Photostable: COSSMA

For the sun protection factor in the finished product to work reliably, a dependable formulation is essential. But not all active ingredients are photostable. Dr Shaher Duchi and Dr Danny Goldstein show how microencapsulation technology improves the performance of avobenzone.

Sunburn and premature skin ageing are caused by continued exposure to sunlight, which is the main source of ultraviolet (UV) radiation. UVB (280 – 320nm) typically induces erythema and direct DNA damage, whilst UVA (320 – 400nm) is associated with tanning, photo-ageing, and the genera-tion of excess reactive oxygen species, resulting in skin ageing and cancer^1-3.
To protect from sunlight, organic UV filters must absorb UV energy. The absorption of a UV energy results in a degradation of the UV filter and formation of new com-pounds that don’t have protective capabilities. The preferred sunscreen product should provide a broad-spectrum protection against both UVB and UVA while maintaining sensorial features that enhance the user’s experience^4-6. Avobenzone (butylmethoxydibenzo-ylmethane) is a very efficient UV filter available and is often used as protection against UVA⁷.

Formulation challenges

However, formuating with avobenzone is still a challenge. One of the major challenges is to photostabilise avobenzone since it undergoes rapid photodegradation when used alone. Therefore, it is often stabilised with the additional UV filter octocrylene⁸.

Yet due to health and safety concerns about the impact of using octocrylene, there is an urgent need to find an alternative and effective way to stabilise avobenzone⁹.

Stabilised by microencapsulation

We have overcome this major challenge – along with others such as recrystallisation, discoloration, solubilisation, and the risk of skin penetration – by developing a microen-capsulated form of avobenzone. Avobenzone is trapped in an unbreakable microcapsule which is made of a transparent polymeric shell that protects and enhances its photostability.

Avobenzone loaded microcapsules were prepared by the solvent extraction method^10,11. Microcapsules are prepared by first dissolving the cellulose acetate polymer and avobenzone in an organic phase.

The organic phase is then mixed with a water phase containing of polyvinyl alcohol as emulsifier to form an emulsion. The resultant emulsion is transferred into the extraction water phase, resulting in sedimentation of the microcapsules, which is collected afterwards via filtration and washed with water to remove any impurities. The microcapsules are freeze dried to obtain a free-flowing powder.

figure 1: Morphology of microencapsulated avobenzone as observed by SEM. Microcapsules are intact and spherical in shape.

Characterisation

Avobenzone content is quantified by Jasco HPLC system. Microcapsules are analysed for their size distribution using a laser diffraction particle size analyser and their morphology was analysed.

To test the efficacy, the encapsulated avobenzone was incorporated into an o/w formulation at a concentration of 5% (equivalent to 3% free avobenzone) following in vitro SPF measurement.

Tests on photostability

Chemical: Formulations (o/w emulsion) containing either 5% encapsulated avobenzone or 3% free avobenzone, were applied to a microscope glass slide at 1 mg/cm². Slides were exposed to direct sun light for up to seven hours. At specific time intervals of two, four, and seven hours post exposure, the slides were washed and analysed for avobenzone content using HPLC method.

Functional: Formulations (same as above) were applied upon PMMA plates at 1.3m/cm². Plates were air dried in the dark for one hour and then scanned at nine distinct locations on the plate to allow rescanning of the same locations. All samples were irradiated using a 1000W solar simulator to approximate natural sunlight. 20J of UV is equivalent to about 1 hour of noon day sun. Exposures were in 20J increments. Average total absorbance bet-ween 360 and 400nm at the nine locations of the plate was calculated. Initial (time 0) averages were used to calculate percentage absorbance remaining for all subsequent scans.

Physical: To evaluate the physical strength, microcapsules were dispersed in water and homogenised for 30 minutes. The size and morphology of the microcapsules were analysed before and after homogenisation.

Size distribution of encapsulated avobenzone

figure 2: Representative graph for the mean size and size distribution of microencapsulated avobenzone.

Results

The microencapsulation process of avobenzone resulted in light yellow colour powder with an average avoben-zone content (%) of 58.2± 0.4 and mean entrapment efficiency (%) of 97.0± 0.6.

Microcapsules were visualised using SEM confirming its integrity and spherical shape as shown in figure 1.

The mean particle size value of the avobenzone microcapsules was 8.02±0.67 µm (figure 2).

Functionality: The measured in vitro SPF value of the tested o/w formulation containing 5% microencapsulated avobenzone was 2.89 representing UV filter efficiency (SPF index, %) of about 3. This value is equivalent to that of the non-encapsulated free avobenzone. Notably, the UVA-PF was 4.97 which is the same expected value as when using free avobenzone at the same concentration level.

Chemical photostability: Photostability results (figure 3) indicates that the mean relative content of avobenzone, following exposure to direct sunlight, remained significantly higher in its encapsulated form compared to free avobenzone, thus enhancing the photostability.After seven hours of direct sun light exposure, 43% of the initial avobenzone was detected in the microcapsules, compared to only 12% in its free form.

Functional photostability: Photostability test results (figure 4) indicate that the functionality of avobenzone as UVA absorber, following exposure to UV irradiation, remained significantly higher in its encapsulated form compared to free avobenzone, showing that microencapsulation has a significant impact on preventing activity loss of avobenzone.

After 100J irradiation, which is equivalent to five hours of sun exposure at 12 noon, the encapsulated avobenzone preserved about 40% of the functionality compared to 5% in its free form. Thus, the microcapsules successfully protect avobenzone from degradation following UV exposure and preserve its ability to absorb UVA.

Physical stability: Avobenzone microcapsules show very high resistance to physical stress and preserve integrity after high shear mixing. The microcapsulesmaintain their spherical shape and size following homogenisation. No broken microcapsules and no change in particle size were obseved before and after homogenisation.

figure 3: Avobenzone content microencapsulated compared to free avobenzone (AvoCap2) incorporated in o/w formulation after sunlight exposure for one, two, four and seven hours. Data is presented as mean % of initial.

Conclusion

Avobenzone is a very efficient and the only globally approved UVA filter, aimed to protect human skin from sun exposure damage such as skin-ageing and UVA-induced skin cancer. However, product developers are still facing formulation challenges when trying to incorporate it into innovative skincare products. Its photostability, along with its low solubility and recrystallisation issues are major obstacles.

Encapsulating avobenzone in an unbreakable and transparent microcapsule is an innovative solution, which can protect and hermetically isolate it from its surroundings, thereby reducing its degradation during UV exposure.

In addition, the avobenzone microcapsules in a powder form can be easily incorporated into any type of formulation at any stage during product preparation. This can be achieved without the need for using solvents (solubilisers) to dissolve the avobenzone and with no risk of recrystallisation. Therefore, innovative sun care formulations, which are safer, stable, and effective, can easily be developed using microencapsulated avobenzone.

figure 4: UVA absorbance of microencapsulated avobenzone (AvoCap2) compared to free avobenzone incorporated in o/w formulation following UV irradiation. Data is presented as mean % of initial absorbance.

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