The dynamic behaviour of supramolecular systems is an important dimension of their potential functions. Here, we report on the use of stochastic optical reconstruction microscopy to study the molecular exchange of peptide amphiphile nanofibres, supramolecular systems known to have important biomedical functions. Solutions of nanofibres labelled with different dyes (Cy3 and Cy5) were mixed, and the distribution of dyes inserting into initially single-colour nanofibres was quantified using correlative image analysis. Our observations are consistent with an exchange mechanism involving monomers or small clusters of molecules inserting randomly into a fibre. Different exchange rates are observed within the same fibre, suggesting that local cohesive structures exist on the basis of β-sheet discontinuous domains. The results reported here show that peptide amphiphile supramolecular systems can be dynamic and that their intermolecular interactions affect exchange patterns. This information can be used to generate useful aggregate morphologies for improved biomedical function.

The surface of solvent cast chitosan membranes was modified using a two-step procedure. Oxygen plasma treatment was used at the first activation step followed by vinyl monomer graft polymerization. Two monomers were used in order to compare the influence of different functional groups on cell adhesion and proliferation; acrylic acid (AA) was used to introduce carboxyl groups and vinyl sulfonic acid (VSA) was used as a source of sulfonic groups. The surface chemistry/energy changes were characterized by means of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR-ATR), and contact angle measurements. Additionally, alterations in the surface morphology were investigated by scanning electron microscopy (SEM). XPS analyses confirmed the polymer grafting on the surface; an S 2s peak appears in the VSA survey spectrum and an O-CLO peak emerges in the C 1s high resolution spectrum after AA grafting. Moreover, contact angle measurements showed an increment in the values of the surface energy polar and Lewis base components for all treated samples, confirming the introduction of additional polar groups by the modification processes. FTIR-ATR spectra showed no significant difference between treated and original materials. These results confirmed that only the very top (a few angstroms) surface layer, but not the bulk of the material, was modified. The effect of modification on the adhesion and proliferation of osteoblast-like cells was studied on a preliminary basis. Direct contact tests were performed using a human osteosarcoma cell line (SaOs-2). Cell morphology (optical microscopy and SEM) and cell viability (MTS test) were evaluated for untreated and surface modified membranes. The results revealed that both plasma treatment, and the presence of sulfonic groups on the surface of chitosan membranes, improve SaOs-2 adhesion and proliferation when compared to untreated or AA-grafted membranes. This effect was strongly related to the polar and Lewis basic components of the total surface energy.