Nature's Packaging: Using Bio-Inspired Liposomes for Mixing in Microfluidic Systems

Wyatt N. Vreeland, Analytical Chemistry Division, Molecular Spectrometry and Microfluidic Methods Group, Stop 8394, x8513.

Central to the implementation of microfluidic or "Lab-on-a-chip" systems for integrated analysis is the ability to precisely control the spatial location of various chemical species.  The control of the location of species within a chip has been accomplished using pressure driven flow or electroosmotic flow to direct plugs of fluid through multichannel microfluidic devices.  To date, mixing in microfluidics has most commonly involved joining two microfluidic channels and allowing for diffusive mixing to blend the two streams together.  However, microfluidic channels, owing to their microscale diameters, engender flows in the low (0.1 to 1) Reynold's number regime and are purely laminar, which results in low efficiency diffusive mixing.  Many strategies for increased mixing have been presented including chaotic fluid mixers, multipatterned surface charge, porous polymeric monoliths, and micropatterned mixers, but all require fabrication of special formations in the microchannel itself to create effective mixing.  However, biology has a much more elegant method of controlling the mixing of various species in the (more complex) microfluidic environment of the biological cell -- using tiny packages called vesicles to traffic species within a cell.

The liposome is a spherical structure composed of a phospholipid bilayer membrane (similar to a cell membrane) that encapsulates a volume of intravesicular aqueous solution.  Liposomes are bathed in an external aqueous solution and are generally 200 nm to 1 µm in diameter.  The ability to encapsulate a species of interest inside liposomes renders that species inert to chemicals residing outside of the membrane.  We have developed a bio-inspired liposome system that allows for the controlled introduction of polar species into a microfluidic system through the modulation of temperature, using thermally triggerable liposomes.  Thermally triggered liposomes take advantage of the dramatically increased bilayer permeability near the lipid chain melting transition temperature (Tm).  Thus at a controlled temperature, a thermally triggerable liposome will release its contents into the extravesicular microfluidic space, allowing for precise deliver of agents to specific regions in the microfluidic environment.  Results from our research to this system will be presented.