3. Surface Functionalization of Microfluidic Devices

Internal surfaces of pharmatechnological or biomedical microfluidic components may be functionalized—i.e., tailored or adapted to fulfill one or more specific physicochemical functions within a lab-on-chip system—by surface-technological methods selected from a number of available coating or modification processes.

Among various potential functions of a surface, its wetting behavior is of particular importance if two different phases (e.g., water and air, water and oil) are involved during operation of the system. Adhesive properties of internal walls are of major relevance in applications where particulate matter (cells, micro- or nanoparticles) plays a role: It may be necessary to prevent the adhesion of such particles on the surfaces in order to prevent clogging; on the other hand, the adhesion of cells may be aspired on certain parts of the surface. Adhesion promotion may, however, not only be an issue for the operation of an MF device but also for its manufacturing, for example for sealing or bonding processes. Frequently an undesired wall deposition of proteins or other constituents of the fluid has to be prevented by an antifouling coating or a suitable pretreatment of the surface. Coatings or surface modifications generating chemically reactive groups may be utilized to bind small molecules, polymers, biomolecules, or nanoparticles covalently to a surface. Controlling the density of charged functional groups, the ζ potential of a surface can be adjusted in order to influence, e.g., the charge of droplets dispensed from a pipette.

While so far mentioned functions of the MF device walls largely depend on their chemical composition close to the interface, specific geometrical and physical characteristics of surfaces and surface coatings may also be desired. Examples are the role of topography and Young’s modulus for the attachment of cells and microorganisms, coatings with specific electrical or optical functions involved in sensing and detection, electrowetting, or electrophoresis, and, last but not least, permeation barriers preventing the leaching of polymer constituents into the fluid or controlling gas transport through a polymer.

The present article gives an introduction to surface modification and coating processes which are established or under development in order to attain the above-mentioned surface functions. An emphasis will be laid on special requirements of microfluidic devices to be used with two-phase fluids and particulate matter.