Research in my group is focused on measuring intermolecular and intersurface forces in complex fluid systems with an emphasis on polymers, polyelectrolytes, biomembranes, and bio-mimetic materials in order to develop new materials with useful properties. These materials are being studied since they can be prepared from renewable resources, they can be biocompatible and biodegradable, and often possess excellent physical properties. We utilize modern principles and techniques of surface chemistry, physics, and biology as the means to achieve our goal of producing materials with superior properties for biomedical and engineering applications. There are currently two main focus areas.

Neutron Scattering Measurements of Confined Complex Fluids

In this work, we use a novel apparatus, Neutron Confinement Cell, to measure the molecular density and orientation of confined, ultra-thin complex fluids under static and dynamic flow conditions. The device couples the utility of the Surface Forces Apparatus - ability to control surface separation and alignment under applied loads - with in-situ structural characterization of the intervening material utilizing neutron reflectivity measurements. The apparatus is thus suitable for studying dynamic and time-dependent interactions, the conformations of molecules trapped between surfaces, and the rheology of thin films. Are initial studies are focused on determining the structure of adsorbed polymer diblock films -polymer brush layers- at the solid-solution interface as a function of confinement and polymer brush overlap. Such polymer layers are used to impart colloidal stabilization, they are used as protective coatings (including mechanical protection of solids against friction and wear), they govern the interactions of biological cell surfaces, and through judicious design they are used to modulate dispersion properties (such as rheology) under a variety of processing conditions. The proposed research should resolve long-standing questions of the anomalous properties of such films, determine their equilibrium structure and should also be the first to measure their behavior at the molecular level under flow. In the area of biomaterials, we will examine the rheology, thin-film viscosity, and structure of the biopolymer hyaluronic acid, a biopolysaccharide. Hyaluronic acid is found in the extracellular matrix and synovial fluids, where it protects and lubricates joints. The material is known to form intra and interchain associations and gel at high concentrations and pressures but rapidly converts to an extremely low friction, good wear interface under conditions of lower pressure and shear. Our aim is to determine the essential properties of this material in bulk, at solid-solution interfaces, and under dynamic conditions of confinement and flow in order to engineer a suitable replacement as an osteoarthritis and sports medicine therapy and for tissue engineering applications.

Membrane Interactions

- Fusion

One pathway for molecules to enter or exit cells or organelles is through membrane fusion. Because of this, membrane fusion plays a key role in a variety of important biological processes, including exocytosis, endocytosis, synaptic transmission, fertilization, and viral infection. However, membranes do not fuse easily under normal circumstances. As a result, membrane fusion frequently requires special proteins and is subject to highly selective controls?a constraint that is crucial, both for maintaining the identity of the cell itself and for maintaining the individuality of each of the intracellular compartments. Research in our group is obtaining a quantitative handle on membrane fusion at the molecular level, by bringing to bear a wide range of theoretical and experimental methods that are not typically applied to biological systems. Experimentally, we study two-dimensional lipid monolayers at the air-water interface, supported lipid bilayers at the solid-solution interface, and vesicles in solution, since they are compositionally easy to alter for studying membrane fusion in realistic environments. By elucidating membrane interactions in model systems at the molecular level, we gain the ability to engineer and manipulate biomembranes that mimic real life systems. This is the key to a better understanding of the fundamental cellular process of fusion as well as the use and design of lipid based (liposomal) drug delivery systems, where the critical concern is how to achieve significant delivery of the therapeutic across biological membranes.

- Lipid Rafts

Do membrane lipids mix uniformly or are they arranged in discrete micro-domains? What is the functional significance of the various types of lipids, their ratios, and protein interactions in membranes? These questions are some of the most intriguing issues in membrane biology. We address these questions by studying model lipid membranes where the composition and packing of the lipids can be precisely controlled while simultaneously determining their structure, orientation, and dynamic motions.

- Ligand-Receptor Interactions and Cellular Adhesion


Dynamic conditions are prevalent in cell adhesion events. For instance, the response of the immune system to infection involves the attachment and migration through tissues of patrolling cells in the circulating blood and lymph. Also, cell spreading and locomotion are achieved by coordinated attachment-detachment sequences. The clarification and relation between the range of attraction of a given ligand-receptor pair and the time required to bind is therefore crucial for our understanding of adhesion mechanisms and our ability to design biomimetic adhesive materials. Using Mother Nature as a template, we design model membrane systems to directly measure the adhesion between membranes as a function ligand-receptor bond strength, density of receptors, contact mechanics and dynamics. We then fold this fundamental physical understanding into simulations in order to obtain predictive abilities for rational engineering of biomimetic surfaces and liposomal based, targeted drug delivery systems.

Biosensing Membranes

- This is a brand new project supported by CPIMA (Center on Polymer Interfaces and Macromolecular Assemblies) an NSF-MERSEC Center bewtween Stanford, IBM and UC Davis. By combining polymer self-assembly and semi-conductor photolithography techniques, we are developing a novel platform technology for high throughput screening. More details will follow as this project evolves.