Liposomes are self-assembled spherical vesicles comprised of a lipid bilayer membrane that segregates an
internal aqueous environment from an external aqueous environment. These nanometer-scale structures
have demonstrated potential for targeted drug delivery applications. For liposomes to be useful in vivo, the
liposome size and dosage of molecules contained within them needs to be controlled. We present here a
fluorescence-based technique for characterizing the relative encapsulation efficiency, leakage rate, and
shelf life of liposome formulations. We report results from three different liposome solutions over a period
of two months that show the liposome brightness remains stable while the background dye concentration
increases. These parameters may prove useful for optimizing the liposome formation process.
We create long polymer nanotubes by directly pulling on the membrane of polymersomes using either optical tweezers or a micropipette. The polymersomes are composed of amphiphilic diblock copolymers and the nanotubes formed have an aqueous core connected to the aqueous interior of the polymersome. We stabilize the pulled nanotubes by subsequent chemical cross-linking. The cross-linked nanotubes are extremely robust and can be moved to another medium for use elsewhere. We demonstrate the ability to form networks of polymer nanotubes and polymersomes by optical manipulation. The aqueous core of the polymer nanotubes together with their robust character makes them interesting candidates for nanofluidics and other applications in biotechnology.
We are developing optically based techniques for the manipulation of nano-containers (containers with sub-picoliter volumes) for handling chemicals in order to perform ultra-small volume chemistry. We are currently investigating three systems, liposomes, polymersomes and hydrosomes, for use as nano-containers. Liposomes and polymersomes are closed structures composed of a lipid and polymer membrane, respectively, that acts as a barrier to separate an aqueous interior environment from an aqueous exterior environment. We are typically working with liposomes or polymersomes that are approximately 10 μm in diameter. Hydrosomes are micron-sized, surfactant-stabilized water droplets that reside in a fluorocarbon environment. The optical techniques we are using include optical tweezers, for trapping and remotely moving the nano-containers, and an "optical scalpel" for localized disruption of lipid and polymer membranes in order to induce fusion of liposomes and polymersomes. In all three systems, we are able to bring together two similar nano-container using optical trapping and subsequently fuse them together, which allows their contents to mix. With the liposomes and hydrosomes we have been able to demonstrate their use for performing a controlled, elementary chemical reaction.
Using optical tweezers and microfluidics, we stretch either the lipid or polymer membranes of liposomes or polymersomes, respectively, into long nanotubes. The membranes can be grabbed directly with the optical tweezers to produce sub-micron diameter tubes that are several hundred microns in length. We can stretch tubes up to a centimeter in length, limited only by the travel of our microscope stage. We also demonstrate the cross linking of a pulled polymer nanotube.