Topography and wettability control in biocompatible polymer for BioMEMS applications

This paper elucidates the extent of changes in surface wettability as influenced by the surface topography on a conducting polymer, polypyrrole. As-deposited, anion-doped polypyrrole is subjected to a series of incremental redox potentials ranging from -0.6 V to +1.5 V (vs. Ag/AgCl), and its surface properties are examined via SEM, AFM, and contact angle measurements. A causal relationship is found to exist between the oxidation states of polypyrrole and its surface topography. At redox potential lower than +0.6 V, the polymer is completely hydrophilic (contact angle <90deg) as ascribed partially to its smooth surface. It becomes less hydrophilic in the range of +0.6-+1.0 V and turns totally hydrophobic beyond +1.0 V. Such a change in surface wettability is attributed in part to the increased surface roughness, as measured by AFM. As redox potentials increase from -0.6 V to +1.5 V, the corresponding average surface roughness rises from 3.1 nm to 31.1 nm - a 10-fold increase. Visual inspection by SEM indicates the formation of islets on the polymer during the oxidation process. Based on our experimental results, we hypothesize that as redox potential increases, changes of surface topography decrease the surface wettability of polypyrrole.