Matej Kanduč from the Department of Theoretical Physics and his collaborators published an article in ACS Nano with the title How the Shape and Chemistry of Molecular Penetrants Control Responsive Hydrogel Permeability in which they uncover the molecular principles of permeability and selectivity in hydrogels. The permeability of small molecules (drugs, toxins, reactants, etc.) through hydrogels is a central property in the design of soft functional materials in biomedical, pharmaceutical, and nanocatalysis applications. Using atomistic simulations, the authors found that dense hydrogels are extremely selective because of a delicate balance between the partitioning and diffusivity of the molecules. These properties are sensitively tuned by the molecular size, shape, and chemistry, leading to vast cancellation effects, which nontrivially contribute to the permeability. The outcomes can be used as approximate guiding (“rule-of-thumb”) principles to optimize penetrant or membrane physicochemical properties for a desired permeability and membrane functionality.