Formulating High SPF Sun Care Products With A - External Factor ...

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SUN CARE LIQUID-MINERAL UV FILTER K. Berg-Schultz, Ch. Mendrok, F. Sit, R. Jermann* Formulating High SPF Sun Care Products with a Liquid-Mineral UV Filter Keywords: Sun Care, Polysilicone UV Filter, High SPF Formulation, Enhanced Sensorial Profile � Introduction Abstract The novel architecture of the new UVB filter, PARSOL ® SLX (INCI: Polysilicone-15) combines the desirable attributes of both polydimethylsiloxane and organic UV filters. It exploits the film-forming characteristics of polydimethylsiloxanes on human skin, ensuring an optimal distribution of the attached UV absorbers when applied as a sun cream. This results in remarkably high performance sun care products which are suitable for all skin types but especially for sensitive skin due to its minimal skin penetration. Formulated products containing Polysilicone-15 also convey appealing sensorial aspects while remaining easy to use and handle. Due to the increased awareness of consumers that excessive sun exposure can lead to damages such as premature skin aging, sunburn, immune suppression, and different types of skin cancer (1,2) the consumption of sun care products has steadily increased in the last decades. Simultaneously, the sun protection factor (SPF) of products found in the marketplace increased significantly. Whereas, 10 years ago an SPF of 4 to 20 was the common practice, today usual sun care products exhibit SPF’s of 20 up to SPF’s of 50+. For formulators the preparation of a sunscreen having a SPF of 40 or more is still a challenging task and needs a lot of experience and skills, especially as it is well known that an increase of the UVfilter concentration does not steadily lead to a higher SPF. Thus, in order to achieve a high SPF different UV-filters have to be combined (Fig. 1) (3). Nevertheless, not all UV-filter combinations lead to expected higher SPF’s and often several trials of different combinations are necessary (4). In a sun care product with a high SPF, the UV-filters represent an important proportion of the total formulation and SPF filter 1+2+3 filter 1+2 Fig. 1 Synergistic effect of UV filter combinations in cosmetic formulations therefore contribute significantly to the sensory profile of the product. However, high levels of traditional organic sun filters can lead to aesthetically unpleasing effects with regard to greasiness and tackiness of the formulation. On the other hand inorganic UV-filters such as minerals and pigments have an undesirable residual whitening effect on the skin which makes the skin look pale (Fig. 2). Fig. 2 Whitening effect on the skin As the consumers dislike such features the choice and combination of UV-filters used in sunscreen formulations is crucial. Additionally, photo-stability, safety aspects, easy handling in production, product stability and costs have to be taken into account in the development of a sunscreen product. For this reason, the cosmetic industry desires UV-filters which provide effective protection against UV radiation reflected by a remarkable SPF performance, which are easy to formulate and have a pleasant skin feel. 2 SÖFW-Journal | 131 | 7-2005 filter 1 UV-filter concentration
� Polysilicone-15 – a liquid mineral UV-filter Polysilicone-15 is the first commercial example of a completely new generation of UV filters which were specifically designed to meet the needs of the cosmetic industry. Unlike conventional monomeric UV filters, this innovative UV filter constitutes a hybrid of silicone technology and classic organic UV-filter chemistry. A closer look at the structure of the molecule reveals, that the mineral silica in form of a polysiloxane chain serves as polymeric backbone for the chromophore units (Fig. 3). This combination leads to a unique polymeric UV filter which besides being liquid at all temperatures has excellent cosmetic properties and minimizes the risk of skin penetration due to its high molecular weight of about 6000g/mol. Although the polymeric backbone represents more than 70% of the molecular mass and does not contribute to the UV absorption as reflected by a low UV specific extinction (E 1%, 1cm), Polysilicone- 15 delivers in vivo SPF performance comparable with industry standards, such as Ethylhexyl Methoxycinnamate (EHMC) (Fig. 4). This, at first sight, surprising result, can be explained by looking into the behavior of different UV-filters on the skin. � In vivo performance of Polysilicone-15 versus EHMC How can it be, that a molecule, which has less than 30% of its molecular weight contributing to sun protection, obtains the same SPF performance as the benchmark molecule EHMC? An explanation of this question needs some insight into the behavior of sun-filters in general and of their behavior on the skin. Numerous attempts have been made to understand how sunscreen preparations protect against UV radiation (5-9). Up to now the measurements of the sun protection factor (SPF) of emulsions containing traditional UV absorbers clearly indicated that the efficacy depends on the absorption characteristics of each single UV filter substance. However, it is also well accepted that the Lambert Beer’s Law is not obeyed at high concentrations SUN CARE LIQUID-MINERAL UV FILTER Fig. 3 Structure of Polysilicone-15 Fig. 4 In vivo SPF comparison of Polysilicone-15 and EHMC (>0.01M) as present in sunscreen formulation due to electrostatic interactions between molecules in close proximity. At high UV absorber concentrations, as commonly found in sunscreen formulations, this leads to a drop of the UV absorption properties and consequently to a loss of the theoretical protection abilities. Additionally, the degree of UV absorption achieved by the UV filters within a sunscreen formulation depends on the irregularities in the geometry of the sunscreen film after application on the skin (5, 8-12). Reason for the uneven distribution of UV-filters on the skin are manifold and can be caused besides the rough- ness of the skin due to wrinkles, cracks, hair and gland shafts, by aggregation of the aromatic UV filter molecules due to π-π interactions (13,14) or agglomeration of nanopigments as well as by microcrystallisation of an ingredient on the skin (15). Various models have been proposed in order to predict the in vivo SPF which are based on the Lambert Beer’s Law and which take the film irregularities into account. In 1983 O’Neill (12) proposed a simple model of irregular film, a step film geometry, which satisfactorily account for the discrepancy between measured in vivo data and simple spectroscopic data. SÖFW-Journal | 131 | 7-2005 3
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