Electrospinning: Current Status and Future Trends (PDF Download ...

Chapter 4

Electrospinning: Current Status and Future Trends Soheila Mohammadzadehmoghadam, Yu Dong, Salim Barbhuiya, Linjun Guo, Dongyan Liu, Rehan Umer, Xiaowen Qi and Youhong Tang

Introduction Fibres with diameters in the range of 1–100 nm are generally referred to as nanofibres in scientific literature. Presently, nanofibre manufacturing is one of the key advancements in nanotechnology. In recent years, various approaches for producing polymeric nanofibres have been employed such as melt blowing [1], template synthesis [2], phase separation [3], self-assembly [4] and electrospinning [5]. Among these methods, electrospinning or electrostatic spinning is recognised as the cheapest and the most straightforward benchtop technique for fabricating continuous nanofibres with diameters ranging from several micrometres down to tens of nanometers [6]. Various outstanding characteristics of electrospun nanofibres are featured including high surface area per unit mass (about 1–100 m2/g), high porosity (about S. Mohammadzadehmoghadam Y. Dong (&) L. Guo Department of Mechanical Engineering, Curtin University, Perth, WA 6845, Australia e-mail: [email protected] S. Barbhuiya Department of Civil Engineering, Curtin University, Perth, WA 6845, Australia D. Liu Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China R. Umer Department of Aerospace Engineering, Khalifa University of Science, Technology & Research, Abu Dhabi, United Arab Emirates X. Qi School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China Y. Tang Centre for NanoScale Science and Technology, School of Computer Science, Engineering and Mathematics, Flinders University, Adelaide, SA 5001, Australia © Springer International Publishing Switzerland 2016 S. Fakirov (ed.), Nano-size Polymers, DOI 10.1007/978-3-319-39715-3_4

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Fig. 4.1 Number of electrospinning publications from 2002 to 2012. Reproduced from Ref. [20]

90 %), light weight, tuneable pore size, flexibility in surface functionalities, relatively good mechanical strength, high permeability, high aspect ratio up to 1000 [7, 8]. Moreover, these properties boost a key role of electrospun nanofibres as an ideal material candidate for a wide range of applications such as biomedicine (e.g. tissue engineering, drug delivery, wound dressing and release control) [9–12], protective clothing [13], filtration [14], reinforcement of composite materials [15], microelectronics (e.g. batteries, supercapacitors, transistors, sensors and display devices) [16, 17], space applications [18] and microwave absorption [19]. Due to ongoing research commitments in this field (Fig. 4.1), significant innovations in set-up, characterisation methods and their applications have been introduced over a short period of time. Hence, there are high demand and great interest in understanding the recent progress in this open-ended field of electrospinning.

Processing and Fabrication Electrospinning History and Principle The technology to produce synthetic filaments with the aid of electrostatic forces has been utilised for over a hundred years. This process is called “electrospinning” that was initially derived from “electrostatic spinning” and developed from the electrospraying method. This fundamental technique was observed by Rayleigh [6] in 1897, which was followed by Morton [21] and Cooley [22] that patented methods to disperse fluids using electrostatic forces. In 1914, Zeleny [23] reported the behaviour of conductive liquid droplets at the end of metallic tubes in the presence of an electrostatic force. Further developments were made by Formhals [6, 24] for the fabrication of textile yarns and described in a sequence of patents from 1934 to 1944. After that, the focus shifted to developing a better

4 Electrospinning: Current Status and Future Trends

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Fig. 4.2 Schematic diagram of a basic electrospinning apparatus. Reproduced from Ref. [29]

understanding of electrospinning process. In 1969, Taylor [25] further studied electrospinning by means of mathematically modelling the conical shape formed by fluid droplets under the effect of an electric field. This cone has since been known as the “Taylor cone”. Several years later, the first investigation of electrospinning melting polymers was carried out by Larrondo and Manley [26]. The method was not well recognised until early 1990s when researchers started to realise the huge potential of electrospinning process in nanofibre production [27]. The basic principle for a polymer solution or melt to be electrospinnable is its ability to carry an electric charge and have sufficient viscosity to be stretched without breaking up into droplets. The basic framework of electrospinning is shown in Fig. 4.2. In a typical electrospinning process, three main components are predominantly utilised, namely a syringe filled with a polymer solution, a high-voltage supplier to provide the required electric force for stretching the liquid jet, and a grounded collection plate to hold nanofibre mats. At the initial stage, polymer melt or solution is introduced into a capillary tube. A high electrical potential (e.g. 10– 50 kV) is applied between droplets of polymer solution or melt at the end of a needle and a grounded collector. By increasing the intensity of electric field, the pendant drop of polymer solution becomes highly electrified, thus inducing the electric charge on the liquid surface, which results in the deformation of liquid drops into a conical shape, known as “Taylor cone”. When the electric voltage reaches a critical value, the electrostatic repulsion forces prevail over the solution surface tension so that a charged jet of solution is ejected. A polymer jet is then ejected from the tip of “Taylor cone” and travels rapidly to the metal collector.

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(d)

(c)

(b)

(a)

(e)

(f)

(g)

Fig. 4.3 Different morphologies of electrospun polymeric fibres: a random fibres [30], b necklace-like fibres [31], c ribbon fibres [32], d porous fibres [33], e hollow fibres [34], f multichannel tubular fibres [35] and g core/shell structure [33]

Before reaching the collector, the jet undergoes a series of electrically driven bending instabilities and gradually becomes thinner in air due to fibre elongation and solvent evaporation. The charged jet is finally collected on a ground collector in the formation of stretched fibrous structures [6, 28]. A typical electrospinning process creates long and continuous fibres that can vary in diameters along the lengths and generally exhibit a solid interior and smooth surface. However, after the recent developments, different nanofibres with specific structures like core/shell, porous, hollow, necklace-like ribbon and multichannel tubular structures can also be prepared (Fig. 4.3) [30–35].

Electrospinnable Polymers A wide range of polymers from solution, sol-gel suspension, or melt can be electrospun into nanofibres. To date, over 200 types of various materials including natural polymers, synthetic polymers and hybrid blends have been used to obtain electrospun fibres [36]. Due to the practicality, mouldability, flexibility, lightness, durability and chemical and physicochemical stability, natural polymers are more preferable to synthetic polymers. Therefore, four major classes of biopolymers including proteins, polysaccharides, deoxyribonucleic acids (DNAs) and lipids as well as their derivatives have been fabricated into electrospun scaffolds [37, 38]. The most popular natural polymers include chitosan, collagen, gelatin, casein, hyaluronic acids, silk protein, chitin and fibrinogen [37–43]. Synthetic polymers often provide many advantages over natural polymers in that they can be tailored according to desired properties for specific applications and


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Table 4.1 Different polymers and solvents used for electrospinning Type of polymer Natural polymers

Synthetic polymers

Polymer

Solvent

Reference

Silk fibroin Chitosan Gelatin Collagen Fibrinogen PCL PLA PVA PVP PAN Nylon-6 PET PU PI EVOH CA PGA PEO

Formic acid Trifluoroacetic acid(TFA) Acetic acid Hexafluoroisopropanol (HFIP) HFIP/10 × minimal essential medium Chloroform/dimethylformamide (DMF) Dichloromethane (DCM) Water Methanol DMF Formic acid TFA/DCM DMF N,N-dimethylace tamide (DMAc) 80 % propan-2-ol/Water Acetic acid/Water Water Water

[42] [39] [41] [40] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]

also present good uniformity. In addition, they are much cheaper and serve as a more reliable source of raw materials than that of natural polymers. These polymers include polycaprolactone (PCL) [44], polylactic acid (PLA) [45], poly(vinyl alcohol) (PVA) [46], polyvinylpyrrolidone (PVP) [47], polyacrylonitrile (PAN) [48], nylon-6 [49], polyethylene terephthalate (PET) [50], polyurethane (PU) [51], polyimide (PI) [52], poly(ethylene-co-vinyl alcohol) (EVOH) [53], cellulose acetate (CA) [54], polyglycolic acid (PGA) [55], polyethylene oxide (PEO) [56] and so on. Several commonly used natural and synthetic polymers are listed in Table 4.1.

Types of Electrospinning A majority of electrospun nanofibres are typically fabricated from polymer solutions and organic solvents have to be used in most cases to form homogeneous polymer solutions [57]. Despite many attempts, electrospinning process suffers from many drawbacks including low productivity (up to 300 mg/hr) [58], the requirement of an extraction process for additional solvents and environmental concerns due to the use of toxic solvents [59]. To tackle these issues, several designs for the types of electrospinning have been employed as highlighted in the following sections.

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Melt Electrospinning Electrospinning of molten polymers, also known as melt electrospinning, is an effective approach to overcome some disadvantages of conventional electrospinning process such as environmental concerns as well as electrospinning of non-soluble polymers like polypropylene (PP) and polyethylene (PE) [23]. Larrondo and Manley [26] applied an electrostatic force to molten polymers for the first time, leading to the production of microfibres. Generally, the set-up of melt electrospinning is similar to that of conventional electrospinning in addition to the provision of polymer melting [59]. More impressively, melt spinning can be used without the requirement for a ventilation system. In some cases, melt electrospinning can be performed without the use of syringe pump where a polymer melt is pushed out of the spinneret using an air pressure system [60] or based on a solid polymer filament that is pushed out of the device [61, 62]. Several heating systems with benefits and disadvantages regarding safety and efficiency can be employed for heating polymers such as circulating fluids, electrical systems, hot air or laser melting devices (Fig. 4.4) [59, 62, 63]. Among these, electrical heating systems are most commonly used [63–65]. However, if high voltage is applied, the heater should be isolated from the high-voltage source that requires a complicated configuration [63]. An efficient hybrid system combining electrical heating and hot air was developed in Joo’s laboratory [66]. In such a combined system, a stream of air was used to assist in maintaining the electrified molten jet and the process was named gas-assisted melt electrospinning (GAME). Various polymers such as PLA [66], polylactide [67], PP [68], PET [69] and PCL [70] were fabricated by melt electrospinning.

Fig. 4.4 Schematic diagram of different heating approaches used for melt electrospinning. Reproduced from Ref. [62]


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Despite numerous advantages of melt electrospinning such as no recycling/removal of toxic solvents, high throughput rate without a mass loss by solvent evaporation, ease of fabricating polymeric fibre blends, this technique is still accompanied by some limitations such as the requirement of a high-temperature melting system, an electric discharge problem associated with the melt and low conductivity of the melt. In addition, it has been noted that diameters of melt electrospun nanofibres are typically over 10 microns owing to the low charge density, high viscosity of the melt as well as rapid solidification of polymers between the needle and collector [59, 71]. Many studies have been carried out to control and reduce the diameter of melt electrospun nanofibres. For example, the use of special additives for viscosity reduction like Irgatec CR 76 to PP can decrease fibre diameter to