Fabrication And Biomedical Applications Of Novel Dynamic Hydrogels Based On Knoevenagel Condensation Reaction

Hydrogels formed by dynamic chemical bonds(termed as dynamic hydrogels)are injectable,self-healing and have the ability to simulate the natural dynamic extracellular matrix(ECM),which have been widely employed in the biomedical fields including cell therapy,drug delivery,wound dressings,tissue engineering,etc.The Knoevenagel condensation(KC)is a classic reaction between aldehydes(or ketones)and compounds containing active methylene groups,resulting in the formation of C=C double bonds.The KC reaction has been widely used for the synthesis of various chemicals and polymers.In addition,the KC reaction can proceed smoothly in aqueous solution,which thus makes it attractive for being used in construction of hydrogels.However,it is unclear whether the C=C double bond formed by KC reaction is a dynamic covalent bond or not.Therefore,in this dissertation,we demonstrated,for the first time,that the C=C double bond formed by KC reaction is a new dynamic covalent bond with unprecedent temperature-responsiveness.Following that,several dynamic hydrogels were fabricated based on the Knoevenagel condensation(KC)reaction and their properties,such as mechanical properties,injectability,and self-healing ability and applications in biomedical fields were systemically studied.The main contents and conclusions of the dissertation are listed as follows:(1)We firstly demonstrated that the C=C double bond formed by KC reaction is a typical dynamic covalent bond with unprecedent temperature-responsiveness,which could be used for construction of thermally reversible dynamic hydrogels.The obtained hydrogel exhibited excellent self-healing,injectable,thermally responsive and thermoplastic properties.In this study,methoxy poly(ethylene glycol)s modified with benzaldehyde and cyanoacetate,denoted as mPEG-BA and mPEG-CA,respectively,were prepared and used as the reactive model compounds and the results confirmed that the C=C double bond formed by KC reaction was a temperature-sensitive dynamic bond.Next,4-arm poly(ethylene glycol)(4-arm PEG)end-capped with benzaldehyde and cyanoacetate,denoted as 4-arm PEG-BA and 4-arm PEG-CA,respectively,were prepared and used for preparation of dynamic hydrogel.In vitro experiments suggested that the hydrogel was biocompatible and was potential to be used as a scaffold for 3D cell culture.(2)To enable the application of dynamic hydrogels in long-term cell differentiation and proliferation,we further design and prepare a double network hydrogel with superior injectable and long-term retention properties.The obtained double network hydrogel showed superior injectability and could effectively protect the cells from mechanical injuries during injection.Moreover,the photo-cross-linked network in the double network hydrogel could enable the retention of hydrogel in injuried site and provie stable scaffold for long-term cell differentiation and proliferation.Firstly,a biocompatible catalyst histidine was applied to accelerate the dynamic exchange of C=C double bond formed by KC reaction,which thus remarkabley improve the injectability of the KC reaction-crosslinked hydrogel(comprised of benzaldehyde and cyanoacetate-functionalized dextran denoted as Dex-BA and Dex-CA,respectively).Next,the acrylated 4-arm PEG(4-arm PEG-A)was incorporated into the hydrogel system and formed a secondary reinforced network under UV irradiation.In general,this injectable double network hydrogel may has great potential for application in cell transplanation.(3)Finally,a self-healing hydrogel with fast on-demand dissolution property was designed and prepared based on tandem dynamic covalent bonds comprised of C=C double bond formed by KC reaction and phenylborate ester bond formed by boronic acid-diol complexation.2-Formylphenylboronic acid(2-FPBA)was used as the cross-linker to react with cyanoacetate end-functionalized 4-arm PEG(4-arm PEG-CA)and poly(vinyl alcohol)(PVA),leading to formation of dynamic hydrogel(<10 s)with excellent injectability and self-healing ability.In addition,the tandem dynamic covalent bonds could be specifically degradaded by cysteine through the formation of thiazolidino boronate compound.As expected,the hydrogel showed fast erosion in the presence of cysteine owing to the formation of thiazolidino boronate.Thereafter,this kind of hydrogel was used as wound dressings for treating full-thickness wounds.In this case,the hydrogel could simultaneously achieve rapid wound closure in seconds and on-demand fast dissolution for dressing changes.Therefore,using 2-FPBA as a crosslinker,we have provided a new strategy to prepare dynamic hydrogel with fast on-demand dissolution property,which may shed a new light on the design and preparation of novel would dressings and further expand the applications of the dynamic hydrogels based on KC reaction.