## 5 Drug Delivery Systems  
### Zeolites  
Zeolites are a type of inorganic material that is commonly used as adsorbent due to their porous structure. Their main constituents are silicon, aluminum, and oxygen [51; 52]. Aside from purely industrial applications like catalysts and adsorbents, they have also been used as drug delivery systems. Their uses included modulation of vitamin absorption, antimicrobial action, as carriers of anticancer and non-steroidal anti-inflammatory drugs, hemostasis, wound healing, and anti-diarrheal properties. They have also been proven to carry organic substances into the cells [53]. Generally speaking, they work based on their increased ion exchange capacity in zeolites with low Si/Al ratios. Such zeolites also tend to be more hydrophilic, which can help with EOCs' water-insolubility. However, it is worth emphasizing that zeolites are quite susceptible to degradation in mild acid pH or the presence of water. Of more industrial and synthetic importance is the possible degradation of EOCs under high temperatures [54]. Much research focuses on the encapsulation of metal ions inside the zeolites to assess their antimicrobial properties. The results have shown that the best results are achieved using silver and zinc ions. Their zeolite composites were also more effective than sole ions [55]. Current studies indicate that various chemicals constituting major EOCs can be encapsulated in synthetic zeolites [54]. So far, there have been studies demonstrating a successful zeolite encapsulation of cinnaldehyde, D-limonene, vanillin [56], eucalypt [57], linalool [58], and numerous EOs [59; 60; 61; 52]. To encapsulate an EO into zeolite, the former has to be in a high-concentration solution, and the latter is to be dried at about 160 \({}^{\circ}\)C. The drying process is crucial as it removes water molecules that reduce the adsorption surface, as the process needs to be separate from adding the EO solution since EOs degrade in these temperatures. The following process involves adding zeolites to EO's solution and removing the solvent, such as ethanol. A couple of studies assessed the effect of such encapsulation and reported higher stability and gaseous phase retention. The addition of cyclodext has been shown to reduce the release of EOs from zeolites and increase their antioxidant properties [63; 64]. The primary characteristics that influence the rate of adsorption of EOs in zeolites are the molecular size, structure, and the presence of functional groups. These characteristics explain various results obtained in studies analyzing different EOs [65]. The main issues that may arise are zeolite toxicity and the antimicrobial effect in vivo. Zeolite toxicity can vary substantially. Some zeolites, like erionite, scolecite, and offretite, were proven to be cytotoxic due to their fibrous structure, and this characteristic was used in possible applications as anticancer drugs [66]. Current studies indicate that zeolite toxicity is mainly influenced by its shape (spherical ones are less toxic) and aluminum content [67]. A couple of studies show that the toxic potential of zeolites depends on the cell type [54]. Studies dealing with the antimicrobial effect of zeolite-encapsulated EOs need to consider possible difficulties regarding proper effect assessment. Namely, measuring the actual bactericidal effect in vivo is not easy. Despite the encapsulation, EOs are difficult to assess in the classical agar plates approach due to their insolubility and volatility [68]. Few studies have researched the effect of EOs encapsulated in zeolites on bacteria directly. The possible antimicrobial effect is usually extrapolated based on the bactericidal properties of a given EO and not assessed directly [59]. An example of a study that directly showed the antimicrobial effect is thymol encapsulated in zeolite 4A [69]. Using the microdilutionmethod, the authors obtained results showing that the composite has bactericidal activity against _Staphylococcus aureus_, _Candida albicans_, and _E.