Preparation And Structure-activity Relationship Of High-strength Alginate-based Hydrogels

Alginate hydrogel is a kind of soft material with a three-dimensional network,surrounding with a large amount of water.Due to the similar structure with human tissue,good biocompatibility,water absorption and water retention ability,it has been widely used in artificial skin,tissue engineering,environmental engineering and other fields.However,due to the solution-like nature of hydrogels（low polymer chain density,small friction between polymer chains,etc.）and the inhomogeneity of polymer network,the mechanical properties of hydrogels are usually poor,which seriously limits their applications.Therefore,simultaneously achieving high strength and toughness for hydrogels,especially for physically cross-linked hydrogels,has always been the focus of researches in the field of high-performance hydrogels.Based on the theory from current researches about highly strong and tough hydrogels,this paper attempts to construct high strength and toughness alginate hydrogels by optimizing their cross-linking network,revealing the relationship between their microstructures and mechanical properties.A series of researches have been listed as follows:1.A hybrid metal ion crosslinked hydrogel with high strength and toughness was successfully prepared by constructing a 2D planar decorated 3D network（2D-3D）crosslinking structural unit in the ionic crosslinked alginate hydrogel.In this strategy,a novel hybrid metal ion crosslinking agent(trivalent/divalent metal cations,such as Fe³⁺/Ca²⁺)was innovatively proposed.When the molar percent of Fe³⁺in the hybrid crosslinking agent was ultralow(X_Fe,c=1%),the multivalent cations had competitive complexion in the hydrogel.In this way,the structural units of loosely two-dimensional planar network and the rigidly three-dimensional network could be combined in a tightest way,and the most stable cross-linked structure endowed the excellent mechanical properties to alginate-based hydrogels.The results showed that for the alginate/polyacrylamide system,the complementary structure obtained by the hybrid metal ion crosslinking method made the mechanical properties of hydrogels better than any single metal ion crosslinked hydrogels,realizing the synergistic mechanics.In addition,by changing the molar percent of each metal ion in the crosslinking agent,the binding ability of each metal ions to the alginate chain could be accurately regulated,where the content of metal ions with different valence in the hydrogel could be effectively controlled,making the varied mechanical parameters in a wide range available.In addition,the hybrid metal ion crosslinking agent can be used for the design of alginate-based materials with complex three-dimensional shapes.This research provides a new path for the structural design and performance optimization of metal ion crosslinked polymers.2.The water content of calcium alginate hydrogel was controlled by vacuum and high temperature assisted water loss method.The relationship between microstructures and mechanical properties induced by water content was systematically studied in calcium alginate hydrogels.The results showed that with the decrease of water content,the polymer chains of alginate hydrogel gradually approached,and the density of polymer chain per unit volume increased.In addition,when the spacing between adjacent polymer chains was reduced,the previously invalid hydrogen bonding sites in the hydrogel were gradually transformed into active sites,and the hydrogen bonding interaction was greatly enhanced,leading to the state transformation of the mechanical parameters.The results showed that when the water content of hydrogel was in the range of 62%to 93%,alginate hydrogel had lower Young’s modulus and fracture strength,but higher ductility.However,when the water content was in the range of 19%to 48%,the hydrogels had high Young’s modulus and fracture strength,but low fracture strain.In other words,when the water content was between 48%and 62%,the hydrogel had a"soft to rigid"transition.Within this range,the hydrogel could achieve both mechanical strength and ductility,whose water content was close to that of human tissue.Therefore,by controlling the drying time of vacuum treatment,the degree of water loss for hydrogels could be effectively controlled,so as to effectively regulate the mechanical properties of calcium alginate hydrogels.The above results revealed the relationship between water content,structure and properties of calcium alginate hydrogels,which also provided theoretical guidance for the preparation of other hydrogels,such as agar and carrageenan hydrogels.3.In this research,an innovative self-reinforcing strategy was proposed to prepare high strength alginate composite hydrogels.The mechanical properties of calcium alginate hydrogels were improved by using different anisotropic alginate materials（calcium alginate fiber（unidirectional orientation structure）,fabric（bidirectional orientation structure）and aerogel（three-dimensional orientation structure））as reinforcement phases.The reinforcing phase was compounded with alginate hydrogel by in situ polymerization.Triggered by the concentration difference of metal ions between the enhancement phase and the precursor solution,the metal ions-Ca²⁺in reinforcing phase could diffuse into the interface between the reinforcement phase and the precursor solution,and simultaneously chelated with alginate chains of both reinforcing phase and hydrogel matrix during the recombination process,thus significantly improving the interfacial bond between the two phases.In the crosslinking process,metal ions acted as interface adhesives.Therefore,under the action of solid interface,the stress could be effectively transferred to the anisotropic reinforcement phase,and the hydrogels matrix could connect the dispersing reinforcement phase into a whole network.The obtained alginate self-reinforcing composite hydrogels showed excellent mechanical properties.