By Dr Xianghong Ma
Microfluidics is a research area within MEMS (Micro-Electro-Mechanical Systems) and is concerned with the control of the flow of fluids measured in micro, nano, or even Pico, litre, quantities. The fluid can be liquid or gaseous in nature, or a mixture of both, and flows through microscale channels, pumps, valves, and filters.
These microfluidic devices can be fabricated in silicon or glass using photolithographic and etching techniques which have been adapted from the semiconductor industry, or from organic materials such as plastics and polymers [1,2].
Microfluidic devices require only a small amount of sample and reagent for processing and posses large surface to volume ratios. In addition, fast reaction times and ease of automation make microfluidic devices ideal for application in biomedical engineering scenarios.
Microfluidics have been widely used in the development of total analysis systems (or lab- on-chip devices) [2, 3], particularly for drug screening in the pharmaceutical industry and in the development of micro-arrays . The technology is rapidly maturing following vigorous research effort over the last 20 years. In the near future, we will see a growing trend towards the production of tailored microfluidic devices which satisfy particular needs, which may be clinical, pharmaceutical, or biotechnological.
One of the most promising applications of microfluidics in biomedical engineering is in point-of-care diagnosis. In the important sample preparation stage, targeted biological cells need to be separated from other substances in the sample. Conventionally, cells can be separated in a fluidic suspension, based on size, density, electrical charge, light-scattering properties, and antigenic surface properties. Separating cells according to these metrics can require complex technologies and specialist equipment. Such techniques include centrifuging, fluorescence activated cell sorting, electrophoresis, chromatography, affinity separation and magnetic separation. Microfluidic solutions have been successfully engineered to either integrate into the above techniques, or to function as a standalone device to execute sample preparation tasks.
An interesting example is that of a microfluidic filtration device for spermato-genetic cell sorting to support IVF (In Vitro Fertilization) and ICSI (intracytoplasmic sperm injection) processes . In certain cases of male factor infertility, a single viable spermatogenic cell has to be retrieved from a biopsy pellet so that it can be directly injected into an oocyte. The biopsy pellet contains a variety of tissues and a range of germ cells from all orders of maturity. The process of finding viable cells for ICSI can be time consuming, requiring hours of intensive work involving manually mincing the pellet, successive cell separation cycles through centrifugation, and individual cell discrimination. The germ cells become smaller as they mature, beginning as a large round spermatogonion of 16~18µm and ending as a small and slender spermatozoon of 4~6µm. Using this characteristic, one can aim to divide the spermatogenic cells into different mature categories according to their sizes in a fast and efficient manner.
As shown in Figure 1, the microfluidic solution is a passive planar DRIE (deep reactive ion etching) microfabricated device  that has separate wells to collect different types of cell that are separated by gradually reducing filter gaps. The fluid suspension is deposited through the central reservoir with conventional micromanipulation tools.
Figure 1: Scanning electron microscopy image of an un-bonded filter device, viewed from the filtration side, showing linear channels and filter segments radiating from the central reservoir.
Through careful control of the properties of the surfaces in contact with the fluid, and by exploiting the laminar nature of microfluidic flow, the filter employs the surface tension of the working fluid to drive the sample through the device.
The device scale allows the utilization of the forces which normally prevent fluid flow at the macro level to actively draw the sample through the filter elements, without the need for external energy sources.
In the experimental testing of the device, it was found that 0.5µl microfluid with about 1500 microparticles could be filtered through the device in less than 1 second.
Figure 2 shows the result of separating a mixture of 3µm and 10µm microspheres, where the majority of the particles were collected in their appropriate reservoirs. Optimal concentrations of particles within their appropriate collection wells were found to be of the order 50% for 3µm and 84% for 10µm particles respectively. The proportion of cells being processed was shown to be a function of their migration through the filter device within the bulk fluid flow.
Figure 2. Confocal image of the separation of a mixed suspension of 3µm (red) and 10µm (green) microspheres using a microfluidic device. In this design, microchannels of different sizes are used both as filter and provider of surface tension force to draw the sample through the device.
This device offers a number of advantages. It is biocompatible due to the use of materials such as silicon and glass and it is economically disposable, because of the low manufacturing cost for large production volumes, thus eliminating sample contamination.
In addition, the hydrophilic nature of the native oxide deposits within the capillaries of the device means that they are virtually harmless to cells and proteins . Self-powered capillary pumping techniques can be used to manipulate the fluid further, reducing the need for external equipment. Such an approach has the potential to enhance reliability and functionality, by eliminating moving components and thus minimizing the potential mechanical damage to cells caused by pumping, or, indeed, potential thermal damage arising from marangoni effect pumping.
In conclusion, the inclusion of physical filtration structures in a passive microfluidic system enables the efficient processing of samples which are free from contamination or damage. The process leads to a marked reduction in processing time and has great potential for automatic and efficient particle separation and bio-sample processing across a wide range of biomedical applications.
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 Prince, M., Ma X., Docker P., Ward M., Prewett P., Design and Modelling a Micro Fluid Filter for Separating Spermatogenic Cells, ASME Design Engineering Technical Conference, California, September, 2005. pp. 475-480. doi:10.1115/DETC2005-85357
 Prince, M., Ma X., Docker P., Ward M., Prewett P., The development of a novel Bio-MEMS filtration chip for the separation of specific cells in fluid suspension, IMechE part H: J of Engineering in Medicine, 221(2), 2007. p113-128. doi: 10.1243/09544119JEIM190
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