Poly(ethylene glycol)-based hydrogels as transparent porous network for diffusivity studies

Although hydrogels are widely studied for their controlled-release properties, their use as model porous media to investigate diffusion in three-dimensional confinement has been limited. The strong opacity usually associated to micron-scale porous networks does not allows the direct visualization of diffusing objects. A better understanding of how micrometric objects move within confined systems would enable huge improvements in the understanding and control of migration processes in disordered, porous media, such as bioremediation, biofertilization, and microbial therapy. Here we present a facile synthesis of transparent porous poly(ethylene glycol)-based (PEG) hydrogels that allows the study of mobility and migration properties of microscopic objects in confined systems. Through polymerization of acrylated-polymeric unit, we synthetized biocompatible hydrogels with high transparency. By freeze-drying we induced the desired porosity without significant effects on the transparency. Thanks to its low opacity, confocal microscopy analysis allowed a deep visualization of the hydrogels’ structure, and the structural characterization of the porous media by morphological image analysis revealing interconnected networks with tunable pore size at the micron-scale. The diffusion properties of thermo-responsive pNIPAM microgels (1.8μm in diameter) through the PEG gels were studied, providing a reference for the investigation of more complex living systems (i.e. bacteria). Positions, trajectories and mean squared displacements of the pNIPAM particles were determined by particle tracking system. Consistent with the structural features, a transition from diffusive to subdiffusive motion was observed when the pore size approaches the particles size. The presented system can be easily modified in terms of the final architecture and interface reactivity allowing to study the effect of the microscopic, internal structure of the gels on bacterial diffusion.