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Development of microfluidics

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The quote, “prediction is very difficult, especially about the future,” by the Danish physicist Niels Bohr, is relevant for almost all modern technology. The concept of the miniaturised total chemical analysis system (µTAS), a microfluidic device that can perform highly efficient, simultaneous analysis of molecules, was introduced nearly 20 years ago.¹ Now, approximately 10 years after the extravagant and excited promises of how this technology would revolutionise chemical, biochemical and diagnostic analyses, it is timely to review subsequent developments. This includes examining what impact microfluidics technology has had and which products benefit from miniaturisation.

Progress in a technology can be seen in terms of a “hype cycle,” that is, a graphical representation of its maturity, adoption and business application.² Figure 1 shows the hype cycle for microfluidics technology and illustrates the development of expectation versus delivery, or the“hype.” It can be argued that the early promise of this technology is being fulfilled in existing products or in products to be released in the near future. Thus, it could be said that the technology is in the “slope of enlightenment” phase, which characterises a stage of development whereby the basis of the technology is widely accepted and the range of applications and products is rapidly growing. This statement is supported by the following observations:

• An industrial infrastructure and manufacturing process for the development and production of microfluidic components and systems is available. Of particular relevance is the growing industrial base for polymer microstructures, because for low cost or biocompatible systems polymers are the only material class to promise economy of scale in the manufacturing cost.³ This is important for diagnostic applications, which are subject to considerable cost pressure. The current manufacturing infrastructure includes automated manufacturing as well as utilising
  certified quality management systems such as EN ISO 13485, Medical Devices, Quality Management Systems, Requirements for Regulatory Purposes.
• The complex technology chain for the production of microfluidic systems (see Figure 2) can be covered by single suppliers, which reduces logistics and interface problems and increases
  production yields.
• An increasing trend towards standardisation of chip-to-world interfaces and external geometries such as microscope slides or Society for Biomolecular Sciences (SBS) size microwell plates allows the seamless integration of microfluidic components into existing laboratory environments.
• Off-the-shelf microfluidic components are commercially available, which allows for quick, low cost implementation of development steps, thereby greatly reducing the entrance barriers
  for the utilisation of microfluidic technologies.
• All the basic reasons for the introduction of microfluidics such as reduced reagent consumption, increased speed and analytical
  performance, multiparameter testing and user friendliness have been demonstrated in commercial products.
• A variety of platforms have been developed, which allows industrial and academic users to quickly develop their own assays, reactions and protocols.⁴

Microfluidics as an enabler

Figure 1: Gartner's hype cycle for microfluidics. (click image to enlarge)

There is one critical element, however, that was forecast during the hype years, which has not yet materialised. It was believed in the early years that a “killer application” would emerge quickly, that is, a commercial product would be developed that would generate millions of euro in turnover and act as a leading product for the whole industrial sector. To understand why this has not happened, it is important to remember that microfluidics is an enabling technology. It provides users with a means of achieving better performance and additional features in their products. It is not a product in itself and is unlikely to become one. Instead, increasing numbers of systems in fields such as clinical, molecular and point of care diagnostics, drug development and discovery, biochemical analysis and biomolecule synthesis will contain critical elements in which functionality is determined by a microfluidic component. This is already visible in a variety of products such as the LabChip produced by Caliper (www.caliperls.com); a diagnostic cartridge from i-Stat Corporation (www.i-stat.com); and a matrix chip architecture that enables a high density of experiments (2304 per chip) and mixing of nanovolume scale fluids developed by Fluidigm Corporation (www.fluidigm.com).

Figure 2: Technology chain for the realisation of microfluidic systems. (click image to enlarge)

The volume of microfluidic products was estimated to be worth approximately US$600 million (e410 million) in 2006. The market was forecast to grow to US$1.9 billion (e1.3 billion) by 2012, the majority of this revenue coming from diagnostic applications.⁵

Extending usage

Figure 3: Serology test cartridge with microstructured features.

A significant boost for microfluidics came in the wake of the 9/11 terror attacks in 2001 for biothreat detection, when simple-to-use, fast analysis systems became much sought after, for example, by the American postal services. One system is the GeneXpert developed by Cepheid (www.cepheid.com). This system demonstrates the potential of full scale functional integration of all relevant process steps such as sample preparation, deoxyribonucleic acid amplification and detection, which can yield a result from a raw sample in 30 minutes or less.

Figure 4: a) Microchannel structure of the serology test cartridge; b) negative (no agglutination, left side) and positive (agglutination, right side) results in this cartridge. (click image to enlarge)

Another example that highlights the adoption of existing standards and the inclusion of a microfabricated element into a conventionally sized cartridge is shown in Figure 3. It is a cartridge test platform for a range of agglutination and particle based tests in serology such as direct blood typing or the indirect antiglobulin test (IAT, “Coombs” test). In state-of-the-art systems, which have identical outer dimensions, small columns are filled with a gel that has the relevant antibodies incorporated in the gel matrix. The columns are then filled with the blood sample. Upon centrifugation, the red blood cells migrate through the gel and in case of a positive response, haemagglutinate on top of the gel or during passage as a result of the antibody–antigen reaction. Negative samples simply pass through the gel uninhibited. In the microfabricated system, which was codeveloped by the Swiss diagnostics company Medion Diagnostics (www.medion-diagnostics.com) and microfluidic ChipShop, the gel matrix is replaced by a microfluidic manifold. The manifold consists of branching microchannels with steadily decreasing cross-sections. These microchannels block an agglutinate while letting unreacted particles pass through. The advantage of the microstructure is the high degree of reproducibility that is possible with the use of microstructure injection moulding. Furthermore, the shelf life of the cartridge is increased because the shelf life determining gel is no longer required. Figure 4a shows a scanning electron micrograph of the microstructured channel. The sample flows from the larger structures, shown at the bottom of the image, through the smaller channels towards the top of the image. Figure 4b shows the result of a positive test result: agglutination reaction, right fluidic column, versus a negative result, left fluidic column.

Off-the-shelf options

Figure 5: Top, micropump for pumping fluids in a volume range between 50 nL/min and 5 mL/min (courtesy of Bartels Mikrotechnik GmbH); bottom, micromixer array for mixing fluids. In both cases, the necessary connectors are already integrated in the devices.

An important step for the dissemination of microfluidics was the availability of off-the-shelf components from a product catalogue. For many years, anyone who wanted to evaluate the possibility of using microfluidics for their applications had to go through an engineering project with the associated cost, time and development risk. However, it is now possible to order a variety of active or passive microfluidic components off-the-shelf, often complete with connector kits or application protocols. This allows for a low cost, low risk approach to implementing microfluidics in a research and development laboratory, because the basic proof-of-principle experiments can often be performed using these comparatively simple components. Only if these principle experiments are successful will the product or assay development enter the next phase, which involves the development of a customer specific microfluidic device. Figure 5 shows examples of off-the-shelf components.

Figure 6: Continuous flow PCR system, left; chip, right.

An example of the use of existing macroscopic laboratory standards for microfluidic systems is shown in Figure 6. A continuous flow polymerase chain reaction (PCR) system^6,7 on the left is the complete instrument; on the right are the PCR chips. The chips have the same outer dimensions as a microscopy slide and are held in place by a carrier frame that has the same dimensions as a SBS microwell plate. They can, therefore, be handled or interfaced with most existing liquid handling systems, which allows an integration of this system into the daily laboratory workflow.

Aiding new product development

There has been a significant development in the field of microfluidics in recent years, which has led to a maturing of the technology and growth in applications and products. Although no single “killer” application has yet been developed, the numerous advantages of microfluidics as an enabling technology means that it has become an essential tool in product development.