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GG8.05 - Brush-Hydrogels with Graded and Gradient-Like Properties: From Organic to Hybrid Biomimetic Coatings 
April 10, 2015   10:15am - 10:30am

The fabrication of organic coatings with well-defined structure and physico-chemical properties is of particular interest in several disciplines. Bio-organic layers on different length-scales, featuring variable composition, structure and modulus have been observed in several biological systems, such as human cartilage, mammalian skin and the nacre of oyster shells [1-3]. These complex materials consist of mechanically graded structures that resist and respond to the external normal and shear forces, protecting underlying tissues from incurring damage. Among the many �natural� coating systems, human epidermis is constituted by different layers of cells which present diverse properties to confer resistance against abrasion. In order to mimic these natural structures, materials scientists have been made numerous efforts to fabricate coatings with graded and gradient-like mechanical properties within a single film. Despite this, the fabrication of a full-organic, polymer-based coating architecture featuring nano-scale variations of properties still represents a challenging task. In order to fabricate polymer films presenting discontinuous and continuous variations of mechanical and chemico-physical properties we applied sequential surface-initiated polymerization (SIP) of different monomer compositions to create polymer brush/hydrogel films with vertically defined structure. Specifically, poly(hydroxyethyl methacrylate) (PHEMA) brush and brush-hydrogel layered films were synthesized by surface-initiated atom transfer polymerization (SI-ATRP) in the presence of different concentrations of diethylene and tetraethylene glycol dimethacrylate (DEG/TEGDMA). Sequential SI-ATRP of monomer mixtures was employed to fabricate brush-hydrogel films presenting vertically graded properties. Alternatively, continuous variation of monomers during the SI-ATRP resulted in brush-hydrogels featuring gradient-like variations of polymer architecture through the film thickness. The chemical, mechanical and tribological properties of the films were characterized by ellipsometry, FT-IR and colloidal probe atomic force microscopy (CP-AFM). All these measures confirmed the influence of polymer architecture on the properties of the films at specific depths. Additionally, graded and gradient-like brush-hydrogels were used as reactors for the synthesis of polymer-inorganic hybrids presenting metal nanoparticles (NPs) embedded within the films. Depending on the vertical crosslinker content, different NPs morphologies were obtained at determined depths. Through this multi-step fabrication, polymer-inorganic hybrids with vertically defined structures and variable NPs loading were obtained. In conclusion we believe these methods will represent an easy and effective mean to form coatings with defined mechanical properties and tunable optical characteristics. 1. R. A. Stockwell et al., Nature 1967 2. J. C. Mackenzie, Nature 1969 3. H. D. Espinosa et al., Nat. Commun. 2011

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