Micro-gel in transparent porous hydrogel: synthesis, morphological characterization, and diffusivity studies

Even though hydrogels are widely studied for their controlled-release properties, their use as model porous media to investigate diffusion in three-dimensional (3D) confinement has been limited. The main problem with the direct visualization of the dynamic motion in micro-porous systems is the large opacity of these networks. A better understanding of how micrometric objects move within confined systems would allow huge improvements in the understanding and control of migration processes in disordered, porous media, such as bioremediation, biofertilization, and microbial therapy. We developed a facile synthesis of 3D transparent porous network of poly(ethylene glycol) (PEG) that allows the study of mobility and migration properties of microscopic objects loaded into this hydrated structure. Through polymerization of the acrylated-polymeric unit (PEGDA) we synthetized biocompatible hydrogels with high transparency1. By freeze-drying, the desired porosity at the micron scale was generated without significantly affecting the transparency of the hydrogel. Confocal microscopy revealed the heterogeneity of the micrometric porous network. The low opacity of the final hydrogel allowed a deep visualization of the structure, even beyond 100μm in depth. Structural and geometrical characterization of the porous media were carried out by morphological image analysis revealing highly interconnected networks structure with elongated pores having a smaller section ranging from 12 to 6 μm. The transport properties of thermo-responsive pNIPAM microgels (1.8μm in diameter) inside 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 using particle tracking. These revealed a transition from diffusive to subdiffusive motion when the pore size approaches the particle 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.