(Nano-)particles play an important role as ingredients in beauty and cosmetic products, as nanosized particles have different properties than larger particles of the same substance. Small particles can easily penetrate the skin and introduce active ingredients into the skin’s deeper layers. The total volume is the same, but the surface area is highly extended, opening up the possibility for various new applications that are useful for cosmetic formulations. Despite their great advantages, it is extremely important to characterize nanoparticles precisely – concerning their application or the security of consumers, e.g. with regard to compliance with certain EU regulations (e.g. Regulation (EC) No 1223/2009, Art. 13 (1)).
Nanoparticles in cosmetics – why should they be examined carefully?
Nanoparticles are small and invisible to the human eye. More precisely, they are between 1 and 100 nanometers in size. Their name is derived from the Greek word "nanós" which means “dwarf”. In the cosmetics industry, nanoparticles serve many different purposes and are used for sun protection, toothpaste, skin care products, and decorative cosmetics. Ingredients are used in nanoscale versions to improve UV protection, skin penetration, long-lasting effects, or color intensity.
The use of nanotechnology in cosmetics is associated with risks concerning health and environmental issues. In fact, this is still an insufficiently explored field. In this article, we will have a look at the use of different nanomaterials in cosmetics, their potential associated risks, and the regulations that have been implemented to minimize them.
Use cases of nanoparticles
Nowadays, almost all major cosmetics manufacturers take advantage of nanotechnology in cosmetics. Many products, including moisturizers, hair care products, make-up, and sunscreen, contain nanosized ingredients. The reduced size of nanoparticles leads to a physicochemical change in their properties. Due to their small size, nanoparticles are more efficient and enhance the product’s performance – one of the main reasons for their application in cosmetics. But which types of nanomaterials are used by the cosmetics industry and in what kind of products?
Liposomes and niosomes are the most prominent examples of nanotechnology in skin care. They are composed of spherical structures of surface-active molecules (e.g. phospholipids), which are arranged in two layers and separate an inner core from an outer phase (see Figure 1). Therefore, the inside and the outside of a liposome are hydrophilic, whereas the inner membrane is lipophilic. Thus, a liposome can absorb both water-soluble active ingredients (e.g. water-soluble vitamins, chemical preservatives, and fat-soluble substances such as fat-soluble vitamins or perfumes). They serve as delivery vehicles and transport active ingredients into deeper skin layers. Depending on the targeted tissue and the application, it is important to precisely define and characterize the particle size of liposomes.
Micelles are nano-sized molecules that arise in mixtures of amphiphilic surfactants and water. When the surfactant concentration reaches a certain level (termed “critical micelle concentration”), the surfactant molecules begin to rearrange themselves. This reaction creates small, spherical micelles, exhibiting both lipo- and hydrophilic properties (Figure 2).
Micelles are often used in facial cleansers or tonics. They gently remove dirt particles, make-up, and excess sebum from the skin, by enveloping the lipophilic material (e.g. make up) to be removed. What’s more, during the cleansing process, nourishing ingredients can be applied and transferred into the skin.
Nanoemulsions are dispersions of nanoscale droplets within another liquid. Due to their particle size of less than 200 nm, they differ from other disperse systems. The emulsion’s structure can be manipulated based on the preparation method. The components used for the preparation of nanoemulsions are generally recognized as safe (GRAS) products. The smaller particle size provides higher stability and better suitability to carry active ingredients; it also increases the shelf life of the product.
Nanocapsules are submicroscopic particles that are made of a polymeric capsule surrounding an aqueous or oily core (see Figure 3). They protect sensitive products in cosmetics and deliver active ingredients to the targeted tissue, for example by only releasing them via diffusion or under certain conditions (e.g. change of the pH value). These capsules can be used in face creams, ointments, or moisturizers. When applied on the skin, active ingredients are released depending on the skin’s individual needs, e.g. by a change in the pH value or by means of specific proteins. Therefore, active components are only released when the skin really needs them.
Solid nanoparticles such as TiO2 and ZnO are probably the best-known representatives of nanoparticles in cosmetics. They are “micronized” for usage in sunscreen, making them more transparent, less greasy, less smelly and more absorbable into the skin. TiO2 particles used as UV filters are between 15 and 150 nm in size compared to ZnO, which has a size range of 30 to 200 nm. Usually, these particles are coated with aluminum or silicon compounds. The coating not only stabilizes the dispersion, but also limits the compound’s photocatalytic properties, as “naked” TiO2 and ZnO mediate the formation of free radicals under the influence of UV light.
Nanosilver and nanogold
Nanosilver comprises particles of elemental silver with a size of less than 100 nm. Its properties are highly different from other forms of silver. Their greatly extended surface makes these particles more reactive. Therefore, silver nanoparticles can also penetrate cell membranes and act like a depot from which particles are released continuously over a longer period of time. Thus, cosmetics manufacturers are harnessing the enhanced antibacterial properties of nanosilver in a range of applications. Some manufacturers are already producing deodorants claiming that the containing silver will provide up to 24 hours of antibacterial protection. Nano-sized gold is declared to be highly effective for mouth disinfection and is also added to toothpaste. Furthermore, it is found in make-up and face powder to give the skin a fresher and glowing look.
Buckminster fullerene, C60 or carbon black, is the most iconic nanomaterial. It is approximately 1 nm in diameter with a truncated icosahedron structure, resembling a soccer ball, as illustrated in Figure 4. Carbon black is used in the cosmetics industry as a black coloring pigment and is characterized by a particularly high color intensity and depth. It is found in mascara, eyeliner, eyeshadow, eyebrow pencils, and nail polishes. Carbon black has been approved as a colorant since the beginning of EU cosmetics legislation and is used worldwide today. It is applied in a highly pure form and is subject to strict quality requirements.
Potential risks and safety regulations
As with other innovative products, there is an ongoing debate regarding the safety of nanomaterials in cosmetics. Since there is a fear that nanoparticles could penetrate the skin showing harmful long-term side-effects, a lot of research is being invested. Still, independent of their nano-sized ingredients, cosmetics products must be safe. Manufacturers are responsible for ensuring that all ingredients and the finished product itself meet strict requirements. Cosmetics that are sold in the European Union are regulated by the EU Cosmetics Regulation No. 1223/2009. The particle sizes of the ingredients must be considered as one part of the safety assessments that must be carried out by manufacturers, importers or independent experts for every cosmetics product in the European Union. In terms of the EU Cosmetics Regulation, “a nanomaterial is an insoluble or biologically stable and intentionally manufactured material with one or more external dimensions or an internal structure in the order of 1 to 100 nanometers”. This regulation stipulates that manufacturers must forward a minimum amount of information to the EU commission, including the specifications of the nanomaterials used as well as their physical and chemical properties and their particle sizes.
Characterization of nanoparticles in cosmetics
Since the particle size of nanomaterials must be submitted to the EU Commission, it is important to precisely characterize and define it. A large number of methods are available for the analysis of cosmetic products and characterization of (isolated) nanomaterials. Nanoparticles used in cosmetics should be specified in terms of their average particle size, taking aggregates into account too. The size of the individual particles and a description of their size distribution (from the smallest to the largest particles occurring in the application) should also be part of the specification.
The EU regulation does not specify the method that must be used for characterization. The most popular methods to measure primary particles and products are laser diffraction, dynamic light scattering, and microscopy.
Many types of nanoparticles with different properties are used in a wide variety of products in the cosmetics industry. In order to ensure consumer safety, the EU stipulates several parameters in its regulations that the particles and products must comply with – particle size being one of them. To characterize nanomaterials in terms of their size, manufacturers can choose between different measurement techniques.
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