Vesicle based home and personal care products constitute a multi-billion dollar business worldwide. Further, products that utilize vesicles for the encapsulation and transport of drugs or other chemical species offer an enormous potential that is only beginning to be exploited. Yet a fundamental understanding of what determines vesicle structure is lacking, and this is both extremely costly in terms of product stability, and limiting in the development of new and more effective encapsulation procedures. The research proposed here is a very significant first step toward addressing this deficiency for the very important class of cationic lipid bilayer vesicles. Such vesicles are observed to exist both in unilamellar and multilamellar ("onion") structures, with a transition from unilamellar to multilamellar structures sometimes occurring spontaneously over periods of hours or days.
This project is aimed at understanding how and when the structure of vesicles may change (or be changed) from a single bilayer membrane to so-called "onion" structures involving multiple imbedded bilayers. Experiments will be done to correlate observations of vesicle structure, and how it may change with time, with measurements of the mechanical properties of the membrane. Membrane properties will be varied via different combinations of lipids/surfactants, and the total concentration of lipids will be changed to control the volume fraction of vesicles in the suspension. Vesicle structure will be studied primarily via Cryo-TEM imaging. Membrane properties will be determined via micropipette aspiration techniques using giant unilamellar vesicles. A theoretical basis will also be developed, using numerical simulation methods, for predicting the changes of vesicle structure, both from a molecular dynamics and continuum point of view.
The combination of experiments and theory proposed here will therefore provide new insight into the relationship between the constituent make-up of bilayer membranes, their measurable mechanical properties, and the structure of corresponding vesicles. The proposed research is transformative as it is a first step in rationalizing the design of vesicles with control over structure with applications in both materials development, and potentially in drug delivery processes. Indeed, our industrial partner views this project as having the long term potential for changing their "research tactics in a revolutionary way". In addition, it quantifies the micropipette aspiration method for the measurement of membrane properties; and it improves the ability to theoretically describe vesicles via continuum-based models.
Broader Impacts: The PhD student will benefit greatly from collaboration with Procter and Gamble via access to a broad range of materials, and facilities, beyond what is available at UCSB, as well as a level of insight about the industrial formulation and production of vesicles, that is simply not available in the open literature. The project also involves important outreach activities including: 1) the development (together with Todd Squires) of a UCSB freshman seminar (for non-science majors) on the science and engineering that goes into various food and personal care products, including those made from vesicles; and 2) funding to allow participation of the PI in the summer undergraduate and teacher intern programs sponsored by the MSERC program at UCSB. These programs are a fantastic opportunity for students involving 8-10 week individually mentored summer research projects, as well as weekly meetings with education staff to develop oral, visual and written communication skills.
Vesicle based products constitute a multi?billion dollar business worldwide, yet a fundamental understanding of what determines microstructure, and thus product stability, is lacking. This project has provided a significant step towards addressing this deficiency, via an improved understanding of why, how and when the structure of vesicles may change (or be changed) from a single bilayer membrane to so?called "onion" structures involving multiple imbedded bilayers. The structure of vesicles has enormous significance; both for the rheological properties and stability of many existing industrial products. Volume fraction measurement technique: The popularity of commercial formulations in the market depends considerably on their shelf life, flow characteristics and appearance. It is desirable to have good control over these properties, all of which are closely linked to the dispersion’s volume fraction. However, a method to measure this property for a concentrated, multilamellar vesicle dispersion has not heretofore existed. Using mass balance relationships in a clever way, we have successfully developed a method that is simple and practical to use. This method is currently being employed at P&G to characterize vesicle based products. Mechanical property measurements: Mechanical properties such as the bending rigidity and area compressibility of surfactant membranes govern not only the aggregate (vesicle) size, but also their shape transformations and interactions with other vesicles in dispersions. These properties depend on a delicate balance of the hydrophobic effect, steric forces electrostatic interactions in the membrane, and the deformation of the external electircal double layer. We have provided the first direct experimental measurements of the effects of counter-ion concentration on these properties, using a novel version of Micropipette Aspiration. Our measurements provide direct experimental evidence of the theoretically predicted opposing effects of counter-ion concentration on the electrical contribution and bare membrane contribution to the bending modulus. Vesicle adhesion and unbinding (Experiments): Vesicles dispersions can phase separate into water-rich and surfactant-rich phases due to vesicle aggregation, a process termed as ‘creaming’. In order to quantify the stability of dispersions against this process, we have studied inter-vesicle interactions using a new force apparatus, developed in our laboratory. We find that hydrodynamic effects play an important role in the adhesion and unbinding of vesicles in dispersion. The force required to separate a pair of tensed, adhering vesicles increases with increasing membrane tension and separation velocity. The measurements reveal that the combined effect of lower adhesion force and the higher global deformability with decreasing membrane tension result in a non-monotonic relationship between the work of adhesion and the membrane tension. Vesicle adhesion and unbinding (Numerics): For a better understanding of these adhesion phenomena, we also developed a continuum fluid-structure interaction code to study them numerically. We confirm an earlier analytical study from this group that found that vesicles’ ability to increase their surface area greatly enhances the interaction energy. We also look at the dynamics of adhesion and determine the rates at which the fluid film separating a pair of vesicles drains during adhesion, for both hydrodynamic and non-hydrodynamic interactions. While the adhesion energy gives some information as to how "strong" the interaction is, it is important to look at the dynamics of unbinding to understand aggregation. We have complemented the experimental studies described earlier by numerical simulations of the unbinding of a pair of adhered vesicles. We find that, depending on the values of various non-dimensional parameters, the unbinding can either follow a "peeling" process or a "pulling" process, with very different dynamics. The existence of these regimes leads to cases where a pair of tensed vesicles with weaker adhesion energy can actually be harder to pull apart than a non-prestressed pair with stronger adhesion energy. Spontaneous shape transformations: Charged unilamellar vesicles in concentrated dispersion can spontaneously deflate and subsequently transition to form bilamellar vesicles. Using cryoTEM imaging, we have quantitatively studied the time-evolution of charged vesicle dispersions and demonstrate that the driving force for spontaneous deflation is the repulsive electrostatic pressure between neighboring charged vesicles. This effect is triggered above a critical volume fraction that depends both on the surfactant and salt concentration in the dispersion. Unlike drops or hard-spheres, vesicles are capable of exchanging liquid with the surrounding. This unique property allows them to respond to ‘crowded’ conditions by deflating and undergoing shape and size transformations in order to reduce their effective volume fraction. Formation and relaxation of vesicle gels: Preparation of concentrated vesicle formulations involves processes wherein dispersions are subject to high shear and extension forces. We find that extrusion of dispersions made from vesicles that are in the solid state at room temperature can result in a dramatic, undesirable transformation into ‘jammed’ dispersions that behave like visco-elastic gels. The rheological behavior of these vesicle gels is found to evolve over time, indicating breakdown of the dispersion microstructure via several complex mechanisms.
