Hydration characteristics of the cross-linked hyaluronan derivative hylan

The ability of hylan, the formaldehyde cross-linked derivative of hyaluronan, to interact with water has been studied using differential scanning calorimetry (DSC). Three types of water can be distinguished: non-freezing, freezing-bound and free. When the water content of the system is increased, even by up to 10%, almost all the water remains in the freezing-bound state, with a ΔH value less than free water. Several metastable states of water can be detected within the structured hylan-water matrix, indicative of defects in the frozen-bound ice structure. The maximum amount of non-freezing water, intimately associated with the hydrophilic groups of hylan, corresponds to 13 mol water per disaccharide unit of the hyaluronan chain. The large capacity shown by hyaluronan entangled networks to build water into their structure could also be responsible for their unusually high viscosity and elasticity after the onset of entanglement. Such viscoelastic properties are the basis for their use in viscosupplementation of arthritic diseased joints.