Yang Yunyun,Liu Tingting

JALA Group Co.,China

Abstract The recent progress in the applications of Pickering emulsion technology in sunscreens has been reviewed,including the Pickering emulsions stabilized by TiO2 particles,Pickering emulsions stabilized by TiO2 and ZnO particles,and composite sunscreen microspheres prepared via Pickering emulsion polymerization.The perspective in future development of this technology in sunscreens has also been given.

Key words pickering;sunscreen;emulsion polymerization

Stabilized by solid particles,pickering emulsion is a new type of dispersion emulsion.[1]With the significantly lowered dosage of emulsifiers in Pickering emulsion,it shows notable advantages in cost saving,irritation reducing and environmental conservation while comparing with traditional emulsions.Since the emerging of pickering technology in the early 20th century,its wide application in the fields of food,cosmetics,medicine and material[2-6]has been witnessed with frequent reports on syntheses[7-10]and stability studies.[11-13]At the same time,the kinds of particles which are used to stabilize the interface are increasing,including clay,titanium dioxide,zinc oxide,starch,chitosan,iron oxide,silica,etc.[14]

In recent years,the applications of pickering emulsion technology in sunscreens have drawn much attention of researchers.Stabilized by surfactants,with oil-soluble organic sun-screening agents as the main sunblock actives,currently marketed sunscreens have shortcomings such as irritation,greasiness,difficult to rinse,etc.[15]The application of pickering emulsion technology in sunscreens can,on the one hand,dramatically reduce the dosage of emulsifies to improve mildness,and on the other hand,achieve efficient sunblock effect by using the composite microspheres synthesized via pickering emulsion polymerization.

1 The application of pickering emulsion stabilized by TiO2 in sunscreens

Featuring low toxicity[16,17]and skin penetration,[18]and broad spectrum UV protection,TiO2can effectively block sunlight damage with its absorption,reflection and scattering of UV.With strong UV blocking efficacy and environmental friendly attribute,Pickering emulsions stabilized by TiO2is a new potential sunscreen system.

Bai Ruixue etc[15]has synthesized a alkylmodified TiO2nanoparticles with moderate hydrophobicity via a reaction of hydroxyl-modified TiO2nanoparticles with dodecyltrichlorosilane in hexane.UV absorption spectrum study indicated that,compared with Pickering sunscreen emulsion with non-modified TiO2nanoparticles(1%)and traditional sunscreen with chemical sun-screening agent(1% benzophenone-3),pickering sunscreen emulsion with alkyl-modified TiO2nanoparticles(1%)showed higher absorption of UV;Applying and rinsing tests on pigskin suggested that compared with traditional chemical sunscreen,this Pickering sunscreen emulsion was easier to be rinsed off.The better dispersion of alkyl-modified TiO2nanoparticles in the system,and more comprehensive UV blocking ability by absorption,reflection and scattering of TiO2nanoparticles might contribute to stronger UV shielding capability and easier rinsing of this system,indicating that Pickering sunscreen emulsion with alkyl-modified TiO2nanoparticles is very efficient.By demulsification to release water-soluble medicine in the inner phase,this Pickering emulsion showed a 12 h controlled medicament release rate of 96.6%,thus proved it a prospective active release system.

Yang Yunyun etc[19]have synthesized Pickering emulsions with various dosages of TiO2of 3 different contact angles.Stability tests of these emulsions showed that because of better wettability and dispersion on interface,TiO2particles with moderate contact angle stabilized emulsion best.Within the dosage of 1～20%,the higher the dosage of TiO2,the better the emulsion stability.However,at high temperature,water loss and conglomeration of inner phase had been witnessed,which might be caused by the poor consistence of interface film formed by rigid TiO2particles.System stability has been improved with the addition of low dosage of co-emulsifies and rheology adjusters and best stability has been achieved when both co-emulsifies and rheology adjusters were added in water phase,oil phase and interface.After the addition of these agents,viscosity has also been lowered.

These researches indicated that functioned both as a physical sun-screening agent and emulsion stabilizer,TiO2particles has increased SPF value and lowered the emulsifier dosage in the emulsion.The Pickering sunscreen emulsions stabilized by TiO2had advantages such as strong UV-blocking efficacy,easy-to-rinse attribute,and long-lasting release of actives.Via formula optimization,such as the addition of low dosage of co-emulsifies and rheology adjusters,system stability has been improved and viscosity has been lowered,which indicated a prospect of this system.

2 The application of pickering emulsion stabilized by TiO2 and ZnO in sunscreens

TiO2is strong at blocking UVB(290～320 nm),while a broad spectrum sunscreen should block not only UVA(320～400 nm)but also UVB effectively,thus both UVA and UVB sun-screening agents are always combined in a formula to achieve better UV protection.In Pickering emulsion,TiO2and ZnO particles can also been combined to stabilize the system,and at the same time,to get good UV sheltering effect.

Marto etc[20]have synthesized two w/o pickering sunscreen emulsions via reactions of TiO2(surface treated with triethoxyoctylsilane),ZnO,aluminum starch octenylsuccinate,ethanol and water with two coffee oils respectively.In vivo and in vitro tests showed that both of them had high SPF values with UVA/B protection,and presented promising water resistance performance.SPF-290S test indicated that SPF values of both emulsions were above 50,with UVA/UVA equaling 0.9±0.1.HRIPT results suggested that both emulsions were safe and mild for topical use.

In the following study,a w/o Pickering sunscreen emulsion was synthesized via the reaction of TiO2(surface treated with triethoxyoctylsilane),ZnO,green coffee oil,ethanol and water.[21]Also,as a safe and mild topical use sunscreen,this emulsion had a high SPF with UVA/B protection,and presented promising water resistance performance.SPF-290S test result of it was SPF 43.4±6.2,with UVA/UVA equaling 0.9±0.1.Besides,it showed no significant difference of appearance and particle size after three months aging tests at room temperature and 40 ℃ with micro test passing of all aging samples.Both the fresh sample and aged sample have passed microbiological tests.With the addition of 5% aluminum starch octenylsuccinate to optimize formula,tests showed that the UV protection efficacy of this system had been reinforced,with SPF of 82.3±10.3 and UVA/UVA equaling 0.9±0.1(SPF-290S result),and strong water resistance efficacy both in vitro and in vivo.These test results indicated that the addition of aluminum starch octenylsuccinate had increased SPF,lowered viscosity and improved spreadability of system.Interactions between aluminum octenylsuccinate group on starch and triethoxyoctylsilane coating on TiO2surface might contribute to these performance enhancements.The silane coating decreased the alumina aggregation,thus decreased the shear stress required to break the flocculation.

With good compatibility of actives,this Pickering sunscreen system can stabilize 1%melatonin.[22]As an effective anti-oxidant and radical scavenger,melatonin can strengthen UV protection efficacy of sunscreen products.Compared with melatonin solution of the same concentration,a Pickering system stabilized by TiO2(surface treated with triethoxyoctylsilane)and ZnO can prevent the degradation of melatonin,and furthermore,promote its skin penetration,showing a potential of this system in biomedical field.

These researches indicated that TiO2and ZnO can stabilize Pickering emulsion.And the addition of surface-treated particles,such as aluminum starch octenylsuccinate has improved spreadability and SPF of the system,making the formula more consumerfriendly.Achieving broad spectrum UV protection while maintaining product aesthetics,this system has represented a green trend.Besides,it showed good compatibility of actives,such as melatonin,whose skin penetration has been enhanced and degradation has been reduced in Pickering emulsion stabilized by TiO2and ZnO,proving it a versatile system in various fields.

3 Composite sunscreen microspheres prepared via Pickering emulsion polymerization

While protecting skin from UV damage,chemical sun-blocking agents in traditional sunscreens might cause adverse reactions,such as irritation,because of the direct contact of skin.Up to now,there have been some researches on the application of polymerized microspheres embedding chemical sun-blocking agents to improve safety and aesthetics of sunscreen products.[23-25]Loading and encapsulating sun-blocking agents with biocompatible materials,Pickering emulsion polymerization,featuring effective interface self-assembly,has reinforced UV protection of sunscreens,and at the same time,improved aesthetics of products for consumers.

Yu Mingying etc[26]have synthesized a kind of organosilane crosslinked microspheres loaded with TiO2nano-particles on surface via emulsiontemplated polymerization,which showed better UVblocking efficacy than the same level of a mixture of organosilane microspheres and TiO2,as UV absorption spectrum indicated.Making sunscreens containing the organosilane crosslinked microspheres loaded with TiO2nano-particles and the mixture of same level of organosilane microspheres and TiO2respectively,and testing their SPF values with UV-2000 also showed that the former sample had higher SPF.Furthermore,in vivo test suggested that the microspheres had soft focal effect,and improved excessive whitening of TiO2and rinse-off easiness.

A kind of PMMA microspheres loaded with TiO2on the surface and embedded with avobenzone(AVB)inside has also has been synthesized.[27]Making sunscreen emulsions with this PMMA microspheres and the mixture of same level of TiO2and AVB respectively,following tests showed that the former sample had higher SPF value,better photo stability,no excessive whitening on the skin,and easy-to-rinse attribute.Special microsphere structure brought by Pickering emulsion polymerization might contribute to these improvements,as this structure could block or scatter UV light,and better stabilized AVB by preventing its contact with TiO2.Besides,via fixing TiO2particles on the surface of microspheres,this structure has also reduced excessive whitening on the skin and made it easier to rinse.With much lowered transdermal absorption,skin penetration test also suggested that AVB embedding was quite firm.

These researches suggested that synthesized via pickering emulsion polymerization,composite microspheres loaded with sun-screening agents has such strengths as higher UV protection efficiency,better photo stability,less excessive whitening,easy to rinse,and lower transdermal absorption.Taking the versatility of Pickering emulsion polymerization technology into consideration,this approach is quite promising in performance enhancing of sun-screening agents.

The global advances in the applications of Pickering emulsion technology in sunscreens in recent years has showed that currently,extensive explorations of applications of pickering emulsion technology in sunscreens has been witnessed,as a host of promising and practical systems have been developed via various agents’ combinations and formulation techniques by researchers.On top of that,compared with traditional sun-blocking agents,Sun-screening microspheres synthesized by pickering emulsion polymerization have exhibited many advantages.Although some limitations in application,e.g.limit on particles selection,not-so-diversified system designs,heavy texture of formulas,etc,we are optimistic with the prospect of this technology as the addition of some ingredients may improve performances.Representing a green trend,this technology will be further developed in cosmetic field.