Click-type Cross-linking Strategy To Prepare Hydrogels For Drug Delivery And Tissue Adhesive

Due to its similar 3D network structure to native extracellular matrix,hydrogels have been widely investigated in 3D cell culture,tissue engineering,drug delivery system and tissue adhesive.The performance of hydrogels is directly associated with the cross-linking strategy.Covalently cross-linked hydrogels,formed by chemical bonds,typically exhibit superior stability,tunable mechanical properties and degradation profiles over a wide range of magnitudes.Various types of reactions have been exploited,including radical polymerization,enzyme-catalyzed reaction,Schiff base reaction,etc.In few decades,the rapid development of click chemistry provides diverse cross-linking strategy for the formation of biomedical hydrogels,such as azide-alkyne cycloadditions,Diels-Alder reactions,hydrazone and oxime reactions.The chosen gelation reactions can have a profound impact on the gelation time,mechanical strength,degradation profiles and biocompatibility of the hydrogels for the targeted applications.For instance,tough hydrogels could be prepared by radical polymerization,due to its high degree of cross-linking density;toxic catalysis and harsh conditions,such as high temperature and ultraviolet radiation,could not be involved for in situ formed hydrogels;bior;thogonal click reaction is preferred for the 3D culture of stem cells,as other reactions can cross-react with functional groups present in cell-surface proteins.In this dissertation,we have focused on the cross-linking strategies for the construction of biomedical hydrogels.Highly efficient cross-linking reactions have been developed and exploited to prepare hydrogels with tunable mechanical properties,degradation profiles and excellent biocompatibility.Potential applications in drug delivery system,tissue adhesive and cell culture have been explored.The main contents and conclusions are summarized as follows:(1)Injectable hydrogels were prepared by inverse electron demand Diels-Alder reactions between norbornene modified poly(L-glutamic acid)and tetrazine end-capped four-arm poly(ethylene glycol).Hydrogels formed within few minutes by mixing the two polymer solutions,and could be delivered by syringe.The gelation kinetics and storage modulus of the hydrogels could be readily tuned by changing the polymer concentration and the molar ratio of reactive groups.Due to the peptide linkages,the hydrogels could be rapidly degraded by elastase.The cisplatin was loaded in the hydrogels through the formation of polymer-metal complexation,and released in a sustained manner by ligand exchange.The cisplatin-loaded hydrogels could effectively suppress the proliferation of MCF-7 tumor cells through cell cycle arrest in vitro.In the MCF-7 subcutaneous tumor model,the cisplatin-loaded hydrogels exhibited inhibitory effect on the tumor growth,with reduced systemic toxicity.Moreover,the in vivo degradation profiles could be accelerated by introducing disulfide bonds between the cross-links and polymer backbone.(2)A fast and versatile cross-linking strategy based on the condensation of o-phthalaldehyde(OPA)and N-nucleophiles was developed.Based on this strategy,hydrogels were formed with excellent gelation kinetics,mechanical properties and bioadhesive ability.OPA end-capped four-arm poly(ethylene glycol)(4P-OPA)was prepared,and hydrogels were formed by mixing 4P-OPA with N-nucleophile end-capped four-arm poly(ethylene glycol).The hydrogels demonstrated much lower gelation concentration,superior gelation rate and mechanical strength,compared with the existing benzaldehyde-based systems.The small molecule model reactions reveal that the key to the cross-linking is the fast formation of phthalimidine or isoindole(bis)hemiaminal intermediates.The rate constant for the reaction between OPA and primary amine is 4.3 M-1 s-1,which is comparable with many click reactions.By using the 4P-OPA as a universal building block,various hydrogels could be easily prepared from hydroxyethyl chitosan,adipic dihydrazide modified hyaluronic acid,bovine serum albumin,gelatin and poly(L-lysine).The gelation rate and storage modulus could be enhanced by increasing the polymer concentration,and the obtained hydrogels could rapidly degrade in the corresponding enzyme.Based on the highly efficient and traceless coupling reaction between OPA and amine,the cell adhesion peptide could be facilely conjugated into the hydrogel network,and the modified hydrogels could well support the adhesion,spreading and proliferation of fibroblasts on the surface.The hydrogels exhibited excellent bioadhesive properties by virtue of linkage formation between OPA and amine in tissue proteins.In the rat skin incision wound model,the hydrogels could effectively close the wound,and promote the wound healing process as compared with commercially available fibrin glue and tert-butyl cyanoacrylate.(3)A superfast cross-linking strategy based on the reaction of 2-formyl phenylboronic acid(FPBA)and 2-acetyl phenylboronic acid(APBA)with?-nucleophiles was developed for instant gelation.In model reactions,FPBA and APBA were screened from aryl aldehydes and aryl ketones with different substituent group.The rate constant for the reaction of FPBA and APBA with hydrazide is 109 M-1 s-1 and 62.4 M-1 s-1,repectively,as the boronic acid in the ortho position greatly accelerate the condensation reaction.The FPBA and APBA end-capped four-arm poly(ethylene glycol)were synthesized,and hydrogels could be formed within seconds even at low polymer concentration by mixing them with ?-nucleophile end-capped four-arm poly(ethylene glycol).The obtained hydrogel exhibited excellent biocompatibility and could degrade in phosphate buffer,cell culture medium,as well as in vivo.Overall,the instant gelation hydrogels hold great potential for rapid wound sealing.