Development and evaluation of a chitosan-coated alginate microsphere system for <i>Thymus vulgaris</i> and <i>Calendula officinalis</i> oil delivery

This study aimed to develop and characterize chitosan-coated alginate microspheres for the encapsulation and controlled delivery of <i>Thymus vulgaris</i> and <i>Calendula officinalis</i> oils, selected for their well-documented antimicrobial, anti-inflammatory, and wound-healing properties. The primary objective was to enhance the stability and controlled release performance of these bioactive oils under simulated gastrointestinal conditions. Microspheres were prepared using water-in-oil (W/O) emulsification followed by external gelation. Morphological and structural characterization was carried out using scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. Swelling behavior was evaluated in simulated gastric (pH 1.2) and intestinal (pH 6.8) fluids. Encapsulation efficiency (EE%) and loading capacity (LC%) were determined by UV-visible spectrophotometry, and <i>in vitro</i> release profiles were subsequently investigated. All data are presented as mean ± standard deviation (<i>n</i> = 3). The optimized microspheres exhibited a mean diameter of 525 ± 25 µm. Encapsulation efficiencies ranged from 72 ± 1.5% to 81 ± 1.8% (w/w) for <i>Thymus vulgaris</i> oil and from 68 ± 2.0% to 72 ± 1.7% (w/w) for <i>Calendula officinalis</i> oil. Swelling ratios increased up to 250 ± 5% at pH 6.8. Release studies demonstrated a controlled release behavior, predominantly following the Higuchi and Korsmeyer-Peppas kinetic models. Overall, chitosan-coated alginate microspheres effectively improved the stability and controlled release of both essential oils compared to uncoated systems, highlighting their potential as a versatile platform for advanced therapeutic and biomedical applications.