9.5 Microdispensers

Microdispensers are important devices for chemical and biomedical analysis. The precise amount of liquid is often required for dosing and analysis applications. Microdispensers can be seen as microdosing systems, microinjectors, or micropipettes. In general, microdispensers are categorized as close-loop controlled dispensers or open-loop controlled dispensers.

Closed-loop controlled dispensers are complex systems with a fluidic actuator and a flow sensor. The fluidic actuator is a micropump or microvalve. Chapter 6 and Chapter 7 discuss these components in detail. In a closed-loop controlled dispenser, the flow rate signal is fed back to a controller where it is compared with the setpoint. The control signal from the controller is connected to the actuator of the pump or the valve to adjust the set-flow rate [106-109]. This type of dispenser is suitable for continuous flow systems. For a fixed amount of fluid, errors can occur in the starting and stopping phases of the system.

Open-loop controlled dispensers are single devices which can be divided into droplet dispensers and in-channel dispensers. The first type generates external droplets with a constant volume, while the second type prepares the fixed volume in the channel for further analysis on the same chip. Each can be categorized by the actuating principle. Common actuating schemes for microdispensers are thermopneumatic (thermal bubble) [110 113], thermomechanic [114], electrostatic [115, 116], piezoelectric [117 125], pneumatic [126, 127], electrochemical [127 129], and electrokinetic. Droplet dispensers have the advantage of massive parallel dosing, which is common in biochemical application. In-channel dispensers are simple and...