Research On Fabrication And Applications Of Silk Fibroin Based Optical And Electronic Sensing Devices

Silk is an ancient material in China with a long history for more than forty centuries.However,it has long been used as the raw materials in textile industry,leaving other applications undeveloped.In recent years,with the development of reverse engineering in chemistry and biology,people have devoted much effort to the research of silk materials in molecular level and greatly extended their applications in a wide range of fields,including biomedicine,physical optics,materials science,flexible electronics,cosmetics,etc.These studies bring new opportunities for silk materials and may provide important guidance for the development,transformation and upgrading of materials in the textile industry.Silk fibroin is the mixtures of P-sheet nanocrystals(Silk II)and noncrystalline regions(Silk I)which determine the physical and chemical properties of silk.The proportions of the two distinct components are highly tunable in regenerated silk materials,therefore the design and customization of silk-based devices have become possible by the route of "structure-property-morphology-application".Based on this route,this thesis will focus on developing new functional devices such as humidity sensor,solvatochromic photonic materials,and on-skin electrodes through the modulation of the secondary structure,properties,and morphologies of silk fibroin materials.The research contents can be mainly summarized in the following aspects:(1)Sub-micron silk fibroin films with high humidity sensibility through color changing have been developed by spin coating of silk fibroin solution with low crystallinity.The optical properties of the silk fibroin film caused by thin film interference can be easily tuned by the coating rates.The film exhibited fast responses to humidity with evident color variation in 5 s when exposure to wet air,and the color changed back even faster,in just 1 s.The humidity-induced color change was mainly originated from the film expansion and subsequent shifts of interference wavelength in increased humidity.The high absorption of water and volumetric expansion abilities of silk fibroin film were owing to the random-coil dominated secondary structures.These low-cost but highly efficient silk fibroin sensors can realize colorimetric detection of humidity and may have great potential in applications for anti-counterfeit labeling and transient optics.(2)Silk fibroin based bioinspired solvatochromic amorphous photonic coatings have been developed by embedding polystyrene(PS)nanoparticles in water stable(Silk II dominated)silk fibroin matrix.The PS nanoparticles have self-assembled into homogeneous amorphous structures in the composite films(10-50 wt%of PS nanoparticles).The films were transparent in dry state due to the low refractive index contrast between PS(nPS=1.59)and silk fibroin(ethanol treated silk,nsilk=1.55).However,after being immersed in water(nwater=1.33),the refractive index contrast was significantly increased due to the water infiltration in hydrophilic silk fibroin matrix.The colors of the composite films were angle-independent and could be tuned by adjusting the size of PS nanoparticles and immersed liquid media.In addition,the composite films exhibited rapid color change in water within 80 s.Moreover,the films could be erased and rewrote multiple times without significant color loss and structural damage.Such bioinspired photonic coatings which prepared in a simple way have showed good reproducibility,offering potential applications for refractive index indicating,identification marking and anti-counterfeiting purposes.(3)Stretchable and breathable silk on-skin electrodes have been developed through the customization of the morphologies and properties of biocompatible silk fibroin.The silk electrodes were prepared by simply coating conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate(PEDOT:PSS)on electrospun silk fiber mats.Glycerol was used for three purposes:Firstly,glycerol induced the change of silk from random coil dominated secondary structure to ? sheet,rendering silk with high stability in water which could make it applicable in subsequent solution processing.Secondly,glycerol absorbed water from ambient environment and plasticized silk.Thirdly,glycerol could lead to a structural reorientation of PEDOT:PSS chains,and then increase the conductivity of silk electrodes.The silk electrodes also showed high electric stretchability and good durability,and the resistance change was stable under 10-30%cyclic strain for 100 times without structural damage.The silk electrodes possessed high breathability with lower thermal insulation and evaporative resistance compared with Au on SEBS polymer and commercial gel electrodes.The water vapor transmission rates were also larger than the transepidermal water loss of the skin both under normal and exercise state.Moreover,the silk electrodes were conformal on skin,and the interfacial impedance was comparable to that of commercial gel electrodes.Electrocardiograph(ECG)recording in calm and sweaty situations showed that the signal quality of silk electrodes was as good as that of commercial gel electrodes before exercise,but it was much better than that of commercial gel electrodes after exercise.