6.2.1.3　Polyesters

Most aliphatic polyesters have low biological toxicity and good compatibility.Hydrophobic polyesters can be used as carriers in the forms of NPs or microcapsules.The aliphatic polyesters that have been approved for drug carriers by the Food and Drug Administration（FDA）of the United States include poly（lactic acid）（PLA），poly（lactic-coglycolic acid）（PLGA），polycaprolactone（PCL），etc.In the 1970s，Pitt and Schindler put forward that PCL can be used as drug delivery carries.At present，it has been widely used in biomedical fields.

PLA has the features of non-toxic，nonirritant，and no immunogenicity，and therefore is used as nano-drug carriers.However，the hydrophilicity of PLA is poor and its degradation speed is very slow.Oppositely，poly（glycolic acid）（PGA）has high polarity and faster degradation speed，but its crystallinity is high.Both PLA and PGA are limited in clinical application due to their features.PLGA is the copolymer of lactic acid and glycolic acid.The solubility，mechanical properties and degradation speed of PLGA can be manipulated by adjusting the ratio of these two monomers to obtain suitable carrier materials.Yoo et al.prepared PLGA NPs containing an anticancer drug doxorubicin.The drug encapsulation efficiency is as high as 96.6％and the drug loading is 3.45％.Moreover，doxorubicin can be slowly released in 25 days.(https://www.daowen.com)

Besides，the hydrophobic polyesters are often linked with hydrophilic polymers to form amphiphilic block copolymer to prepare the micellar-typed nano-drug carriers.Poly（ethylene glycol）（PEG）is often used as the hydrophilic block，which can increase the solubility and stability of nano-drug carriers.In general，the phagocytosis speed increases with the hydrophobicity.Therefore，enhancing the surface hydrophilicity of the nano-drug carriers could be able to prolong the circulation time of drugs in the body.