Continuity and Growth Mechanism of Polydopamine Thin Films



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With their ability to adhere to a wide variety of underwater surfaces, mussels have inspired a new line of investigation into surface chemistry and adhesives. This unique ability is due to byssal threads that mussels excrete, composed of proteins with a high concentration of the amino acid 3,4-dihydroxyphenlalanine (DOPA). Dopamine (DA), a small molecule mimic of DOPA, is able to polymerize under oxidizing conditions. Polydopamine (PDA) is known as a universal adhesive, although its adhesive properties are affected by many different experimental variables, including pH, buffer type, oxidizing agent, substrate, and coating method. Previous research in the Chen lab has determined the optimized conditions for spin coating PDA. These include DA concentration of 4 mg/mL, sodium periodate (SP) as the oxidizing agent with [SP]:[DA] = 2, 0.25 M pH = 6 acetate buffer, and 1 min adhesion time prior to spin. This project focuses on the differences in PDA adhesion on substrates with varying functional groups. PDA is able to form many different chemical interactions, but it is limited by the chemistry of the substrate. This project examines PDA adhesion on native silicon wafers (SiO2) and substrates containing polydimethylsiloxane (PDMS) and aminopropyl methylsiloxane – dimethylsiloxane copolymer (PADMS). PDMS and PADMS are chemically similar; however, continuous PDA morphology is obtained on SiO2 and PADMS, while the PDA films contain cracks on PDMS. When PDMS and PADMS polymers were mixed to form composite substrates, only a minuscule fraction of PADMS, 1/20,000, is required for the morphology of the adhered PDA film to utilize the same growth mechanism as that on pure PADMS. In order to form a continuous film on pure PDMS, the buffer solution was diluted to reduce interference from salt ions. Dip coating is the traditional method of PDA film deposition; however, the research for this project was conducted using spin coating. Therefore, a modified dip coating method was created in order to ensure that any conclusions drawn from this research would be relevant for both spin and dip coating. Additionally, in an attempt to extend the lifespan of PDA solutions, the pH was lowered at the aging time with the most adhesive particles in solution. This would theoretically halt additional polymerization and allow one PDA solution to be used multiple times.



Polydopamine, Surface Chemistry, Thin Film, Adhesives