# Dental Hydrogels with Essential Oils with Potential Activity in Periodontitis
## 3 UV-Vis Absorption Spectroscopy of the Hydrogels
### Antimicrobial Activity of the Experimental Gels Containing Natural Essential Oils
All essential oils with a high D-limonene content not only have a potent anti-inflammatory action but also significantly reduce the generation of reactive oxygen species by scavenging free radicals [27]. Several studies indicated the antioxidant potential of D-limonene inhibits lipid peroxidation and fights cell damage induced by free radicals [28]. The monoterpenes include the well-known compound--\(\alpha\)-pinene, which is present in the essential oils of numerous plants. Antibiotic resistance modulation, anticoagulant, anti-cancer, antibacterial, antimalarial, antioxidant, anti-inflammatory, anti-Leishman, and analgesic properties are only a few of the many pharmacological activities that have been identified [29]. The most important effects of \(\alpha\) and \(\beta\) -pinene are outlined, including their cytogenetic, gastroprotective, anxiolytic, cytoprotective, anticonvulsant, and neuroprotective effects, as well as their effects on pancreatitis, stress-induced hyperthermia, pulpal pain, and H\({}_{2}\)O\({}_{2}\)-stimulated oxidative stress [29]. The three primary constituents of the last tested oregano essential oil are carvacrol, p-cymene, and \(\gamma\)-terpinene. The monoterpenes carvacrol and cyrene are frequently present in Origanum essential oils. These show antibacterial and antifungal effects, as well as anti-inflammatory and analgesic properties [30]. These characteristics make these products excellent candidates for the creation of tissue-healing solutions. All the experimental hydrogels present an important absorption peak around 200 nm and a large band between 260 and 300 nm due to the presence of carvacrol and eugenol, which are the majority constituents of the essential oils extracted from oregano and clove plants. In order to evaluate the potential of the main constituents of the essential oils regarding specific effects in the oral cavity, further in vitro tests were performed, whereas the cytotoxic effect of hydrogels was tested on mesenchymal stem cells (D1 MSCs) derived from the dental papilla of human wisdom teeth germs (D1 MSCs). No cytotoxic effects were observed in any of our experimental gels. In contradiction with earlier findings [31], where a drop of viability (75%) was recorded in similarly tested gels filled with natural essential oils, our findings prove a lack of in vitro toxicity in the new gel formulations. Even though these results differ from some published studies [21], where the tested gelswith essential oils having the same compounds identified by us (carvacrol, thymol) but also cinnamaldehyde were cytotoxic, reducing fibroblast viability below 70%, they are consistent with [32], where hydrogels having Clove essential incorporated showed good cell viability rates to the same human gingival fibroblast cells. Even if our major interest is to have non-cytotoxic gels, the antibacterial or even bacteriostatic effect is truly desired because the basic etiology of periodontal diseases is infectious and originates from the bacteria found in dental plaque, which forms a specific microbial biofilm made up of a complex population of microorganisms. Approximately 500 bacterial species have been identified in these biofilms, both in healthy and diseased oral cavities, from dogs and cats [33]. Therefore, the best performance was seen in Oregon(r) essential oil gel. This highlights the usefulness of carvacrol as an antibacterial agent. The antibacterial action of carvacrol, which is stronger against Gram-positive than Gram-negative bacteria, principally relies on bacterial membrane damage; it results in the dissolution of the proton motive force and subsequent reduction in ATP synthesis that leads to a reduction in other energy-dependent cell processes, including synthesis of enzymes and toxins [34].