Surface modifications, microfluidic devices

Keywords Polydimethylsiloxane (PDMS), microfluidic devices, surface modification, physical adsorption, inner migration, high... 

Surface treatments and surface interactions are the challenges ahead for the research community in relation to the development of advanced, sophisticated microfluidic devices. Stuface modification techniques need to be optimized for various applications and particularly for disposable microfluidic devices. PDMS will be a popular material for future microfluidic devices, and some issues associated with most of the currently employed treatments, such as the hydrophobic recovery of PDMS surfaces, need to be adequately addressed. 

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Surface modification for microchannels in microfluidic and nanofluidic devices. 

The CVD method is an important method for the deposition of highly pure thin-layer films for microfluidic and nanofluidic devices. The most common use of CVD techniques in microfluidic and nanofluidic devices is for surface modification. With the rapid development in microfluidic... 

Chang et al. [5] utilized microtubes to generate micro-segmented flow. Upon surface modification, the prepared nanoparticles were mixed with a monomer and emulsified into uniform droplets in a capillary-based microfluidic device. The microchannel-based reactor offered reliable control over the nanocomposite products by precisely adjusting the interfacial tension. 

As glass and quartz exhibit the same surface property as fused-silica capillary, the monolithic materials could be conveniently prepared in a glass- or quartz-based microfluidic device via the same way of monoliths in the capillary. However, glass/quartz devices are rather expensive, and the need for specialized facilities for their fabrication with conventional photolithography technology hinders any rapid modification of the chip architecture. An attractive alternative is using a variety of polymeric materials, such as poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), polycarbonate (PC), and cyclic olefin copolymer (COC), to fabricate microchips for their mechanical and chemical properties, low cost, ease of fabrication, and high flexibility. 

In 2004, Hisamoto and coworkers [19] have presented a simple approach, called capillary-assembled microchips (CAs-CHIP), for assembling a commercial square fused-silica capillaiy into a PDMS microfluidic device. The capUlaiy could be completely functionalized off-chip and cut into required size and then integrated raito a chip without any solution leakage (as shown in Fig. 3). As many methods for surface modification of capillary have been well established, the CAs-CHIP method offered a way to fabricate different microfluidic devices having various functions for analytical applications including sample pretreatment, biochemical sensors, and so on. 

Microchannels that range in diameter from tens to hundreds of microns have emerged as potentially powerful tools for a variety of biomedical applications. They can be used to minimize sample volume and reduce costs, as well as to increase throughput and analysis sensitivity. Since microfluidic devices operate at small length scales, their functionality is greatly dependent on their surface properties, which vary depending on the type of material that is used to fabricate the channel and the subsequent surface modifications. 

The materials used for the fabrication of most microfluidic chips include glass, silicon, quartz, and plastics ( Materials Used in Microfluidic Devices) In addition to cost and optical, electric, and physical properties, careful consideration must be given to the surface chemistry of the material ( Surface Modification, Methods). In fact, surface chemistry plays a major role in chemical cytometry, as protein adsorption to the channel walls can degrade the separation performance and make the electroosmotic flow unreproducible. 

The surfaces of devices often exhibit different wettability characteristics, depending on the manufacturing approach adopted. Surface modification techniques can be used to alter the wettability behavior of microfluidic devices. This difference in wettability can be used to control the flow rate in devices. There are many other benefits of hydrophilic surface treatments, including the ability to increase adhesion and capillary effects [2]. Irrespective of the material used in the device, the primary requirement that a material needs to fulfill is biocompatibifity in various applications. Therefore, it is also necessary to use surface modification techniques to render materials biocompatible. It is believed that future devices of increasing sophistication will often require programmable surface properties, including control of the spatial distribution of charge and polarity [3]. 

Conventional polymers do not always possess the combination of desired bulk and surface properties for a specific application. The polymer materials used for microfluidic devices are innately hydrophobic, low-surface-energy materials and thus do not adhere weU to other materials brought into contact with them. This necessitates their surface modification/treatment to render them adhesive. This has prompted the development of a variety of polymer modification techniques, with the aim of developing new materials from known and commercially available polymers that have desirable bulk properties (elasticity, thermal stability, permeability, etc.) in conjunction with newly tailored surface properties (adhesion, biocompatibUity, optical reflectivity, etc.). 

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