Analysis Of Influencing On The Strength Of Silk From Bombyx Mori

As an important natural protein fiber that can be produced on a large scale,silk has been developed and widely used in textile materials,biomedical materials,flexible electronics and other fields due to its excellent mechanical properties,good biocompatibility and low immunogenicity.Strength is an important index to evaluate the performance of silk.How to improve the strength of silk is a hotspot in raw silk biology research.At present,measures to improve the strength of silk mainly include silkworm species selection,sericin treatment,rearing environment improvement,artificial physical modification,gene modification and feeding functional materials.Due to the limitations of the methods,there are not many applications in production.Studies have shown that the filament size and strength of silk have a certain inherent relationship.For example,fine-denier silk is stronger than ordinary silk,the strength of silk of trimolter is greater than that of four-moulter,and silk of young larva is stronger than mature larva.In these studies,in addition to filament size,it also includes many variables such as silkworm species,exogenous substances(such as antijuvenile hormone),and the developmental stage of tissues and organs.In order to in-depth study of the influencing factor of silk strength,the present study used mulberry leaves,mulberry leaves+artificial feed and artificial feed to rear the same silkworm species,and silks with three filament size was obtained from the same species,in which the filament size is a single variable.The influence of filament size on the strength of silk was analyzed;the reason for the formation of high-strength silk was explored and verified by analyzing the fibrillation process of silk;at the same time;at the same time,it was found that the strength of the silk is different at different positions,and in the current research on the strength of the silk,the strength is obtained by calculating the average value after randomly selecting small samples,which will cause a deviation in the determination of the strength of the silk.In view of this,a method for segmented testing of the mechanical properties of silk was proposed,and natural silk and modified silk were measured.The main results obtained are as follows:1.The effect of filament size on the strength of silkCompared with mulberry group(Mul group),the cocoon weight and cocoon shell weight of the silkworms reared by artificial diet were significantly reduced.Among them,the whole-instar artificial diet group(Ⅰ-Ⅴ group)decreased the most,1-3 instars artificial feed group(Ⅰ-Ⅲ group)followed(P≤0.05 or P≤0.001).Compared with the Mul group,the filament size of the silk in the Ⅰ-Ⅴ and Ⅰ-Ⅲ groups was significantly reduced(P≤0.001).The decrease in the filament size was caused by the decrease in the content of silk protein per unit length of the silk,which had nothing to do with its density.By analyzing the molecular conformation and secondary structure of the silk,it was found that the fine-denier silk has more β-sheet content,and the smaller the filament size,the higher the content.The analysis results of the crystallinity and molecular orientation of the silk showed that the fine-denier silk has higher crystallinity and molecular orientation.The test results of mechanical properties showed that fine-denier silk have higher strength and elongation.The strength of silk in Ⅰ-Ⅲ group was 6.22%higher than that in Mul group(P≤0.05).The strength of silk inⅠ-Ⅴ group increased by 21.87%(P≤0.001).The results showed that the filament size is an important factor affecting the strength of the silk.The smaller the filament size,the higher the strength of the silk.2.The degree of stretching determines the filament size and strength of the silkBy analyzing the content of 17 main amino acids of the silk,it was found that the amino acid composition is not the reason for the high crystallinity,high β-sheet and high strength of fine-denier silk.By analyzing the fibrillation process of silk,it was found that the difference in structure and properties may be caused by the different degree of stretching of silk with different filament size during the fibrillation process.The anterior silk gland lumens of the silkworms in groups Ⅰ-Ⅲ and Ⅰ-Ⅴ are smaller,which may restrict the flow of silk protein and cause the silk protein to be more fully stretched during the process of fibrosis.By measuring the filament size of the silk at different positions,it is found that the filament size of the silk increases to the maximum,and then gradually decreases to the end,indicating that the secretion rate of silk protein in the silkworm secretion process is also constantly changing.The calculation results of the degree of stretching on different positions of the silk showed that the change in the secretion rate of silk protein will affect the degree of stretching and caused the variation in filament size.Finally,the strength of different positions of the silk was measured,and it was found that the change trend of the silk strength along the length was the same as the degree of stretching,which verified the viewpoint that the degree of stretching determines the strength of the silk.3.Segmented measurement of silk strengthThere is variation in the strength of the silk at different positions,which has an impact on the related research on the strength of the silk.The deviation of the measurement of the silk strength will affect the accuracy of the evaluation of the silk strength modification effect.In view of this,modified silk was obtained by feeding silkworm with two doses of graphene oxide(GO)of 1‰(1 g/L)and 2‰(2 g/L).Compared with ordinary cocoons,the cocoon shape,total cocoon weight and cocoon shell weight of the GO treatment group were not significantly different,and the surface characteristics of the silk were basically the same.The structural characterization results showed that the feeding of GO will cause the decrease of β-sheet content in the silk fiber,the increase of α-helix or random coils structure,and the increase of the thickness of the mesophase region.The mechanical properties of natural silk and GO modified silk were measured segmented by segmented.The results showed that the mechanical properties of the two kind of silk varies in a wide range along the length.Their strength and Young’s modulus gradually increase along the length,and the load and elongation gradually decrease along the length.The last 20%of the 1‰ GO modified silk has the highest strength.In summary,the present study proved that filament size is a key factor affecting the strength of silk;it was proposed and verified that the degree of stretching of silk during the fibrillation process is the main factor that determines the filament size and strength of silk;The strength of the silk was measured segmented by segmented,which improved the accuracy of strength evaluation and the distribution area of the highest strength in the silk was found.The research results provide new ideas for the development of high-strength raw silk which is helpful for improving the strength of silk.