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nanofibers overlap in a mat. A second observation is the advancement in large‐scale nanofiber manufacture. Development of flexible nanofiber mats that might be integrated into fabric to support efforts at developing wearable electronics is under way. Innovations of nanofibers in textile science are discussed in Chapter 2. Large‐scale fabrication of nanofiber mats using electrodeless systems such as already‐ commercialized Nanospider™ (ElMarco) will further advance to bring down the cost of the material. Chapter 5 reviews the status of commercial production of nanofibers.

Nanofiber mats are porous with up to 90% of the volume being void spaces and their average pore diameters are less than about ten times the fiber diameter. Because of these features the mats are highly permeable and can be readily adapted for air filtration. Multilayer filter media with one layer being made of nanofiber mats has already been commercialized. Applications of nanofibers in filtration are discussed in Chapter 3. Even more porous nanofiber materials are made of aerogel materials, a class of nanomaterials that shows promise in several of the application areas as discussed in Chapter 9. Nanofibers significantly contribute to innovation in medical technology, specifically in tissue engineering, wound healing and controlled release applications. Nanofibers have a large surface area to volume ratio and can be fabricated with biodegradable polymers compatible with body tissue, mimicking protein fibrils or the chemical structure of native extracellular matrix as well as synthetic ‘protein’ nanofibers of polyamino acids. Chapters 6 and 7 discuss their biomedical uses, including tissue scaffolding by nanofibers that can also serve as highly compatible implantable materials. An especially interesting application in the biomedical aren a is controlled delivery of bioactive agents such as proteins and DNA. Successful DNA delivery with nanofibers holds the possibility of its use a as a vehicle for clinically relevant gene‐delivery in genomic treatment modes. A third related area is biological sensors based on nanofiber materials with the surface functionalized to specifically interact with a biomolecule. The reaction with the biomolecule results in a physical or chemical change in the fiber that allows the quantification of its concentration in terms of a change in fluorescence or conductivity of the fiber mat. Chapter 4 discusses the use of nanofibers in sensor technology. Nanofibers are finding uses in energy technology, especially in battery, fuel cell, and solar energy. Chapter 5 of the book reviews these developments.

      The book addresses the basic science behind fabrication and nanofiber characterization with a clear emphasis on practical aspects of electrospinning. An attempt has been made to compile the more recent information and cover the different application areas where novel uses will be found for nanofiber materials.

      Anthony L. Andrady

      Saad A. Khan

      Department of Chemical and Biomolecular Engineering

      North Carolina State University

      Note

      1 1 Silk threads from spider species Nephila clavipes Vehoff, T., Glisović, A., Schollmeyer, H., Zippelius, A. et al. (2007). Mechanical properties of spider dragline silk: humidity, hysteresis, and relaxation. Biophysical Journal 93 (12): 4425–4432.

1 Electrospinning Parameters and Resulting Nanofiber Characteristics : Theoretical to Practical Considerations

       Christina Tang1, Shani L. Levit1, Kathleen F. Swana2, Breland T. Thornton1, Jessica L. Barlow1, and Arzan C. Dotivala1

       1Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, VA, USA

       2U.S. Army Combat Capabilities Development Command Soldier Center, Natick, MA, USA

1.1 Electrospinning Overview

Electrospinning has been widely used to produce nonwoven nanofibers for applications in biomaterials, energy materials, composites, catalysis, and sensors (Agarwal et al. 2008, 2009; Ahmed et al. 2014; Cavaliere et al. 2011; Chigome and Torto 2011; Ma et al. 2014; Mao et al. 2013; Yoon et al. 2008; Thavasi et al. 2008). On a bench scale, it is a simple, inexpensive process. To generate nanofibers by electrospinning, an electric potential is applied between a capillary containing a polymer solution or melt and a grounded collector (Figure 1.1). The applied electric field leads to free charge accumulation at the liquid‐air interface and electrostatic stress. When the electrostatic stress overcomes surface tension, the free surface deforms into a “Taylor cone.” Balancing the applied flow rate and voltage results in a continuous fluid jet from the tip of the cone. As the jet travels to the collector, it typically undergoes nonaxisymmetric instabilities such as bending and branching leading to extreme stretching. As the fluid jet is stretched, the solvent rapidly evaporates to form the polymer fibers that are deposited onto a grounded target (Reneker and Chun 1996; Helgeson et al. 2008; Rutledge and Fridrikh 2007; Thompson et al. 2007; Teo and Ramakrishna 2006; Li and Xia 2004). As a complex electrohydrodynamic process, the final fiber and mat/membrane properties depend on process parameters by process parameters, setup parameters, and solution properties.

Figure 1.1 Schematic of conventional electrospinning setup and overview of process, setup, and solution parameters that affect fiber and mat properties.

      Source: Photograph of mat reprinted from Dror et al. (2008). Copyright (2008). American Chemical Society.

1.2 Effect of Process Parameters

      Electrospun fibers from 30 nm to 10 μm in diameter have been reported (Greiner and Wendorff 2007). Despite its widespread use, electrospinning of new materials is typically done ad hoc varying polymer concentration and process variables. Although the nanofiber properties, namely fiber diameter, could be ideally controlled by varying the process parameters, precise control over the fiber diameter remains a technical bottleneck. The effect of process variables on fiber characteristics has been widely examined theoretically and experimentally.

      1.2.1 Theoretical Analysis

      To avoid the cost and time of experimental trial and error, modeling and theoretical analysis have been applied to predict how process parameters affect fiber diameter. Reneker and coworkers have developed a theoretical model based on simulating jet flow as bead‐springs. Their model describes the entire electrospinning process and accounts for solution viscoelasticity, electric forces, solvent evaporation and solidification, surface tension, and jet–jet interactions. Performing sensitivity analysis of 13 model input parameters, they determined that initial jet radius, tip‐to‐collector distance, volumetric charge density, and solution rheology, i.e. relaxation time and elongational viscosity, had strong effect on final fiber size. Initial polymer concentration, perturbation frequency, solvent vapor pressure, solution density, and electric potential had a moderate effect, whereas vapor diffusivity, relative humidity, and surface tension had minor effects on fiber diameter (Thompson et al. 2007).

Using

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