Passive Noramlly Closed Micro Check-valves

Normally closed (NC) micro check-valve is used to stop liquid flow when liquid pressure difference across the membrane is lower than the designed cracking pressure. As shown in Figure 2-1, when the applied pressure P is lower than the designed cracking pressure, P c , the NC check-valve is closed, and vice versa. Generally the cracking pressure can be introduced electrically such as electromagnetic force, mechanically such as MEMS springs, or other physical approaches such as stiction force. Passive check-valves usually use mechanical approaches to reduce complexity and thus no controlling circuit is needed. To mechanically implement controllable cracking pressures onto the NC micro check-valves, many different types of pre-stress technique have been developed, such as cantilever type, diaphragm type, and bivalvular type, etc. [105]. These NC check-valves use deformation of covering materials geometrically designed to provide the desired controlling force based on the requirements of applications. The material selection would also depend on the application of the micro check-valves. For example, if high cracking pressure is necessary, materials with large Young's modulus are used to provide the required pre-stressed force. Moreover, if the device is for human body implantation use, the biocompatible material such as parylene-C is needed to