What do we understand by nanodispersions?

Nanodispersions, in other words, liquids with particles in nano size meaning < 1 µm = < 1000 nm, are widely used in skin care preparations and in dermatological cosmetic applications.^1,2,3,4,5,6 Optimal haptic properties can be achieved with nanodisperse lipids and oils. Thus the inconvenient oily and fattening components can be converted into aqueous lotions that rapidly penetrate into the skin. Water-soluble active agents and particularly such with high polarity are encapsulated into liposomes which also belong to the category of nanodispersions.

Regardless of their biodegradability and their structure, either lipophilic (oils, fats, waxes, organic active agents), hydrophilic (liposomes with aqueous interior) or inorganic, nanodispersions imply a high availability of active agents and accordingly allow using low concentrations of active agents in the formulations. Just to mention some examples: Liquid nanoparticles loaded with retinyl palmitate (vitamin A) sooner than conventional emulsions reach the irritation threshold that is critical for the formation of vitamin A acid. Liposomes with very low concentrations (≤ 1%) of sodium ascorbyl phosphate (vitamin C) inhibit the melanin formation.

In comparison to conventional formulations, nanodispersions with particles smaller than 1 µm allow a reduction or even elimination of cosmetic additives such as emulsifiers, spreading agents and penetration enhancers. Cosmetic additives often stress the skin and can even be counterproductive when facilitating washout effects or irritations. Moreover, the combination of nanodispersions with conventional formulations often fails due to the incompatibility with emulsifiers.⁷
Since the preservatives listed in the annex of the EU Cosmetic Directive imply the risk of sensitisations when permeating, nanodispersions are produced under sterile conditions and filled in ampoules, or alternatively produced without any preservatives whatsoever. The advantage, of course, is that physiological concepts are realized to a large extent.

An important criterion with topically applied nanoparticles is their biodegradability in the human body. The following nanoparticles are easily biodegraded:

• Nanocrystals of difficultly soluble or high-melting organic substances such as boswellic acids (frankincense), phytosterines, flavonoids and their glycosides as for instance rutin, as well as ceramides.  
• Liquid nanoparticles based on phosphatidylcholine – in this case not the particles are penetrating into the skin but the molecular individual components. They can contain fat-soluble active agents such as vitamins, coenzyme Q₁₀, ceramides, herbal oils and essential fatty acids.
• Liposomes with water-soluble vitamins, antioxidants, glycosides, moisturizers etc.

There are other nanoparticles with carrier substances and sometimes also active agents that are non-biodegradable, as for instance:

• Lipid nanoparticles made of waxes, poly-alpha-olefins and other hydrocarbons. On the skin they combine into occlusive films from which lipophilic active agents such as coenzyme Q₁₀ are released.
• Polymer beads made of polyamides, polypeptides or polysaccharides with embedded, mostly pharmaceutical active agents.
• Silica nanoparticles (silicic acid) that integrate and stabilize amorphous active agents and pigments in their pores.
• Inorganic substances such as titanium dioxide, zinc oxide and carbon (carbon black) for sun screen purposes and decorative cosmetic products.
• Elementary precious metals such as silver and gold. Gold forms red dispersions; silver has antibacterial and anti-inflammatory effects.

Aqueous nanodispersions with small particles look like aqueous solutions while particles larger than 400 nm show an opaque or even milky consistency. They can also be coloured, as for instance gold dispersions.  

In the field of dermatological cosmetics they are used for the supportive prevention and the adjuvant skin care in the case of skin irritations.⁸ For this purpose individual substances but also substance compounds and herbal extracts are used. The focus is on biodegradable nanodispersions that are compatible with the human physiology.

Nanoparticles with a size of 100 nm or smaller have to be labeled as nanomaterial according to the Regulation EC1223/2009 of the EU Cosmetic Directive, in force since 11.07.2013. They are subject to strict requirements. Exempt of the directive are nanoparticles with a particle size > 100 nm as well as liquid, biodegradable nanoparticles that already decompose into their individual components in the skin barrier.

Table: Examples of frequently used nanoparticles and active agents
Liposomes (L); liquid (N), solid (SN) nanoparticles; biodegradable (+), non-degradable (-)

References

1. H. Lautenschläger, Nanopartikel von fest bis flüssig, medical Beauty Forum 2016 (2), 12-16
2. H. Lautenschläger, Cosmeceuticals, medical Beauty Forum 2014 (4), 16-18
3. H. Lautenschläger, Indikationsgemäße Anwendungen von Nanodispersionen, Vortrag auf der 19. Jahrestagung der Gesellschaft für Dermopharmazie (GD) in Berlin am 18.3.2015
4. H. Lautenschläger, Geschichte und aktuelle Gesichtspunkte der Korneotherapie, Kosmetische Medizin 26 (2), 58-60 (2005)
5. H. Lautenschläger, Mikrokosmos modularer dermaler Zusammensetzungen, Vortrag auf der 18. Jahrestagung der Gesellschaft für Dermopharmazie (GD) in Berlin am 9.4.2014
6. J. Cornier, C.M. Keck, M. van de Voorde, Nanocosmetics – From Ideas to Products. Springer. (2019).
7. H. Lautenschläger, Huckepack – Übersicht Trägersysteme, medical Beauty Forum 2013 (1), 16-18
8. H. Lautenschläger, Nutzen von lamellaren Präparaten in der Hautpflege, im Hautschutz und in der dermatologischen Therapie, Vortrag auf der 17. Jahrestagung der Gesellschaft für Dermopharmazie (GD) in Mainz am 23.3.2013

Hans Lautenschläger & Cornelia Keck