Simple method for high-performance stretchable composite conductors with entrapped air bubbles
© Hwang et al. 2016
Received: 26 October 2015
Accepted: 5 January 2016
Published: 12 January 2016
We integrate air bubbles into conductive elastic composite-based stretchable conductors to make them mechanically less stiff and electrically more robust against physical deformations. A surfactant facilitates both the formation and maintenance of air bubbles inside the elastic composites, leading to a simple fabrication of bubble-entrapped stretchable conductors. Based on the unique bubble-entrapped architecture, the elastic properties are greatly enhanced and the resistance change in response to tensile strains can clearly be controlled. The bubble-entrapped conductor achieves ~80 % elongation at ~3.4 times lower stress and ~44.8 % smaller change in the electrical resistance at 80 % tensile strain, compared to bare conductor without air bubbles.
Conductive elastic materials that can retain their electrical performance under various deformations have recently received great attention due to the potential applications as stretchable conductors and interconnects in stretchable electronics [1–5]. Many efforts have been made to demonstrate various types of conductive elastic materials. Examples include electrical nanonetworks mainly made of conductive carbon nanotubes (CNTs) [6–8] and metallic nanowires (NWs) [9–11], buckled architectures of metallic thin films [12–14], three-dimensional conductive foams [15–17], and conductive composites synthesized by doping elastomer matrices with conductive nanofillers [18–22].
Among these methods, conductive composites have been considered as one of the most efficient ways of preparing conductive elastic materials due to the superior advantages including fabrication simplicity and easy control of the intrinsic electrical properties. In this regard, to date, this method has been widely used to fabricate functional stretchable devices. However, two critical issues can potentially arise in the conductive composite-based approaches: (1) considerable increase of the stiffness of the composites from excessive filler doping and (2) steep change in the electrical conductivity when stretched. These would lead to significant degradation of the mechanical and electrical performance of the stretchable devices.
In this work, we present a new class of CNT-doped stretchable conductor with embedded air bubbles (hereafter, bubble conductor) based on conductive elastic composites. The air bubbles inside the composite matrix play a key role in (1) decreasing the stiffness of the composites and (2) sustaining the electrical properties, mainly by allowing considerable reduction of the resistance change of the CNT network located at the boundary regions upon stretching. The advantages of the proposed approach include process simplicity, cost-effectiveness, and potential scalability for large area fabrication.
Fabrication of Bubble Conductors
The surface morphologies of the fabricated bubble conductors were observed using an optical microscope (OM; BX60M, OLYMPUS) equipped with a CCD module. A field emission scanning electron microscope (FESEM; S4700, HITACHI) was used to characterize the detailed morphologies of the air bubbles and CNT networks in the conductors. The stress-strain relationships of the bare conductor and bubble conductor were examined using an automatic testing stand (JSV-H1000, JISC) equipped with a digital push-pull force gauge (HF-10, JISC). All stretching tests were performed using a custom-made jig while monitoring the change in the electrical resistance using a digital multimeter (U1253B, Agilent Technologies). The electrical resistances of the conductors under stretching were recorded after imposing a stabilization time of 20 s in each strained state.
Results and Discussion
When gradually increasing the tensile strain, ΔR/R 0 of the conductor was increased accordingly due to the resulting decrease in the current paths in the CNT networks, eventually leading to a resistance change of ~39.5 % at a strain of 80 %. The electrical robustness of the stretchable conductor against mechanical deformation was significantly improved by the air bubbles, with a negligible resistance change of ~0.4 % at a strain of 40 %. Moreover, ΔR/R 0 of the bubble conductor at a tensile strain of 80 % was decreased by ~44.8 % (from ~39.5 to ~21.8 %) compared to that of the bare conductor. This is mainly attributed to the change in the shape of the air bubbles in response to mechanical deformation.
The air bubbles in the conductor gradually become distorted into an oval shape upon stretching as shown in Fig. 4b. Up to a certain level of strain, most of the CNT networks located at the boundary regions of the air bubbles are just shifted along the deformed profiles of the bubbles rather than experiencing the applied strain directly. Moreover, the inter-CNT contact junctions in the bubble conductor can be maintained better than those of the bare conductor, even under high strains, as illustrated schematically in Fig. 4c. This is probably because that the load applied to the stretchable conductor is inevitably distributed to deform the air bubbles in the polymer matrix. For this reason, ΔR/R 0 of the bubble conductor is much smaller than that of the bare conductor at the same strain levels.
In addition, the rate of increase in the electrical resistance of the bubble conductor was also lower than that of the bare conductor, as shown in Fig. 4a. This also means that the morphology of the CNT networks in the bare conductor is greatly affected by the applied tensile strain. In contrast, the air bubbles play an important role in alleviating the resistance change of the device by allowing the CNT networks to retain the networked architecture even at high strains.
Figure 4d shows ΔR/R 0 for the bubble conductor under repetitive strain loading (0 to 80 %) and unloading (80 to 0 %) for up to 10 cycles. The ΔR/R 0 values were quite uniform for each loading state, representing a minimal standard deviation of ~0.58 %. Importantly, the electrical resistances increased in response to the applied strains and almost returned to the initial state when removing all the strains, as shown in Fig. 4d. In this state, the standard deviation was as small as ~0.36 %. This means that the bubble conductors can operate quite reliably with considerable reversibility even under repetitive high strains.
We have demonstrated air-bubble-entrapped conductors with both enhanced elastic properties and electrical robustness for stretchable electronics applications. The functional air bubbles can be formed easily and sustained in a CNT-doped PDMS matrix with the aid of a small amount of surfactant. It was experimentally found that the bubble conductor requires ~3.4 times lower stress (~66 kPa) compared to the bare conductor (~225 kPa) to achieve ~80 % elongation. The change in the electrical resistance of the stretchable conductor upon stretching to 80 % was decreased by ~44.8 % (from 39.5 to 21.8 %) by integrating air bubbles into the matrix. The bubble conductor also showed quite reliable and reversible performance under repetitive operations, with low standard deviations of 0.58 and 0.36 % in the loading and unloading states, respectively. The bubble conductors could find many potential applications in stretchable electronics due to the advantages that include simple fabrication, enhanced elastic properties, and electrical robustness against physical deformations.
This research was supported by the Basic Science Research Program (No. 2015R1A2A2A01004038) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning. HH and DGK contributed equally to this work.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Ahn JH, Je JH (2012) Stretchable electronics: materials, architectures and integrations. J Phys D: Appl Phys 45:103001View ArticleGoogle Scholar
- Park M, Im J, Shin M, Min Y, Park J, Cho H, Park S, Shim MB, Jeon S, Chung DY, Bae J, Park J, Jeong U, Kim K (2012) Highly stretchable electric circuits from a composite material of silver nanoparticles and elastomeric fibres. Nat Nanotechnol 7:803–809View ArticleGoogle Scholar
- Sekitani T, Nakajima H, Maeda H, Fukushima T, Aida T, Hata K, Someya T (2009) Stretchable active-matrix organic light-emitting diode display using printable elastic conductors. Nat Mater 8:494–499View ArticleGoogle Scholar
- Sekitani T, Someya T (2010) Stretchable, large-area organic electronics. Adv Mater 22:2228–2246View ArticleGoogle Scholar
- Hu L, Pasta M, Mantia FL, Cui L, Jeong S, Deshazer HD, Choi JW, Han SM, Cui Y (2010) Stretchable, porous, and conductive energy textiles. Nano Lett 10:708–714View ArticleGoogle Scholar
- Cohen DJ, Mitra D, Peterson K, Maharbiz MM (2012) A highly elastic, capacitive strain gauge based on percolating nanotube networks. Nano Lett 12:1821–1825View ArticleGoogle Scholar
- Lipomi DJ, Vosgueritchian M, Tee BC-K, Hellstrom SL, Lee JA, Fox CH, Bao Z (2011) Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nat Nanotechnol 6:788–792View ArticleGoogle Scholar
- Liu K, Sun Y, Liu P, Lin X, Fan S, Jiang K (2011) Cross-stacked superaligned carbon nanotube films for transparent and stretchable conductors. Adv Funct Mater 21:2721–2728View ArticleGoogle Scholar
- Yun S, Niu X, Yu Z, Hu W, Brochu P, Pei Q (2012) Compliant silver nanowire-polymer composite electrodes for bistable large strain actuation. Adv Mater 24:1321–1327View ArticleGoogle Scholar
- Xu F, Zhu Y (2012) Highly conductive and stretchable silver nanowire conductors. Adv Mater 24:5117–5122View ArticleGoogle Scholar
- Lee P, Lee J, Lee H, Yeo J, Hong S, Nam KH, Lee D, Lee SS, Ko SH (2012) Highly stretchable and highly conductive metal electrode by very long metal nanowire percolation network. Adv Mater 24:3326–3332View ArticleGoogle Scholar
- Lacour SP, Jones J, Suo Z, Wagner S (2004) Design and performance of thin metal film interconnects for skin-like electronic circuits. IEEE Electron Device Lett 25:179–181View ArticleGoogle Scholar
- Wang X, Hu H, Shen Y, Zhou X, Zheng Z (2011) Stretchable conductors with ultrahigh tensile strain and stable metallic conductance enabled by prestrained polyelectrolyte nanoplatforms. Adv Mater 23:3090–3094View ArticleGoogle Scholar
- Bowden N, Brittain S, Evans AG, Hutchinson JW, Whitesides GM (1998) Spontaneous formation of ordered structures in thin films of metals supported on an elastomeric polymer. Nature 393:146–149View ArticleGoogle Scholar
- Chen Z, Ren W, Gao L, Liu B, Pei S, Cheng HM (2011) Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition. Nat Mater 10:424–428View ArticleGoogle Scholar
- Park J, Wang S, Li M, Ahn C, Hyun JK, Kim DS, Kim DK, Rogers JA, Huang Y, Jeon S (2012) Three-dimensional nanonetworks for giant stretchability in dielectrics and conductors. Nat Commun 3:916View ArticleGoogle Scholar
- Ge J, Yao HB, Wang X, Ye YD, Wang JL, Wu ZY, Liu JW, Fan FJ, Gao HL, Zhang CL, Yu SH (2013) Stretchable conductors based on silver nanowires: improved performance through a binary network design. Angew Chem Int Ed 52:1654–1659View ArticleGoogle Scholar
- Huang YY, Terentjev EM (2010) Tailoring the electrical properties of carbon nanotube-polymer composites. Adv Funct Mater 20:4062–4068View ArticleGoogle Scholar
- Lu N, Lu C, Yang S, Rogers J (2012) Highly sensitive skin-mountable strain gauges based entirely on elastomers. Adv Funct Mater 10:4044–4050View ArticleGoogle Scholar
- Kong JH, Jang NS, Kim SH, Kim JM (2014) Simple and rapid micropatterning of conductive carbon composites and its application to elastic strain sensors. Carbon 77:199–207View ArticleGoogle Scholar
- Woo SJ, Kong JH, Kim DG, Kim JM (2014) A thin all-elastomeric capacitive pressure sensor array based on micro-contact printed elastic conductors. J Mater Chem C 2:4415–4422View ArticleGoogle Scholar
- Liu CX, Choi JW (2012) Precision patterning of conductive polymer nanocomposite using a laser-ablated thin film. J Micromech Microeng 22:045014View ArticleGoogle Scholar