9.6.3.4　Polymer nanoparticles（PNs）

PNs are colloidal carrier systems composed of a polymer core with a diameter of submicron level.According to their structural organization，composition and distribution of the active compound，they can be classified into nanocapsules（NCs）and nanospheres（NSs）.In the former one，the active compounds are entrapped inside，whereas in the latter，they are adsorbed on the surface.The polymer lipid walls of NCs are ideal for sustained release of lipophilic compounds and protect them from photodegradation phenomena.The properties of PNs depend on the polymer composition，organic solvent and surfactant dosage，which can affect the particle size，surface properties and release rate of UV active.

The main advantages of PNs are that they can provide controlled/sustained release of the UV absorbents，hence prolonging their residence time on the skin，improving photostability of the sunscreen agents and preventing skin absorption.(https://www.daowen.com)