Methylparaben

Methylparaben was purchased from FAGRON IBERICA (Barcelona, Spain) and PEG-400 from Thermo Fisher Scientific (Barcelona, Spain).
Trypto-Casein Soy Agar (TSA) (Oxoid, Madrid, Spain) and Sabouraud Dextrose Agar (Oxoid, Madrid, Spain) were used as culture media. We used sodium chloride-peptone buffer at pH 7.7 for sample neutralising and as a sterile suspending fluid (Scharlau, Barcelona, Spain). Beerens Diluent 3% was used to neutralise parabens.

Morphine FHC® 20 mg·mL⁻¹ (aqueous injectable solution) was purchased from FHC Farmacêutica, Lda (Mortágua, Portugal) and used as source of morphine hydrocloride (MH). Carboxymethylcellulose sodium (NaCMC; high viscosity: 700–1500 mPa·S) was acquired from Sigma Aldrich (Darmstadt, Germany). Methylparaben and propylparaben were obtained from Fagron Iberica (Barcelona, Spain). Glycerol was purchased from Vencilab (Vila Nova de Gaia, Portugal) and sterile water for injections was obtained from B. Braun Medical, Lda. (Queluz de Baixo, Portugal). IntraSite® Gel (Smith & Nephew, London, UK) and Varihesive® Hydrogel (Convatec, Reading, UK) were also used for comparison purposes. Acetonitrile HPLC-gradient grade (Panreac, Castellar del Vallès, Spain) and ethanol (Merck, Darmstadt, Germany) were also used. Tuffryn® membranes (polisulfone membrane disc filters, 0.45 µm) were purchased from the Pall Corporation (East Hills, NY, USA). All other ingredients used were of analytical grade or equivalent.