Partially Insoluble Silk Fibroin Film Induced By UV Irradiation For Flexible Strain Sensors

Electronic skin(e-skin)can be attached to the surface of the human skin to perceive external signals and has the advantages of lightness,softness,low cost,stretchability and good durability,which has important applications in human health monitoring.An e-skin is usually constructed with silicone elastomer,rubber,polyurethane(PU),polyethylene terephthalate(PET),polyimide(PI),poly(vinylidene fluoride)(PVDF)and other base materials.However,the e-skins based on those materials cannot be bio-degraded after use,posing great threats to the environment.Regenerated silk fibroin(RSF)has been regarded as an ideal substrate for the flexible strain sensors due to its excellent biocompatibility,bio-degradability,and flexibility.The RSF films prepared by conventional methods are usually brittle and cannot be tightly adhered on the human skin.At present,it is still a great challenge to balance the flexibility and solubility of the RSF film by adjusting its secondary structures.In response to this problem,this study prepared a partially insoluble RSF film by UV treatment.The insoluble part of the film can be used as the base of the flexible strain sensor,while the soluble part can be used as an adhesive to tightly adhere the device to the skin.Based on this principle,a stretchable and optically transparent strain sensor was constructed on the UV-induced partially insoluble RSF films using silver nanowires(Ag NWs)with good optical transmittance and conductivity as conductive filler.The main contents of this thesis are as follows:1.Preparation of UV-induced partially insoluble RSF filmDue to the water solubility of the newly made RSF film,it is necessary to modify the film to obtain a water-insoluble RSF film.In this study,the Li Br-dissolved SF solution was treated with hydrolysis for 0,2,4,6,8,and 10 hours.The molecular weight of SF in solution decreased gradually with the increase of hydrolysis time by SDS-PAGE.The RSF film was prepared by casting the hydrolyzed solution in a petri dish.Through the characterization of structure,mechanical properties and light transmittance,it was found that the β-sheet content of the RSF film decreases and the transmittance gradually increases With the increase of hydrolysis time.When the hydrolysis time is 6 hours,the film exhibits the highest ultimate elongation,indicating its excellent flexibility.However,the hydrolyzed RSF film can quickly dissolve in water,which cannot meet the requirements for the preparation and application of flexible strain sensor substrates.Therefore,the film hydrolyzed for 6 hours was further treated with UV irradiation,and the transformation from being soluble to partially insoluble was realized.FTIR spectra showed that there is no significant change in the secondary conformation of the film before and after UV treatment.The Gel permeation chromatography-multi-angle laser light scattering(GPC-MALLS)tests shows that the molecular weight of the RSF solution hydrolyzed for 6 hours gradually increases as the UV treatment time increases from 0 to120 minutes,which was caused by the UV-induced crosslinking of the SF molecules.These results indicate that UV-induced crosslinking of SF molecules leads to partial insolubility of RSF films.After optimization,the partially insoluble film obtained with a UV irradiation time of 60 minutes and a distance of 1 cm was used as the substrate to prepare the strain sensor in the following section.2.Construction of flexible strain sensors based on UV-induced partially insolubleRSF filmIn this part of work,Ag NWs were synthesized by a one-pot method,and characterized by field emission scanning electron microscope(FESEM),UV-Vis spectrometer(UV-VIS)and X-ray diffraction test(XRD).It was proved that the uniform and pure Ag NWs were successfully prepared.Subsequently,Ag NWs/RSF conductive composite film was constructed on the UV-induced partially insoluble film using newly prepared Ag NWs as conductive fillers.FESEM images showed that the Ag NWs presented a cross-linking network structure and partially embedded in the RSF film.This structure facilitates the construction of electron transfer pathways,while partially dissolved SF facilitates the tight adhesion of conductive networks to the film.When the concentration of Ag NWs is 10 mg/m L,the composite membrane has good transmittance and conductivity.An electronic skin strain sensor was constructed by dropping a drop of water onto the latex glove and pasting the composite film onto it through dissolved SF.The sensor has a fast response time(2.01 s),high sensitivity(GF=1.03 in the 40% strain range),and good stability(Tolerating 300 stretching-release cycles at 20% strain).Furthermore,the device can be firmly attached to human skin to real-time monitor body movements,such as the bending of fingers,wrists,elbows,and knees.In addition,when the electronic skin is attached to the neck and cheeks,nodding and mouth opening can also be sensed.In summary,in this paper,the low molecular weight RSF solution was obtained by hydrolyzing the SF-Li Br solution.Subsequently,a flexible,transparent,and soluble RSF film was prepared by a casting method.The RSF film was modified by ultraviolet irradiation,a flexible strain sensor was constructed using the UV-treated RSF film as a flexible substrate and Ag NWs as a conductive filler,further demonstrating its application in human motion monitoring.This thesis not only provides a simple,low-cost,and pollution-free method for the construction of flexible strain sensors,but also is expected to expand the potential application of silk materials in human health monitoring.