Injectable Hydrogels Synthesized By Copper-Free Click Chemistry

Hydrogels have a highly water-swellon three-dimensional network structure, which is similar to biological tissues. Hydrogels are widely applied in drug controlled release, sensing technology and tissue engineering. They are used as tissue filling agent, drug carrier for slow release, carrier of embedded enzyme, artificial skin, artificial plasma and tissue engineering scaffold materials. Click reactions have advantages of mild reaction conditions, insensitive to oxygen and water, good stereoselectivity, high yield, good stability of product, and little by-products. Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is the most widely used click reaction. Its application in the biomedical field, however, is limited by the potential toxicity of the copper catalysts. Increasing attention is attracted on click reactions without using metal catalysts which are named copper-free click reactions.This thesis synthsized a series of water-soluble macromolecules functionalized by cyclooctyne and azide groups. They formed hyrogels via Strain-Promoted Alkyne Azide Cycloaddition (SPAAC) under mild conditions. The hydrogels can form without metal catalysts after injection, which may be widely used in the biomedical field due to low toxicity.In Chapter 1, we reviewed the research progress of hydrogels and click chemistry. We proposed a research direction that hydrogels can form efficiently in aqueous solution by cyclooctyne-funcationalized and azide-funcationalized water-soluble macromolecules such as polyethylene glycol (PEG) and chitosan without need of metal catalysts. This kind of injectable hydrogels might be used as biomaterials such as embolic agents.In Chapter 2, we designed and synthsized a series of dicyclooctyne-functionalized and diazide-functionalized PEGs, in which the alkyne and azide groups are linked to the PEG backbone by ether or ester linkages. The products were characterized by FT-IR, 1H NMR,13C NMR, ESI-MS, MTT assay and Dynamic Light Scattering (DLS). The difunctional PEGs can form hydrogels under physiological conditions without catalysts and present low toxicity in vitro. The gelation time, however, was not as short as expected, so we proposed to functionalized PEG with more functional groups to shorten the gelation time in the next chapter.In Chapter 3, we designed, synthsized and characterized multi-functional PEGs. Ring opening polymerization (ROP) of cyclocarbonate and epoxy monomers bearing azide and cyclooctynyl groups, respectively, were initiated with PEG to functionalize PEG with multiple azide and cyclooctyne groups. We also prepared a multi-hydroxyl derivatived PEG and further attached azide and cyclooctyne groups through the hydroxyl groups. As expected, the multifunctional PEG derivatives formed hydrogels rapidly. The hydrogels containing ester linkages can degrade under physiological conditions.In Chapter 4, chitosan was modified by azide and cyclooctyne groups in order to study the hydrogels formed by functional natural polymers via SPAAC. The modified chitosans at low concentration kept a good liquidity and formed hydrogels upon mixing, which may extend applications of natural polymers in a wide area.In Chapter 5, multi-cyclooctyne and multi-azide functionalized PEGs were chosen to form injectable hydorgel for use as embolic agents due to its shortest gelation time and biodegradability. The cytotoxicity of the multifunctional PEGs precursors was evaluated on the basis of MTT assay on L929 cells. The gelation time determined by dynamic rheological measurements matched the time measured by tilt test tube method. In situ formation and in vivo biocompatibility of the hydrogel were investigated on mouse. The properties of the hydrogel as embolic agents were investigated by injection of the hydrogel emblolization in rabbit ear vessels. Injection of a mixture of the gel precursors into the auricular central artery of rabbits blocked the fast-flow vessel rapidly without any invasive operation, and the vessel flow restored spontaneously by gel degradation in two days. This kind of injectable hydrogels would be useful as biomaterials such as embolic agents and stanching materials.