EFFECT OF PREPARATION METHOD ON THE HYDRATION CHARACTERISTICS OF HYLAN AND COMPARISON WITH ANOTHER HIGHLY CROSS-LINKED POLYSACCHARIDE, GUM-ARABIC

The water binding characteristics of hylan are compared with another c rosslinked polysaccharide Acacia senegal gum exudate (A. senegal) usin g differential scanning calorimetry. Both polysaccharide systems bind water effectively, and the transitions characteristic of two types of freezing-bound water can be distinguished from the melting or freezing of free water. There is evidence for the existance of metastable stat es of freezing-bound water within the two systems. Gum arabic binds co nsiderably less freezing-bound water than hylan systems. A. senegal do es not have the same ability as hyaluronic acid to form structured ent angled networks which can incorporate water within the matrix. The hyl an samples are of two types: hylan fluid where the hyaluronan chains a re crosslinked with formaldehyde, and hylan gel where the cross-linkin g agent is vinyl sulphone. The hylan gel retains the freezing-bound st ate of water as a stable thermodynamic state ca 20-50% more effectivel y than hylan prepared from the freeze-dried solid prepared from either concentrated or dilute hylan fluid. The traps formed from freeze-drie d hylan gel are also more stable. Hylan gel prepared by precipitation with isopropanol and freeze-dried is the most effective hylan sample f or stabilising the freezing bound state. For this material even in sim ilar to 6% solution the vast majority of the water is retained in the freezing-bound form.