Modifications of the surface of the device are essential for the designed functionality of microfluidic devices in (nano)biotechnology. Often, these modifications are to be achieved locally, and therefore different areas of the device will require different modifications. These modifications must be achieved on all surfaces, including the sidewalls of high aspect ratio microstructures, for example in deep channels.

The objectives for modifications of the fluidic device surface include the modification of wetting characteristics (hydrophobic/hydrophilic), increased biocompatibility, reducing or eliminating solute interactions with the device surfaces, modifying electroosmotic flow, immobilizing the reagents, enzymes, antibodies, proteins, DNA, etc. to carry out chemical reactions or detection mechanisms, or to provide a proper surface for immobilization, increasing the surface area for catalytic reactions, and tethering sieving matrices or stationary phases for separation devices.

The surface modifications may be achieved by a variety of techniques, including CVD and PVDmethods, spin coating and solution casting, plasma processes (e.g., plasma etching and plasma polymerization), grafting, chemical self-assembly, the Langmuir–Blodgett technique, printing, and others. In some cases, these surface modifications will involve nanotechnology. The thickness of the modification layer is in the nanometer range; thicker layers might modify the device geometry, and its function, and so for the objective of the functionalization, often only a few monolayers are sufficient.

For example, when multilayer films containing ordered layers of protein species are assembled by means of alternate electrostatic absorption with positively charged PEI, PAH, chitosan or with negatively charged PSS, DNA and heparin, the enzymatic activity of the films does not increase with layer number for more than 10–15 layers [1].

Requirements which the surface modifications must meet include good adhesion, chemical stability against the media used in the device, and a time stability which is better than the lifetime of the device.

One very important surface modification is that of modifying the wetting characteristics of the surface. As the interfacial tensions cannot be monitored directly, measurement of the contact angle between the surface and a droplet of liquid is widely used to characterize the wetting characteristics of the surface.

Materials such as glass, Si and SiO2 have many OH-groups on their surfaces, and this causes hydrophilic behavior. Especially in the case of silicon, the wettability depends strongly on the pre-treatment and history of the surface. Hydrophobic surfaces may be produced using octadecyltrichlorosilane (OCTS), and hydrophilic behavior may be stabi lized using hexamethydisilazane (HMDS).

Polymer surfaces are hydrophobic in most cases. Hydrophilic surfaces may be easily generated using O₂ plasma treatment, but such surfaces are stable only for a few days. More stable surface modifications are obtained by plasma polymerization of layers involving OH-groups at the surface.

The wetting characteristics of the surface may also be modified by a nanostructured sur face. This principle of nanobiotechnology is found in nature, for example, in the cuticular structure of leaf surfaces [2] and in fractal surfaces [3]. Such water-repelling surfaces have self-cleaning properties (the Lotus effect), as particles on nanostructured hydrophobic surfaces are more readily wetted and washed away (Figure 1).

Large surface areas are required for both catalytic reactions and separation assays, and this may be achieved by coating microfluidic chips with a porous material. In the case of silicon, porous silicon with pore sizes in the nanometer to micrometer range may be generated.

Another important surface functionalization is the binding of specific molecules to designated areas of the chip. Such applications include DNA-, proteomics-, cell-, and tissue-chips. Generally, by using various surface chemistries, linkers for such molecules must be provided in designated areas, while the remaining surface should be non-binding.

Methods to immobilize the specific molecules include adsorption, crosslinking, covalent binding, microencapsulation, and entrapment. A thin, sputtered gold film can be used to immobilize a dense molecular film of thiols [4], providing a high density of alkyl groups as binding sites for surface reactions.

Fig1. Left: Nelumbo nucifera, the Lotus flower. Right: a double-structured surface optimized for self-cleaning. Contact areas are minimized through the combination of micro- (cells) and nanostructures (wax crystals). (Courtesy University of Bonn.)

One example of polymer substrates is the building of a functional chemical scaffold on PMMAusing an ethylene diamine foundation [5]. In this way, various materials such as oligonucleotides, enzymes, or stationary phases may be attached to the device surface.

References

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1- M. Onda, K. Ariga, T. Kunitake, J. Ferment. Bioeng. 1999, 87, 87.

2- C. Neinhuis, W. Barthlott, Ann. Bot. 1979, 79, 667.

3- S. Shibuichi, T. Onda, N. Satoh, K. Tsuij, J. Phys. Chem. 1996, 100, 19512.

4- M. Mrksich, G.M. Whitesides, Trends Biotechnol. 1995, 13, 228.

5- S. Soper, in: J. Göttert (ed.), 2001 CAMD Summer School Micro-and Nanotechnologies, Chapter 10, Baton Rouge, CAMD/LSU Publishers, 2001.