Fig 4. Tobramycin diffusion.

Numerical solution of the reaction-diffusion equation with power-law binding to the biofilm matrix in a spherical bead with radius R and diffusion constant D for the free tobramycin. The free tobramycin is displayed as a function of radius and time. Time is expressed in units of R²/D. a) With binding. The external concentration of tobramycin is kept at a = 4 mg/l from time t = 0. The maximum time shown is t = 5R²/D. b) Without binding. The maximum time shown is t = 0.5R²/D. c) With binding, external concentration is kept at a = 4 mg/l from time t = 0 to t = R²/D. d) Without binding, external concentration is kept at a = 4 mg/l from time t = 0 to t = R²/D.