Design And Analysis Of Composite Nanofibrous Mats For Tissue Engineering Scaffold Based On Silk Fibroin

In recent years,with the rise of tissue engineering,the research on cell scaffold material has become the focus of the researchers.Silk Fibroin?SF?is a natural polymer material,which not only has good biocompatibility,biodegradability,lower immunity and tissue reaction,no toxic and no irritation,but also owns good environmental stability,reproducibility as well as unique physical and chemical properties,so it is widely used in biomedical field.The electrospinning SF nanofiber scaffold has a very high specific surface area and can better simulate the reticular structure of extracellular matrix?ECM?in morphology,which is conducive to cell adhesion,proliferation and migration,and which is most suitable to be used as the scaffold material for tissue engineering cell growth.However,the molecular structure of SF nanofiber obtained by electrospinning is mainly characterized by?-helix and random coil,so it belongs to an amorphous structure and it has low crystallinity.Meanwhile,it is water-soluble,brittle and hard,and its mechanical properties are poor.Therefore,the wide application of SF nanofiber scaffold for tissue engineering cell growth is limited.The mechanical properties of scaffold materials are the key that the scaffold plays a role in tissue engineering and regenerative medicine.As far as the microscale is concerned,the nanofiber in the scaffold must have enough strength and toughness to support the adhesion,growth and migration of the cells and the deposition of ECM.At the same time,as far as macroscale is concerned,nanofiber membrane scaffold must have mechanical properties which can be matched with the substitute tissue.Therefore,the construction of cell scaffold is a challenging work.In view of the existing relevant theories of polymer structure and performance,the mechanical properties of nanofiber scaffold are determined by the single fiber properties of the scaffold materials and the microstructure of the scaffold.SF blends with other biocompatible and biodegradable polymers,such as polycaprolactone?PCL?,can adjust the mechanical properties of the fiber through the synergistic effect of the polymer blend system,without affecting the biological properties.For specific substitute tissues,from the fibers that make up the scaffolds to the fiber membrane scaffold as a whole,the mechanical properties of the scaffolds have certain requirements.Obviously,in order to realize the construction of ideal tissue engineering scaffolds,the performance and structure of the fibers as well as the mechanical properties of the scaffold should be regulated.For traditional materials,the continuous scale is usually close to the molecular level,and the continuous scale of the electrostatic spinning fiber membrane scaffold material is close to the micron level.When the integration of nonlinear fiber stress and strain response,it can cause a complex response in macro-scale of the samples,but the classical material mechanical model is no longer applicable.Therefore,it needs to establish such scaffold structure tensile mechanical relation model which is composed of fiber network to characterize and predict the mechanical properties of scaffold materials.Therefore,in this thesis,SF/PCL composite nanofiber membrane was prepared by using electrospinning technology through the synergistic effect of polymer blend system and taking SF and PCL as materials.The orthogonal experimental design and multi-index normalization method were adopted to analyze the impact of solute concentration,solute mass ratio and spinning flow rate on the structure and mechanical properties of SF/PCL composite nanofiber membrane,and the optimal spinning process of SF/PCL composite nanofiber scaffolds was obtained.Based on this,the Agilent T150 micro/nanofiber tensile machine was adopted to test the mechanical properties of SF/PCL nanofiber and study the mechanical properties of SF/PCL composite nanofiber membrane under the condition of different tensile states.Then the modulation laws of the structure and performance of SF/PCL composite nanofiber membrane were revealed,and the tensile mechanical model between single fiber and fiber membrane scaffold was established.Meanwhile,according to the microstructure characteristics of nanofiber membranes,Matlab and Abaqus software were adopted to establish a three-dimensional stretch finite element?FE?model of SF/PCL composite nanofiber membranes.The study will provide the theoretical and experimental basis for the deep understanding of the relationship between the composition,structure and performance of SF blend system and the mechanism of mechanical properties change,the prediction of tensile mechanical properties of nanofiber scaffold,as well as the research and development of SF nanofiber tissue engineering cell scaffold with practical use value.Meanwhile,it is of great significance for promoting the application of SF in tissue engineering.The main conclusions of this thesis are as follows:?1?The optimal process of electrospinning SF/PCL composite nanofiber membrane:environment temperature 25±2?,relative humidity 35±5%,solvent hexafluoroisopropanol?HFIP?,solute concentration 6%,solute mass ratio(W_SF/W_PCL)3:2,spinning voltage 15 kV,spinning distance 12 cm,spinning flow rate 1.2 ml·h^-1.?2?SF/PCL composite nano single fiber presented an elastoplastic mechanical response in the stretching process.The linear elastic stage of the curve accounted for a very small part.Then,the fiber entered the plastic stage,and the stretch curve showed an obvious segmentation.The nonlinearity of the tensile deformation of single fiber must be considered in the case of fiber network structure composed of single fiber.?3?When the fiber receiver roller didn't rotate,the distribution of SF/PCL composite nanofiber was generally uniform,with the diameter of fibers around 300 nm.The tensile failure mechanism of the fiber membrane was mainly the irreversible re-orientation arrangement and damage failure of SF/PCL composite nanofiber.The SF/PCL composite nanofiber membrane showed a good fatigue resistance in the small strain cyclic stretching process,so it is an ideal material for tissue engineering scaffold.When the fiber receiver roller rotated,the rotating speed had no obvious effect on the diameter of the fiber.The pore size and distribution of fiber membrane were related to the orientation distribution of nanofiber.The higher the orientation degree of fiber layer was,the closer the arrangement between fibers would be,and the smaller and more concentrated the pore size of fiber membrane would be.When the speed of the roller was lower than 2.38 m·s^-1,the fibers were irregularly and randomly distributed.When the speed of the roller was 11.88 m·s^-1,the orientation arrangement degree of the fibers along the rotational direction of the roller?MD?reached the maximum,and the composite nanofiber film presented an obvious mechanical anisotropy.When the SF/PCL composite nanofiber membrane of fiber orientation distribution stretched in the uniform biaxial direction,the increase of the roller speed made the tensile curve of the fiber membrane along with the orthogonal direction of the roller's rotation direction?CD?become smooth.At the same time,the initial modulus,yield stress and fracture stress of the fiber membrane reduced,and the situation of MD was on the contrary,which was consistent with the variation trend of fiber membrane during uniaxial stretching.?4?The tensile mechanical properties of the fiber membrane were mainly determined by the following structural parameters:length-width ratio of samples,porosity,fiber diameter and fiber curvature.There was a quantitative relationship between the mechanical properties of the single fiber material and the uniaxial tensile mechanical properties of the fiber membrane.For non-uniformly distributed bending fiber,the stretching stress of the fiber membrane only needs to introduce a fiber distribution probability density functionin the fiber membrane tensile stress relationship in which the fibers are uniformly distributed.If there is no shear deformation for the fiber membrane in the biaxial stretching process,the study on the tensile mechanical properties can be completely replaced by the uniaxial stretching study.?5?Matlab numerical analysis software and Abaqus FE software were adopted to establish the simulation model of three-dimensional network structure SF/PCL composite nanofiber membrane stretching under the two situations?the fiber's random distribution and orientation distribution?.The elastic modulus of the model is negatively correlated with the structural parameters such as length-width ratio,porosity and fiber curvature;In the case of uniaxial stretching,the simulation analysis values,experimental values and theoretical values of the FE model of the uniformly distributed fiber have been well verified.In the case of biaxial stretching,the simulation results of the FE model of uniformly distributed fiber verified the correctness of the theoretical analysis of biaxial tensile mechanics?The study of biaxial stretching of fiber membrane can be replaced by uniaxial study without considering the shear deformation in the stretching process?.Meanwhile,the simulation results showed that the tensile modulus of the fiber membrane was inversely proportional to the width of the tensile loading terminal,and the effect of porosity on the tensile modulus was the same as that of uniaxial tension,namely,the larger porosity of fiber membrane was,the lower tensile modulus was.In the case of small strain,the simulation analysis values of SF/PCL composite nanofiber membrane with biaxial tensile of fiber orientation distribution were better consistent with the experimental values.The innovation points of this thesis mainly include the following aspects:?1?The relationship between macroscopic mechanical behavior and microstructure of electrospinning SF/PCL composite nanofiber scaffolds was studied,and the tensile mechanical model between single fiber and fiber membrane was established,which provided theoretical basis for the design of mechanical properties of electrospinning nanofiber membrane materials for tissue engineering scaffolds.?2?Based on the actual microstructure of electrospinning nanofiber membrane,the uniaxial and biaxial stretching elastoplastic FE model of SF/PCL composite nanofiber membrane was developed by simulating one nanofiber?random distribution or orientation distribution?with interlaced beam elements.?3?The mechanical tensile deformation of SF/PCL composite nanofiber membrane was numerically simulated by using FE tensile model,the effective regulation of the structure and mechanical properties of the electrospinning nanofiber membrane scaffold for tissue engineering was realized.