During recent decades, nanotechnology has become an integral part of research and development activity for cosmetic applications such as emulsions and sun care products. Investigations in the field of nanoparticle assembly as well as efforts in the development of their industrial fabrication processes allowed for the emergence of various new types of cosmetic applications.
For the development of a new cosmetic product, the most important attribute of nanotechnology is that a seemingly simple change of a fundamental physical property, the particle dimension, can give rise to a significant improvement in the final product, such as enhanced skin penetration ability. Nanoparticle production follows two main principles, either the “bottom-up” or the “top-down” approach. The first principle describes the assembly of particles starting from the molecular level, the second one the subsequent downsizing of macro-material. Very often, the pathway for molecular assembly is not accessible as, for example, raw products provided by nature exist as macro-sized and “pre-formed” materials. Here, new technologies are required which allow for efficient and economical dispersing, with respect to homogenisation of such materials for nanoscale products. The search for a suitable type of transport vector (e.g. emulsion, liposome) is as important as its physicochemical parameters, such as particle size and size distribution. Adequate drug dosage and location are crucial for effective delivery of ingredients, but small particle size is essential and its advantages basically are seen in two major criteria. In the first place, small particles are generally known to be more stable than their larger analogues. Secondly, the smaller the size, the better is the skin penetration and thus the delivery effect. However, progress is often hampered as technologies “introducing” the required mechanical and thermal stress may trigger damage in the structure of active ingredients. Here, results from low-pressure homogenisation as a gentle and energysaving way for particle size reduction will be presented.
Low pressure homogenisation
Following is an example illustrating the importance of size and homogeneity of an emulsion. Emulsions are well-known as efficient cosmetic delivery systems and frequently used for skin care products. In certain cases, particle size has to be as small as possible so as to obtain optically pleasant emulsions with enhanced delivery qualities. Generally, emulsions appear increasingly transparent when the droplet size is continuously reduced. The optical aspect of the sample can be visualised by laser light transillumination, which allows observation of the light scattered by the nanoparticles. Figure 1 (inset yellow curve) shows an emulsion with a broad spectrum of particles sizes in the submicron range before homogenisation. At first sight, the sample appears indeed homogeneous on laser light transillumination, but it is perceptible that the opalescent character is limited due to the blearing effect resulting from the fraction of large particles (those above circa 300 nm). The inset shows the result from dynamic light scattering (DLS) analysis revealing a hydrodynamic mean diameter of about 300 nm and particle sizes ranging from circa 80 nm to 800 nm (yellow curve). During the homogenisation process, the mean diameter of the particles is strongly reduced (after few homogenisation passages only). When laser light transilluminates the homogenised sample, the enhanced transparency and the less diffuse character of the scattered light become apparent (Fig. 2). This is due to the fact that small particles scatter light less efficiently than large ones. The size distribution clearly shifts to smaller particle sizes (Fig. 2 inset, red curve). However, likewise important is the effect on the shape of the size distribution, which undergoes a significant narrowing on homogenisation. The homogenised sample completely lacks particles above circa 200 nm, which is important as the particle properties (e.g. stability, skin permeation) are strongly correlated with their dimension. A narrow size distribution of particles corresponding to an enhanced level of homogeneity therefore promises products with more clearly pronounced advantageous properties and a smaller spectrum of side effects as imposed by “odd” particles.
Efficient and energy-saving emulsification
The Serendip low pressure homogeniser (LPH) technology (Fig. 3) is based on well-known high pressure homogenising technology. The core of the company’s technology is a new pressure cell that is designed according to latest scientific know-how, considering the optimised effects of friction, turbulence and cavitation. The technology combines the advantages of valve and nozzle systems allowing for very low working pressures and thus gentle process conditions. As the LPH technology works at moderate pressures (e.g. 500 bar) compared to high-pressure homogenisers (up to 2500 bar), the energy consumption is considerably reduced. The technology opens the door to high quality nanoemulsions and microemulsions with superior properties such as those relating to penetration of natural barriers, solubility, drug load, shelf life and skin feel.
