Construction And Characterization Of Selfadhesive And Conductive Hydrogels Based On Resilin-like Proteins

Protein-based adhesive and conductive hydrogels have significant applications in green wearable strain sensors.However,the preparation of protein hydrogels that could satisfy the mechanical properties and conductivity required by strain sensors is still a great challenge.In this study,genetically engineered resilin-like proteins（RLPs）were designed and synthesized,and hydrogels with high ductility,recoverability,adhesion,strain sensitivity and stability were obtained through step-by-step functionalization.And the novel adhesive and conductive hydrogel has been initially applied to wearable strain sensors.Firstly,in order to tune the mechanical properties of RLP hydrogels,three recombinant expression plasmids encoding different molecular weight of RLPs were constructed by recursive cloning method,and were expressed efficiently with high solubility in host Escherichia coli BL21（DE3）,the yield reached up to 370-770 mg L^-1.Notably,R64 with a molecular weight of 93 k Da is currently reported the largest molecular weight of RLP.It is found that the strength,stiffness and toughness of the three RLPs hydrogels formed by photochemical crosslinking are positively correlated with the protein molecular weight.Secondly,in order to improve the extensibility of the hydrogel,we used glycerol-water as binary solvent to dissolve the R64 protein for photochemical crosslinking into hydrogels.Addtion of 10-30%glycerol significantly enhanced the extensibility of the hydrogels.Surprisingly,the addition of glycerol imparted adhesive properties to the hydrogels,and the maximum adhesion strength reached approximately 24 k Pa when 20%glycerol was added,which can support self-adhesion of the hydrogel to different substrate surfaces such as skin,latex,glass,metal,and cellulose paper.This method of adding a multihydroxyl solvent such as glycerol provides a simple and effective new strategy for tuning hydrogel extensibility and adhesion.Finally,in order to impart the RLP hydrogel conductivity,the graphene-R64 conjugate was synthesized by using EDC/NHS activation.The conjugate was mixed with a R64 glycerol precursor solution and photocrosslinked to form a hybridhydrogel network.The hybrid hydrogel not only can be stretched to four times its original length,but also maintained the excellent extensibility and adhesion strength as observed in the R64-20%glycerol hydrogel.Moreover,the hybrid hydrogel exhibited high strain sensitivity with a gauge factor of 3.415 at 200%strain.The sensor based on the hybrid hydrogel described above can generate resistance changes in response to tensile or compressive deformation,and convert these mechanical stimuli into electrical signals,thereby not only being used for real-time monitoring human activities of large movements of finger bending,but also for swallowing and vocalization.The results showed that the sensor also exhibited good stability in the condition that simulated its use on human skin for three days,with only about 30%of weight loss due to water evaporation,and the hydrogel can still be used for human activity monitoring.In summary,based on the natural RLP elastomer,a novel type of self-adhesive and conductive protein hydrogel was designed and constructed by regulating the solvent and adding graphene-resilin conjugate.The hydrogel with characteristics such as high elongation,resilience,adhesion,sensitivity and stability meets the application requirements of wearable sensors,and it is expected to have great application in the fields of soft robots,tissue engineering and human-machine interfaces.The strategies of design and functionalization of the protein hydrogel established in this study may be applicable for the development of other functional materials.