Figure 4. Molecular properties influence enrichment and longevity within mucus layers.

Particles with concentrated regions of positive charge or hydrophobicity can bind to and become enriched in mucus layers (left, red dots). However, binding to mucus leads to faster clearance from the body due to sloughing off of the outer mucus layer. Particles that do not bind to mucus do not become enriched in mucus layers but are able to penetrate deeper and are not cleared as quickly from the body (right, blue dots). Both mucoadhesive and muco-inert particles present advantages and tradeoffs in terms of drug design and delivery. (This figure was adapted from Lai et al. (2009).)