Research On High-strength Conductive Hydrogels Based On Ionic Cross-linking

As a moist and soft material with a three-dimensional network structure,hydrogel has unique swelling properties,toughness,porous structure and biocompatibility.It is widely used in drugs delivery,wound dressings,dye adsorption,flexible sensors and other fields.However,the low strength limits the application of hydrogels.Among them,conductive hydrogel,as a special flexible conductive material,has promoted the emergence and development of flexible electronic devices.However,the application of conductive hydrogels is limited because of its low strength.Ionically cross-linked hydrogels have been proved to have good electrical conductivity,deformation responsiveness and freeze resistance on the basis of high strength in recent years.However,the application scenarios of single deformation-responsive conductive hydrogels are still very limited,and some researchers have turned their attention to multi-responsive hydrogels.Currently known multiple responses,especially hydrogels containing temperature responses,are generally low in strength,and it is still a challenge to prepare dual temperature/deformation response hydrogels with high strength.Facing these challenges,we prepared conductive hydrogels using a two-step ionic crosslinking strategy:firstly,the hydrogel matrix was prepared by chemical crosslinking,and then the ions penetrated into the gel to form ionic crosslinks by soaking.The main research contents include:（1）HNTs@CS was prepared by using short-chain CS-modified HNTs,mixed with AM,AA monomers,and polymerized to form a nanocomposite hydrogel matrix.The hydrogel matrix was soaked in Fe³⁺and SO₄^2-solutions respectively by a two-step method to prepare ionically cross-linked DN-gel.The optimal ratio of hydrogel was explored.The maximum tensile strength of the hydrogel is 3.96 MPa,the elongation at break is 494%,and the modulus is 4 MPa.The product has ultra-high compression resistance,the compressive strength under 85%strain can reach 13.4MPa,and it has good self-recovery performance.DN-gel exhibits a unique resistance to swelling,with almost unchanged mass after soaking in deionized water.And the cross-linked network structure of the hydrogel in deionized water is reorganized.Rigidity of hydrogel increases,and the tensile strength can reach 5.64 MPa,and the modulus increases to 15 MPa,when the elongation at break drops to 149%.There are a large number of free ions inside the hydrogel,which can migrate under an external electric field,which contribute to the good electrical conductivity and deformation response performance of the hydrogel.The hydrogel has antifreeze properties and can keep flexibility at-20℃.（2）A semi-interpenetrating CS/Poly（NIPAm-co-AA）hydrogel matrix was prepared:polymer network was formed by chemical crosslinking of monomers NIPAm and AA,and CS chains were interspersed in the network.Conductive hydrogels PNIDN with dual response properties of temperature and deformation were prepared by immerse the hydrogel matrix into Fe³⁺and SO₄^2-solutions respectively.The effects of monomer ratio,ion concentration and soaking strategy on the mechanical properties of hydrogels were studied,and the crosslinking process and crosslinking mechanism of PNIDN were further explored.PNIDN possesses high tensile strength（2.04 MPa）,modulus（0.43 MPa）,and elongation at break（225.3%）.Its mechanical properties and electrical conductivity have good cycle stability.PNIDN can monitor some simple movements of the human body in real time,such as flexing the fingers,frowning,making a fist,etc.The monitoring response fast（0.6 s）,and the monitoring results are repeatable.The waveforms of different sports have certain specificity,which make this hydrogel can be used to accurately monitor some subtle muscle movements.At the same time,the addition of PNIPAm endowed the hydrogel with good temperature-responsive properties.When the temperature changes,the UV transmittance and resistance of the PNIDN response obviously.PNIDN will change from transparent to opaque when exposed to body temperature.When the temperature is lower than 24℃,the increase of temperature has little effect on the light transmittance of the system.But when temperature rises from 24℃ to 26℃,the light transmittance drops sharply from 42%to 20%.The resistance of hydrogel exhibits different responses to different temperature changes.The resistance decreased rapidly when the hydrogel temperature increased from 25℃ to 50℃.But the resistance increases rapidly after a reaction time when the temperature drops from50℃ to 0℃.