Figure 2.

A scaled representation of the mucus gel depicted as a complex, porous network, and some relevant foreign constituents. Large bacteria, which are several micrometres in width and length, do not migrate through the mucus gel but may infect tissues where the mucus surface is disrupted or thin. Antibodies (~10 nm) diffuse readily, as do plasmid DNA; however, these small and potentially therapeutic molecules may be degraded by acidic environments or enzymes in the mucus. Long and narrow carbon nanotubes, and other particulate matter (pollutants) may be present, but are typically sequestered due to entanglement and binding to mucin fibres. Some virus particles, typically <200 nm, can migrate quickly through the mucus gel by reducing their surface interaction with mucin fibres. Fluorescent polystyrene particles that are surface-modified by PEG can mimic this behaviour (A), but still adhere if the particle surface is too sparsely coated (B) or entangle if coated with PEG polymers that are too long (C)¹. PEG coatings have also been shown to enhance diffusion of polymer particles that are endowed with the potential to carry drugs through the mucus gel⁸.