Surface and tribological behaviors of the bioinspired polydopamine thin films under dry and wet conditions.

Dopamine is a "sticky" biomolecule containing the typical functional groups of mussel adhesive proteins. It can self-polymerize into a nanoscale thin film on various surfaces. We investigated the surface, adhesion, friction, and cracking properties of polydopamine (PDA) thin films for their effective transfer to functional devices and biocompatible coatings. A series of surface characterizations and mechanical tests were performed to reveal the static and dynamic properties of PDA films coated on glass, polydimethylsiloxane (PDMS), and epoxy. We found that PDA films are highly hydrated under wet conditions because of their porous membrane-like nanostructures and hydrophilic functional groups. Upon dehydration, the films form cracks when they are coated on soft substrates due to internal stresses and the large mismatch in elastic modulus. The adhesive pull-off force or the effective work of adhesion increased with the contact time, suggesting dynamic interactions at the interface. A significant decrease in friction forces in water was observed on all three material surfaces coated with PDA; thus, the film might serve as a water-based lubrication coating. We attributed the different behavior of PDA films in air and in water to its hydration effects. These research findings provide insight into the stability, mechanical, and adhesive properties of the PDA films, which are critical for their applications.