This document discusses biological hydrogels and their use as selective diffusion barriers. It defines hydrogels as three-dimensional polymer networks that can swell in water, giving them both solid and liquid properties. Hydrogels are biocompatible and can be designed to respond to environmental stimuli. They are classified based on origin, water content, porosity, and degradability. The document also examines hydrogel properties, fabrication methods, applications for drug delivery, and filtering mechanisms related to mesh size and electrostatic interactions.