(189c) Micro-Rheology of Biologically Relevant Interfaces

Interfaces serve a variety of functions in the biological world -- partitioning organisms into distinct organs, organs into cells, and cells into organelles. Some trans-membrane proteins act as sensors and signals for cells, while others either prevent or facilitate transport across the phospholipid bilayers. A molecular layer of lung surfactant plays an essential role for respiration, by substantially reducing surface tension within the alveoli.

The mechanical properties of these interfaces are often essential to their function: cell and organelle membranes must remain fluid for membrane proteins to diffuse. Lung surfactant must be viscous enough to stay in the deep lung against the surface tension gradients that act to pull it out. Protein adsorption onto interfaces -- involving specific membrane-bound receptors or air-water interfaces -- can change the properties of those interfaces in important ways.

It is frequently difficult to measure the static and dynamic properties of these interfaces, for a number of reasons. First, it can be difficult to measure the viscosity and elasticity of a molecularly-thin layer without being swamped by the viscosity or elasticity of the (three-dimensional) materials on either side. Nonetheless, the shear viscosity of lipid bilayers determines membrane protein diffusivity, and the viscosity (or visco-elasticity) of lung surfactants keep them in the lung. Second, synthetic or purified biomaterials are often available only in minute quantities, and are generally quite expensive. Practically, it is often impossible to procure enough of these materials to make conventional measurements of surface properties. Finally, biologically-relevant interfaces often exhibit complex, multi-phase structures, including liquid crystalline domains, and lipid 'rafts' within bilayers.

Here we describe a new suite of novel micro-scale techniques we have developed for the sensitive measurement of the static and dynamic properties of biologically relevant interfaces:

1) Interfacial microrheology. We have developed a technique to microfabricate interfacially-active, ferromagnetic probes to measure the viscous and elastic response of fluid-fluid interfaces. By controling the shape, (Janus) surface chemistry, and magnetic properties of our probes, we make amphiphilic, ferromagnetic disk probes that straddle surfactant interfaces. Externally torquing the disk enables us to probe the mechanical (viscous and elastic) response of the intefaces to various stresses. Significantly, our technique enables the simultaneous visualization of the interface itself as it is being deformed -- allowing the mechanical (visco-elastic) response to be related directly to the behavior and evolution of the structure of the interface itself. We illustrate with a molecular monolayer of DPPC, a ubiquitous phospholipid that is the predominant component of lung surfactant. We show that the mechanical response of DPPC can be far more complicated -- and interesting -- than "simple" viscosity measurements would indicate. In particular, we directly visualize liquid crystalline domains of DPPC as they are deformed by the probe. Surprisingly, we show a strong history-dependence of these films, direct evidence of "shear-banding" (inhomogeneous flow profiles) and yield stress behavior (wherein the interface does not flow until a critical stress is applied). Furthermore, we find DPPC to have an extremely long "memory", recoiling for nearly an hour following an imposed flow. The ability to deform on demand, while directly visualizing the interface, was essential to properly understand this material. Finally, we note that our technique uses ~10 micron probes, and is thus amenable to very small interfaces.

2) Interfacial microtensiometry. Conventional techniques to measure the surface pressure (or tension) of surfactant monolayers often requre significant quantitites of surfactant. To enable measurements on sample-limited interfaces, we have developed a microfabricated tensiometer that directly sits on a fluid-fluid interface, and deflects in response to the surface pressure of an insoluble surfactant film external to the device. The sensitivity, dynamic range, and surface chemistry of the device can be tuned and designed in a straightforward fashion. We demonstrate the efficacy of our technique against conventional techniques, and comment on extensions that would enable ?Langmuir micro-troughs? and kinetic measurements of surfactant or protein adsorption.