What are chitosan hydrogels?

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Due to their chemical structure, chitosans can easily form hydrogels: when chitosan powder is dissolved in water (at slightly acidic pH), many of the water molecules are more or less tightly adhering to the chitosan, making chitosan solutions viscous. The higher the concentration of the chitosan, and the higher its degree of polymerization, the more viscous will be the solution. Eventually, when there is no free water left, a gel will be formed (i.e. kind of an intermediate state between a solution and a dry powder). Chitosan gels are held together by the hydrophobic interactions between N-acetylglucosamine residues. The gels can be stabilized by the addition of polyanions (molecules with multiple negative charges) such as proteins or negatively charged polysaccharides (e.g. pectin or alginate) which can ionically bind to the positively charged glucosamine residues present in the chitosan. The nature and properties of the gel, thus, will be determined by the balance of hydrophobic and hydrophilic interactions. Due to their net positive charge, chitosan hydrogels appear to be superior materials to support the growth of human cells, making them ideally suited for tissue engineering.