ABSTRACT 
   This thesis deals with the biological activity of fibronectin (FN) as interface protein in the interaction between cells and materials. It investigates protein response, in terms of adsorbed amount and conformation, to different physico-chemical properties of the material. Moreover, early cell response and cellular functionality are correlated to the state of the protein adsorbed onto the material.
   For that, different sets of materials with tailored physico-chemical properties were prepared. FN distribution on the different surfaces was characterized via atomic force microscopy (AFM) and its surface density was quantified by radiolabelling and western blotting. Cell response was evaluated in terms of initial adhesion to the surfaces, and of the subsequent processes of differentiation, proliferation, extracellular matrix reorganization and secretion. 
   The effect of nanotopography on FN adsorption and cell behavior was investigated using a set of topographies tailored at the nanoscale, obtained by spin casting of poly(L-lactic) acid/polystyrene (PLLA/PS) solutions of different concentrations. PLLA migration to the top of the film during the spin casting process provides PLLA surfaces with nanopits of different sizes (14, 29 and 45 nm). The size of the nanostructure affects the density of adsorbed FN, which is higher on the nanotopography with smaller nanostructures, whereas FN is evenly distributed through the peaks and valleys of the different nanotopographies when FN adsorption takes place from solutions of concentration of 10 ug/ml or higher (thus including the concentration employed in cell cultures, 20 ug/ml). With regard to initial cell response, more developed focal adhesions and stronger cell-mediated reorganization of the adsorbed FN layer are observed on surfaces with higher nanostructures (29 and 45 nm), resulting in enhanced a greater production and organization of new matrix.
   On the other hand, a family of materials with subtle variations in their chemical composition were employed: acrylic polymers (polymethyl, ethyl, butyl acrylate -PMA, PEA and PBA respectively-) which only differ in the length of the side chain (number of carbons). This change in surface chemistry provides materials of different stiffness and surface mobility, allowing to identify the latter as a new physical parameter able to regulate protein adsorption and cell differentiation. Transition from PMA to PEA drastically alters FN distribution at the material interface, from a globular conformation on PMA to the formation of a well-interconnected FN network on PEA. At increasing surface mobility a FN network is still formed, but with a faster adsorption dynamics (on PBA). Cell adhesion and differentiation to the osteoblastic lineage is enhanced with the surface mobility of the material.
   PEA was further studied due to its ability of triggering FN organization into a physiological fibrillar network with enhanced biological activity in the absence of cells. The effect of vitronectin (VN), an extracellular adhesion protein, on FN adsorption and cell response was investigated. FN surface density and distribution onto PEA surfaces is altered when FN is adsorbed competitively with VN. The presence of VN during FN adsorption is likely to provide higher mobility to the FN network, contributing to improve cell-mediated FN reorganization.
   FN reorganization on PEA was analyzed at the nanoscale via AFM in order to analyze the observed variations in the adsorbed FN layer. A less interconnected FN network is observed around cells, while a denser network is observed on regions far from cells: the occurrence of matrix degradation (among other possibilities) is discussed in order to explain the effect of cells on the protein network in the initial stages of cell adhesion on PEA.
   Finally, preliminary assays were performed via AFM in liquid environment in order to assess fibronectin adsorption and distribution under conditions more similar to physiological ones.