Porous Hydrogels: Present Challenges and Future Opportunities

In this feature article, we critically review the physical properties of porous hydrogels and their production methods. Our main focus is nondense hydrogels that have physical pores besides the space available between adjacent cross-links in the polymer network. After reviewing theories on the kinetics of swelling, equilibrium swelling, the structure-stiffness relationship, and solute diffusion in dense hydrogels, we propose future directions to develop models for porous hydrogels. The aim is to show how porous hydrogels can be designed and produced for studies leading to the modeling of physical properties. Additionally, different methods that are used for making hydrogels with physically incorporated pores are briefly reviewed while discussing the potentials, challenges, and future directions for each method. Among kinetic methods, we discuss bubble generation approaches including reactions, gas injection, phase separation, electrospinning, and freeze-drying. Templating approaches discussed are solid-phase, self-assembled amphiphiles, emulsion, and foam methods.

Polyfoam: Foam-Templated Microcellular Polymers

High internal phase emulsion (HIPE) templating has been developed as a robust method for the synthesis of porous polymers with designed porosity and void size. However, it has low starting material efficiency, requiring the removal of over 74% of the starting material to form interconnected porous structures. Foam templating methods attempt to resolve this issue by replacing the internal liquid phase with a gas phase. The current challenge for foam templating is to reach small void sizes (<100 μm), especially when using free-radical polymerization. We use a rapid gas dispersion method to form foam-templated hydrogels, which is an energy-effective method in comparison to the typical HIPE templating approach. We report hydrogels with an average void diameter of 63 μm at 70% porosity. The morphology and properties of foam-templated hydrogels are evaluated to show comparability to the HIPE-templated hydrogels of the same material.