The Design,Mechanical Properties And Applications Of Metal Coordination Crosslinked Hydrogel

The mechanical properties of hydrogel play an important role in its applications.For example,when hydrogels serve as wound healing materials,they provide not only strong adhesion to organic tissues,but also high mechanical strength and toughness to bear mechanical loads.Recently,many efforts have been devoted to designing hydrogels with tunable static mechanical properties,including toughness and mechanical strength.However,the dynamic mechanical response of hydrogels is also vital.A hydrogel that is tough and strong at one strain rate may be brittle and weak at another.To obtain more functional hydrogels,the design of dynamic mechanical properties has attracted more and more attention.Here,we investigated how the physical environment,chemical environment and structural conformations of ligands affect the dynamic mechanical response of metal-coordination hydrogels.These studies provide a general method to tune the dynamic mechanical properties of hydrogels.Specifically,the thesis includes:(1)Tuning the dynamics of hydrogels by the physical/mechanical environments of ligands.The type of metal ions and ligands could cause great effects on the dynamic mechanical response of metal coordination crosslinked hydrogels.However,the dynamics of hydrogel cannot be merely attributed to the intrinsic dynamics of the crosslinkers.The hydrogel network considerably contributes to the dynamic properties of hydrogel.By simply altering the molecular weights and concentrations of PEG polymers,the hydrogel dynamics can be shifted by several folds.Then,we established a model of fibrous network hydrogel based on the elasticity theory of polymers,and analyzed our experimental results at the micro level.The crosslinking complexes experience stretching forces due to the swelling of the hydrogel network.Both molecular weight and concentration of PEG polymer can change the stretching forces acting on crosslinking complexes.This model offers a novel route to engineer complex time-dependent mechanical properties of soft materials.(2)Tuning the dynamics of metal hydrogels using the chemical environment of ligands.Besides the physical environments,chemical modification of ligands could also cause considerable effects on the dynamic mechanical response of hydrogels.We showed that the halogen substituent groups of dopa could decrease the association/dissociation dynamic of crosslinkers,and extend the characteristic relaxation time of the hydrogels.As a result,the dynamic mechanical stability of hydrogels would be enhanced.Such observations could be explained by the change of protonation constant due to the change of the chemical environment,which would reduce the protonation of ligands and further influence the dynamic mechanical response of hydrogels.(3)An injectable hydrogel based on tandem metal-ligands complexes.The cooperative binding motif is a fundamental structure in functional metalloproteins.This is the key factor for metal ion transportation.Inspired by the special conformation,we designed a hydrogel with high mechanical strength and tunable dynamic mechanical properties.We showed that the dynamic mechanical response of hydrogel was correlated with the dynamics of metal-ligand bonds and this was not affected by the formation of cooperative binding motif.However,the mechanical strength of hydrogel was significantly improved due to the increased thermodynamic stability of crosslinkers.This method could tune mechanical strength and dynamic mechanical property of hydrogels independently.(4)Bio-mimic hydrogel with gradient stress-relaxation behaviors based on coordination interactions.To mimic natural biomaterials,the gradient of mechanics and composition was introduced in hydrogel engineering.But tuning the gradient of dynamic mechanical properties,such as stress relaxation properties,is still a fundamental challenge.Based on the investigation of the dynamic mechanical properties of hydrogels and the design theory of hybrid network hydrogel,we successfully engineered a series of hydrogels with stress relaxation gradient.By simply controlling the density of coordinate crosslinkers and covalent crosslinkers,the gradient of stress-relaxation was achieved.Meanwhile,the Young's modulus and mechanical strength of the hydrogel remained uniform.Then,fibroblast culture experiments further confirmed the biological application value of these hydrogels.The mechanical strength and dynamic mechanical properties of hydrogels are influenced by their polymer network structures and crosslinkers.However,they often change simultaneously in the engineering process and it is difficult to control one of them independently.How to adjust them separately is one of huge difficulties encountered in the design of functional hydrogels.By controlling the physical environment,chemical environment and conformational structure of ligands,we offered several convenient approaches to independently regulate the mechanical strength,dynamic properties and stress relaxation behavior of metal-coordination hydrogels.Meanwhile,based on the polymer physics,the mechanism was explained by a theoretical model.These results provided several new methods for the design of new functional hydrogels with complex structures.