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.
KeywordsEntrapped air bubbles Conductive elastic composites Surfactant Stretchable conductor CNT networks
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.
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