A Highly Ordered Material from Magnetically Aligned Peptide Amphiphile Nanofiber Assemblies

Abstract

Materials that are built up out of molecular building blocks via a spontaneous hierarchical self-assembly process have intrigued scientists for many years. Nature provides, in this respect, beautiful examples, where, through extreme control over recognition and consequent directed self-assembly on a molecular level, discrete objects are constructed, such as the triple helical rods found in collagen, virus capsids, or the ribosome. Our present synthetic capabilities allow us to introduce many noncovalent interactions into molecules to construct supramolecular assemblies. Besides hydrogen bonding and ionic interactions, one of the main driving forces for self-assembly in an aqueous environment is hydrophobic forces. It is the subtle interplay and balance between all forces that enable a system to smoothly generate well-defined assemblies. In this respect, peptide amphiphiles have gained much interest as molecular building blocks for the development of new surfactants with unprecedented properties, leading to novel applications in the fields of materials and biomedical research. Peptide amphiphiles have been shown to provide a starting point for the generation of materials with a high degree of order on a nanoscopic level that imparts a functional scaffold that can be applied for, for example, biomineralization or cell proliferation. However, the designer process normally stops at the stage of spontaneous assembly, and macroscopic control over the organization of the assemblies is often not obtained. One of the methods to introduce this next level of organization is a further processing step of the supramolecular materials using external forces. There are many different external forces that can be applied. Traditionally, mechanical alignment and electric forces for orientation of materials have often been used. Another interesting, although less often used directional force, is a strong magnetic field. Magnetic alignment is an extremely versatile technique that exploits the anisotropy in diamagnetic susceptibility of assemblies of molecules. Such a procedure is very attractive as it is contact free, homogeneously effective over the whole sample, and can be used to produce structured thin films as well as bulk material. One of the drawbacks of the use of magnetic fields is that the aligning force is very small. The ability to align structures depends on the anisotropy of the diamagnetic susceptibility of the molecules and the strength of the applied magnetic field, which results in an energy difference DE between parallel and perpendicularly aligned molecules determined by using Equation 1