APALIGHT: Biomimetic SPF mineral booster for sustainable sunscreen

Some organic filters, when affected by radiation, could potentially produce dangerous chemical compounds known to be allergenic and to
cross the skin barrier due to their molecular weight and lipophilic character; additional studies prove their photo-instability that may cause damage induced by ROS formation. Moreover, their potential estrogenic effects are very controversial. Therefore, organic filters are expected to induce more toxicity than inorganics.

Besides safety concerns, some organic UV filters contribute to contamination of the environment, specifically through their bio-accumulation in seawater with repercussions for aquatic ecosystems. The various methods of removing UV filters from the water are poorly effective and the best ones are very expensive. Mineral filters report low environment risk being mineral substances, they are preferred for their sea eco-system aspect.

In order to find an appropriate compromise between safety and functional parameters, the industry has addressed growing attention to sunscreen containing only inorganic mineral filters or based on a hybrid use of inorganic and organic filters with the target of reducing the overall amount of filters content in the formulation and take advantage of their synergistic UV screening features to increase SPF.

This article explains how a SPF mineral booster based on biomimetic hydroxyapatite (Apalight, by Kalichem; INCI name:hydroxyapatite) used in sun care formulation, acting with a mechanism comparable to inorganic filters like zinc oxide and titanium dioxide, offers proven benefits in terms of multifunctionality, texture improvement, safety and low environmental impact.

Hybrid sunscreen formulations containing blends of mineral and organic filters were taken into evaluation to assess the contribution of biomimetic HAP to increase organic filters in vivo SPF and compare it to the SPF increase induced by titanium dioxide. In vitro UV-A protection factor was also assessed. In detail, two formulations were analyzed, with the composition details and final results summarised in Table 2.

Formula 1

The organic filters blend SPF value reported corresponds to 30. By adding to such system 4% of titanium dioxide or 4% of biomimetic HAP,
the final SPF recorded of the formulations was 34 after the addition of titanium dioxide and 40 after the addition of HAP. The UV-A/UV-B ratio (data not shown in table) was kept in both cases, with values of 0.5 for titanium dioxide-based formula and 0.35 for HAP-based formulation.

Formula 2

The organic filters blend SPF value reported corresponds to 50. By adding to such system 5% of titanium dioxide or 5% of biomimetic HAP,
the final SPF recorded of the formulations was 64 after the addition of titanium dioxide and 65 after the addition of HAP. The UV-A/UV-B ratio (data not shown in table) was kept in both cases, with values of 0.38 for titanium dioxide-based formula and 0.33 for the HAP-based formulation.

The results suggest therefore that the +1:1 replacement of titanium dioxide with HAP to organic filters based formulations is expected to keep comparable SPF increase values, while optimizing texture, reducing the formulation whitening and maintaining the UVA/UVB ratio.

Biomimetic HAP differentiates from other minerals frequently used in sun care for its in vivo solubility, triggered by the skin acidic pH and its peculiar external porous surface able to create many anchor points with external substrates. In such a context, HAP time- controlled and partial ions release is suggested to promote several skin benefits, especially following long-term application, given by increased signalling of key ions like calcium and phosphates.

The treatment with biomimetic HAP highlighted an increase of the skin elasticity (R2) parameter by 10.3% (Figure 6), and a decrease of visco-elasticity by 17.1%, (value R6). The visco-elasticity is a key parameter to assess skin stretchability, whose value is inversely proportional to skin firmness increase (reduction of skin stretchability corresponds to higher skin keratinocytes compactness, hence increased firmness). Elasticity and firmness are key aspects to consider in sun care products, keeping in mind that prolonged sun exposure dries out the skin.

Anti-wrinkle

In order to evaluate the anti-wrinkle action, skin replicas were used: in detail a fast-hardening synthetic polymer (SILFLO by Flexico, UK) and adhesive disc were chosen with the aim of having an identical imprinting on the skin. To analyze the results, according to the Corcuff method, some images were taken through a processing software (Quantilines, Monaderm, Monaco).

The silicone replica of the skin surface was illuminated by a light source with an incidental angle of 35°. The images (covering 12×9 mm of each skin replica) were acquired through a video camera (high performance CCD camera, COHU, Monaco). This instrument enables to analyze the following parameters: Ra (roughness value), Rz (maximum roughness value, namely deep wrinkles).

The anti-wrinkle efficacy is given by a decrease of Ra and/or Rz values. The treatment with biomimetic HAP (Figure 6) showed statistically significant deep wrinkle reduction (Rz) compared to the placebo treatment by around 10%.

Sunscreens market has experienced a consistent increase of interest toward the use of mineral filters and boosters as well as ingredients with multifunctional features, high naturalness, sustainably sourced and with low environmental impact.

Apalight biomimetic HAP is a specialty ingredient based on a mineral comparable to that occurring in the human body and in the sea ecosystem. This specific type of mineral is traditionally used in medical applications (as coating to promote bone ingrowth into prosthetic implants, and as a rejuvenating solution in aesthetic medicine etc.). It represents an interesting innovation for sun care, as it enables the formulation of fully natural mineral sunscreens with optimized sensorial features and hybrid sunscreens in combination with organic filters to achieve high SPF protection values.

HAP mineral does not give any applicative whitening effect and shows a light texture that drastically differentiates it from micron sized inorganic filters, enabling to either reduce their amount in mineral sunscreens or totally replace them in hybrid sunscreens. Furthermore, its biomimetic nature, its peculiar external surface and pH dependent oligo-elements release triggered by skin acidic values promote multiple benefits, including elasticity enhance, maintenance of skin barrier integrity and anti-wrinkle features.

In the wake of the studies previously shown, biomimetic HAP represents therefore a promising candidate for the development of heterogeneous multifunctional sun care products in the name of environmental sustainability.