  Bacterial cellulose biofilms are complex networks of strong interwoven
nanofibers that control transport and protect bacterial colonies in the film.
Design of diverse applications of bacterial cellulose films also relies on
understanding and controlling transport through the fiber mesh, and transport
simulations of the films are most accurate when guided by experimental
characterization of the structures and the resultant diffusion inside.
Diffusion through such films is a function of their key microstructural length
scales, determining how molecules, as well as particles and microorganisms,
permeate them. We use microscopy to study the unique bacterial cellulose film
structure and quantify the mobility dynamics of various sizes of tracer
particles and macromolecules. Mobility is hindered within the films, as
confinement and local movement strongly depend on void size relative to
diffusing tracers. The biofilms have a naturally periodic structure of
alternating dense and porous layers of nanofiber mesh, and we tune the
magnitude of the spacing via fermentation conditions. Micron-sized particles
can diffuse through the porous layers, but can not penetrate the dense layers.
Tracer mobility in the porous layers is isotropic, indicating a largely random
pore structure there. Molecular diffusion through the whole film is only
slightly reduced by the structural tortuosity. Knowledge of transport
variations within bacterial cellulose networks can be used to guide design of
symbiotic cultures in these structures and enhance their use in applications
biomedical implants, wound dressings, lab-grown meat, and sensors.
