C O M M U N I C A T I O N S
1 µm greater than the target width. The narrowest features (0.8, 1,
and 2 µm) did not appear at this writing power. At a power of 40
mW (Figure 3b), the feature width was 2.5-4 µm larger than the
defined width. The increase in width is due to both the increase in
the effective spot size at higher powers and the higher concentration
of the photoacid, which diffuses to some extent away from the
region in which it is formed, leading to an expanded zone of
deprotection of the PTHPMA. The fact that hydrophilic channels
narrower than ∼2.5 µm did not appear is perhaps a function of the
diffusion length of the photogenerated acid. Because acid diffuses
away into the surrounding areas for smaller feature sizes, below a
certain exposure dose there is no longer a sufficient acid concentra-
tion within the exposed regions to make the polymer brush
hydrophilic.
In conclusion, we have demonstrated a facile method for
fabricating 3D microfluidic channels by using two-photon-activated
chemistry to locally switch the interior surface of a porous solid
from a hydrophobic state to a hydrophilic state. These 3D structures
can be infilled selectively with water and/or hydrophobic oil with
a minimum feature size of only a few micrometers. We envision
that this approach may enable the fabrication of complex microf-
luidic structures that cannot be formed via current technologies.
Acknowledgment. This material is based on work supported
by the U.S. Department of Energy, Division of Materials Sciences,
under Award DE-FG02-07ER46471 through the Frederick Seitz
Materials Research Laboratory at the University of Illinois at
Urbana-Champaign. We thank D. V. Gough, and Dr. D. G. Yu
for experimental assistance. J.-T.L. gratefully acknowledges partial
support from the Ministry of Education of the ROC (Taiwan).
Supporting Information Available: Detailed experimental proce-
dures and characterizations. This material is available free of charge
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