Release characteristics of lidocaine from local implant of polyanionic and polycationic hydrogels

In this study, the release characteristics of lidocaine conveyed in base (LB) and salt (LS) forms from an anionic hydrogel composed of carbopol and a cationic hydrogel composed of chitosans were examined for optimizing hydrogel formulation as a sponge filler to stop the bleeding and as a carrier for delivering lidocaine to relief pain after a tooth extraction. A Franz cell was used to simulate the in vivo environment and evaluate the drug release kinetics. It was confirmed that the release profiles of LB and LS from both the carbopol and chitosan hydrogels were best described by the Higuchi model, and values of the release rate constant (K) calculated from the slope of the linear portion of the plot were compared. Results demonstrated that the K value increased with increasing LB concentration at the same three carbopol levels of the hydrogels, whereas it increased with a decreasing level of carbopol for the same concentration of LB in the hydrogels. A minimum in the value of K was observed near neutral pH, which was attributed to two influencing factors of the viscosity and the complexing effect of carbopol gel. However, K values at the same concentration of lidocaine were larger for those formulations using the salt form compared to those using the base form. Results further revealed that the K value increased with an increasing amount of LB added to chitosan hydrogels with the same 0.5% concentration of C1000 (Chitosan, viscosity 1000 cps). K values increased with a decreasing MW of chitosan at the same level in the same concentration of an acetic acid solution with the same amount of LB added. K values for the release from chitosan hydrogels prepared at a lower level were lower than those of hydrogels prepared at a higher level. However, similarities in the release profiles between LB and LS were observed. In conclusion, the viscosity of the gel matrix and the ionic complexing effect between the anionic acid groups of hydrogels and basic groups with lidocaine were two main factors influencing regulation of the diffusion coefficient for controlling drug release.