The controlled exchange of molecules between biological entities (organelles, cells, or organisms) and their environment is critical for life. Biological gels appear well suited to achieve such selective exchange: A gel within the nuclear pore controls the passage of material between nucleus and cytoplasm. Mucus that lines our lungs allows us to expel ingested particles and defends the cells beneath from a variety of pathogens. The extracellular matrix in connective tissue regulates the exchange of ions and proteins with the epithelium and between fibroblasts. The extracellular matrix within biofilms forms a barrier toward, for example, antibiotics. Biological gels represent a whole class of molecular assemblies that are understudied, with many fascinating and important structure-function problems to investigate and medical/engineering applications to invent.
Our focus is on basic mechanisms by which mucus barriers exclude, or allow passage of different molecules and pathogens, and the mechanisms pathogens have evolved to penetrate mucus barriers. We hope to provide the foundation for a theoretical framework that captures general principles governing selectivity in mucus, and likely other biological hydrogels such as the extracellular matrix, and bacterial biofilms. Our work may also be the basis for the reconstitution of synthetic gels that mimic the basic selective properties of biological gels.