Design Of Silk Fibroin Self-healing And Self-adhesive Conductive Hydrogels And Research On Low-temperature/underwater Strain Sensors

In recent years,hydrogels have shown great advantages and attracted much attention in the field of flexible electronic devices due to their ’soft and wet’ properties similar to human tissues and flexible mechanical properties.However,at present,functional hydrogels are mainly composed of synthetic polymer materials,and most of them have environmental problems or biocompatibility problems in use.Silk is a natural biomass material,safe,non-toxic,and has excellent biocompatibility.With people’s in-depth understanding of its structure and performance,its application range has broken through the traditional textile field.SF hydrogel material designed by regenerated silk fibroin(referred to as silk fibroin,SF)formed after degumming and dissolution of silk combines the advantages of silk and hydrogel.In particular,SF hydrogel has the characteristics of biocompatibility and easy functional design,which makes it have great potential in the development of flexible electronic devices in recent years.Based on the current problems:(1)the basic characteristics of the hydrogel wearable strain sensor,it is difficult to integrate other necessary functions(such as self-repair,selfadhesion,etc.)at the same time,(2)the multifunctional hydrogel wearable strain sensor directly adhered to the skin surface mostly has no biological safety,and(3)the hydrogel wearable strain sensor has poor moisture retention and cannot be applied to lowtemperature,underwater and other environmental.In this paper,SF prepared by natural biomass polymer material mulberry silk was used as the substrate,combined with inorganic salts,conductive polymers and other active materials,a multifunctional SF conductive hydrogel based on SF was prepared,which has self-healing,self-(wet)adhesion,wide working strain range,good biocompatibility,antibacterial property,high moisture retention and low-temperature resistance,stable electrical properties and strain sensing,so as to design a flexible wearable strain sensor that can adapt to different application environments.The main research contents and conclusions are as follows:(1)Polyvinyl alcohol(PVA),borax,silk fibroin(SF)and tannic acid(TA)were selected as biocompatibility components to design a SF conductive hydrogel with complete network structure and multiple functions(named PBST).Using the dynamic borate ester bond,catechol-boronic acid ester bond and the synergistic effect of multiple hydrogen bonds formed by cross-linking of the four,PBST has rapid self-healing ability of mechanical properties(60 s,self-healing efficiency of 82.26%)and electrical properties(1.3 s)without external stimulation.The catechol group of TA endows PBST with adhesion easy to peel without residue and shows repeated adhesion to different material surfaces.SF was used as a substrate and reinforcing agent to stabilize the non-Newtonian behavior of PVA and borax.At the same time,SF was skillfully as a crosslinking agent combined with TA to reduce the dissociation of TA on the hydrogel network and improve the mechanical properties and viscoelasticity of the hydrogel.With the combination of SF and TA,the viscoelasticity of hydrogel can be adjusted without loss of other properties,which makes PBST conformally contact with skin and has good stretchability(strain>1000%).In addition to the above functions,borate and sodium ions provide conductivity for PBST.PBST has both wide strain sensing range(>650%)and high strain sensitivity(650%,GF=1.66),stable electrical performance and cyclic durability under large strain(100%,>250 cycles).PBST has good biocompatibility and antibacterial properties.It can be directly adhered to the human body surface to realize the motion monitoring of joints(large strain)and facial micro-expression(small strain).The PBST conductive hydrogel with multiple synergistic functions has great potential as a flexible wearable strain sensor.(2)The hydrogel strain sensor becomes brittle and hard at low-temperature,loses mechanical flexibility and electrical properties,and has poor moisture retention at room temperature,which affects the service life and performance stability.and rarely sees reports of antifreeze hydrogel has self-healing,self-adhesion and biosafety reports at the same time.In view of the above problems,on the basis of the study in the first part,PSEBT(PVA,SF,EG,borax and TA)hydrogels were designed and prepared by the binary solvent(ethylene glycol(EG)/water)with antifreeze mechanism,combined with the freeze-thaw treatment that can balance the internal multiple properties.The gel mechanism and structure,realization mechanism of hydrogel functionality(self-healing,self-adhesion,antifreeze,high moisture retention,antibacterial)were discussed,and the conductivity and sensing properties were characterized.The results showed that EG had good compatibility with the other four components,and a three-dimensional hierarchical gel network was constructed.The abundant hydrogen bonding ability between EG and water molecules endows PSEBT with frost resistance(phase transition temperature as low as-39℃)and high moisture retention(standard atmosphere,24 h,97%).Freeze-thaw treatment makes up for the loss of mechanical properties of hydrogel when balancing its self-healing,selfadhesion and stretchability,and improves the mechanical properties of the hydrogel.The antifreeze mechanism and balance mechanism retain the stretchability,rapid self-healing(mechanical,electrical,strain sensing),self-adhesion,antibacterial,electrical conductivity,and the ability to monitor different strain movements of the hydrogel endowed by functional components.PSEBT can monitor the movement of the entire human epidermis,and at the same time,it can identify changes in movement speed and distinguish movement directions.In addition,when the TA content is constant,the mechanical properties and adhesion ability of PSEBT can be improved by EG combined with freeze-thaw treatment.Compared with common antifreeze hydrogels,PSEBT not only has excellent mechanical flexibility at low temperature,but also has low-temperature self-healing,low-temperature adhesion,low-temperature conductivity and strain sensitivity.In the low-temperature environment of-23℃,human motion monitoring can still be carried out.Based on the stable electrical performance,at low-temperature,the expression of Morse code can be realized by controlling the body movement to achieve silent communication.This part successfully designed a hydrogel wearable strain sensor that can be applied to lowtemperature environment and has self-healing,self-adhesion and biological safety.(3)The filling of water molecules at the interface between the sensor and the tissue will lead to the weakening or elimination of the self-healing,self-adhesion,strain sensing and other properties of most hydrogel materials,which will affect the stability of the use and limit the underwater use of hydrogel flexible strain sensors.The current research status of most underwater flexible strain sensors reported have biological compatibility and environmental problems(using hydrophobic fluorine-containing ionic liquids).Based on the in-depth understanding of SF and TA easy crosslinking and their respective functional properties in the first two parts,as well as the stronger underwater stability of electronic conductive hydrogel than that of ionic conductive hydrogel.In the third part of this paper,silk fibroin(SF)combined with tannic acid(TA)was used as the gel substrate,and the conductive polymer polypyrrole(PPy)was introduced into the same gel network by in-situ polymerization.The SF/TA@PPy electronic conductive hydrogel was designed to be more suitable for underwater applications than the ionic conductive hydrogel.SF/TA@PPy electronic conductive hydrogel was designed,which was more suitable for underwater application than ionic conductive hydrogel.The gel-forming mechanism,structure,performance and motion monitoring ability as well as its application as a strain sensor under water were comprehensively analyzed.The results showed that the synergistic effect of dynamic reversible hydrogen bonding,metal coordination bond(TA-Fe3+)and hydrophobic interaction enabled SF/TA@PPy to have draftability(tensile strain>500%);no matter in air or underwater,without external stimulation,has rapid self-healing ability(electrical,mechanical,strain sensing);Exhibit adhesion and wet adhesion to different materials(72 h after water,still adhesive and skin compliance.The wet adhesive strength of the bonded PTFE plate did not decrease significantly after immersion in water for 5 days).SF/TA@PPy has excellent antibacterial property and biocompatibility.PPy endows hydrogel with conductivity,which enables it to have universal monitoring ability for all joints and regions of human body.It still has stable and accurate monitoring ability for human movement in water.It is directly attached to the body surface,and Morse code can be expressed through regular and rhythmic movement in water to realize underwater communication.This part has successfully designed a hydrogel wearable strain sensor which can be applied to the underwater environment with self-healing,self-adhesion and biosafety.The research shows that wearable strain sensors with multiple functions and biocompatibility can be designed and prepared with SF as the substrate,and provides a new way for its application in low-temperature environment and underwater environment.