PDMS

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Remarkable Conductive Anisotropy of Metallic Microcoil/PDMS Composites Made by Electric Field Induced Alignment Xinghao Li,† Jun Cai,*,† Yingying Shi,† Yue Yue,† and Deyuan Zhang† †

School of Mechanical Engineering and Automation, Beihang University, No. 37 Xueyuan Road, Haidian District, Beijing 100191, China S Supporting Information *

ABSTRACT: We successfully fabricated a highly anisotropic electrical conductive microcoil/polydimethylsiloxane (PDMS) composite based on helical Spirulina-templated metallic particles using an electric field-induced alignment method. The optimized AC electric field (2 kV/cm, 1 kHz) could efficiently assemble the lightweight conductive microcoils into continuous long chains and form unique end-to-end physical contacts between adjacent particles in the alignment direction, leading to highly conductive channels. Furthermore, the electrical conductivity in the alignment direction reached up to ∼10 S/m for 1 wt % loading and exhibited almost 7−8 orders of magnitude higher than that in perpendicular directions, which is by far the most remarkable conductive anisotropy for anisotropic conductive composites (ACCs). In addition, the anisotropic composites exhibit excellent current-carrying capability in a functional light emitting diode (LED) circuit. Therefore, due to the superior conductive anisotropy and high conductivity, the composites have promising potential in high reliability electrical interconnections and subminiature integrated circuits. KEYWORDS: conductive anisotropy, microcoil, alignment, directional assembly, current-carrying capability

1. INTRODUCTION Currently, anisotropic functional materials have attracted much attention due to their unique anisotropy in electrical and/or thermal conductivity, wetting, and optical activity, which are promising for wide applications in microelectronics, bioengineering, biological sensing, and energy conversion.1−5 In particular, as electronic systems become more subminiature, reliable, and intelligent, a variety of materials with electrical conductive anisotropy will be widely employed for electrical interconnection, miniaturized sensing devices, and electrochemical actuators.6−11 This is mainly attributed to anisotropic conductive material advantages:6 electrically conductive along expected directions (we denote this direction as the X direction and the other two perpendicular directions are respectively denoted as the Y and Z direction) and insulative along the Y and Z directions to avoid short-circuiting; high average density of conductive pathways and predesigned electrical connections in narrow spaces. Targeted for specific practical applications, numerous methods to fabricate ACCs have been developed, mainly including external electric field-induced alignment,3,12,13 shearinduced self-assembly,14−16 magnetic field-assisted orientation,17,18 and other novel methods such as template-guided growth19 and special electrospinning technology.6 Generally, it is the well-ordered alignment or the directional orientation of conductive fillers that endows the composites with electrical anisotropic conductivity. Among these feasible methods, shearinduce alignment is a facile and efficient method. However, © XXXX American Chemical Society

continuous conductive pathways or effective end-to-end contacts between conductive fillers are difficult to construct, consequently, leading to poor conductivity and/or low anisotropy. As for magnetic field-directed alignment, the conductive fillers normally need to be decorated with magnetic nanoparticles such as γ-Fe2O3 nanoparticles, the decoration seems time-consuming and reduces the intrinsic electrical conductivity of fillers. To solve these problems, the electric field-induced alignment20 could be a preferred method. Since the electric field can assemble the conductive particles into continuous chain-like channels21−24 even at low filler loadings, this method would possibly further enhance the conductivity along the alignment direction and achieve highly conductive anisotropy. Besides the alignment techniques, some researchers have tried to develop various advanced conductive fillers and use them to fabricate ACCs, such as carbon nanofibers,13 single/ multiwalled CNTs,3,7,14 and graphene.11,25−27 Mao et.al25 used graphene nanosheets as conductive fillers to prepare an anisotropic conductive film; the electrical resistivity along the X direction was ∼3.5 × 105 Ω·m and almost 4 orders of magnitude lower than that along the Y and Z directions. Gao et al.7 reported an anisotropic conductive polymer composite based on MWCNTs and polycarbonate/polyethylene blend; Received: October 23, 2016 Accepted: December 22, 2016 Published: December 22, 2016 A

DOI: 10.1021/acsami.6b13505 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the resulting materials showed 3 orders of magnitude difference between the perpendicular (∼109 Ω·cm) and parallel stretch directions (∼106 Ω·cm). Moreover, Li et al.16 fabricated ultrathin 3D SWNT films and studied the anisotropy in electrical transport properties; the conductivity in the vertical direction was about 1−2 times higher than that in horizontal direction. The ACCs described above are all composed of conductive carbon fillers and insulative polymer matrix, and show obvious electrically conductive anisotropy. However, it is noted that their highest conductivity (i.e., along the X direction) is insufficient for high performance interconnection,1 and, to some degree, would not meet the superior currentcarrying requirements. Furthermore, the conductive anisotropy between the X direction and the other directions is not high enough (mostly lower than 4 orders of magnitude) and may lead to the unreliable interconnections (i.e., insufficient conductivity along the X direction and/or poor insulation along the perpendicular directions). Therefore, it is urgent to explore novel conductive fillers to fabricate ACCs with high conductivity along the X direction and remarkable conductive anisotropy. The traditional conductive metallic filler (i.e., silver particles) may serve as an appropriate substitute, yet undesired spontaneous sedimentation28,29 of heavy metallic particles in the fluidic polymer matrix is an obvious problem, which would lead to material defects and severely degrade the comprehensive properties. Therefore, to maintain high electrical conductivity and reduce unwanted sedimentation, we introduce silver-coated Spirulina-templated particles30 as comparable conductive fillers for fabricating ACCs. The particles are prepared by electroless sliver plating and show the complicated helical soft-core structure (i.e., the interior biotemplate is totally covered by metallic silver film). Thus, the metallic microcoils combine excellent electrical conductivity and unique lightweight properties, which could positively improve overall electrical anisotropy and effectively diminish sedimentation. In this work, we fabricate a novel high-performance anisotropic conductive microcoil/PDMS composite using electric field-induced alignment. The specific AC electric field is optimized to efficiently assemble the conductive microcoils into continuous networks. The directional alignment process and physical-contact formation are observed and analyzed in detail. We also measure both the AC impedance and the DC conductivity of the microcoil-based composites in three directions and evaluated the conductive anisotropy. Furthermore, a functional light emitting diode (LED) circuit is developed to demonstrate the reliable interconnection and current-carrying capability of the ACCs.

Figure 1. Optical microscope image of Spirulina. (Dow Corning, Sylgard 184) with elastomer to cross-linker ratio 10:1, was used as the matrix due to its excellent physical properties and electrical isolation. 2.2. Fabrication of Anisotropic Conductive Microcoil/PDMS Composites. Figure 2 shows the fabrication process for anisotropic conductive microcoil/PDMS composites by using alternating electric field-induced alignment. The electric field was generated using a custom-made system composed of a function waveform generator (Agilent, 33522A), an amplifier (XE-50500), a two-channel digital storage oscilloscope (Tektronix, TDS2012C) and two panel copper electrodes. Parylene-C thin films were previously deposited onto the electrode surfaces to isolate them from the mixture, avoiding short circuit and partial discharge during the assembly process. First, the conductive microcoils were added to PDMS and mechanically stirred for 30 min to achieve homogeneous dispersion; the particle weight fractions were 0.1%, 0.5%, 1%, and 5%, respectively. The mixture was injected into the gap (1 mm in width and depth) between the two panel electrodes, and a coverslip was placed on the electrodes to eliminate external disturbance (Figure 2a). Then, an AC electric field was applied to directionally assemble the microcoils at room temperature (Figure 2b). Subsequently, the PDMS mixture was cured at 120 °C for 10 min while the electric field was maintained (Figure 2c). 2.3. Characterization. A JSM-6010LA (JEOL Ltd.) scanning electron microscope (SEM) was employed to characterize the morphology of the prepared particles and a matched energy dispersive spectrometer (EDS) was used to analyze their main elements. The whole alignment process was observed from the top view using an optical microscopy (Olympus, BX51) with a digital camera (Canon, 600D) in the transmission mode. The AC impedance analysis of the as-prepared composites was conducted by using an automatic component analyzer (Tonghui, TH2818) over a sweeping frequency range from 20 Hz to 200 kHz with an applied voltage of 100 mV. We also evaluated the DC conductivity using a multimeter (UNI-T, UT61E) and an automatic component analyzer (only for high resistance). The samples were connected to the wires using conductive silver adhesives.

3. RESULTS AND DISCUSSION 3.1. Characterization of Metallic Spirulina-Templated Microcoils. Figure 3 shows the morphology and elemental mapping images for a silver-coated Spirulina-templated microcoil. The helical configuration of Spirulina template is perfectly retained after electroless plating and the coating surface is smooth (Figure 3a), which implies that the coating is continuous and homogeneous. The microcoil cross section (marked with the dashed line) shows an obvious interface between silver film and the biotemplate (Figure 3a inset), confirming the soft-core structure. Therefore, compared to a complete silver helix with the same volume, the Spirulinatemplated metallic microcoils are much more lightweight, which can effectively reduce the spontaneous sedimentation and positively contribute to efficient assembly. Furthermore,

2. MATERIALS AND METHODS 2.1. Materials. Spirulina (Arthrospira platensis) is a natural microscopic organism that has a three-dimensional spring-like structure (Figure 1 and inset). Normally, the whole length (L) of Spirulina is about 100−400 μm, the diameter (D) about 26−36 μm, the wire diameter (d) 5−10 μm and the turn number (n) 4−10. The structural size can be artificially controlled by adjusting the temperature and light intensity during the cultivation process. Moreover, Spirulina is easily cultivated and obtained in large quantities, thereby providing abundant helical biotemplates for synthesizing functional microcoils. After the biotemplates were fixed with glutaraldehyde, electroless silver plating was used to fabricate the metallic microcoils. Details for the procedure and chemicals can be found in our previous research.30 Herein, the silver-coated Spirulinatemplated metallic microcoils serve as the conductive fillers. PDMS B


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Figure 2. Schematic illustration for the fabrication process of anisotropic conductive microcoil/PDMS composites: (a) random dispersion of microcoils in PDMS; (b) electric field-induced alignment; and (c) curing process.

to the intrinsic microcoil conductivity and essentially serves as the conductive channels in the percolated networks. Thus, the metallic microcoils can be employed as promising conductive fillers in the fabrication of functional conductive composites. 3.2. Directional Assembly of Conductive Microcoils. An AC electric field was employed to directionally assemble the prepared conductive microcoils, so the voltage and frequency are two key factors. Figure 4 shows the effects of AC electric field magnitude and frequency on the microcoil alignment. The microcoil loading was 1 wt %, far lower than the percolation threshold (∼35 wt %) reported in our previous research.30 As demonstrated in Figure 4a, the conductive microcoil alignment and chain-formation are highly dependent on the electric field magnitude and frequency. Furthermore, the rod-like microcoils can be oriented and assembled into the long chains bridging the two electrodes at the specific voltages and frequencies (marked in the blue line). We also note that the alignment efficiency and/or velocity varies significantly during the alignment process. The deviation angles were measured in AutoCAD Software based on the pictures captured by a digital camera

Figure 3. Characterization of metallic microcoils: (a) SEM image of a metallic Spirulina-templated microcoil and (b) elemental mapping images of the microcoil; the yellow, green, and red points represent silver, silicon, and oxygen, respectively.

the elemental mapping (Figure 3b) demonstrates the main elements in the region marked with the dashed box. The yellow dots representing the silver element follow the same helical shape shown in the SEM images, which suggests that the compact silver films were successfully deposited onto the biotemplate surface. The continuous metallic coating is crucial

Figure 4. Effects of AC electric field magnitude and frequency on microcoil alignment: (a) alignment and chain-formation; (b) and (c) percentage of particle deviation angle |θ| ≤ 20° as a function of alignment time at different voltages and frequencies, respectively. Angle θ is the deviation angle of the microcoil major axis with respect to the electric field direction. Solid lines are given as a visual guide. C


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Figure 5. Particle alignment process in an AC electric field: (a) random dispersion in PDMS; (b) microcoil rotation and translation; (c) directional assembly and chain formation; (d) physical contact of adjacent microcoils; and (e) deviation angle, θ, distribution. The solid line (Gaussian fit) is given as a visual guide.

assembly of metallic microcoils, we prefer relatively lower frequencies ranging from 100 Hz to 1 kHz, compared with the high frequency 10 kHz and 100 kHz. On the basis of the optimized AC electric field (2 kV/cm, 1 kHz), we expect to better understand the metallic microcoil alignment process. This understanding could significantly contribute to the formation of unique long microcoil-lines and the resulting composite anisotropic characteristics. The particles are physically isolated from each other at the beginning stage (shown in Figure 5a). When the electric field was applied, the microcoils rotated to orient their major axis parallel to the electric field and translated toward the nearest particles to form long microcoil-lines (shown in Figure 5b and see as well as Video S1 in the Supporting Information, SI). Indeed, the electric field-induced alignment is attributed to two factors: rotation and translation. First, the microcoils and the suspending medium are polarized when they are subjected to the applied electric field.33 The direction of induced dipole moment is along the major axis of the rod-like microcoils. Each charge on the dipole experiences an opposite force in the electric field and, consequently, the microcoil experiences an inplane torque,31 which rotates the particle and directionally orients its major axis parallel to the field (shown in Figure S1 and Video S2). The statistical results (Figure 5e) show the batch microcoil angular distribution. The percentages of deviation angle |θ| ≤ 20° increased from ∼10% to ∼60% (aligning for 2 min), and finally to ∼90%, implying the rotation and orientation behavior of collective microcoils in the electric field. Second, due to the nonuniform field and particle−particle interaction in the suspending medium,36 the microcoil would also experience a DEP force, which can overcome viscous forces and spatially translate the particles. Since the effective polarizability of metallic microcoils is far larger than that of PDMS medium, the resulting DEP force propels the particles toward the higher electric field magnitude regions,37 (i.e., the regions near the two electrodes or at each end of particles). Moreover, the polarized microcoils could interact with each other and change the local field intensity,38 especially when two particles are close enough. This interaction could contribute to

mounted on an optical microscopy. The particle motion and alignment process were observed from the top view. The statistical results (shown in Figure 4b) further illustrate the effects of field magnitude and time on the alignment. The percentage of deviation angle |θ| ≤ 20° monotonically increases from ∼10% to ∼90% using the electric field-induced assembly, indicating that random distribution of the microcoils evolves into the uniform orientation. As the electric field magnitude increases from 100 V/cm to 2 kV/cm (at 1 kHz frequency), the particles require less and less time to orient the major axis along the field (the viscosity change of PDMS is little during the experiment ∼1 h, 20 °C). Notably, the alignment finished in less than 10 min at 2 kV/cm, which is far faster than other smaller field magnitudes. However, for the relatively low electric field at 100 V/cm, the microcoils are still randomly dispersed after 40 min and 1 h (not shown). This is mainly because the gradient of the field magnitude squared31,32 was too low, and the resulting dielectrophoretic (DEP) force could not drive the particles.33 Therefore, properly high voltage is crucial for microcoil alignment, and the increase of electric field strength could largely enhance the alignment efficiency. Moreover, we predict that the alignment efficiency and/or velocity in our experiments could be further improved if the electric field strength is appropriately increased. Additionally, we further studied the effects of varying the frequency from 10 Hz to 100 kHz on the alignment. Figure 4c shows the percentage of deviation angle |θ| ≤ 20° at different frequencies as a function of alignment time. When the frequency is lower than 1 kHz, the monotonic increasing trend in percentage of |θ| ≤ 20° seems nearly the same and the directional alignment can be finished in less than 10 min. However, for higher frequencies, the alignment requires 30 min or longer to complete at 10 kHz, while the assembly did not occur at 100 kHz (2 kV/cm). This frequency behavior can be attributed to the mechanism of particle and medium polarization. At high frequencies, the mobile charges have insufficient time to respond to the electric field, or there is no time to charge the electrical double layer induced at the metal−electrolyte interface.34,35 As a result, for the directional D


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ACS Applied Materials & Interfaces particle motion as well. Generally, the rotation and translation took place at the same time, yet there is some specific conditions in the alignment process (the motion and corresponding mechanism are shown in Figure S2). For some other particles such as nanowires28,39 and Janus particles,40 the similar phenomenon is also observed. Therefore, the electric field-induced force can serve as an effective and significant driving force for orienting and assembling particles. In addition to the rotation and translation behavior, it is noteworthy that the long microcoil-lines bridged the two electrodes and were composed of several end-to-end linked microcoils (shown in Figure 5c). This structure mainly results from the particle−particle interaction and positive dielectrophoresis. For polarized microcoils with major axis along with the field, the high field strength region would be formed between the two particles.31 Due to positive dielectrophoresis, the two microcoils would move toward this region and, thus, form the end-to-end linked microcoil-chains. More importantly, the unique physical contact can be constructed between the two adjacent microcoils during the alignment process (shown in Figure 5d). This phenomenon positively contributes to contact conductance enhancement due to the electron tunneling effect at two adjacent particle junctions.41 On the contrary, if the tunneling distance is too far between the two adjacent points of microcoils, then the insulating layer around the particles would prevent the electric tunneling effect and immensely increase the contact resistance.42 Also, the end-toend-link and oriented assembly can be advantageous for lowering the loadings requirements of metallic microcoils for developing the percolation networks,12,43 which may improve the filler utilization rate and could be severely crucial for the anisotropic performance of the resulting composites. Compared to other alignment methods (e.g., the shear-induced-assembly), the orientation could be achieved, yet physical contact formation may be difficult at low loadings (shown in Figure S3), leading to poor conductivity. The hierarchical distribution of long microcoil-lines was also identified by the depth of field of optical microscope (i.e., the conductive pathways were established in different layers along the Z-axis). The results are consistent with the lightweight properties of the complicated soft-core structure (as illustrated in Figure 3), which could diminish the sediment of conductive fillers in PDMS and contribute to composite uniformity. Similarly, we achieved the directional assembly of conductive microcoils with different weight contents (i.e., 0.1%, 0.5%, and 5%) using the AC electric field (shown in Figure 6). Also, samples of larger area (1 × 20 mm2) were also produced (shown in Figure S4). Compared to the percolation threshold of conductive microcoils,30 the particle loadings in the experiments were too low to establish cross-linked conductive networks. Before alignment, the particles were randomly dispersed in the polymer matrix without agglomeration. After alignment was completed, it was clear that particles exhibited well-ordered orientation with their major axis parallel to the electric fields. However, when the weight fraction was 0.1 wt %, the microcoils did not form the end-to-end continuous microcoil-lines due to the low filler content and relatively long distance between separated particles. Furthermore, as the loadings increased, conductive pathway density could also gradually improve. However, if the weight fraction is higher than 10 wt %, then the microcoils would not always orient their major axis parallel to the field or form well-ordered end-to-end long microcoil-lines. This is because high filler concentration

Figure 6. Directional assembly of microcoils with different loadings: 0.1, 0.5, and 5 wt %.

tends to cause the interconnection or aggregation of microcoils, which severely restricts the particle free motion and directional assembly. Nevertheless, the physical contact and percolation networks could still form. In addition, the formed conductive pathways were stabilized and permanently locked in the cured PDMS matrix. Therefore, the AC electric field-induced alignment can be applied to fabricate composites with low filler loadings but excellent conductive and anisotropic properties. 3.3. Electrical Conductive Anisotropy. Considering the well-ordered and continuous long microcoil-lines, we predict that the resulting microcoil/PDMS composites (1 wt % filler content) would exhibit highly conductive anisotropy. To confirm this prediction, we measured both the AC impedance and the DC conductivity of the prepared composites in the alignment direction (X direction) and the perpendicular directions (Y and Z directions). The composites were cut into the sample of uniform size (1 × 10 × 1 mm3). Figure 7 shows the complex impedance for the as-prepared nonaligned and aligned microcoil/PDMS composites along the three directions. For the randomly dispersed samples (Figure 7a and inset), the impedance in all directions is nearly the same, meaning that the nonaligned composites are electrically isotropic. Also, the composites exhibit predominantly capacitive nature that the AC impedance keeps higher than 108 Ω at low frequencies (i.e., between 20 and 300 Hz) and monotonically decreases to ∼106 Ω from 300 Hz to 0.2 MHz. These results are attributed to low loadings of conductive fillers and the resulting physical isolation between particles. However, for the aligned materials (shown in Figure 7b), the impedance in X direction exhibits about 7−8 orders of magnitude lower than that without alignment. Furthermore, this impedance is maintained at ∼10Ω for all the measurement frequencies, indicating that the aligned samples are predominantly conductive in X direction. This change is mainly due to two key factors: One is the directional alignment or chain-like conductive pathways, and the other is the unique physical contact between adjacent particles formed by electric fieldinduced force. However, the AC impedances for aligned samples in Y and Z directions are nearly the same as that for nonaligned composites, and still as high as 108 Ω. The results E


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Figure 7. Absolute value of the complex impedance |Z| for isotropic (a) and anisotropic (b) microcoil/PDMS composites.

Figure 8. Measurement of DC conductivity: (a) nonaligned sample; (b) aligned sample; and (c) samples (1 wt %) prepared at different frequencies and electric field strengths.

Table 1. Comparison between Electrical Anisotropies in the Previous Literature ref.

conductive filler

18 13 12 25 27 14 6 this work

maghemite/MWCNTs carbon nanofiber graphene nanoplatelet graphene graphene/CNTs MWCNTs Fe3O4 metallic microcoil

filler content 1 3 0.52 1.5 9 1.5

wt % wt % vol % wt % wt % wt %

1 wt %

method

anisotropy index

conductivity in x axis (S/m)

magnetic field-induced alignment electric field-induced alignment electric field-induced alignment shear-induced self-assembly electrospinning shear-induced self-assembly electrospinning electric field-induced alignment

4.1 10 103 104 106 106 108 108

4.1 × 10−7 10−4 10−5 3 × 10−6 8.3 × 10−2 4.1 × 10−4 −4 10 10

the X direction is about 7−8 orders of magnitude higher than that along the Z direction, meaning that the outstanding conductive anisotropy index is about 107∼108 (for filler content exceeding 0.5 wt %). However, for the sample with 0.1 wt % filler content, the conductivity along the two directions is nearly the same. This is because the loading is too low to build the physically connected conductive network in the alignment direction. Also, Figure 8c shows the DC conductivity (along the X direction) for prepared samples aligned with different voltages and frequencies corresponding to Figure 4a. As expected, only the samples with long microcoil-chains exhibit high DC conductivity. Interestingly, the conductivity of the two samples (green bars) seemed still low though the long microcoil-lines were formed and observed. The possible reason is that the long microcoil-lines formed in the relative low field of 500 V/cm may have some unstable contact points or defects between adjacent particles. Additionally, Table 1 summarizes the conductive anisotropy indexes of some comparable polymer-based multifunctional composites fabricated using different strategies in the prior studies.6,12−14,18,25,27 From the aspects of anisotropic electrical conductivity, the results and data indicate that the anisotropy of aligned microcoil/PDMS composites produced in this study is,

confirm that multiple long microcoil-lines were constructed only in the X direction and that the PDMS matrix isolated each conductive pathway along Y and Z directions. Thus, we can conclude that the long microcoil-lines are crucial for largely enhancing the AC conductivity in the alignment direction and, consequently, lead to excellent conductive anisotropy. In addition, Knaapila et al. also achieved the conductivity enhancement (at least 2−3 orders of magnitude) of carbon nanocone adhesive in a similar way.21 Herein, we not only reveal the dominant underlying factor (i.e., end-to-end physical contact between each particle) (Figure 5d), but also further introduce the anisotropic conductive properties. Apart from the AC impedance, the DC conductivity for prepared composites was also measured to further investigate the conductive anisotropy. Figure 8 shows the DC conductivity along X and Z directions (the measurement in the Y direction is nearly the same as that in Z direction) as a function of microcoil loading. For the nonaligned materials (shown in Figure 8a), the conductivity along both the X and Z directions is slightly enhanced as the loadings increase, but still remains quite low (∼10−7 S/m), indicating that the composites are isotropic and nonconductive. In contrast, for the aligned composites (shown in Figure 8b), the DC conductivity along F


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Figure 9. Application of microcoil/PDMS composites in functional LED integrated circuits. (a) A nonaligned sample with loading of 5 wt %; (b) a nonaligned sample with loadings of 50 wt %; and (c) an aligned sample with loadings of 5 wt %.

direction). The results are mainly attributed to the oriented assembly of conductive microcoils and the resulting remarkable conductive anisotropy. And it indicates that composites can exhibit superior current-carrying capability in the alignment direction and good insulation in the other directions, which may be exploited for highly reliable electrical interconnects.

to our knowledge, the most remarkable for functional ACCs. Compared with other ACCs, the metallic microcoil-based anisotropic composites also exhibit higher conductivity (∼10 S/ m) along the X direction. The results indicate that the composites exhibit not only an outstanding conductive anisotropy but also highly electrical conductivity, which could improve the current-carrying capability and electrical reliability of the electrical interconnection.1 This is mainly ascribed to fact that the homogeneous silver coating on the surface of Spirulina has low intrinsic resistivity44 with the value of about 10−6 Ω·cm and, more significantly, the unique physical contact formed by electric field-induced alignment can substantially contribute to enhancing contact conductance of percolation networks. Nevertheless, the conductivity values in the X direction could not reach the same order of magnitude as the bulk silver, which is regarded as a general feature45 for conductive composites composed of conductive fillers and polymer matrix. This phenomenon results from the contact resistance between the connected microcoils, which is likely dependent on effective contact area between two particles, morphology of silver coating and lubricated property of the matrix.22 In addition, the intrinsic properties of conductive particles, such as the thickness and composition of the silver coating, also exert effects on the electrical conductivity of the aligned composites. Therefore, considering the excellent conductive anisotropy and high electrical conductivity, the composites may be applied for high performance interconnects and miniaturized sensing devices. Due to the remarkable conductive anisotropy and high electrical conductivity only in the X direction, the aligned microcoil/PDMS composites can be directly employed for electrical interconnects. Figure 9 shows the application of such microcoil/PDMS composites in LED integrated circuits. The measured samples were all roughly 1 mm in length and width (shown in Figure 9a inset) and connected with Cu wires by conductive silver paint. Here, we preferred the composites with 5 wt % loading content owing to the higher density of conductive pathways. The red and blue wires are two independent cycle circuits with two separate DC voltage supplies (4 V). For the nonaligned composites, the sample (shown in Figure 9a) with the loadings of 5 wt % exhibited isotropic properties and seemed to be isolated in both X and Y directions. Similarly, the composites with a higher content of 50 wt % (shown in Figure 9b) acts as an isotropic conductor, and the conductivities along the two directions are approximately equal, which can be clearly identified by LED brightness. However, as for the aligned composites with 5 wt % loadings (shown in Figure 9c), the LED was well operational only in the red circuit, where the aligned sample was in series connected with the sample along the X direction (i.e., the alignment

4. CONCLUSIONS We introduced Spirulina-templated metallic particles to prepare highly anisotropic electrical conductive composites using electric field-induced assembly. The alignment process shows that the specific AC electric field (2 kV/cm, 1 kHz) can be applied to directionally assemble lightweight microcoils into continuous long microcoil-lines. More importantly, unique endto-end physical contact between adjacent particles can be achieved at low weight fractions and largely contribute to the enhancement of contact conductance. The electrical measurements illustrate that the aligned composites exhibit remarkable conductive anisotropy; the electrical conductivity in the alignment direction reaches up to ∼10 S/m and displays almost 7−8 orders of magnitude higher than that along the perpendicular directions. Moreover, the anisotropic composites exhibit excellent current-carrying capability and conductive anisotropy in the functional LED circuit, which may resolve many deficiencies of the existing ACCs, such as the insufficient conductivity and low conductive anisotropy. The results indicate that this anisotropic conductive composite may have promising potential in highly reliable electrical interconnection and subminiature integrated circuits. Additionally, we expect that longer chain-like microcoil-lines may be obtained by generating a suitably higher electric field and further optimizing the parameters of the electric field.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13505. Video S1 showing the directional assembly of conductive microcoils into longer microcoil-lines (AVI) Video S2 showing the microcoil rotation in the electric field (AVI) Figure S1 showing rotation of a microcoil in the electric field. Figure S2 showing schematic diagram of the torque and external force acting on a microcoil. Figure S3 showing the shear-induced alignment of microcoils in PDMS. Figure S4 showing the larger area alignment of microcoils with different loadings (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.C.). ORCID

Jun Cai: 0000-0001-5167-5524 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51322503 and No. 51275025) and the Fundamental Research Funds for the Central Universities.

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