Supplementary Materials
Formation of fully closed microcapsules as microsensors by microfluidic
double emulsion
Bo Wu and Hai-Qing Gong*
School of Mechanical & Aerospace Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore 639798
E-mail: MHQGONG@ntu.edu.sg; Fax: (+65) 67911859
1. Fabrication of microfluidic device
The microfluidic device to form the microcapsules was made by soft lithography of
polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning, USA). Firstly, a mold of the
microchannel network was made on a silicon wafer by photolithography of SU-8
photoresists (MicroChem, USA) as illustrated in Fig. S1(a~d). Two alignment marks
were made on the silicon wafer by deep reactive-ion etching (DRIE). Then, a SU-8
structure of the microchannel network in thickness of 25 µm was made on the silicon
wafer with the two alignment marks as the first layer of the mold. Another SU-8 structure
of the serpentine section and the microchannel carrying the outer-phase fluid was made in
thickness of 175 µm above the first layer structure with the two alignment marks to form
the second layer of the mold. The mold for soft lithography was obtained by removing
the uncured photoresists by SU-8 developer.
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Fig. S1. Schematic illustration of the microfluidic device fabrication. (a) Two alignment marks made by
DRIE. (b) Formation of 1st-layer structure of the microchannel network. (c) Formation of 2 nd-layer structure
of the serpentine section. (d) Removal of uncured SU-8. (e) Soft lithography of PDMS. (f) Device assembly
by O2 plasma. (g) Embedding PI molecules in the PDMS wall of the microchannels. (h) Filling of AA
monomer solution in the microchannels. (i) Grafting PAA on the PDMS wall by UV light. (j) Flushing the
microchannel network by ethanol and water.
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Secondly, the microfluidic device was assembled by a PDMS cast and a piece of
acrylic substrate coated with a thin layer of PDMS. PDMS base was mixed with its
curing agent in a ratio of 10:1 (w/w) and degassed before being poured on the mold.
After curing at 80 oC for 4 hours in an oven, the cured PDMS cast was peeled off from
the mold (Fig. S1(e)). The cured PDMS cast was bonded covalently to the thin PDMS
film on the acrylic substrate by oxygen plasma (Fig. S1(f)). The acrylic substrate has a
much high transmittance for UV light than the glass substrate to cured the ETPTA outer
droplets as shells in the serpentine section. The microfluidic device was placed in the
oven at 80 oC for 2 days to reverse the original hydrophobic PDMS surface.
Thirdly, the serpentine section downstream of the Y-junction was patterned to be
hydrophilic to form water-in-oil-in-water (W/O/W) double emulsions by UV-initiated
graft polymerization of poly(acrylic acid) (PAA) (Schneider et al. 2010), as illustrated in
Fig.
S1(g~j).
A
photoinitiator
(PI)
solution
of
10 %
(v/v)
2-hydroxy-2-
methylpropiophenone (Daracure 1173) in acetone flushed through the microchannel
network. The PI molecules penetrate into the PDMS matrix. After 10 min, the PI solution
in the microchannels was removed by air and the microchannels were dried by vacuum
for 10 min. Then these microchannels were filled with a monomer solution of 10 % (v/v)
acrylic acid (AA) in deionized water. To graft PAA at the PDMS inner wall, the
serpentine channel was exposed to UV light. The region above the other microchannels
was covered by a piece of aluminum foil to avoid the UV exposure. Finally, the
microchannel network was flushed by ethanol for 30 min and water at pH 10 (adjusted by
1 mol/L NaOH solution) for another 30 min to remove the unreacted AA monomer.
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Reference
Schneider MH, Willaime H, Tran Y, Rezgui F, Tabeling P (2010) Wettability Patterning
by UV-Initiated Graft Polymerization of Poly(acrylic acid) in Closed Microfluidic
Systems of Complex Geometry. Anal Chem 82 (21):8848-8855. doi:10.1021/ac101345m
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