Poloxamer 407

All solvents, reagents, and standards used were of analytical grade. The standards used in the HPLC analysis were the following: hyperoside, rutin, quercitrin, phlorizin, procyanidin B2, and chlorogenic acid, obtained from Extrasynthese (Genay, France); (+)-catechin and (–)-epicatechin, purchased from Sigma-Aldrich GmbH (Buchs, Switzerland), and avicularin and isoquercitrin, obtained from Chromadex (Santa Ana, CA, USA). The chemicals applied in the modeling of semi-solid pharmaceutical forms were methanol, glycerin, olive oil, sorbitan monolaurate (Span 20), polyethylene glycol sorbitan monolaurate (Tween 20), Vaseline, sodium chloride obtained from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). Poloxamer 407 was obtained from Fagron (St. Paul, MN, USA). PIONIER^® PLW was obtained from Hansen & Rosenthal KG^® (Hamburg, Germany). Ethanol from AB Stumbras (Kaunas, Lithuania). Purified deionized water used in the tests was prepared with the Milli-Q^® (Millipore, Bedford, MA, USA) water purification system. The reagents used in the antioxidant activity assay were 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) acquired from Scharlau (Barcelona, Spain).

Ophthalmic gels were formulated using different concentrations of polymers. Poloxamer 407 (Fagron, St. Paul, MN, USA), hydroxypropyl methylcellulose (HPMC) (Sigma-Aldrich, Steinheim, Germany), propane-1,2-diol (AppliChem GmbH, Darmstadt, Germany), purified water, and a 1% solution of lyophilized balsam poplar buds extract (L1) were used to form the experimental in situ gels. We chose to add 10% (w/v) of 1% solution of balsam poplar extract (L1) to each formulation. Poloxamer 407 and HPMC gels were prepared separately. The appropriate amount of poloxamer was weighed (10%, 12%, or 15% (w/v)) and mixed with the appropriate amount of purified water, and the mixtures left in a refrigerator (5 °C) for 24 h. The HPMC mixture was prepared by weighing an appropriate amount (0.5% or 0.75% (w/v)) of polymer and adding an appropriate amount of water, with the mixture placed on a magnetic stirrer at 50 °C until a homogeneous gel form was obtained. In the preparation of the in situ gels, the Poloxamer 407 and HPMC gels were mixed with a magnetic stirrer to form a homogeneous structure. Next, 10% (w/v) propylene glycol and 10% (w/v) [46 (link)] L1 were added dropwise to the formulations, and the gels were mixed to form a homogeneous structure. All experimental formulations were stored in the refrigerator (5 °C).