Introduction
A number of different micropumps have been developed over the last decades. Among the various kinds of working principles, the micropump using membrane actuation is known as a promising approach. Therefore, a lot of membrane micropumps have been studied with various actuators. The valve is another issue for the development of micropumps. Since a piezoelectric micropump with check valves was first presented by Van Lintel et al. , passive or active check valves have been widely used for micropumps. The micropump with check valves can prevent unintended backflow while the pump is turned off. Moreover, the pumps with check valves have self-priming and bubble- tolerance characteristics and they can be used for both liquid and gas, if the volume stroke ratio is large enough. But the pressure drop at the valve is relatively large and the valves are very sensitive to the bubbles or small particles. Therefore, valveless diffuser micropump has been presented to overcome these problems and theoretical analyses were also presented. Since the diffuser and nozzle has no moving part, most of the problems caused by the valves are eliminated. The pump stroke efficiency of the diffuser micropump is about 19?25%, which is the ratio of the pumped volume to the total stroke volume .

A bidirectional micropump has been a new challenge to broaden the application range of the micro fluidic devices. The bidirectional characteristic has been investigated with the micropump with nozzle and diffuser, the peristaltic micropump and the conventional micropump with check valves. The peristaltic micropump can generate forward and backward flows by changing working sequence of three actuators. However, the peristaltic micropump needs at least three pumping chambers and actuators, which are not suitable for miniaturization.

In this seminars, a new valveless micropump for bidirectional use has been presented, the designing ,fabrication and testing also described. Since there are no valves, this pump is expected to be free from wear, fatigue and clogging problems. Moreover, this pump can be made in a simple design, which is helpful for miniaturization. A piezoelectric ceramic plate (PZT) and silicon membrane were used for the actuator, and the membrane and channels were fabricated by micromachining process. The performance characteristics were investigated by the experiments in order to find out the optimum geometry and operating conditions