Formation of Fully Closed Microcapsules As Microsensors by Microfluidic Double Emulsion

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, 50Nanyang Avenue, Singapore 639798
E-mail: ; 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 waferby 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.

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 2nd-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 thePDMS 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.

Secondly, the microfluidic device was assembled by a PDMS cast and a piece of acrylicsubstrate 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 tothe 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 80oCfor 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 10min, the PI solution in the microchannels was removed by air and the microchannels were dried by vacuum for 10 min. Then thesemicrochannels 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 networkwas flushed by ethanol for 30 min and water at pH10 (adjusted by 1 mol/L NaOH solution)for another 30 min to remove the unreacted AA monomer.

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