The recent discovery that block copolymers, having a tendency to form lamellae, can also be organized into vesicles has opened the new field of polymer vesicles. Copolymer membranes are roughly twice as thick as their phospholipid counterparts, and can sustain areal strains as much as 10 times those which destroy liposomes. The robustness of polymeric vesicles presents the opportunity for membrane modification so that more sophisticated behaviors can be engineered. We envision that dispersions of polymer vesicles will exceed their obvious applications as drug and gene delivery agents and will find use as sensors and chemical actuators, scavengers, and a communication mechanism between cells and other entities in aqueous environments. Work in the Santore lab focuses on imparting tunable and dynamic functionality into these membranes, so that their potential roles in medicine, sensors, aquatic scavengers, and on-chip chemical plants may ultimately be realized.
Insertion and release
The insertion of molecules into a vesicle membrane is of interest because it provides a means of membrane manipulation. Indeed the insertion of additional species into cell membranes is a critical step in naturally-occurring processes such as the infection of cells and bacteria by viruses and the attack of toxins (bee-sting and beetle poison) on cells, causing them to leak or rupture. Mimicking insertion processes in an artificial system will provide a deeper understanding of similar processes in nature and will also allow us to control release from polymer vesicles using classical surfactants and peptide-mimics.
Using micropipette manipulation, polymer vesicles are transferred from a neutral solution to a solution of the insertion molecules. The insertion rates are measured by the areal change of the vesicle “skin” as it is held in the micropipette under constant membrane tension, in Figure 2.
• M. Santore*, D.E. Discher , Y.Y. Won, F.S. Bates, D.A. Hammer, Langmuir, 18, 7299-7308 (2002).
Insertion of species which do not traverse the bilayer isolates the inserted molecules in the outer leaflet of the bilayer, inducing membrane stresses, and leading to curvature. Figure 3 illustrates how inserted molecules induce budding in a polymer membrane.
Adhesion and recognition
Living cells interact with other cells and the extra-cellular matrix via their surface receptors. In the Santore lab, students implement chemistries to modify the surfaces of polymer vesicles in ways that will make them adhesive towards specific targets. Studies then measure contact angles, adhesion rates, and formation of the adhesion plaque (the process by which receptor-bearing polymers concentrate diffusively in the contact zone). Adhesion of polymer vesicles to target objects such as cells, bacteria, viruses, and other particles will comprise a means of disease treatment that is unprecedented. Imagine the possibility of treating metatstatic cancer cells anywhere in the body by specific recognition, without the side effects of more general exposure to toxic pharmaceuticals. Santore group members are also working on the development of adhesive vesicles for sensor applications and bacterial and viral scavengers.
Figure 4 displays vesicle pairs, bearing complimentary receptors and ligands. The vesicle held at lower tension slowly engulfs that at higher tension. The fluorescent image below shows concentration of complimentary receptors and ligands in the contact zone, forming an adhesive plaque. Click on image for movie
The two-pipette method comprises a fascinating means of quantifying engulfment rates of one vesicle over a second object, as a function of the tension in the vesicle membrane. In the case of reversible adhesion, the contact angle relates to the thermodynamic work of adhesion, by analogy to the Young Equation. An example where this applies is the measurement of depletion forces, on the right using a free solution of polyethylene oxide between two unfunctionalized polymer vesicles.Click on image for movie
Multicomponent Membranes and Phase separation
In cell membranes, separation of phospholipids into raft phases facilitates adhesion by local concentration of receptors and is important in membrane trafficking and secretory processes. Phase separation of monolayers and bilayers of phosopholipids has been established for decades, but phase separation in phospholipid vesicle systems has been documented relatively recently. Figure 6 shows, as an example, phase-separating liposomes in a fluorescence microscope in the Santore lab: Here the vesicle surface is in liquid-solid equilibrium and a fluorescent tracer dye concentrates in the liquid phase. Ongoing work in our group focuses on liquid-liquid equilibrium and critical phenomena in polymer vesicles, invoking phase separation as a means of altering vesicle permeability and stability. Ultimately, we envision using phase separation to trigger adhesion, release, membrane trafficking, and engulfment.