MRSEC Seminar

Physicochemical Mechanics in Asymmetric Nanomembranes

James K. Ferri

Department of Chemical Engineering, Lafayette College, Easton, Pennsylvania
Max Planck Institute for Colloids and Interfaces, Golm/Potsdam, Germany

Wednesday, August 13, 10:00 am

Cook Hall room 2058
2220 Campus Drive, Evanston IL

Naturally occurring materials have long served as an inspiration for the development of new materials and technology. For example, biological macromolecules are ideal building blocks for engineering self-assembled nanostructured membranes at fluid-fluid interfaces. These systems present interesting potential for reversible and reconfigurable fabrication, and the implication that specially engineered structures can undergo self-repair, self-replication, or self-degradation. Although an increasing number of investigations describe new routes to produce these surface materials, detailed studies of both mechanical and transport properties and the constitutive parameters that describe them are relatively scarce. This is because quantitative experimental techniques are limited due to the length scales involved.

We discuss fabrication and characterization of polysaccharide and protein-based nanomembranes having thicknesses less than 100 nm and present data for hyaluronic acid (HA)-poly-L-lysine (PLL), fibrin, and elastin-based nanomembranes. In most cases, electrostatic complexation, hydrophobic association, or covalent cross-linking of these molecules at fluid-fluid interfaces lead to the formation of supramolecular networks which confer properties such as mechanical rigidity that are outside of the description provided by equilibrium surface thermodynamics. We describe an experimental approach and accompanying theoretical framework to measure the intrinsic mechanical properties of the materials.

For the mechanical properties, the polysaccharide nanomembranes display linear elasticity when (HA) and (PLL) are cross-linked to each other; otherwise, essentially all deformations of these materials are plastic. In the cases of the protein-based membranes, our measurements indicate strain-stiffening nonlinear elasticity and a nearly perfect elastic recovery for total strain up to 70% which is consistent with recent results reported by W. Lui et al. Science 313 (634) 2006.

The rates of enzymatic degradation of each nanomembrane are also measured as a function of the concentration of enzyme appropriate to each. We demonstrate that although externally trigged degradation proceeds more rapidly, it is possible to program auto-degradation by incorporation of enzymes during membrane assembly. We also measure the impact of strain on the enzyme-mediated degradation and show a direct relationship between the rate of degradation and strain.

Host: Professor Kenneth Shull, MSE