Nanotribological properties, nanoppatterning and biological interactions of poly(cysteine methacrylate) brushes

Abstract The goal of this project was to develop a strategy to fabricate various nanometre and micrometre scale structures for biocompatible poly(Cysteine methacrylate) (PCysMA) brushes by photopatterning of 3-(2-bromoisobutyramido)propyl triethoxysilane (APTES-BIBB) film on a silicon surface. Micrometre scale structures were formed by exposure through a mask, and nanometre scale structures by Interference lithography. UV exposure causes debromination of the surfaces. Surface-initiated atom transfer radical polymerization (SI-ATRP) was used to grow the brushes from the ATRP initiators. Friction force microscopy (FFM) was used to investigate the conformations of the brushes as a function of footprint sizes in different environments. The friction - load relationship was found to be dependent on the conformation of the brushes, which is then was found to be influenced by the solvent medium. In a good solvent, sublinear friction-load relationships were acquired that were fitted with the Derjaguin-Muller-Toporov (DMT) model of contact mechanics, for nanostructured materials the contact mechanics were found to depend on the periodicity of the patterns. For large periods, the grafted polymer molecules exhibit a brush conformation, but this starts to collapse when the period is reduced below the critical value. In contrast, in poor solvent linear friction-load relationships were acquired and the friction coefficient increased as the period increased. In a poor solvent the polymer molecules collapse and form mushroom or pancake structures. These observations were rationalised by considering the friction values as a sum of an interfacial (shear) and plowing (load) dependent term. Unpatterned brushes of controlled, but varying densities were fabricated by maskless UV exposure of brominated films followed by SI-ATRP. By varying the exposure, the density of initiator sites was changed. The contact mechanics were studied as a function of brush coverage. In a good solvent, sublinear friction-load relationships were observed at high density, but the relationship became linear as the density decreased. The change is attributed to a change in the conformation, from brushes at high grafting density to mushrooms and the pancakes as the density decreased. In a poor solvent, the friction-load relationship was linear regardless of grafting density. This means that energy dissipation by molecular ploughing was 2 dominate for poorly solvated brushes. The friction coefficient increased initially as the grafting density decreased, then started to decrease as the density further reduced. The impact of nanotopography, chemistry and utility of non-biofouling of PCysMA brushes on the cultivation of mesenchymal stem cells (MSC) has been explored. AFM, ellipsometry, XPS, contact Angle goniometry, and fluorescence microscopy have been utilised to characterise the surfaces. Interference lithography (IL) was used to pattern three different types of SAM, including 2-nitrophenylpropyloxycarbonyl-protected aminosiloxane (NPPOC-APTES), (chloromethyl)phenyltrichlorosilane (CMPTS) and APTES-BIBB film. The patterned SAMs were used as templates to control SI-ATRP. Uniform polymer brush structures showed excellent resistance toward nonspecific adsorption of the cells, after 7 days of culture. The cells attached successfully on the patterned surfaces, but their alignments and organisations were different, depending on the feature sizes of grafted polymer lines and the chemistry of adhesive regions. Immunocytochemical methods were used to characterise the effect of nanoscale surface cues on cytoskeletal organisation in MSCs grown on these surfaces. A method was developed for the fabrication of lipid bilayers supported on a PCysMA brush layer with integral gold nanostructures, to facilitate spectroscopic characterisation using plasmonic techniques. Gold nanostructures were formed using IL to pattern a resist consisting of SAM of octadecanethiol on gold. Regions between nanostructures were functionalised with an aminosilane which was brominated and used as an initiator for SI-ATRP. The polymer growth was controlled so that swollen brushes in water had heights identical to the heights of the Au nanostructures. Supported lipid bilayers were formed on these materials and lipid mobility was characterised using fluorescence recovery after photobleaching (FRAP). Mobilities were achieved that were similar to those obtained for positive control surfaces (glass).