**1.3 Aqueous two-phase system**

In this study, we employed aqueous two-phase system (ATPS) to realize the microextraction in the microfluidic system. The liquid-liquid system, called ATPS or aqueous biphasic system was first studied by a Swedish biochemist P. Å. Albertsson (Albertsson, 1986). ATPS has become a powerful tool for separation of a range of biomaterials, including plant and animal cells, microorganisms, fungi, virus, chloroplasts, mitochondria, membrane vesicles, proteins, and nucleic acids. It can also be an appealing system for microfluidic droplet application, since the two phases are all aqueous (Walter et al., 1985; Hatti-Kaul, 2000). An ATPS consists of two immiscible phases formed by dissolving two incompatible polymers, such as poly(ethylene glycol) and dextran, or one polymer and an appropriate inorganic salt, or a cationic surfactant and a salt. The phase separation, which finally leads to equilibrium, is thought to be due to the incompatible physicochemical properties of components. Thus one phase is predominantly rich with one component and the second phase is enriched by the other component. ATPS is highly advantageous because the high water content (usually 70- 90%) provides biocompatibility and selectivity for the stable pre-concentration of hydrophilic molecules with relatively low interfacial tension compared to that of organic/water twophase system. It can be a versatile partitioning system for the separation of many kinds of dyes, metal ions, silica particles, proteins and cells (Walter & Johansson, 1986; Walter et al., 1991). Various factors such as concentration and type of phase-forming polymer or salts and the choice and addition of affinity ligands can affect the distribution of a solute over the two phases, thus giving more flexibility for the customized systems.

Several applications of ATPS into microfluidic systems have been reported for continuous partitioning of cells (Yamada et al., 2004; Nam et al., 2005), taking the advantage of PDMS (polydimethylsiloxane)-compatibility avoiding swelling problem that is commonly caused when organic/water two-phase system is applied to PDMS microfluidic device. The ATPSbased partitioning systems developed in these research works have almost same protocol because the direction and configuration of mass transfer where the material being extracted is transported from one phase to the other are almost identical with those of the two-phase system based on oil and water. Although laminar nature of liquid-liquid flow in microfluidic channel makes continuous separation possible, control of the interface has been known as a difficult task because the immiscible nature of the two liquid phases causes competition between interfacial tension and the viscous force. As Dreyfus et al. reported (Dreyfus et al., 2003), only in a certain regime of flow rates of liquid phases, stratified structure of flows, that is necessary for continuous microextraction, can be maintained.
