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This thesis work is dedicated to my husband, Sotirios, who has been a source of love and encouragement during the challenges of graduate school and family life and to my son Vaios, the biggest joy in my life. I am truly thankful for having you both in my life. This work is also dedicated to my parents, Georgios and Stamatia Valai, who have always loved me unconditionally and whose good examples have taught me to work hard with patience and love for the things that I aspire to achieve.