The Study On Properties Regulation Of Conductive Hydrogels And Their Applications In Flexible Wearable Strain Sensors

Hydrogel is a family of three-dimensional network polymer material with high moisture content.It is widely used in numerous fields such as biomedical,tissue engineering,actuators,and flexible electronic devices,because of its similar structure to natural biological tissue and good flexibility and biocompatibility.Especially in the field of flexible wearable strain sensors,hydrogel has received extensive attention and in-depth research as an ideal flexible base material.However,traditional hydrogels have poor mechanical properties and lack of comprehensive properties such as adhesion,self-healing and freeze-resistance,which limit their applications in the field of flexible wearable strain.Focusing on the above problems,in this paper,a series of multifunctional hydrogels with conductivity,high stretchability,anti-fatigue,self-healing,adhesion,high transparency,and freeze-resistance were prepared,and their applications as flexible wearable strain sensors were studied in detail.The main research contents are as follows:1.The poor mechanical properties of most natural hydrogels limited their applications in flexible wearable strain sensors.In this part,we compounded the prepared silver-coated copper（Ag@Cu）nanoparticles with gelatin,and obtained a nano-composite hydrogel with high conductivity,toughness and fatigue resistance based on salting-out effect.The effect of salting-out made two kinds of physical cross-linking（hydrophobic cross-linking and triple helical cross-linking）in the composite hydrogel produce synergistic effect,which endowed the composite hydrogel with excellent mechanical properties and fatigue resistance.The hydrogel exhibited excellent electrical conductivity（1.35 S/m）due to the uniform distribution of Ag@Cu nanoparticles.The hydrogel-based flexible wearable strain could be used for real-time monitoring of human movements,such as bending of joints,swallowing,throat vocalization,etc.,and exhibited excellent tensile strain sensitivity and stability over a wide strain range.2.In this part of the work,we were committed to improving the stretchability of hydrogels and developing hydrogels with high tensile and electrical conductivity.The silver/tannic@graphene-oxide nanocomplex（Ag/TA@GO）was introduced into the polyacrylamide hydrogel system as a crosslinking agent and conductive filler.The multifunctional nanocomposite hydrogel（Ag/TA@GO-PAM）with a unique organic/inorganic network structure was formed by hydrogen bonding and physical adsorption between the polyacrylamide chains and Ag/TA@GO nanocomplexes.The Ag/TA@GO-PAM hydrogel showed excellent electrical conductivity and high stretchability（1250%）.The Ag/TA@GO-PAM hydrogel based flexible wearable strain sensor had a fairly stable tensile strain sensitivity（GF=3.108）over a large strain range（0-1000%）,and could monitor both large human movements（such as joint bending）and small movements（such as facial expressions and pronunciation）in real time.In addition,this part of work also extended the application scenarios of the previous hydrogel,and explored the application of hydrogels in the fields of information encryption and identification,bionic robot electronic skin and human-computer interaction sensors.3.This part of work mainly aimed to solve the lacking of self-healing and adhesion of hydrogels in the above work.A novel two-dimensional nanomaterial MXene was prepared and introduced into the system composed of anionic monomer and cationic monomer,and the MXene-polyampholytes hydrogel（MXene/PMN）was obtained by one-step free radical polymerization.There were a large number of strong ionic bonds and weak ionic bonds in the hydrogel system:the strong ionic bonds served as permanent crosslinking to maintain the integrity of the hydrogel;the weak ionic bonds served as sacrificial crosslinking,endowing the hydrogel with good stretchability（>1200%）,toughness and self-healing property.The synergistic toughening effect of MXene and its unique electrical properties further enhanced the mechanical and electrical properties of the hydrogel.In addition,the hydrogel showed repeatable self-adhesion on some substrate surfaces,including plastic,glass and metal and pig skin.The wearable strain sensor based on this hydrogel demonstrated its long-lasting accuracy and sensitivity in human motion monitoring.Moreover,the strain sensor could be equipped with an alarm device for monitoring the daily activities of patients with attention deficit hyperactivity disorder,reminding patients to pay attention to their own behaviors,to achieve the effect of adjuvant treatment.This work shed a new light on the development of wearable strain sensors for the personalized healthcare monitoring,human–machine interfaces,and artificial intelligence.4.On the basis of the previous part of work,the structure of anion-cation cross-linking network was improved,and a new conducting double-network hydrogel was constructed by introducing the Ca²⁺-alginate network.Compared with the single-network hydrogel,the double cross-linking network could effectively dissipate energy and endowed the hydrogel with excellent tensile property（1375%）,rapid self-recovery and anti-fatigue property.In addition,the introduction of Ca²⁺could enhance the mechanical properties of the hydrogel and further improved the conductivity of the hydrogel.The hydrogel showed good self-healing and adhesion,as well as excellent optical transparency（92.2%）,so that it could accurately pinpoint specific parts of the human body in monitoring.The hydrogel based wearable strain sensors could monitor various joint bending（such as fingers,elbows and knees）and local muscle movements（such as eyebrows and mouth）in real time,all of which confirmed its great potential in personalized medical monitoring,soft robotics,and human-machine interface.5.In order to further enrich the properties of hydrogels,we focused on the improvement of dry resistance and freeze-tolerance for hydrogels.Hyaluronic acid（HA）and Chitosan（CS）were selected as the main chain polymers of hydrogels,and hydrazide HA（HA-ADH）and aldehyde-modified CS（OCS）were synthesized by modifying HA and CS.Based on the moderate and rapid Schiff base reaction between hydrazine and aldehyde,the main network of hydrogel could be rapidly formed by one-step mixing injection.At the same time,KCl and glycerol were introduced into the hydrogel system,and the strong hydrogen bond between water and glycerol made the water molecules firmly fixed in the hydrogel network,prevented the freezing and volatilization of the water molecules,endowing the hydrogel with anti-drying and anti-freezing properties.In addition,the hydrogel exhibited good mechanical properties and self-healing properties driven by reversible arylhydrazones bonds,ionic crosslinking and hydrogen bond crosslinking.The experimental results showed that the hydrogel-based strain sensor could maintain good flexibility,extensibility and conductivity in cold environment（-23℃）,thus effectively improving the long-term stability of the hydrogel.Therefore,the study of the hydrogel further extended the application scenarios of conductive hydrogel as flexible wearable strain sensors,especially in dry or cold environments.Collectively,a series of multifunctional hydrogels with conductivity,high stretchability,anti-fatigue,self-healing,self-adhesion and freeze-resistance were prepared from the perspective of preparation of nano-composite materials and network structure design in this project.On this basis,a variety of multifunctional hydrogel-based flexible wearable strain sensors with sensitivity and stability were designed,and used in the fields of human activity and physiological health monitoring.The research work of this thesis will further promote the research progress of conductive hydrogel flexible wearable strain sensors,and has important reference significance in the fields of artificial intelligence,bionic robot electronic skins,and so on.