IN-SITU FORMATION OF HYDROGEL MEMBRANES AND GROWTH OF COLLOIDAL CRYSTALS IN MICROCHANNELS USING ONE STEP STAMPING Eunpyo Choi 1 and Jungyul Park 1* 1 Department of Mechanical Engineering, Sogang University, Seoul, Korea ABSTRACT This paper reports a novel method for in-situ formation of hydrogel membranes and geometrically controlled self- assembly of colloidal crystals in microchannels using one step stamping. The simple elastomeric stamp was made of PDMS from SU-8 patterned master using standard soft lithography. Then the selective hydrophobic pattern on the PDMS stamp sur- face was achieved using Pluronic F127 coating and staircase design. Finally, with this PDSM stamp, not only the formation of the In-situ hydrogel membranes with closed loop, but also the growth of colloidal crystals within microchannels was carried out. KEYWORDS: One step stamping, In-situ hydrogel membrane, Closed loop, Photonic crystal, Microfluidic INTRODUCTION The formation of hydrogel membranes in microchannels has attracted considerable interest due to various applications, such as separating of components and tissue engineering [1]. Self assembly of colloidal crystals with controlled size, shape, and position is essential for fabricating a large-scale integrated microfluidic system [2]. However, so far, the existing methods for in-situ formation of hydrogel membranes are intrinsically impossible to form the closed loop and to grow colloidal crystals in the microchannels has only been achieved through the uncontrolled evaporation-induced self-assembly [3]. Moreover, there has been no possible common technique for formation of hydrogel membranes and growth of colloidal crystals in microchan- nels. The proposed one step stamping method can not only resolve previous problems, but also be applied into two different areas. FABRICATION PROCESS Figure 1: Schematic illustration of fabrication process Figure 1(a) - (f) shows the fabrication process for the surface modified stamp with multilayered design: (a) A 4 inch silicon wafer was patterned with SU-8 resist (Microchem Inc.). The height of first layer for the shallow channel that can form the desired patterns was 7 μm for formation of hydrogel membranes and 25 μm for growth of colloidal crystals. Subsequently, SU-8 2050 was patterned on the first SU-8 layer (target: 200 μm, this layer is for the deep channel) (b) A PDMS precursor (Sylgard 184 Silicone Elastomer, Dow Corning) and a curing agent were mixed at the ratio of 10 to 1, based on weight. The PDMS mixture was poured onto the master and cured at 95 °C for 1 hr. (c) The PDMS was then peeled off. (d) Pluronic F127 was spin-coated on Si substrates. (e) PDMS was placed on the Pluronic F127 coated wafer and maintain at 35 ˚C temperature. (f) PDMS was stained with Pluronic F127 only outer surface. This stamp was applied to two cases as follows: (g, h) In hydrogel case, photo-curable poly (ethylene glycol)-diacrylate (PEG-DA) solution was spin coated on the silicon wafer or in col- loidal case, the diluted silica bead (diameter: 300 nm) was coated on the wafer. Photo-curable PEG-DA solution was prepared from 99 wt% PEG-DA and 1 wt% of a water soluble photoinitiator 2- hydroxy-2-methylpropiophenone. (i, j) Pluronic F127-stained PDMS was placed on the surface and was pressed carefully. (k, l) Since the Pluronic F127 is very hydrophobic, PEG-DA liquid drop- let or diluted silica bead were stained on the only shallow-channels. (m) In hydrogel case, the PDMS was stamped on the glass and ex- posed to UV for a few tens of seconds 90 mW/cm 2 . (n) In colloidal case, the PDMS was stamped on a PDMS-coated glass. 978-0-9798064-4-5/μTAS 2011/$20©11CBMS-0001 236 15th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 2-6, 2011, Seattle, Washington, USA