Recent Advances on Nanofiber Fabrications: Unconventional State-Of-The-Art Spinning Techniques

Review Recent Advances on Nanoﬁber Fabrications: Unconventional State-of-the-Art Spinning Techniques

Jinkyu Song 1, Myungwoong Kim 2,* and Hoik Lee 3,*

1 Division of Nano-Convergence Material Development, National Nano Fab Center (NNFC), Daejeon 34141, Korea; [email protected] 2 Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Korea 3 Research Institute of Industrial Technology Convergence, Korea Institute of Industrial Technology, Gyeonggi-do, Ansan 15588, Korea * Correspondence: [email protected] (M.K.); [email protected] (H.L.)

Received: 25 May 2020; Accepted: 18 June 2020; Published: 20 June 2020

Abstract: In this review, we describe recent relevant advances in the fabrication of polymeric nanofibers to address challenges in conventional approaches such as electrospinning, namely low throughput and productivity with low size uniformity, assembly with a regulated structure and even architecture, and location with desired alignments and orientations. The efforts discussed have mainly been devoted to realize novel apparatus designed to resolve individual issues that have arisen, i.e., eliminating ejection tips of spinnerets in a simple electrospinning system by effective control of an applied electric field and by using mechanical force, introducing a uniquely designed spinning apparatus including a solution ejection system and a collection system, and employing particular processes using a ferroelectric material and reactive precursors for atomic layer deposition. The impact of these advances to ultimately attain a fabrication technique to solve all the issues simultaneously is highlighted with regard to manufacturing high-quality nanofibers with high- throughput and eventually, practically implementing the nanofibers in cutting-edge applications on an industrial scale.

Keywords: nanoﬁber; electrospinning; needleless spinning; mass production; structural regulation

1. Introduction Nanofibers belong to a group of most interesting and promising materials due to their unique physicochemical properties, i.e., extraordinary porosity with interconnectivity between pores in mats, mechanical flexibility and strength, high surface area, and high applicability to fabricate composites with other materials [1–3]. These characteristics enabling its use for a variety of applications have attracted tremendous scientific and technological interests [4–8]. Among various nanofiber fabrication techniques, such as melt spinning [9], wet spinning [10], dry spinning [11] and others, an electrospinning utilizing electrical force to form polymeric fibers with a diameter ranging from nanometer to sub-micrometer has been considered as an important workhorse academically as well as industrially. In the early days, Lord Rayleigh0s studies showed that an electrical force can overcome the surface tension of a small drop of solution [12]. While the studies have been an important background for developing electrospinning, Formhals registered a crucial patent related to electrospinning technique for the first time in 1934 [13]. In 1969, Taylor observed the formation of cone type jets from viscose fluids with an electrical field, the-so-called “Taylor cone” [14]. Since the 1980s up to now, the electrospinning technique has widely been utilized due to the accessibility and applicability to nanotechnology. More importantly, it has been rapidly evolved in its practical use on the basis of fundamental studies on significant aspects of the electrospinning process [15].

Polymers 2020, 12, 1386; doi:10.3390/polym12061386 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 19

Polymersapplicability2020, 12, to 1386 nanotechnology. More importantly, it has been rapidly evolved in its practical use2 of 20on the basis of fundamental studies on significant aspects of the electrospinning process [15]. In general, electrospinning is regarded as an easily accessible, highly effective and efficient nanofiberIn general, fabrication electrospinning method. One is regardedof its significant as an benefits easily accessible, is the versatility highly to e ffuseective a number and e ffiofcient types nanofiberof materials, fabrication e.g., organic method. mate Onerials of its ranging significant from benefits small ismolecules the versatility to macromolecules, to use a number inorganic of types ofspecies materials, and e.g.,particles, organic and materials organic/inorganic ranging from hybrids. small moleculesA typical toelectrospinning macromolecules, apparatus inorganic consists species of anda high particles, voltage and power organic supply,/inorganic a spinneret hybrids. connected A typical to electrospinningthe power supply, apparatus and a collector consists having of a high the voltageopposite power charge supply, to the a spinneretejection spinneret connected charge to the power[16]. The supply, fiber andsource a collector is typically having a homogeneous the opposite chargepolymer to thesolution ejection which spinneret is injected charge into [16 the]. Thespinne fiberret. source A polymer is typically solution a homogeneous droplet at the polymer end of a solutionspinneret which is exposed is injected to an into applied the spinneret. electric field, A polymer and the solutionelectric field droplet induces at the charges end of aon spinneret the droplet is exposedsurface to[17]. an appliedWhen the electric voltage field, is andsufficiently the electric high field to inducesovercome charges the droplet on the dropletsurface surfacetension, [17 the]. Whenrepulsive the voltageelectrical is suforcefficiently stretches high out to the overcome droplet. the The droplet stretched-out surface jet tension, is rapidly the repulsiveaccelerated electrical to form forcethe structure stretches of out the the nanofiber droplet. [2]. The The stretched-out morphology jet of is rapidlythe resulting accelerated nanofibers to form is effectively the structure controlled of the nanofiberby many specific [2]. The processing morphology parameters, of the resulting i.e., solution nanofibers viscosity, is eff appliedectively voltage, controlled solution by many injection specific rate, processingdistance between parameters, spinneret i.e., solution tip and viscosity,collector, appliedsolvent voltage,conductivity, solution temperature, injection rate, and distancehumidity between [18]. spinneretDespite tip and the collector,outstanding solvent characteristics conductivity, of temperature,electrospinning, and improvement humidity [18]. to address important issuesDespite is in progress the outstanding by partially characteristics modifying of the electrospinning, spinning process improvement or even developing to address new important types of issuesspinning is in process progress and by apparatus. partially modifying Most notably, the spinningthe throughput process of or conventional even developing electrospinning new types has of spinningbeen a serious process bottleneck and apparatus. in its practical Most notably, applications the throughput. Furthermore, of conventional it is still challenging electrospinning to achieve has beenthe fabrication a serious bottleneck of nanofibers in its with practical (i) a highly applications. regulated Furthermore, structure, (ii) it the is stilldesired challenging fiber alignment to achieve and theorientation, fabrication and of nanofibers(iii) three-dimensional with (i) a highly complexity regulated in the structure, current (ii) electrospinning the desired fiber process. alignment (Figure and 1) orientation,In the perspective and (iii) of three-dimensional the materials, there complexity has been in a the significant current electrospinning limitation on the process. range (Figure of materials1) In theused perspective in the fabrication; of the materials, for example, there hassome been polymers a significant, such limitation as polyolefins, on the with range poor of materials solubility used in a inrange the fabrication; of solvents, for or example,with high some electrical polymers, resistivity, such ascannot polyolefins, be utilized with as poor a homogeneous solubility in apolymer range ofsolution solvents, where or with reasonable high electrical electrical resistivity, conductivity cannot is be required utilized as[19]. a homogeneousConsidering these polymer issues, solution much whereresearch reasonable effort has electrical been devoted conductivity to achieve is required advanc [ed19 ].spinning Considering systems. these To issues, date, researches much research have efffocusedort has on been the devoted invention to of achieve new spinneret advanced systems spinning with systems. unique To designs, date, researches e.g., blow-jet have spinning focused [20], on thecentrifugal invention spinning of new spinneret [21], microfluidic systems with spinning unique designs,[22], bubble e.g., blow-jet electrospinning spinning [20[23],], centrifugal core-shell spinningelectrospinning [21], microfluidic [24,25], and spinning the combination [22], bubble of electrospinning several current [ techniques.23], core-shell In electrospinningthis review, we [explore24,25], andseveral the combinationnotable recent of several nanofiber current fabrication techniques. techniques, In this review, and wehighlight explore novel several and notable remarkable recent nanofiberapproaches fabrication which potentially techniques, offer and highlightmore efficient novel and remarkableeffective pathways approaches towards which high potentially quality onanofiberffer more estructuresfficient and in e ffa ectiverapid pathwaysmanner with towards regard high to regulated quality nanofiber structure structures and architecture in a rapid with manner high withcomplexity. regard to We regulated envision structure that strategies and architecture covered in with the highcurrent complexity. review should We envision have the that potential strategies to coveredstimulate in the the development current review of shouldother ne havew techniques the potential as well to stimulateas to facilitate the development beneficial applications of other new that techniqueshave impact as on well our as daily to facilitate lives. beneficial applications that have impact on our daily lives.

FigureFigure 1. 1. ChallengesChallenges inin thethe conventionalconventional nanofibernanofiber fabricationfabrication techniquetechnique andand didifferentfferentnotable notable approachesapproaches for for advanced advanced nanofiber nanofiber production. production. [Reproduced [Reproduced with with permission permission from from 42, 42, 63, 63, 98, 98, 81, 81, 87, 87, andand 91. 91. Copyright Copyright (2017, (2017, 2016, 2016, 2019, 2019, 2019, 2019, 2019 2019 and and 2019) 2019) American American Chemical Chemical Society]. Society].

Polymers 2020, 12, 1386 3 of 20

2. Needleless Spinning for Large Scale Production In typical electrospinning to fabricate nanofiber mats, a single spinneret is employed because it provides a convenient and cost-effective process. However, it does not offer high throughput which is one of the important requirements for practical and industrial applications. The typical production rate of the single spinneret electrospinning falls in the range of 0.01–0.1 g h 1 [26], which highly limits · − its applicability in practice. One of the simplest and easiest approaches to increase the throughput and enhance the productivity is the use of multiple nozzles [27,28]. However, there have still been challenges in multi-tip electrospinning; for example, undesirable interaction between formed jets which is caused by the interference of the electric fields, and difficulties in the cleaning process for a number of tips eventually degrades the quality of the fabricated nanofibers. In addition, clogging of the needle tip frequently occurs due to its small diameter, preventing continuous spinning. The multi-tip system also requires huge space for the equipment, leading to increase of cost of the production process. To overcome this issue, needleless electrospinning has been considered as an effective method to maximize the continuity of the fabrication process and therefore, the productivity. Contrary to conventional electrospinning, in the needleless electrospinning technique, an electrical force is applied on the liquid surface directly without using a needle nozzle [23,29]. To date, many needleless electrospinning techniques have been reported with systematic variation in the spinneret geometry, e.g., spiral coil [30,31], rotary cone [32], sprocket wheel disk [33], rotating disk [34], self-cleaning threaded rod [35], twisted wire [36,37], conical wire coil [38], metal dish [39], double-ring [40], curved slot [41,42], bowl [43], and tube [44]. The open spinneret in the needleless electrospinning suppresses the clogging of the tip, facilitating multiple jet operations. Consequently, far higher fabrication productivity is feasible: the quantitative range is from 0.5 g h 1 to 600 g h 1 which is much higher than · − · − that of conventional electrospinning with a single needle [45–47]. Representative improvements are summarized in Table1. In this section, we describe the recently developed new techniques related to needleless electrospinning that can achieve an outstanding throughput.

Table 1. Summary of productivity of diﬀerent spinning techniques.

Voltage Productivity Type Spinneret Polymer Ref (kV) (g h 1) · − Secondary coil (T-coil) PVA 85 ~9.72 [31] Double-ring slit PVA 30 ~2.25 [40] Twisted wire PVP 20 ~1.023 [37] Rotating-disk PCL 25 ~10.611 [34] Threaded rod PEO 60 ~5.2 [35] Rotating spiral wire coil PVA 60 ~9.42 [30] Needleless Curved convex slot PVA 70 ~2 [41] electrospinning Foam Naﬁon 25 ~9.73 [45] Bowl edge PEO 16 ~0.684 [43] Rotating cone PVP 30 ~600 [32] Umbrella nozzle PLLA 30 ~180 [46] Curved slot with PVA 60 ~1.98 [42] temperature elevation Needle Roller PVA 40 12.8 [47] Multi-nozzle (19 nozzle) PEO 15 ~0.712 [27] Nozzle Coaxial with air-blowing PAN(Core)/TPU(Shell) 38 ~3.6 [25] electrospinning Porous hollow tube (13 PVP 40–60 0.3–0.5 [28] cm long, 20 holes) Polymers 2020, 12, 1386 4 of 20

2.1. Double-Ring Slit as Electrospinning Spinneret Contrary to conventional electrospinning which is a closed system, in a needleless spinneret, a polymer solution is exposed to air, inducing solvent vaporization during electrospinning and hence, resulting in inconsistency in the quality of fabricated fibers due to the changes in polymer solution concentration [23]. Recently, Wei et al. introduced a double-ring slit as a spinneret for Polymers 2020, 12, x FOR PEER REVIEW 4 of 19 electrospinning to address this challenge and to improve fabrication productivity by simultaneous formation2.1. of Double-Ring multiple Slit jets as during Electrospinning electrospinning Spinneret [40]. The schematic diagrams for the double-ring slit needlelessContrary spinneret to conventional (DRSNS) electrospinning setup are presented which is a inclosed Figure system,2A,C, in a andneedleless the photographs spinneret, a from front andpolymer top views solution also is areexposed shown to air, in inducing Figure 2solvD,E.ent Thevaporization double-ring during basedelectrospinning electrospinning and hence, setup is quite similarresulting to conventional in inconsistency single in the needlequality of spinneret fabricated (SNS) fibers due electrospinning; to the changes in spinneret, polymer solution syringe pump, concentration [23]. Recently, Wei et al. introduced a double-ring slit as a spinneret for electrospinning high-voltageto address generator, this challenge and metal and collector to improve (Figure fabrication2B);however, productivity the by key simultaneous component formation for DRSNS of based electrospinningmultiple is jets the during use ofelectrospinni a double-ringng [40]. slit The as schematic spinneret. diagrams As shown for the in double-ring Figure2C, slit the needleless double-ring slit consists ofspinneret four parts, (DRSNS) i.e., setup outer are ring, presented inner in ring, Figure inner 2A,C, core, and the and photographs shell. The from inner front core and and top shell are made fromviews polytetrafluoroethylene also are shown in Figure 2D to,E. insulate The double-ring the current, based but electrospinning the outer and setup inner is quite rings similar are to composed conventional single needle spinneret (SNS) electrospinning; spinneret, syringe pump, high-voltage of copper to effectively apply an electrical field to the polymer solution. The width of the inner generator, and metal collector (Figure 2B); however, the key component for DRSNS based and outerelectrospinning slit was 0.5 is mm, the use and of a the double-ring diameter slit andas spinneret. height As of shown the spinneret in Figure 2C, was the 35 double-ring mm and 40 mm, respectively.slit consists The DRSNS of four parts, was i.e., mounted outer ring, on inner a pedestal, ring, inner and core, the and polymer shell. The solution inner core was and injectedshell are into the inner coremade of a double-ringfrom polytetrafluoroethylene spinneret, and to the insulate solution the transportedcurrent, but intothe outer the double-ringand inner rings slit. are The ejected polymercomposed solution of from copper the to innereffectively and apply outer an circleelectrical slit field is highlyto the polymer charged solution. owing The towidth the of copper the rings inner and outer slit was 0.5 mm, and the diameter and height of the spinneret was 35 mm and 40 mm, impartingrespectively. an electrostatic The DRSNS repulsion was mounted force on that a pedestal, counteracts and the thepolymer effect solution of surface was injected tension. into the As a result, the polymerinner solution core of a isdouble-ring allowed spinneret, to be stretched and the so outlution to formtransported nanofibers. into the Indouble-ring this process, slit. The the loss of polymerejected solution polymer is limited solution eff fromectively, the inner and and the outer multiple circle slit formation is highly charged of a Taylor owing coneto the ofcopper the polymer solution occursrings imparting and therefore, an electrostatic enhancing repulsion the force mass that production counteracts ofthe nanofiber. effect of surface The tension. productivity As a of this techniqueresult, was the compared polymer tosolution the SNS, is allowed with to experiments be stretched out to to fabricate form nanofibers. poly(lactic In this acid) process, (PLLA) the loss nanofibers. of polymer solution is limited effectively, and the multiple formation of a Taylor cone of the polymer Noticeably,solution the productivity occurs and therefore, of the DRSNS-basedenhancing the mass electrospinning production of nanofiber. process wasThe productivity improved aboutof this 22 times, 1 1 from 0.10technique0.03 g wash tocompared2.25 0.25to the g hSNS,(Figure with experiments2F), under to the fabricate same spinningpoly(lactic conditionsacid) (PLLA) including ± · − ± · − feed rate,nanofibers. voltage, andNoticeably, rotating the speed. productivity Two of aspects the DRSNS-based made this electrospinning improvement process feasible: was improved (i) precise control −1 −1 of the exposedabout 22 area times, of the from polymer 0.10 ± 0.03 solution g·h to to 2.25 the atmosphere± 0.25 g·h (Figure using 2F), a control under the pump same which spinning reduced the conditions including feed rate, voltage, and rotating speed. Two aspects made this improvement loss of polymerfeasible: solution (i) precise during control theof the spinning exposed process, area of the and polymer (ii) concurrent solution to formation the atmosphere of multiple using a jets with the circularcontrol narrow pump slit. which DRSNS reduced was the applicable loss of polymer to other solution types during of polymers the spinning such process, as polyacrylonitrile, and (ii) poly(vinylconcurrent alcohol), formation poly(methyl of multiple methacrylate), jets with the poly(DL-lactic circular narrow slit. acid), DRSNS and was polycaprolactone, applicable to other to achieve a multilayeredtypes of nanofiberpolymers such structure as polyacrylonitrile, which may poly(vinyl be implemented alcohol), poly(methyl further methacrylate), for bio-related poly(DL- applications such as druglactic loadingacid), and systems. polycaprolactone, to achieve a multilayered nanofiber structure which may be implemented further for bio-related applications such as drug loading systems.

Figure 2.FigureSchematic 2. Schematic diagrams diagrams of ( Aof )(A the) the double-ring double-ring slit slit needleless needleless spinneret spinneret (DRSNS) (DRSNS) setup, (B setup,) single ( B) single needle electrospinning setup, and (C) diagram of the internal structure of the DRSNS apparatus, needle electrospinningphotographs of DRSNS setup, from and (D)( Cfront) diagram view and of(E) thetop view, internal and ( structureF) a plot showing of the comparison DRSNS of apparatus, photographsnanofiber of DRSNS productivities from of (D DRSNS-) front and view single and need (Ele) topspinneret view, (SNS)-based and (F) a electrospinning plot showing systems. comparison of nanoﬁber[Adapted productivities with permission of DRSNS- from 40. and Copyright single (2019) needle American spinneret Chemical (SNS)-based Society]. electrospinning systems. [Adapted with permission from 40. Copyright (2019) American Chemical Society]. Polymers 2020, 12, 1386 5 of 20

2.2. Two-Level Coil Edge Electrospinning Among a variety of possible spinneret structures, a curved geometry has attracted attention due to its high electrospinning process efficiency. Yan et al. investigated the effect of a slot line shape on electrospinning productivity and the quality of resulting nanofiber by comparing four convex slot spinnerets with different line shapes, i.e., straight, rectangular, triangular, and curved shapes [41]. In the curved slot, higher electric field with more uniform distribution could be applied along the slot length direction, resultingPolymers 2020, in 12, structuralx FOR PEER REVIEW uniformity as well as improved throughput. The profile5 of of19 the formed electric field2.2. was Two-Level critical; Coil WangEdge Electrospinning et al. utilized a finite element method (FEM) to analyze the electric field intensity profilesAmong of a a cylinder,variety of possible disc, and spinneret coil spinneretsstructures, a curved [30]. Theygeometry found has attracted that larger attention curvature due leads to higher electricto its fieldhigh electrospinning on a coil surface. process Maximizing efficiency. Yan this et al. e investigatedffect for needleless the effect of electrospinning a slot line shape on was feasible with coil-basedelectrospinning electrospinning. productivity and It typically the quality uses of resulting a smooth nanofiber wire by coil comparing as a spinneret four convex [ 48slot]. However, there are alsospinnerets challenges with different such as line corona shapes, discharge i.e., straight, and rectangular, breakdown triangular, of the and electric curved shapes field [41]. to make In a curved the curved slot, higher electric field with more uniform distribution could be applied along the slot structure withlength the direction, spinneret resulting [49 ].in Particularly,structural uniformity the electric as well as field improved is often throughput. unevenly The formedprofile of at a certain position inthe the formed curvature, electric inducingfield was critical; corona Wang discharge et al. utilized which a finite leads element to unnecessary method (FEM) energy to analyze consumption and therefore,the electric process field eintensityfficiency profiles becomes of a cylinder, lower disc, than and expected.coil spinnerets The [30].additional They found that use larger of energy also curvature leads to higher electric field on a coil surface. Maximizing this effect for needleless can result inelectrospinning unexpected was equipment feasible with damage.coil-based electrospinning. Therefore, it It is typically important uses a tosmooth design wire acoil curvature as a shape of the spinneretspinneret to [48]. realize However, an etherefficient are also electrospinning challenges such as system corona discharge for ensuring and breakdown the mass of productionthe of high-qualityelectric nanofibers field to make with a curved the desirable structure with size the uniformity. spinneret [49]. Particularly, the electric field is often Niu etunevenly al. recently formed presented at a certain aposition new typein the ofcurv coil-basedature, inducing electrospinning, corona discharge whichwhich leads uses to a secondary unnecessary energy consumption and therefore, process efficiency becomes lower than expected. The coil structureadditional on the use coil of energy spinneret, also can to result overcome in unexpe thected aforementionedequipment damage. Therefore, challenges it is important [31]. The two-level coil (T-Coil)to design electrospinning a curvature shape apparatus of the spinneret was builtto realize with an efficient a high electrospinning voltage power system supply, for ensuring metal collector, helical coil,the and mass plastic production solution of high-quality container, nanofibers as shown with in the Figure desirable3a. size The uniformity. helical coil is partially immersed Niu et al. recently presented a new type of coil-based electrospinning, which uses a secondary in the polymercoil structure solution on the bath, coil andspinneret, it rotates to overcome along the the aforementioned axis during challenges the spinning. [31]. The The two-level T-Coil spinneret is made bycoil winding (T-Coil) electrospinning the second apparatus coil onto was a built normal with a coil high (N-coil) voltage power with supply, a spacer metal template collector, to keep a consistenthelical coil pitch. coil, and Theplastic electrical solution container, power as is shown supplied in Figure by 3a. immersing The helical coil an is partially electrode immersed into the plastic solution bath.in the With polymer this solution setup, bath, the and eff itect rotates of the along secondary the axis during coil the using spinning. FEM The analysis T-Coil spinneret was investigated, is made by winding the second coil onto a normal coil (N-coil) with a spacer template to keep a as shown inconsistent Figure 3coilb. Interestingly,pitch. The electrical the power electric is supplied field in theby immersing T-Coil is an predominantly electrode into the centralized plastic on the secondarysolution coil rather bath. thanWith this the setup, primary the effect coil, of whereasthe secondary that coil in using the N-CoilFEM analysis is centralized was investigated, on the primary coil. The secondaryas shown in coilFigure enhances 3b. Interestingly, the electric the electric field field on in itsthe surface,T-Coil is predominantly inducing a reductioncentralized on of threshold the secondary coil rather than the primary coil, whereas that in the N-Coil is centralized on the voltage forprimary jet initiation. coil. The Insecondary addition, coil enhances the secondary the electr coilic field provides on its surface, an additional inducing a reduction area for of jet formation on its surface,threshold resulting voltage infor thejet initiation. electrospinning In addition, productivitythe secondary coil increasing provides an upadditional to 170% area comparedfor jet to the 1 1 N-coil (N-coilformation productivity: on its surface, 2.94 resulting g h− ,in T-coil the electr productivity:ospinning productivity 8.1 g h− increasing). These up results to 170% highlight the · −1 · −1 importancecompared of the designto the N-coil of a (N-coil novel productivity: electrospinning 2.94 g·h spinneret, T-coil productivity: with regard 8.1 g·h to). massThese results production with highlight the importance of the design of a novel electrospinning spinneret with regard to mass high throughput.production with high throughput.

Figure 3. (Figurea) Schematic 3. (a) Schematic illustration illustration of of the the two-level two-level coil coil electrospinning electrospinning setup, and setup, (b) electric and ( bfield) electric field intensity profiles of two-level coil (T-Coil) and normal coil (N-Coil). [Adapted with permission from intensity profiles of two-level coil (T-Coil) and normal coil (N-Coil). [Adapted with permission from 31. 31. Copyright (2018) American Chemical Society]. Copyright (2018) American Chemical Society]. 2.3. Rotary Cone as Electrospinning Spinneret 2.3. Rotary Cone as Electrospinning Spinneret Lu et al. demonstrated a super-high-throughput electrospinning technique employing an Lu etelectriferous al. demonstrated rotary cone as a a super-high-throughputspinneret [32]. This technique improved electrospinning the productivity technique of nanofiber employing an −1 electriferousfabrication rotary up cone to 600 as g· ah spinneret. This achievement [32]. Thisis remarkable technique as theimproved typical productivity the productivity of single nozzle of nanofiber electrospinning is in the range of 0.01–0.1 g·h−1 [26]. This electrospinning system has similar 1 fabricationcomponents up to 600 to ga htypical− . This electrospinning achievement apparatu iss remarkable except for the as cone-type the typical spinneret. productivity Upon the of single · 1 nozzle electrospinning is in the range of 0.01–0.1 g h [26]. This electrospinning system has similar · − components to a typical electrospinning apparatus except for the cone-type spinneret. Upon the injection Polymers 2020, 12, 1386 6 of 20 of polymer solution into the cone, a droplet is formed and charged immediately. The charged polymer droplet flows along the rotating surface of the cone under the coaction of gravity, the moment of inertia, Polymers 2020, 12, x FOR PEER REVIEW 6 of 19 and the electric force. Once the liquid droplet reaches the edge of the cone, it starts to deform and stretch out toinjection the fiber ofstructure. polymer solution A number into of the jets cone, are formeda droplet around is formed the edgeand charged of the rotating immediately. cone asThe feeding of thecharged polymer polymer solution droplet continues, flows along resulting the rotating in an increase surface of productionthe cone under rate. the The coaction spinning of gravity, rate is high enoughthe tomoment be applied of inertia, to an and industrial the electric production force. Once process. the liquid It isdroplet notable reaches that the though edge thisof the technique cone, it was reportedstarts in to 2010, deform the and productivity stretch out is to the the highest fiber structure. gain among A number nanofiber of jets productionare formed around techniques the edge so far. of the rotating cone as feeding of the polymer solution continues, resulting in an increase of 2.4. Foamproduction Based rate. Needleless The spinning Electrospinning rate is high enough to be applied to an industrial production process. It is notable that though this technique was reported in 2010, the productivity is the highest gain Solutionamong nanofiber blow spinning production which techniques uses so concentric far. nozzles and compressed gas also has attracted attention in nanofiber fields [50–55]. The high pressure gas such as air, nitrogen, and argon is released from2.4. the Foam outer Based nozzle, Needleless and theElectrospinning polymer solution is ejected through the inner nozzle. This technique has also beenSolution considered blow spinning as an which effective uses way concentric to achieve nozzles a higher and compressed production gas rate also than has thatattracted of typical electrospinningattention in nanofiber [55]. This fields process [50–55]. is kindredThe high pressure to the melt-blown gas such as air, technique. nitrogen, and The argon polymer is released solution is stretchedfrom the by theouter co-flowing nozzle, and gas the jetpolymer from solution the outer is ejected jet, leading through to the formation inner nozzle. of a This polymer technique nanofiber structure.has also Similar been considered to blow spinning, as an effective bubble way spinning to achieve (which a higher is named production foam rate electrospinning) than that of typical also uses a gas.electrospinning However, the [55]. gas isThis injected process with is kindred the submerged to the melt-blown nozzles intechnique. solution The making polymer bubbles, solution leading is to the possibilitystretched by of the fibers co-flowing initiating gas from jet from the bubblethe outer surface. jet, leading This to type formation of spinning of a polymer using single nanofiber or several structure. Similar to blow spinning, bubble spinning (which is named foam electrospinning) also uses bubbles in a vast bath of solution has been presented in various reports [56–58]. a gas. However, the gas is injected with the submerged nozzles in solution making bubbles, leading Anto the interesting possibility approach, of fibers initiating by Hwang fromet the al., bubble to exploit surface. the This foam type of of polymer spinning solution using single has recentlyor exhibitedseveral improved bubbles in productivitya vast bath of solution in Nafion has nanofiberbeen presented fabrication in various [45 reports]. Nafion [56–58]. is a widely utilized polymer inAn the interesting field of approach, electrolyte by membranes Hwang et al., owing to exploit to its the high foam water-saturated of polymer solution proton has recently conductivity (0.1 Sexhibitedcm 1), as improved well as its productivity thermal, mechanical, in Nafion nanofiber and chemical fabrication stability [45]. [ 59Nafion]. Nafion is a haswidely been utilized engineered · − in manypolymer different in the physical field of electrolyte forms, e.g., membranes bulk such owing as pellet, to its high film, water-saturated and dispersion proton in a mixedconductivity solution of alcohol(0.1 and S·cm water.−1), as well Its as nanofibrous its thermal, mechanical, structurehas and attractedchemical stability much attention[59]. Nafion because has been it engineered could improve protonin conductivitymany different and physical subsequently, forms, e.g., the bulk performance such as pellet, in film, certain and applicationsdispersion in [a60 mixed,61]. solution In most of studies, alcohol and water. Its nanofibrous structure has attracted much attention because it could improve electrospun Nafion/polymer composite nanofibers formed by a single nozzle electrospinning have proton conductivity and subsequently, the performance in certain applications [60,61]. In most beenstudies, demonstrated electrospun [62]. Nafion/polymer However, the composite fabricated nanofibers Nafion typically formed by exhibited a single nozzle low purity electrospinning and as a result, low protonhave been conductivity demonstrated in [62]. its composite However, the structure. fabricated Foam Nafion electrospinning typically exhibited addresses low purity these and issues as by usinga compressedresult, low proton gas, e.g., conductivity CO2, to form in its polymeric composite foamsstructure. where Foam multiple electrospinning polymer addresses jets are ejected these from the surfaceissues by of using the foam compressed toward gas, the collector.e.g., CO2, to In form the setup,polymeric a copper foams electrodewhere multiple is connected polymer to jets a funnelare to supplyejected electrical from the power surface to of the the polymer foam toward solution the coll (Figureector. In4 thea). setup, The applied a copper electricelectrode field is connected disturbs the polymerto a solutionfunnel to foamsupply formed electrical by power injected to the gas polymer and as asolution consequence, (Figure 4a). multiple The applied polymer electric jets arefield ejected towarddisturbs the collection the polymer plate solution directly, foam as formed shown by in injected Figure 4gasb. and The as multiple a consequence, jets led multiple to a high polymer production jets are ejected toward the collection plate directly, as shown in Figure 4b. The multiple jets led to a rate of approximately 9 g h 1 m 2 which is far higher than that of single nozzle electrospinning for high production rate of· −approximately· − 9 g·h−1·m−2 which is far higher than that of single nozzle Nafion nanofiber fabrication (typically 0.0014 g h 1 m 2). In addition, the purity of Nafion nanofiber electrospinning for Nafion nanofiber fabrication· (typically− · − 0.0014 g·h−1·m−2). In addition, the purity of was aboutNafion 98%, nanofiber which was has about not been 98%, achievedwhich has with not been traditional achieved electrospinning. with traditional electrospinning. The method was The capable of resolvingmethod was both capable productivity of resolving and both purity productivity issues in an Nafiond purity nanofiber issues in Nafion fabrication. nanofiber fabrication.

FigureFigure 4. (a 4.) Schematic(a) Schematic illustration illustration of of foam foam based electrospinning, electrospinning, (b) ( photographb) photograph of fiber of fibergeneration generation from the foam surface, and (c) Nafion nanofiber production rate of traditional electrospinning and from the foam surface, and (c) Nafion nanofiber production rate of traditional electrospinning and foam based needleless electrospinning. [Adapted with permission from 45. Copyright (2019) foam based needleless electrospinning. [Adapted with permission from 45. Copyright (2019) American American Chemical Society]. Chemical Society]. Polymers 2020, 12, 1386 7 of 20 Polymers 2020, 12, x FOR PEER REVIEW 7 of 19

3.3. The Use of Mechanical Force for NanofiberNanofiber FabricationFabrication DespiteDespite thethe simplicitysimplicity andand versatilityversatility ofof electrospinning,electrospinning, it hashas intrinsicintrinsic limitationslimitations forfor makingmaking nanofibersnanofibers owingowing to to the the electrostatic electrostatic repulsion repulsion force force that counteractsthat counteracts the surface the surface tension tension of the droplet. of the Itdroplet. has recently It has beenrecently demonstrated been demonstrated that the electricalthat the electrical force can force be replaced can be replaced with a mechanical with a mechanical force to fabricateforce to fabricate nanofibers. nanofibers. The use of The mechanical use of mechanic force overcomesal force theovercomes conventionally the conventionally accepted limitations accepted of electrospinning,limitations of electrospinning, e.g., requirement e.g., for highrequirement electrical for potential high andelectrical electrically potential conductive and electrically targets to driveconductive the stretching-out targets to drive of the the droplets stretching-out in the fiber of elongation the droplets process. in the In contrastfiber elongation with electrospinning, process. In thecontrast use of with mechanical electrospinning, force does the not use limit of mechanic the type ofal polymersforce does and not solventslimit the because type of theirpolymers electrical and propertiessolvents because are no longertheir electrical relevant. Furthermore,properties are it no avoids longer the excessiverelevant. useFurthermore, of energy init productionavoids the contributingexcessive use to of high energy cost. in In production this section, contributing the efforts on to using high anothercost. In typethis section, of force the than efforts electrical on using force toanother make high-qualitytype of force nanofiberthan electrical are discussed. force to make high-quality nanofiber are discussed. 3.1. Handspinning 3.1. Handspinning As an alternative to electrical force, Lee et al. developed the-so-called handspinning method [63]. As an alternative to electrical force, Lee et al. developed the-so-called handspinning method [63]. It only relies on simple mechanical stretching forces instead of high electrical voltage during the It only relies on simple mechanical stretching forces instead of high electrical voltage during the fabrication process of nanofiber, which is advantageous in reducing the high cost as well as excessive fabrication process of nanofiber, which is advantageous in reducing the high cost as well as excessive energy use for fiber production. Figure5 shows the setup of the handspinning apparatus. Nanofibers energy use for fiber production. Figure 5 shows the setup of the handspinning apparatus. Nanofibers are formed by simple attachment–detachment of two plates where polymer solution is sandwiched by are formed by simple attachment–detachment of two plates where polymer solution is sandwiched the plates, resulting in well-oriented nanofiber along a single axis. The nanofiber formation can be by the plates, resulting in well-oriented nanofiber along a single axis. The nanofiber formation can be controlled by varying the process parameters of pulling-away speed, pulling-away distance, and plate controlled by varying the process parameters of pulling-away speed, pulling-away distance, and area. The unique approach using the uniaxial stretching-out force offers a handle to control the internal plate area. The unique approach using the uniaxial stretching-out force offers a handle to control the structure of nanofiber composites: carbon nanotube (CNT) was evenly distributed in a polymer fibrous internal structure of nanofiber composites: carbon nanotube (CNT) was evenly distributed in a matrix minimizing its agglomeration, which is commonly observed in electrospinning. Furthermore, polymer fibrous matrix minimizing its agglomeration, which is commonly observed in CNTs are aligned along the direction of the pulling-out motion, which is quite challenging to achieve electrospinning. Furthermore, CNTs are aligned along the direction of the pulling-out motion, which with electrical force. Consequently, a controlled structure was integrated for the enhancement of is quite challenging to achieve with electrical force. Consequently, a controlled structure was mechanical and thermal properties of the nanofiber composites. These observations highlight the integrated for the enhancement of mechanical and thermal properties of the nanofiber composites. potential of mechanical force to fabricate a nanofibrous structure with controlled complexities which These observations highlight the potential of mechanical force to fabricate a nanofibrous structure can be directly related to the functional materials platform. with controlled complexities which can be directly related to the functional materials platform.

FigureFigure 5. 5. (a–(ca)– Thec) The nanofiber nanofiber fabrication fabrication process process via handspinning, via handspinning, (d) representative (d) representative SEM image SEM of imagehandspun of handspunnanofibers nanofibers(scale bar = (scale10 μm), bar and= 10(e) photographµm), and ( eshowing) photograph handspun showing CNT-reinforced handspun CNT-reinforcednanofibers using nanofibers two fingers. using [Adapted two fingers. with permi [Adaptedssion from with 63. permission Copyright from (2016) 63. Nature Copyright Publishing (2016) NatureGroup]. Publishing Group].

PolymersPolymers 20202020, ,1212, ,x 1386 FOR PEER REVIEW 8 8of of 19 20

3.2. Needle Spinning 3.2. Needle Spinning Although handspinning offers many advantages, unevenly supplied polymer solution on Although handspinning offers many advantages, unevenly supplied polymer solution on pulling pulling plates leads to low size uniformity in the resulting nanofibers. This issue was addressed by plates leads to low size uniformity in the resulting nanofibers. This issue was addressed by changing changing the plate to needle as a tool for drawing the polymer solution, the-so-called needle spinning the plate to needle as a tool for drawing the polymer solution, the-so-called needle spinning [64]. [64]. Needle spinning is capable of achieving highly uniform nanofibers by supplying a certain Needle spinning is capable of achieving highly uniform nanofibers by supplying a certain amount of amount of polymer solution by generating a uniform meniscus on the needle tips. The process is polymer solution by generating a uniform meniscus on the needle tips. The process is illustrated with a illustrated with a photograph of a home-built apparatus in Figure 6. By simply dipping a needle in photograph of a home-built apparatus in Figure6. By simply dipping a needle in the polymer solution the polymer solution and drawing out the solution, a single nanofiber is readily fabricated. On and drawing out the solution, a single nanofiber is readily fabricated. On multiple needles, a highly multiple needles, a highly regular shape of the meniscus at the tips of the different needles is realized. regular shape of the meniscus at the tips of the different needles is realized. Processing parameters, Processing parameters, i.e., the pulling-away speed, pulling-away distance, needle size, and polymer i.e., the pulling-away speed, pulling-away distance, needle size, and polymer concentration were concentration were effective in controlling the size of the resulting nanofiber. Clear correlation effective in controlling the size of the resulting nanofiber. Clear correlation between the nanofiber between the nanofiber diameter and processing parameters was observed: the nanofiber becomes diameter and processing parameters was observed: the nanofiber becomes thicker as both polymer thicker as both polymer concentration and needle size increase, and it becomes thinner as both concentration and needle size increase, and it becomes thinner as both pulling-away speed and pulling-away speed and pulling-away distance increase. The needle-spun nanofibers exhibited a pulling-away distance increase. The needle-spun nanofibers exhibited a smooth surface and uniformly smooth surface and uniformly controlled diameter as shown in Figure 6. The length of nanofiber can controlled diameter as shown in Figure6. The length of nanofiber can be controlled up to the meter scale, be controlled up to the meter scale, which is also a remarkable improvement of mechanical force- which is also a remarkable improvement of mechanical force-based spinning techniques. These studies based spinning techniques. These studies elucidate the fundamental principles for a mechanically elucidate the fundamental principles for a mechanically driven nanofiber formation process, and show driven nanofiber formation process, and show an effective way to produce well-defined nanofiber an effective way to produce well-defined nanofiber with the desired dimension. with the desired dimension.

FigureFigure 6. 6. TheThe needle needle spinning spinning apparatus, apparatus, and and SEM SEM images images of of nanofibers nanoﬁbers fabricated fabricated from from needles needles at at differentdiﬀerent locations locations in in the the same same batch batch (scale (scale bar bar == 22 μµm).m). [Adapted [Adapted with with permission permission from from 64. 64. Copyright Copyright (2018)(2018) MDPI]. MDPI].

3.3.3.3. Track Track Spinning Spinning TheThe drawing-based techniquetechnique has has been been further further improved improved towards towards a continuous a continuous fabrication fabrication process processusing an using automated an automated single-step single-step drawing drawing system [65 system]. The [65]. process, The calledprocess, track called spinning, track isspinning, based on is a basedsimple on drawing a simple method drawing using method two oppositely using two angled oppositely rotating angled tracks. rotating The equipment tracks. The consists equipment of two consistsrotating of tracks two rotating touching tracks each touching other at theeach top, othe ar solution at the top, dispenser a solution located dispenser above located the tracks, above and the a tracks,collection and racka collection located rack below located the tracks below (Figure the tracks7a). (Figure Similar 7a). to handspinningSimilar to handspinning and needle and spinning, needle spinning,the driving the force driving to formforce nanofibersto form nanofibers is a mechanical is a mechanical force, which force, allows which aallows range a of range options of options for the forselection the selection of polymer of polymer and solvent. and solvent. Upon Upon feeding feeding the polymer the polymer solution solution by dispenser by dispenser at the at top the of top the ofapparatus, the apparatus, fibers arefibers continuously are continuously spun by spun mechanical by mechanical drawing drawing with the with increase the increase of the distance of the distancebetween between the tracks the due tracks to the due geometry to the geometry of the apparatus. of the apparatus. The polymer The solutionpolymer is solution coated uniformlyis coated uniformlyover the surface over the of twosurface tracks of two at the tracks initial at stage, the initia andlit stage, is steadily and it stretched is steadily out stretched across the out gap across between the gapthe between tracks around the tracks the third–fourtharound the third–fourth stage. Pulled-out stage. fibersPulled-out are subsequently fibers are subsequently solidified bysolidified solvent byevaporation solvent evaporation as the fibers as move the downfibers along move the down track. al Asong this the process track. isAs repeated this process by continuous is repeated rotation by continuous rotation of the tracks, a large amount of fibers can be fabricated. The fabricated nanofibers

Polymers 2020, 12, 1386 9 of 20 Polymers 2020, 12, x FOR PEER REVIEW 9 of 19 areof thecollected tracks, aonto large the amount collection of fibers frame can beat fabricated.the bottom. The The fabricated size of nanofibers the nano arefibers collected is effectively onto the controlledcollection framein a nanometer at the bottom. scale Thewith size the of processing the nanofibers parameters is effectively of the controlledangle of the in track, a nanometer the vertical scale collectionwith the processing distance, and parameters the track ofspeed. the angle This approach of the track, maximizes the vertical the collection use of mechanical distance, force and thecapable track ofspeed. highly This aligning approach nanofibers maximizes with the high use ofthroughp mechanicalut owing force capableto its continuity. of highly aligningFurthermore, nanofibers the setup with canhigh be throughput easily modified: owing for to itsexample, continuity. the size Furthermore, and texture the of setup tracks can improve be easily the modified: production for rate example, and thethe feasibility size and texture to control of tracks the location improve of thedrawing production and therefore, rate and the feasibilitylocation of to fibers control (Figure the location 7d). As ofa proofdrawing of concept, and therefore, poly(vinyl the acetat locatione) and of fibers polyurethane (Figure7 d).nanofibers As a proof were of fabricated concept, poly(vinylwith a diameter acetate) of aboutand polyurethane 500 nm and the nanofibers length of were 255 mm. fabricated Therefore, with spinning a diameter with of a aboutmechanical 500 nm force and clearly the length proves of the255 feasibility mm. Therefore, of mass spinning production with aof mechanical well-defined force nanofibers, clearly proves which the have feasibility an impact of mass in the production field of nanofiberof well-defined applications. nanofibers, which have an impact in the field of nanofiber applications.

FigureFigure 7. 7. (a()a )The The setup setup for for track track spinning, spinning, (b) ( bschematic) schematic illustration illustration of continuous of continuous fiber fiber formation, formation, (c) photographs(c) photographs of track of track spinning spinning process process with with 8 cm 8 cm wi widede tracks tracks and and of ofresulting resulting aligned aligned fibers, fibers, and and (d (d) ) photographphotograph of of a a 3D 3D spinning spinning system system with with a a pattern patterneded array array of of bristles bristles on on one one track track towards towards precise precise locationlocation of of fibers. fibers. [Adapted [Adapted with with permission permission from from 65. 65. Copyright Copyright (2019) (2019) American American Chemical Chemical Society]. Society].

4.4. Structural Structural and and Architectural Architectural Regulations Regulations MostMost researches researches conducted conducted with with typical typical electros electrospinningpinning have have demonstrat demonstrateded randomly randomly oriented oriented nanofibersnanofibers exhibiting exhibiting a a circular circular cross-section cross-section and and smooth smooth surface. surface. To To expand expand the the applicability applicability of of nanofibers,nanofibers, however, however, constructing constructing particular particular architectures architectures in in two two and and even even three three dimensions dimensions with with structurallystructurally well-defined well-defined nanofibers nanofibers is is critical. For For example, example, nanofiber nanofiber patterns patterns with with controlled controlled alignmentalignment on on a a two- two- or or three-dimensional three-dimensional substrate substrate were were demonstrated demonstrated for for ce certainrtain purposes purposes [66–69]. [66–69]. SpecificSpecific surface surface morphologies morphologies with with highly highly regulated regulated fiber fiber structures structures are are often often desirable desirable [68,70–74]. [68,70–74 In]. thisIn this section, section, recent recent unique unique demonstrations demonstrations on on co controllingntrolling nanofiber nanofiber morphology, morphology, alignment, alignment, pattern pattern formation,formation, and and constructing constructing a a three-dimensional three-dimensional structure structure with with electrospinning electrospinning are introduced.

4.1.4.1. Bipolar Bipolar Pyroelectrospinning Pyroelectrospinning for for the the Formation Formation of of Nanofiber Nanofiber Arrays Arrays AsAs aforementioned, aforementioned, electrospun electrospun nanofiber nanofiber is is gene generallyrally collected collected as as a a nonwoven nonwoven type type due due to to the the uncontrollableuncontrollable nature nature of of jet jet formation, formation, which which limits limits its its applicability. applicability. Enormous Enormous interest interest in in controlling controlling thethe nanofiber nanofiber deposition deposition process process has has arisen arisen to toim improveprove the the performance performance in inspecific specific applications. applications. A numberA number of ofstudies studies dealt dealt with with patterning patterning techniques techniques with with electrospun electrospun nanofiber, nanofiber, including including precise precise positioning,positioning, stacking, stacking, and and orientation orientation [75,76], [75,76], select selectiveive deposition deposition [77], [77], and and templated templated patterning patterning [78]. [78]. TheseThese techniques techniques provide provide significant significant impacts impacts especially especially in in the the field field of of biomedical biomedical engineering, engineering, such such as as tissue engineering [79] and regenerative medicine [80]. These efforts have evolved to find an easy and

Polymers 2020,, 12,, 1386x FOR PEER REVIEW 1010 of of 19 20 efficient method to achieve the desired pattern formation using electrospun nanofibers in high tissue engineering [79] and regenerative medicine [80]. These efforts have evolved to find an easy and precision. efficient method to achieve the desired pattern formation using electrospun nanofibers in high precision. Recently, Grilli et al. invented bipolar pyroelectrospinning, which is capable of generating Recently, Grilli et al. invented bipolar pyroelectrospinning, which is capable of generating ordered ordered arrays of nanofibers by exploiting periodically poled lithium niobate (PPLN) as a substrate arrays of nanofibers by exploiting periodically poled lithium niobate (PPLN) as a substrate [81]. First, [81]. First, PPLN crystal was patterned in a hexagonal shape, where ferroelectric domains are formed PPLN crystal was patterned in a hexagonal shape, where ferroelectric domains are formed with with reversed polarization, achieved by a standard electric field poling onto a template sample reversed polarization, achieved by a standard electric field poling onto a template sample defined defined with photolithography. The optical microscopic image of hexagonal PPLN crystal pattern is with photolithography. The optical microscopic image of hexagonal PPLN crystal pattern is shown in shown in Figure 8a. Heat is an important stimulus for this pattern: charges are homogeneously Figure8a. Heat is an important stimulus for this pattern: charges are homogeneously distributed on the distributed on the surface having the PPLN crystal patterns when the heating is off. In this state, only surface having the PPLN crystal patterns when the heating is off. In this state, only the crystal surface the crystal surface exposed to the c ends has a spontaneous polarization state. On the opposite side exposed to the c ends has a spontaneous polarization state. On the opposite side from the PPLN crystal from the PPLN crystal side, a drop of polymer solution is formed on the metallic tip, similar to the side, a drop of polymer solution is formed on the metallic tip, similar to the typical electrospinning typical electrospinning process. When the heat is switched on to render the plate at a temperature of process. When the heat is switched on to render the plate at a temperature of approximately 60 C, approximately 60 °C, the charge state of the PPLN crystal pattern becomes different, resulting in◦ a the charge state of the PPLN crystal pattern becomes different, resulting in a heat-induced pyroelectric heat-induced pyroelectric effect. As a result, the regions near the hexagons experience reversed effect. As a result, the regions near the hexagons experience reversed polarization to exhibit an excess polarization to exhibit an excess of negative charges, and the hexagons exhibit an excess of positive of negative charges, and the hexagons exhibit an excess of positive charge. This bipolar electric charge. This bipolar electric field patterns guide the location of the deposited nanofibers during field patterns guide the location of the deposited nanofibers during electrospinning. If the ejected electrospinning. If the ejected nanofibers are positively charged, they effectively interact with the nanofibers are positively charged, they effectively interact with the negatively charged area outside of negatively charged area outside of the hexagons and therefore, patterned nanofiber mat is formed. the hexagons and therefore, patterned nanofiber mat is formed. (Figure8d). The resulting patterns (Figure 8d). The resulting patterns prove the validity of this technique to realize periodically aligned prove the validity of this technique to realize periodically aligned arrays. arrays.

Figure 8. Schematic illustration of the bipolarbipolar pyroelectrospinningpyroelectrospinning process. ( a) Optical microscope image of of the the periodically periodically poled poled lithium lithium niobate niobate (PPLN) (PPLN) crystal crystal plate, plate, (b) the (b) pyroelectrospinning the pyroelectrospinning with withheat-off heat-o andff (cand) with (c) heat-on with heat-on where where the PPLN the PPLNplate become plate becomess charged. charged. The polymer The polymer droplets droplets exhibit positiveexhibit positive charges charges to deform to deform into Taylor into Taylor cone. cone. (d) (Thed) The ejected ejected nanofibers nanofibers collected collected on on the the region surrounding the hexagons, and and the the SEM SEM image image of of th thee resulting resulting pattern pattern array array in in the the nanofiber nanofiber mat. mat. [Adapted with permission from 81.81. Copy Copyrightright (2019) American Chemical Society].

4.2. Near-Field Electrospinning to Construct Nanoarchitecture The electrospunelectrospun jet jet is is highly highly uncontrollable uncontrollable due due to the to chaoticthe chaotic whipping whipping jet formation, jet formation, which makeswhich makesa randomly a randomly orientated orientated nonwoven nonwoven type nanofiber type nanofib mat. Toer suppressmat. To suppress the chaotic th whippinge chaotic whipping jet formation jet formationand to deposit and nanofibers to deposit on ananofibers substrate withon a desiredsubstrate pattern, with thea near-fielddesired pattern, electrospinning the near-field (NFES) electrospinningtechnique has been (NFES) developed technique in recent has years. been NFES developed provides in a straightforwardrecent years. andNFES versatile provides means a straightforwardto precisely control and the versatile location means of nanofiber to precisely and therefore, control the to fabricate location nanofiberof nanofiber patterns. and therefore, It features to a fabricate nanofiber patterns. It features a short tip-to-collector distance less than 3 mm, which leads to a reduction of bending instability of the electrospun jet, and a lower voltage of several hundreds

Polymers 2020, 12, 1386 11 of 20 short tip-to-collector distance less than 3 mm, which leads to a reduction of bending instability of the electrospun jet, and a lower voltage of several hundreds of volts is required. The first proof-of-concept was demonstrated by Lin et al. in 2006 [82]. The NFES technique requires a relatively low voltage for processing, and enables the fabrication of pattern arrays with excellent position control, while excessive consumption of polymer solution can be minimized. Initially, it was employed to attain only 2D structures; however, recently building a 3D structure has been extensively undertaken. The most use of this method is for the layer-by-layer additive printing method [83]. Hutmacher et al. showed highly ordered scaffold architecture using multilayer additive printing by spinning similar to NFES with highly viscous polymer solution [84]. As a result, fibers with several micrometer dimension were stacked to form the scaffold. Three dimensional architecture in nanometer dimension was presented by Lee et al. in 2014 [85]. In this work, a pre-defined pattern on the conducting electrode guides the location of spun nanofibers, which could however limit the expansion of its applicability. Another 3D architecture, a hollow pottery shape, was fabricated by Kim et al. using the spontaneous coiling of electrospun nanofiber, demonstrating a hollow cylindrical structure [86]. The a layer-by-layer additive printing method with the NFES was successful in achieving the desired shapes, though a pre-defined structure or specifically a pre-designed collector is required. Recently, Cho et al. successfully fabricated nanofiber stacks built with high-resolution control using the NFES technique [87] (Figure9a). In their method, a high aspect ratio in a variety of architectures such as curved nanowall, grid pattern, and nanobridge, was achieved without any pre- designed collector because of the self-alignment behavior of the nanofibers. The self-alignment feature was possible by the simple addition of salt, i.e., poly(ethylene oxide) (PEO)/sodium chloride (NaCl) aqueous solution. When pure PEO solution is electrospun, the charge does not fully dissipate, leaving a weak positive charge on nanofiber surface. The weak positive charge induces the repulsive interaction in jet streams and as a consequence, stacking of nanofibers becomes challenging. On the other hand, additional salt enhances the conductivity of the polymer solution, and the surface of the resultant deposited nanofibers becomes negatively charged. The increase of the interaction between polymer solution jet and deposited fibers leads to effective stacking of the nanofibers (Figure9b). This approach facilitated the fabrication of various nanoarchitectures including nanowalls, curved nanowalls, grids, and nanobridges at the desired locations with pre-programmed X-Y stage motion of the spinneret, and subsequent metal coating to enhance mechanical stability (Figure9c).

4.3. Sequential Metal Deposition for Multilayered Nanofiber Cross-section morphology control of nanofibers, e.g., fabrication of core-shell type or multilayered nanofiber which can be utilized for a variety of applications, has been an attractive but challenging issue [88–90]. Recently, an interesting facile fabrication approach for multilayered nanofiber was demonstrated by Aydin et al. [91]. They fabricated strong, flexible, and centimeter- long nanowires through multiple metal depositions on a polymeric nanofiber, as shown in Figure 10. Unlike a conventional core-shell fiber formation method which is enabled by ejecting two polymer solutions into one spinning tip, this method involves a physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). First, electrospun nanofibers of poly(m-phenylene isophthalamide) (PMIA) were collected on a spoked-drum collector for fabricating the aligned and free-standing nanofibers. (Figure 10a) Then, the fibers were transferred onto a rectangular frame to suspend them. Subsequently, the fixed nanofibers were coated with various functional metals or oxides using CVD, ALD, and PVD. Fabricated nanofibers exhibited various cross-section images, i.e., round-, oval-, and ribbon-shaped, controlled by the chamber humidity. (Figure 10b) This is due to the poor solubility of PMIA in water inducing a phase separation in the jet when water molecules from the atmosphere diffuse into the spinning jet. The number of layers were controlled by the number of deposition cycles, which is clearly assured in Figure 10b. This method opens up the possibility to finely control the internal structure of nanofibers, and therefore, eventually to achieve high structural complexities in nanofibers. Polymers 2020, 12, x FOR PEER REVIEW 11 of 19 of volts is required. The first proof-of-concept was demonstrated by Lin et al. in 2006 [82]. The NFES technique requires a relatively low voltage for processing, and enables the fabrication of pattern arrays with excellent position control, while excessive consumption of polymer solution can be minimized. Initially, it was employed to attain only 2D structures; however, recently building a 3D structure has been extensively undertaken. The most use of this method is for the layer-by-layer additive printing method [83]. Hutmacher et al. showed highly ordered scaffold architecture using multilayer additive printing by spinning similar to NFES with highly viscous polymer solution [84]. As a result, fibers with several micrometer dimension were stacked to form the scaffold. Three dimensional architecture in nanometer dimension was presented by Lee et al. in 2014 [85]. In this work, a pre-defined pattern on the conducting electrode guides the location of spun nanofibers, which could however limit the expansion of its applicability. Another 3D architecture, a hollow pottery shape, was fabricated by Kim et al. using the spontaneous coiling of electrospun nanofiber, demonstrating a hollow cylindrical structure [86]. The a layer-by-layer additive printing method with the NFES was successful in achieving the desired shapes, though a pre-defined structure or specifically a pre-designed collector is required. Recently, Cho et al. successfully fabricated nanofiber stacks built with high-resolution control using the NFES technique [87] (Figure 9a). In their method, a high aspect ratio in a variety of architectures such as curved nanowall, grid pattern, and nanobridge, was achieved without any pre- designed collector because of the self-alignment behavior of the nanofibers. The self-alignment feature was possible by the simple addition of salt, i.e., poly(ethylene oxide) (PEO)/sodium chloride (NaCl) aqueous solution. When pure PEO solution is electrospun, the charge does not fully dissipate, leaving a weak positive charge on nanofiber surface. The weak positive charge induces the repulsive interaction in jet streams and as a consequence, stacking of nanofibers becomes challenging. On the other hand, additional salt enhances the conductivity of the polymer solution, and the surface of the resultant deposited nanofibers becomes negatively charged. The increase of the interaction between polymer solution jet and deposited fibers leads to effective stacking of the nanofibers (Figure 9b). This approach facilitated the fabrication of various nanoarchitectures including nanowalls, curved nanowalls,Polymers 2020 ,grids,12, 1386 and nanobridges at the desired locations with pre-programmed X-Y stage motion12 of 20 of the spinneret, and subsequent metal coating to enhance mechanical stability (Figure 9c).

Polymers 2020, 12, x FOR PEER REVIEW 12 of 19

PEO/NaCl on insulating plate, and (c) fabricated various 3D nanoarchitecutres, i.e., nanowalls, curved nanowalls, grid pattern, and nanobridges. [Adapted with permission from 87. Copyright (2019) American Chemical Society].

4.3. Sequential Metal Deposition for Multilayered Nanofiber Cross-section morphology control of nanofibers, e.g., fabrication of core-shell type or multilayered nanofiber which can be utilized for a variety of applications, has been an attractive but challenging issue [88–90]. Recently, an interesting facile fabrication approach for multilayered nanofiber was demonstrated by Aydin et al. [91]. They fabricated strong, flexible, and centimeter- long nanowires through multiple metal depositions on a polymeric nanofiber, as shown in Figure 10. Unlike a conventional core-shell fiber formation method which is enabled by ejecting two polymer solutions into one spinning tip, this method involves a physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). First, electrospun nanofibers of poly(m-phenylene isophthalamide) (PMIA) were collected on a spoked-drum collector for fabricating the aligned and free-standing nanofibers. (Figure 10a) Then, the fibers were transferred onto a rectangular frame to suspend them. Subsequently, the fixed nanofibers were coated with various functional metals or oxides using CVD, ALD, and PVD. Fabricated nanofibers exhibited various cross-section images, i.e., round-, oval-, and ribbon-shaped, controlled by the chamber humidity.FigureFigure (Figure 9. ((aa)) SchematicSchematic 10b) This illustration illustration is due to of the of experimental poorexperimental solubi setuplity setup of of NFSE,PMIA of NFSE, (inb) water diagram (b) diagram inducing showing showing a surface phase surface chargeseparation in thechargedensity jet when density for pure water for poly(ethylene moleculespure poly(ethylene oxide)from the (PEO) oxide) atmosphere and (PEO) PEO/NaCl anddiffuse onPEO/NaCl conducting into the on spinning plate,conducting and jet. PEO Theplate,/NaCl number and on of layersinsulating were controlled plate, and by (c) fabricatedthe number various of deposition 3D nanoarchitecutres, cycles, which i.e., is nanowalls, clearly assured curved nanowalls,in Figure 10b. This gridmethod pattern, opens and up nanobridges. the possibility [Adapted to finely with permissioncontrol the from internal 87. Copyright structure (2019) of nanofibers, American and therefore,Chemical eventually Society]. to achieve high structural complexities in nanofibers.

FigureFigure 10.10.( a()a Schematic) Schematic illustration illustration of theof processthe process for multi-layered for multi-layered fibrous fibrous structures, structures, (b) cross-sectional (b) cross- SEMsectional images SEM of images of poly( ofm of-phenylene poly(m-phenylene isophthalamide isophthalamide (PMIA) nanofibers (PMIA) nanofibers with various with cross-sectional various cross- structuresectional structure by humidity by humidity control, and control, (c) layer-by-layer and (c) layer-by-layer metal deposited metal deposited PMIA nanofibers PMIA nanofibers via atomic via layeratomic deposition layer deposition (ALD) (ALD) and chemical and chemical vapor vapor deposition deposition (CVD). (CVD). [Adapted [Adapted with with permission permission from from 91. Copyright91. Copyright (2019) (2019) American American Chemical Chemical Society]. Society].

4.4. Transformation of 2D Nanofiber Mat to 3D Object Nanofiber-based 3D scaffolds have been considered as a material platform in tissue engineering as they can be implemented to mimic the extracellular matrix (ECM) in native tissues. However, despite the development of various useful fabrication techniques, building a three-dimensional structure with nanofibers still remains a tough challenge. Widely utilized methods for transforming a 2D structure into a 3D object are the folding, rolling, bending, cutting, and buckling of 2D objects

Polymers 2020, 12, 1386 13 of 20

4.4. Transformation of 2D Nanofiber Mat to 3D Object Nanofiber-based 3D scaffolds have been considered as a material platform in tissue engineering as they can be implemented to mimic the extracellular matrix (ECM) in native tissues. However, despite the development of various useful fabrication techniques, building a three-dimensional structure with Polymers 2020, 12, x FOR PEER REVIEW 13 of 19 nanofibers still remains a tough challenge. Widely utilized methods for transforming a 2D structure into[92–94]. a 3D Recently, object are Jiang the folding, et al. reported rolling, the bending, establis cutting,hment andof a 3D buckling structure of 2D from objects 2D nanofiber [92–94]. Recently, mats of Jiangpoly(ε et-caprolactone) al. reported the using establishment a gas flow of to a 3Dexpand structure electrospun from 2D nanofibernanofiber mats along of the poly( fiberε-caprolactone) deposition usingdirection a gas [95–97]. flow to expandWith this electrospun technique, nanofiber the thickn alongess theand fiber porosity deposition in the direction highly [ordered95–97]. Withscaffold this technique,architecture the were thickness controlled. and These porosity nanofiber-based in the highly 3D ordered architectures scaffold consist architecture of multilayers were controlled.of aligned Thesenanofibers nanofiber-based where the layers 3D architectures have gaps ranging consist offrom multilayers several micrometers of aligned nanofibers to millimeters. where Fabricated the layers havescaffolds gaps have ranging shown from the severalfeasibility micrometers for utilization to millimeters. in tissue engineering Fabricated [96] sca. ffMoreolds recently, have shown the gas the feasibilityflow technique for utilization was further in tissue developed engineering to achieve [96]. Morea hierarchical recently, theassembly gas flow of techniquenanofibers was with further pre- developeddesigned shapes to achieve having a hierarchical high complexity assembly [98]. of nanofibersThe key process with pre-designed of the gas-assisted shapes having expansion high complexitytechnique is [ 98to ].fix The one key side process of the of2D the nanofiber gas-assisted mat expansionduring the techniqueprocess, as is presented to fix one sidein Figure of the 11a. 2D nanofiberBoth processes mat during include the thermal process, treatment as presented at in85 Figure°C for 11 1 a.s before Both processes the expansion include step thermal to selectively treatment atharden 85 ◦C one for 1side s before of the the nanofiber expansion mat. step Depending to selectively on harden the cutting one side shape ofthe of the nanofiber 2D nanofiber mat. Depending mat, i.e., onrectangle, the cutting triangle, shape half of thecircle, 2D and nanofiber arch, and mat, on i.e., th rectangle,e position triangle, of thermal half treatment, circle, and the arch, 2D and nanofiber on the positionmat was oftransformed thermal treatment, into a cylinder, the 2D nanofibercone, sphere mat, wasand transformedhollow sphere, into respectively, a cylinder, cone,as shown sphere, in andFigure hollow 11b. sphere,Furthermore, respectively, changing as shown the axis in Figure for rotation11b. Furthermore, during the changing process theallowed axis for the rotation fiber duringalignment the direction process allowed in the resulting the fiber alignment3D object directionto be controlled. in the resulting In the resulting 3D object materials, to be controlled. shape Inrecovery the resulting upon compression materials, shape was recoverypossible, upon and its compression porous and was layered possible, structure and itseffectively porous and guided layered the structureorganization effectively of seeded guided cells, the enabling organization a highly of seededordered cells, artificial enabling tissue. a highly ordered artificial tissue.

Figure 11. (a) Representative process for transformation of of 2D nanofiber nanoﬁber mats into 3D object, and ( b) design and realization of thethe desireddesired shapes.shapes. [Adapted[Adapted with permission from 98. Copyright (2019) American Chemical Society].

4.5. Yarn-Spinning Although electrospunelectrospun nanofibers nanofibers have have various various unique unique and valuableand valuable properties, properties, they are they typically are knowntypically not known to be not suffi tociently be sufficiently mechanically mechanically robust torobust replace to replace common common textiles. textiles. A few A reports few reports have shownhave shown that individual that individual electrospun electrospun nanofibers nanofibers can exhibit can exhibit a reasonable a reasonable mechanical mechanical strength strength despite theirdespite size their in nanoscale size in nanoscale [1,99,100], [1,99,100], however, thehowever, final strength the final of strength the nanofiber of the mat nanofiber is not enough mat is to not be utilizedenough practically.to be utilized For practically. actual applications, For actualnanofiber applications, mats nanofiber need to be mats supported need to bybe nonwovenssupported by or othernonwovens substrates. or other Recently, substrates. Kim et Recently, al. demonstrated Kim et aal. yarn demonstrated spinning technique a yarn tospinning fabricate technique a nanofiber to basedfabricate single-stand a nanofiber yarn based by single employing-stand ayarn dual-spinneret by employing electrospinning a dual-spinneret process electrospinning on a support process wire (Figureon a support 12a) [101 wire]. They (Figure used 12a) a funnel [101]. as They a collector used toa windfunnel the as electrospun a collector nanofiber to wind ontothe electrospun the support nanofiber onto the support wire, and the dual spinning nozzle for electrospinning. Simultaneously, the electrospun nanofibers were collected as a yarn on the winder like a thread. The fabricated nanofiber yarn has a core-shell type structure where the core is a conventional thread yarn, and the shell consists of electrospun nanofibers with high density. In this technique, it is important to control the surface charge on the polymer droplets formed by electrospinning, as positively and negatively charged electrospun nanofibers are intermixed on the thread which is located in the funnel. This flexible yarn platform was further implemented for hydrogen sensing by coating the yarn with

Polymers 2020, 12, 1386 14 of 20 wire, and the dual spinning nozzle for electrospinning. Simultaneously, the electrospun nanofibers were collected as a yarn on the winder like a thread. The fabricated nanofiber yarn has a core-shell type structure where the core is a conventional thread yarn, and the shell consists of electrospun nanofibers with high density. In this technique, it is important to control the surface charge on the polymer droplets formed by electrospinning, as positively and negatively charged electrospun nanofibers are Polymersintermixed 2020, 12 on, x theFOR threadPEER REVIEW which is located in the funnel. This flexible yarn platform was14 further of 19 implemented for hydrogen sensing by coating the yarn with sequential sputter depositions of Pd sequentialand Pt. The sputter fabricated depositions metal-polymer of Pd and composite Pt. The fabricated single-strand metal-polymer yarn showed composite a wide detection single-strand range yarnfrom showed 4 to 0.0001% a wide with detection long-term range stability from toward 4 to 0.0001% repeated with exposure long-term of high-concentration stability toward repeated H2 (4%). exposureIt also exhibited of high-concentration dramatically rapid H2 (4%). sensing It also speed exhibited and strong dramatical mechanically rapid bending sensing strength, speed and ensuring strong mechanicalthe potential bending as a usable strength, sensor ensuring platform. the Thispotential approach as a usable is not sensor complicated platform. for fabricatingThis approach a flexible is not complicatedand free-standing for fabricating single-stand a flexible yarn scaandff oldfree-stand with high-densitying single-stand polymer yarn nanofibers,scaffold with but high-density it provides polymerdifferent functionalnanofibers, core-shell but it provides (thread-nanofiber different fu yarn)nctional type core-shell fabrics with (thread-nanofiber high flexibility, highyarn) surface type fabricsarea, and with open high porosity. flexibility, high surface area, and open porosity.

FigureFigure 12. 12. ((aa)) Schematic Schematic illustration illustration of of yarn yarn spinning spinning followed followed by by a a metal metal deposition deposition process, process, and and ( (bb)) photographsphotographs of as-fabricated as-fabricated nanofiber nanofiber yarn and and Pd Pd-coated-coated yarn. yarn. [Adapted [Adapted with with permission permission from from 101. 101. CopyrightCopyright (2019) (2019) American Chemical Society]. 5. Closing Remarks and Outlook 5. Closing Remarks and Outlook Nanofibers have attracted tremendous technological interest, and related researches are rapidly Nanofibers have attracted tremendous technological interest, and related researches are rapidly moving forward to exploit significant characteristics of nanofibers such as high porosity with excellent moving forward to exploit significant characteristics of nanofibers such as high porosity with pore interconnectivity between pores in mats, flexibility with reasonable strength, high surface-to-mass excellent pore interconnectivity between pores in mats, flexibility with reasonable strength, high ratio, and the ability to incorporate other materials into actual applications. However, their use in surface-to-mass ratio, and the ability to incorporate other materials into actual applications. industries have been still challenging due to the poor throughput and productivity of the current However, their use in industries have been still challenging due to the poor throughput and fabrication processes, and the difficulties in regulating the structure, alignment, and orientation. productivity of the current fabrication processes, and the difficulties in regulating the structure, In the productivity perspective, electrospinning, one of the popular fabrication techniques, exhibits alignment, and orientation. In the productivity perspective, electrospinning, one of the popular productivity in the range of 0.01–0.1 g h 1, and the principle for fiber formation does not allow the fabrication techniques, exhibits produc·tivity− in the range of 0.01–0.1 g·h−1, and the principle for fiber realization of complex nanofiber structures or patterns. However, other approaches to resolve these formation does not allow the realization of complex nanofiber structures or patterns. However, other issues have recently been proposed. The most important issue is the increase of productivity. The use approaches to resolve these issues have recently been proposed. The most important issue is the of multiple nozzles can be the easiest way to do so; however, it brings several issues for processing, increase of productivity. The use of multiple nozzles can be the easiest way to do so; however, it and requires an increase of apparatus size, which also increases the cost of equipment as well as of the brings several issues for processing, and requires an increase of apparatus size, which also increases the cost of equipment as well as of the space. As an alternative, the use of an open spinneret system such as needleless spinning has been shown. This system allows direct application of an electrical force to the liquid surface, generating multiple jets and eventually, achieving an impact on productivity with up to 600 g·h−1. However, it is typically conducted in an open system with a complicated setup requiring the control of many different processing parameters. Consequently, the consistency of the nanofiber morphologies is largely affected, and the complexity in the system negates its applicability in industry. Another issue to have arisen in the spinning technique is the regulation of structure regulation with the control of alignment and orientation. Uniquely designed,

Polymers 2020, 12, 1386 15 of 20 space. As an alternative, the use of an open spinneret system such as needleless spinning has been shown. This system allows direct application of an electrical force to the liquid surface, generating multiple jets and eventually, achieving an impact on productivity with up to 600 g h 1. However, · − it is typically conducted in an open system with a complicated setup requiring the control of many different processing parameters. Consequently, the consistency of the nanofiber morphologies is largely affected, and the complexity in the system negates its applicability in industry. Another issue to have arisen in the spinning technique is the regulation of structure regulation with the control of alignment and orientation. Uniquely designed, but complicated processes and specially outfitted apparatus for patterning or constructing specific structure of nanofibers have been proposed; however, they still suffer from low throughput. Finding an innovative and creative fabrication technique is still ongoing in order to solve all the issues simultaneously to realize high-throughput and easy operation and eventually, to bring the use of nanofibers to the real world. We anticipate that the currently devoted efforts by a number of researchers will eventually bring advanced technologies into this field, with regard to practical implementation in industrial applications.

Author Contributions: The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding: This research was funded by the Korea Institute of Industrial Technology (Grant number KITECH EO200029). Acknowledgments: We acknowledge the support from the Korea Institute of Industrial Technology (KITECH EO200029). Conflicts of Interest: The authors declare no conflict of interest.

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