The TRI Guide to Sunscreen Testing for Claim Substantiation

Dr Samuel Gourion-Arsiquaud, Director at TRI

Dr Philippa Cranwell, Technical Content Creator at TRI

The Skin and Bio-substrates team at TRI have worked since 2018 on understanding how organic UV filters penetrate the skin.  The team have presented and published work on the use of encapsulation to prevent sunscreen penetration into the skin, the effects of film-formers on sunscreen absorption, and on the effects of skin temperature on sunscreen penetration.  In this bite-size review we use our specialist knowledge to provide a general background understanding of sunscreen testing methods.  TRI does not offer SPF testing, but, as you will see, we can assist formulators with other very useful tests.

Image from https://skinpluspharmacy.com.au  

In words of Australian Baz Luhrmann, circa 1999, ‘if I could offer you only one tip for the future, sunscreen would be it’. As well as being lyrics to a chart-topping song, this maxim is a good one to follow as wearing sunscreen can help prevent skin damage by the UV rays that can cause premature aging and skin cancer. 

Until recently, it was widely believed that wearing sunscreen was only necessary when in strong sunlight. For most people this was while on holiday, however as research has started to show that sun protection is advisable even in winter, consumer-demand means that everyday beauty products such as moisturizer, foundation and lipstick have all started to include UV-blocking materials.

When developing sunscreen-containing products for everyday use, quantifying and substantiating claims is important, and is an area in which the Skin and Bio-substrates team at TRI Princeton can support clients. In this extended blog we will briefly consider the regulations around sunscreen in both the US and EU, the different types of active ingredients, visualization of skin penetration, the development of safety improvements around ingredients, and how to evaluate sunscreen efficacy in vivo. 

But first: why do we even need sunscreen?

It is well-known that exposure to sunlight, or, more specifically the ultraviolet (UV) component of sunlight, can accelerate the signs of aging and lead to skin cancer. This is of particular importance for those with lighter skin tones who may burn easily.  The UV component of sunlight can be sub-divided into three categories: UVA, UVB and UVC, Figure 1:

• UVA light is longer in wavelength and has less energy than UVB but can penetrate deep into the skin and interact with DNA, which can cause mutations that lead to skin cancers. In addition, UVA is associated with skin ageing;

• UVB is higher in energy than UVA, with a shorter wavelength, and only penetrates the outermost layer of skin: the epidermis. However, UVB rays are linked with skin burning and also play a role in formation of skin cancers, such as malignant melanoma;

• UVC rays are stopped by the ozone layer and pose no risk to human health. 

Figure 1: Schematic showing how UVA, UVB and UVC penetrate different layers within the skin. 

Application of sunscreen is a simple way to reduce risk from the sun’s UV rays, providing some protection against harmful UVA and UVB radiation. Importantly, when formulating a sunscreen, it must protect against the skin damage associated with sun exposure while also being safe and providing a sensory and visually pleasant experience for customers upon application. 

Regulation: the great EU and US divide

Within the EU sunscreen is classified as a cosmetic item, whereas in the US it is classified as a non-prescription drug. This may look like semantics, but this difference in designation means there are some challenges in relation to the development of new products and the active ingredients that can be included when formulating for the EU and US markets. 

For example, because the US designates sunscreen as a non-prescription drug, any newly developed active ingredients require testing on animals to develop a pharmacological profile, akin to any other drug developed by a pharmaceutical company. In contrast, designation within the EU as a cosmetic item means that active ingredients still need to be deemed safe and comply with rigorous EU legislation but do not need to pass the same tests required for pharmaceutical drugs: gaining approval for a cosmetic item is less involved than a pharmaceutical. At the time of writing, within the EU there are 34 different UV filters approved for use whereas in the US there are only 16. 

As well as the designation between the definition of a sunscreen, there are also differences in the protection offered between EU and US sunscreens, which are mainly due to the active ingredients used. In the US, the UVA protection provided by sunscreen products is often significantly lower than that provided by EU products, especially with non-mineral sunscreens (see below). This is primarily due to the smaller range of active ingredients available that are specifically designed to block UVA rays. In addition, the EU recommends that manufacturers offer UVA protection that is 1/3 of the overall SPF, so if a product in the EU is marketed as SPF 30, the UVA protection should be at least 10. A ‘broad spectrum’ sunscreen is a product that protects against both UVA and UVB rays. 

Active ingredients: mineral vs organic

There are two types of active ingredient present in sunscreen: inorganic (or mineral) and organic (containing carbon atoms), also known as physical and chemical filters, respectively. Inorganic ingredients include zinc oxide (ZnO) and titanium dioxide (TiO₂) and physically act as a shield on skin by forming a layer that both blocks and scatters UVA and UVB harmful rays. These products can leave a white residue and often have a thicker texture, which can be undesirable.

Organic actives work by absorbing light energy from the sun and converting it to heat energy. Formulations containing organic actives are often lighter and easier to apply than mineral sunscreens as they can be in aerosol or cream form, leading to popularity with consumers and increased ease of incorporation into cosmetic products such as daily moisturizers. Commonly used organic actives include avobenzone, cinoxate, meradimate and octocrylene, Figure 2.  

Figure 2: Structure of some commonly used organic actives within sunscreen. 

Drawbacks of organic sunscreens

Although organic sunscreens offer superior UVA protection when compared with mineral sunscreens, there are several drawbacks including photodegradation of the active species, skin irritation, potential absorption into the body (either through the skin, ingestion, or inhalation as an aerosol), limited evidence of effects on the endocrine system and mechanical removal throughout the day, as formulations containing organic actives are often thinner and easier to spread. These pathways can lead to lower efficacy of a sunscreen-containing product, discoloration, and have the potential to lead to human health implications. This means that testing of formulations containing active ingredients is essential, and allows for probing of active behavior, both chemically and physically. 

Safety improvements within organic sunscreens

While there are known safety concerns with organic-based UV filters, there are moves within the cosmetics industry to improve their safety profile. Microencapsulation is one such innovation, where an active ingredient is confined within a capsule either permanently or temporarily. In this way, UV filters are no longer in direct contact with skin, reducing the risk of toxicity. Research undertaken by TRI using FT-IR imaging has shown that encapsulation of avobenzone, an organic UVA filter, with octocrylene, a photo-stabilizer, showed no penetration of the organic material into the stratum corneum (SC), whereas a formulation containing “free” avobenzone showed penetration by the active ingredient up to layer six of the SC after only one application. In addition, the durability of the sunscreen was markedly improved when using encapsulation technology upon comparison with the formulation containing the “free” active ingredient, and the photostability of the active compounds was also improved. 

Other innovations include the use of film formers, where a formulation is specifically designed to form a layer or film that covers the surface of the skin, and SPF boosters such as SunSpheres^TM, where a polymer is added that causes scattering of the sun’s radiation therefore reducing the amount of harmful UV radiation reaching the skin’s surface. 

SPF Testing

The Sun Protection factor (SPF) is a measure of a sunscreen’s ability to prevent UV radiation from damaging the skin.  The SPF relates to sun exposure needed to induce skin inflammation (minimal erythemal dose).  A better SPF allows users to sit in the sun longer without getting sunburn (for method details see ISO 24444).  Whilst in vivo SPF tests are routinely performed by clinical testing laboratories, it is also now possible to test sunscreens using in vitro SPF tests (see ISO 23698).   TRI does not normally perform SPF testing.

Sunscreen Testing: How TRI Princeton can support you

Testing of sunscreen active ingredients, in particular the evaluation, comparison and visualization of penetration by organic filters into the skin, is of great importance when developing a new product. At TRI Princeton, there are three main approaches that we usually recommend: 

1. Use of a Franz cell in conjunction with HPLC to monitor the efflux of small organic compounds, or their metabolites, across a membrane;

2. FT-IR imaging to probe lipid disorganization in the stratum corneum (SC), which acts a proxy for the efficacy of a sunscreen), and; 

3. FT-IR in conjunction with confocal Raman spectroscopy to image deposition of materials onto, and ingress into, the skin. 

   
   

The Franz cell is a stalwart for in vitro testing, Figure 3, but when coupled with HPLC becomes an extremely powerful analytical technique allowing trace amounts of material to be monitored. This gives in-depth information relating to the penetration of materials through a membrane, as well as their metabolism and break-down. TRI Princeton offers a range of test protocols, as well as several skin substrates and receptor phases. In all cases our experts will provide advice and guidance on choosing the right test for your products. 

Figure 3: TRI's PermeGear Franz diffusion cells

FT-IR spectroscopy can be used to show how well a sunscreen product can protect the stratum corneum (SC) from UV light through analysis of the SC lipid barrier, with an increase in lipid disorder indicating increased UV exposure, Figure 4a. When FT-IR is used in combination with confocal Raman spectroscopy, a clear picture of surface deposition and retention of a product can be uncovered. In addition, both FT-IR and Raman spectroscopy allow monitoring of sample penetration into the skin through use of characteristic chemical fingerprints of the molecules under study, Figure 4b. Confocal Raman is especially useful, as penetration of a sample can be easily monitored by simply adjusting the spectrometer focal point rather than preparing numerous skin samples. 

Figure 4: (a) The effect of UV light on lipid packing in the SC. Exposure to UV leads to disruption in the lipid organization in the skin. The red colors in the infrared image indicate a shift to “looser” packing of lipids, which increases with UV exposure time. (b) The LUMOS II FT-IR microscope at TRI Princeton. 

Evaluation in vivo and in situ

As well as evaluation of sunscreen performance either in vitro or ex vivo, TRI Princeton also offers the opportunity to evaluate product performance in vivo through the use of 3D facial mapping in combination with FT-IR spectroscopy. This state-of-the-art technique allows quantification of the amount of sunscreen on the skin at a given location, then comparison with amount of sunscreen at a particular location after a specified timeframe or activity, Figure 5. 

Figure 5: The degree of redness denotes the concentration of sunscreen detected on the skin surface by the FT-IR.  The first image is the control, taken before sunscreen application.  The second image shows the sunscreen distribution on the face 15 minutes after application.  Note that the distribution is not uniform, with sunscreen protection being lower on the nose, chin and temples.  The third image shows the sunscreen distribution after 1 hour.  Clearly levels have dropped significantly in some areas.

Summary

In summary, the development of sunscreen formulations and UV filters for everyday products is a burgeoning industry, with significant scope for development and innovation in future. Testing of formulations for skin penetration, durability, chemical degradation of active ingredients and differences in performance between “free” and encapsulated actives is an extremely important part of product development, and a key area where the TRI can support your research endeavors. 

Contact us today for more information.