Silicon Microspheres as UV, Visible and Infrared Filters for Cosmetics

Sep 1, 2010 | Contact Author | By: Isabelle Rodriguez, PhD, Roberto Fenollosa, PhD, and Francisco Meseguer, Universidad Politécnica de Valencia
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Title: Silicon Microspheres as UV, Visible and Infrared Filters for Cosmetics
silicon microspheresx UV/infrared filterx shapex smoothnessx refractive indexx
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Keywords: silicon microspheres | UV/infrared filter | shape | smoothness | refractive index

Abstract: Silicon microspheres are described as UV, visible and infrared (IR) radiation filters. Parameters of these spheres including shape, smoothness, refractive index and size are examined for their potential benefits in cosmetic formulations. Finally, the ability of the spheres to block IR radiation is evaluated for thermo-regulatory effects.

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I Rodriguez, R Fenollosa and F Meseguer, Silicon microspheres as UV, visible and infrared filters for cosmetics, Cosm & Toil 125(9) 42-50 (Sep 2010)

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As is generally known, the solar radiation spectrum extends from wavelengths of 200 nm to 3,000 nm. Within this spectrum, the different types of radiation can be classified by wavelength and energy content. Ultraviolet (UV) radiation, from 200 nm to 380 nm, produces burning and erythema, in the case of UVB, or skin aging and photocarcinogenesis possibly leading to skin cancer, in the case of UVA. Visible light falls in the range of 380 nm to 700 nm; and infrared (IR) radiation, from 700 nm to 3,000 nm, is responsible for heat and also appears to be involved in skin aging and cancer.

Solar radiation that reaches the earth’s surface is composed of approximately 7% UV. The remaining 93% is roughly divided between visible and IR radiation. While 7% is seemingly small in comparison, this level of UV radiation is sufficient to cause skin damage. Moreover, the ozone layer depletion during the past few decades has enhanced the levels of UV radiation that reach the Earth’ s surface. Therefore, efforts have been devoted to the development of organic and inorganic UV filters, and the sunscreen industry has benefited from the introduction of new active ingredients to enhance UV protection.

Organic chemical molecules including salicylates, cinnamates, camphor, triazone derivatives (UVB) or benzophenones, avobenzone, bemotrizimol (UVA), etc. absorb UV radiation, whereas inorganic particulates like titanium dioxide (TiO2) and zinc oxide (ZnO) reflect and scatter UV rays. Micronized particles of these latter compounds have been used to improve protection against UVA since they scatter light efficiently in the 320–400 nm range, provided they are present in sufficient quantities. Cosmetic manufacturers currently use these materials in conjunction with organic UV-absorbing chemicals to boost sun protection in the UVA region or to broaden the spectral coverage. However, while these particles effectively scatter UV, their use in sun care formulas poses a challenge since they appear white on skin. This is due to the attenuation of visible light, which is particle-size dependent, and is generally aesthetically unacceptable.


Lab Practical: Using Silicon Microspheres

  • Silicon microspheres are dispersible in aqueous phases; further processing enables their inclusion in oil phases.
  • The microspheres are slightly acidic with a pH of 5-6, which ensures the stability of the dispersions and minimizes particle aggregation.
  • The microparticles are obtained as aggregates of many colloids, i.e., a photonic sponge; previous grinding is required to improve the efficiency of the dispersions in emulsions as well as their optical properties.

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Figure 1. SEM image of a 2 micrometer-diameter spherical polycrystalline silicon microparticle28

Figure 1. SEM image of a 2 micrometer-diameter spherical polycrystalline silicon microparticle<sup>28</sup>

The spherical morphology of this silicon microparticle enhances light scattering.

Figure 2. Size distribution of silicon microspheres

Figure 2. Size distribution of silicon microspheres

Size distribution of silicon microspheres employed in sunscreen tests

Figure 3. Photonic sponge and optical transmittance

Figure 3. Photonic sponge and optical transmittance

a) SEM image of a photonic sponge made of silicon colloids, and b) the measured optical transmittance in the IR range of the photonic sponge and that of a silicon wafer, for comparison

Figure 4. Attenuation in the UV range

Figure 4. Attenuation

Attenuation of silicon microsphere-based sunscreens, compared with those made of TiO2 particles in the UV region (2 mg/cm2)

Figure 5. Attenuation in the IR range

Figure 5. Attenuation in the IR range

Attenuation of silicon microsphere-based sunscreens as compared to those made of TiO2 particles in the IR region (2 mg/cm2)

CVD Techniques

Chemical Vapor Deposition (CVD) is a technique used wildly in the semiconductor industry to produce high-purity solid materials. During the CVD process, the substrate to be coated is exposed to one or more precursor gases that react and/or decompose on the substrate surface to produce the desired deposit. For depositing silicon, precursor gases can be silane, disilane or trichlorosilane. In a typical CVD process, the precursor gas is decomposed by means of a heat source, such as a tubular oven in the simplest version, and is called hotwall thermal CVD.

Footnotes [Rodriguez 125(9)]

a Silicon Colloids are a product of Unidad Asociada ICMM-CSIC/UPV.

b The MATLAB algorithm using circular Hough transform is registered to The Mathworks Inc.

c The emulsion ingredients used for this study were obtained from Guinama S.L.U.

d The P-25 TiO2 nanoparticles used for this study are a product of Degussa.

e The PMMA Helioplates-HD6 substrates used for this study were obtained from HelioScreen Labs.

f The Oriel 77250 monochromator is a device from Newport Co.

g The xenon lamp used for this study is produced by Hamamatsu, JP.

h The silicon detector used for this study is a device from Thorlabs Inc.

j The IFS66 Fourier transform infrared spectrophotometer (FT-IR) equipped with an MCT detector used for this study is produced by Brucker Co.

Formula 1. Test sunscreen emulsions

Test formulas

Test sunscreen emulsions a) with silicon microparticles and b) with TiO2 nanoparticles

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