Porous matrix, freeze drying

One of the most practical modifications of freeze-drying synthesis is the cryoimpregnation process. This process is often considered as an independent synthesis technique. In this case, the object of freeze-drying is a porous matrix of fibers or powders soaked/wetted with a solution of microcomponents and frozen by fast cooling. The purpose of the cryoprocessing method in this case is prevention of macro- and microscopic redistribution of the microcomponents within the substrate, which is usually observed during air or thermal drying. 

There are, however, other ways in which conventional drying could, in principle, be performed at temperatures above Tg, without deleterious results. If a solution could be treated so that one or several components can undergo crystallisation, then the crystals formed would serve as a substitute for the subliming ice crystals and could mechanically protect the porous, amorphous matrix against structural collapse. The efficacy of this type of formulation for freeze-drying purposes has been demonstrated for the system water-sucrose-NaCl. " Its practical applications may, however, be limited to products where the bioactive component is not chemically degraded in the freeze-concentrate at temperatures above Tg. 

Oxidized hyaluronic acid was coupled with chitosan to form porous scaffolds after freeze drying. The proportion of porosity of the freeze-dried chitosan-hyaluronic acid dialdehyde composite (CHDA) gels enhanced with augmentation in oxidation. Fibroblast cells seeded onto CHDA porous scaffold adhered, proliferated and offered extracellular matrix components on the scaffold [99]. Chondrocytes encapsulated in CHDA gels retained their viability and specific phenotypic features. The gel material is therefore projected as a scaffold and encapsulated material for tissue engineering applications. Films of hyaluronan (HA) and a phosphoryl choline-modified chitosan (PC-CH) were constructed by the electrolyte multilayer (PEM)... 

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