Synthesis Of Multiple-responsive PNIPAAm Hydrogels By Enzyme-mediated Crosslinking Method

Stimuli-Responsive hydrogels which have the capability to respond to external stimulis have attracted much attention.Poly?N-isopropylacrylamide??PNIPAAm?hydrogel is a most popular thermal-responsive hydrogel since it undergoes a dramatic phase transition at its Lower Critical Solution Temperature?LCST?.However,the applications of PNIPAAm hydrogels were limited due to its single thermal responsiveness and weak mechanical properties.Multiple-responsive hydrogels are more desirable for the application in drug delivery system.Enzyme-mediated hydrogelation has received extensive attention,because it can be conducted under mild conditions that compatible with drugs and living cells.Horseradish peroxidase?HRP?enzyme-catalytic system had been proposed as an effective method for preparing hydrogel.Recently,graphene oxide?GO?has been demonstrated to exhibit extraordinary photothermal conversion efficiency and excellent biocompatibility.It is an attractive precursor for the production of nanocomposites materials.Hence,the nanocomposite?NC?hydrogel by incorporating photothermal conversion material?GO?into the PNIPAAAm hydrogel exhibits the near-infrared?NIR?-responsive properties,which can be remotely and non-invasively activation by NIR.In this paper,near-infrared photothermal-responsive Graphene oxide/Polymer nanocomposite?NC?hydrogel based on poly?NIPAAm-co-AAc-g-Tyr?and GO was prepared by enzyme-catalyzed cross linking method through HRP/H₂O₂ system.A temperature and redox-sensitive disulfide-cross-linked hydrogel was prepared through HRP/Tyr enzymatic gelation of thiol-functionalized poly?NIPAAm-co-SH?.The main results of the work are as follows:In the first part,we preapred a new photothermal-responsive polymer/GO nanocomposite?NC?hydrogel by enzyme-catalyzed cross-linking method using HRP/H₂O₂ enzymatic system.The tyramine-modified copolymerpoly?NIPAAm-co-AAc-g-Tyr?and GO was used as the gel precursor and nano filler respectively.The NC hydrogel exhibited strong temperature and pH sensitivity simultaneously owing to the NIPAAm and AAc units,and GO endowed the NC hydrogel with NIR responsiveness.Results showed that the NC hydrogel can be quickly formed under physiological conditions.The gelation rate could be tuned by varying the concentrations of gel precursor,HRP and H₂O₂.The rheological experiments confirmed that mechanical property of the NC hydrogel was enhanced by the introduction of GO.The thermal,pH and ionic strength sensitivity of the NC hydrogel were confirmed by swelling experiments.SEM analysis revealed a highly homogeneous 3D porous structure.More importantly,the NC hydrogel exhibited excellent NIR-responsive property as we had originally conceived.Topotecan?TPT?was loaded into NC hydrogel efficiently by in situ gelation,and pH/NIR triggered controlled release of TPT was obtained in the NC hydrogel.Therefore,the injectable and NIR/pH-responsive NC hydrogel might be an efficient drug carrier for pH and NIR controlled release of drug in clinical treatment.In the second part,we describe a HRP-mediated preparation of temperature and redox-sensitive disulfide-cross-linked hydrogel through the gelation of thiomers with the addition of HRP and Tyrosamine.Thiol-functionalized poly?NIPAAm-co-SH?was used as a hydrophilic and cytocompatible gel precursor.The explorations on the formation conditions of disulfide bonds indicate that the formation rate of disulfide bonds can be accelerated by increasing of temperature and pH value.Hydrogel formation was confirmed in situ during the network evolution by rheological analysis.The SEM images showed that the hydrogel has a multi-layer microporous network with the average pore size is between 3⁵ um.The redox sensitivity was demonstrated by degradation of the hydrogel in the presence of DTT,the disulfides can readily cleave to free thiols.The results of RB release showed that the release profile can be controlled by temperature and the concentration of DTT.The release rate can be accelerated by increasing temperature and DTT.