Role of Grounded Liquid Collectors in Precise Patterning of

Dec 7, 2017 - Liquid collectors are applicable as ground collectors in electrospinning, which fabricates complex nanofiber architectures. However, the...
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Role of Grounded Liquid Collectors in Precise Patterning of Electrospun Nanofiber Mats Sang Min Park, Seongsu Eom, Wonkyoung Kim, and Dong Sung Kim* Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Pohang, Gyeongbuk 37673, South Korea S Supporting Information *

ABSTRACT: Liquid collectors are applicable as ground collectors in electrospinning, which fabricates complex nanofiber architectures. However, the influence of the electrical properties of liquid collectors on the controlled deposition of electrospun nanofiber mats has received little attention. Here, we prepare two types of liquid collectors (electrolyte solutions and dielectric liquids) and newly scrutinize their roles in the patterning of electrospun nanofiber mats in experiments and in numerical simulations. By simulating the concentrations of the electric fields around the liquid collectors, we indirectly evaluated the patternability of the collectors. The patternability trends were verified by the patterning of nanofiber mats on line-array-shaped liquid collectors fabricated by electrospinning. The deposition accuracy of the electrolyte solution collector was very high, equivalent to that of a conventional metal collector even at low salt concentrations (e.g., 0.01 M KCl). However, the nanofiber mats fabricated by electrospinning with the dielectric liquid collector showed retarded patternability.

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INTRODUCTION Electrospun nanofiber mats have demonstrated remarkable potential in various applications such as filters,1,2 catalysis,3 sensor,4 and tissue scaffold5,6 owing to their high porosity, large specific area, and native extracellular matrix-mimetic structure.7 The functionalities of electrospun nanofiber mats can be further extended by microscale patterning, which has consequently received much attention. For example, micropatterning can increase the stretchability of the mat8,9 or provide topographical cues to the cells.6,10 However, the inherent bending instability in electrospinning hampers the controlled deposition of nanofibers, resulting in a disorganized, randomly oriented nanofiber mat.7 To better control the deposition of electrospun nanofibers, researchers have developed many alternative fabrication methods based on patterned collectors,11 direct writing,5,12 and photolithography.6,13 Patterned nanofiber mats are most commonly fabricated on properly designed solid conductive collectors that modulate the electric field between a metal needle and the collector and subsequently guide the electrospun nanofibers to a specific region on the collectors, where they are selectively deposited. To date, the electric field of the collector has been concentrated in two ways. In the first approach, the collector is constructed from heterogeneous materials with distinguishable electrical properties.11,14 For © XXXX American Chemical Society

example, a collector comprising a metal (with high electrical conductivity) and an insulator (with low electrical conductivity) induces a distorted electric field toward the metal surface region. Under such a field, the electrospun nanofibers selectively deposit on the metal side, eventually forming a patterned nanofiber mat. In the second approach, geometrical features are imposed on a homogeneous collector, for example, protrusions on a metal collector.8,15−17 Given that the Coulomb force is inversely proportional to the square of the distance between the two static electric charges, the highly charged electrospun nanofibers are preferentially attracted toward the protrusion rather than the lower part of the collector. Both of these approaches achieve a patterned nanofiber mat, but the mat is strongly adhered to the solid collector and is not easily detached. Even when successfully detached, the nanofiber mat has weak mechanical properties that limit its integration into a complex architecture or with other devices. As liquid collectors are fluidic by nature, they can dramatically increase the flexibility of fabricating spatially Received: October 11, 2017 Revised: December 4, 2017 Published: December 7, 2017 A

DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic for the fabrication of a line-array-shaped nanofiber mat. The dashed rectangular in (a) is the field of view of the nanofiber mats for (b−d). Line-array-shaped nanofiber mats fabricated on a patterned AgNW (b), 3 M KCl solution collector (c), and ethylene glycol (d), and their magnified images (the inset in (b−d)) and diameter histogram (e). The scale bars are 1 mm (b−d) and 5 μm (the inset in (b−d)). obtained from Sigma-Aldrich and used without further purification. Methanol was purchased from Samcheon Chemicals. A polyethylene terephthalate (PET) adhesive tape was purchased from Daehyun ST Co. in Korea, and AgNW solution was purchased from N&B Inc. (Korea). PCL was dissolved in a mixture of chloroform and methanol (3:1 by volume) to a concentration of 7.5 wt %. The polymethyl methacrylate (PMMA) substrates were purchased from Acryl Choika (Korea). Electrospinning with a Liquid Collector. A PET adhesive tape mask was patterned by a laser cutter (IS350, INNOSTA, Korea) and transferred to the PMMA substrates. The PMMA surface with the adhesive tape mask was rendered hydrophilic by exposing it to oxygen plasma (VITA1, Femto Science, Korea) at a plasma power and time of 50 W and 30 s, respectively. Various KCl solutions and dielectric liquids were selectively positioned on the hydrophilic pattern on the PMMA surface. The PCL solution was ejected through a 23 gaugesized metal needle at a flow rate of 0.5 mL/h. A 19 kV electric potential was applied between the metal needle and the collector by a high voltage supplier (HV30, NanoNC, Korea). The nanofibers were imaged by scanning electron microscopy, and their diameters were measured by ImageJ software (>150 fibers per image). Numerical Simulation. The numerical simulations on the electric field were conducted using COMSOL Multiphysics v5.0 assuming that (1) the metal needle and the collector system are separated by 20 cm, (2) the shape of both the collectors is the segment of a circle with a diameter of 1 mm in 0.2 mm height, (3) the gap distance between the collectors is 1 mm, (4) the applied electric potential is 20 kV, and (5) the collector is connected by a grounded copper metal wire located at the left side. The electrolyte solution was modeled based on the space charge density. The mobile ions in the electrolyte solution can be described by a Boltzmann distribution, and thus, for a symmetric 1:1 electrolyte, the space charge density ρ(x) is represented as follows

controlled nanofiber mats. In one study, electrospun nanofiber mats were deposited in a liquid reservoir of ethanol or methanol (which has low surface tension), forming a threedimensional nanofiber mesh.18 Elsewhere, electrospun nanofibers were assembled into microscale yarns by a water vortex collector.19,20 Recently, our group has reported a novel electrospinning process using an electrolyte solution, named electrolyte-assisted electrospinning, which directly fabricates a patterned, free-standing nanofiber mat on a premade microchannel.21 Likewise, liquid collectors are highly promising in the production of spatially controlled nanofiber mats, although the role of their electrical property in the controlled deposition of electrospun nanofibers is little understood. How liquid collectors influence the electrospinning fabrication of patterned nanofiber mats is therefore worthy of investigation. Here, we numerically and experimentally scrutinize the electrical properties of grounded liquid collectors and their role in the patterning of electrospun nanofiber mats. As the liquid collectors, we selected the two most common types reported in the literature, namely, electrolyte solutions21,22 and dielectric liquids.18−20 In the numerical simulations, the patternability of electrospun nanofiber mats was mainly governed by the salt concentration of the electrolyte solutions and the dielectric constant of the dielectric liquids. To compare the patterning resolutions of nanofiber mats produced by electrospinning with the grounded liquid collector and conventional electrospinning, we prepared a line array of collectors comprising silver nanowires (AgNWs; metal collector) along with various types of liquid collectors. Line-array-shaped nanofiber mats were then fabricated by the electrospinning process. We numerically and experimentally confirmed that when the KCl concentration exceeds 0.01 M, the patterning resolution is analogous to that of the metal collector, whereas in dielectric liquids such as deionized (DI) water, ethylene glycol, and glycerol, the patternability of the nanofiber mats is slightly retarded.

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⎞ ⎛ e ρ(x) = − 2ec0 sinh⎜ ϕ(x)⎟ ⎠ ⎝ kBT

(1)

where c0 is the electrolyte concentration, kB is Boltzmann’s constant, and e is the charge on the electron. For the dielectric liquid, we only considered the dielectric constant of the liquid in the simulation to estimate the generated electric field. Evaluation of Patternability. A tape mask with a line-array pattern of 1 mm width and 1 mm distance was cut by a laser cutter and transferred to the PMMA substrate. Oxygen plasma treatment

EXPERIMENTAL SECTION

Material. Polycaprolactone (PCL, Mw = 80 000), potassium chloride (KCl), chloroform, ethylene glycol, and glycerol were B

DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX

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Figure 2. (a) Electric field simulations for the case of a metal collector grounded by a metal wire (a(i)) and a magnified image of the dashed rectangular in a(i),a(ii). The dashed rectangular in a(ii) is the field of view of the collector for (b). A magnified image of simulation results presenting electric field vectors on the reference line l(θ) is plotted for the case with a metal (b(i)) and 1 M KCl solution collector (b(ii)), ethylene glycol (b(iii)), without collector (b(iv)), and (b(v)) their measured normal electric field along the reference line l(θ). followed by electrospinning was conducted as previously described. A conductive collector of AgNW was prepared by pouring AgNW solution onto the PMMA substrate with the tape mask. After removing the solution, the sample was dried. To investigate the influence of dielectric constant on the patternability of the electrospun fibers, we prepared glycerol, ethylene glycol, and mixed ethylene glycol and DI water (6:4 by volume) with dielectric constants of 36.80, 39.22, and 58.97 (at 30 °C), respectively. The interface between the nanofiber mat and the bare surface on the substrate (back plate) was imaged by a digital camera. The changing light intensity was measured along 50 equally spaced lines perpendicular to the interface. To obtain the edge response, we averaged the light intensities over 200 pixels in each line. Statistical Analysis. Statistical differences between the compared groups were determined by the single-factor ANOVA and Tukey’s post hoc test. A p value below 0.05 indicated statistical significance.

diameters of the PCL nanofibers on the KCl solution collector (454 ± 178 nm) and the AgNW collector (443 ± 147 nm) were not significantly different, whereas those produced on ethylene glycol were significantly larger (an average diameter of 571 ± 272 nm). According to these results, the electrolyte solution and metal collectors play analogous roles in the production of electrospun nanofibers. The dielectric liquid collector is also available, but it has lower capability in the fabrication and patterning of electrospun nanofibers. The spatial deposition of electrospun nanofibers is mainly governed by the Coulomb forces resulting from the electric field.16 Therefore, by calculating the electric field distribution between the metal needle and the grounded collector during the electrospinning process, we can indirectly evaluate the spatial controllability of the electrospun nanofibers. Before the systematic investigation of the spatial controllability of the electrospun nanofibers depending on the material properties of the liquid collector, we performed a numerical simulation of a simplified model of two line-shaped metal collectors to determine the line width and the distance between the lines of the line-array-shaped collector (Figure S1). The numerical simulation results indicated that when the line width was less than 2 mm, the concentration of the electric field was greatly enhanced, which implied that decreasing the line width could promote the selective deposition of electrospun fibers, whereas the level of concentrated electric field was slightly decreased when the line width exceeded 2 mm. It was also found that the relationship between the electric field and the distance was inverse to the case of the line width. The decrease in the distance between the line-shaped collectors resulted in lowering the concentration of the electric field, which implied that the closer the line-shaped collectors are, the worse the patterning is. However, although we could find a trend for the patterning behavior of electrospun fibers depending on the variations of width and distance with numerical simulations, it was not able to define the critical line width and distance between the lineshaped collectors to obtain successfully patterned nanofiber mats because of the monotonically increasing or decreasing behavior of the magnitude of the electric field without any

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RESULTS AND DISCUSSION Figure 1a schematizes the electrospinning configuration for fabricating line-array-shaped PCL nanofiber mats. Figure 1b shows a line-array-shaped nanofiber mat produced by conventional electrospinning with the patterned AgNW collector and a magnified image of the electrospun nanofibers (inset). Figure 1c,d shows the line-array-shaped nanofiber mats produced by electrospinning with patterned liquid collectors of a 3 M KCl electrolytic solution and an ethylene glycol dielectric liquid, respectively. The fabrication proceeds by the following steps: (1) a hydrophilic line array is formed on a PMMA surface by plasma surface treatment with a tape mask, (2) the electrolyte solution or dielectric liquid is positioned on the hydrophilic pattern, and (3) a line-array-shaped nanofiber mat is fabricated by electrospinning. The 3 M KCl solution collector formed nanofiber mats with spatial resolution and nanofiber morphology similar to those formed on the AgNW collector, whereas the ethylene glycol collector formed mats with coarse resolution and a slightly different morphology with retarded patternability (Figure 1b−d). Figure 1e shows a measured diameter histogram of the nanofibers. The PCL nanofibers produced by electrospinning with the 3 M KCl solution collector and the AgNW collector showed similar diameter distributions, but those produced on the ethylene glycol collector had a wider diameter distribution. The averaged C

DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX

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Langmuir critical point. Given that the main purpose of this study is to elucidate the influence of the material properties of the liquid collector on the patternability of the nanofiber mat, we decided to carry out the electrospinning experiments with an array of lines with 1 mm width and 1 mm distance between the lines that could stably produce a patterned nanofiber mat. As the electrolyte and dielectric liquid collectors produced patterned nanofiber mats with different spatial resolutions, we investigated whether their electrical properties affect the patternability of electrospun nanofiber mats. To quantitatively evaluate the electric field concentration around the collector, we consider the normal component of the electric field, En, as

En = E ·nl

(2)

Figure 3. Normalized electric field intensity graph depending on the dielectric constant is plotted with a metal, electrolyte solution, and dielectric liquid.

where E is the electric field, and nl is the outward normal vector to the reference line l surrounding the collector (see Figure 2a(ii)). En is positive when the normal points outward from the collector and negative when the normal points toward the collector. Figure 2b shows the electric fields (Figure 2b(i−iv)) and magnitudes of En (Figure 2b(v)) along the reference line l for the AgNW (i), 3 M KCl solution (ii), and ethylene glycol (iii) collectors and without a collector (iv). Intriguingly, the metal and 3 M KCl collectors produced identical intensities and distributions of the normal electric field. In both cases, the normal electric field on the reference line l pointed inward and toward the collectors (Figure 2b(i,ii)). However, in the case of ethylene glycol, the intensity of the normal electric field on l was slightly lower than that in the previous cases and a small amount (at θ < −26° along l) was directed outward (Figure 2b(iii)). Without the collector, all of the electric field vectors were directed toward the grounded copper metal wire at the left side of the collector system (see Figure 2b(iv)). Consequently, the En measured along l was reduced in magnitude and was positive at θ < −22° along l, implying an outward-directed electric field. These results revealed that during electrospinning, the electrolyte solution collector can generate an electric field distribution analogous to that of metal. However, the electric field concentration of the dielectric (ethylene glycol) was retarded toward the collector. Therefore, the electrolyte solution collector (but not the dielectric liquid collector) is expected to achieve the same precise patterning of electrospun nanofibers as the metal collector. To further quantify how the electrical properties of the liquid collectors affect the electric field concentration, we calculated the normalized electric field intensity E̅n as follows En̅ =

when their salt concentrations exceeded 10−8 M, the electrolyte collectors concentrated the electric field similarly to the conductive metal collector, that is, E̅n = 1 (see Figure S2). Therefore, DI water containing only a few free ions can behave as a metal collector, selectively depositing the electrospun nanofibers. By contrast, a dielectric liquid collector without free ions retarded the concentration of the electric field (E̅ n < 1). Furthermore, lowering the dielectric constant of the dielectric liquid reduced the E̅n, implying that the dielectric constant largely affects the electric field focusing by a dielectric liquid collector. The numerical results of the concentrated electric fields around line arrays of various collector types were validated in electrospinning experiments. The collectors were shaped as shown in Figure 1a. The electric field concentration induced by the collector is reflected in the patterning resolution of the electrospun nanofiber mat. Figure 4 shows line-array-shaped PCL nanofiber mats fabricated on a PMMA substrate using three electrolyte solution collectors with different salt concentrations (1, 0.1, and 0.01 M KCl; Figure 4a) and three dielectric liquids (DI water, mixed ethylene glycol and DI water, and glycerol; Figure 4b). In the electrolyte cases, the patterning resolution of the nanofiber mats was almost independent of KCl concentration. However, as the dielectric constant decreased in the dielectric liquid collector, the patternability reduced. To quantifiably analyze the patternability between the samples, we examined the interface between the nanofiber mat and the bare surface. The nanofiber mat on the PMMA substrate was placed on a black back plate, creating a distinguishable edge between the mat and the bare surface of the PMMA substrate (see Figure 5a(i)). To avoid the effect of color on the measured light intensity, we adopted a transparent AgNW electrode as the conductive metal collector rather than a conventional opaque metal. The light intensity across the linearray-shaped nanofiber mat was then measured along the x direction from −0.5 to 0.5 mm, as indicated in Figure 5a(i). Typical light intensity data are plotted in Figure 5a(ii). Figure 5b shows the light intensity data along the x direction of the electrospun nanofiber mats fabricated with the AgNW, 1 M KCl solution, DI water, and glycerol collectors. At the edge, the light intensity sharply increased in the AgNW, 1 M KCl solution, and DI water cases, but it increased much more slowly in the glycerol case. This corroborates the lower patternability of a dielectric liquid collector with a low dielectric constant, as observed in the numerical simulations. To quantify the patternability, we determined the edge response dER, a new

∫l En ds ∫l En,M ds

(3)

where En,M is the normal component of the electric field around a metal collector. E̅n gives the normalized concentration level of the electric field toward the collector. Strictly, it defines the magnitude of the net electric field toward the collector, relative to that of the metal collector. Note that E̅ n = 1 when the collector concentrates the same net electric field as a metal collector, and E̅n = 0 when no net attraction toward the collector occurs. To clarify how the electrical properties of the liquid collectors affect the shape controllability of the electrospun nanofibers, we prepared AgNW and KCl solutions with different salt concentrations (10−8 to 3 M) and dielectric liquids with different dielectric constants (1 to 80) as collector materials. Figure 3 shows the calculated E̅n values of the metal, electrolyte solutions, and various dielectric liquids. Notably, D

DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX

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Figure 4. Line-array-patterned nanofiber mats fabricated by changing the electrolyte concentration and the dielectric constant of the liquid collector including 1, 0.1, and 0.01 M KCl solution (a) and DI water, a mixture of ethylene glycol and DI water, and glycerol (b). All scale bars are 1 mm.

Figure 5. Evaluation of patternability depending on the AgNW electrode and various liquids. (a) A line array of spatially controlled nanofiber on a PMMA substrate (a(i)) and a light intensity graph along the line perpendicular to the edge between a nanofiber mat and a back plate ((aii)). (b) Light intensity−distance curve for the case of AgNW electrode, 1 M KCl solution, DI water, and glycerol. (c) Electrolyte concentration−edge response curve and (d) dielectric constant−edge response curve. Scale bar is 1 mm [** denotes the statistical significance (p < 0.05) over all other groups].

5d). In summary, an electrolyte solution collector containing a few free ions can concentrate the electric field sufficiently for the controlled deposition of electrospun nanofibers; moreover, the quality of the electrospinning matches that of a conventional conductive metal collector. By contrast, electrospinning with a dielectric liquid collector retards the patternability from that of a metal collector, and the patternability worsens with decreasing dielectric constant. These findings indirectly confirm the simulation results in Figure 3. However, there are several

measure defining the distance between the two points corresponding to a 10 and 90% rise in light intensity from the back plate to the nanofibers. Figure 5c shows the calculated dER of nanomats fabricated on AgNW, electrolyte solutions with various salt concentrations, and DI water collectors. Consistent with the simulation results, the dER of the 0.01, 0.1, 1, and 3 M KCl solution collectors did not significantly differ from that of the AgNW collector. By contrast, the dER of the dielectric liquids was influenced by the dielectric constant (see Figure E

DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX

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Langmuir limitations in the numerical simulation. First, the ion impurities of the dielectric liquids were not considered, although they could influence the generation of the electric field. DI water contains H+ and OH− ions because of autoprotolysis with a concentration around 10−7, and dielectric liquids also usually possess ionic impurities. Second, we adopted a simple model for a space charge density of the mobile ion in the electrolyte solution that omitted the ion size effects. It is known that the finite size of the ions keeps them at a certain distance and influences the distribution of the mobile ions in the electrolyte solution, which leads to the alteration in the space charge density. Although these limitations possibly arose the inaccuracy of the numerical simulation, both the experimental and simulation results showed similar tendency in the controlled deposition of the electrospun nanofiber on liquid collectors. The numerical and experimental results confirmed that electrolyte solution collectors can modulate the electric field and thereby control the deposition of electrospun nanofibers, similarly to the metal collectors that are widely utilized in conventional electrospinning. This implies that when an electric potential is applied between the metal needle and the ground, the free ions in the electrolyte solution are reorganized until the internal electric potential of the solution becomes nearly uniform. The same phenomenon occurs in metal collectors. At this time, the electric potentials of the ground and the solution surface are very similar. Consequently, the electric field distribution in the electrolyte solution is similar to that established in a metal collector, enabling a high patterning resolution of the electrospun nanofiber mat. The dielectric liquid collector also forms a patterned nanofiber mat but with retarded patternability. A dielectric liquid collector is thought to concentrate an electric field mainly through electrostatically induced polarization.15 However, the electric field formed by polarization was insufficient to homogenize the internal electric potential of the dielectric liquid; consequently, the electric potential at the liquid surface was above the ground potential. This surface−ground potential difference reduced the potential difference between the metal needle and the dielectric liquid, lowering the concentration of the electric field and hence the patternability of the electrospun nanofiber mats. Decreased polarization also explains why the patternability of the nanomats fabricated on dielectric liquid collectors deteriorated with decreasing dielectric constant. Figure 6 demonstrates a potential application example of liquid collector in electrospinning where the letter-shaped nanofiber mats (in this case, POSTECH and Langmuir) were directly patterned on a PMMA substrate by means of liquid collectors of 3 M KCl solution and glycerol without any patterned conductive electrode. The liquid collector in this patterning played a role as a temporal collector, which was utilized as a collector during electrospinning and readily removed after electrospinning. Electrospinning with the liquid collector of 3 M KCl solution produced the letter-shaped nanofiber mat with a clear boundary, whereas the nanofiber mat produced with glycerol had a blurred boundary, although both letters are recognizable (Figure 6). These results showed the potential of facile integration of a patterned nanofiber mat with various devices or structures made up of insulators such as dielectric polymers or ceramics and also implied that it is important to select an appropriate liquid collector depending on the desired application. Several research groups have demonstrated application examples of patterned electrospun

Figure 6. Letter-shaped nanofiber mats fabricated by electrospinning with a liquid collector of a 3 M KCl solution (a) and glycerol (b) on a PMMA substrate. All scale bars are 2 mm.

fibers including cell patterning,6 cell colonization,10 and highly stretchable strain sensor.23 For the biomedical applications such as cell patterning and colonization, it is recommended to utilize a cell culture medium that contains mobile ions without any cytotoxic element as a liquid collector to produce a patterned nanofiber mat. With a stretchable strain sensor application in mind, residual conductive impurities after electrospinning might result in an error during sensing, and thus, DI water is recommended as a liquid collector for electrospinning. Likewise, we expected that the utilization of liquid collectors in patterning of electrospun nanofibers would provide great opportunities in various applications. It has been known that electrospinning with two metal strips separated by an insulating gap could produce aligned fibers on the insulating gap. Likewise, we confirmed that two line-shaped liquid collectors, especially electrolyte solution collectors, also enabled to align electrospun fibers on an insulating gap between the two line-shaped liquid collectors, as shown in Figure S3. Li et al. stated that the alignment of electrospun fibers was governed by the electrostatic force stretching the electrospun nanofibers perpendicular across the gap.24 Also, Pokorny et al. found that the magnitude of the transversal electric field is a key parameter to determine the stretching force and alignment.25 Thus, we could also expect that the electrolyte solution collectors generated sufficient transversal electric field and electrostatic force to align the nanofibers on the insulating gap. The line width and the distance between lines of the line-array-shaped liquid collector are expected to influence the degree of nanofiber alignment. The investigation on the fiber alignment depending on the variation of the width and distance would be a valuable future study by considering the transversal electric field strength and direction.

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CONCLUSIONS In summary, we numerically investigated the role of liquid collectors on the patterning of nanofiber mats during electrospinning and experimentally validated the simulation results. Most importantly, in nanomats fabricated on electrolyte solution collectors with salt concentrations above 0.01 M, the patternability was analogous to those fabricated on conventional metal collectors. Dielectric liquids such as DI water also achieved electrospun nanofiber mats with relatively low patterning resolution. Given their fluidic nature, we expect that liquid collectors will provide a more versatile and effective tool for fabricating complex nanofiber architectures than solid F

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Langmuir collectors. Such flexibility will advance various biomedical and industrial applications.

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(10) Jia, C.; Yu, D.; Lamarre, M.; Leopold, P. L.; Teng, Y. D.; Wang, H. Patterned electrospun nanofiber matrices via localized dissolution: potential for guided tissue formation. Adv. Mater. 2014, 26, 8192− 8197. (11) Li, D.; Ouyang, G.; McCann, J. T.; Xia, Y. Collecting Electrospun Nanofibers with Patterned Electrodes. Nano Lett. 2005, 5, 913−916. (12) Brown, T. D.; Dalton, P. D.; Hutmacher, D. W. Direct writing by way of melt electrospinning. Adv. Mater. 2011, 23, 5651−5657. (13) Carlberg, B.; Wang, T.; Liu, J. Direct Photolithographic Patterning of Electrospun Films for Defined Nanofibrillar Microarchitectures. Langmuir 2010, 26, 2235−2239. (14) Cho, S. J.; Kim, B.; An, T.; Lim, G. Replicable multilayered nanofibrous patterns on a flexible film. Langmuir 2010, 26, 14395− 14399. (15) Zhao, S.; Zhou, Q.; Long, Y.-Z.; Sun, G.-H.; Zhang, Y. Nanofibrous patterns by direct electrospinning of nanofibers onto topographically structured non-conductive substrates. Nanoscale 2013, 5, 4993−5000. (16) Zhang, D.; Chang, J. Patterning of Electrospun Fibers Using Electroconductive Templates. Adv. Mater. 2007, 19, 3664−3667. (17) Ding, Z.; Salim, A.; Ziaie, B. Selective nanofiber deposition through field-enhanced electrospinning. Langmuir 2009, 25, 9648− 9652. (18) Hong, S.; Kim, G. Fabrication of size-controlled threedimensional structures consisting of electrohydrodynamically produced polycaprolactone micro/nanofibers. Appl. Phys. A: Mater. Sci. Process. 2011, 103, 1009−1014. (19) Teo, W.-E.; Gopal, R.; Ramaseshan, R.; Fujihara, K.; Ramakrishna, S. A dynamic liquid support system for continuous electrospun yarn fabrication. Polymer 2007, 48, 3400−3405. (20) Wang, L.; Wu, Y.; Guo, B.; Ma, P. X. Nanofiber Yarn/Hydrogel Core−Shell Scaffolds Mimicking Native Skeletal Muscle Tissue for Guiding 3D Myoblast Alignment, Elongation, and Differentiation. ACS Nano 2015, 9, 9167−9179. (21) Park, S. M.; Kim, D. S. Electrolyte-Assisted Electrospinning for a Self-Assembled, Free-Standing Nanofiber Membrane on a Curved Surface. Adv. Mater. 2015, 27, 1682−1687. (22) Park, S. M.; Eom, S.; Choi, D.; Han, S. J.; Park, S. J.; Kim, D. S. Direct fabrication of spatially patterned or aligned electrospun nanofiber mats on dielectric polymer surfaces. Chem. Eng. J. 2018, 335, 712−719. (23) Yu, G.-F.; Yan, X.; Yu, M.; Jia, M.-Y.; Pan, W.; He, X.-X.; Han, W.-P.; Zhang, Z.-M.; Yu, L.-M.; Long, Y.-Z. Patterned, highly stretchable and conductive nanofibrous PANI/PVDF strain sensors based on electrospinning and in situ polymerization. Nanoscale 2016, 8, 2944−2950. (24) Li, D.; Wang, Y.; Xia, Y. Electrospinning of polymeric and ceramic nanofibers as uniaxially aligned arrays. Nano Lett. 2003, 3, 1167−1171. (25) Pokorny, M.; Niedoba, K.; Velebny, V. Transversal electrostatic strength of patterned collector affecting alignment of electrospun nanofibers. Appl. Phys. Lett. 2010, 96, 193111.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03547. Determination of the line width and the distance between the lines of the line-array-shaped collector; simulation results of electric field with various electrolyte solutions; and alignment of electrospun nanofibers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dong Sung Kim: 0000-0003-4780-9635 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (nos. 2017R1A2A1A05001090 and 2011-0030075).

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ABBREVIATION ELES, electrolyte-assisted electrospinning REFERENCES

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DOI: 10.1021/acs.langmuir.7b03547 Langmuir XXXX, XXX, XXX−XXX