della Vecchia, Nicola Fyodor (2014) Biomimetic Polymers and Related Molecular Systems: Synthesis, Structure, Physico-chemical Properties and Applications. [Tesi di dottorato]

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Abstract

Harnessing Nature’s chemical principles and logics for designing efficient, sustainable and biocompatible multifunctional molecular systems is an important goal in materials science steering advanced technological solutions for various applications [1-3]. Unique opportunities in this connection are offered by a variety of structurally complex and heterogeneous polymeric systems derived from the oxidative polymerization of phenolic and polyphenolic substrates. Besides their use in food industry and biomedicine, phenolic biopolymers and their synthetic mimics attract growing interest in various areas of materials science as they can be obtained at low cost from renewable natural sources and are biocompatible, biodegradable and non-toxic. Eumelanins, the black insoluble pigments of human skin, hair, eyes and melanomas, are a typical case in point. Synthetic eumelanin polymers modeled to the black functional human pigments, and polydopamine (pDA), a black insoluble biopolymer produced by autoxidation of the catecholamine neurotransmitter dopamine (DA), have burst into the scene of materials science as versatile bioinspired functional systems for a broad range of applications. Synthetic eumelanins from dopa or 5,6-dihydroxyindoles are the focus of increasing interest as UV-absorbing agents, antioxidants, free radical scavengers and water-dependent hybrid electronic-ionic semiconductors. pDA is characterized by extraordinary adhesion properties providing efficient and universal surface coating for e.g. drug delivery, microfluidic systems, and water-condensing devices. Because of their peculiar physicochemical properties, eumelanins and pDA hold considerable promise in nanomedicine and bioelectronics, as they are biocompatible, biodegradable and exhibit suitable mechanical properties for integration with biological tissues. So far, however, progress in pDA and eumelanin applications has been based more on empirical approaches than on a solid conceptual framework of structure-property relationships. The objective of the present PhD Thesis was dual: on one side it was directed to address key structural issues that have remained so far unsettled, on the other side it was aimed at rationally tailoring eumelanin properties to engineer new synthetic biopolymers for various applications. Main outcomes of the PhD project include: a) The identification of key structural units of pDA and their relationship with adhesion properties; b) The definition of the role of the buffer on the particle growth and mode of aggregation of pDA during the autoxidation of dopamine; c) The synthesis and characterization of novel copolymers of dopamine with aromatic amines, leading to filmable semiconductor materials with n-type behavior in metal-insulator-semiconductor (MIS)-type devices; d) The synthesis of cysteinyldopamine (CDA) conjugates as the building block for a novel type of biomaterial with a peculiar UV-dependent conductivity in MIS devices; e) The realization of polyCDA films as interfaces for studies of cell growth; f) The definition of structural and aggregation features determining the superior antioxidant and free radical scavenger properties of synthetic eumelanin for 5,6-dihydroxyindole-2-carboxylic acid (DHICA) compared to polymer from 5,6-dihydroxyindole (DHI). Part of the work relating to research line e) was carried out during a three-month stage in the laboratory of Professor Davide Bonifazi at Namur University (Belgium). Overall, the results described in this thesis provide unprecedented advances toward an understanding of the correlation between eumelanin and pDA properties and their micro- and nanometer-scale characteristics, and show how it is possible to translate chemical knowledge about eumelanin complexity into new practical guidelines for selective manipulation of eumelanin/pDA structure and properties for the fabrication of novel functional materials.

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