Preparations And Properties Of Polyethylene Glycol Hydrogels By Controlled Azide-Alkyne Cycloaddition

Hydrogel has received considerable attention as a new kind of soft and wet material in recent years due to its widespread applications in various fields such as sensing,drug delivery,tissue engineering,soft robot,etc.In comparison with other“hard”polymeric materials,hydrogels can respond more easily to external stimuli in different ways.However,they usually do not have enough mechanical strength due to their intrinsic structural inhomogeneity or lacking effective energy dissipation mechanism,which largely limit their application in many fields.Exploring ultrahigh strength hydrogels behaving like natural load-bearing soft tissues like tendon,cartilage,muscle,and blood vessels have many application potentials.During the past decades,many efforts have been devoted in this regard by introducing effective energy dissipation mechanism or designing distinctive structure to fabricate tough hydrogels.However,at present,most polymer networks are irregular,and there are a lot of stress defects in polymer networks,resulting in poor mechanical properties of polymer networks,which limit their application in biomaterials,especially in biomimetic devices.Therefore,in this paper,the controlled"click-chemistry"method was used to prepare the perfect and well-defined hydrogels,and the energy was dissipated during the tensile process by introducing recoverable physical crosslinking points.?1?For most Copper-Catalyzed Azide-Alkyne Cycloaddition?CuAAC?reactions,the influence of the diffusion can be ignorable because they were carried out under stirring.However,for the preparation of hydrogel,stirring can not be carried out because the catalysts of CuAAC reaction have high reactivity and are easily oxidized.Once the catalyst was added,the gelation point could be reached in few seconds,thus the uneven distribution of the hydrogel in the reaction system and incomplete click reaction appeared due to the poor diffusion,which led to the largely presence of the molecularly defects and macro-defects in the prepared hydrogel.Thus,the traditional CuAAC reaction is not available to the preparation of hydrogel devices with large size and specific 3-dimensional structures.In this project,we develop a controlled CuAAC reaction,in which,the organometallic were produced in a dormant state.The dormant state catalyst was easy to dissolve and had no catalytic activity,which could be dissolved in the reaction system uniformly.Under a given condition,the activity of the catalyst is waken up to finish the CuAAC.The controlled CuAAC can eliminate the effects of diffusion completely and make the reaction process be controlled.Controlled CuAAC allows the preparation of high performance hydrogel devices with large size and desired 3-D structure with more regular molecular structure and less defects in molecular structure.The hydrogels prepared by controlled CuAAC have better processability,which is of great significance to promote the industrialization of high performance hydrogel devices.?2?The chain extension of azido-pendent polyurethane-polyethylene glycol?PU-PEG?were synthesized using hydroxyl and diisocyanate and then reacted with?,?-dialkynyl PEG for preparing PU-PEG hydrogel by“click-chemistry”.In PU-PEG hydrogel,due to the presence of polyurethane?PU?segments which is hydrophobic and easy formation of hydrogen bonds.Under the action of external force,the introduction of hydrogen bonds can effectively disperse energy,thus improving the mechanical properties of PU-PEG hydrogel.PU-PEG hydrogels have unique hydrophilic,hydrophobic and temperature-sensitive properties.Meanwhile,the cytotoxicity test of PU-PEG hydrogels has been carried out,and the results showed that the hydrogels had good biocompatibility.?3?Azido-pendent polyethylene glycol?PEG?was synthesized by ring opening reaction of amino-epoxide,and then reacted with linear PEG with alkyl functional groups at the end of the chain and n-alkyl propargyl ether under controlled"click-chemistry".PEG hydrogel with hydrophobic chain molecular brush was synthesized.These hydrophobic chains gather together in water to form physical crosslinking.Under the action of external force,the hydrophobic crosslinking point breaks.When the external force is removed,the hydrophobic crosslinking point is back to its original state.We call this process a mechanical fuse connection,which effectively dissipates energy,and the mechanical properties of PEG hydrogel are improved.PEG hydrogel also had unique hydrophilic,hydrophobic and thermo-sensitive properties at a certain temperature.Meanwhile,the cytotoxicity test of the mechanically fused PEG hydrogel was carried out.The results showed that the hydrogels had good biocompatibility.?4?Alkynyl-pendent PEG was synthesized by ring opening reaction of amino-epoxide,then reacted with the linear PEG with azido and adamantane functional groups and6-N₃-?-cyclodextrins?CD-N₃?to synthesize double crosslinked PEG hydrogel under controlled"click-chemistry"conditions.In the double crosslinked hydrogel,the physical crosslinking point of self-assembly of cyclodextrin?CD?and adamantine?AD?will be dissociated by external force.When the external force is removed,the physical crosslinking point of self-assembly will return to its original state,and the introduction of the physical crosslinking point of self-assembly can effectively dissipate energy.At the same time,the cytotoxicity test of double crosslinked PEG hydrogels was carried out.The results showed that the hydrogels had good biocompatibility.The"controlled click"reaction enables the preparation of well-defined,perfectly structured hydrogels,which enrich the preparation of polymer networks,and we use controlled click to prepare PU-PEG hydrogel using hydrogen bond to dissipate energy,mechanical fuse PEG hydrogel using hydrophobic interaction to disperse energy and double cross-linked network PEG hydrogel using self-assembly to disperse energy.Because the three kinds of hydrogels were prepared from polyethylene glycol,a biocompatible material,the hydrogels synthesized have good biocompatibility and can be used in biomaterials.The synthesis of these polymers provides an important synthetic approach for the applications of polymer networks in bionic devices.