Tunable Exfoliation Of Single Layer Of Silk Nanoribbons And Construction Of Flexible Silk Based Energy Devices

With the development of new-generation biomedical electronics,more and more implantable devices,such as heart pacemaker,brain sensor,and dorsal column stimulator,have been used in biomedical field.However,there are still existing many problems,for example,inflammatory response,external power source,and second surgery.Therefore,the research about implantable self-powered electronic devices and wires with biocompatibility,and biodegradability has significant meaning.Ttriboelectronic nanogenerator (TENG) can convert the mechanical energy to electrical energy,and power other electric devices.However,it's a challenge to fabricate the fully biodegradable bio-TENG with excellent output performance,and high sensitivity.Silk,a biodegradable natural polymer with excellent biocompatibility,and mechanical properties.Silk has complex hierarchical structures,such as,?-crystalline and fibrillar structures.The nanoscale fibrillar structure plays an important role in the mechanical properties.Silk nanofibrils(SNF) maintain the mesoscale structure and outstanding properties of natural silk.Hence,there are advantages to use SNF for the fabrication of silk-based materials and implantable devices.However,the existing SNF fabricating methods have many disadvantages,such as high toxic solvent system,unstable aqueous suspension,and SNF based materials with low properties.In view of the above problems,the following research was carried out.Firstly,a low temperature solvent system and a oxidization system for cellulose were used to dissolve degummed silk(DS) and fabricate single layer of silk nanoribbons(SNRs) with the same thickness of single?-sheet layer.The new hierarchical structure was presented.Secondly,silk based TENG was fabricated basing SNR film(SNRF) and regenerated silk fibroin film(RSFF) with different crystalline structure.The working principle and the output performance were investigated.Thirdly,SNR and konjac glucomannan composite film (SKCF)-aurum(Au) wire was prepared by using vacuum filtration and thermal resistance evaporation,and integrated with SNRF/RSFF TENG to explore the electrical transmission efficiency of SKCF-Au wire.Finally,the biocompatibility of films and the biodegradability of devices were investigated.The single layer of SNRs were extracted from natural Bombyx mori silk by partially dissolving DS in NaOH/urea solution at -12?.A nontoxic solvent system was developed to prepare stable SNF suspension via solving the problems of low stability of SNR suspension and high toxicity of solvent system.The solubility increases with the increasing of NaOH content within certain limits.The highest yield of SNR is about 21.4%.The SNR suspension has a negative charge.This stable SNR suspension can be stored at 4? more than 6 months.With the increasing weight ratio of NaOH to urea,the?-sheet content and crystallinity of freeze-dried suspension are found to decrease,illustrating the decreasing ratio of SNR to RSF.When the total weight of NaOH and urea is constant,with the increasing weight ratio of NaOH to urea,the width and thickness of SNR decrease.When the weight ratio of NaOH to urea is 0.19,and the total weight of NaOH and urea is 19g,the width and thickness are about 21.8nm,and 0.44nm,respectively.The similar thickness about 0.4nm is also obtained from small-angle X-ray scattering result.The thermostability curve,species and ratio of amino acids of SNR are accordance with those of DS.The computer simulation result shows that the single?-sheet layer is easily maintained in the NaOH/urea solvent system.Hence,the SNR may consist of many single?-sheets and amorphous structure.In order to further prove the existence of single layer of SNR,SNR was obtained by oxidizing DS using 2,2,6,6-tetramethy-pipelidine-l-oxyl radical(TEMPO)/sodium bromide(NaBr)/sodium hypochlorite(NaClO)system.SNR has a negative charge and can be stored at 4? under stable conditions for more than 13 months.As the NaClO content increases(while the TEMPO and NaBr concentrations remain fixed),the?-sheet content and crystallinity of SNR increase,and the width and thickness of SNR decrease.The width and thickness of SNR are about 20.2 nm,and 0.4 nm,respectively,using 0.02g TEMPO,0.1g NaBr,and 0.016 mol NaClO pergram DS.According to the dimension of SNRs derived from two solvent systems,a new hierarchical structure model was proposed.The SNF is composed of the assembly of SNR having a uniform thickness about 0.4 nm.The SNR contains?-crystalline and amorphous phase.Compared with NaOH/urea solvent system,TEMPO/NaBr/NaClO solvent system has many advantages,such as energy saving,simpler experiment operation,more stable suspension,and higher yield of SNR.Therefore,SNR fabricated with TEMPO/NaBr/NaClO system was used to prepare flexible energy devices.Due to the different secondary structure,crystallinity,and work function of SNRF and RSFF,they were used as friction layers to fabricate silk based TENG with excellent output performance,and biodegradability,improving the output performance and sensitivity of bio-based TENG.During the process of applying and removing external force,the electrons transferred from SNRF to RSFF,and electric energy generated.The SNRF loses electrons and becomes positively charged,and the RSFF gains electrons and becomes negatively charged.As the thickness of SNRF increased,the output voltage of SNRF/RSFF TENG increased firstly and then decreased.When the SNRF thickness is 37?m,the output voltage reaches highest voltage.The upward compressing output voltage remains constant while the downward releasing output voltage gradually decreases with increasing frequency.At a constant frequency of 5 Hz,the compressing output voltage generally tends to increase with increasing press force.The highest output voltage and power density are 41.6V,and 86.7 mW m^-2,respectively,under a 100M? load resistance.The SNRF/RSFF TENG has high sensitivity,and can realize real-time monitoring of different parts of the body,such as pulse.Compared with other fully biodegradable TENGs,SNRF/RSFF TENG has greater output performance and higher sensitivity.The friction layers,SNRF and RSFF,have good biocompatibility.In vitro biodegradation test of SNRF/RSFF TENG was conducted in PBS at 37?.The biodegradable mass is about 66% after 114d.The SKCF-Au wire was prepared from natural polymer and chromium(Cr)/Au.The silk based biodegradable energy device was integrated.Konjac glucomannan(KGM) contributes to the elongation at break of SKCFs in dry and wet condition.S8K2 has an excellent toughness of 0.6MJ m^-3in wet condition.In order to prepare a conducting wire,S8K2 was coated with a thin metal layer of Cr/Au with a thickness of 10/140 nm,respectively,by evaporation and then cut into wires with 1 mm in width.The evaporated Au contributes to the improvement of the strength of SKCF-Au wire,and the slight decrease of the elongation at break and toughness.This SKCF-Au wire exhibites a resistance of approximately 8? per centimeter.The impedance of SKCF-Au film at 1000 Hz remains largely unchanged at 140? after immersing in the PBS for different time.The square resistance of the film(5×5 mm²) was 20–24? per square after immersion in PBS for up to 10d.When SKCF-Au wire was connected with SNRF/RSFF TENG,the highest output power density was 314.3 mWm^-2.The output power density of the TENG constructed with the SKCF-Au wire was 2.8–4.7 times larger than that of the TENG constructed with Cu wire,Cu foil wire,or Al foil wire.The result showed that this conducting wire could effectively transfer the generated power.Cells have similar proliferation,attachment,and growth on the SKCF,and SKCF-Au,reflecting good biocompatibility.After immersing in protease solution,the degradation quality of SKCF-Au wire and integrated device is 45% and 33%,respectively.The weight ratio of Au to the integrated device is less than 0.07%.Therefore,the biodegradable percentage of integrated energy device is over 99.9%.And the integrated energy device can be thought as fully biodegradable device.Two solvent systems were proposed to tunably exfoliate DS and prepare stable SNR suspension.According to the size and structure of SNR,a new silk hierarchical model was created,having direct meaning in exploring silk-based materials and devices with excellent properties.The fully biodegradable silk-based TENG prepared using SNR has outstanding output performance and realizes the pulse-driven self-power.The biodegradable natural polymer based SKCF-Au wire has high electrical transmission property.It was connected with SNRF/RSFF TENG to construct all biodegradable self-powered device,showing promising application potentials in the fields of flexible wearable materials and biomedicine.