Poly(dimethylsiloxane) (PDMS)-based microfluidic devices are increasing in popularity due to their ease of fabrication and low costs. Despite this, there is a tremendous need for strategies to rapidly and easily tailor the surface properties of these devices. We demonstrate a one-step procedure to covalently link polymers to the surface of PDMS microchannels by ultraviolet graft polymerization. Acrylic acid, acrylamide, dimethylacrylamide, 2-hydroxylethyl acrylate, and poly(ethylene glycol)monomethoxyl acrylate were grafted onto PDMS to yield hydrophilic surfaces. Water droplets possessed contact angles as low as 45 degrees on the grafted surfaces. Microchannels constructed from the grafted PDMS were readily filled with aqueous solutions in contrast to devices composed of native PDMS. The grafted surfaces also displayed a substantially reduced adsorption of two test peptides compared to that of oxidized PDMS. Microchannels with grafted surfaces exhibited electroosmotic mobilities intermediate to those displayed by native and oxidized PDMS. Unlike the electroosmotic mobility of oxidized PDMS, the electroosmotic mobility of the grafted surfaces remained stable upon exposure to air. The electrophoretic resolution of two test peptides in the grafted microchannels was considerably improved compared to that in microchannels composed of oxidized PDMS. By using the appropriate monomer, it should be possible to use UV grafting to impart a variety of surface properties to PDMS microfluidics devices.
We demonstrate a simple procedure to coat the surfaces
of enclosed PDMS microchannels by UV-mediated graft
polymerization. In prior applications, only disassembled
channels could be coated by this method. This limited the
utility of the method to coatings that could easily and
tightly seal with themselves. By preadsorbing a photoinitiator onto the surface of PDMS microchannels, the rate
of polymer formation at the surface was greatly accelerated
compared to that in solution. Thus, a gel did not form in
the lumen of enclosed microchannels. We demonstrate
that the photoinitiator benzophenone remained on the
surface of PDMS even after extensive washing. After
addition of a variety of monomer solutions (acrylic acid,
poly(ethylene glycol) monomethoxyl acrylate, or poly(ethylene glycol) diacrylate) and illumination with UV light,
a stable, covalently attached surface coating formed in the
microchannels. The electroosmotic mobility was stable in
response to air exposure and to repeated cycles of
hydration−dehydration of the coating. These surfaces also
supported the electrophoretic separation of two model
analytes. Placement of an opaque mask over a portion of
the channel permitted photopatterning of the microchannels with a resolution of ∼100 μm. By using an appropriate mixture of monomers combined with masks, it should
be possible to fabricate PDMS microfluidic devices with
distinct surface properties in different regions or channels.
We have developed a strategy using ultraviolet light to polymerize mixed monomer solutions onto the surface of a poly(dimethylsiloxane) (PDMS) microdevice. By including monomers with different chemical properties, electrophoretic separations were optimized for a test set of analytes. The properties of surfaces grafted with a single neutral monomer, a neutral and a negative monomer, or a neutral, negative, and cross-linking monomer were assessed. The highest quality separations were achieved in channels with cross-linked coatings. The separation efficiency for biologically relevant peptides (kinase substrates) on these surfaces was as high as 18 600 theoretical plates in a 2.5 cm channel. The test peptides were fluorescein-AEEEIYGEFEAKKKK, fluorescein-GRPRAATFAEG, fluorescein-GRPRAA(T-PO(3))FAEG, fluorescein-DLDVPIP GRFDRRVSVAAE, and fluorescein-DLDVPIPGRFDRRV(S-PO(3))VAAE. Separations between two different peptides occurred in as little as 400 ms after injection into the separation channel. The simultaneous separation of five kinase and phosphatase substrates was also demonstrated. By carefully selecting mixtures of monomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different electrophoretic separations.
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