The Optimized Preparation And Application Of Electrospun Aligned Nanofibers On The Basis Of Surface Tension

As a simple method to produce nanofibers, electrospinning is attracting more and more attention from the scholars. Electrospun nanofibers have higher specific surface area and larger surface-to-volume ratio, possess a lot of pores. These advantages endow the electrospun nanofibers improved performance and wide range of applications, including composite materials, filtering membrane, sensor drug release and tissue engineering. However, the random electrospun nanofibers have some disadvantages such as inferior mechanical properties, and these disadvantages would limit their application. On the other hand, electrospun aligned nanofibers possess higher mechanical properties, additionally, aligned nanofibers have anisotropic morphology and hydrophilicity, thus would target drug release and guide the orientation of cell growth. Hence, aligned nanofibers have wider range of application in drug release and tissue engineering. In this study, we give force analysis of infinitesimal jet firstly and study the influence of solution surface tension on the jet formation and jet stretching. Secondly, study the jet movement and mechanism of collecting oriented nanofibers on the basis of jet stretching analysis, design equipment that can control jet movement and collect aligned nanofibers and then produce aligned nanofibers using the designed equipment. Finally, seed vascular cells on aligned nanofibers and study the application of aligned nanofiber on vascular tissue engineering.During this study, we analyze the forces exert on the electrospun jet according to the mass conservation, charge conservation and momentum equation, investigate the effects of solution surface tension on the jet formation and jet stretching theoretically. The relationship between surface tension of solution γ and critical voltage Φc is Φc～γ. The relationship between jet diameter R and the vertical distance of jet away the interception point z was R～CZ-1/2+C1.C is a constant related to surface tension of solution. To further confirm the effects of solution surface tension on electrospinning, we added different content of anion surfactant into PVA solution to decrease the surface tension of solution. Experimental results showed that the critical voltages are proportional to surface tension of solution, reduced surface tension would result in decreased critical voltage. In addition, under different surface tension, the relationship between jet diameter and the vertical distance are R～z-1/2This result indicated that the solution surface tension have no influence on jet stretching, however, the surface tension have a great impact on jet diameter, reduced surface tension would lead to decreased jet diameter. To further investigate the effect of surface tension on morphology and properties of nanofibers, we added four different kinds of surfactant (cationic surfactant, anionic surfactant, anionic surfactant, amphoteric surfactant) with different content into PVA solution. Surface tension of the PVA solution decreased significantly when the surfactant content was less than1%. The solution viscosity increased with the increasing of cationic and anionic surfactant content. The electric conductivity of PVA solutions increased with surfactant content increasing, except for the PVA solution with non-ionic surfactant. Additionally, the average diameter of nanofibers decreased significantly when surfactant were added into PVA solution, especially the fiber diameter of PVA remarkably decreased from405to100nm as1%non-ionic surfactant was added into PVA solution. Moreover, the thermal properties and crystallinity of nanofibers are also improved with the addition of surfactants.On the basis of jet stretching analysis, we utilized Maxwell equation and Coulomb force between charges to study the mechanism of nanofiber orientation. In the process of electrospinning, the distorted electric field generated by the pair of electrodes would change the electrostatic potential distribution and the direction of coulombic force. According to the lowest energy principle, nanofibers would arrange perpendicularly to the electrodes. The coulombic force generated by the deposited fibers would pull the fiber’s movement and enhance the nanofiber orientation. Based on the theoretical analysis, we design two new kinds of devices including parallel metal plate device and cylinder-electrodes devices to collect aligned nanofibers. Two parallel metal plates is equivalent to a capacitor, the two metal plates were connected to the same negative voltage, in the electrospinning process, the parallel plates would have identical stretching force to the jet, hence the nanofibers would oriented themselves vertical to the parallel plates. The cylinder-electrodes device can increase the collecting-area, it can also collected aligned nanofibers with hierarchical structure, that is the aligned nanofibers have different oriented direction. Finally, we used a software named Ansoft Maxwell to simulate the electric field distribution for the parallel metal plates device and cylinder-electrodes device, further confirm the nanofibers orientation on these two kinds of devices.Many types of tissue in the body, such as nerve, muscle and ligament, have highly organized microstructure. In vascular tissue engineering, the key feature of arterial microarchitecture is the alignment of smooth muscle cells (SMCs) elongating their axis towards the circumferential direction of the medial layer, aligned polymer nanofibers can be used to take on the role of natural ECM fibers to provide mechanical strength, sites for cell attachment, modulation of cell behavior via morphological cues. This study produced pure polyurethane (PU) and composite polyurethane/collagen (PU/Coll) nanofibers with different morphology (randomness and alignment) for vascular scaffolds, and investigated the impact of aligned fiber morphology on orientation of cell growth. Due to the stretching force loaded on the fiber, produced by the rotating drum during the electrospinning process, the aligned nanofibers possess decreased fiber diameter compared to random nanofibers. In addition, the aligned nanofibers showed anisotropic wetting characteristics and mechanical properties, matching the anisotropic behavior of the native artery. Vascular smooth muscle cells (SMCs) were cultured on electrospun PU and PU/Coll nanofibers. Cell experiment results showed that insertion of collagen into PU enhanced cell proliferation. In addition, aligned PU/Coll scaffolds can greatly promote SMC orientation through contact guidance. Moreover, SMCs grown on aligned PU/Coll nanofibers showed strong, uniform, aligned SMA and MHC expressions. All in all, aligned PU/Coll nanofibers have reduced fiber diameter, anisotropic wetting characteristics and mechanical properties, can guide cell orientation, enhance protein expression, are better choices for vascular scaffolds.Vascular endothelial cell (EC) is another important cell that exists in blood vessel wall, seeding EC on vascular scaffolds will endow the scaffolds to prevent thrombus and enhance graft survival. However, matured ECs are not always available and have limited proliferation ability. On the other hand, Mesenchymal stem cells (MSCs) are multipotent stem cells and they have the ability to differentiate into many mesodermal lineages cells, and under suitable environment in vivo or in vitro, they can differentiate into vascular ECs. Hence, investigate the differentiation of MSCs into ECs have a great significance for vascular tissue engineering. During this paper, we produced poly (L-lactic acid)(PLLA) and PLLA/Coll nanofibers with different morphologies (randomness and alignment), and study the impact of aligned fiber morphology on the differentiation of MSCs into ECs. Aligned PLLA/Coll nanofibrous scaffolds possess reduced fiber diameter, anisotropic hydrophilicity and mechanical properties, are more mimic the microstructure of native ECM in vascular wall. MSCs were cultured on electrospun PLLA and PLLA/Coll nanofibers and the differentiation of MSCs into ECs were investigated through cell morphology and protein expression. Cell morphology results showed that MSCs grown on aligned PLLA/Coll scaffolds have differentiated into ECs and expressed ECs’cobblestone morphology, are similar to the native ECs in blood vessel. Moreover, MSC differentiated cells on aligned PLLA/Coll (1:1) scaffolds expressed proteins specific of endothelial cells such as the platelet endothelial cell adhesion molecule-1(PECAM-1or CD31) and Von Willebrand factor (vWF). From the results of cell morphology and protein expression studies, we concluded that the aligned PLLA/Coll nanofibers could mimic the native vascular ECM environment and may be promising substrates for potential application towards vascular regeneration.Aligned nanofibers have wider range of application because of their better mechanical properties, unique fiber morphology and anisotropic hydrophilic performance. In this paper, we study the mechanism of nanofiber orientation during electrospinning process, design new device through introducing auxiliary electrodes to collect aligned nanofbiers. Culture vascular cells on aligned nanofibers and study the application of aligned on vascular tissue engineering.