Optimization of hydrogel containing toluidine blue O for photodynamic therapy by response surface methodology

Photodynamic therapy with toluidine blue O (TBO) hydrogel exhibits antibacterial activity against Staphylococcus aureus and Escherichia coli in this paper. The response surface methodology is employed to optimize formulations for antibacterial activity. The optimal formulations are carbomer concentration of 3% (w/v), TBO concentration of 0.1mg/mL and the quality ratio of NaOH and carbomer of 0.4 (w/w). Under the optimized formulations, the log-transformed of CFUmL^-1 on the Staphylococcus aureus and Escherichia coli are 0.84 and 1.26 (the log-transformed of CFUmL^-1 of negative control groups on the Staphylococcus aureus and Escherichia coli are 8.21 and 8.47), respectively. In comparison with photodynamic therapy with TBO aqueous solution, the proposed formulations provide a much stronger antibacterial activity against Staphylococcus aureus and Escherichia coli. TBO hydrogels are stable during 6weeks at three different temperatures (4, 25 and 40°C) with respect to no change of color, transparency, pH and viscosity. 50% and 68.26% of TBO are released from carbomer hydrogel after 4h and 24h, respectively. TBO hydrogel alone or light alone (630nm) treatment is incapable of showing antibacterial activity against Staphylococcus aureus and Escherichia coli. Therefore, photodynamic therapy with the novel optimized TBO hydrogel formulations is a promising treatment strategy for periodontitis.