Carbomer Hydrogels with Microencapsulated α-Tocopherol: Focus on the Biocompatibility of the Microcapsules, Topical Application Attributes, and In Vitro Release Study

The microencapsulation of α-tocopherol based on the complex coacervation of low-molecular-weight chitosan (LMWC) and sodium lauryl ether sulphate (SLES) without harmful crosslinkers can provide biocompatible carriers that protect it from photodegradation and air oxidation. In this study, the influence of the microcapsule wall composition on carrier performance, compatibility with a high-water-content vehicle for topical application, and release of α-tocopherol were investigated. Although the absence of aldehyde crosslinkers decreased the encapsulation efficiency of α-tocopherol (~70%), the variation in the LMWC/SLES mass ratio (2:1 or 1:1) had no significant effect on the moisture content and microcapsule size. The prepared microcapsule-loaded carbomer hydrogels were soft semisolids with pseudoplastic flow behavior.

The integrity of microcapsules embedded in the hydrogel was confirmed by light microscopy. The microcapsules reduced the pH, apparent viscosity, and hysteresis area of the hydrogels, while increasing their spreading ability on a flat inert surface and dispersion rate in artificial sweat. The in vitro release of α-tocopherol from crosslinker-free microcapsule-loaded hydrogels was diffusion-controlled. The release profile was influenced by the LMWC/SLES mass ratio, apparent viscosity, type of synthetic membrane, and acceptor medium composition. Better data quality for the model-independent analysis was achieved when a cellulose nitrate membrane and ethyl alcohol 60% w/w as acceptor medium were used.

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Materials

The components of the microcapsule wall were low-molecular-weight chitosan (LMWC) (50,000–190,000 g/mol) (Sigma-Aldrich, Beijing, China) and sodium lauryl ether sulphate (SLES) with 2–3 ethylene oxide groups (Clariant, München, Germany). The degree of deacetylation of LMWC was determined to be 81.8% using the potentiometric titration method developed by Yuan et al. [57]. SLES was in the form of a sticky paste containing 70% of the active ingredient. A 36.5% formaldehyde solution (Sineks Medical, Belgrade, Serbia) or a 50% glutaraldehyde solution (Fisher Chemical, Loughborough, UK) was used to prepare the microcapsules containing a crosslinking agent. The oily core of the microcapsules consisted of caprylic/capric triglyceride (Saboderm TCC, Sabo S.p.A, Levate, Italy) and (±)-α-tocopherol (Sigma-Aldrich, Merck, Darmstadt, Germany). The purity of α-tocopherol was 96%.

The following reagents were also used to prepare the microcapsule: acetic acid (Zorka-Pharma, Šabac, Serbia), sodium acetate (Centrohem, Stara Pazova, Serbia), silicon dioxide (Aerosil 200 Pharma, Evonik, Hanau, Germany), and purified water. The following substances were used to produce blank hydrogel: carbomer (Carbopol® Ultrez 10 polymer, Lubrizol, Wickliffe, OH, USA), triethanolamine (Sigma-Aldrich, Merck, Darmstadt, Germany), isopropyl alcohol (Fagron, Capelle aan den Ijssel, The Netherlands), propylene glycol (Fagron, Capelle aan den Ijssel, The Netherlands), and purified water. The ingredients of artificial sweat (sodium chloride (NaCl), calcium chloride (CaCl2), magnesium sulfate (MgSO4), potassium dihydrogen phosphate (KH2PO4), and sodium hydroxide (NaOH)) were purchased from Sigma-Aldrich, Merck (Darmstadt, Germany). In the preparation of the acceptor media for the in vitro release tests were used: ethyl alcohol 96% v/v (Zorka-Pharma, Šabac, Serbia), polysorbate 20 (Fagron, Capelle aan den Ijssel, The Netherlands) and purified water

Đekić, L.; Milinković Budinčić, J.; Stanić, D.; Fraj, J.; Petrović, L. Carbomer Hydrogels with Microencapsulated α-Tocopherol: Focus on the Biocompatibility of the Microcapsules, Topical Application Attributes, and In Vitro Release Study. Pharmaceutics 2024, 16, 628. https://doi.org/10.3390/pharmaceutics16050628

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