6.2.2.3　Polymer micelles/vesicles

Polymer micelles are spherical aggregates formed via self-assembly of amphiphilic block copolymers with hydrophobic chain as core and hydrophilic chain as corona.The hydrophobic chains commonly used include PLA，PLGA，PCL and poly（amino acids），etc.，and PEG is used as the hydrophilic block.The size of polymer micelles is generally between 10-100 nm and it is usually monodispersed.Drugs are supposed to be loaded in the hydrophobic core of micelles via physical interaction while hydrophilic shell can maintain the stability of the micelles and prolong the circulation time in vivo as well.In addition，drug molecules also can be bonded onto hydrophobic chains of block copolymer through chemical interaction，followed by self-assembling into micelles.The size of the hydrophobic core and its interaction with drugs have important effects on drug loading.Currently，most polymer nano-drug carriers in clinical researches are polymer micelles.

Polymer vesicles are aggregates with capsule shape and bilayer structure formed via the self-assembly of amphiphilic block copolymers.Unlike the spherical micelles，vesicles are filled with water.The block ratio of hydrophobic chains to hydrophilic chains of block copolymer used for the preparation of micelles is smaller than that for vesicles.The size of vesicles is usually larger than that of micelles but the uniformity of the former is no better than that of the latter.Similar with polymer nano-capsules，vesicles are also a kind of nanoscale liquid capsule，which not only are able to load traditional small-molecule drugs but also can be used to deliver NPs and biological macromolecules such as protein and nucleic acid.Unlike nano-capsules，both the internal and external walls of vesicles are hydrophilic polymer chains.Vesicles are suitable for loading hydrophilic drugs.But owing to the restriction of the preparation methods，the encapsulation efficiecy of drugs is very low.Unlike the spherical micelles/NPs，the drugs loaded in the cavity of vesicles/nano-capsules not only can release slowly via penetrating their wall，but also are able to release instantaneously with the breaking up of the wall to achieve local high concentration of drugs.It allows to regulate and control the speed of drug release according to different requirements.(https://www.daowen.com)