Understanding Sunscreen: A Comprehensive Guide
Updated: Aug 24
The Importance of Sunscreen in Daily Life
Dr Samuel Gourion-Arsiquaud, Director at TRI
Dr Philippa Cranwell, Technical Content Creator at TRI
Image from https://skinpluspharmacy.com.au
The Skin and Bio-substrates team at TRI has worked since 2018 to understand how organic UV filters penetrate the skin. The team has 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 the effects of skin temperature on sunscreen penetration. In this bite-sized 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.
In the words of Australian Baz Luhrmann, circa 1999, "if I could offer you only one tip for the future, sunscreen would be it." This maxim is not just lyrics to a chart-topping song; it is a good guideline to follow. Wearing sunscreen can help prevent skin damage caused by UV rays, which can lead to premature aging and skin cancer.
Why Sunscreen is Essential
Until recently, many believed that sunscreen was only necessary in strong sunlight. Most people thought this only applied while on holiday. However, research now shows that sun protection is advisable even in winter. Consumer demand has led to everyday beauty products, such as moisturizer, foundation, and lipstick, including UV-blocking materials.
When developing sunscreen-containing products for daily use, quantifying and substantiating claims is crucial. This is an area where 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.
The Science Behind Sunscreen
But first: why do we even need sunscreen?
It is well-known that exposure to sunlight, specifically the ultraviolet (UV) component of sunlight, can accelerate the signs of aging and lead to skin cancer. This is particularly important 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.
UVA light has a longer wavelength and less energy than UVB. It can penetrate deep into the skin and interact with DNA, causing mutations that may lead to skin cancers. UVA is also associated with skin aging.
UVB light has a shorter wavelength and higher energy than UVA. It only penetrates the outermost layer of skin, the epidermis. UVB rays are linked with skin burning and play a role in forming skin cancers, such as malignant melanoma.
UVC rays are stopped by the ozone layer and pose no risk to human health.
Application of sunscreen is a straightforward way to reduce risk from the sun’s UV rays. It provides protection against harmful UVA and UVB radiation. Importantly, when formulating a sunscreen, it must protect against skin damage associated with sun exposure while also being safe and providing a pleasant sensory experience for customers upon application.
Regulation: The Great EU and US Divide
Within the EU, sunscreen is classified as a cosmetic item. In contrast, in the US, it is classified as a non-prescription drug. This distinction may seem semantic, but it presents challenges in developing 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, similar to any other drug developed by a pharmaceutical company. In contrast, the EU's designation as a cosmetic item means that active ingredients must 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 for a pharmaceutical. Currently, within the EU, there are 34 different UV filters approved for use, while in the US, there are only 16.
Additionally, there are differences in the protection offered between EU and US sunscreens, mainly due to the active ingredients used. In the US, UVA protection provided by sunscreen products is often significantly lower than that offered by EU products, especially with non-mineral sunscreens. This is primarily due to the smaller range of active ingredients available that specifically block UVA rays. The EU recommends that manufacturers offer UVA protection that is one-third of the overall SPF. For instance, if a product in the EU is marketed as SPF 30, the UVA protection should be at least 10. A ‘broad spectrum’ sunscreen protects against both UVA and UVB rays.
Active Ingredients: Mineral vs. Organic
There are two types of active ingredients 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). They physically act as a shield on the skin by forming a layer that blocks and scatters harmful UVA and UVB 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. 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.
Drawbacks of Organic Sunscreens
Although organic sunscreens offer superior UVA protection compared to mineral sunscreens, there are several drawbacks. These include 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. 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 potential human health implications. Thus, testing of formulations containing active ingredients is essential. It 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, the cosmetics industry is working to improve their safety profile. Microencapsulation is one such innovation. Here, an active ingredient is confined within a capsule, either permanently or temporarily. This way, UV filters are no longer in direct contact with the 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). In contrast, a formulation containing “free” avobenzone showed penetration by the active ingredient up to layer six of the SC after just one application. Additionally, the durability of the sunscreen improved significantly when using encapsulation technology compared to the formulation containing the “free” active ingredient. The photostability of the active compounds also improved.
Other innovations include the use of film formers. These formulations are specifically designed to form a layer or film that covers the skin's surface. SPF boosters, such as SunSpheresTM, are polymers added to scatter the sun’s radiation, thereby reducing the amount of harmful UV radiation reaching the skin’s surface.
SPF Testing
The Sun Protection Factor (SPF) measures a sunscreen’s ability to prevent UV radiation from damaging the skin. The SPF relates to the sun exposure needed to induce skin inflammation (minimal erythemal dose). A higher SPF allows users to stay in the sun longer without getting sunburned (for method details, see ISO 24444). While in vivo SPF tests are routinely performed by clinical testing laboratories, it is now also possible to test sunscreens using in vitro SPF tests (see ISO 23698). TRI does not typically perform SPF testing.
Sunscreen Testing: How TRI Princeton Can Support You
Testing of sunscreen active ingredients, particularly the evaluation, comparison, and visualization of penetration by organic filters into the skin, is crucial when developing a new product. At TRI Princeton, we usually recommend three main approaches:
Use of a Franz cell in conjunction with HPLC to monitor the efflux of small organic compounds or their metabolites across a membrane.
FT-IR imaging to probe lipid disorganization in the stratum corneum (SC), which acts as a proxy for the efficacy of a sunscreen.
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, but when coupled with HPLC, it becomes an extremely powerful analytical technique. This combination allows for monitoring trace amounts of material, providing in-depth information about penetration through a membrane, as well as metabolism and breakdown. TRI Princeton offers a range of test protocols, along with 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 show how well a sunscreen product can protect the stratum corneum (SC) from UV light. This is done through analysis of the SC lipid barrier. An increase in lipid disorder indicates increased UV exposure. 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. Additionally, both FT-IR and Raman spectroscopy allow monitoring of sample penetration into the skin through the use of characteristic chemical fingerprints of the molecules under study. 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
In addition to evaluating sunscreen performance either in vitro or ex vivo, TRI Princeton also offers the opportunity to evaluate product performance in vivo. This is achieved through the use of 3D facial mapping combined with FT-IR spectroscopy. This state-of-the-art technique allows quantification of the amount of sunscreen on the skin at a given location. It also allows comparison with the amount of sunscreen at a specific location after a specified timeframe or activity.
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 one hour. 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. There is significant scope for development and innovation in the future. Testing 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. This is a key area where TRI can support your research endeavors.
