Synthesis, Characterization And Properties Of Biodegradable Chitosan-Oligo(Lactic Acid) Graft Copolymers

Chitosan (CS) is a kind of natural macromolecules with excellent biocompatibility, biodegradability and bioactivity, and can be obtained from a wide range of natural sources. However, the intermoleculer hydrogen-bonding interactions contribute to its poor solubility in water and in many organic solvents, thus constraining its potential applications. Modification of CS by branding is an effective approach to improve its solubility. Poly(lactic acid) (PLA) presents outstanding mechanical strength, biocompatibility, and biodegradability, and has been attracting growing attention for various applications. Nevertheless, the high crystallinity and hydrophobicity of PLA greatly constrains its applications as drug-release carriers or tissue engineering scaffolds. Low molecular weight chitosan exhibits excellent hydrophilicity. Chitosan-Oligo(lactic acid) Graft Copolymers (CS-OLA), prepared by attaching OLA to CS backbone, will combine the mechanical strength of PLA with the hydrophilicity of CS. Therefore, CS-OLA presents great potential as controlled drug release carriers and tissue engineering scaffolds, which is of major importance for both fundamental research and pratical applications.Two novel reaction routes were proposed to prepare CS-OLA, using N,N'-carbonyldiimidazole (CDI) or N,N'-Dicyclohexylcarbodiimide (DCC) as the coupling agent to attach OLA to the amino and/or hydroxy groups of CS. The reaction conditions of CS-OLA synthesis were investigated in detail. CS-based physical hydrogel systems were prepared through stereocomplexation between ODLA and OLLA segments of CS-OLA. The stereocomplexed structure and drug release behavior of hydrogels were evaluated.A series of monodisperse hydroxyl-capped OLA (OLA-OH) were first synthesized using low toxic zinc lactate as initiator. OLA-OH was then activated by CDI to yield OLA-CI active intermediate. OLA-CI was finally coupled to the CS backbone to yield CS-OLA, usins DMAP as catalyst. In the meantime, carboxyl-capped OLA (OLA-COOH) was synthesized under similar conditions. Following the activation by DCC, OLA-COOH is readily attached to C2-NH2 and C6-OH of CS with DMAP as catalyst, thus yielding CS-OLA. Enhanced reactivity of C2-NH2 with respect to C6-OH was observed from 1H-NMR and FT-IR analyses. The degree of substitution of the CS-OLA was close to the molar ratio of [CSunit] to [OLA] in the feed, and could be adjusted by varying the feed ratio. The resulting CS-OLA graft copolymers present variable solubility, facilitating preparation of nano-particlcs or hydrogels. Comparison of the results obtained from CDI and DCC routes shows that OLA was attached to C2-NH2 on CS, especially when CS was in large excess, in agreement with the higher reactivity of C2-NH2 as compared to C6-OH of CS. Furthermore, the degree of substitution of CS-OLA obtained by DCC approach is higher than that by CDI approach. Altogether, DCC approach is preferable for the synthesis of CS-OLA with shorter OLA segments, while CDI approach is preferable with longer OLA segments.The cytocompatibility of CS-OLA was determined via MTT assay. All the copolymers present excellent cytocompatibility, especially well water-soluble 10#-CS5K-OLA20C and 6#-CS5K-OLA20D. The latters were then selected to prepare CS-OLA stereocomplex hydrogels. The swelling ratio, DSC, WAXD, SEM measurements were employed to determine the hydrogel structure. Data showed that the formation of hydrogels results from stereocomplexation between ODLA and OLLA segments of CS-OLA. A three-dimensional network structure is formed with CS as framework and OLLA/ODLA stereo-complex as physical crosslinks. With increasing degree of substitution, the gel structure becomes denser and the swelling ratio decreases.The drug release behavior of the hydrogels was investigated using thymopentin (TP-5) as model drug. A more compact hydrogel structure leads to lower drug release rate and total released ratio. The drug load also affects the release behavior:hydrogels with higher load exhibit an increased release rate at the later stage with respect to those with lower drug load. CS-OLA physical hydrogel is injectable and can be formed at the injection site in situ, which is very promising in the field of controlled drug release.