Hydrophobic forces as a key factor in crystalline biofilm formation on ureteral stents

Current discussions about biofilm formation focus on the solid/liquid interface between a medical device and body fluids. Yet it has been shown that gas bubbles (GB) can stably form on ureteral stents in artificial urine and that their fate depends on the stent's surface properties. The liquid/gas interface constitutes an adhesion site for precipitating salts as well as hydrophobic organic molecules.

The surface wettability of polyurethane stents is varied by coating with amorphous hydrogenated carbon (a-C:H). GB and crystalline biofilm formation on the stents are investigated in a novel encrustation device which avoids gravitation- or sample-position-related influences on the results.

Bigger and more stable GB form on hydrophobic stents than on hydrophilic, coated stents. Appearance and amount of crystalline deposits differ significantly between the surfaces. With decreasing wettability the number of hollow crystalline spheres and the mass of precipitate increase.

On hydrophobic surfaces, stable GB increase precipitation of salts and become incorporated in the growing encrustation layer in vitro. In contrast, GB quickly lift off from hydrophilic surfaces taking part of the precipitate with them. This self-cleaning mechanism slows down the encrustation process. A similar effect may explain the prolonged complication-free indwelling time of amorphous-carbon coated stents in vivo.