A Critique on Multi-Jet Electrospinning: State of the Art and Future Outlook Clothing and Fabrics Manufacture

Home , Electrospinning, Spinning (polymers)

Nanotechnol Rev 2019; 8:236–245

Review Article

Hosam El-Sayed*, Claudia Vineis, Alessio Varesano, Salwa Mowafi, Riccardo Andrea Carletto, Cinzia Tonetti, and Marwa Abou Taleb A critique on multi-jet electrospinning: State of the art and future outlook https://doi.org/10.1515/ntrev-2019-0022 clothing and fabrics manufacture. In 1738 Lewis Paul was Received Jan 29, 2019; accepted Jul 01, 2019 granted a patent for roller drafting spinning machinery [1]. Toward the end of the nineteenth century, the ring process Abstract: This review is devoted to discuss the unique char- was fairly well perfected, and its use was becoming stan- acteristics of multi-jet electrospinning technique, com- dard throughout the world. Ring spinning is about 250% pared to other spinning techniques, and its utilization in more productive than mule spinning and is simpler and spinning of natural as well as synthetic polymers. The less expensive to operate so; the mass production arose in advantages and inadequacies of the current commercial the 18th century with the beginnings of the industrial revo- chemical spinning methods; namely wet spinning, melt lution. spinning, dry spinning, and electrospinning are discussed. Yarns are usually spun from a various materials that The unconventional applications of electrospinning in tex- could be natural fibers viz., animal and plant fibers, or syn- tile and non-textile sectors are reported. Special empha- thetic ones. sis is devoted to the theory and technology of the multi- jet electrospinning as well as its applications. The current status of multi-jet electrospining and future prospects are outlined. Using multi-jet electrospinning technique, vari- 2 Types of spinning ous polymers have been electrospun into uniform blend nanofibrous mats with good dispersibility. In addition to Spinning process is classified according to the fiber types the principle of multi-jet electro electrospinning, the differ- which we want to process into: chemical spinning and me- ent devices used for this technique are also highlighted. chanical spinning. Keywords: Multi-jet electrospinning, biopolymers, nanofibres, nonwoven mats Mechanical spinning

Mechanical spinning is a multistep process in which fibers 1 Introduction are spun into yarns physically. Viz., rotor, ring, friction, or self-twist spinning. Spinning is an art that has been utilized in formation of yarn by drawing out and twisting natural or synthetic fibers. Thousands of years ago, fibers were spun manu- Chemical spinning ally with simple tools as distaff and spindle. The spindle was the first tool used for spinning all the threads usedfor Making filament yarns from man-made fibers is feasible using chemical spinning operations. This is achieved by extruding the viscous polymer solution through a nozzle *Corresponding Author: Hosam El-Sayed: Industrial Textile Re- (spinneret). There are three main types of production pro- search Division, National Research Centre, El-Behouth St., Dokki, cesses to form synthetic fibers [2]. Giza, Egypt; Email: [email protected] The wet-spinning process: in which solidification of Claudia Vineis, Alessio Varesano, Riccardo Andrea Carletto, the soluble polymer occurs after a counter current diffu- Cinzia Tonetti: CNR-ISMAC, National Research Council of Italy- Institute for Macromolecular Studies, C. so G. Pella, 16-13900 Biella, sion between the spinning dope and the coagulation bath. Italy The dry-spinning process: where the evaporation of Salwa Mowafi, Marwa Abou Taleb: Industrial Textile Research the solvent contained in the spinning dope leads to solidi- Division, National Research Centre, El-Behouth St., Dokki, Giza, fication. Egypt

Open Access. © 2019 H. El-Sayed et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License A critique on multi-jet electrospinning: State of the art and future outlook Ë 237

Figure 1: Wet-spinning device [3]

The melt-spinning process: where phase transforma- tion is due to the solidification from a molten mass.

3 Different techniques of chemical spinning

3.1 Wet spinning technique

In this process a very viscous polymer solution is extruded through the small holes of a spinneret dipped in a liquid bath. Technological analysis of the wet spinning process is much more complicated than dry spinning where the spinning process is accompanied by chemical reaction. Solid- ification of the polymer occurs as a result of a diffusional exchange between the freshly prepared fluid filaments and this bath. During this process coagulation one or more of Figure 2: Melt-spinning process [8] the bath components diffuse into the filament, while the solvent diffuses out of it [3]. The polymer precipitates or 3.2 Melt spinning technique crystallizes as a consequence of this exchange, because it is rendered insoluble by chemical reaction of the polymer Melt spinning is the most appropriate and economic pro- or by an excessive buildup of non-solvent or by both [4] cess for polymer fiber manufacturing at industrial scales. (Figure 1). Spinning of polyester, or any other molten polymers like Wet spinning technique is found to have the advan- nylon and polypropylene, begins upon cooling the molten tage of producing reelable fiber that can be drawn and filament once it is withdrawn from the spinneret. Atthe tested for mechanical properties. Its main disadvantages same time, the filament is drawn towards the take-up sec- are (a) the slow rate of processing, as the produced fila- tion and the resulting tension offers a stretching in the ments need to be washed to remove impurities, and (b) molten filament itself [7]. low productivity. Polymers can be regenerated by wet spin- Because the apparent viscosity of the molten filament ning technique like, silk [5] and cellulose [6] by dissolu- increases as it cools, most of the stretching takes place tion in ionic liquids, followed by spinning in a coagulating in a region relatively close to the spinneret hole, whereas baths of methanol, acetonitrile, or water. Acrylic polymer most of the cooling takes place after leaving the hole as can be spun by wet spinning technique by dissolving the shown in Figure 2. The comparatively simple and easy pro- polymer in dimethyl formamide which can be coagulated cessing is the most important advantage of melt spinning. in water [4]. However, this technique has some drawbacks; Viz. possi- 238 Ë H. El-Sayed et al. ble fiber breakdown, heterogeneous thickness of the fila- fibril) or hollow shape (a tubule). The importance ofthis ment, and limit to the fineness of fiber and spinneret clog- method arises from the fabrication of nanometer tubules ging. and fibrils of various raw materials such as semiconduc- tors, metals, and electrically conducting polymers. Unlike drawing process, this method cannot make one-by-one 3.3 Dry spinning technique continuous nanofibers. The phase separation process consists of several steps; namely dissolution, gelation, extrac- In the process of dry spinning, the fiber structure is formed tion with various solvent, freeze-drying which leads to for- by forcing the polymer solution out through fine nozzle mation of porous nano-foam. The phase separation pro- and then evaporating the solvent. Technological analysis cess is found to take a relatively long time to convert the of the dry spinning process is much more difficult than polymer into the nano-porous foam. In self-assembly pro- of melt spinning which is treated as a problem of struc- cess, the pre-existing components are self-organized to the tural formation in a one-component system. Unlike melt required forms and functions. Like the phase separation spinning, in both dry and wet spinning, the polymer dis- process, the self-assembly is a time-consuming process solves in different solvents and the resulting solution or in spinning continuous polymer nanofibers. Electrospin- suspension is viscous spin dope [9]. This process mainly in- ning is a type of dry spinning technique where the solvent troduces another species, which is consequently removed, is evaporated under high voltage. Thus, the electrospin- and therefore it is more expensive than conventional melt ning process turns out to be the most developing process spinning processes. It is used in cases where the poly- for mass production of one-by-one continuous nanofibers mer may decompose thermally when melted or in cases from different polymers. where specific surface properties of the filaments are desired where melt spinning produces smooth surface filament and dry spinning produces rough surface filament. 5 Electrospinning and its The rough surfaces may be necessary for better dyeing steps or for special yarn features (Figure 3). applications

The process of polymer fiber formation within an electric field has been known since the 1930s. This technique was named electrostatic spinning or electro-spinning in the 1990s. Electro-spinning is the proper method to produce nanofibers [19]. Since 1980s and especially in recent years, the electrospinning process especially similar to that de- scribed by Baumgarten [20], has acquired more attention probably due to the excessive interest in nanotechnology, where ultrafine fibers for various polymers with submi- Figure 3: Dry-spinning process [10] crons or nanometer scale can be easily fabricated with this technique. A variety of polymers can be spun each from a specific solution and collected in the form of nonwoven mats. In addition, the ability to produce highly porous 4 Innovative processes for fiber nano-fibrous membranes with structural integrity is also an attractive feature of electrospinning [21]. When the di- manufacture ameters of spun fibers are decreased from micro-scale (e.g. 10–100 µm) to submicro-scale or nano-scale (e.g. 10–100 A number of processing techniques such as drawing [11], × 10−3 µm), then the fibers exhibit new technically fasci- template synthesis [12, 13], phase separation [14], self- nating features; Viz. up to 100% increase in the surface assembly [15, 16], electrospinning [17, 18] etc., have been area to volume ratio, enhanced superficial functions, im- used to prepare nanofibers in recent years. The draw- proved mechanical performance, and high porosity. These ing process is similar to dry spinning in fiber manu- outstanding properties make the polymer nanofibers to be facture, as it can make very long single nanofibers. As optimal candidates for many important applications for ex- per its name, the template synthesis uses templates of ample in medical applications [22] (viz., wound dressing) nanoporous membranes which make solid nanofibers (a [23], membrane separation [24–26] filtration [25], compos- A critique on multi-jet electrospinning: State of the art and future outlook Ë 239 ite reinforcement [27], scaffolding used in tissue engineer- 6 Device and theory ing [28] and protective clothing [29]. Electrospinning works with both polymer solutions From 1934 to 1944, Formhals published a series of (solution electro-spinning) and molten polymers (melt- patents [36–40], describing an experimental setup for the electrospinning). Melt electro-spinning currently seems to production of polymer filaments using the electrostatic be less efficient and requires several additional pieces of force. A polymer solution was subjected to an electric field equipment (e.g. heating devices, vacuum chamber, and between two electrodes of opposite polarity to produce fil- higher voltage power supplies), although it would be more aments. One of the electrodes was placed into the solu- appealing for industrial production because of the ab- tion and the other on a collector. Once the polymer left sence of solvents [30]. Mowafi et al. have developed a the spinneret, evaporation of the charged solution jets take non-woven keratin-based mats extracted from waste ker- place forming the fibers. The molecular mass and viscos- atinous materials; namely wastes of combing process of ity of the spinning polymer solution are the main parame- coarse wool fleece and feather [31]. In another investiga- ters which assign the appropriate potential difference be- tion, some biopolymers; such as keratin and cellulose in tween the two electrodes. The distance between the spin- the nano-fiber form, were used successfully to purify wa- neret and the collector is also an important factor where ter from heavy metal ions. Extraction of cellulose was car- short distance caused the spun fibers to stick to the col- ried out by cost-effective environmental friendly method lecting device as well as to each other, due to incomplete from cotton stalks, banana leaves, and rice straw. Similar solvent evaporation. There are three basic components to method was adopted to extract keratin from coarse wool. achieve the process: a high voltage supply, a needle of fine The said polymers were electro-sputtered on the surface diameter, and a metal collector. In the electrospinning pro- of untreated viscose fabrics or polyester fabric pretreated cess, a high voltage power supply is used to make an elec- with polyurethane [32]. trically charged jet of polymer out of the needle. Within the Membranes of electrospun keratin/polyamide 6 distance between the needle and the collector, the polymer blended nanofiber were prepared and used as adsorbents jet solidifies, and an interconnected web of very fine fibers of Cu+2 ions. The adsorption tests showed that keratin- is found [41]. Usually, the collector is earthed, as shown in based nanofibers adsorb2+ Cu ions and the adsorption Figure 4. capacity increases as the specific surface area of the nanofiber membranes increase. The adsorption onto the keratin rich nanofibers depend on pH. The optimal pH was found to be above the isoelectric point of keratin [33]. Pure keratin nanofibers can be also electrospun from formic acid solution. The obtained membranes showed higher adsorption capacity and removal efficiency with re- spect to a wool fabric due to the high specific surface area of keratin nanofibers [34]. Electrospun keratin/polyamide 6 nanofibers were also used for water purification from chromium ions and air depuration from formaldehyde. Infrared analysis and viscosity suggested a negligible interaction between keratin and polyamide 6 despite the similar chemical structure Figure 4: Nanofibers formation by electrospinning [41] characterized by amide bonds. However, all the blend solutions were suitable for electrospinning producing thin nanofibers with a mean diameter of ~150 nm. Keratin- The charged polymer solution jet is unstable and un- based nanofibers demonstrated a good capacity to ad- dergoes elongation, which allows the fiber to become ex- sorb chromium ions from water. Moreover, keratin-based tremely long and thin. In case of melt polymers, solidifica- nanofibers showed a chromium adsorption capacity of tion of the charged jet takes place while being in the air. one order of magnitude higher than that of the films with Up to our knowledge, around 50 polymeric materials the same composition. Keratin-based nanofibers showed a have been efficaciously electrospun into nano-fibers ofdi- good formaldehyde absorption capacity reducing airborne ameters in the range 3 nm - 1µm [42, 43]. Before elctrospin- formaldehyde concentration up to 70% [35]. ning, these polymers were solubilized in the respective appropriate solvents. 240 Ë H. El-Sayed et al.

7 Multi-jet electrospinning up to80 KV. The electrospun mat is collected on a nonwoven sheet of polyproplene with width 30cm (Figure 6).

Electro-spun nanofibers have many potential applications as shown in Figure 5, and the production rate of a conventional electrospinning system is less than 10 g h−1 of nanofibers [44], depending on polymer concentration and process conditions (e.g. flow rate of solution). Multi-jet electrospinning systems are designed to increase both productivity and cover area for large scale nanofiber production.

Figure 6: Picture of needleless electro-spinning device at the Textile Research Division, National Research Centre (TRD-NRC) and the jets during the electro-spinning experiment (40 kV nozzles, 13 cm distance, polyamide 6, in a mixture of formic acid and acetic acid)

Multi-jet electrospinning device affords the opportu- nity for electrospinning of multi-component polymers to obtain blend of nanofibrous mats of uniform thickness and adequate dispersibility. This approach also can be used to fabricate blend nanofibrous mats with multi-polymers Figure 5: Potential applications of electrospun polymer nanofibers. which cannot be dissolved in the same solvent or kept in the same container [46]. CNR-ISMAC developed a multi- It has been reported that, in multi-needles electrospin- nozzle electrospinning plant with a bottom-up configura- ning can be used to prepare skin-core structures. Forma- tion. It consists of 31 to 62 nozzles and a 50-cm wide metal tion of nanofiber filaments included two processes: form- collector (Figure 7a) [47, 48]. The developed configurations ing process of spun nanofiber filaments and post-drawing produce overlapped deposition zones in order to have an process. In the forming process of as-spun nanofiber fila- even nanofiber deposition on the collector. ments, when the auxiliary electrode was added, the elec- In multi-nozzle electrospinning, jet-jet repulsion may trostatic field interference between needles reduced, in- occur and can lead to a loss of nanofibers with a decrease ducing the decrease of jet offsets and the enhancement of in productivity [47], i.e. not all the solidified jets can be col- Taylor cone and jet stability, and nanofibers with skin-core lected by the system. In the present plant, each electrospin- structure were finally deposited on the bath in good condi- ning nozzle of plant produces an electrified jet. In order to tion [45]. tackle this effect, the metal collector is electrically charged Electrospun nanofibre jets can be obtained from either with a polarity opposite to that of the spinneret and jets. multi-nozzle electrospinning device or nozzleless electro- In this way nanofibers gathering is more efficient and elec- spinnnig device. In other words, nanofibre mats can be trospinning is steady. Figure 7b shows the jets from the produced using any of the aforementioned devices. straight path to the whipping motion zones before reach- In our laboratories, we adopted a needleless electro- ing the negatively charged collector. spinning technique with a bottom up configuration. This technique is based on dipping a 30 cm wire with the electrospun polymer and subjecting the wire to a high voltage A critique on multi-jet electrospinning: State of the art and future outlook Ë 241

Figure 7: (a) Picture of 31-nozzle configuration plant at CNR-ISMAC Biella. (b) Picture of jet and whipping motion during a multi-nozzle electrospinning experiment (+35 kV nozzles, −20 kV collectors, 30 cm distance, 9 wt.% PEO solution in water).

7.1 Different multi-jet devices

There are two main types of multi-jet electrospinning devices; namely nozzleless electrospinning and nozzles electrospinning. Different setups of these devices were de- Figure 9: Scheme of multi-jet electrospinning setups: (a) electrically signed by controlling the tips-to-collector distance and charged solution and grounded collector, (b) electrically charged the applied voltage (working distance). Either the solution collector and grounded solution, and (c) electrically charged solu- or the collector was electrically charged with positive or tion and grounded collector with secondary electrode [47]. negative polarity. Also, varying the number of nozzles or the shape of the multi-jet electrospinning heads were de- movable multi-jet and movable earthed tubular signed for different applications. Some samples of the de- layer is adopted to obtain a uniform thickness of signed devices for different purposes and applications are blend nanofibrous mats with adequate dispersibility presented below. of multi-component as in (Figure 10) [46]. 1. A nozzleless electrospinning device was mainly de- 4. A multi-jet electrospinning device was designed ei- veloped for increasing the yield of nano-fibrous ther with movable or immovable collector. The ro- mats (Figure 8) [49]. tatable multi-jet and movable earthed tubular layer 2. As it was mentioned, the yield of conventional elec- is adopted to obtain a uniform thickness of blend trospinning process is low compared to the applica- nanofibrous mats with adequate dispersibility of tions. So, a multi-jet electrospinning system was de- multi-component [50]. signed to increase the productivity (Figure 9) [47]. From the previous data about the different types of 3. A device was designed to produce mats of blend multi-jet devices, it can be noticed that: nano-fibers with multi-component polymers. The 242 Ë H. El-Sayed et al.

4. In 2004, a biodegradable blended nanofibrous mat composed of polyvinyl alcohol (PVA) and cellulose acetate (CA) was prepared by multi-jet electrospinning [46]. 5. In 2006, polyamide-6 fibers with average diameters below 500 nm were electrospun on conventional air filter media using a multi-nozzle head [51]. 6. Since 2007, wool keratin/poly (ethylene oxide) nanofibers were electrospun from aqueous solutions of polymer blends under different operating conditions [19, 52]. 7. In 2008, novel cellulose fibers (Biocelsol) were spun by wet spinning technique from alkaline solution. The fibers were spun into a coagulating bath contain- ing sulphuric acid and sodium sulphate [53]. 8. In 2008, a direct approach to fabricate ceramic fibres of hydrous alumina from alkoxide-based pre- cursors. These fibres were made for various appli- Figure 10: Scheme of the multi-jet with multi-nozzles [46]. cations that require faster electrospinning rates and lack of impurities from additives. Alkoxide-based solutions formed the backbone of these fibes, where Each device was built to be used in the production of the utilization of the hydrolysis and condensation ki- nanofibres of specific properties. Also, the type of device netics allowed for stabilization of the jetn and hence may differ according to the type of spun polymer andits continous fibre formation [54]. properties viz., molecular weight, viscosity and conductiv- 9. In 2011, mats of random patterns of nano-filaments, ity. So, it would be more p referable to use nozzleless multi- from electrospun keratin/polyamide 6 blends in jet electrospinning technique for the spinning of highly vis- formic acid, were utilized as adsorbents of Cu2+ ions cous polymers to avoid the clogging of the needles in the [33, 34]. multi-nozzle systems. On the other hand, it is more con- 10. In 2012, activated carbon fibre webs (ACF) are pre- venient to use the multi-nozzle system for the spinning of pared from electrospun PAN by air-oxidation sta- more than one polymer that could be soluble in different bilization and CO2-activation at different tempera- media. tures [55]. 11. In 2013, fabrication and characterization of silk sericin (SS) nanofibers via electrospinning was 8 Different substrates spun by made [56] and silk fibroin (SF) [57]. 12. In 2016, wet spinning and drawing of human recom- different spinning techniques binant collagen which lead to significant progress in the field of advanced bio textile-based tissue engi- 1. In 1968, fiber formation of gelled solution of acrylic neering and regenerative medicine [58]. polymer was performed by wet spinning method. 13. In 2017, electrospinning was used for the first time This process was called coagulation process, where to produce nanofibers including a host/guest com- diffusional interchange between the freshly pre- plex in a protein-based matrix. A lipid binding pro- pared fluid filaments and the liquid bath results in tein was the host and Irgasan, an insoluble biocide solidification of the polymer [4]. molecule, was bound within the protein internal cav- 2. In 1971, fibers from dimethylformamide (DMF) so- ity as the guest [59]. lution of acrylic resin were spun by electrostatic means [20]. 3. In 2001, fiber mats and yarns were electrospun from Future outlook polyethylene oxide which are of interest for a variety of applications including semipermeable mem- Electrospinning technique can be utilized to produce ex- branes, filters...etc [17]. tremely fine fibers in the form of a non-woven mat.It A critique on multi-jet electrospinning: State of the art and future outlook Ë 243

(a) Electrospun acrylic resin in DMF [20] (b) Electrospun fibroin fibers [57] (c) Electrospun keratin nanofibers [60]

(d) Electrospun keratin/PEO nanofibers (e) Electrospun keratin/PA6 nanofibers (f) Electrospun PEO fibers [17] 50/50 [52] 70/30 [33]

(g) Electrospun PA6 fibers [61] (h) Electrospun PVA/CA nanofibrous mats 3/1 [46]

Figure 11: (a-h) show the scanning electron micrographs of some electro-spun polymers. has been suggested that nanofibers have better mechani- mats can be utilized in the following possible future appli- cal properties than microfibers. So, high productivity us- cations: a) capture of heavy metal ions from highly con- ing the multi-jet systems is required for practical applica- taminated euents, b) absorption of dyes from dye-house tions of polymer nanofibers where most of these applica- drainage water. Within the frame of bio-medical applications are only considered as future prospect. The possi- tions; peculiar properties of nanofibrous materials such as bility of blending more than biopolymer through multi-jet high surface to volume ratio and high porosity, make them electrospinning would produce nanofibers exhibiting tai- promising candidates for several biomedical applications, lored properties for certain desired future applications. such as cell-growth scaffolds, wound dressing and drug de- The huge surface area of the electrospun nanofibers livery systems. would produce an appropriate candidate for many applications; filtration for instance. Multi-jet electrospun polymer 244 Ë H. El-Sayed et al.

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