【作者】 贾琳；

【导师】 覃小红；

【作者基本信息】 东华大学 ， 纺织材料与纺织品设计， 2013， 博士

【摘要】 静电纺丝是制备纳米纤维最简单易行的方法,静电纺纳米纤维具有较大的比表面积,较高的孔隙率以及可控的纳米纤维直径等特征。因此受到了越来越多的关注,被应用到增强复合材料,过滤膜,传感器,药物缓释和组织工程等各个方面。但是目前应用的随机排列的纳米纤维强度较低,在很大程度上限制了纳米纤维的应用。静电纺取向纳米纤维可以有效地提高纳米纤维的机械强度,而且取向的纳米纤维形态可以引导药物和蛋白等的定向释放,引导细胞的定向生长,在药物释放和组织工程方面有更加广阔的应用。本文在前人研究的基础上,分析静电纺过程中射流的受力,研究溶液的表面张力对静电纺射流的形成和拉伸的影响。然后在射流拉伸分析的基础上,研究射流的运动,并通过自行设计的装置控制射流的运动,制备取向的纳米纤维。最后研究取向的纳米纤维在血管组织工程方面的应用。本文首先利用质量守恒定律,电荷守恒定律和动量守恒定律分析静电纺射流的受力,从理论方面研究溶液的表面张力对静电纺射流形成和拉伸的影响。结果表明溶液的表面张力与射流形成的临界电压之间的关系为Φc～γ；射流的半径R与射流距离截取点之间的纵向距离z之间的关系为：R-Cz-1/2+C1其中C是关于溶液表面张力的一个常数。然后在聚乙烯醇(PVA)中加入了不同含量的阴离子表面活性剂,降低溶液的表面张力,从实验方面验证溶液的表面张力对射流形成和拉伸的影响。实验结果与理论分析的结果是一致的,即射流形成的临界电压与溶液的表面张力成一个正比例的关系,溶液的表面张力越小,射流形成的临界电压越小。溶液的表面张力对射流的拉伸没有影响,不同的表面张力下,射流的直径与纵向距离之间的关系都是R～z-1/2；但是溶液的表面张力对射流的直径有很大的影响,即溶液的表面张力越小,射流的直径越小。为了进一步研究溶液的表面张力对静电纺纳米纤维性能的影响,本文还在PVA溶液中加入了不同种类(阳离子,阴离子,非离子和两性表面活性剂)不同含量的表面活性剂,研究表面活性剂对溶液性质以及纳米纤维形态和性能的影响。结果表明当表面活性剂的含量小于1%时,PVA溶液的表面张力会随着表面活性剂含量的增加而减小；PVA溶液的粘度会随着阳离子和阴离子表面活性剂含量的增加而增加；除了非离子表面活性剂,在PVA溶液中加入其它三种表面活性剂都会有效地增加PVA溶液的电导率。另外,在PVA溶液中加入表面活性剂,可以有效地降低纳米纤维的直径,当加入1%的非离子表面活性剂以后,纳米纤维的直径从405nm降低到了100nm。而且纳米纤维的热学性能和结晶度都随着表面活性剂的加入而提高了。在分析射流拉伸运动的基础上,本文利用麦克斯韦电场方程以及电荷之间的库伦斥力研究纳米纤维取向排列的机理。平行电极对纳米纤维产生的附加电场改变了接收装置的静电场分布,根据最低能量定理,纳米纤维最终将沿着与电极垂直的方向取向排列。由于沉积下来的纳米纤维还带有一些电荷,已经沉积的纳米纤维对刚沉积下来的纳米纤维有一个库伦斥力,纳米纤维之间存在的库伦斥力进一步增加纤维的取向排列。在理论分析的基础上,改进现有的静电纺设备,通过引入辅助电极,自行设计了平行金属板和滚筒平行电极等取向纳米纤维的收集装置。两块平行排列的金属板相当于一个电容,将两块金属板接相同的负电压,在静电纺过程中,两块金属板会同时对射流有一个相同的拉伸力,纳米纤维最终将沿垂直于金属板的方面取向沉积下来。而滚筒平行电极组合收集装置不仅可以提高纳米纤维的收集面积,而且通过改变滚筒和平行电极的位置,还可以制取取向方向不同的分层的取向纳米纤维。利用平行金属板收集装置和平行电极滚筒组合的收集装置进行静电纺丝,收集取向排列的PVA和聚乳酸(PLLA)纳米纤维。最后本文还利用Ansoft Maxwell电磁场模拟软件,模拟平行金属板装置和滚筒平行电极组合装置的电场强度分布,从电场方面验证了纳米纤维在平行金属板收集装置和滚筒平行电极组合收集装置中的取向排列。人体的许多组织,例如神经组织,肌肉组织和心脏组织中,其微观结构都是高度取向的。在血管组织工程中,平滑肌细胞(SMCs)沿血管壁中间膜的取向排列是本体动脉血管的一个关键特征,取向的聚合物纳米纤维可以被用作细胞外基质(ECM)的替代材料,为细胞的粘附生长提供支架材料和机械强度,并通过纳米纤维的形态调节细胞的生长行为。本论文利用静电纺丝的方法制备非取向和取向排列的聚氨酯(PU)和混合的聚氨酯/胶原蛋白(PU/Coll)纳米纤维,用作血管组织工程的支架材料,研究取向纳米纤维对引导细胞取向生长的作用。由于静电纺过程中旋转的滚筒所产生的拉伸力,取向的纳米纤维的直径略小于非取向的纳米纤维,取向的纳米纤维还具有各向异性的亲水性能和力学性能,其性能特征与血管ECM的微观结构更相似。在PU和PU/Coll纳米纤维表面培养SMCs,结果表明：胶原蛋白的嵌入有效地提高了SMCs的增殖率,取向的PU/Coll纳米纤维通过纤维的接触引导作用促使了SMCs的取向生长,且SMCs的延伸的纺锥形态与体内的SMCs的纺锥形态更接近。培养在取向的PU/Coll纳米纤维表面的SMCs表现出了较强的,均匀的,取向的SMA和MHC蛋白表达。取向的PU/Coll纳米纤维具有更小的直径,各向异性的亲水性和力学性能,还可以促使SMCs的取向生长,提高SMCs的蛋白表达,是更好的血管组织工程支架材料。血管内皮细胞(ECs)是血管壁中一种非常重要的细胞,在血管支架材料表面培养内皮细胞(ECs)将赋予了支架的抗血栓性,有效地阻止血管支架的血栓形成,提高血管移植物的成功率。但是ECs是一种成熟的细胞,其增殖能力有限,间充质干细胞(MSCs)是一种多功能干细胞,具有强大的增殖能力和多向分化潜能,在适宜的体内或体外环境下可分化为血管内皮细胞。因此研究MSCs向ECs的分化对于血管组织工程具有非常重要的意义。本文制备取向的聚乳酸(PLLA)和混合的聚乳酸／胶原蛋白(PLLA/Coll)纳米纤维,进一步研究取向的纳米纤维形态对MSCs向ECs的分化的影响。取向的PLLA/Coll纳米纤维具有更小的纳米纤维直径,各向异性的亲水性和机械性能,与血管壁ECM的微观性能更接近。在非取向和取向纳米纤维支架表面培养MSCs,发现培养在取向的PLLA/Coll纳米纤维表面的MSCs表现出来取向的鹅卵石形态,与体内的内皮细胞的形态最接近。蛋白染色结果表明培养在取向纳米纤维表面的MSCs显现出了更强的ECs的特征蛋白CD31和vWF,取向的纳米纤维形态有效地提高了MSCs的增殖率和向ECs方向的分化。较好的机械性能,特殊的纳米纤维形态以及各向异性的亲水性能等赋予了取向纳米纤维更广的应用范围,本文首先从理论上分析静电纺射流的受力,控制射流的运动,进而通过自行设计的接收装置制备取向的纳米纤维,并将取向的纳米纤维应用到血管组织工程中,为进一步扩宽取向纳米纤维的应用奠定了基础。更多还原

【Abstract】 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.更多还原