We report here on the formation of a bioactive hierarchically structured membrane by self-assembly. The membrane is formed with hyaluronic acid and peptide amphiphiles with binding affinity for heparin, and its hierarchical structure contains both an amorphous zone and a layer of fibrils oriented perpendicular to the membrane plane. The design of bioactivity is based on the potential ability to bind and slowly release heparin-binding growth factors. Human mesenchymal stem cells seeded on these membranes attached and remained viable. Basic fibroblast growth factor (FGF2) and vascular endothelial growth factor (VEGF) were incorporated within the membrane structure prior to self-assembly and released into media over a prolonged period of time (14 days). Using the chicken chorioallantoic membrane (CAM) assay, we also found that these membranes induced a significant and rapid enhancement of angiogenesis relative to controls.

The mechanical properties and water permeability of hierarchical self-assembling membranes and sacs formed from oppositely charged high molecular weight hyaluronic acid (HA) and small molecule peptide amphiphiles (PAs) were studied. Techniques to make reproducible 2D planar membranes and 3D spherical sacs from these materials were developed while membrane inflation and osmotic swelling were used to quantify the mechanical properties and water permeability of these structures. It was found that incubation time and concentration of HA used had an effect on the area modulus and water permeability of the membranes. These factors also affected the kinetics of membrane growth as evidenced in SEM micrographs, which showed differences in the structure. Area modulus of membranes changed from about 6 N m À1 for the lower weight percent HA system at the shortest incubation time of 3 minutes, up to 12 N m À1 for the higher weight percent HA system at the longest incubation time of 60 minutes. Water permeability decreased with incubation time, but the lower weight percent HA system showed a lower water permeability when compared to the higher weight percent HA system at the same incubation time. This type of characterization and understanding of the structure-property relationships in self-assembling systems are necessary steps in both using these structures for specific applications and applying this knowledge to design new and better materials in the future.

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Self-assembly in the presence of external forces is an adaptive, directed organization of molecular components under nonequilibrium conditions. While forces may be generated as a result of spontaneous interactions among components of a system, intervention with external forces can significantly alter the final outcome of self-assembly. Superimposing these intrinsic and extrinsic forces provides greater degrees of freedom to control the structure and function of self-assembling materials. In this work we investigate the role of electric fields during the dynamic self-assembly of a negatively charged polyelectrolyte and a positively charged peptide amphiphile in water leading to the formation of an ordered membrane. In the absence of electric fields, contact between the two solutions of oppositely charged molecules triggers the growth of closed membranes with vertically oriented fibrils that encapsulate the polyelectrolyte solution. This process of self-assembly is intrinsically driven by excess osmotic pressure of counterions, and the electric field is found to modify the kinetics of membrane formation, and also its morphology and properties. Depending on the strength and orientation of the field we observe a significant increase or decrease of up to nearly 100% in membrane thickness, as well as the controlled rotation of nanofiber growth direction by 90 degrees, resulting in a significant increase in mechanical stiffness. These results suggest the possibility of using electric fields to control structure in self-assembly processes involving diffusion of oppositely charged molecules.

The binding of streptavidin to biotin located at the terminal ends of poly(ethylene oxide) tethered to a planar surface is studied using molecular theory. The theoretical model is applied to mimic experiments (Langmuir 2008(Langmuir , 24, 2472 performed using drop-shape analysis to study receptorligand binding at the oil/water interface. Our theoretical predictions show very good agreements with the experimental results. Furthermore, the theory enables us to study the thermodynamic and structural behavior of the PEO-biotin+streptavidin layer. The interfacial structure, shown by the volume fraction profiles of bound proteins and polymers, indicates that the proteins form a thick layer supported by stretched polymers, where the distribution of bound proteins is greater than the thickness of the height of one layer of proteins. When the polymer spacer is composed of PEO (3000), a thick layer with multi-layers of proteins is formed, supported by the stretched polymer chains. It was found that thick multi-layers of proteins are formed when long spacers are present or at very high protein surface coverages on short spacers. This shows that the flexibility of the polymer spacer plays an important role in determining the structure of the bound proteins due to their ability to accommodate highly distorted conformations to optimize binding and protein interactions. Protein domains are predicted when the amount of bound proteins is small due to the existence of streptavidinstreptavidin attractive interactions. As the number of proteins is increased, the competition between attractive interactions and steric repulsions determines the stability and structure of the bound layer. The theory predicts that the competition between these two forces leads to a phase separation at higher protein concentrations. The point where this transition happens depends on both spacer length and protein surface coverage and is an important consideration for practical applications of these and other similar systems. If the goal is to maximize protein binding, it is favorable to be above the layer transition, as multiple layers can accommodate greater bound protein densities. On the other hand, if the goal is to use these bound proteins as a linker group to build more complex structures, such as when avidin or streptavidin serves as a linker between two biotinylated polymers or proteins, the optimum is to be below the layer transition such that all bound linker proteins are available for further binding. II. INTRODUCTIONLigand-receptor interactions are an important control mechanism in many biological systems. For example, the binding of extracellular matrix proteins to specific receptors on the cell surface triggers signal transduction pathways allowing the cell to sense and react to its environment in Correspondence to: Igal Szleifer. NIH Public Access Author ManuscriptLangmuir. Author manuscript; available in PMC 2010 October 20. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptways such as aggregating with ot...