Modification and Utilization of Nanotube-vesicle Networks

Doctoral thesis, 2007

Methods based on self-assembly and self-organization for construction of lipid bilayer networks, consisting of vesicles (3-100 μm in diameter) connected by conduits (50-300 nm in diameter), have previously been developed. Control over network geometry, topology, content, and membrane composition has been demonstrated. Since the lipid bilayer behaves as a two-dimensional fluid, the network parameters mentioned above can be dynamically altered, rendering quite different possiblities compared to devices produced with conventional microfabrication methods. Such lipid bilayer networks are the foundation for the work described in this thesis, where methods for modification, manipulation, and utilization of the networks are described. A protocol where nanotube-vesicle networks function as fluid mixing-devices, based on Marangoni flows, was developed. Controlled mixing of femtoliter to picoliter volumes of fluid was achieved. A novel method for initiation of reactions in lipid bilayer vesicles was successfully developed and utilized to study enzymatic reactions with as few as approximately 15 enzyme molecules. The method is based on dynamic shape changes of the network geometry, permitting combination of two or more vesicles’ content without creation of pores in the membrane. Modification of the vesicles internal volume by microinjection of different polymers was also studied. A suspension of the ionomer mixture poly(ethylene dioxythiophene) / poly(styrene sulfonate) was utilized to change the diffusion rate of particles inside the vesicles. This was achieved through cross-linking of the ionomer suspension with calcium ions, thereby creating a hydrogel. Three methods for introduction of calcium ions into the vesicles, i.e. vesicle fusion, electroporation, and microinjection, were demostrated, showing the versatility of the system. Furthermore, solutions of the thermosensitive polymer poly(N-isopropyl acrylamide) (PNIPAAm) were shown to display the characteristic sol-gel phase transition inside vesicles at the lower critical solution temperature (LCST). The phase transition was used to reversibly immobilize nanometer-sized particles suspended in the PNIPAAm solution, demonstrating temperaure-controlled, dynamic modification of the vesicles’ internal volume. At high concentrations and combined with certain additives, the PNIPAAm chains were shown to form a single hydrogel compartment when heated above the LCST. The hydrogel compartment displayed temperature-dependent shrinking and swelling, generating extremely heterogenous distribution of polymer inside the vesicle. By merging two or more vesicles containing PNIPAAm compartments, a single vesicle with multiple compartments could be created. Moreover, it was demostrated that nanometer-sized particles could be immobilized within the PNIPAAm compartments, thereby providing a generic method to functionalize and differentiate them (given that the particles carry the function). Both the compartment formation and the particle immobilization were shown to be reversible and repeatable. The ability to aggregate the PNIPAAm chains into a single compartment was also utilized to make vesicles in networks function as valves. By alternating between the dissolved state and the shrunken hydrogel state, the passive diffusion rate through a PNIPAAm-filled vesicle could be changed. This provides a novel means to control passive transport between different nodes in the network.