| Flexible electronic devices have great application potential in human motion monitoring,early disease diagnosis,non-invasive treatment,electronic skin and soft robots.In the past few years,with the rapid development of flexible electronic devices,soft materials with stretchability,biocompatibility,self-healing and electrical conductivity have attracted extensive attention.Hydrogels are a kind of smart and biocompatible soft materials composed of hydrophilic three-dimensional polymer networks,and can absorb a large amount of water without structural damage.Owing to their soft,operable,rubbery and highly swelling features,hydrogels can be endowed with good mechanical properties,self-healing performance and electrical conductivity,and have emerged as promising candidates for flexible and wearable devices.However,traditional hydrogels prepared by chemical crosslinking are usually of poor mechanical properties and lack of self-healing,adhesion and biocompatibility,which cannot meet the requirements of flexible wearable devices.Based on these problems,this work introduces multiple dynamic interactions(including host-guest interaction,Schiff-base and borate ester bonds)into polymer networks to prepare multifunctional hydrogels with good self-healing performance,biocompatibility,conductivity,stretchability,adhesion and remodeling property.The main work is listed as follows:(1)By the living and controlled ring opening metathesis polymerization(ROMP),ferrocene-containing homopolymers,binary copolymers and terpolymers were synthesized successfully.First,ferrocene-containing monomer 3 is synthesized,and its ROMP kinetics are studied.The polymerization kinetics analysis shows that the polymerization rate of 3 is fast.Polymer 4 can be obtained within 10 minutes even at the degrees of polymerization up to 100.Then,the“one-pot”ROMP of hydrophilic monomer 10 and ferrocene-containing monomer 3 was used to fabricate amphiphilic ferrocene-containing binary copolymers 11.The polymerization kinetics results indicate that all the monomers can be polymerized completely within 10minutes.Furthermore,amphiphilic ferrocene-and aldehyde-containing terpolymers16 also were prepared by the“one-pot”ROMP of hydrophilic monomer 10,aldehyde-containing monomer 15 and ferrocene-containing monomer 3.Polymerization kinetics analysis indicates that all the monomers are polymerized fully within 20 minutes.The structures,molecular weights and molecular weight distribution of these ferrocene-containing polymers were determined by NMR,FT-IR,GPC and Uv-vis spectrum.(2)ROMP was used as a polymerization tool to integrate dendritic oligoethylene glycol moieties and ferrocenyl groups into the hydrogels that have response to both temperature andβ-cyclodextrin(β-CD).Four ferrocene-based hydrogels with different compositions were prepared by the“one-pot”ROMP polymerization at different feed ratios of hydrophilic monomer 10,ferrocene-containing monomer 3 and cross-linker 17.It is found that the hydrophobic ferrocenyl moiety plays an important role in maintaining the structure of hydrogels.The tensile test results show that the fracture stress of these hydrogels decreases with the increase of hydrophilic moiety(0.32 MPa→0.16 MPa),while the fracture strain of these hydrogels gradually increases with the increase of hydrophilic moiety(70%→129%).Compression experiments indicate that the structure of these hydrogels is not destroyed at compression strain of 90%,and the compression strength of these hydrogels can reach up to 0.78 MPa.In addition,the cyclic compression tests also show that these hydrogels have good compression-resilience property.These results indicate that the increase of the hydrophobic ferrocenyl moiety can enhance the strength of hydrogels,while the increase of the hydrophilic part can improve the toughness of hydrogels.Thermogravimetric analysis shows that these hydrogels have good thermal stability and their decomposition temperatures are over 400 ~oC.Interestingly,these hydrogels have response to both temperature andβ-CD.The dendritic oligoethylene glycol moieties play a decisive role in the thermo-responsive behavior of hydrogels.With the increase of the hydrophilic oligoethylene glycol moieties,the swelling rate of hydrogels increases gradually and its response to ambient temperature becomes more obvious.When the content of the hydrophilic oligoethylene glycol moieties is high,the volume of the hydrogel will shrink significantly at higher temperatures(50 ~oC),leading to partial dehydration of the hydrogel.Based on the host-guest interaction betweenβ-CD and ferrocene,when these ferrocene-based hydrogels are immersed into theβ-CD solution,the ferrocene groups of the hydrogels can form host-guest inclusion complexes withβ-CD,which will increase the hydrophilicity of hydrogels and result in the increase of volume and swelling rate.(3)Multifunctional dual-network composite hydrogels are parepared by the combination of the host-guest interaction ofβ-CD in poly(β-CD)and ferrocene in copolymers 11 with the dynamic borate ester bonds of polyvinyl alcohol(PVA)and borax.The effect of the composition in copolymers 11 on the mechanical properties of the double network hydrogels was studied.The structure of the dual-network hydrogels was determined by 2D NOESY,FT-IR and SEM.Thermogravimetric analysis confirms that hydrogels have good thermal stability.The test results of mechanical properties show that the hydrogels have good tensile property.Based on the host-guest interaction and the dynamic borate ester bonds,the hydrogels exhibit excellent self-healing properties(healing efficiency up to 95.0%)and remodeling property(different shapes can be formed).Adhesion tests show that hydrogels have appropriate adhesion to various substances,such as PTET,rubber,metal,wood,paper,plastic,glass,skin and pig lung.Cell viability test on RAW 246.7 cells indicates that the hydrogel has good biocompatibility.By adding carbon nanotubes,the composite hydrogel can be endowed with good electrical conductivity and may serve as a conductor to build a conductive circuit that makes LEDs lighting.When the hydrogel is stretched,the brightness of the LEDs would dim significantly,indicating that the resistance of the hydrogel could vary with its length.In addition,when the hydrogel is cut off to form an open circuit,the LEDs do not light up.After self-healing,the circuit is rebuilt and the brightness of LEDs is restored to their original state.Due to its excellent self-healing and remodeling properties,the composite hydrogel can be remolded into any shape desired,indicating that it can be used to build flexible electronic devices.And the hydrogel is shaped into a cylinder that can be used as a touch pen to write on a smart phone screen.The composite hydrogel was then made into flexible strain sensors whose elative electrical resistance changes can vary with the change of strain.The hydrogel strain sensor has high sensitivity with gauge factor of 5.9.The relative electrical resistance changes of sensor show good stability and repeatability under cyclic loading/unloading small strains(10%,20%and 50%),confirming that the hydrogel sensor has good repeatability and durability.In addition,the hydrogel sensor was used to monitor various large-scale and subtle human movements.In large-scale human motions(including finger bending,wrist bending,elbow bending,knee bending,and mark time),the real-time relative electrical resistance changes of the hydrogel strain sensor range from 25%to 110%.It is worth noting that the hydrogel strain sensor can also recognize the motion states(rapid and slow).In subtle human motions(such as blink and speaking),the real-time relative electrical resistance changes of the hydrogel strain sensor range from 2.5%to 10%.After cutting,the healed hydrogels also can monitor human motions with durable stability.In addition,the hydrogel sensors may track the respiratory movement of a pig lung in vitro.(4)Multifunctional triple-network composite hydrogels were fabricated by multiple dynamic interactions.The host-guest interaction ofβ-CD in poly(β-CD)and ferrocene in copolymers 16 was used to construct the first network.The Schiff-base bonds between copolymers 16 and biocompatible gelatin acted as the second network.And the dynamic borate ester bonds between PVA and borax served as the third network.The multiple dynamic interactions make the triple-network composite hydrogels have excellent self-healing performance(healing efficiency up to 95.0%),mechanical strength(40.1 k Pa),stretchability(705%)and remodeling properties.Carbon nanotube was used as a filler to give this triple-network hydrogel good electrical conductivity,and can improve its mechanical properties(mechanical strength of 54.6 k Pa and stretchability of 778%).Good biocompatibility of this triple-network composite hydrogel was confirmed via cell viability tests.To study the electrical conductivity,this triple-network composite hydrogel was used as a conductor to construct a conductive circuit,which can lighten a LED.The brightness of the LED decreases after this composite hydrogel is stretched,indicating that it has the potential to act as strain sensor.A strain sensor fabricated by this triple-network composite hydrogel exhibits good strain sensitivity with gauge factor up to 9.4.This strain sensor is also applied to monitor all kinds of human movements(finger bending,wrist bending,elbow bending,knee bending,mark time,bite and speaking,etc.).Significantly,the healed strain sensor is also able to monitor movements with almost the same strength of electrical signals as the original one.Furthermore,the hydrogel-based strain sensor has the ability to detect the breathing movements of a pig lung in vitro. |
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