Microfluidic Devices

Microfluidics is a sub division of MEMS which consists of devices designed for fluid flow. An important application for such devices is in “miniaturised total chemical analysis systems” (µTAS) or “lab-on-a-chip” devices. Such devices incorporate a variety of integrated fluidic components, including valves, pumps and nozzles, which can be fabricated together on a single substrate. The principal components are described below:

Microfluidic Components

Valves: Micro-valves generally rely on a diaphragm or flap opening and closing against a duct face, using piezoelectric (such as that demonstrated by Koch et al. [45, 47]), electromagnetic, thermopneumatic or other actuation methods compatible with the micron scale of the device. Al- though not preventing negative fluid flow, diffuser/nozzle ducts can be used to encourage positive fluid flow with the advantage of having no moving parts and zero power consumption, relying on the different fluid losses incurred when orientated as a diffuser or nozzle; i.e. for the same pressure drop, the nozzle will permit a greater flow rate than orientated as a diffuser. Figure 2.8 illustrates a pair of such ducts within a pump system, rectifying the flow [28].

Pumps: There are two principal mechanical methods employed to pump liquid through micro- channels, namely actuation of a plug along a channel and actuation of a diaphragm [48]. The former method uses either a liquid metal or bubble plug actuated by electrodes and an array of heaters, respectively. In the case of the bubble plug the heaters also serve to form the bubble. With

the diaphragm method, the diaphragm is actuated by a piezoelectric layer, the motion of which causes variations in the internal volume of the device and therefore fluid displacement. A pair of one-way valves ensures positive fluid displacement; these are either etched flap valves or diffuser ducts (see figure 2.8). Another method to ensure positive fluid displacement demonstrated by Husband et al. [49] is to use a series of diaphragms which operate peristaltically. Non-mechanical pumps also exists for certain fluids, for example, taking advantage of electrostatic forces [50].

Injectors: Injectors are pumps which deliver droplets using the growth of a bubble to isolate a discrete volume of liquid and force it through a nozzle [51]. Again, a heater is used to control the formation of the bubble.

Flow sensors: An important component in Microfluidic systems is a flow sensor. Examples of how the devices operate include measuring the transit time of a heat pulse and sensing the pressure drop across a restriction [51]. Nguyen presents a good introduction to the variety of sensing techniques available [52].

Mixers: In Microfluidic applications laminar flow is dominant; therefore mixing, for the pur- poses of say drug delivery and chemical tests, generally relies on diffusion processes [53, 54]. However, micromixing has been realised on a microscale using turbulent flow induced by the use of jets, increasing the efficiency of mixing [43].

Reaction Chambers: Reaction chambers realised on a microfluidic scale have applications in polymerase chain reaction (PCR) DNA analysis systems. In such systems, the microfluidic chip will include microchannels and heaters which facilitate controlled reactions between DNA sam- ples and reagents. DNA analysis relies on multiple reactions, therefore a chip incorporating mul- tiple reaction chambers and complete fluid handling, e.g. pumps, is desirable [55, 56].

Silicon Micro Turbines: Although unlikely to be incorporated into aµTAS device, micro-scaled gas turbine engines have been created, and are an interesting concept. These can produce 10-50W of electrical power and have rotation speeds of ∼1.2x106rpm, such examples developed by the Massachusetts Institute of Technology [57, 58]. As for conventional turbines, micromachined turbines include bearings, compressor, turbine and combustion chamber elements and potentially have applications in powering portable equipment and in space vehicle propulsion systems.

Applications of Microfluidic Devices: Injectors are found in commercial inkjet printer heads and have been considered for use in biomedical and chemical handling for the accurate dispensing of substances [20]. However, applications for microfluidic devices become far more diverse when a selection of devices can be integrated, so that a fluid sample can be prepared and processed by a single device (µTAS). For example, medical applications, including drug delivery systems, may incorporate micropumps, valves, micromixers and sensors on a single chip. This is also the aim with DNA analysis systems [55, 56], chemical and biochemical detectors, where processing of clinical samples or bacterial agents through a micromachined device may be achieved quickly and require only small samples. An introduction toµTAS devices is given in the following subsection.

Micro Total Analysis Systems (µTAS)

With the development of microfluidic components, analysis systems composed of a series of such component devices fabricated on a single chip becomes feasible. Manz et al. [59] gave an early review of such a concept, miniaturising total analysis systems (TAS) in order to improve the perfor- mance of the process, for example, in environmental monitoring. They explain that the reduction of length scales reduces the transport times, the consumption of liquid samples and reagent, and also increases the speed of sample processing.

More recent reviews [60, 61] clearly associateµTAS with micro-fabricated microfluidic devices. Auroux et al. [61] describe how various microfluidic processing stages such as injectors and mixers have been applied to sample processing, especially in biochemical analysis, referring to a vast number of publications. For example, the analysis of DNA within microfluidic devices has been investigated by Bu et al. [56] describing the development of a chip for DNA amplification, a device which is portable and provides rapid analysis when compared to the conventional macro- scale equivalent. This particular device incorporates a micropump, reaction chambers and various sensors, demonstrating their integration into the single chip.

Reyes et al. [60] discuss the development of micro-fabricated devices with reference toµTAS

and also how the integration of various devices or processing stages may be achieved, and several types of fluid interconnections are mentioned, including the use of capillary tubing between micro- fabricated system components. Schabmueller [62] describes how wafers may be stacked in order to incorporate multiple fluid processing and operation stages, and this was demonstrated with the fabrication of a chemical reaction system incorporating pump, mixing and sensor components on a single chip. This avoids additional fabrication steps which would otherwise be required to

fabricate individual devices and to form fluid connections between these devices. It also avoids introducing excessive dead volume as the entire fluid network consists of etched micro-channels.

Certainly, the growing interest inµTAS fuels the development of microfluidic devices and research into microfluidic flow, the development of individual microfluidic components providing more possibilities and solutions forµTAS devices.