Tea Tree Essential Oil in Hot Spring Water: History
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Hsiao Hsien Lin
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Chin-Hsien Hsu
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Tzu-Yun Lin
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Chien jung-hul

The composite microcapsules of alginate/tea tree essential oil have an obvious antibacterial effect on microorganisms in hot spring water, while the composite microcapsules of alginate/chitosan have no antibacterial effect in hot spring water. When the concentration of the cross-linking agent is fixed, the longer the cross-linking time is (10 min > 5 min > 1 min), the longer the release equilibrium time of the essential oil in the microcapsules in the hot spring water is. When the cross-linking time is fixed, the higher the concentration of the cross-linking agent (1 M > 0.5 M > 0.1 M) and the longer the release equilibrium time of the essential oil in the microcapsules in the hot spring water is. When the concentration of the cross-linking agent and the cross-linking time are fixed, the higher the metal activity of the cross-linking agent (Ca > Zn) is and the longer the release equilibrium time of the essential oil in the microcapsules in the hot spring water is.

  • alginate
  • tea tree essential oil
  • microcapsule
  • Escherichia coli
  • Staphylococcus aureus

1. Microcapsule Technology

Research on microcapsule technology began in the United States in 1940; however, it was used from the 1950s after commercial product availability and patent applications [1][2]. Capsules refer to protective capsule-like containers formed by filling fine, coated materials, such as nuclei and core materials, into one or more skin or shell walls to isolate them from the external environment [3]. Capsules are prepared using physicochemical, chemical, and mechanical methods. Natural or synthetic polymer materials are typically used to prepare shells, and highly dispersed nuclear substances are used as coating materials. The particle size ranges between 1 μm and 100 mm [4]. Low-cost polymer materials that easily form films are typically used for shells. Such shell materials should be more stable than nuclear substances, should not cause decomposition or side reactions, and should be able to surround nuclear substances, such as dispersed fine solid or liquid, and polymerize to form strong capsules [5]. Moreover, the oil-soluble liquid protoplasm is often used as the nuclear substance. If the solid protoplasm is used as the fine coating material, an appropriate solvent can be added according to dissolution characteristics to liquidize it and fully mix it with oily shell material monomers. This process can improve the reaction rate and is favorable for the interface contact for the polymerization reaction and the formation of high-quality capsules [4][5]. Therefore, encapsulated materials exhibit improved physical properties, stability, and compatibility; demonstrate reduced photosensitivity; and are isolated from air [2]. Microcapsules are widely used in many industrial products.

2. Capsule Preparation Technology and Dru-Release System

Capsules can be prepared using physicochemical, chemical, and mechanical methods; among them, interfacial condensation polymerization and in situ condensation polymerization are the most commonly used [6]. Capsules with different coated states and particle size distributions can be prepared through interfacial condensation polymerization and in situ condensation polymerization by using synthetic polymer monomers as raw materials and by controlling formulation and operating conditions [7].
Various polymer compounds can be used as shell materials. The selected polymers should not react with nuclear substances chemically and should be stable, permeable, tough, and low-cost. The polymers polyamide, polyester, and cellulose are commonly used as shell materials, and different polymer materials can form shell layers of different materials [6][7].
Controlled-release drug-carrier materials are beneficial for disease treatment [8]. Controlled-release drugs are transmitted to the target point regularly and quantitatively. Thus, reduced drug decomposition during the controlled transmission process avoids the loss of drugs, increasing drug efficacy and preventing drug decomposition due to high temperature [9]. Therefore, capsule preparation should be conducted according to the drug-release system (Figure 1).
Water 14 01391 g001 550
Figure 1. Drug of controlled release and dosage of tradition.

3. Sodium Alginate, Chitosan, and Tea Tree Essential Oil

Sodium alginate, a natural polysaccharide substance extracted from seaweeds, can cross-link with divalent cations and can be concentrated in solutions and form gels and films [10]. Therefore, sodium alginate is widely used for immobilization in foods, medical dressings, drug carriers, heavy metal adsorption, and biological enzymes and is employed as a liquid homogenizer and thickener in the food industry [11][12]. Furthermore, calcium alginate exhibits high biocompatibility and moisture absorption capacity; it can easily remove substances and adsorb heavy metals. Thus, calcium alginate is suitable for coating living organisms to prevent their harm under special environmental conditions or when transferring matter into living organisms.
Chitosan, a positive cation, electrostatically interacts with anions on the surface of bacteria, changes the permeability of bacterial cell membranes, causes abnormal access of materials in and out of cells, and exerts an antibacterial effect. Moreover, chitosan promotes the synthesis of inducible nitric oxide synthase (iNOS) by macrophages to produce nitric oxide (NO) [13], enabling macrophages to engulf bacteria and attack cancer cells [14] and exerting antibacterial effects. In particular, chitosan is the most effective in controlling the growth of Staphylococcus aureus and Salmonella. Chitosan with an MIC50 value of > 5mg/mL exerted the strongest inhibitory effect on E. coli, Pseudomonas aeruginosa, S. aureus, Candida albicans, Streptococci, and S. enteritidis [15].
Tea tree, also known as Melaleuca alternifolia, originates from Australia and belongs to the family Myrtaceae. Tea tree oil is composed of terpenes, mainly monoterpenes, sesquiterpenes, and related alcohols, which are volatile aromatic hydrocarbons. The antibacterial activity of tea tree oil is mainly attributed to terpinene-4-ol, the main component in the essential oil. Tea tree essential oil has poor hydrophilicity. At a high concentration, tea tree oil exerts a bactericidal effect and can inhibit tumor necrosis factor [16]; thus, tea tree oil can be used as a natural bactericide. Moreover, tea tree oil exerts antiviral effects and can inhibit mold growth, stimulate leukocyte proliferation, increase immune cell activity, enhance immunity, prevent infection, inhibit infection spread, promote wound healing, relieve pain and discomfort, and reduce inflammation [16][17][18]. Tea tree oil is often used for anti-inflammatory purposes in surgery and dental operations [19][20] and is widely used in aromatherapy [21].
Because of the antibacterial and anti-inflammatory properties of chitosan, polycaprolactone nonwoven mats (PCLNM) prepared using tea tree essential oil (TTO) exhibit anti-inflammatory and bactericidal effects [22][23][24]. After being injected into capsules and placed in water at 50 °C, the time required for the release of tea tree essential oil is similar to that at 70 °C–90 °C [25]. Therefore, sodium alginate, chitosan, and tea tree essential oil can eliminate pollutants from used hot spring water and thus achieve the goal of the sustainable management of hot spring water.
In summary, microcapsules prepared using sodium alginate, chitosan, and tea tree essential oil exert a bactericidal effect without causing any side effects to humans. Thus, they can be used for water resource management to achieve the sustainable management of hot spring water resources.

4. The Antibacterial Effect of Alginate/Tea Tree Essential Oil Microcapsules and Alginate/Chitosan Microcapsules

Sodium alginate is a natural polysaccharide with the ability to concentrate solutions, form gels, and form films [10]. Chitosan also has inhibitory effects on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans, streptococcus, and salmonella [15]. However, tea tree essential oil can damage cell membranes, prevent cell adhesion, cause irregular aggregation of cytoplasm, and lead to cell disintegration [26]. Therefore, compared with the antibacterial liquid prepared by simply combining alginate substances, the antibacterial effect of microcapsules made of tea tree essential oil is better than that of sodium alginate. This result has been confirmed by [10][15].

5. Discussion on the Release of Microcapsules Obtained with the Same Concentration of Cross-Linking Agent for Different Cross-Linking Time

The experiment objective was to meet the properties of “not increasing its redox potential”, “not changing the water quality of hot springs”, and “having a high efficiency in antibacterial effect”. The experiment showed that the longer the cross-linking time, the thicker the zincization thickness and the smaller the porosity, and the longer the tea tree essential oil encapsulated in the microcapsules may be released in the hot spring water. It also revealed that it could be completely cross-linked with the alginate after 3 h. However, since the metal activity of calcium is higher than that of zinc, the time to reach complete cross-linking in the experimental group added with CaCl2 can last for 3.4 h during the experiment. Therefore, in the experiment of adding different concentrations of CaCl2 and ZnCl2 into the microcapsules, the release time of the CaCl2 group was longer, and it could maintain the effective antibacterial effect for a longer time.

6. Discussion on the Release with Different Concentrations of Cross-Linking Agents for the Same Cross-Linking Time

The microcapsules extracted from alginate have high water absorption. They can easily remove stains and can absorb heavy metals [27][28]. Moreover, the metal activity of CaCl2 is higher than that of ZnCl2, so it is helpful for the release of microcapsules in hot spring water. However, the temperature of hot spring water is generally at least 40 degrees or above. Although substances such as alginate, chitosan, and tea tree essential oil have the effect of removing heavy metals and sterilizing, their chemical substances are prone to qualitative change under long-term immersion in high temperatures, which reduces their existing functions. As a result, although each of 0.1 M, 0.5 M, and 1 M has antibacterial benefits, the release time of 1 M is longer. In addition, considering factors such as production technology, cost, release time, and antibacterial benefit, when the concentration of the substance loaded in the microcapsules is set above 1 M, the release time and antibacterial effect of 3 h can be achieved.

This entry is adapted from the peer-reviewed paper 10.3390/w14091391

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