- Email: [email protected]
Selectively plated stretchable liquid metal wires for transparent electronics
Sensors and Actuators B 221 (2015) 1114–1119
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Selectively plated stretchable liquid metal wires for transparent electronics Guangyong Li, Xuan Wu, Dong-Weon Lee ∗ MEMS and Nanotechnology Laboratory, School of Mechanical Engineering, Chonnam National University, Gwangju 500757, South Korea
a r t i c l e
i n f o
Article history: Received 16 May 2015 Received in revised form 10 July 2015 Accepted 14 July 2015 Available online 20 July 2015 Keywords: Liquid metal Plating Stretchable circuit board Advanced electronics
a b s t r a c t This paper presents an innovative and versatile selective liquid-metal plating (SLMP) process to realize a transparent stretchable circuit board (TSCB) with the capability of large deformation and self-healing. The TSCB consists of polydimethylsiloxane (PDMS) thin films and non-toxic liquid-metal patterns that are embedded into the PDMS substrate. Complex liquid-metal patterns with various aspect ratios are achieved by using the selective wetting behavior of the reduced liquid metal on the Au/Cr patterns of the PDMS substrate. The smallest liquid-metal pattern is approximately 3 m in width. The TSCB integrated with a light-emitting diode exhibit stable performance, even though it is twisted more than 180◦ or is stretched up to ∼60% 6000 times. To evaluate the feasibility of the SLMP process, an electrical filter was fabricated on the PDMS substrate and the current–voltage (I–V) characteristics are successfully demonstrated under various conditions. The experimental results reveal that the proposed idea has the potential to enable advanced electronics such as wearable computers, wall-scale displays, electronic paper, and other stretchable electronics. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A printed circuit board (PCB) is widely employed as the carrier of integrated circuits (ICs) in modern electronics that are fundamentally rigid and planar. When mechanical and electrical functionalities of PCBs are preserved even under high strain values, their application areas could be extended to the fields of conformal antenna arrays [1,2], wearable tactile interfaces [3,4], stretchable solar-cell arrays [5,6], stretchable displays [7–10], and electronic circuits fabricated in clothing [11,12]. The stretchability of the PCB provides circuit designers with a new degree of freedom [13]. One of the most difficult challenges in the development of stretchable circuit boards (SCBs) is the simultaneous achievement of both excellent mechanical robustness and electronic performance. Many concepts and demonstrations have been presented to realize the stretchable or flexible circuit boards [14,15]. Wagner et al. showed numerous examples of wavy-pattern-based stretchable devices by prestretching elastomeric substrates before metal deposition [16–19]. Stretchable electronics using serpentine interconnection technology were reported by Rogers et al. [20–25]. However, the prestrain limits the maximum stretchability of the electronics, which restricts the available applications of wavy-pattern-based
∗ Corresponding author. Tel.: +82 62 530 1684. E-mail address: [email protected] (D.-W. Lee). http://dx.doi.org/10.1016/j.snb.2015.07.062 0925-4005/© 2015 Elsevier B.V. All rights reserved.
technology [26]. Some complex processes are required to protect the device in which all of the strain is concentrated on the serpentine interconnection [27,28]. In recent years, stretchable circuits have been realized by a silicone elastomer embedded with microchannels of a liquidphase gallium–indium (Ga–In) alloy. In studies by the Wu group, liquid-metal-filled polydimethlysiloxane (PDMS) using a microfluidic approach was presented, which inspired the application of stretchable radio frequency (RF) electronics [29,30]. Several studies on elastomer circuits using direct printing techniques have been also carried out [31–33]. Owing to the extra demand of a needle and syringe for liquid-metal injection or printing, these manual methods have constraints in geometry and high-volume manufacturing. Hyperelastic circuits have been produced by other techniques, such as laser patterning, stencil masks, and masked deposition [34–37]. Despite these remarkable achievements, the developed liquidmetal printing techniques cannot satisfy the increasing demand of SCB applications owing to the low resolution (pattern planar dimension >100 m) and irregular edge [34–37]. In this study, we demonstrate a simple and practical way for producing stretchable circuits by using the selective wetting behavior of reduced liquid-metal on metal patterns, which eliminates the need for a syringe and a stencil mask. The wetting behavior of hydrochloric acid (HCl)-treated Galinstan (a gallium–indium–tin alloy) on different substrates is discussed (shown in the Supplementary data, Fig. S1) in details. The reduced liquid-metal droplet
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
1115
has different behaviors on metal and non-metal substrates [38]. The current–voltage (I–V) characteristics of a stretchable circuit integrated with a light-emitting diode (LED) are obtained under various conditions such as bending, twisting, and stretching. The capability, effectiveness, and usability of the selective liquid-metal plating (SLMP) process are evaluated with an SCB implanted with conventional electrical elements (resistor, inductor, capacitor, and LED).
Same processes that were employed in the previous experiment were repeated to form liquid-metal patterns on the PDMS surface. After rinsing the sample with acetone and IPA, the PDMS surface with liquid-metal patterns was covered with a PDMS solution and finally cured at 80 ◦ C for 90 min. Finally, the PDMS film with liquidmetal patterns was peeled off from the Si substrate.
2. Experimental
Fig. 1 shows the fabrication process for transparent stretchable circuits using the selective wetting behavior of a reduced liquid metal. The Au/Cr thin film selected as an adhesion layer for reduced Galinstan is deposited onto a PDMS substrate (Fig. 1(b)), and the thin film is patterned using conventional microfabrication techniques (Fig. 1(c)). HCl-treated Galinstan droplets are then rolled over the PDMS substrate (Fig. 1(d and e)) to form desired patterns. Selectively plated liquid metals are covered with a PDMS solution for the purpose of protection from contamination (Fig. 1(f)). Fig. S2 (Supplementary data) shows optical micrographs of various liquid metal patterns realized by the SLMP process, which are used to demonstrate the accuracy of the manufacturing process. The complex patterns, shown in Fig. 2(a–e), demonstrate the versatility of this approach to rapidly pattern liquid metal with milli/microscale resolution over a large area. The liquid-metal-embedded PDMS can be arbitrarily bent and stretched, which are demonstrated in Fig. 2(f–j). Under conditions that guarantee the uniformity of the liquid-metal pattern edge, the minimum width of a liquid-metal pattern may be as low as 10 m, as shown in Fig. S3 (Supplementary data). The line features are spaced with edge-to-edge separations of 10 m. Fig. 3 shows a 10- and 50-m-wide stretchable liquid-metal wire produced by the SLMP process, which is for characterization of the mechanical and electrical properties. Considering the fabrication and dimension of stretchable liquid-metal wires formed on a PDMS substrate, the practical stretchability was about 120%. The stretchability can be improved by further optimization of the fabrication process and the use of different polymer substrates [39]. As can be observed from Fig. 3(a) and (b), the increase in the resistance of the stretchable liquid-metal wires with 10 and 50 m in width is ∼130 and 4 , respectively, when the SCB is stretched up to 100%. After releasing the strain, the resistance of the wire can recover to its initial value. As can be seen from Fig. 3(a3) and (b3), some cracks are generated on the liquid-metal surface, which are attributed to the oxidation layer of the liquid metal [40]. These cracks have no influence on the electrical characteristics of the stretchable circuit. The liquid metal under the cracked oxidation surface can still fully satisfy the electrical connection of the circuit because the oxide layer is relatively thin compared to the diameter of the liquid-metal wires. Compared with the stretchable liquid-metal wire, cracks on the gold wire (produced using the steps in Fig. 3(c2)) cause a connection failure, even after fully releasing the applied strain, which is shown in Fig. 3(c3). It should be noted that the high aspect ratio of the cross section can benefit the extensibility of the liquid-metal wire during stretching, thereby avoiding disconnection of the wire. Fig. 3(d1–d4) shows a flexibility of the proposed circuit board integrated with a LED. Stable electrical performance is successfully maintained, even for an applied bend of 180◦ , a twist of 180◦ , or a strain of 60%. Fig. 3(e) presents the I–V characteristics of an integrated circuit consisting of a liquid-metal wire and an LED, which displays stable performance under various types of mechanical deformation (bending, twisting and stretching). This is due to the small change of a total resistance value in LED-integrated stretchable circuits. Fig. 3(f) shows the change of the normalized resistance (Ra /R0 ) as a function of strain for a pre-twisted (180◦ ) stretchable liquid-metal wire, where Ra and R0 represent the resistance under a certain applied strain and the original resistance, respectively. The
2.1. Preparation of the liquid metal Galinstan is a commercially available (from Geratherm Medical AG, Germany) eutectic GaInSn metal alloy (68.5%, 21.5%, and 10% by weight, respectively), which has outstanding properties such as a low melting point (−19 ◦ C), a high electrical conductivity (3.46 × 106 S m−1 ), a high boiling point (1300 ◦ C), a favorable thermal conductivity (16.5 W m−1 K−1 ), and an ultralow vapor pressure. It was well-known as a replacement of previously employed the toxic liquid mercury or the reactive NaK (sodium–potassium alloy) due to the low toxicity and low reactivity of its component metals. A typical eutectic composition of the Galinstan is 68 wt% Ga, 22 wt% In and 10 wt% Sn, though it varies between 62 and 95 wt% Ga, 5 and 22 wt% In, 0 and 16 wt% Sn while keeping eutectic ability. Based on its favorable properties, gallium-based liquid metals have been investigated for a variety of applications including microfluidic wireless strain, stretchable microfluidic antenna, hyperelastic pressure sensor, etc. However, gallium is readily oxidized in air, forming a gallium oxide layer at the outer skin of the gallium-based liquid metal alloys. This causes the gallium-based liquid metals to be viscoelastic and to adhere to almost any surface, which is an extremely challenging problem to solve. Here we employ HCl-treated Galinstan for the fabrication of transparent stretchable circuit boards because Galinstan can temporarily maintain a true liquid phase at room temperature when it is treated with HCl solution. 2.2. Fabrication of liquid-metal patterns on a silicon wafer First, 10-nm-thick Cr thin layer was deposited onto a Si wafer with a 500-nm-thick silicon dioxide layer using an electron beam (e-beam) evaporator. This layer enhances the adhesion ability between Au and the silicon dioxide layer. Au layer with a thickness of 100 nm is continuously deposited on the Cr layer in a same process chamber. Various Au micro-patterns were obtained using a conventional photolithography and Au/Cr wet-etching processes. The metal patterns were then covered with a Galinstan droplet treated with a 16-wt% HCl solution. Next, the reduced Galinstan droplet was rolled over the patterns for approximately 5 s, which ensures that the reduced Galinstan fully wets the pattered Au layer. Excessive liquid metal could be easily removed using a conventional syringe with a needle if there is some residue after rolling. This way is effective due to the hydrophobic characteristic of reduced Galinstan on a PDMS substrate. After rinsing with an acetone solution, the liquid-metal patterns were treated with isopropyl alcohol (IPA) and finally placed on a hot plate at 60 ◦ C for 10 min. 2.3. Fabrication of a liquid-metal pattern embedded in PDMS A PDMS solution known as optically clear, inert, non-toxic, and non-flammable material was spin-coated on a Si wafer (800 rpm for 30 s). The sample was then placed on a hot plate at 80 ◦ C for 90 min. After curing process, the PDMS layer was exposed to oxygen plasma at 60 W for 90 s, which enhance the adhesion ability between the metal and PDMS. A thin Au/Cr layer was deposited on the sample.
2.4. Stretchable circuit board with optical transparency
1116
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
Fig. 1. Fabrication process of the liquid-metal pattern embedded in PDMS. (a) Spin-coating the PDMS solution onto a Si wafer. (b) PDMS curing, oxygen plasma treatment or parylene deposition, and Au/Cr (100 nm/10 nm) deposition using an electron beam (e-beam) evaporator. (c) Photolithography and Au/Cr wet-etching. (d) Dropping HCltreated liquid metal onto the adhesion patterns. (e) Rolling of the liquid-metal droplet over the PDMS substrate and removing the excess liquid-metal droplet. (f) Covering the liquid-metal pattern with a PDMS solution. (g) PDMS curing and separation of the Si wafer. (h) Bending test.
Fig. 2. Complex patterns of liquid metal embedded in PDMS. (a–e) Square, lines and letters, Chinese characters, picture and signature, and line and square patterns embedded into PDMS. (f and g) Bending. (h–j) Off-axis stretching of an SCB.
total resistance value of the LED-integrated stretchable circuits is maintained a nearly constant value under the tensile strain in a range of 0–100%. This indicates that the stress-induced resistance change in liquid metal wires has negligible influences to the performance of LED-integrated stretchable circuits. Fig. 3(g) shows the I–V characteristics of the 180◦ twisted stretchable liquid-metal wire when stretching of the SCB is repeated 6000 times with a strain of 60%. Fig. 4 shows the capability of self-healing for the stretchable liquid metal wires. The stretchable wire is severed with scissors and self-healed electrically at ambient conditions. No additional force is required for the reconnection of the electrically broken wire. To verify the utility of the SLMP process further, the embedded liquid metal has been employed to connect passive elements for the fabrication of functional devices such as RLC filters. As shown in Fig. 5(a–f), the fabricated SCB is optically transparent and exhibits excellent electrical and mechanical behaviors under the conditions of bending, twisting, or off-axis stretching. Fig. 5(g) demonstrates the characteristics of the SCB-based low-pass filter with a 50-m-wide liquid-metal wire. An input square waveform (amplitude of 5 V) is applied using a function generator to the series combination, and the output signal (Vout ) is measured using an oscilloscope across the capacitor. The voltage responses
of the filter at different frequencies of the square-wave-shaped signal, 10 kHz, 0.1 and 1 MHz, are measured under conditions of non-stretching and stretching. At low frequencies (10 kHz), the output waveform is almost similar to the input waveform. The slight differences between the input and output waveforms are due to charging of the capacitor [41]. At higher frequencies (1 MHz), the output voltage response is converted into a triangular-shaped waveform with respect to the input square-wave signal. This shaping of the waveforms in low-pass RC filters can be attributed to the frequency-dependent reactance of the capacitor [41]. The transparent SCB-based low-pass filter maintains stable electrical performance at an applied strain of 30%. The stretchability of the SCB-based low-pass filter is limited due to the fabrication and structure (commercial passive elements embedded in the PDMS) issues. It is also influenced by the position of passive elements on the stretchable PCB board. Further optimization in fabrication and structure design could improve the stretchability. 3. Discussion In our previous studies, the wetting behaviors of oxidized or HCl-treated Galinstan droplets on PDMS substrates are described [38,40]. Reduced Galinstan exhibits superlyophobicity on a PDMS
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
1117
Fig. 3. Characterization of the mechanical and electrical properties of a PDMS substrate with conventional metal wires and liquid-metal-based wires (width: 10 and 50 m, length: 20 mm). (a and b) Liquid-metal wires fabricated by the SLMP process (tensile stress; Fig. b1–3 and c1–3). (c) Conventional metal patterns formed on the PDMS substrate (tensile stress; Fig. c1–3). (d) LED-integrated stretchable circuit board (SCB) with an external power supply of 3 V (3D deformation; Fig. d1–4). (e) Current–voltage (I–V) characteristics of an LED-integrated SCB under various types of mechanical deformation (bending, twisting, and stretching). (f) Normalized R of an LED-integrated SCB as a function of the strain after twisting (180◦ ). (g) I–V curves of an LED-integrated SCB with repeated stretching and an applied strain of 60% after twisting (180◦ ).
Fig. 4. Self-healing characteristics of selectively plated stretchable liquid metal wires. (a) SCB implanted with a LED. (b and c) Physical separation of SCB with a blade and (d) reconnection of stretchable liquid metal wires.
substrate, as shown in Fig. S1(a2) of the Supplementary data. Conversely, oxidized Galinstan behaves more like a gel than a true liquid, which adheres to the PDMS surface with an ∼125◦ contact angle. It has been previously reported that a Galinstan droplet, reactively wetting Sn foil for 24 h, has a contact angle that is less than 20◦ [37]. The reason is that the adsorption of the liquid metal onto the solid metal induces dissolution described by the dissolution–diffusion model of Robertson [42]. This phenomena can be applied even to different metal materials such as Au, Cu, and Zn. However, a Galinstan droplet has different behaviors on those materials and provides much bigger contact angles compared to a Sn surface, as shown in Fig. S1(b1), (c1) and (d1) of the Supplementary data. The lyophilic characteristics of the liquid metal on the metal patterns are attributed to the fact that any exposed region of Galinstan to an air environment or even as little as 0.2% volume of oxygen will form a thin solid film at the Galinstan surface [43]. The thin oxide layer disturbs that the ability of Galinstan to fully
wet on metal patterns. This is one of the reasons that a long time is required for Galinstan to fully wet Sn. However, oxidized Galinstan becomes a true liquid when it is treated with a diluted HCl solution. As observed in Fig. S1(b2), (c2) and (d2) of the Supplementary data, Galinstan treated with a 37 wt% HCl solution fully wets the patterned metal layer within 1 s. The experiment here is focused on the selective wetting behavior of reduced Galinstan because a reduced Galinstan droplet that is in contact with the metal pattern and PDMS substrate exhibits superlyophilic and superlyophobic characteristics, respectively. The concentration of HCl is an important factor during the SLMP process because the 37 wt% HCl solution strongly reacts with the thin Cr layer, and a higher concentration causes damage to the adhesion layer at the edge (shown in the Supplementary data, Fig. S4(a)). Various concentrations of HCl solutions (37, 25, and 16 wt%) are evaluated to effectively remove the oxide skin of Galinstan during the SLMP process. It can be seen that a uniform
1118
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
Fig. 5. Transparent and stretchable low-pass filter (liquid-metal wire width: 50 m). (a) Optical image of fabricated low-pass filter. (b) Circuit diagram of low-pass filter. (c–f) Optically transparent low-pass filter under various conditions. (g) Input and output voltage waveforms of low-pass filter at frequencies of 10 kHz, 0.1 MHz, and 1 MHz with a voltage amplitude of 5 V.
pattern edge is still obtained for a lower HCl solution concentration (16 wt%) (shown in the Supplementary data, Fig. S4(c)). It should be noted that an HCl solution concentration less than 16 wt% cannot completely remove the oxidation layer of Galinstan at room temperature [38]. To minimize corrosion of the adhesion layer (Cr) and ensure the uniformity of the pattern edge, a lower HCl solution concentration (16 wt%) is employed in our experiments. The clear shape of the liquid-metal patterns is experimentally confirmed by the use of line and space patterns having a pitch distance of 20 m, as shown Fig. S5(b1) of the Supplementary data. On the other hand, the rolling time of the liquid metal over the adhesion layer also influences the shape of the pattern edge. The edges of line patterns with a width more than 10 m become rough as the rolling time increases, shown in Fig. S5(b2 and b3) of the Supplementary data. In order to avoid any damage to the adhesion layer, 5 s is desirable for the SLMP process. Damage to the patterns is caused by the chemical reaction between diluted HCl and the thin Cr layer. However, the formation of liquid-metal lines within 30 s is not possible when the adhesion patterns are less than 10 m in width (shown in the Supplementary data, Fig. S5(c1)). As observed in Fig. S5(c2) and (c3) of the Supplementary data, reduced Galinstan requires more than 30 s to wet the adhesion layer, which has a width of 5 m. The increased rolling time still affects the edges of the adhesion layers, and this causes the edges of the liquid metal to become worse. Various adhesion materials or surface modification techniques could be used to overcome this issue [40]. Even though a smaller feature width (5 m) for the conductive pattern could not be achieved with
a high accuracy using the proposed SLMP, the resolution of 10 m is enough to satisfy the requirements of SCB applications. 4. Conclusion In closing, we introduce a novel approach for producing stretchable circuits by liquid-metal plating, which is based on the selective wetting behavior of reduced Galinstan on metal patterns. The SLMP process can produce various conductive complex micropatterns containing features that have planar dimensions less than 10 m with higher resolution and uniform edges. Compared with the previous liquid metal printing methods, the accuracy of manufacture is significantly improved [34–37]. The I–V characteristics demonstrate that the LED-integrated stretchable circuit has stable electrical and mechanical properties, even after twisting (180◦ ) and stretching (60%) for 6000 repetitions. To further demonstrate the feasibility of the SLMP process, an SCB consisting of passive elements and a diode is fabricated and evaluated under various conditions such as bending, twisting, or off-axis stretching. Future investigations will focus on improving the resolution of the conductive pattern, enhancing the stretchability of the wire, and increasing the compatibility between the liquid-metal wire and the rigid electronic element for satisfying the demand of SCBs. For example, a Cu foil adheres to PDMS very well, which could replace the Au adhesion layer [15], and the Young’s modulus of PDMS can be decreased by changing the mass ratio of elastomer base to curing agent. Furthermore, the compatibility between the liquid-metal wire and the
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
electronic element can be enhanced using conductive elastomer electrodes (NuSil R-2637) [34]. We expect that further improvement of this approach will yield the ability to combine the functions of multiple electronic elements on an SCB for the creation of novel electronic devices such as robotics, wearable electronic clothing, skin sensors, medical devices, and other advanced electronics. Acknowledgments This study was supported by the Korean Health Technology R&D Project (HI13C1527), International Collaborative R&D Program (N0000894) and the National Research Foundation (No.2015R1A2A2A05001405) funded by the Korea government. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.07.062 References [1] N. Tiercelin, P. Coquet, R. Sauleau, V. Senez, H. Fujita, Polydimethylsiloxane membranes for millimeter-wave planar ultra flexible antennas, J. Micromech. Microeng. 16 (2006) 2389–2395. [2] J.M. Cannata, J.A. Williams, Q. Zhou, T.A. Ritter, K.K. Shung, Development of a 35-MHz piezo-composite ultrasound array for medical imaging, IEEE Trans. Ultrason. Ferroelect. Freq. Control 53 (2006) 224–236. [3] R.K. Kramer, C. Majidi, R.J. Wood, Wearable tactile keypad with stretchable artificial skin, in: 2011 IEEE International Conference on Robotics and Automation (ICRA), 2011, pp. 1103–1107. [4] R.D.P. Wong, J.D. Posner, V.J. Santos, Flexible microfluidic normal force sensor skin for tactile feedback, Sens. Actuators, A 179 (2012) 62–69. [5] G. Dennler, N.S. Sariciftci, Flexible conjugated polymer-based plastic solar cells: from basics to applications, Proc. IEEE 93 (2005) 1429–1439. [6] K. Otte, L. Makhova, A. Braun, I. Konovalov, Flexible Cu(In,Ga)Se2 thin-film solar cells for space application, Thin Solid Films 511–512 (2006) 613–622. [7] K.J. Allen, Reel to real: prospects for flexible displays, Proc. IEEE 93 (2005) 1394–1399. [8] A. Sobel, Electronic paper: high-speed inks, Nat. Mater. 2 (2003) 643–644. [9] Y. Chen, J. Au, P. Kazlas, A. Ritenour, H. Gates, M. McCreary, Electronic paper: flexible active-matrix electronic ink display, Nature 423 (2006) 136. [10] J.A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V.R. Raju, et al., Paper-like electronic displays: large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 4835–4840. [11] S. Park, S. Jayaraman, Smart textiles: wearable electronic systems, MRS Bull. 28 (2003) 585–591. [12] D. De Rossi, C.E. Piaggio, Electronic textiles: a logical step, Nat. Mater. 6 (2007) 328–329. [13] A.C. Siegel, S.T. Phillips, M.D. Dickey, N. Lu, Z. Suo, G.M. Whitesides, Foldable printed circuit boards on paper substrates, Adv. Funct. Mater. 20 (2010) 28–35. [14] R. Vieroth, T. Löher, M. Seckel, C. Dils, C. Kallmayer, A. Ostmann, et al., Stretchable circuit board technology and application, ISWC ‘09 International Symposium on Wearable Computers (2009) 33–36. [15] J. Vanfleteren, M. Gonzalez, F. Bossuyt, Y. Hsu, T. Vervust, I. De Wolf, et al., Printed circuit board technology inspired stretchable circuits, MRS Bull. 37 (2012) 254–260. [16] J. Jones, S.P. Lacour, S. Wagner, Z. Suo, Stretchable wavy metal interconnects, J. Vac. Sci. Technol. 22 (2004) 1723–1725. [17] T. Li, Z. Suo, S.P. Lacour, S. Wagner, Compliant thin film patterns of stiff materials as platforms for stretchable electronics, J. Mater. Res. 20 (2005) 3274–3277. [18] S.P. Lacour, D. Chan, S. Wagner, T. Li, Z. Suo, Mechanisms of reversible stretchability of thin metal films on elastomeric substrates, Appl. Phys. Lett. 88 (2006) 204103. [19] C. Keplinger, J.-Y. Sun, C.C. Foo, P. Rothemund, G.M. Whitesides, Z. Suo, Stretchable, transparent, ionic conductors, Science 341 (2013) 984–987. [20] T. Kim, W.M. Choi, D. Kim, M.A. Meitl, E. Menard, H. Jiang, et al., Adv. Mater. 20 (2008) 2171–2176. [21] R. Kim, M. Bae, D.G. Kim, H. Cheng, B.H. Kim, D. Kim, et al., Stretchable, transparent graphene interconnects for arrays of microscale inorganic light emitting diodes on rubber substrates, Nano Lett. 11 (2011) 3881–3886. [22] S. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, et al., Stretchable graphene transistors with printed dielectrics and gate electrodes, Nano Lett. 11 (2011) 4642–4646. [23] J. Lee, J. Wu, M. Shi, J. Yoon, S. Park, M. Li, et al., Stretchable GaAs photovoltaics with designs that enable high areal coverage, Adv. Mater. 23 (2011) 986–991.
1119
[24] D. Kim, J. Viventi, J.J. Amsden, J. Xiao, L. Vigeland, Y. Kim, et al., Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics, Nat. Mater. 9 (2010) 511–517. [25] J. Jeong, W. Yeo, A. Akhtar, J.J.S. Norton, Y. Kwack, S. Li, et al., Materials and optimized designs for human-machine interfaces via epidermal electronics, Adv. Mater. 25 (2013) 6839–6846. [26] X. Feng, B.D. Yang, Y. Liu, Y. Wang, C. Dagdeviren, Z. Liu, et al., Stretchable ferroelectric nanoribbons with wavy configurations on elastomeric substrates, ACS Nano 5 (2011) 3326–3332. [27] G. Shin, C.H. Yoon, M.Y. Bae, Y.C. Kim, S.K. Hong, J.A. Rogers, et al., Stretchable field-effect-transistor array of suspended SnO2 nanowires, Small 7 (2011) 1181–1185. [28] S. Xu, Y. Zhang, J. Cho, J. Lee, X. Huang, L. Jia, et al., Stretchable batteries with self-similar serpentine interconnects and integrated wireless recharging systems, Nat. Commun. 4 (2013) 1543. [29] S. Cheng, Z.G. Wu, Microfluidic electronics, Lab Chip 12 (2012) 2782–2791. [30] S. Cheng, Z.G. Wu, Microfluidic stretchable RF electronics, Lab Chip 10 (2010) 3227–3234. [31] C. Ladd, J. So, J. Muth, M.D. Dickey, 3D printing of free standing liquid metal microstructures, Adv. Mater. 25 (2013) 5081–5085. [32] J.W. Boley, E.L. White, G.T. Chiu, R. Kramer, Direct writing of gallium-indium alloy for stretchable electronics, Adv. Funct. Mater. 24 (2014) 3501–3507. [33] Y. Zheng, Z.Z. He, J. Yang, J. Liu, Personal electronics printing via tapping mode composite liquid metal ink delivery and adhesion mechanism, Sci. Rep. 4 (2014) 4588. [34] T. Lu, L. Finkenauer, J. Wissman, C. Majidi, Rapid prototyping for soft-matter electronics, Adv. Funct. Mater. 24 (2014) 3351–3356. [35] S.H. Jeong, A. Hagman, K. Hjort, M. Jobs, J. Sundqvist, Z.G. Wu, Liquid alloy printing of microfluidic stretchable electronics, Lab Chip 12 (2012) 4657–4664. [36] S.H. Jeong, K. Hjort, Z.G. Wu, Tape transfer printing of a liquid metal alloy for stretchable RF electronics, Sensors 14 (2014) 16311–16321. [37] R.K. Kramer, C. Majidi, R.J. Wood, Masked deposition of gallium-indium alloys for liquid-embedded elastomer conductors, Adv. Funct. Mater. 23 (2013) 5292–5296. [38] G. Li, M. Parmar, D. Kim, J.B. Lee, D.W. Lee, PDMS based coplanar microfluidic channels for the surface reduction of oxidized Galinstan, Lab Chip 14 (2014) 200–209. [39] A. Fassler, C. Majidi, Soft-matter capacitors and inductors for hyperelastic strain sensing and stretchable electronics, Smart Mater. Struct. 22 (2013) 055023. [40] G. Li, M. Parmar, D.W. Lee, An oxidized liquid metal-based microfluidic platform for tunable electronic device applications, Lab Chip 15 (2015) 766–775. [41] N. Kurra, G.U. Kulkarni, Pencil-on-paper: electronic devices, Lab Chip 13 (2013) 2866–2873. [42] W.M. Robertson, Propagation of a crack filled with liquid metal, Trans. Met. Soc. AIME 236 (1966) 1478–1482. [43] T. Liu, P. Sen, C. Kim, Characterization of nontoxic liquid-metal alloy galinstan for applications in microdevices, J. Microelectromech. Syst. 21 (2012) 443–450.
Biographies Guangyong Li received the B.E. and M.E. degrees in mechanical engineering from Chongqing University of Technology, Chongqing, China, in 2009 and 2012, respectively. He is currently working toward the Ph.D. degree in mechanical engineering in the MEMS and Nanotechnology Laboratory, Department of Mechanical Engineering, Chonnam National University, Korea. His current research interests include liquid metal based microfluidic platform, energy-harvesting technology, MEMS device design and fabrication and tunable FSS. Xuan Wu received the B.E. and M.E. degrees in mechanical engineering from Chongqing University of Technology, Chongqing, China, in 2009 and 2012, respectively. He is currently working toward the Ph.D. degree in mechanical engineering in the MEMS and Nanotechnology Laboratory, Department of Mechanical Engineering, Chonnam National University, Korea. He is the holder of two patents in China. He is also a member of the Korean Sensors Society. His current research interests include energy-harvesting technology, MEMS device design and fabrication, piezo-materials for various applications, and graphene local anodic lithography technology. Dong-Weon Lee received the Ph.D. degrees in mechatronics engineering from Tohoku University, Sendai, Japan, in 2001. He has been a professor of Mechanical Systems Engineering at Chonnam National University (CNU), Republic of Korea since March of 2004. Previously, he was with the IBM Zurich Research Laboratory in Switzerland, working mainly on micro cantilever devices for chemical AFM applications. At CNU, his research interests include liquid metal based microfludic platform, smart cantilever devices, miniaturized energy harvester, smart materials and structures, and nanoscale transducers. He is a member of the technical program committee of IEEE Sensors Conference, Transducers, and Micro processes and Nanotechnology Conference (MNC).
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Selectively plated stretchable liquid metal wires for transparent electronics Guangyong Li, Xuan Wu, Dong-Weon Lee ∗ MEMS and Nanotechnology Laboratory, School of Mechanical Engineering, Chonnam National University, Gwangju 500757, South Korea
a r t i c l e
i n f o
Article history: Received 16 May 2015 Received in revised form 10 July 2015 Accepted 14 July 2015 Available online 20 July 2015 Keywords: Liquid metal Plating Stretchable circuit board Advanced electronics
a b s t r a c t This paper presents an innovative and versatile selective liquid-metal plating (SLMP) process to realize a transparent stretchable circuit board (TSCB) with the capability of large deformation and self-healing. The TSCB consists of polydimethylsiloxane (PDMS) thin films and non-toxic liquid-metal patterns that are embedded into the PDMS substrate. Complex liquid-metal patterns with various aspect ratios are achieved by using the selective wetting behavior of the reduced liquid metal on the Au/Cr patterns of the PDMS substrate. The smallest liquid-metal pattern is approximately 3 m in width. The TSCB integrated with a light-emitting diode exhibit stable performance, even though it is twisted more than 180◦ or is stretched up to ∼60% 6000 times. To evaluate the feasibility of the SLMP process, an electrical filter was fabricated on the PDMS substrate and the current–voltage (I–V) characteristics are successfully demonstrated under various conditions. The experimental results reveal that the proposed idea has the potential to enable advanced electronics such as wearable computers, wall-scale displays, electronic paper, and other stretchable electronics. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A printed circuit board (PCB) is widely employed as the carrier of integrated circuits (ICs) in modern electronics that are fundamentally rigid and planar. When mechanical and electrical functionalities of PCBs are preserved even under high strain values, their application areas could be extended to the fields of conformal antenna arrays [1,2], wearable tactile interfaces [3,4], stretchable solar-cell arrays [5,6], stretchable displays [7–10], and electronic circuits fabricated in clothing [11,12]. The stretchability of the PCB provides circuit designers with a new degree of freedom [13]. One of the most difficult challenges in the development of stretchable circuit boards (SCBs) is the simultaneous achievement of both excellent mechanical robustness and electronic performance. Many concepts and demonstrations have been presented to realize the stretchable or flexible circuit boards [14,15]. Wagner et al. showed numerous examples of wavy-pattern-based stretchable devices by prestretching elastomeric substrates before metal deposition [16–19]. Stretchable electronics using serpentine interconnection technology were reported by Rogers et al. [20–25]. However, the prestrain limits the maximum stretchability of the electronics, which restricts the available applications of wavy-pattern-based
∗ Corresponding author. Tel.: +82 62 530 1684. E-mail address: [email protected] (D.-W. Lee). http://dx.doi.org/10.1016/j.snb.2015.07.062 0925-4005/© 2015 Elsevier B.V. All rights reserved.
technology [26]. Some complex processes are required to protect the device in which all of the strain is concentrated on the serpentine interconnection [27,28]. In recent years, stretchable circuits have been realized by a silicone elastomer embedded with microchannels of a liquidphase gallium–indium (Ga–In) alloy. In studies by the Wu group, liquid-metal-filled polydimethlysiloxane (PDMS) using a microfluidic approach was presented, which inspired the application of stretchable radio frequency (RF) electronics [29,30]. Several studies on elastomer circuits using direct printing techniques have been also carried out [31–33]. Owing to the extra demand of a needle and syringe for liquid-metal injection or printing, these manual methods have constraints in geometry and high-volume manufacturing. Hyperelastic circuits have been produced by other techniques, such as laser patterning, stencil masks, and masked deposition [34–37]. Despite these remarkable achievements, the developed liquidmetal printing techniques cannot satisfy the increasing demand of SCB applications owing to the low resolution (pattern planar dimension >100 m) and irregular edge [34–37]. In this study, we demonstrate a simple and practical way for producing stretchable circuits by using the selective wetting behavior of reduced liquid-metal on metal patterns, which eliminates the need for a syringe and a stencil mask. The wetting behavior of hydrochloric acid (HCl)-treated Galinstan (a gallium–indium–tin alloy) on different substrates is discussed (shown in the Supplementary data, Fig. S1) in details. The reduced liquid-metal droplet
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
1115
has different behaviors on metal and non-metal substrates [38]. The current–voltage (I–V) characteristics of a stretchable circuit integrated with a light-emitting diode (LED) are obtained under various conditions such as bending, twisting, and stretching. The capability, effectiveness, and usability of the selective liquid-metal plating (SLMP) process are evaluated with an SCB implanted with conventional electrical elements (resistor, inductor, capacitor, and LED).
Same processes that were employed in the previous experiment were repeated to form liquid-metal patterns on the PDMS surface. After rinsing the sample with acetone and IPA, the PDMS surface with liquid-metal patterns was covered with a PDMS solution and finally cured at 80 ◦ C for 90 min. Finally, the PDMS film with liquidmetal patterns was peeled off from the Si substrate.
2. Experimental
Fig. 1 shows the fabrication process for transparent stretchable circuits using the selective wetting behavior of a reduced liquid metal. The Au/Cr thin film selected as an adhesion layer for reduced Galinstan is deposited onto a PDMS substrate (Fig. 1(b)), and the thin film is patterned using conventional microfabrication techniques (Fig. 1(c)). HCl-treated Galinstan droplets are then rolled over the PDMS substrate (Fig. 1(d and e)) to form desired patterns. Selectively plated liquid metals are covered with a PDMS solution for the purpose of protection from contamination (Fig. 1(f)). Fig. S2 (Supplementary data) shows optical micrographs of various liquid metal patterns realized by the SLMP process, which are used to demonstrate the accuracy of the manufacturing process. The complex patterns, shown in Fig. 2(a–e), demonstrate the versatility of this approach to rapidly pattern liquid metal with milli/microscale resolution over a large area. The liquid-metal-embedded PDMS can be arbitrarily bent and stretched, which are demonstrated in Fig. 2(f–j). Under conditions that guarantee the uniformity of the liquid-metal pattern edge, the minimum width of a liquid-metal pattern may be as low as 10 m, as shown in Fig. S3 (Supplementary data). The line features are spaced with edge-to-edge separations of 10 m. Fig. 3 shows a 10- and 50-m-wide stretchable liquid-metal wire produced by the SLMP process, which is for characterization of the mechanical and electrical properties. Considering the fabrication and dimension of stretchable liquid-metal wires formed on a PDMS substrate, the practical stretchability was about 120%. The stretchability can be improved by further optimization of the fabrication process and the use of different polymer substrates [39]. As can be observed from Fig. 3(a) and (b), the increase in the resistance of the stretchable liquid-metal wires with 10 and 50 m in width is ∼130 and 4 , respectively, when the SCB is stretched up to 100%. After releasing the strain, the resistance of the wire can recover to its initial value. As can be seen from Fig. 3(a3) and (b3), some cracks are generated on the liquid-metal surface, which are attributed to the oxidation layer of the liquid metal [40]. These cracks have no influence on the electrical characteristics of the stretchable circuit. The liquid metal under the cracked oxidation surface can still fully satisfy the electrical connection of the circuit because the oxide layer is relatively thin compared to the diameter of the liquid-metal wires. Compared with the stretchable liquid-metal wire, cracks on the gold wire (produced using the steps in Fig. 3(c2)) cause a connection failure, even after fully releasing the applied strain, which is shown in Fig. 3(c3). It should be noted that the high aspect ratio of the cross section can benefit the extensibility of the liquid-metal wire during stretching, thereby avoiding disconnection of the wire. Fig. 3(d1–d4) shows a flexibility of the proposed circuit board integrated with a LED. Stable electrical performance is successfully maintained, even for an applied bend of 180◦ , a twist of 180◦ , or a strain of 60%. Fig. 3(e) presents the I–V characteristics of an integrated circuit consisting of a liquid-metal wire and an LED, which displays stable performance under various types of mechanical deformation (bending, twisting and stretching). This is due to the small change of a total resistance value in LED-integrated stretchable circuits. Fig. 3(f) shows the change of the normalized resistance (Ra /R0 ) as a function of strain for a pre-twisted (180◦ ) stretchable liquid-metal wire, where Ra and R0 represent the resistance under a certain applied strain and the original resistance, respectively. The
2.1. Preparation of the liquid metal Galinstan is a commercially available (from Geratherm Medical AG, Germany) eutectic GaInSn metal alloy (68.5%, 21.5%, and 10% by weight, respectively), which has outstanding properties such as a low melting point (−19 ◦ C), a high electrical conductivity (3.46 × 106 S m−1 ), a high boiling point (1300 ◦ C), a favorable thermal conductivity (16.5 W m−1 K−1 ), and an ultralow vapor pressure. It was well-known as a replacement of previously employed the toxic liquid mercury or the reactive NaK (sodium–potassium alloy) due to the low toxicity and low reactivity of its component metals. A typical eutectic composition of the Galinstan is 68 wt% Ga, 22 wt% In and 10 wt% Sn, though it varies between 62 and 95 wt% Ga, 5 and 22 wt% In, 0 and 16 wt% Sn while keeping eutectic ability. Based on its favorable properties, gallium-based liquid metals have been investigated for a variety of applications including microfluidic wireless strain, stretchable microfluidic antenna, hyperelastic pressure sensor, etc. However, gallium is readily oxidized in air, forming a gallium oxide layer at the outer skin of the gallium-based liquid metal alloys. This causes the gallium-based liquid metals to be viscoelastic and to adhere to almost any surface, which is an extremely challenging problem to solve. Here we employ HCl-treated Galinstan for the fabrication of transparent stretchable circuit boards because Galinstan can temporarily maintain a true liquid phase at room temperature when it is treated with HCl solution. 2.2. Fabrication of liquid-metal patterns on a silicon wafer First, 10-nm-thick Cr thin layer was deposited onto a Si wafer with a 500-nm-thick silicon dioxide layer using an electron beam (e-beam) evaporator. This layer enhances the adhesion ability between Au and the silicon dioxide layer. Au layer with a thickness of 100 nm is continuously deposited on the Cr layer in a same process chamber. Various Au micro-patterns were obtained using a conventional photolithography and Au/Cr wet-etching processes. The metal patterns were then covered with a Galinstan droplet treated with a 16-wt% HCl solution. Next, the reduced Galinstan droplet was rolled over the patterns for approximately 5 s, which ensures that the reduced Galinstan fully wets the pattered Au layer. Excessive liquid metal could be easily removed using a conventional syringe with a needle if there is some residue after rolling. This way is effective due to the hydrophobic characteristic of reduced Galinstan on a PDMS substrate. After rinsing with an acetone solution, the liquid-metal patterns were treated with isopropyl alcohol (IPA) and finally placed on a hot plate at 60 ◦ C for 10 min. 2.3. Fabrication of a liquid-metal pattern embedded in PDMS A PDMS solution known as optically clear, inert, non-toxic, and non-flammable material was spin-coated on a Si wafer (800 rpm for 30 s). The sample was then placed on a hot plate at 80 ◦ C for 90 min. After curing process, the PDMS layer was exposed to oxygen plasma at 60 W for 90 s, which enhance the adhesion ability between the metal and PDMS. A thin Au/Cr layer was deposited on the sample.
2.4. Stretchable circuit board with optical transparency
1116
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
Fig. 1. Fabrication process of the liquid-metal pattern embedded in PDMS. (a) Spin-coating the PDMS solution onto a Si wafer. (b) PDMS curing, oxygen plasma treatment or parylene deposition, and Au/Cr (100 nm/10 nm) deposition using an electron beam (e-beam) evaporator. (c) Photolithography and Au/Cr wet-etching. (d) Dropping HCltreated liquid metal onto the adhesion patterns. (e) Rolling of the liquid-metal droplet over the PDMS substrate and removing the excess liquid-metal droplet. (f) Covering the liquid-metal pattern with a PDMS solution. (g) PDMS curing and separation of the Si wafer. (h) Bending test.
Fig. 2. Complex patterns of liquid metal embedded in PDMS. (a–e) Square, lines and letters, Chinese characters, picture and signature, and line and square patterns embedded into PDMS. (f and g) Bending. (h–j) Off-axis stretching of an SCB.
total resistance value of the LED-integrated stretchable circuits is maintained a nearly constant value under the tensile strain in a range of 0–100%. This indicates that the stress-induced resistance change in liquid metal wires has negligible influences to the performance of LED-integrated stretchable circuits. Fig. 3(g) shows the I–V characteristics of the 180◦ twisted stretchable liquid-metal wire when stretching of the SCB is repeated 6000 times with a strain of 60%. Fig. 4 shows the capability of self-healing for the stretchable liquid metal wires. The stretchable wire is severed with scissors and self-healed electrically at ambient conditions. No additional force is required for the reconnection of the electrically broken wire. To verify the utility of the SLMP process further, the embedded liquid metal has been employed to connect passive elements for the fabrication of functional devices such as RLC filters. As shown in Fig. 5(a–f), the fabricated SCB is optically transparent and exhibits excellent electrical and mechanical behaviors under the conditions of bending, twisting, or off-axis stretching. Fig. 5(g) demonstrates the characteristics of the SCB-based low-pass filter with a 50-m-wide liquid-metal wire. An input square waveform (amplitude of 5 V) is applied using a function generator to the series combination, and the output signal (Vout ) is measured using an oscilloscope across the capacitor. The voltage responses
of the filter at different frequencies of the square-wave-shaped signal, 10 kHz, 0.1 and 1 MHz, are measured under conditions of non-stretching and stretching. At low frequencies (10 kHz), the output waveform is almost similar to the input waveform. The slight differences between the input and output waveforms are due to charging of the capacitor [41]. At higher frequencies (1 MHz), the output voltage response is converted into a triangular-shaped waveform with respect to the input square-wave signal. This shaping of the waveforms in low-pass RC filters can be attributed to the frequency-dependent reactance of the capacitor [41]. The transparent SCB-based low-pass filter maintains stable electrical performance at an applied strain of 30%. The stretchability of the SCB-based low-pass filter is limited due to the fabrication and structure (commercial passive elements embedded in the PDMS) issues. It is also influenced by the position of passive elements on the stretchable PCB board. Further optimization in fabrication and structure design could improve the stretchability. 3. Discussion In our previous studies, the wetting behaviors of oxidized or HCl-treated Galinstan droplets on PDMS substrates are described [38,40]. Reduced Galinstan exhibits superlyophobicity on a PDMS
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
1117
Fig. 3. Characterization of the mechanical and electrical properties of a PDMS substrate with conventional metal wires and liquid-metal-based wires (width: 10 and 50 m, length: 20 mm). (a and b) Liquid-metal wires fabricated by the SLMP process (tensile stress; Fig. b1–3 and c1–3). (c) Conventional metal patterns formed on the PDMS substrate (tensile stress; Fig. c1–3). (d) LED-integrated stretchable circuit board (SCB) with an external power supply of 3 V (3D deformation; Fig. d1–4). (e) Current–voltage (I–V) characteristics of an LED-integrated SCB under various types of mechanical deformation (bending, twisting, and stretching). (f) Normalized R of an LED-integrated SCB as a function of the strain after twisting (180◦ ). (g) I–V curves of an LED-integrated SCB with repeated stretching and an applied strain of 60% after twisting (180◦ ).
Fig. 4. Self-healing characteristics of selectively plated stretchable liquid metal wires. (a) SCB implanted with a LED. (b and c) Physical separation of SCB with a blade and (d) reconnection of stretchable liquid metal wires.
substrate, as shown in Fig. S1(a2) of the Supplementary data. Conversely, oxidized Galinstan behaves more like a gel than a true liquid, which adheres to the PDMS surface with an ∼125◦ contact angle. It has been previously reported that a Galinstan droplet, reactively wetting Sn foil for 24 h, has a contact angle that is less than 20◦ [37]. The reason is that the adsorption of the liquid metal onto the solid metal induces dissolution described by the dissolution–diffusion model of Robertson [42]. This phenomena can be applied even to different metal materials such as Au, Cu, and Zn. However, a Galinstan droplet has different behaviors on those materials and provides much bigger contact angles compared to a Sn surface, as shown in Fig. S1(b1), (c1) and (d1) of the Supplementary data. The lyophilic characteristics of the liquid metal on the metal patterns are attributed to the fact that any exposed region of Galinstan to an air environment or even as little as 0.2% volume of oxygen will form a thin solid film at the Galinstan surface [43]. The thin oxide layer disturbs that the ability of Galinstan to fully
wet on metal patterns. This is one of the reasons that a long time is required for Galinstan to fully wet Sn. However, oxidized Galinstan becomes a true liquid when it is treated with a diluted HCl solution. As observed in Fig. S1(b2), (c2) and (d2) of the Supplementary data, Galinstan treated with a 37 wt% HCl solution fully wets the patterned metal layer within 1 s. The experiment here is focused on the selective wetting behavior of reduced Galinstan because a reduced Galinstan droplet that is in contact with the metal pattern and PDMS substrate exhibits superlyophilic and superlyophobic characteristics, respectively. The concentration of HCl is an important factor during the SLMP process because the 37 wt% HCl solution strongly reacts with the thin Cr layer, and a higher concentration causes damage to the adhesion layer at the edge (shown in the Supplementary data, Fig. S4(a)). Various concentrations of HCl solutions (37, 25, and 16 wt%) are evaluated to effectively remove the oxide skin of Galinstan during the SLMP process. It can be seen that a uniform
1118
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
Fig. 5. Transparent and stretchable low-pass filter (liquid-metal wire width: 50 m). (a) Optical image of fabricated low-pass filter. (b) Circuit diagram of low-pass filter. (c–f) Optically transparent low-pass filter under various conditions. (g) Input and output voltage waveforms of low-pass filter at frequencies of 10 kHz, 0.1 MHz, and 1 MHz with a voltage amplitude of 5 V.
pattern edge is still obtained for a lower HCl solution concentration (16 wt%) (shown in the Supplementary data, Fig. S4(c)). It should be noted that an HCl solution concentration less than 16 wt% cannot completely remove the oxidation layer of Galinstan at room temperature [38]. To minimize corrosion of the adhesion layer (Cr) and ensure the uniformity of the pattern edge, a lower HCl solution concentration (16 wt%) is employed in our experiments. The clear shape of the liquid-metal patterns is experimentally confirmed by the use of line and space patterns having a pitch distance of 20 m, as shown Fig. S5(b1) of the Supplementary data. On the other hand, the rolling time of the liquid metal over the adhesion layer also influences the shape of the pattern edge. The edges of line patterns with a width more than 10 m become rough as the rolling time increases, shown in Fig. S5(b2 and b3) of the Supplementary data. In order to avoid any damage to the adhesion layer, 5 s is desirable for the SLMP process. Damage to the patterns is caused by the chemical reaction between diluted HCl and the thin Cr layer. However, the formation of liquid-metal lines within 30 s is not possible when the adhesion patterns are less than 10 m in width (shown in the Supplementary data, Fig. S5(c1)). As observed in Fig. S5(c2) and (c3) of the Supplementary data, reduced Galinstan requires more than 30 s to wet the adhesion layer, which has a width of 5 m. The increased rolling time still affects the edges of the adhesion layers, and this causes the edges of the liquid metal to become worse. Various adhesion materials or surface modification techniques could be used to overcome this issue [40]. Even though a smaller feature width (5 m) for the conductive pattern could not be achieved with
a high accuracy using the proposed SLMP, the resolution of 10 m is enough to satisfy the requirements of SCB applications. 4. Conclusion In closing, we introduce a novel approach for producing stretchable circuits by liquid-metal plating, which is based on the selective wetting behavior of reduced Galinstan on metal patterns. The SLMP process can produce various conductive complex micropatterns containing features that have planar dimensions less than 10 m with higher resolution and uniform edges. Compared with the previous liquid metal printing methods, the accuracy of manufacture is significantly improved [34–37]. The I–V characteristics demonstrate that the LED-integrated stretchable circuit has stable electrical and mechanical properties, even after twisting (180◦ ) and stretching (60%) for 6000 repetitions. To further demonstrate the feasibility of the SLMP process, an SCB consisting of passive elements and a diode is fabricated and evaluated under various conditions such as bending, twisting, or off-axis stretching. Future investigations will focus on improving the resolution of the conductive pattern, enhancing the stretchability of the wire, and increasing the compatibility between the liquid-metal wire and the rigid electronic element for satisfying the demand of SCBs. For example, a Cu foil adheres to PDMS very well, which could replace the Au adhesion layer [15], and the Young’s modulus of PDMS can be decreased by changing the mass ratio of elastomer base to curing agent. Furthermore, the compatibility between the liquid-metal wire and the
G. Li et al. / Sensors and Actuators B 221 (2015) 1114–1119
electronic element can be enhanced using conductive elastomer electrodes (NuSil R-2637) [34]. We expect that further improvement of this approach will yield the ability to combine the functions of multiple electronic elements on an SCB for the creation of novel electronic devices such as robotics, wearable electronic clothing, skin sensors, medical devices, and other advanced electronics. Acknowledgments This study was supported by the Korean Health Technology R&D Project (HI13C1527), International Collaborative R&D Program (N0000894) and the National Research Foundation (No.2015R1A2A2A05001405) funded by the Korea government. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.07.062 References [1] N. Tiercelin, P. Coquet, R. Sauleau, V. Senez, H. Fujita, Polydimethylsiloxane membranes for millimeter-wave planar ultra flexible antennas, J. Micromech. Microeng. 16 (2006) 2389–2395. [2] J.M. Cannata, J.A. Williams, Q. Zhou, T.A. Ritter, K.K. Shung, Development of a 35-MHz piezo-composite ultrasound array for medical imaging, IEEE Trans. Ultrason. Ferroelect. Freq. Control 53 (2006) 224–236. [3] R.K. Kramer, C. Majidi, R.J. Wood, Wearable tactile keypad with stretchable artificial skin, in: 2011 IEEE International Conference on Robotics and Automation (ICRA), 2011, pp. 1103–1107. [4] R.D.P. Wong, J.D. Posner, V.J. Santos, Flexible microfluidic normal force sensor skin for tactile feedback, Sens. Actuators, A 179 (2012) 62–69. [5] G. Dennler, N.S. Sariciftci, Flexible conjugated polymer-based plastic solar cells: from basics to applications, Proc. IEEE 93 (2005) 1429–1439. [6] K. Otte, L. Makhova, A. Braun, I. Konovalov, Flexible Cu(In,Ga)Se2 thin-film solar cells for space application, Thin Solid Films 511–512 (2006) 613–622. [7] K.J. Allen, Reel to real: prospects for flexible displays, Proc. IEEE 93 (2005) 1394–1399. [8] A. Sobel, Electronic paper: high-speed inks, Nat. Mater. 2 (2003) 643–644. [9] Y. Chen, J. Au, P. Kazlas, A. Ritenour, H. Gates, M. McCreary, Electronic paper: flexible active-matrix electronic ink display, Nature 423 (2006) 136. [10] J.A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V.R. Raju, et al., Paper-like electronic displays: large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 4835–4840. [11] S. Park, S. Jayaraman, Smart textiles: wearable electronic systems, MRS Bull. 28 (2003) 585–591. [12] D. De Rossi, C.E. Piaggio, Electronic textiles: a logical step, Nat. Mater. 6 (2007) 328–329. [13] A.C. Siegel, S.T. Phillips, M.D. Dickey, N. Lu, Z. Suo, G.M. Whitesides, Foldable printed circuit boards on paper substrates, Adv. Funct. Mater. 20 (2010) 28–35. [14] R. Vieroth, T. Löher, M. Seckel, C. Dils, C. Kallmayer, A. Ostmann, et al., Stretchable circuit board technology and application, ISWC ‘09 International Symposium on Wearable Computers (2009) 33–36. [15] J. Vanfleteren, M. Gonzalez, F. Bossuyt, Y. Hsu, T. Vervust, I. De Wolf, et al., Printed circuit board technology inspired stretchable circuits, MRS Bull. 37 (2012) 254–260. [16] J. Jones, S.P. Lacour, S. Wagner, Z. Suo, Stretchable wavy metal interconnects, J. Vac. Sci. Technol. 22 (2004) 1723–1725. [17] T. Li, Z. Suo, S.P. Lacour, S. Wagner, Compliant thin film patterns of stiff materials as platforms for stretchable electronics, J. Mater. Res. 20 (2005) 3274–3277. [18] S.P. Lacour, D. Chan, S. Wagner, T. Li, Z. Suo, Mechanisms of reversible stretchability of thin metal films on elastomeric substrates, Appl. Phys. Lett. 88 (2006) 204103. [19] C. Keplinger, J.-Y. Sun, C.C. Foo, P. Rothemund, G.M. Whitesides, Z. Suo, Stretchable, transparent, ionic conductors, Science 341 (2013) 984–987. [20] T. Kim, W.M. Choi, D. Kim, M.A. Meitl, E. Menard, H. Jiang, et al., Adv. Mater. 20 (2008) 2171–2176. [21] R. Kim, M. Bae, D.G. Kim, H. Cheng, B.H. Kim, D. Kim, et al., Stretchable, transparent graphene interconnects for arrays of microscale inorganic light emitting diodes on rubber substrates, Nano Lett. 11 (2011) 3881–3886. [22] S. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, et al., Stretchable graphene transistors with printed dielectrics and gate electrodes, Nano Lett. 11 (2011) 4642–4646. [23] J. Lee, J. Wu, M. Shi, J. Yoon, S. Park, M. Li, et al., Stretchable GaAs photovoltaics with designs that enable high areal coverage, Adv. Mater. 23 (2011) 986–991.
1119
[24] D. Kim, J. Viventi, J.J. Amsden, J. Xiao, L. Vigeland, Y. Kim, et al., Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics, Nat. Mater. 9 (2010) 511–517. [25] J. Jeong, W. Yeo, A. Akhtar, J.J.S. Norton, Y. Kwack, S. Li, et al., Materials and optimized designs for human-machine interfaces via epidermal electronics, Adv. Mater. 25 (2013) 6839–6846. [26] X. Feng, B.D. Yang, Y. Liu, Y. Wang, C. Dagdeviren, Z. Liu, et al., Stretchable ferroelectric nanoribbons with wavy configurations on elastomeric substrates, ACS Nano 5 (2011) 3326–3332. [27] G. Shin, C.H. Yoon, M.Y. Bae, Y.C. Kim, S.K. Hong, J.A. Rogers, et al., Stretchable field-effect-transistor array of suspended SnO2 nanowires, Small 7 (2011) 1181–1185. [28] S. Xu, Y. Zhang, J. Cho, J. Lee, X. Huang, L. Jia, et al., Stretchable batteries with self-similar serpentine interconnects and integrated wireless recharging systems, Nat. Commun. 4 (2013) 1543. [29] S. Cheng, Z.G. Wu, Microfluidic electronics, Lab Chip 12 (2012) 2782–2791. [30] S. Cheng, Z.G. Wu, Microfluidic stretchable RF electronics, Lab Chip 10 (2010) 3227–3234. [31] C. Ladd, J. So, J. Muth, M.D. Dickey, 3D printing of free standing liquid metal microstructures, Adv. Mater. 25 (2013) 5081–5085. [32] J.W. Boley, E.L. White, G.T. Chiu, R. Kramer, Direct writing of gallium-indium alloy for stretchable electronics, Adv. Funct. Mater. 24 (2014) 3501–3507. [33] Y. Zheng, Z.Z. He, J. Yang, J. Liu, Personal electronics printing via tapping mode composite liquid metal ink delivery and adhesion mechanism, Sci. Rep. 4 (2014) 4588. [34] T. Lu, L. Finkenauer, J. Wissman, C. Majidi, Rapid prototyping for soft-matter electronics, Adv. Funct. Mater. 24 (2014) 3351–3356. [35] S.H. Jeong, A. Hagman, K. Hjort, M. Jobs, J. Sundqvist, Z.G. Wu, Liquid alloy printing of microfluidic stretchable electronics, Lab Chip 12 (2012) 4657–4664. [36] S.H. Jeong, K. Hjort, Z.G. Wu, Tape transfer printing of a liquid metal alloy for stretchable RF electronics, Sensors 14 (2014) 16311–16321. [37] R.K. Kramer, C. Majidi, R.J. Wood, Masked deposition of gallium-indium alloys for liquid-embedded elastomer conductors, Adv. Funct. Mater. 23 (2013) 5292–5296. [38] G. Li, M. Parmar, D. Kim, J.B. Lee, D.W. Lee, PDMS based coplanar microfluidic channels for the surface reduction of oxidized Galinstan, Lab Chip 14 (2014) 200–209. [39] A. Fassler, C. Majidi, Soft-matter capacitors and inductors for hyperelastic strain sensing and stretchable electronics, Smart Mater. Struct. 22 (2013) 055023. [40] G. Li, M. Parmar, D.W. Lee, An oxidized liquid metal-based microfluidic platform for tunable electronic device applications, Lab Chip 15 (2015) 766–775. [41] N. Kurra, G.U. Kulkarni, Pencil-on-paper: electronic devices, Lab Chip 13 (2013) 2866–2873. [42] W.M. Robertson, Propagation of a crack filled with liquid metal, Trans. Met. Soc. AIME 236 (1966) 1478–1482. [43] T. Liu, P. Sen, C. Kim, Characterization of nontoxic liquid-metal alloy galinstan for applications in microdevices, J. Microelectromech. Syst. 21 (2012) 443–450.
Biographies Guangyong Li received the B.E. and M.E. degrees in mechanical engineering from Chongqing University of Technology, Chongqing, China, in 2009 and 2012, respectively. He is currently working toward the Ph.D. degree in mechanical engineering in the MEMS and Nanotechnology Laboratory, Department of Mechanical Engineering, Chonnam National University, Korea. His current research interests include liquid metal based microfluidic platform, energy-harvesting technology, MEMS device design and fabrication and tunable FSS. Xuan Wu received the B.E. and M.E. degrees in mechanical engineering from Chongqing University of Technology, Chongqing, China, in 2009 and 2012, respectively. He is currently working toward the Ph.D. degree in mechanical engineering in the MEMS and Nanotechnology Laboratory, Department of Mechanical Engineering, Chonnam National University, Korea. He is the holder of two patents in China. He is also a member of the Korean Sensors Society. His current research interests include energy-harvesting technology, MEMS device design and fabrication, piezo-materials for various applications, and graphene local anodic lithography technology. Dong-Weon Lee received the Ph.D. degrees in mechatronics engineering from Tohoku University, Sendai, Japan, in 2001. He has been a professor of Mechanical Systems Engineering at Chonnam National University (CNU), Republic of Korea since March of 2004. Previously, he was with the IBM Zurich Research Laboratory in Switzerland, working mainly on micro cantilever devices for chemical AFM applications. At CNU, his research interests include liquid metal based microfludic platform, smart cantilever devices, miniaturized energy harvester, smart materials and structures, and nanoscale transducers. He is a member of the technical program committee of IEEE Sensors Conference, Transducers, and Micro processes and Nanotechnology Conference (MNC).