Surface-initiated polymerization, microfluidic

Surface-initiated polymerization is an attractive method for fabrication of microfluidic devices [244—247]. For example, Xu et al. reported surface-initiated ATRP inside microchannels to produce flat gradient and patterned surfaces [248]. 

Recently, surface modification techniques for polymer chains have progressed a great deal with the development of a new polymer synthesis method. In particular, surface-initiated atom transfer radical polymerization (SI-ATRP) is one of the most effective modification methods for preparing a well-defined dense polymer brush structure, or polymer brush, on solid substrates. Thus, a self-oscillating polymer brush prepared by SI-ATRP can be expected to create a novel self-oscillating surface with autonomous function, which will lead to potential applications in transporting systems for nanomaterials of flow control in microfluidics. 

SIP-driven polymer brush library fabrication leverages the fact that the polymerization initiation species are permanently bound to the substrate. Since the initiators are tethered, controlled delivery of monomer solution to different areas of the substrate results in a grafted polymer library. In NIST work, initiators bound via chlorosilane SAMs to silicon substrates were suitable for conducting controlled atom transfer radical polymerization (ATRP) [53] and traditional UV free radical polymerization [54, 55]. Suitable monomers are delivered in solution to the surface via microfluidic channels, which enables control over both the monomer solution composition and the time in which the solution is in contact with the initiating groups. After the polymerization is complete, the microchannel is removed from the substrate (or vice versa). This fabrication scheme, termed microchannel confined SIP ([t-SIP), is shown in Fig. 10. In these illustrations, and in the examples discussed below, the microchannels above the substrate are approximately 1 cm wide, 5 cm long, and 300-500 [tm high. 

The use of a focused infrared laser beam on a surface results in local heating, which has been applied to synthesize MIPs locally by thermal polymerization [72]. In this case, a common thermo-initiator, azobisisobutyronitrile, was used together with a standard MIP recipe, to generate MIP microdots in a microfluidic device. 

Some commercially available protein-inert polymers commonly used in microfluidic applications, all of which require permanent surface modification, are polyacrylamide, poly (N-hydroxyethyl acrylamide), poly(N,N -dimethyl acrylamide) (PDMA), polyvinylpyrrolidone (PVP), poly(vinyl alcohol) (PVA), hydroxyethyl cellulose (HEC), and hydroxypropyl methylceUulose (HPMC). To permanently attach protein-resistant materials to the channel sinface, high-energy sources, special chemistries, or even strrMig physical adsorption have been employed to introduce reactive functionalities. After activatirm, protein-resistant polymers can be anchored via UV-initiated free-radical polymerization. Polymeric materials... 

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