Unidirectional freezing as a tool for tailoring air permeability in macroporous poly(ethylene glycol)-based cross-linked networks

Unidirectional freezing followed by photopolymerization at subzero temperatures was used to obtain highly air-permeable monoliths with ordered porous structures. Scaffolds were obtained from aqueous solutions of a poly(ethylene glycol)dimethacrylate (PEGDMA) oligomer, a photosensitizer and a reducing agent. Solutions were vertically frozen in liquid nitrogen at a controlled rate to induce the oriented growth of ice crystals and then cryo-photopolymerized under blue-light irradiation. Ice crystals were finally removed under vacuum producing macroporous hydrophilic networks with aligned pores. Porosities ranged between 80 and 95%, depending on the initial concentration of PEGDMA. The influence of processing variables on the final properties of the materials was addressed, concerning particularly the effect of porosity and freezing directionality on air permeability. Compared to porous PEGDMA-based monoliths with non-aligned macropores, gas permeability was two to three times higher for oriented scaffolds at the same porosity level, a fact explained by the easier transport of gas molecules through the aligned structures. However, the role of pore orientation on gas permeability was shown to be less marked as porosity increased. The results demonstrate that the use of unidirectional freezing strongly increases the permeability of monolithic samples up to values usually required, for instance, in tissue engineering applications (higher than 2D). These findings provide new perspectives on pore design principles toward future scaffolding of polymeric cross-linked matrices.