Physical Gelation of Chitosan in the Presence of β-Glycerophosphate:  The Effect of Temperature

Abstract

When adding beta-glycerophosphate (beta-GP), a weak base, to chitosan aqueous solutions, the polymer remains in solution at neutral pH and room temperature, while homogeneous gelation of this system can be triggered upon heating. It is therefore one of the rare true physical chitosan hydrogels. In this study, physicochemical and rheological properties of chitosan solutions in the presence of acetic acid and beta-GP were investigated as a function of temperature in order to gain a better understanding of the gelation mechanisms. The gel structure formed at high temperature was only partially thermoreversible upon cooling to 5 degrees C because of the existence of remaining associations, confirmed by the spontaneous recovery of the gel after breakup at low temperature. Increasing temperature had no effect on the pH values of this system, while conductivity (and calculated ionic strength) increased. Values from the pH measurements were used to estimate the degree of protonation of each species as a function of temperature. The decreasing ratio of -NH3+ in chitosan and -OPO(O-)2 in beta-GP suggested reduced chitosan solubility along with a diminution of ionic interactions such as ionic bridging with increasing temperature. On the other hand, the increased ionic strength as a function of temperature, in the presence of beta-GP, enhanced screening of electrostatic repulsion and increased hydrophobic effect, resulting in favorable conditions for gel formation. Therefore, our study suggests that hydrophobic interactions and reduced solubility are the main driving force for chitosan gelation at high temperature in the presence of beta-GP.

In vitro and in vivo evaluation of thermosensitive chitosan hydrogel for sustained release of insulin

“…The gelation mechanism of CS/b-Gp solution has previously been reported (Cho et al, 2005;Li et al, 2010). Briefly, electrostatic attraction between oppositely charged ammonium groups of chitosan and negatively charged phosphate moieties of b-Gp salt allows for attractive hydrophobic and hydrogen bonding between chitosan chains, which are the main molecular forces responsible for the sol/gel transition in CS/b-Gp solutions.…”

Viscoelastic Behavior and In Vivo Release Study of Microgel Dispersions with Inverse Thermoreversible Gelation

“…This is the common behavior of a polymer solution. [21][22][23] For polymer solutions, dynamic modulus decrease can be ascribed to increase in chain flexibility and compactness of polymer molecules in solution with increasing temperature. Therefore, rheological parameters decrease with increasing temperature.…”

Chitosan-Based Thermosensitive Materials

“…While any change in pH could not be appreciated, the conductivity displayed a monotonous increment as the temperature rises. This behaviour was associated to a reduction of chitosan solubility and an enhancement on the screening of electrostatic repulsion, therefore increasing hydrophobic interactions and improving conditions for gel formation [5]. According to the same authors, temperature most probably modifies hydrogen bond distribution and favours polymer-polymer interactions over those of polymer-solvent interactions.…”

“…The mechanism of gelation of chitosan in the presence of glycerophosphate involves several interactions such as screening of electrostatic repulsion, ionic cross-linking and hydrophobic and hydrogen-bonding interactions [5,8].…”

“…According to the same authors, temperature most probably modifies hydrogen bond distribution and favours polymer-polymer interactions over those of polymer-solvent interactions. Ionic strength: an increment of the ionic strength gives favourable conditions for gel formation due to the higher screening of electrostatic repulsions, thus promoting the hydrophobic interactions in the polymer physical network [5].…”