Chitins and chitosans for the repair of wounded skin, nerve, cartilage and bone

This review provides a balanced integration of the most recent chemical, biochemical and medical information on the unique characteristics of chitins and chitosans in the area of animal/human tissue regeneration. Hemostasis is immediately obtained after application of most of the commercial chitin-based dressings to traumatic and surgical wounds: platelets are activated by chitin with redundant effects and superior performances compared with known hemostatic materials. To promote angiogenesis, necessary to support physiologically ordered tissue formation, the production of the vascular endothelial growth factor is strongly up-regulated in wound healing when macrophages are activated by chitin/chitosan. The inhibition of activation and expression of matrix metalloproteinases in primary human dermal fibroblasts by low MW chitosans prevents or solves problems caused by metalloproteinase-2 such as the hydrolysis of the basement membrane collagen IV. Experimental biocompatible wound dressings derived from chitin are today available in the form of hydrogels, xerogels, powders, composites, films and scaffolds: the latter are easily colonized by human cells in view of the restoration of tissue defects, with the advantage of avoiding retractive scar formation. The growth of nerve tissue has been guided with chitin tubes covalently coated with oligopeptides derived from laminin. The regeneration of cartilage is also feasible because chitosan maintains the correct morphology of chondrocytes and preserves their capacity to synthesize cell-specific extracellular matrix: chitosan scaffolds incorporating growth factors and morphogenetic proteins have been developed. Impressive advances have been made with osteogenic chitosan composites in treating bone defects, particularly with osteoblasts from mesenchymal stem cells in porous hydroxyapatite-chitin matrices. The introduction of azido functions in chitosan has provided photo-sensitive hydrogels that crosslink in a matter of seconds, thus paving the way to cytocompatible hydrogels for surgical use as coatings, scaffolds, drug carriers and implants capable to deliver cells and growth factors. The peculiar biochemical properties of chitins and chitosans remain unmatched by other polysaccharides.