Preparation Mechanism And Application In Biomedical Engineering Of Electrospun Uniaxially Aligned Nanofiber Yarns

While the global competition is increasing dramatically, the textile industry needs more innovative technology to produce high value-added products. In the traditional textile industry, fiber and its products have been used for thousands of years, but the diameters of these fibers reach to at least several microns or more. With the development of science and technology, nanotechnology has attracted a great deal of attention in the past few decades, which can fabricate fibers with diameters of roughly 1 to 1000 nm. As known that nanometer scale can endow the material with several amazing characteristics such as very large surface area to volume ratio and high aspect ratio and so on. Therefore, the nanotechnology may bring a new way to update traditional textiles for traditional textile industry.Electrospinning has been recognized to be the simplest method to produce nanofibers from polymer solutions or melts in terms of its versatility, flexibility and ease of fiber production. Electrospun nanofiber yarns can be easily applied into textile techniques like braiding, weaving and knitting, et al. Recently many modified electrospinning approaches have been reported to obtain nanofiber yarns, but further improvements are still needed in terms of high fiber alignment, more fiber production, uniform evenness, enough long yarn length and excellent mechanical properties. In this thesis, we designed and implemented a novel and modified electrospinning system to efficiently fabricate continuous uniaxially aligned nanofiber yarns(UANYs) in the laboratory level. The mechanism of the novel electrospinning system was investigated based on theory and experiments. The effects of the different solution properties and spinning parameters on yarn formation and physical characteristics of the nanofibers and the yarns were analytically investigated. The UANYs were fabricated into 3D heterogeneous constructs using textile techniques, i.e. braiding, weaving, and knitting, demonstrating the feasibility and versatility for designing 3D nanofibrous fabrics with tunable multi-scale structure and material properties. Finally, we applied the UANYs and its fabrics into biomedical engineering field, such as tissue engineering and bioactuators.In Chapter 2, according to the mechanism of conjugated electrospining, electrostatic induction and point discharge, a modified electrospinging setup, mainly composed of two oppositely placed metal needles, oppositely placed metal disc and hollow metal rod, was patented to manufacture UANYs. We employed a software named Ansoft Maxwell to simulate the 3D electric field distribution of our device, and the influences of different needle configuration and voltage regulation mode on electric field distribution were investigated. The formation mechanism of spinning triangle cone was also analyzed, and the effects of different needle configuration and voltage regulation mode on the stability of spinning triangle cone were studied. It was found that the best needle configuration and voltage regulation mode was that the two metal needles were oppositely placed and were applied applied with positive and negative voltages respectively, and the placed metal disc and hollow metal rod are neutral, and not grounded. Several different polymers, i.e. polyacrylonitrile(PAN), polycaprolactone(PCL), polyvinylidene fluoride(PVDF) and polyurethane(PU), were employed to fabricate yarns successfully, which supports the versatility and feasibility of the designed electrospinning system. Importantly, all the polymer yarns possessed highly uniaxially aligned structure and uniform yarn evenness. Moreover, our device could be modified to fabricate nanofibers coated yarns. The traditional textile yarns or filaments were used as core yarns and coated with functional polymer nanofibers, which can add specific properties for traditional yarns or filaments and produce high value-added products.In Chapter 3, PAN was chosen as a model polymer to systematically investigate the effects of spinning parameters, including polymer solution concentration, applied voltage, distance between two needles, guiding distance between neutral metal disc(NMD) and neutral hollow metal rod(NHMR), solution flow rate, rotation speed of NMD, addition of single-walled carbon nanotubes(SWCNTs) on yarn fabrication process, yarn morphology(i.e. fiber alignment, fiber diameter and yarn diameter) and yarn mechanical properties. With the increasing of PAN concentration, the solution viscosity increased and the diameters of both nanofibers and yarns increased. The applied voltage and rotation speed of NMD had very slight effect on diameters of nanofibers. The diameters of both nanofibers and yarns increased to a threshold value and started to reduce thereafter with the distance between two needles increasing. The diameters of both nanofibers and yarns presented the increasing trend at the higher flow rate. Meanwhile, the yarn diameters decreased with increasing the distance between NMD and NHMR and rotation speed of NMD. The distance between two needles, distance between NMD and NHMR, and rotation speed of NMD were found to be three main parameters affecting nanofiber alignment of the resultant yarns. Besides that, the nanofiber alignment apparently depended on the distance between two needles, distance between NMD and NHMR and rotation speed of NMD. With the distance between two needles or the distance between NMD and NHMR increasing, the nanofiber alignment was enhanced. Meanwhile, the nanofiber orientation became inferior at a higher rotation speed of NMD. Moreover, the increase of nanofiber alignment could improve the yarn mechanical property. The results also showed that the SWCNTs addition didn’t affect the nanofiber aligment. SWCNTs /PAN composite nanofiber yarns showed great morphology and were uniaxially aligned. With SWCNTs concentration increasing, the diameters of the composite nanofibers and yarns reduced, but the quantity of beaded nanofibers increased. The mechanical properties of composite yarns increased to a threshold value and started to reduce thereafter with the SWCNTs content increasing.In Chapter 4, some tissues in human being, such as smooth muscle, blood vessel, myocardium, and nerve, possess the characteristics of functional and mechanical anisotropy, due to their specific anisotropy structure. Therefore, an excellent tissue scaffold can not only provide cells with an optimal extracellular matrix(ECM) that mimics the structure and organization of natural ECM, but also provide a template to form a function- anisotropic tissue. UANYs may be perfect candidates to fabricate scaffolds for anisotropic tissues. We evaluated the cellular behaviors of human adipose derived stem cells(HADSCs) on UANYs. The generated UANYs had excellent fiber alignment and efficiently directed the alignment of cells along the UANYs, favoring the formation of engineered tissues with anisotropic features. The UANYs supported high cell viability and displayed unique abilities to promote the cell adhesion, proliferation and smooth muscle and osteogenic differentiation of HADSCs. Moreover, these UANYs were explored successfully as building blocks for generate 3D fibrous scaffolds with complex heterogeneous structures by using various major textile processes including braiding, weaving and knitting either by hand or with machines. The UANYs and related scaffolds could combine with hydrogel systems(i.e. hyaluronic acid, gelatin, collagen, and alginate) to form core-shell composite living fibers and 3D composite scaffolds for designing and making advanced tissue engineering constructs with complex architectures. These results suggested that the continuously fabricated UANYs had excellent cell-responsive properties and processability, which showed excellent promise in tissue engineering and regenerative medicine applications.In Chapter 5, heart valve related diseases are one of major causes of death worldwide. Excellent tissue engineered heart valves have the potential to serve as permanent replacements of diseased valves in clinical therapy. Despite their chemical similarity to heart valve ECM, most hydrogel scaffolds were not mechanically suitable for the dynamic stresses of the heart valve microenvironment. PAN UANYs were used as the weft yarns to pass through PAN microfiber yarns(MY)(the wrap yarns) to form the woven fabric. The woven fabric possessed notable anisotropic mechanical characteristics and nanofiber structure, which could mimic the fiber scale and anisotropy of heart valve ECM. We integrated PAN woven fabrics within a hybrid hydrogel made from methacrylated hyaluronic acid(Me-HA) and methacrylated gelatin(Me-Gel) as the composite scaffolds for aortic valve tissue engineering application. The human aortic valve interstitial cells(HAVICs) were encapsulated into hydrogel/woven fabric composites. The HAVICs-laden composite scaffolds combined the advantageous properties of ECM-mimicking hydrogels and anisotropic woven scaffolds to mimic the cellular environment and anisotropic mechanical characteristics of native heart valve tissue. The composite scaffolds showed similar mechanical properties with the native valve. From the view of ECM formation and HAVICs phenotype, the composite scaffolds were more excellent compared with pure hydrogels or woven fabrics. Moreover, the composite scaffolds could obviously restrain the cell phenotype transformation into myofibroblasts and osteoblasts, which could prevent the diseased HAVICs from continuing to calcification.In Chapter 6, PAN based uniaxially aligned gel nanofiber yarns(UAGNYs) were prepared from PAN UANYs by two activation steps, namely preoxidation and saponification. In order to obtain excellent PAN based UAGNYs, the preoxidation and saponification conditions were systematically investigated. It was found that the ideal preoxidation temperature was 230 oC, and the ideal saponification process was 0.5 N Na OH solution for 20 min. The PAN based UAGNYs were intelligent materials, which could strongly expand(contract) in response to a change in p H of environmental solution. The expansion/contraction behavior is similar to the spontaneous motion of human muscles. It was found that the UAGNYs showed strong p H response in weak acid solution i.e. p H=4-7). The length and diameter variations could be reached to 100 % and 900 %. Compared with traditional PAN based gel microfibers, the UAGNYs presented more sensitive p H response and no hysteresis phenomena could be observed. Importantly, The UAGNYs effectively promoted the cell viability, adhesion and proliferation, supported smooth muscle differentiation of HADSCs and induce the cell alignment. The p H-driven microactuator showed excellent promise in linear actuators, artificial muscles and drug controlled release for biological application. Although many microactuator designs have been developed and studied in recent years, bioactuators are especially interesting due to the exploitation of living cells, their self-actuating behavior, and the potential promise for in vivo applications. We designed and built compact and miniaturized UAGNYs that actuated in response to the spontaneous contraction of Chick Embryonic cardiomyocytes. We chose cardiac cells because they did not require application of an electric field for stimulation. The UAGNYs effectively promoted the cell viability, adhesion and proliferation and maintain the cardiomyocytes’ phenotype. Importantly, the UAGNYs could induce the cardiomyocytes’ alignment, which could mimic the architecture of oriented cardiac bundles in human heart. When creating bioactuators powered by cell movement, cell alignment on the device was a critical design component for maximizing the output force. The cardiomyocytes alignment could maximize the output force in the UAGNYs. The UAGNYs that were seeded with well-arranged cardiomyocytes successfully underwent rhythmic bending, which was driven by synchronous beating of the heart cells. Until 7 days, the beating frequency could still maintain at 61 times/min. The cardiomyocyte-driven UAGNYs showed excellent promise in bioreactors, biorobotics and drug screening applications.