Preparation Of Poly(γ-Glutamic Acid)/Polyvinyl Alcohol Composite Conductive Hydrogel And Its Applications In Flexible Strain Sensor

Conductive hydrogels have the electrochemical properties of conductive fillers and the flexibility of hydrogels.Because of their high water content and good biocompatibility,they are considered as ideal biosensing materials.Flexible sensors based on conductive hydrogels have adjustable mechanical and electrical properties,and have broad application prospects in electronic skin,soft robots,wearable devices and implantable devices.Today,conductive hydrogels in air environment have high conductivity and sensing sensitivity,enabling personalized sensing applications such as human motion sensing,health monitoring,humancomputer interaction,etc.However,achieving liquid-phase sensing of conductive hydrogels remains challenging.On the one hand,the hydrogel in the liquid phase environment is easy to absorb water swelling,dilute the conductive network and lead to a significant decrease in mechanical properties;On the other hand,the presence of hydration layer can seriously weaken the adhesion between the hydrogel and the substrate.Unnecessary swelling and lack of interfacial adhesion in liquid phase environment can lead to distortion of sensing signal and hysteresis in signal sensing.In order to solve the problems of easy swelling,low adhesion and unstable sensing signal in liquid environment,this study based on the strategies of " Freeze-crystallization method","Huffman effect" and "Catechol chemistry",using polyglutamic acid(γ-PGA),polyvinyl alcohol(PVA)as hydrogel matrix,conductive polymer PEDOT:PSS as conductive filler,γ-PGA/PVA/PEDOT:PSS/TA conductive composite hydrogels were prepared by using tannic acid(TA)as modifier.The swelling resistance,mechanical properties and liquid phase sensing properties of γPGA/PVA/PEDOT:PSS/TA hydrogels were investigated.The results showed that the hydrogels exhibited high tensile property and high adhesion during the liquid phase sensing process,and the signal changes could be clearly observed under different strain frequencies and amplituaries,which could be used for real-time feedback sensing information.It avoids the drastic change of sensing signal caused by large-scale swelling of traditional conductive hydrogels in the liquid-phase sensing process,and provides a practical idea for the liquidphase sensing application of γ-PGA/PVA-based hydrogels.The main research content of this paper is as follows:(1)Preparation and characterization of γ-PGA/PVA composite conductive hydrogel:Good mechanical stability and anti-swelling performance are the basis for expanding the application of hydrogel underwater sensing.In this part,γ-PGA and PVA as the matrix,based on the strategy of "Freeze-crystallization method " and " Hydrogen bond ",γPGA/PVA composite conductive hydrogels with low swelling and high mechanical stability were successfully prepared.The results show that the equilibrium water absorption of γPGA/PVA hydrogels prepared by directional freezing decreases from 165.5 ± 1.5%to 0.3±0.1%,and the swelling resistance is improved by 550 times compared with that prepared by traditional freeze-thaw method.The hydrogel has good mechanical properties:mechanical tensile strength of 0.5 ± 0.1 MPa,elongation at break of 605.9±4.3%.Human motion monitoring results show that γ-PGA/PVA composite hydrogels show good human motion sensing performance,which can achieve high frequency sensing in the range of 0.4 HZ-1.1 HZ,and different strain amplitudes of conductive hydrogels sensing signals have significant differences,can be used to collect human motion sensing signals.However,the γPGA/PVA composite conductive hydrogels prepared by this method have low conductivity and poor liquid phase sensing stability,which need to be further improved.(2)Preparation and characterization of γ-PGA/PVA/PEDOT:PSS composite conductive hydrogels:In view of the problems of weak mechanical strength and low electrical conductivity of γ-PGA/PVA composite hydrogel prepared in the first part,this part of the study uses conductive polymer PEDOT:PSS as the hydrogel conductive filler,combined with the synergistic effect of "Hofmeister effect" and "Electrostatic interaction".γPGA/PVA/PEDOT:PSS composite conductive hydrogels with excellent conductivity and mechanical properties were successfully prepared.The results show that compared with γPGA/PVA composite conductive hydrogel,the conductivity of γ-PGA/PVA/PEDOT:PSS composite conductive hydrogel is improved from 0.2 ± 0.1 S/m to 6.6± 0.3 S/m,and the conductivity is improved by about 33 times.In addition,the mechanical tensile strength of the hydrogel was increased from 0.5 ± 0.1 MPa to 1.2 ± 0.2 MPa by about 2.4 times.The elongation at break was 605.9±4.3%.The balanced water absorption rate of the hydrogel is only 11.2 ± 0.9%,the anti-swelling performance is excellent.However,its liquid phase sensing still lacks stability.On the one hand,PEDOT:PSS has poor dispersion in the hydrogel system,and the conductive network is not uniform.On the other hand,γ-PGA/PVA/PEDOT:PSS conducting composite hydrogels lack of underwater adhesion,and the liquid phase environment can not closely fit the target,resulting in distortion of sensing signals.Therefore,it is necessary to further improve the underwater adhesion and liquid phase sensing stability of γ-PGA/PVA/PEDOT:PSS conductive composite hydrogels.(3)Preparation and characterization of γ-PGA/PVA/PEDOT:PSS/TA composite hydrogel:In view of the problems of uneven conductive network and lack of underwater adhesion of γ-PGA/PVA/PEDOT:PSS conductive composite hydrogels prepared in the second part,this part of the study aims to introduce tannic acid(TA)to improve the dispersion of the conductive network of the hydrogels and give the hydrogels underwater adhesion.Based on the strategy of " Catechol chemistry ",γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel with underwater adhesion and liquid phase sensing stability was prepared successfully.The results show that compared with the γ-PGA/PVA/PEDOT:PSS composite hydrogel prepared in the second part,the mechanical tensile strength of γPGA/PVA/PEDOT:PSS/TA composite hydrogel is 0.9±0.1 MPa,and there is no loss.The elongation at break was increased to 1495.5 ± 15.5%,which was increased by about 2.5 times.The conductivity of the hydrogel was 0.94 ± 0.05 S/m,the equilibrium water absorption was 13.0± 0.9%,and the conductivity and anti-swelling properties were not lost.And γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel has excellent underwater adhesion,can easily adhere to a variety of substrates in the water,such as human rubber,plastic,its adhesion strength to human skin up to 1.1 KPa.The hydrogel has the stability of liquid phase sensing,in the liquid phase environment can clearly observe the human body sensing signals of different motion frequency and amplitude.Underwater communication can also be achieved by sending Morse code signals such as " SOS " and "SAFE " through liquid-phase sensing.The γ-PGA/PVA/PEDOT:PSS/TA composite conductive hydrogel showed low swelling rate,high mechanical strength,strong underwater adhesion and sensing stability,which is expected to be used in the preparation of wearable sensing devices that maintain long-term sensing stability in liquid phase environment.