Supercritical CO₂-assisted impregnation of cellulose microparticles with R-carvone: Effect of process variables on impregnation yield

Highlights

• Carvone loading was influenced by all process variables and some binary interactions.

• The addition of cosolvent enhanced ∼2.5-times the carvone impregnation yield.

• The high crystallinity of MCC remained unchanged after the supercritical process.

• Supercritical impregnation induced the rupture of MCC fibers agglomerates.

• Carvone-impregnated MCC showed slower release compared to physical mixture.

Abstract

In this work, the supercritical CO₂-assisted impregnation of microcrystalline cellulose (MCC) particles with R-(−)-carvone, an antimicrobial monoterpene, was investigated. The effect of CO₂ density (278–600 kg m⁻³), temperature (40–60 °C), carvone:MCC mass ratio (0.5–1.0), and depressurization mode (cold–hot), on the impregnation yield was studied. Carvone loadings were obtained in the range of 0.6–3.6 wt%, and the addition of ethanol (2 wt%) as cosolvent enhanced the impregnation yield up to 10 wt%. The high crystallinity of cellulose (81–84%) remained unchanged after the supercritical process, although SEM analysis showed a partial separation of fiber agglomerates. Carvone release in simulated saliva solution from particles impregnated using cosolvent was slightly slower than from the pure CO₂-impregnated particles. The observed release profile could be useful for pharmaceutical and biomedical applications, such as the prevention of oral cavity infections or their treatment at early stages.

Introduction

In the last few decades, supercritical carbon dioxide (scCO₂)-assisted impregnation has been extensively investigated for the development of active materials, not only because it allows incorporating different natural and synthetic active compounds in a wide range of polymers, but also based on its advantages over traditional incorporation methods, such as moderate operating conditions (low critical point of CO₂: T_c=31.4 °C; P_c=7.38 MPa) and a final solvent-free impregnated material [1], [2].

The versatility of scCO₂-assisted impregnation lies mainly in its properties as a solvent. The combination of low viscosity, low surface tension, and high diffusivity makes it an excellent solubilizing agent with high penetrating capability as well as good mass and heat transport properties [3], [4]. Additionally, CO₂ is non-toxic, non-flammable, non-polluting, odorless, colorless, and readily available [5]. Due to these advantages, this technology has been applied for several purposes in the pharmaceutical and biomedical research fields. The development of biomedical devices or drug delivery systems using scCO₂-assisted impregnation has increased in the last years. In general, research has been focused on the development of ocular implants [6], [7], [8], wound healing/tissue engineering [9], [10], [11], [12], drug-eluting sutures [13], and drug delivery systems [14], [15], [16], [17], among other applications. However, the employment of supercritical fluids for the preparation of biomedical materials for buccal health management or the treatment of oral cavity diseases has been less explored [18], [19]. Infections of this area are most commonly odontogenic in origin and are caused by microorganisms (or their products) inhabiting the oral cavity. These infections include dental caries, pulpitis, periapical abscess, gingivitis, periodontal disease, and infections in the deep fascial spaces [20]. Most of these diseases are treated with synthetic drugs, such as wide-spectrum antibiotics and antiseptics. In this sense, topical drug delivery has some advantages compared to oral administration, such as more direct access to target diseases, the enhanced therapeutic efficacy of the drug, the avoidance of gastrointestinal tract-related problems, and the improvement of patient compliance [21].

In particular, the employment of microparticles as porous carriers in drug delivery systems is attractive because they can be incorporated into different dosage forms, such as solids (tablets and capsules), semisolids (creams and gels), and liquids (solution, suspensions). Besides, they can protect the active ingredient from the environment, reduce the unpleasant taste of many drugs, and help to tune the drug release profile under various conditions [22]. In this context, the scCO₂-assisted impregnation of particulate systems with active ingredients appears as a promising technology. Compared with other supercritical fluid-based techniques for particle synthesis, direct impregnation of pre-specified particles (obtained by other methods) avoids difficulties associated with size and morphology control and requires simpler equipment [2]. In this sense, the impregnation of different commercially available polymeric particles with active compounds, including modified starches [23], hydroxymethylcellulose and poly(N-vinyl-2-pyrrolidone) [24], polymethylmethacrylate [25], etc., has been proposed. Microcrystalline cellulose (MCC) particles are widely used in the pharmaceutical industry as a binder or diluent in the production of tablets and capsules for oral formulations [26]. MCC is a high molecular weight homopolymer of β-1,1-linked anhydro-ᴅ-glucose units occurring in wood, hemp, and other plant-based materials [4], and it has also been investigated as a drug delivery carrier of lidocaine [27], paracetamol [28], ranitidine [29], and cephalexin [30], among others, using conventional preparation methods and different pharmaceutical formulations. However, to the best of our knowledge, there is only one report regarding the incorporation of an active ingredient in MCC particles by scCO₂-assisted impregnation. In this recent work, Krivokapic et al. [31] loaded MCC particles obtained from agricultural waste with ibuprofen, with a maximum impregnation yield of 9.43% at 25 MPa and 40 °C, after 24 h of contact. These results suggest the potential of scCO₂-assisted impregnation for obtaining oral drug delivery systems based on MCC as a carrier.

Furthermore, the use of active compounds derived from natural sources in the development of biomedical devices or drug delivery systems by scCO₂-assisted impregnation has attracted much attention in the last years. Some recent examples include the loading of plant extracts onto cotton fabrics or alginate aerogels with potential antioxidant, antibacterial and anti-inflammatory properties [32], [33], and the preparation of poorly water-soluble vitamins supplements into starch and alginate aerogels for oral delivery [34], [35] or the topical treatment of glaucoma and other eye diseases [36]. Among them, some common constituents of essential oils (such as eugenol, thymol, carvone, and other volatile compounds) have interesting potential applications in the pharmaceutical field due to their anti-inflammatory, antioxidant, antimicrobial, antifungal, and anthelminthic properties, and are readily soluble in scCO₂, allowing impregnation at moderate conditions. For instance, several authors have studied the supercritical impregnation of thymol into different materials for its controlled release, including polyamide nanofibers, cellulose acetate films, poly-(L-lactic acid) (PLA) and poly-(L-lactic-co-glycol acid) (PLGA) foams, low density polyethylene (LDPE) nanocomposites, among others [9], [37], [38], [39]. Eugenol, another essential oil component commonly used in odontology as an antiseptic, has been incorporated into polyamide fibers under supercritical conditions, obtaining an active dental floss with antibacterial activity against Escherichia coli and Staphylococcus aureus [18].

R-(−)-carvone is a natural monoterpene present in the essential oils of caraway (Carum carvi L.) and spearmint (M. spicata L.), among other species, which have demonstrated significant in vitro antimicrobial, antifungal, and antihelmintic activity [40], [41]. The study of the supercritical impregnation of carvone into polymeric materials has been scarcely studied. The incorporation of carvone into commercial LLDPE and PLA films for active packaging applications was investigated by Goñi et al. [42] and Miranda-Villa et al. [43], respectively, whereas Coutinho et al. have evaluated the simultaneous impregnation of carvone and anti-inflammatory compounds in PLLA and LDPE films [44]. However, the incorporation of carvone in MCC particles under supercritical conditions remains unexplored.

Based on this information, this contribution aims to investigate the incorporation of R-(−)-carvone into MCC particles using scCO₂-assisted impregnation, as a potential drug delivery system for the treatment of buccal cavity infections, with three main objectives: a) to evaluate the effect of different operation variables (CO₂ density, temperature, carvone:polymer mass ratio, depressurization mode, and cosolvent) on the impregnation yield; b) to characterize the morphology of the particles subjected to the supercritical process; c) to determine the release kinetics of carvone from the impregnated samples in a simulated saliva solution.