Microfluidics Devices and Systems

Miniaturisation of biomedical and environmental analysis systems offers many advantages including; low sample volumes, low chemical consumption, fast response time, multiple simultaneous assays, and portability. The greatly reduced volume of chemicals, typically in the microlitre and nanolitre ranges, and the ability to simultaneously perform multiple different assays leads to a reduced cost per assay. Portability of the overall system allows the analysis to be performed close to the sample source; for example, at the patient’s bedside or on the riverbank. The most significant advantages would be gained where the total analysis, (sample preparation, pre-treatment, analytical reactions, detection, and results) are combined in a single device or chip. The complete integrated form of such systems is commonly known as a laboratory-on-a-chip (LOC) or a micro-total-analysis-system (µTAS). Microfluidic components such as pumps, valves, sensors, active filters, dispensers and mixers form important parts for such microsystems.

The medical and biomedical markets show strong promise for MEMS solutions in a wide variety of applications. Some examples of BIOMEMS applications are listed in Table 1. These devices are used as life saving diagnostic sensors, accelerating drug discovery and improving drug delivery.

A MEMS micro-total-analysis-system (µTAS) can also be extended into the field of environmental monitoring applications in the area of sensors for detection of air and water pollution. Examples of such instruments are mass spectrometers, micro gas chromatography systems, electrochemical and optical gas sensors and ion mobility spectrometer.

Microvalves

Microvalves are a  key component for fluid flow control in many microfluidic systems; e.g. chemical microanalyser or drug delivery system. These valves can be fabricated as stand alone devices in flow channels or integrated with other microfluidic components, such as micropumps or micromechanical sensors. The microvalves developed to date can be classified in two categories; passive check valves and active valves driven with an actuator.

Passive valves (without actuators) are mainly used as check valve components in micropump and microfluidic systems. In micropump applications, the integrated valves minimise leakage under reverse pressure and have a high forward to reverse flow ratio. These valves can be fabricated by bulk etching of silicon, mirolamination of thin metals and surface machining of polysilicon or polymer materials, such as polyimide, parylene or silicone rubber. 

Research at the centre focused on the development of two models of passive microvalves based on silicon micromachining. Fluid flows through the valves is in the opposite direction to each other and they are thus of considerable potential in integrated microfluidic devices or systems. In micropump applications, the valves form the inlet and outlet components and can be fabricated on the same surface with common process steps, removing the need for precision processing on both wafer surfaces, hence enabling further miniaturisation of the overall pump system. As individual components, the valves offer quality sealing and operate successfully with negligible reverse leakage. Fluid flows through the valves are comparable to developed models, and produce reverse flow rates as low as 0.5 % of the forward flow.

Micropumps

The incorporation of  two valves into and cavity that can be deformed using a piezoelectric material allows the formation of a simple pump.

A methanol pump rate of 100Hz:- 0.3ml/min was achieved

Electro-osmotic Pump

Fluid pumping in capillary electrophoresis is achieved by a phenomena known as Electro-osmotic flow (EOF). EOF is fluid motion induced by the interaction between the fluid charge at the wall of the channel and the external applied electric field. The rate of flow is proportional to applied voltage, pH and conductivity of the solution, and material of channel wall. It’s primary advantage over other pumping schemes is the fact that valves are not required to control fluid flow; the voltage magnitude and polarity are used to do this.

Click here to view  a video clip of liquid being electrically 'pumped' along a 100µm wide channel fabricated in the surface of silicon.

Chemotaxis systems

The work was carried out in collaboration with Biological Sciences at QUB. Pores of a precise dimension are micromachined into a silicon substrate, see figures below. The pore array is placed in a chemo-attractant solution, enhancing the cell transfer through the pore array.