Microvalves are one the most important microfluidic components. Besides pumps and flow sensors, active valves are critical components in microfluidic systems. Today, the industry requirements for microfluidic systems continue to force an evolution and a revolution in valve design because of the new effects in microscale. Smaller device size, higher pressures, biocompatibility, response, and, most importantly, the microtechnology are all contributing to the valve design in microscale. Since passive valves or check-valves are a part of micropumps, this chapter only deals with active microvalves for flow control.

With the conventional design, an active valve is a pressure-containing mechanical device used to shut off or otherwise modify the flow of a fluid that passes through it. The working state of the valve is determined by a closure element the valve seat, which is driven by an actuator. This definition reveals that a valve is a very simple device. It has a body to contain the fluid and its pressure, a valve seat to manipulate the fluid, and an actuator to control the position of the valve seat.

There are many ways to categorize active microvalves. Based on their initial working state, there are three microvalve types: normally open, normally closed, and bistable. Bistable microvalves can actively open and close the valve seat; an off-state is not defined.

Similar to an electronic transistor, valves can control flow in two ways: analog and digital. In the analog, or proportional, mode, at a constant inlet pressure, the valve actuator varies...