Investigation of charge effects on drug release behavior for ionic thermosensitive hydrogels

The influence of various drugs with different charges on the drug release behavior in porous ionic thermosensitive hydrogels was investigated. The present hydrogels were prepared from N-isopropylacrylamide (NIPAAm) and cationic monomer, trimethyl (acrylamido propyl) ammonium iodide (TMAAI), or anionic monomer, acrylic acid (AA), or zwitterionic monomer, N′,N′-dimethyl (acrylamido propyl) ammonium propane sulfonate (DMAAPS), or nonionic monomer, poly(ethylene glycol) methylether acrylate (PEGMEA), and pore-forming agent, poly(ethylene glycol) (PEG) with different molecular weights. Caffeine as a nonionic drug, crystal violet (CV) as a cationic drug solute, and phenol red as an anionic drug solute were chosen as model drugs to perform the drug release experiment. Results show that the release ratio of caffeine in the hydrogels is not affected by the ionicity of hydrogels. The CV strongly interacted with the anionic hydrogel; thus, the CV release ratio is very low. CV is only adsorbed on the skin layer of the cationic hydrogel due to charge repulsion and is released rapidly. The result of phenol red (anionic solute) release in the hydrogels is contrary to CV. In addition, the partition coefficients (Kd) and the drug delivery behavior of the present gels were also investigated.