MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO REVIEW ON ELECTROSPINNING FOR NANOFIBER DESIGN AND MANUFACTURING Jorge Alberto Wadgymar Gutiérreza, José Israel Martínez Lópezb, Alex Elías Zúñigac, Héctor R. Siller Carrillod, Ciro A. Rodríguez Gonzáleze a, c Department of Mechanical Engineering, Tecnológico de Monterrey, Eugenio Garza Sada 2501 Sur, C.P. 64849 Monterrey, NL, México b,d, e Center for Innovation in Design and Technology, Tecnológico de Monterrey, Eugenio Garza Sada 2501 Sur, C.P. 64849 Monterrey, NL, México george_albert86@hotmail.com, tuertoartillero@gmail.com, aelias@itesm.mx, hector.siller@itesm.mx, ciro.rodriguez@itesm.mx RESUMEN. ABSTRACT. El método de electrohilado ha sido reconocido como una técnica eficiente para la fabricación de nanofibras poliméricas. Gran cantidad de polímeros han sido electrohilados exitosamente en finas fibras, las cuales tienen potenciales aplicaciones en el desarrollo de nanocompuestos, ingeniería del tejido, industria textil, industria militar, cosméticos y otras aplicaciones industriales. En este artículo se presenta un estado del arte del método de electrohilado para la fabricación de nanofibras, los parámetros que influyen en este proceso para la obtención de fibras, algunos materiales que han sido electrohilados con potenciales aplicaciones en la industria, modelación y caracterización de nanofibras. Finalmente, se presenta una breve descripción de los avances en el diseño y la manufactura de nanofibras en el Tecnológico de Monterrey. Electrospinning has been recognized as an efficient technique for the fabrication of polymer nanofibers. Several polymers have been successfully electrospun into fine fibers, which have potential applications in nanocomposite development, tissue engineering, textile and military industry, cosmetics and other industrial applications. In this paper is presented a state-ofthe-art of electrospinning for nanofiber fabrication, the parameters that have an influence on this process, and some materials that have been electrspun with potential applications in industry, modeling and characterization of nanofibers. Finally, a brief description of the advances on design and manufacturing of nanofibers at Tecnológico de Monterrey is presented. Nomenclature Vc H L R γ P σzz R0 λ3 c1, c2 critical voltage distance between the capillary tip and the collecting screen length of the capillary tube radius of the tube surface tension of the fluid total axial tensile force true axial tensile stress current fiber radius after stretching principal stretch of the material element in the axial direction two independent material constants ISBN: 978-607-95309-3-8 R A, R B τrr, τzz, τθθ f(R) Ri τ b M m T initial interior and exterior radio of the fiber respectively shear stress components material incompressibility initial radii centered at a point Cauchy stress-softened tensor softening parameter maximum strain intensity strain intensity Cauchy stress tensor Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO Introduction Electrospinning, a broadly used technology for electrostatic fiber formation which utilizes electrical forces to produce polymer fibers with diameters ranging from 2 nm to several micrometers using polymer solutions of both natural and synthetic polymers has seen a tremendous increase in research and commercial attention over the past decade. This growth in interest can be shown in the increase over 1000% of the number of publications in less of ten years in articles regarding this process (Fig. 1) and in the development of an industry focused on new commercial products that take advantages of the properties of the nanofibers produced with electrospinning. No. of Publications 800 separation, and electrospinning, which are briefly described in Table 1. Electrospinning generates continuous, uniform, long fibers with diameters down to the nanoscale dimension. The advantages of the electrospinning technology make it suitable for small quantity production for laboratory research use and mass production for industrial use. Electrospinning seems to be the only method which can be further developed for mass production of one-by-one continuous nanofibers from various polymers. In this paper, different setups for nanofiber fabrication are shown; materials that have been electrospun for different applications, parameters that influence the process, modeling and characterization of nanofibers are described. Process 600 400 200 0 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 Publication Year Fig. 1 Number of scientific publications of electrpspinning per year in the last decade. (Source: ISI Web of Knowledge from Elsevier) This process offers unique capabilities for producing novel natural nanofibers and fabrics with controllable pore structure. Electrospun fibers have been successfully applied in various fields, such as, nanocatalysis, tissue engineering scaffolds, protective clothing, filtration, biomedical, pharmaceutical, optical electronics, healthcare, biotechnology, defense and security, and environmental engineering. A diagram in Fig. 2 shows the different potential applications of electrospun nanofibers. In the electrospinning process a high voltage is used to create an electrically charged jet of polymer solution or melt out of the pipette. One electrode is placed into the spinning solution/melt and the other attached to the collector. The electric field is subjected to the end of the capillary tube that contains the solution fluid held by its surface tension. This induces a charge on the surface of the liquid. Before reaching the collecting screen, the solution jet evaporates or solidifies, and is collected as an interconnected web of small fibers. Dong et al. (1) has shown the different kinds of electrospinning set-ups (Fig. 3). By using these different setups, electrospinning can produce different nanofibrous structures with various twoor three-dimensional shapes, including aligned nanofibers, nanofibrous yarn, tubular structures, and core-shell nanofibers. Nanofibrous structure can be generated using mainly three methods: self-assembly, phase ISBN: 978-607-95309-3-8 Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO Filtration Energy generation Nanocomposites NANOFIBER APPLICATIONS Cosmetics Nano-sensors Tissue Engineering Scaffolding Biomedical Protective clothing Fig 2. Applications of electrospun nanofibers (References: (2) and (3)). Process Scalable Convenient to process Control on fiber dimension Fiber diameters Types of materials Advantages Disadvantages Phase Separation Solvent extraction from gelated polymer solution to form nanofibers Self-Assembly Molecules organize and arrange themeselves into an ordered structure Electrospinning Uses static electricity to draw fibers from polymer solution X X X X X 50 – 500 nm Organic, Biopolymers 7 – 100 nm Organic Minimum equipment required Batch-to batch consistency Tailorable mechanical properties and pore size Limited to specific polymer Matrix directly fabricated Good for obtaining small nanofibers 3 nm – 1 µm Organic, Inorganic, Metallic, Biopolymers Cost effective Long continuous fibers Production of aligned fibers Tailorable mechanical properties, size and shape Jet instability Large nanometer to micron scale fibers Use of organic solvents Complex process Limited to a few polymers Table 1. Comparison of three different methods of nanofiber fabrication (adapted from (1), (4) and (5)) ISBN: 978-607-95309-3-8 Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO A B C D E Fig. 3 Schematic diagrams of different electrospinning set-ups. (A) Standard electrospinning setup. (B) Aligned electrospinning. (C) Nanofibrous yarn. (D)Tubular structure. (E) Core-shell nanofiber.(Taken from (1)). Other Electrospinning Setups Multi-jet Several modifications in the general setup of the electrospinning process have been made in order to improve the efficiency of this process. Some researchers have tried to increase productivity and covering area by creating arrays of nozzles with or without additional electrodes. Despite this technique results are contradictory; the general consideration is that the mutual interactions between jets and the additions of elements make this technique becomes more difficult. (Theron et al., 2005 (6)). 2002 (8)). Meanwhile, this setup offers a continuously production with high production capacity (1 g min-1 m-1) (He et al., 2008 (9)). Fig. 4 NanospiderTM technology (Taken from www.elmarco.com) Nanospider (Needless electrospinning) Based in the form free surface electrospinning of Yarin (Yarin et al., 2004 (7)) there is a patented alternative called NanospiderTM (Fig. 4), a polymer solution is for spinning supplied into the electric field using a surface of a rotating charged cylindrical electrode. No syringes, capillaries or nozzles are needed. The production rate of a conventional electrospinning system is in the range of 10–100 mg fiber per min (Tsai et al, ISBN: 978-607-95309-3-8 Electrospinning Parameters Many parameters can influence the transformation of polymer solutions into nanofibers. The parameters can be classified into process parameters, solution parameters and ambient parameters. Process Parameters Solution Parameters Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO Ambient Parameters A review about the effects of parameters on electrospinning is presented in (2) and (3). A summary of this review is presented in this article and Table 2 shows a brief description of the variables affected by each parameter. Process Parameters Applied Voltage There is a little dispute about the behaviour of applied voltage in the electrospinning process. Some investigations have showed that there is not much effect of electric field on the fiber diameter; other researchers have suggested that higher voltages facilitate the formation of a larger diameter fiber. Other authors have reported that an increase in the applied voltage favours the narrowing of fiber diameter. In most cases, a higher voltage causes a reduction in the fiber diameter.. At a higher voltage there is also greater probability of beads formation. The voltage influences fiber diameter, but the level of significance varies with the polymer solution concentration and on the distance between the tip and the collector (N. Bhardwaj, S.C. Kundu, 2010 (3)). Flow rate It has been observed that the fiber diameter and the pore diameter increase with an increase in the polymer flow rate in the case of polystyrene (PS) fibers. A lower feed rate is more desirable as the solvent will get enough time for evaporation. If the flow rate is high, beaded fibers are obtained due to the unavailability of proper drying time before reaching the collector. Types of collectors Generally, aluminum foil is used as a collector but due to the need for aligned fibers for various applications, other collectors such as, conductive paper, conductive cloth, wire mesh, pin, parallel or gridded bar, rotating rod, rotating wheel, liquid non solvent such as methanol coagulation bath and others are also common types of collectors nowadays. The fiber alignment is determined by the type of the target/collector and its rotation speed. ISBN: 978-607-95309-3-8 Distance tip-collector It has been found that a minimum distance is required to give the fibers sufficient time to dry before reaching the collector, otherwise with distances that are either too close or too far, beads have been observed. Needle diameter X. M. Mo et al. (10) found that the formation of beads was influenced by the needle diameter. With larger needle diameters beads and clogging were observed meanwhile with a thicker needle diameter no beads or clogging were found. Solution Parameters Concentration It has been found that at low solution concentrations, beads are obtained meanwhile at higher concentrations results in fewer beads but larger nanofiber diameters (Z.-M. Huang et al. (2), N. Bhardwaj, S.C. Kundu 2010 (3)). X. M. Mo et al. (10) found that a low concentration of the P(LLA-CL) copolymer solution (less than 3wt%) caused the formation of beads in the electrospun nanofibers. Also the concentration affected the fiber diameter, by increasing the concentration the larger the fiber diameter. Molecular weight This parameter has an important effect on some properties such as viscosity and surface tension. It has been observed that a low molecular weight solution tends to form beads instead of fibers and a high molecular weight solution tends to form fibers with larger average diameter (N. Bhardwaj, S.C. Kundu 2010 (3)). Viscosity The solution viscosity is an important parameter that influences the fiber size and morphology during electrospinning. With low viscosity there is no continuous fiber formation and with high viscosity difficulty in the jet ejection from polymer solution is presented. Researchers have reported maximum spinning viscosities ranging Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO from 1 to 215 poise (N. Bhardwaj, S.C. Kundu 2010 (3)). Surface tension The formation of droplets, bead and fibers depends on the surface tension of solution and a lower surface tension of the spinning solution helps electrospinning to occur at a lower electric field (N. Bhardwaj, S.C. Kundu 2010 (3)). Ambient parameters Parameter Voltage Flow rate Type of collector Tip- Collector distance Apart from solution and processing parameters, there are also ambient parameters that include humidity, temperature etc. Researchers have found that with increase in temperature, there is a yield of fibers with decreased fiber diameter. The variation in humidity while spinning polystyrene solutions has been studied and shows that by increasing humidity there is an appearance of small circular pores on the surface of the fibers; further increasing the humidity leads to the pores coalescing. Variables affected Process variables Fiber diameter Rate of evaporation of solvent Jet velocity Material transfer rate Fiber alignment parallelism Whole process Solution variables Jet velocity Concentration Molecular weight Whole process Fiber diameter Viscosity Whole process Surface tension Minimum electric field An increase typically causes Reduction (-) Increase (+) Increase (+) Increase (+) Increase (+) (using rotating device) A minimum distance is necessary Reduction(-) Low: formation of beads; High: interruption of fibers formation. Increase (+) Low: difficulties in fiber formation; High: not allow the jet formation Reduction (-) Table 2. Typical effect of process parameters in electrospinning. Characterization Geometrical Characterization Geometric properties of nanofibers such as fiber diameter, diameter distribution, fiber orientation and fiber morphology can be characterized using: Scanning electron microscopy (SEM) Field emission scanning electron microscopy (FESEM) Transmission electron microscopy (TEM) Atomic force microscopy (AFM) availability of accurate testing apparatus, few papers discuss about the experimental characterization for the mechanical properties of a single nanofiber. Three testing apparatus are available to measure tensile and bending properties of a single nanofiber: 1.Cantilever technique, 2.AFM-based nanoindentation system 3.Nano tensile tester. Fig. 5 shows a schematic diagram of the cantilever technique for tensile testing of single nanofibers. Mechanical characterization Single Nanofiber Due to its extremely small diameter, the handling difficulty to extract a single fiber from the electrospun nanofiber web and also non- ISBN: 978-607-95309-3-8 Fig. 5 Cantilever techinique for tensile testing of single nanofibers (Taken from (5)). Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO solution through a millimeter diameter capillary tube into solid fibers. A bending test of single nanofibers can be performed using AFM-based nanoindentation system, as shown in Fig. 6. Fig. 6 AFM-based nanodindentation system for bending test (Taken from (5)). A tensile test of continuous fibers from the range of few millimeters to several centimeters in length can be performed by a commercial nano tensile testing system. Tan and Lim (11) conduct a tensile test of a single polymer nanofiber using a nano tensile tester (Nano Bionix System, MTS, USA). Nanofiber Membrane Mechanical tests of nanofibrous nonwoven membranes can be performed using conventional testing techniques. Xu et al. (12) obtained the typical stress-strain curve (Fig. 8) of P(LLA-CL) nanofibrous scaffold under tensile loading. Li et al. (13) obtained a stress-strain curve of PLGA electrospun nanofibers, which were found to be suitable for soft tissue. When the applied electrostatic forces overcome the fluid surface tension, the electrified fluid forms a jet out of the capillary tip towards a grounded collecting screen. Jet initiation The formation of fine threads from viscous liquid drops in an electric field is due to the maximum instability of the liquid surface induced by the electrical forces. An issue related with the initiation of the jet is the strength of the electrostatic field required. Taylor also showed that the critical voltage Vc (expressed in kilovolts) at which the maximum jet fluid instability develops is given by Eq. 1 2 H 2L 2 Vc 4 ln 1.5 0.117R 2 R L (1) Modeling of single nanofiber Some studies have been focused on the development of a model that describes the behavior of polymer nanofibers under tension. Wu and Dzenis (14) studied the size effect on the elastic behavior of solid and hollow polymer nanofibers subjected to uniaxial stretching. In their work, a one-dimensional nonlinear elastic model was developed, considering the coupling effect of surface tension and fiber radius on the tensile response of these nanofibers. Considering a thin solid cylindrical polymer fiber subjected to uniaxial stretching (Fig. 9a) as hyperelastic isotropic material, Wu and Dzenis (14) obtained the relation for the total axial tensile force P (Eq. 2) and for the true axial tensile stress σzz (Eq. 3). Fig. 8 Stress-strain curve of electrospun P(LLA-CL) nanfibrous scaffold (Taken from(12)). Modeling and Simulation c R 1 P R02 3 2 c1 2 0 3 3 3 Modeling of Electrospinning Process zz It is necessary to understand how the electrospinning process transforms the fluid ISBN: 978-607-95309-3-8 c P 1 3 2 c1 2 2 3 R0 3 (2) (3) R0 3 Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO Organic PA 6 PA 6/12 PAI PAA PUR PES PVA And for a thin hollow cylindrical fiber subjected to uniaxial stretching (Fig. 9b), the relations for the total axial tensile force P (Eq. 4) and for the true axial tensile stress σzz (Eq. 5) are: P π RAB 2 τ zz τ rr τ θθ Rf(R) Rf'(R) f(R) dR R πγ R A f(RA ) RB f(RB ) zz R R B 2 zz rr Rf ( R ) Rf ' ( R ) f ( R ) dR A R B f ( R B ) R A f ( R A ) 2 R A f ( R A ) RB f ( RB ) 2 2 RB f ( RB ) RA f (RA ) (4) 2 (5) The discovered size effect on the nanofiber deformation should be taken into account for precise prediction of their properties and mechanical response (14). (a) (b) PAN PEO PS PVDF PVP PVP-I Inorganic* Metallic Biopolymer TiO2 SiO2 Al2O3 ZnO Li4Ti5O12 ZrO2 MgAl2O4 Pt Cu Mn Gelatin Chitosan Collagen Cellulose PLA PCL * After calcination or post-processing treatment Table 3. Materials for nanofiber fabrication with NanospiderTM technology (Source: www.elmarco.com) Applications Electrospun nanofibers are broadly applied in many applications shown previously in Fig. 1. In the medical field it has been seen that electrospinning can bring a suitable substitute to the natural extracellular matrix, and soft tissues as skin or cartilage due the features of a wide range of pore size distribution, high porosity, and high surface area-to-volume ratio, which are favorable parameters for cell attachment, growth and proliferation. (Li et al., 2002 (13)). Nowadays, nanofiber membranes are commercially available (Fig. 10) for this kind of applications showing good properties, which make them suitable for tissue engineering applications. Fig. 9 Coordinate systems for the analysis of (a) solid cylindrical nanofibers and (b) hollow nanofibers (Taken from (14)). Electrospun Materials There are a wide variety of polymers that have been used to produce electrospun nanofibers, including natural polymers, synthetic polymers and blends of both, biopolymers. Besides polymers, inorganic and metallic materials have been used to produce nanofibers by electrospinning. Table 3 shows some materials that have been processed to produce nanofibers with the NanospiderTM technology. ISBN: 978-607-95309-3-8 Fig. 10 Commercial nanofiber membrane (Nanosan®, SNS Nano Fiber Technology, LLC) Advances on Nanofiber Design and Manufacturing at Tecnológico de Monterrey Nanofiber Design The modeling of a single nanofiber considers it as a hyperelastic isotropic material, which is a good approach for rubber-like polymers. In the topic of Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO rubber-like materials, studies have been performed in order to describe phenomenological models that complement the softening phenomenon exhibited by rubber-like materials. The Mullins effect in isotropic, hyperelastic, incompressible rubber-like materials was investigated by Zúñiga and Beatty (17) considering a rubber cord under uniaxial extension. The uniaxial stress in the elastic stresssoftened material is given by: e b M m Τ (6) Calva (18) proposed a model to predict analytically the softening phenomenon of rubberlike materials, showing a good prediction when compared to experimental data for uniaxial extension. This type of material models are the basis for the design of nanofiber devices for applications in tissue engineering and biomedical fields. Nanofiber Manufacturing Tecnológico de Monterrey is working in the design and fabrication of an electrospinning apparatus in order to produce nanofibers, characterize and control the parameters; and optimizes nanofiber production, trying to get good properties for applications in tissue engineering scaffolds. The main components of a basic electrospinning apparatus are: High Voltage Power Supply Syringe Pump Ground Collector The high voltage will be applied with the Gamma Model ES20P-5W/DAM (Gamma High Voltage Research). A Single-Syringe Infusion Pump KDS 100 (KD Scientific) was selected to pump the solution, however, by now instead of using a commercial syringe pump, a stepper motor and a ball screw will be used to pump the polymer solution out of a syringe, and the ground collector will be an aluminum disc having a radius of about 15 cm. A schematic diagram of the design of our electrospinning apparatus is shown in Fig. 11. ISBN: 978-607-95309-3-8 Fig. 11 Schematic diagram of electrospinning apparatus by Tecnológico de Monterrey Conclusions Electrospinning has had a tremendous development in the last two decades due to the unique capabilities to produce nanofibers, which are lately being of great interest for industry in many applications such as tissue engineering, biomedical applications, protective clothing among others. Desirable properties of the nanofibers of interest include primarily their mechanical behavior and biological characteristics such as biocompatibility. Different setups have been developed in order to reach a higher productivity and investigations changing different parameters have been done in order to get better properties in nanofibers. Because of the limitations that electrospinning has, practical applications of nanofibers are limited so far. However, recent advances in this technique are promising to improve the efficiency of this method, so more applications can be realized. Acknowledgments This work supported by the Research Chairs of Materials and Nanotechnology and of Intelligent Machines of the Tecnológico de Monterrey, Campus Monterrey, and also, to the CONACYT project: Synthesis and Constitutive Modeling of Biocompatible Polymers for Microfluidic Devices # 61061. Derechos Reservados © 2010, SOMIM MEMORIAS DEL XVI CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 22 al 24 DE SEPTIEMBRE, 2010 MONTERREY, NUEVO LEÓN, MÉXICO References (1) Yixiang Dong, Susan Liao, Michelle Ngiam, Casey K. Chan and Seeram Ramakrishna. “Degradation Behaviors of Electrospun Resorbable Polyester Nanofibers”, Tissue Eng (2009). (2) Zheng-Ming Huang, Y.-Z. Zhang, M. Kotaki, S. Ramakrishna. “A review on polymer nanofibers by electrospinning and their applications in nanocomposites”, Composites Sci and Tech (2003). smooth muscle cell and endothelial proliferation”, Biomaterials (2004). cell (11) E.P.S. Tan, C.T. Lim, “Mechanical characterization of nanofibers – A review”, Comp Sci and Tech (2006). (12) Chengyu Xu, Ryuji Inai, Masaya Kotaki and Seeram Ramakrishna. “Electrospun Nanofiber Fabrication as Syhntetic Extracellular Matrix and Its Potential for Vascular Tissue Engineering”, Tissue Eng (2004). (3) Bhardwaj N, Kundu SC. “Electrospinning: A fascinating fiber fabrication technique”, Biotechnol Adv (2010). (13) Wan-Ju Li, Cato T. Laurencin, Edward J. Caterson, Rocky S. Tuan, Frank K. Ko. “Electrospun nanofibrous structure: A novel scaffold for tissue engineering”, J Biomed Mater Res (2002). (4) Catherine P. Barnes, Scott A. Sell, Eugene D. Boland, David G. Simpson, Gary L. Bowlin. “Nanofiber technology: Designing the next generation of tissue engineering scaffolds”, Adv Drug Delivery (2007). (14) X.-F. Wu and Y. A. Dzenis. “Size effect in polymer nanofibers under tension”, J. Appl. Phys. (2007). (5) Ramakrishna, Seeram; Fujihara, Kazutoshi; Teo, Wee-Eong; Lim, Teik-Cheng; Ma, Zuwei. “An Introduction to Electrospinning and Nanofiber” Singapore: World Scientific Printers Pte Ltd, 2005. 396 p. ISBN 981-256-415-2. (6) Theron S A, Yarin A L, Zussman E and Kroll E. “Multiple jets in electrospinning: experiment and modeling”, Polymer (2005). (7) Yarin, A. L., and E. Zussman. “Upward needleless electrospinning of multiple nanofibers”, Polymer (2004). (8) P. P. Tsai, H. Schreuder-Gibsonb, P. Gibson. “Different electrostatic methods for making electret filters”, J. Electrostat. (2002). (9) Ji-Huan He, Yong Liu, Lan Xu, Jian-Yong Yu, Gang Sun. “BioMimic fabrication of electrospun nanofibers with high-throughput”, Chaos, Solitons & Fractals (2008). (15) Eugene D. Boland, Branch D. Coleman, Catherine P. Barnes, David G. Simpson, Gary E. Wnek, Gary L. Bowlin. “Electrospinning polydioxanone for biomedical applications”, Acta Biomat, (2005). (16) Scott A. Sell, Michael J. McClure, Koyal Garg, Patricia S. Wolfe, Gary L. Bowlin. “Electrospinning of collagen/biopolymers for regenerative medicine and cardiovascular tissue engineering”, Adv. Drug Deliv. Rev. (2009). (17) Alex Elías-Zúñiga and Millard F. Beatty. “Stress-softening Effects in the Transverse Vibration of a Non-Gaussian Rubber String”, Meccanica, (2003). (18) Víctor Calva Mendoza. “Characterization of the Stess-Softening and Permanent Set Effects of Elastomeric Materials”, Thesis for Master of Science in Manufacturing Systems, Tecnológico de Monterrey, Campus Monterrey, (2008). (10) X.M. Mo, C.Y. Xu, M. Kotaki, S. Ramakrishna. “Electrospun P(LLA-CL) nanofiber: a biomimetic extracellular matrix for ISBN: 978-607-95309-3-8 Derechos Reservados © 2010, SOMIM