Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability

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Photoreactive coating for high-contrast spatial patterning of microfluidic device wettability

Authors: Adam R. Abate, Amber T. Krummel, Daeyeon Lee, Manuel Marquez, Christian Holtzed and David A. Weitz

Lab Chip, 2008, 8, 2157–2160

Soft matter keywords

Microfluidics, PDMS, wettability, sol-gel, patterning, graft polymerization, double emulsion

By Alex Epstein

Abstract from the original paper

For many applications in microfluidics, the wettability of the devices must be spatially controlled. We introduce a photoreactive sol–gel coating that enables high-contrast spatial patterning of microfluidic device wettability.

Soft matters

Fig. 1 SEM images of channel cross sections; scale bars denote 5 mm. (a) Uncoated PDMS channel cross-section and (b) magnified view of upper right corner. (c) Coated, PAA functionalized PDMS channel and (d) magnified view of upper right corner; the corner is rounded because the sol–gel wets the surface and collects in regions of high curvature.

The authors, principally members of the Weitz group, report a chemical and lithography based approach to independently and precisely pattern sections of microfluidic devices such as channels for both surface chemistry and wettability. They use the standard microfluidic fabrication platform of PDMS soft lithography, noting that the inertness of PDMS makes it very difficult to functionalize. However, their technique leads to high contrast chemistry and wettability patterns along the PDMS channels. Also these patterns can be scaled to large parallelized arrays of channels, so industrialization is possible.

Fig. 2 Contact angle measurement of water droplets in air on (a) sol–gel coated substrate with contact angle 105 deg and (b) PAA grafted substrate with contact angle 22 deg. AFM images of (c) sol–gel coated and (d) PAA grafted microchannels. The images show a 10 deg 20 mm area at high resolution; the dark to light color scale maps to feature heights of -150 to 150 nm. (e) Surface concentrations of atoms on sol–gel coated and PAA functionalized substrates, measured with XPS; fluorine, oxygen, carbon, and silicon.

Brief experimental

The authors apply a photoreactive sol-gel coating with fluorinated silanes onto the PDMS. This dense network of functionalized silanes makes the coated surface hydrophobic by default. The photoinitiator allows for spatial patterning as follows: through lithographic UV exposure, the initiator releases radicals that cause polymerization of any present monomer. Then the polymers grow from the sol-gel interface (“grafting”) and give the interface the selected chemical properties and wettability.

Fig. 3 (a) Photomicrograph of a sol–gel coated channel. PAA has been grafted to the right half of the channel using UV-initiated graft polymerization. The grafted polymer is dyed with toluidine blue, a dye that preferentially stains PAA. (b) Average grayscale intensity across the channel as a function of location along the channel.
Fig. 4 (a) Diagram of sol–gel coated device. The upper half of the device is hydrophilic due to graft-polymerization of PAA. The bottom half of the device is hydrophobic due to the default properties of the sol–gel coating. (b) Photomicrograph of double emulsions being formed (R1) and flowing out of the microfluidic device (R2 and R3). (c) Magnified view of the double emulsion flow-focusing junction in R1. (d) Photomicrograph of O/W/O double emulsions; the drops crystallize due to their high monodispersity. The scale bars for all figures denote 100 mm.

Why we care

We see a neat demonstration of the technique when the authors graft patches of hydrophilic polyacrylic acid along PDMS microfluidic channels, cross sections of which are shown in Fig 1. (Interestingly, the coating and grafting leads to edge rounding in the channels.) The contact angle of water on the grafted areas drops from 105±1° to 22±5° (Fig. 2), a change of 83°via polymerization alone. This is much larger than can be done by grafting polymers directly to PDMS, and it is enough to emulsify fluorocarbons, hydrocarbons, and silicon oils. Moreover, the grafting changes the surface chemistry as measured by x-ray photoelectron spectroscopy (XPS). In Fig. 2, we see that the PAA treated areas adsorb more oxygen and carbon but less fluorine and silicon, compared to the native sol-gel coating. Fig. 3 also shows the stark contrast at the interface of the treated regions. Armed with this method of spatial wettability patterning via polymerization, the authors demonstrate the production of monodisperse double emulsions of oil-water-oil through a sol-gel-coated device (Fig. 4), showing the potential of this technique to the potential industrialization of templated core-shell structures. Channels with different functional properties in different areas can also be powerful in the fabrication of sensing devices and the separation of analytes.