Rheological study of a thermoreversible morphine gel

The rheological behavior of a poloxamer 20% thermoreversible gel with morphine or without was studied. Its behavior was newtonian below the transition temperature and became non newtonian above this temperature. The non newtonian part was best fitted with the Herschel-Bulkley equation in comparison with the Casson equation. The sol-gel transition was associated with a drastic increment of the apparent viscosity and a brutal modification of the Herschel-Bulkley equation parameters. Likewise, the oscillatory parameters (the lag phase, the storage modulus and the loss modulus) revealed the great influence of the temperature on the viscoelastic properties of the sample. Different rheological methods have been described in order to determine the transition temperature usable for the control of preparations. The temperature interval corresponding to the sol-gel transition ranged between 22-25°C for the solution with morphine and between 23-26°C for the solution without. Thereby, the addition of morphine acetate did not induce the loss of thermoreversible properties and this preparation could be used as a controlled release system.