SHEAR-FLOW DIRECTED SELF-ASSEMBLY OF DIACETYLENE LIPOSOMES OF CONTROLLED SIZE

Abstract

This study addresses a novel microfluidic method to fabricate diacetylene (DA) liposomes and control their size. Polydiacetylene (PDA) is a conjugated polymer sensor with unique optical properties which are color (blue-to-red) and fluorescence change under the application of thermal or chemical stress. The liposome size distribution is important because they significantly affect the phase transition and the fluorescent emission. So far, DA liposomes are prepared by mixing of bulk phases. As a result, they are polydisperse in size and require post-processes to improve the size uniformity. Here, we report a novel strategy to generate uniform PDA sensor liposomes and control their size using a microfluidic chip and hydrodynamic focusing technique. PDA liposomes fabricated on a chip are analyzed using scanning electron microscope (SEM) and dynamic light scattering (DLS). The results present that the microfluidic strategy generates more monodispersed liposomes than a bulk method. It is also observed that the PDA liposomes size becomes smaller and their size distribution becomes more homogeneous as the flow rate ratio of inlet flows increases INTRODUCTION Liposomes, a class of vesicles that consist of spherical lipid bilayers, have attracted great interest for a wide range of chemical, biological, pharmaceutical and industrial applications due to their abilities to encapsulate and keep apart the aqueous component in its core and the lipophilic substance between two lipid layers. Among various liposomes, polydiacetylene (PDA), a conjugated polymer sensor, has unique optical characteristics that it changes its visible color and fluorescence upon the stimulus (Ji et al., 2003). Translucent diacetylene (DA) liposomes become non-fluorescent blue-phase PDA liposomes by exposing them to 254 nm UV light. Then, the blue PDA liposomes change their color to the “fluorescent” red when thermal or chemical stresses are applied, as shown in Figure 1. A PDA sensor uses these unique properties of the blue-to-red color and fluorescence change. PDA liposomes are commonly fabricated through mixing of bulk phases, but it results in heterogeneous and polydisperse distribution in size (Kim et al., 2005). The polydispersity, however, deteriorates the quality of PDA sensor. For example, individual liposome emits different intensity in visible red and fluorescence in spite of applying the same stress to them. Therefore, additional postprocesses required such as micro-filtering and sonication to improve the size uniformity even though they often cause the fracture of the liposome configuration. Thus, it is required to discover a novel strategy to form uniform PDA liposomes to improve the PDA sensor quality. Recently, the size control of microor nano-particles has been actively studied using a microfluidic chip. Zourob et al. (2006) reported a micro-reactor using droplets to produce monodisperse polymer beads instead of using the suspension. In addition, Jahn et al. (2004 and 2007a) used a hydrodynamic focusing technique for better size distribution, and controlled the self-assembly process of liposomes by altering a flow rate and a flow rate ratio in a microfluidic chip. We presented a microfluidic method to generate uniform PDA liposomes and control their size. They are generated by self-assembly in a microfluidic channel with hydrodynamic focusing. The flow rate ratio of injected solutions for sample and sheath flows appears to control the self-assembly of PDA liposomes. We show the detailed analysis on the liposome size using SEM and DLS. EXPRIMENTAL SECTION Preparation of Bulk PDA Liposomes A common procedure for the preparation of DA liposome in aqueous solution is shown in Figure 2. In the first step, 10, 12-pentacosadiynoic acid (PCDA) powder is dissolved in a little amount of dimethyl sulfoxide (DMSO) in a test tube. This solution is dropped by a syringe into 80 C deionized (DI) water in a flask, and is thoroughly mixed for 20~30 min to yield a total lipid concentration of 1 mM. The resulting solution is sonicated for 30 min and is cooled at 4 C at least for 4 hrs. Then, polymerization is Sixth International Symposium on Turbulence and Shear Flow Phenomena Seoul, Korea, 22-24 June 2009