New Applications for the Aboriginal Remedy: Tea Tree Oil – Wrapped up in Sugar
Australian tea tree oil is an extremely effective antiseptic.
However, the Aboriginal remedy has its disadvantages: It evaporates quickly, has a strong smell and is chemically sensitive. In the past, these properties posed an obstacle to the oil's use in cosmetic formulations, personal care products and coatings. Now, the new WACKER technology of microencapsulation in cyclodextrin has opened up a broad range of uses for this valuable oil.
For the last ten years or so, natural cosmetic preparations have proved a big hit with consumers – reason enough for product developers to be constantly looking for new plant-based actives. Now, their attention has turned to tea tree oil – an essential oil known for its powerful action against a broad spectrum of bacteria and other microorganisms. Many use tea tree oil as a general-purpose household remedy. Although not permitted as a pharmaceutical in Germany, Austria and Switzerland, tea tree oil can be used in cosmetics.
All the same, tea tree oil has certain drawbacks that prevent its use in cosmetics. For one thing, it is highly volatile. As soon as the container is opened, a considerable amount of the tea tree oil evaporates and the benefit is lost. Many are also put off by the oil's pungent smell. Moreover, tea tree oil is sensitive to oxidation. This process produces compounds that can trigger local skin irritations and contact allergies. For these reasons, product developers have been wary of using tea tree oil. Many of the potential applications for this essential oil proved impracticable – until WACKER succeeded in solving the problem.
Tea tree oil is extracted from the leaves and branch tips of the Australian tea tree of the genus Melaleuca alternifolia by steam distillation. The colorless to pale yellowish-green oil has a characteristic, spicy smell redolent of eucalyptus, camphor and nutmeg. Tea tree oil evaporates quickly, i.e. it is highly volatile. This type of essential oil, unlike fatty oils such as sunflower oil, does not leave greasy stains on paper or textiles.
Like all essential oils, tea tree oil is a complex mixture of substances. It comprises over 100 compounds, about 60 of which have been identified so far. Tea tree oil's chief constituents include the monoterpenes terpinen-4-ol, γ-terpinene and α- terpinene. The oil's composition is influenced by many factors, such as the variety of tea tree, its location and prevalent climatic conditions, the age of the harvested leaves, and the time of harvest. Even the distillation procedure affects the oil composition. Tea tree oils with a high terpinen-4-ol content are considered especially effective, biologically speaking.
What we know about tea tree oil's antimicrobial effects ultimately dates back to the Australian Aborigines. They traditionally live in the northern part of present-day New South Wales – the tea tree's natural habitat – and have been using tea tree leaves for medicinal purposes since ancient times. The oil's antiseptic effect was first investigated by scientists in the 1920s. They found that tea tree oil's disinfecting action was about eleven times as strong as that of phenol, which was the most powerful disinfectant in common use at that time. In the 1940s, this traditional remedy lost ground to modern antibiotics such as penicillin. When it was shown in the 1970s that bacteria could develop resistance to modern antibiotics, the time was ripe for tea tree oil's renaissance.
Recent lab studies and clinical trials confirm the antimicrobial activity of terpinen-4-ol-rich tea tree oil. They showed that tea tree oil is also effective against bacterial strains resistant to antibiotics. Additionally, it is unlikely that bacteria will build up resistance to tea tree oil. Our natural skin bacteria are less sensitive to tea tree oil than pathogenic organisms.
What sets tea tree oil apart is its truly broad spectrum of action: it exhibits antiviral activity and acts as a bactericide, fungicide and insect repellent. When applied topically, preparations containing tea tree oil counteract, for example, acne, dandruff, athlete's foot, fungal nail infections, yeast infections and cold sores; in low concentrations, the oil is suitable for preserving cosmetic and personal-care preparations. Tea tree oil repels flies, mosquitoes, head and body lice, ticks and itch mites; in higher concentrations, it even kills these harmful arthropods.
Tea tree oil contains chemically-sensitive substances. Several oil constituents oxidize on contact with air at room temperature, especially when there's light, too. In this way, air greatly reduces the terpinen-4-ol content (main active). What's worse is that the gradual loss of active constituents is accompanied by a dramatic rise in the concentration of substances, such as p cymol, ascaridol and 1,2,4-trihydroxymenthane, which irritate the skin and are conducive to allergic reactions.
To minimize the risk of skin irritation and allergic contact eczema, the German Federal Institute of Risk Assessment (BfR) recommends a maximum tea tree oil concentration in cosmetics of 1 wt%. “In many instances, such a low tea tree oil content makes it impossible to reliably obtain the desired effect,” says Marlies Regiert, responsible for cyclodextrin-product development at WACKER FINE CHEMICALS.
One way out of this predicament is the molecular inclusion of tea tree oil in a suitable cyclodextrin – a method that has proved effective for fragrances, vitamins A, E and F as well as other lipophilic substances. Cyclodextrins are ring-shaped sugar molecules comprising several interlinked glucose units. Each cyclodextrin molecule can house a lipophilic guest molecule in its cavity, and will release it again under suitable conditions. Regiert explains the principle: “It's best to imagine a cyclodextrin molecule as a tiny beauty case in which an individual molecule is kept safe and protected against the influence of oxygen, light and heat. When necessary, the case is opened and the molecule emerges completely unchanged – as fresh as when it was put in the case. The key to opening these molecular cases is moisture.”
For as long as the tea tree oil remains enclosed in the cyclodextrin, it enjoys perfect protection – it can neither evaporate nor be altered chemically. “The skin's natural moisture and temperature are sufficient to release the tea tree oil. The oil thus reaches the skin in juvenile form. There are no skin-irritating and sensitizing oxidation and degradation products,” she adds.
Regiert conducted a battery of experiments to ascertain the extent to which the molecules of the tea tree oil constituents are protected within the cyclodextrin complexes. She not only tested the inclusion compounds themselves, but also examined the behavior of the complexes in cosmetic formulations and surface coatings, such as emulsion paints with repellent action.
β-Cyclodextrin, which comprises seven glucose units, plays a key role in affording protection against air, UV light and moderate temperatures: its cavity is exactly the right size for the inclusion of effective tea tree oil components. As a result, a lipstick containing the β-cyclodextrin complex will not smell of tea tree oil even after several years' storage. The oil is not released until it makes contact with moist lips. The β-cyclodextrin tea tree oil complex is just as stable in wall paint. In the presence of moisture, paint containing this complex will continue to release the volatile oil for over four years.
The smaller α-cyclodextrin, comprising six glucose units, likewise turned out to be an excellent candidate for protecting the essential oil – provided that the guest molecule is enclosed by two α-cyclodextrin molecules. “This 2:1 complex even withstands 60 minutes of heat treatment at 220 °C,” explains Regiert. “Enclosed in two α-cyclodextrin molecules each, the sensitive active-ingredient molecules are totally safe.” This inclusion compound can be used, for example, to coat razor blades. During shaving, the blades release the antiseptic tea tree oil, thereby preventing the sensitive facial skin from becoming infected.
Cyclodextrins are based on starch. WACKER uses a biotech process that selectively produces the desired cyclodextrin. “Inclusion within cyclodextrin is a promising and low environmentalimpact technology that exemplifies the principle of sustainability,” notes Marlies Regiert. Cyclodextrins have no known adverse health effects. Dermatologically speaking, there's nothing stopping their use in cosmetics, skin-care agents and personal care products or in surface coatings. Consistency is a further benefit of the inclusion compounds marketed by WACKER FINE CHEMICALS under its CAVAMAX® brand: they are colorless, odorless and free-flowing powders that are easy to store, handle and process.
This enables the cosmetics and consumer goods sectors to take advantage of tea tree oil's entire spectrum of activity – without having to worry about undesirable side-effects, a loss of effectiveness or the intrusive smell of the free essential oil. The gates are wide open for totally new applications.
The Background to Cyclodextrins
Cyclodextrins are non-reducing chiral sugars, whose molecules are made of several glucose building blocks linked into a ring. According to the number of glucose units – and therefore the ring size – a distinction is made between α, β and γ-cyclodextrin. a-Cyclodextrin has six, β- cyclodextrin seven, and γ-cyclodextrin eight glucose units. Cyclodextrins are natural degradation products of starch. WACKER FINE CHEMICALS produces cyclodextrin from phytomaterials by a bio-engineering method. In the cyclodextrin molecules, the glucose building blocks are arranged so that they have a lipophilic cavity (i.e. one with an affinity for fat) in their interior. This cavity can receive another lipophilic molecule as “guest,” provided that it has the correct size and shape. The cohesion between the two molecules is relatively weak (van der Waals forces), so that the guest molecule can be liberated again under suitable conditions. The weak van der Waals forces in such inclusion compounds leave the two counterpart molecules unchanged. This ability to enclose other substances reversibly makes cyclodextrins invaluable in many products and industries, such as household and personal care, pharmaceutical and cosmetic preparations, textiles and foods.