Preparation And Properties Of Double Network Hydrogels With High Strength And Flexibility

As a “soft and wet” materials composed of 3D network,hydrogels are widely used in various fields,such as agricultural drought-resistance,drug delivery,and tissue engineering.However,in some applications such as wearable sensors,muscle bionics,etc.,it is often necessary to combine various mechanical properties of the hydrogels,including strength,stiffness,toughness,and ductility.These applications are limited for conventional hydrogels due to their poor mechanical properties.Double network(DN)hydrogels are composed of two asymmetric networks of different properties with extremely stiffness and toughness.The rigid network,which acts as a sacrificial key,could effectively dissipates energy,while the flexible network maintains the integrity of the hydrogels during the deformation process.At present,the research on DN hydrogels still faces some challenges.For example,DN hydrogels could not recover quickly when subjected to external force and most DN hydrogels lack self-healing properties.In view of these problems,we have developed two DN hydrogels and evaluated their properties.1.Polypyrrole(PPy)was grafted onto gelatin by Michael addition reaction to prepare a gelatin-polypyrrole(G-PPy)polymer,which was the first network of hydrogel,then polyacrylamide(PAAm)was used as the second network of hydrogel,and G-PPy/PAAm DN hydrogel was successfully prepared.Then,we introduced tannic acid(TA)into G-PPy/PAAm hydrogel,and the prepared G-PPy/PAM-TA DN hydrogel has higher mechanical strength.Fourier transform infrared spectroscopy(FTIR)was used to characterize the structure of the polymer.Dynamic light scattering(DLS)was employed to test the particle size of polymer.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to observe the morphology of polymer and hydrogels.Macroscopic observation was employed to observe the stability of polymer,and the shape memory properties and conductivity of hydrogels were also investigated.Electronic universal tensile tester and rheometer were used to study the mechanical properties of hydrogels.Conductivity meter was employed to test the conductivity of hydrogels.Electrochemical workstation was used to investigate the impedance change of hydrogels during the stretching process.The experimental results showed that we have successfully prepared a G-PPy/PAAm-TA DN hydrogel with high strength and toughness.In addition,shape memory and electrical conductivity also were existed.When the hydrogel was subjected to large deformation,it can quickly return to its original shape after a certain thermal stimulus.In addition,the hydrogel has excellent conductivity,and exhibited strain sensitivity.These excellent properties allow G-PPy/PAAm-TA hydrogels be used in construction of wearable strain sensors.2.We prepared a noncovalent muscle-inspired hydrogel.Polyvinyl alcohol(PVA)microcrystalline domains,generated by freezing/thawing,were used to mimic folded titin immunoglobulin domains and random-coil-like poly(N-methylol acrylamide)(PNMA)chains were used to mimic unstructured sequences.X-ray diffraction(XRD)and FTIR were used to characterize the structure of hydrogels.Polarized light microscopy was employed to investigate the anisotropic properties of hydrogels.Electronic universal tensile tester and rheometer were used to test the mechanical properties of hydrogels.Gravity method was employed to determine the water content of hydrogels.Macroscopic observation and electronic universal tensile tester were used to evaluate the self-healing properties of hydrogels.Finally,a mechanism was proposed to explain why the noncovalent muscle-inspired hydrogel exhibit excellent fatigue resistance under cyclic stress.The experimental results showed that the noncovalent muscle-inspired hydrogel had high mechanical properties.In addition,the noncovalent muscle-inspired hydrogel also had self-healing,self-recovery and excellent fatigue resistance properties.The toughness of the hydrogels was maintained or even improved during the continuous stretching process due to the strain-induced orientation,which led to an increase in the mechanical properties of hydrogels.This hydrogel promises to be among the most relevant drivers for the development of new-generation muscle-inspired hydrogels in the next decade.