Small-Scale Sample Handling for Studies of Liquid Crystals and Lipid-Based Soft Matter

Doctoral thesis, 2012

In the field of sensing in general and biosensing in particular, there is a growing need for integrating miniaturised senor elements in microfluidic systems. The advantages are several, some of which being that the consumption of, often very expensive, sample material can be reduced; sample exchange can be optimized; sensor performance in terms of detection limits can be improved and phenomena occurring on a very small, and even single-molecule, scale can be studied. In this work different microfluidic solutions have been designed and integrated with a number of different sensing techniques chosen based on their capacity to unravel a number of different physically, chemically or biologically relevant phenomena in connection with lipid-based soft matter and liquid crystals. The first part of the thesis is focused on the advantages of miniaturizing one out of three dimensions of a reaction chamber designed for quartz crystal microbalance with dissipation (QCM-D) monitoring, while keeping the size of the radial extension macroscopic. With a high radius- to-height ratio, surface induced effects could be identified and taken advantage of in studies of liquid-crystal phase transitions in confined geometries. A similar approach was implemented to improve the performance of aqueous-phase QCM-D studies of the self assembly of supported cell-membrane mimics, revealing improved reaction rates and reduced sample consumption. The ability of QCM-D to unravel the viscoelastic properties of thin soft matter films was also used to study changes in the viscoelastic properties of surface-immobilized lipid vesicles undergoing a transition from gel to liquid phase during a temperature sweep. By varying buffer salt and pH, it was shown that the vesicle-surface interaction plays an important role in determining the characteristics of the shape fluctuations, phase transition temperature and hysteresis around the phase transition. In the last part of the thesis work, two different microfluidic systems were designed and used to study rapid transport of water and glycerol across the lipid membrane of surface-immobilized lipid vesicles. In this case, fluorescence imaging was employed to resolve transport reactions on the level of individual liposomes. The membrane-transport reactions were induced by rapidly exchange the solution outside the liposomes, for which purpose two conceptually different approaches were developed and compared. The first approach took advantage of the fact that small sample volumes can be moved rapidly over a predetermined probe area containing surface- immobilized lipid vesicles. In the second approach the vesicles were suspended and moved into a mixing region, while the liquid flows were kept constant in order to reduce the limitation imposed by external equipment such as electric valves on the system. Both methods generated lipid-membrane permeability data close to what has been reported in the literature, and statistics from single vesicle data revealed a new means to characterize the efficiency of membrane-protein incorporation.