chitosan and plga microspheres as drug delivery ... - UniCA Eprints

2. Chitosan And PLGALactic AcidGlicolic AcidHOCOCHCH 3O CmOH 2C O HnFigure 2.3: PLGA Chemical StructureThey can degrade by non-enzymatic hydrolysis of the ester backbone in body fluid. Thedegradation products (i.e. lactic and glycolic acids) are metabolic compounds (227). Mostimportantly, PLA and PLGA have been approved by the United States Food and DrugAdministration (FDA) for drug delivery.The biodegradation of the PLGA occurs through random hydrolitic chain scissions of theswollen polymer. The cleavage of ester bond linkages yields carboxylic end groups andhydroxyl groups. The formed carboxylic groups then could catalyze and accelerate thehydrolysis of other ester bonds, a phenomenon referred as autocatalysis. The polymer erosionin delivery devices is the degradation of polymers to water-soluble fragments, accompaniedby a progressive weight loss of the matrix. Generally, the polymer erosion could be classifiedinto two mechanisms, namely surface or bulk erosion (228). In the case of surface erosion, thedegradation is faster than the water diffusion. Thus the degradation and erosion take place onthe surface of the matrix; in contrast, with bulk erosion, the water penetration is faster and thedegradation and erosion affect all the polymer bulk. PLGA are bulk erosion polymers. Theweight loss of the polymer devices doesn’t take place at the beginning of the degradation ofthe PLGA. Accompanying with the produced water-soluble oligomers, significant weight lossoccurs when the molecular weight of the PLGA reaches certain threshold (229). Theheterogeneous degradation of the large size PLGA devices has been reported recently (230). Itwas found that after subcutaneously implantation, the molecular weight of the outer phase ofthe polymer plate was higher than that of the inner phase. The outer phase was solid but theinner phase was sometimes semisolid (230). During the degradation of the polyester, theformed soluble acidic oligomers inside the matrix may not easily diffuse out, which may leadto a more acidic microenvironment inside the matrix. Therefore the autocatalysis is moreprominent in the bulk than at the surface, which leads to the surface-interior differentiation.The physical and chemical characteristics of PLGA such as molecular weight, glass transitiontemperature, and copolymer ratios are crucial to the biodegradation behavior of the polymers.41