Exposure to high levels of UV increases the risk of developing the three main forms of skin cancer [1]. In fact, squamous cell carcinomas and basal cell carcinomas develop almost exclusively on sun-exposed areas of the skin, and estimates suggest that approximately 65–90% of melanomas are also caused by exposure to UV rays. As the occurrence of several types of skin tumors appears to be related to sun exposure, their risk can be reduced by limiting exposure to sunlight, which is the main source of ultraviolet radiation [2, 3]. In this context, the use of topical sunscreens is considered the best preventive measure, especially for parts of the body that are directly exposed to solar radiation. Suscreens applications effectively prevent sunburn and are one of the most common practices to prevent skin cancer [4]. In fact, clinical trials have found that sunscreens effectively reduce the incidence of actinic keratosis, squamous cell carcinoma precursors, and the number of moles (the most important precursors and risk factor for melanoma) [4–6].

To protect humans from sun damage, scientists have developed sunscreen formulations containing different filters [7, 8]. Filters are compounds able to absorb and/or reflect specific wavelengths, such as UVA (320–400 nm), UVB (280–320 nm), and visible radiation, thereby preventing the radiation from reaching and penetrating the skin surface and causing damage. As the UV part of the spectrum is considered to have the most deleterious effect in the skin, currently all sun protection offered by photoprotective products is based either on curbing the generation of skin erythema (mainly due to the UVB band) or the biological effects dependent on the UVA part of the electromagnetic solar spectrum, such as the generation of permanent pigmentation [9, 10]. Nowadays, the high capacity of some filters to absorb UV rays is well demonstrated, and the success and differentiation of the various sunscreens rely on the molecules added to this already established formula. On the other hand, any photoprotector that comes on the market must meet the efficacy and safety requirements dictated by regulatory agencies [10], and the efficacy and safety of most artificial sunscreen components are hindered by their photostability, toxicity, and damage to marine ecosystems [11–13]. Thus, maintaining high efficacy with increasing photostability, along with insignificant toxicity, is an uncovered need of the field and one of the goals of these formulations.

In this scenario, the use of natural compounds in sunscreens is attracting considerable attention. Natural selection and evolution have ensured that plants and animals developed effective protective mechanisms against the harmful effects of UV radiation and the related oxidative stress [14, 15]. Thus, there are substances found in living organisms, for instance, naturally occurring plant products rich in polyphenols, with photoprotective capabilities. Some of those compounds exert their photoprotective activities through their antioxidant effects, controlling skin inflammation, alleviating skin barrier deterioration, and preventing UV-induced aging [14–16]. Hence, the incorporation of natural substances and ethnobotanical antioxidants into sunscreen formulas is on the rise.

Aspalathus linearis (Brum.f) Dahlg. (Fabaceae) (AL), is a fynbos shrub native to the Cape Floristic Region of South Africa [17]. While it is widely consumed globally as an herbal tea, it is increasingly being incorporated into skincare products. Two typical types of AL can be found, known as green and red. The green (unfermented form of AL, ALU hereinafter) has a chemical profile rich in chalcones, including the unique chalcone, aspalathin. The red type (fermented form of AL, ALF hereinafter) results from the oxidative/enzymatic fermentation of green AL form under certain conditions, which alters the plant's chemical profile and reduces the content of dihydrochalcone derivatives, especially aspalathin [18]. Therefore, ALU is preferred for isolating aspalathin and other polyphenolic compounds of interest, which are active ingredients in cosmeceutical products. Extracts from ALU exhibit potent antioxidant, anti-inflammatory, and antitumor properties, with evidence supporting their photoprotective effects on the skin [19–21]. Indeed, different studies demonstrated the protective effects of compounds such as linearthin, aspalathin, and nothofagin from Aspalathus linearis against UVB-induced oxidative stress and toxicity in human skin cells [22, 23]. In a recent study by our group, we explored the photoprotective properties of both ALF and ALU, assessed individually and combined. ALF showed higher photoprotective capabilities than ALU in terms of increasing human dermal fibroblast survival, with synergistic properties observed when both extracts were combined [24]. Therefore, AL extracts appear as attractive candidates for exploring their inclusion in topical and systemic photoprotective strategies. Since their role in photoprotection has been studied using cellular models, no references have been reported on their photoprotection capabilities in terms of radiation absorption properties. Plant-based compounds that possess antioxidant, anticarcinogenic, and anti-inflammatory effects, such as flavonoids or other polyphenols, usually can absorb UVA and UVB rays, which contribute to their photoprotective effect [25, 26]. Apart from flavonoids, other natural products such as certain vegetable oils, carotenoids, stilbenes, and ferulic acid also have UV-absorbing properties [7]. These UV-absorbing properties support their use as UV blockers, and if those are combined with other cellular and antioxidant photoprotective activities, the cosmeceutical potential of the ingredient to be used in sunscreen formulations would be enviable. Therefore, analyzing the optical properties of A. linearis, which is enriched with those potentially absorbing substances, is of considerable scientific and practical interest.

Another potential photoprotective role of active compounds of plant extracts can be related to the stabilization of UV absorbing molecules. The photostability of a sunscreen formulation has a profound influence on its performance, as its effectiveness depends on its stable protection attributes for as long as possible on the skin. This is explicitly important considering that the UV filters can experience degradation when exposed to high doses of solar UV radiation. In Europe, the list of approved UV filters remains limited [27], and classically, most sunscreen formulas are based on combinations of UVB and UVA filters, such as octyl-methoxycinnamate (OMC) and butyl methoxy dibenzoyl methane (BMDBM), respectively. Both filters are still the basis of the vast majority of formulas, as they are very effective and inexpensive, but they have very limited photoinstability [28, 29]. Other UV filters, such as octocrylene, have been added to maintain the formula's photostability, but they have been shown to cause photosensitivity pathologies in many patients [30]. So, the search for safer and more effective new filters is still ongoing, and testing natural substances can be of central interest for their potential photostability role, due their radiation absorption properties as well as their antioxidant capabilities.

Therefore, the present study aims to explore the UV absorption properties of Aspalathus linearis extracts (fermented and unfermented), their synergistic effects when combined with organic sunscreens such as OMC and BMDBM, and their potential to enhance the photostability of these standard filters under high doses of solar simulated UV radiation.

The present study demonstrates that all Aspalatus linearis (AL) extracts tested, including fermented (ALF), non-fermented (ALU), and their combination (ALU/ALF), exhibit enviable optical properties supporting their consideration for the development of future photoprotective formulas.

Initially, a broad UV absorption spectrum increasing gradually from UVA up to UVB, with the highest absorption peak around 285 nm was observed for AL water extracts. This effect of natural extracts has also been reported in our previous work [12, 42], and could be due to the presence of UV-absorbing phytochemicals within the plant tissue [34, 35]. One of the most probable groups of compounds contributing to the observed effect is flavonoids. Flavonoids possess a typical UV-VIS spectra including two bands, corresponding with the absorption of the B ring (band I: 310–385 nm), and the A ring (band II 250–285 nm) [36]. They are demonstrated natural UV filters [15, 41], and some of them have been tested against keratinocyte damage in experimental settings [22]. The water extracts of AL contain high concentrations of different phenolic compounds known to absorb UV radiation, including 3 dihydrochalcones (aspalathin, nothofagin, and linearthin), and other flavonoids such as orientin, isoorientin, quercetin, and luteolin [34]. Given their structure, the presence of the C-glycoside dihydrochalcones could confer a high absorption peak at 286 nm, while the presence of rutin, quercetin, or other flavonols could explain the UVA peak at 353 nm. Interestingly, our data showed that the UV absorbance of ALF was higher than that of ALU. It is well known that during the plant processing (known as fermentation), the dihydrochalcones are rapidly oxidized by nonenzymatic mechanisms to flavanones and polymeric brown products [37] [38, 39] [40], then noticeable browning takes place [18]. Therefore, the appearance of the oxidized forms from the pool of flavonoids and hydrochalcones may change the absorption properties of the extracts, increasing ALF UV absorbance. It is worth noticing that the autooxidation process decreases the antioxidant potential of the extracts, underlaying the lower antioxidant capability ascribed to ALF vs ALU [24, 41]. Thus, considering the important benefits attributed to antioxidant ingredients in sunscreens (ref), plus the recent synergistic biological photoprotective effect shown by a specific combination ALU/ALF (ref), we decided to include the spectral properties analysis of that combination. As expected, we found intermediate absorbance properties, lower than ALF but significantly higher than ALU, which would support the use ALU/ALF combination, to take advantage of maintaining high antioxidant capacities, optimal absorption properties, and the synergistic biological photoprotective effect.

We are in a new era, where we aim natural compound combination providing the sunscreen formula absorption properties as “real biological” filters for UVB and UVA bands of the solar spectrum. Walking towards the practical inclusion of AL extracts in commercial sunscreens, our next step was to assess the performance of the extracts when incorporated into real sunscreen formulas, tested their absorption properties following the international standards outlined in ISO 24443:2022, and calculated SPF, UVAPF, and HIF values. SPF and UVAPF are reference parameters commonly tested in sunscreens, and account for the specific spectrum of action for erythema and permanent pigmentation darkening, respectively. However, Damian et al. 2011, determined 2 peaks of UV radiation (310 and 370nm), key for immunosuppression, which is one of the most relevant factors in the steps previous to skin carcinogenesis [32]. Then, a biological protection factor against human photoimmunosupression can be calculated (named as HIF). Since natural products included in cosmetics (e.g. Polypodium leucotomos extracts) have been reported as immunomodulators [42]; and AL extracts show a broadband absorption covering other biological factors beyond erythema and permanent pigmentation darkening, we decided to include HIF analysis in this study. AL extracts, individually and combined, showed a significant absorbance increment from UVA to UVB, leading to SPF values close to 2 at concentrations of 2.5%, reinforcing their properties as radiation filters. Few works in the literature have reported the absorption properties of phenolic compounds for photoprotection purposes. For instance, quercetin and especially rutin offer high UV absorption potential, reaching SPFs above 35 [49], and total polyphenols extracted from some plant leaves can achieve SPF values above 20 [50]. The traditional herbal formulation, Ubtan, based on different plant seeds (rich in flavonoids), can reach SPF values above 30 [51]. Although those works showed higher SPF values, it is worth mentioning they used methanol and other organic extractions, which yield much higher concentrations. Conversely, the water extractions used in this work are considerably less toxic, affordable, and environmentally sustainable, which represents a strong advantage for their industrial implementation. Additionally, those works did not include the extracts in galenic formulations or test the regulated standards for real sunscreens. Therefore, the results presented here demonstrate not just the enviable broadband absorbing properties of aqueous extracts of AL, but also its performance in complex formulations analyzed under regulated standards, which strongly support and facilitate its practical inclusion as natural filters in current sunscreens.

Interestingly, the most surprising results were obtained when AL extracts were combined with other filters (OMC and BMDBM). The addition of 2.5% ALF was able to increase the SPF of the reference filters from 15.43 to 27.24, an increase of 76.52%. Similarly, the UVAPF increased 50% and the HIPF increased 56%. ALU extract showed lower values than ALF, yet conferred a very important increase in all biological protection factors, and the combination ALU/ALF showed intermediate photoprotection factors closer to ALF (increment of 45,57% SPF, 35% UVAPF, and 37.5% HIPF). Given the potential adverse effects of certain organic filters on human health and the environment, the incorporation of natural products into cosmetic formulations is a growing trend, aiming to reduce reliance on high concentrations of synthetic UV filters [52]. In this scenario, this work demonstrates a booster effect on commonly used synthetic UV-filters when AL is included in the formula. This would allow reducing the amount of filters used, maintaining the efficacy of the sunscreen while improving safety and sustainability.

Finally, the additional role of the AL extracts as photostabilizers was demonstrated. It is well known that the combination of BMDBM and OMC, with very extended use as UVB and UVA filters in commercial sunscreens, is photo-unstable. This process is initiated by BMDBM, which after absorbing UVA radiation, reacts with OMC to form cycloaddition products [53]. To palliate this effect, other molecules are included in sunscreen formulations not just as UV filters, but as stabilizers. This is the case of bisethylhexyloxyphenol methoxyphenyltriazine (tinosorb S) [54], methylbenzylidene camphor [55], diethylhexyl 2,6-naphtalate [56], or octocrylene (ref). However, in the current scenario where synthetic filters are under scrutiny, octocrylene, due to its toxic potential in humans and animals, is currently being phased out by industry, justifying the need for alternatives [30]. As the light-induced degradation mechanism of BMDBM and OMC proceeds through the initial formation of free radicals and singlet oxygen [57, 58], scavenging these reactive species by antioxidants has been proposed as an approach for photostabilization [59, 60]. In this context, triplet–triplet energy transfer is an important photostabilization mechanism for organic UV filters [54], and the stabilizing potential of flavones and flavonoids, including quercetin, has been described [61] [62]. In this study, both AL extracts, especially ALF and the combination ALU/ALF, were able to significantly prevent the absorbance decrease of the reference filters under UV radiation. This importantly reinforces their double role, not just as filtering ingredients but as alternative photostabilizers.

In conclusion, the evidence presented herein highlights the promising potential of natural extracts as components for advanced solar photoprotection. Up to now, none of the commercially available sunscreen molecules presents ideal behavior as a photoprotection ingredient, which implies very high absorption properties in the UV region, high photostability, and negligible toxicity or phototoxicity. AL extracts combination demonstrated in the present work that they can play this role, acting as biological filters and photostabilizers, allowing the development of future topical photoprotective agents based on natural sources, effective, and safer for human health and the environment, which in current times could potentially enhance user adherence to the recommended photoprotection protocol.