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Preparation of Porous Poly(L-lactic acid) Honeycomb Monolith Structure by Unidirectional Freezing and Freeze-Drying

Jin-woong Kim, Kentaro Taki, Shinsuke Nagamine, and Masahiro Ohshima. Department of Chemical Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto, Japan

For the application of porous polymers as scaffolds for tissue engineering, the porosity, biodegradability, interconnectivity, and morphology are very important. Organs such as nerves, bones, blood vessels, and ligaments require a scaffold with an aligned porous structure for their cell proliferation. Thus, the controlled preparation of aligned porous materials in the form of 2D surface patterns or a 3D monolithic structure is critical for this purpose. A promising and novel technique for creating aligned porous structures is a solvent unidirectional freezing method, which is relatively simple and cost effective.

A honeycomb monolith structured porous PLLA, (a porous PLLA with smooth-wall-micro tube aligned in parallel to the freezing direction, and a honeycomb structure perpendicular to the aligned direction in cross section) was prepared by combining unidirectional freezing and freeze-drying methods. Dehydrated 1,4-dioxane was used as a solvent for PLLA and the single-phase mixture was frozen by soaking the sample tube into liquid nitrogen at a constant rate. The 1,4-dioxane crystal was nucleated in the solution and grown in the freezing direction. Using the crystalline structure as a template, the PLLA was solidified and structured. Then, the crystal was allowed to sublimate by freeze-drying and the aligned structure, which has either smooth wall micro tubes or ladder-like micro tubes aligned along the freezing direction and a honeycomb structure in its vertical direction, was prepared. The effects of polymer concentration, sample tube soaking rate and water contents on the aligned structure were thoroughly investigated. We found that the smoothness of the micro tube wall and its interconnectivity could be controlled by the polymer concentration and water content in solution.

Furthermore, the addition of Poly (ethylene glycol) (PEG) into the PLLA 1,4-dioxane solution enabled to prepare a similar honeycomb monolith structure with mirco/nanoscale porous wall. Phase separation of PLLA and PEG induced during crystallization of 1,4-dioxane was exploited to create such a cellular structure. The porous structure in the wall of honeycomb monolith was prepared by selective leaching of PEG. The results show that the morphology was affected by PLLA and PEG blend ratio and molecular weight of PEG.