the visibility of passive microvalves among researchers and enhance their comprehension by classifying them according to the aforementioned three aspects, facilitating the practical applications and further developments of passive microvalves. Additionally, this paper is expected to serve as a comprehensive and systematic reference for interdisciplinary researchers that intend to design related microfluidic systems.
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