Analyzing How Spreading Impacts Sunscreen Efficacy

The unequal spreading of sunscreen can impact its efficacy. This article describes an in vitro, Raman-based technique to explore this relationship.

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Renowned cosmetics company L’Oréal has gained a reputation over the years for giving great importance to research and innovation. The Advanced Research Division that is located just outside Paris (France) is at the heart of that specific component of L’Oréal’s mission. The constant improvement of sunscreens is one of the various subjects studied by the division’s team of specialized researchers. Florian Formanek, head of the Microscopy & Microanalysis Laboratory, gives a brief about this.

Photoprotection – a Key Cosmetic Segment

Sun Protection Factor, or SPF, is a measure of the efficacy of sunscreens to give protection to the skin against harmful UV radiation. SPF is determined via the in vivo monitoring of erythema (reddening) caused on human skin by solar-simulated sunlight. This is the approved method.

However, there are also alternative in vitro measurements that are performed by diffuse UV transmission spectroscopy through a sunscreen-covered substrate, such as rough polymethyl methacrylate (PMMA) plates which imitate skin texture and surface free energy.

Cosmetic formulations usually consist of several physical (inorganic) and chemical (organic) filters that either reflect, scatter, or absorb UVA and UVB radiation to provide an efficient broad-spectrum protection. Other ingredients include glycerin (moisturizer), water, emulsifiers (to mix water with oil), emollients (for smoothing properties), film formers (water resistancy), thickeners (to confer viscosity), and sensory enhancers.

Apart from the obvious parameters like film thickness and homogeneity, the performance of sunscreen can also be affected by the localization of the different filters within the bulk. Based on the physicochemical properties of the substrate surface, processes like dewetting, coalescence, or phase migration take place upon drying and evaporation of volatile compounds, thus affecting the distribution of filters as well as oily/aqueous phases.

Raman imaging is used in tandem with other microscopy techniques to get a better understanding of the distribution of UV filters in various formulation bases and compared with in vitro SPF assessment.

Experiments and Data Processing

A Renishaw inVia™ Qontor™ Raman microscope equipped with a 532 nm laser is used to obtain 3D or 4D hyperspectral maps containing - at each pixel - a Raman spectrum in the fingerprint region (400-2000 cm^-1). A real-time focus tracking unit, which was recently introduced, simultaneously provides the topography of the sample surface with micrometric axial resolution.

MountainsMap^® software is first used to remove the outliers (non-measured points obtained during auto-focus), and then to offset the sample’s general slope with the leveling or form removal functions. The next step is to do morphological filtering, before using sophisticated analysis methods.

The colocalization module allows the relief map to be overlaid with 2D chemical images acquired following chemometric processing of spectroscopy information. Part of this processing involves the deconvolution of the measured spectrum using reference spectra of each individual ingredient, in order to extract their relative concentration at the laser focus spot.

3D height profile of a bare PMMA plate with theoretical Sq roughness of 6.6 μm, measured with MountainsMap^® at 6.18 μm.

The principle of the spectral decomposition processing to retrieve the relative contribution of the different compounds in the formulation.

Reconstructed view of a sunscreen-covered PMMA plate, combining topographic and chemical data.

• 10% height amplification was applied to enhance 3D rendering.
• Gray areas show uncovered PMMA regions, red and green areas reveal the distribution of two different UV filters.
• Calculated Sz parameters (67.4 μm → 50.3 μm) indicate smoothing of the surface through filling of the valleys by the cream.

Other applications of the MountainsMap^® software include extracting quantitative surface textures from 3D samples (skin, hair, nail, materials) measured by optical coherence tomography (OCT), optical profilometry, or fringe projection, creating stereo views from tilted scanning electron microscopy (SEM) images and also extracting and processing intensity profiles from energy-dispersive X-ray (EDX) spectroscopic data generated by TEM and SEM.

This information has been sourced, reviewed and adapted from materials provided by Digital Surf.

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