Designing Of Tough Hydrogels Based On Formation Of Carboxyl-Fe³⁺ Coordination Bonds And Their Fracture Behavior And Processability

Hydrogel is a type of soft and wet materials composed of hydrophilic polymer networks and water.Due to its tissue-like properties and remarkable biocompatibility,hydrogel has promising applications in drug delivery,soft actuators,stretchable devices,etc.However,conventional hydrogels are usually considered as a type of weak and brittle material due to the heterogeneous networks,and only used in the scenarios where the mechanical properties are not highly concerned.In the past two decades,researchers have developed a series of tough hydrogels by designing the structure of network and introducing effective energy dissipation mechanisms.However,in order to apply the hydrogel in biomedical and engineering fields,following issues still remain to be solved:（i）Preparation of hydrogels with high strength and high Young’s modulus.The high strength of the tough hydrogels usually relies on their high extensibility,while the Young’s moudulus（usually<1 MPa）of them is still lower than the cartilage and skin（10-100 MPa）.（ii）Understanding the fracture behaviors of the tough hydrogel.The rupture of the hydrogel usually occurs via the origination and propagation of the cracks from the intrinsic flaws.However,understanding the fracture of viscoelastic hydrogels is still very rare.（iii）Processing the hydrogel into desired shapes or structures.In many engineering applications such as tissue engineering and soft robotics,hydrogels with specific structure are desired.However,hydrogels are usually prepared through reacting and gelating of its precursor solution in a reaction cell.As the result,the strucutre of the obtained hydrogel is highly restricted by the shape of the cell.To solve the issues mentioned above,we did the following investigations:（i）We developed a metallosupramolecular Poly（acrylic acid-co-acrylamde）[P（AAc-co-AAm）]hydrogel by casting.The hydrogel shows excellent mechanical properties,with Young’s modulus of 80 MPa,breaking stress of 18 MPa,breaking strain of 1100%,fracture work of 40 MJ/m³ and fracture energy of 1300 J/m².The mechanical properties of the gel can be well tuned in a wide range by adjusting the p H values of the incubated solution,the molar ratio of acrylic acid unit（AAc）and the concentration of the Fe Cl₃ solutions.Because of the dynamic nature of coordination bonds,the hydrogel shows rate-dependent mechanical performance and self-recovery behavior.Taking advantage of the reversibility of the coordination bonds,sol-gel transitions,shape memory and stimuli-triggered healing are realized.（ii）We investigated the fracture behaviors of the P（AAc-co-AAm）hydrogel and determined a specific length for fracture,fractocohesive length L_f,by pure shear tests.We found that the L_f scales three other critical lengths with explicit physical meanings,including:the length of cut-sensitivity L_c,which is measured by stretching gels with different initial crack lengths to rupture;the extension length of the crack before catastrophic advancing L_ss,which is determined from the resistance curve of the hydrogel;the size of large deformation zone during fracture L_i,which is calculated theoretically and also observed under a polarizing microscope.Besides,the fracture performance of the P（AAc-co-AAm）hydrogel is viscoelastic,which is investigated by performing pure shear tests and rheological tests with different deformation rates;the characteristic time of the coordination bonds is determined.（iii）We prepared the ultrathin P（AAc-co-AAm）hydrogel film by spin-coating the polymer solution with subsequent gelation in Fe Cl₃ solution.The thickness of gel film is uniform and is tunable from 5μm to 110μm by controlling the viscosity of the polymer solution,spin speed and spin time.Because of the constrained diffusion of ferric ions during the gelation process,gradient structure formed along the direction of thickness,leading to the thickness-dependent mechainical performance of the gel film.The Poly（acrylic acid-co-N-isopropylacrylamide）[P（AAc-co-NIPAm）]gel films were prepared in a similar way.A P（AAc-co-NIPAm）/P（AAc-co-AAm）bilayer hydrogel film was prepared by two-step spin-coating,which would bend into a roll in 3 M saline solution within 38 s.（iv）We fabricated P（AAc-co-AAm）hydrogels with designable structures through3D printing technology.We investigated the printability of the P（AAc-co-AAm）solution and P（AAc-co-NIPAm）solution systematically.Considering the binding strength between the hydrogel fibers and the shrinkage of the responsive hydrogels in saline solution,we choosed three different hydrogel fibers and integrated them into a four-layer hydrogel constructs.The printed composite hydrogel could transform into complex 3D configurations in saline solution.The configurations of the hydrogel in saline solution were tunable by controlling the alignment direction of the active gel fiber.Because the gel constructs were composed of gel fibers with diameter of～0.5 mm and possessed excellent mechanical properties,the deformation speed of the gel was fast（～1 min）and the holding force of the four-armed gel gripper was large.（v）We developed a rigid yet fully recoverable hydrogel by combining movable slide-ring（SR）cross-links with reversible and robust coordination bonds.The coordination cross-links endow the hydrogel with high Young’s modulus up to 18.3MPa and also the possibility for self-recovery,while the SR cross-links move along the chains to redistribute the stress and activate the rebinding process of the dynamic bonds.As revealed by experiments,the SR hydrogel is more stretchable and shows faster self-recovery process than the reference hydrogel.By analyzing the stress relaxation behaviors of the hydrogel by a three-dimensional continuum theory,we found the coordination bonds in the SR hydrogel possessed longer characteristic breaking time and shorter characteristic reattaching time.