5 minute read
A new particle design for mineral sunscreens
Driven by our purpose – to use “smart science to improve lives” – Croda is tackling some of the biggest challenges the world is facing today and has committed to be “climate, land and people positive” by 2030. One of our hero targets within our “people positive” commitment is to protect at least 60m people annually from potentially developing skin cancer from harmful UV rays, through the use of our solar protection ingredients. By Helene Hine, Croda
Croda’s latest UV filter launch, Solaveil MicNo, is helping us to meet this hero target, extending our range of Solaveil mineral filters with a patented particle design that has all the benefits of a micron-sized particle, yet minimal whitening on skin.
PARTICLE DESIGN AND HOW IT WORKS
The patented technology behind Solaveil MicNo is its micron-sized platelet-shaped particles. This unique morphology is formed from primary crystals. Figure 1 shows scanning electron microscopy (SEM) images of this unique platelet structure. At high magnification, the SEM images show the platelet structure in a stacked formation. This stacked packing and the separation of structural layers are why Solaveil MicNo has unrivalled transparency on skin.
Measurement of mean particle size of uncoated Solaveil MicNo by X-ray disc centrifuge (XRDC) by mass, gives a mean particle size of 516nm. Based on both the REACH definition of “nano” and the Cosmetics Europe interpretation of “nano”, Solaveil MicNo would therefore be considered to be “non-nano”.1, 2 Furthermore, studies have revealed that the platelet structure of this UV filter is not broken down during use, even when extremely high shear, such as bead milling, is applied to the product.
Solaveil MicNo consists of platelets that form an ordered stacked structure (see Figure 1). This structural composition results in less scattering and reflection back to the surface of the skin, and ultimately results in unrivalled transparency on skin. The mechanism of action is summarised in Figure 2.
Did you know?
Mineral inorganic sunscreens attenuate UV by two mechanisms: absorption and scattering. Conventional ZnO sunscreens scatter and reflect visible light to the surface of the sunscreen film, which can lead to a white appearance on the skin.
APPEARANCE ON DIFFERENT SKIN PHOTOTYPES
To visualise the skin appearance, skin whitening was evaluated using a basic W/O emulsion containing 20% active ZnO. Subjects with skin phototypes from I to V were selected, with two subjects with skin phototype V being included in the study. Of the sample, 0.05ml was applied to the subjects in a 5cm square. After application, the subjects were instructed to rub in the sample 40 times; the skin was left to dry for five minutes before images were taken.
Figure 3 shows the results for Solaveil MZP3 (uncoated ZnO powder from the Solaveil MicNo range) compared with the market-leading uncoated ZnO powder which is spherical and has a small particle size. In all cases, Solaveil MicNo has a lower level of whitening on the skin.
SAFE AND SUSTAINABLE MINERALS
ZnO occurs naturally as the mineral zincite, but in cosmetics, all ZnO used is of synthetic origin. Solaveil MicNo is classified as a nature-identical mineral and categorised by ISO16128 as a “derived mineral”. In contrast, organic (or chemical) sunscreens which are widely used in sunscreens are not considered natural as they are fossilderived, synthetic compounds. In the Solaveil MicNo range, three products are COSMOS-approved. These are the uncoated powder Solaveil MZP3, stearic acid coated powder Solaveil MZP8, and an uncoated powder dispersed in caprylic/capric triglyceride, Solaveil MZ3-300. In addition, the entire range is vegan-suitable and halaal-approved.3
ZnO is well-known as a safe material that has been used in cosmetics for decades. In more recent times extensive safety reviews have been conducted by the US Food & Drug Administration (FDA) and the EU Scientific Committee for Consumer Safety (SCCS) and these reviews have concluded that whatever its size (nanomaterial or not), ZnO largely does not penetrate the skin.4, 5 The FDA classify them as GRASE (Generally Recognised as Safe & Effective) Category 1. ZnO is approved by regulatory bodies globally at levels up to 25% active, which is the highest active level permitted of any UV active, along with titanium dioxide (TiO2). ZnO also has a very low potential for skin irritation and has a long history of use in topical treatments for skin conditions, making Solaveil MicNo a great choice for formulators working on products for any skin type, including babies, children and sensitive skin.
There is some evidence of allergens with chemical/organic sunscreens, specifically benzophenone, cinnamates and dibenzylmethanes.6 In addition, the FDA states there is evidence that they are/ or may be absorbed through the skin, and the consequences of this absorption are not known.4 Finally, their permitted use levels are limited to 5% to 10% in most cases, depending on the country.
SPF AND UVA EFFICACY
Solaveil MicNo is broad spectrum and comparable in UVA performance with conventional ZnO grades. It has a UVA PF per 1% solids of around 0.41. Used alone it passes the EU requirements for UVA (that is SPF/UVA PF ratio of approximately 1.2, so less than 3) and meets the US requirements for broad-spectrum claims (that is a critical wavelength of around 376nm, so greater than 370nm). It also meets the new proposed US requirement for broad spectrum, which is a UVAI/UV ratio of at least 0.70. Solaveil MicNo achieves a UVAI/UV ratio of around 0.875, which would achieve a rating of ‘high’.
The SPF efficacy of Solaveil MicNo has been studied in a wide range of formulations and tested in vivo with independent laboratories. Results show that the UV filter has efficacy equal to, or better than, conventional ZnO.
Solaveil MicNo will typically achieve at least 0.52 SPF units for 1% solids, while higher performance is obtained with dispersions of the UV filter. SPF efficacy is, however, highly formulation dependant and some studies show SPF efficacy of 1 SPF unit or higher for 1% active. Further work is ongoing in Croda’s global formulation laboratories to understand these findings and to establish greater efficiencies for Solaveil MicNo. •
