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A Rubberlike Stretchable Active Matrix Using Elastic Conductors

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Science  12 Sep 2008:
Vol. 321, Issue 5895, pp. 1468-1472
DOI: 10.1126/science.1160309

Abstract

By using an ionic liquid of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, we uniformly dispersed single-walled carbon nanotubes (SWNTs) as chemically stable dopants in a vinylidene fluoride-hexafluoropropylene copolymer matrix to form a composite film. We found that the SWNT content can be increased up to 20 weight percent without reducing the mechanical flexibility or softness of the copolymer. The SWNT composite film was coated with dimethyl-siloxane–based rubber, which exhibited a conductivity of 57 siemens per centimeter and a stretchability of 134%. Further, the elastic conductor was integrated with printed organic transistors to fabricate a rubberlike active matrix with an effective area of 20 by 20 square centimeters. The active matrix sheet can be uniaxially and biaxially stretched by 70% without mechanical or electrical damage. The elastic conductor allows for the construction of electronic integrated circuits, which can be mounted anywhere, including arbitrary curved surfaces and movable parts, such as the joints of a robot's arm.

The creation of stretchable electronics is one of the most interesting challenges in materials science and engineering. Stretchability is an entirely different concept from the miniaturization trend pursued by conventional electronics, and thus has the potential to open exciting opportunities, particularly in the area of large-area electronics (16). In the past decade, large-area electronic devices have become thin and light enough to allow the fabrication of large-area solar cells (7) and displays easily hung on roofs and walls. It is expected that large-area electronic devices will now be developed further, making the realization of bendable and rollable displays possible (8). At the same time, large-area flexible sensors (9, 10) and actuators (11, 12) are another emerging frontier. Although these achievements represent valuable advances, the utility of flexible electronics is limited to nearly flat substrates such as walls or paper. In contrast, stretchable electronics can cover arbitrary curved surfaces and movable parts such as the joints of a robot's arm, and thus would substantially expand where electronics can be used.

Efforts toward stretchable electronics have originated from electronics using metal electrodes on rubber substrates (1316). Khang et al., Sun et al., and Kim et al. have embedded active components, such as transistors and diodes, in rubber sheets and integrated them with wavy metal wires by carefully controlling the strains in thin films (1416). Their electrical circuits have high mechanical durability and show good electrical performance under stretching because all the circuit components are stretchable. In an alternative cost-effective approach, integrated circuits have also been directly fabricated on plastic films and mechanically processed to form perforated films with net-shaped structures, which serve as stretchable artificial skins (10). Net-shaped integrated circuits are flexible but have inelastic wirings; their mechanical robustness can be substantially improved by using elastic wirings.

The simultaneous incorporation of excellent mechanical robustness and electronic performance is the key to realizing rubberlike stretchable electronics. Rigid materials usually exhibit good electronic performance and controllability or stability, but they exhibit poor mechanical robustness. On the other hand, soft materials show good mechanical properties while exhibiting poor electronic properties. For example, a conductivity of 0.1 S/cm for the best elastomers (rubbers filled with carbon particles) is insufficient to operate integrated circuits.

Besides metals and carbon-particle composites, carbon-nanotube–based conducting materials have also been produced (1720). Their conductivities are typically 10–3 to 101 S/cm (1719). Furthermore, carbon-nanotube and/or conducting-polymer (polyaniline) composites have been reported (20); however, these materials cannot be stretched.

We developed rubberlike transistor-active matrices that can be stretched biaxially by 70%. The key was the fabrication of a highly elastic conductor and gel with high conductivity composed from millimeter-long single-walled carbon nanotubes (SWNTs) (21, 22), an ionic liquid, and a compatible fluorinated copolymer (23). We used SWNTs as a conducting dopant because they are chemically inert and can improve the mechanical properties of polymer matrices. It has been reported that fine bundles of SWNTs can be produced by grinding with imidazolium ion-based ionic liquids (24, 25).

SWNT composite films (referred to as SWNT films henceforth) were fabricated using millimeter-long SWNTs, an ionic liquid, and a fluorinated copolymer. A schematic representation of the fabrication process of the elastic conductors is shown in Fig. 1. We used super-growth SWNTs (>99.98% in purity, >1 mm in length, and 3 nm in diameter) (22) unless otherwise specified. Typically, we mixed the SWNTs (50 mg) with 50 mg of an ionic liquid [1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI)] and subjected the resulting suspension to an automatic grinding system for 1 hour, giving rise to a black pastelike substance we refer to as “bucky gel.” To the gel (100 mg) we successively added 4-methly-2-pentanone (8 ml) and a fluorinated copolymer, vinylidene fluoride-hexafluoropropylene copolymer [100 mg; Daiel-G801 (Daikin, Osaka, Japan), weigh average molecular weight (Mw) of 150,000; referred to as G801 henceforth]. The mixture was stirred at 25°C (1 hour) and sonicated [UH-50 (SMT, Tokyo, Japan)] at 30°C (1 hour). After stirring again at 80°C (1 hour), the resulting swollen gel was poured onto a glass plate by drop casting and then air-dried for 24 hours to obtain a composite film (SWNT film), as shown in Fig. 2, A and B. With this method, we dispersed SWNTs uniformly into a fluorinated copolymer matrix to fabricate a SWNT film.

Fig. 1.

Manufacturing process of SWNT film, SWNT elastic conductor, and SWNT paste.

Fig. 2.

SWNT films and elastic conductor. (A) Photograph and (B) scanning electron microscope images of an optimized SWNT film composed of SWNTs, polymer matrix G801, and ionic liquid BMITFSI. SWNTs were uniformly dispersed into the fluorinated copolymer as the base matrix. (C) Conductivity as a function of uniaxial tensile strain. Measurements were performed on three samples in ambient air: SWNT film (1), SWNT elastic conductor (2), and commercially available carbon-particle–based conducting rubber (3). (D) Change in conductance of SWNT elastic conductors under uniaxial stretching cycles. Stretchability is 25, 50, 70, 110, and 130%. (E) Conductivity of SWNT films as a function of the content of BMITFSI, in which the amount of G801 was 100 mg and that of the SWNTs was 50 mg. (F) Conductivity of SWNT films as a function of the content of SWNTs, in which the BMITFSI content was the same as the SWNTs, and the amount of G801 was 100 mg.

The advantage of fabrication using ionic liquids is that after the dispersion of SWNTs, ionic liquids can be recovered quantitatively (99%) by Soxhlet extraction and recycled for the next batch process. After the extraction of ionic liquids, conductivity decreases; however, it is as high as 10 S/cm, as shown later. In the following experiments, we used elastic conductors with ionic liquids.

The SWNT film is flexible and tensile, but it has low elasticity. In order to improve its tensibility and elasticity, it was mechanically processed with a numerically controlled punching system and transformed into a perforated film with a net-shaped structure (fig. S1). Subsequently, it was coated with dimethyl-siloxane–based silicone rubbers [polydimethylsiloxane (PDMS), Sylgard 184 or SH9555 (Dow Corning, Midland, MI)]. The resulting composite material is referred to as an “elastic conductor.” Its elasticity is determined by the elasticity of PDMS, which is 6 N/mm2. PDMS is nonadherent to almost all materials; however, our SWNT film has an excellent capillary surface (meaning a significantly large surface area), the same as SWNTs inherently possess. Therefore, by dip-coating diluted PDMS around SWNT films, PDMS was attached firmly to SWNT material.

We investigated the electrical and mechanical properties of the SWNT film and elastic conductor under tensile stress. The stretch test was performed with a high-precision mechanical system [Autograph/AG-X (Shimadzu, Kyoto, Japan)], and conductivity was measured by means of the four-probe method with a high-precision multimeter [34401A (Agilent, Santa Clara, CA)]. Figure 2C shows conductivity as a function of uniaxial tensile strain. For comparison, the stretchability and conductivity of a commercial conducting rubber containing carbon particles (Kinugawa Rubber Industrial, Chiba, Japan) are also shown in Fig. 2C. Although the stretchability of the commercial conducting rubber exceeded 150%, its conductivity was approximately 0.1 S/cm for all strains, which is insufficient for electronic circuit applications. In contrast, the SWNT film exhibited an extraordinarily high conductivity of 57 S/cm, and it did not show significant changes in conductivity or mechanical damage when uniaxially stretched by 38% or less. Furthermore, the SWNT elastic conductor exhibited a high conductivity of 57 S/cm, and it could be uniaxially stretched up to 134%; however, the conductivity decreased moderately with the application of tensile strain. Even under such a large strain, the material showed good conductivity (6 S/cm). We observed that the shapes of holes of elastic conductors changed during stretching. The enhancement in the stretchability of the net-shaped structure was realized because the interconnecting struts buckled and twisted out of plane upon tension (fig. S1).

To investigate the reversibility of the elastic conductor, several stretching cycles were applied. Measurements were performed at each stretching cycle by using the four-probe method. Figure 2D shows the normalized conductance as a function of the number of uniaxial stretching cycles. There was no significant change in conductance even after 4000 25%-stretching cycles, 500 50%-stretching cycles, 20 to 50 70%-stretching cycles, or 1 to 2 110%-stretching cycles. Further increase in strain beyond 110% caused an irreversible change in conductance, although it was greater than 1 S/cm. Similar to other stretchable materials, either conducting or nonconducting, our elastic conductor exhibits irreversible mechanical and electrical changes after many stretching cycles (Fig. 2D). However, our conductor with a conductivity of 50 S/cm can be stretched approximately 500 times at a high level of strain (50%) without notable degradation. The feasibility of such a robust conductor has been demonstrated unambiguously by using the net-shaped structure with a PDMS coating. In our study, the electrical and mechanical characteristics of the conductor did not deteriorate over time—not over at least 1 year—because its components were chemically stable.

The fabrication process is fully optimized to achieve the best mechanical and conductive properties of the SWNT composite film. To establish this, we examined various conditions by changing process parameters and materials. For instance, as a polymer matrix, we examined vinylidene fluoride-hexafluoropropylene copolymer with composition ratios of 0.78:0.22 (G801) and 0.88:0.12 [KYNAR-FLEX2801 (Arkema, Paris, France), molecular weight of 30,000; referred to as KYNAR henceforth]. For ionic liquids, we used, in addition to BMITFSI, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) (fig. S2).

One of the most important experiments in this study was to determine a combination of elastic polymers and ionic liquids that are compatible with each other. When G801 and BMITFSI were used, the resulting SWNT films exhibited a very smooth, flat, and uniform surface (Fig. 2, A and B). In contrast, all other combinations tested in this study (fig. S2) did not exhibit good compatibility. For example, KYNAR, which is a hard resin, was not compatible with BMITFSI, and the SWNT film obtained with this combination was readily crumpled or deformed.

Besides the compatibility of the materials, the correct mixing ratios of SWNT, BMITFSI, and G801 are needed to fabricate mechanically robust and highly conductive SWNT films. First, the BMITFSI content was changed from 12 to 47 weight percent (wt %), and the amounts of SWNT and G801 were 50 and 100 mg, respectively (Fig. 2E and fig. S4). When the BMITFSI content was greater than 40 wt %, the SWNT film became discontinuous; when it was less than 10 wt %, the film was thick and porous, and thus was fragile and had low conductivity. We found that the highest conductivity can be obtained when the contents of SWNT and BMITFSI are both 20 wt %, without sacrificing the mechanical flexibility of the copolymer (Fig. 2F and figs. S5 and S6). Next, the bucky gel with optimized SWNT and BMITFSI contents was mixed with G801 by varying the mixture ratios between the bucky gel and G801 (fig. S6). Figure 2F displays the plots of the conductivity of the film as a function of the SWNT content. When the SWNT content was less than 10 wt %, the SWNT film was discontinuous (fig. S5). In conformance with the results shown in Fig. 2E, when the SWNT content was 16%, the conductivity was as high as 53 S/cm, indicating excellent reproducibility and controllability of our fabrication method for highly elastic conductors. Meanwhile, when the SWNT content was greater than 30 wt %, the resultant film was thick and porous, thereby giving rise to brittleness and low conductivity.

The extremely high aspect ratio of the super-growth SWNTs (22) is important not only for acquiring higher conductivity but also for the stability of conductivity under tensile strain, because the conductivity of the super-growth SWNTs does not change with strain (fig. S8). In sharp contrast, the electrical properties of films fabricated with high-pressure carbon monoxide process (HiPco) SWNTs showed a strong dependence on stretchability; their conductivity rapidly decreased with an increase in the applied tensile stress (fig. S8).

A previous study on circuits containing stretchable interconnects (13) revealed that the contact points between stretchable and rigid materials are locations where mechanical failures can occur. We fabricated a SWNT-based paste by using the G801-containing bucky gel via cross-linking the polymer matrix (Fig. 1). A peroxide cross-linking initiator [Perhexane-25B (NOF, Tokyo, Japan)] (1.3 mg) and a cross-linker Triallylisocyanurate (TAIC) [(Nippon Kasei Chemical, Tokyo, Japan)] (4 mg) were added to the SWNT-dispersed solution. The mixture was stirred at 80°C for 1 hour and air dried, producing a paste with a partially cross-linked fluorinated copolymer. The paste exhibited a high conductivity of 5 to 10 S/cm and high adhesion capability. The adhesive strength increased up to 1.5 N/mm2 (fig. S9). Therefore, the SWNT paste can be used to form interconnections between the contact pads of organic transistors (812, 2631) and SWNT-based elastic conductors that require considerably more mechanical robustness than other components, such as wires (Fig. 3).

Fig. 3.

Images of large-area stretchable active matrix comprising 19-by-37 printed organic transistors and wiring using the SWNT elastic conductor. The printed organic transistors function as active components, and the SWNT elastic conductor functions as word lines and bit lines for interconnection among the transistors. (A) Image of the curved surface covered with the stretchable active matrix. (B) Magnified image of one cell. A SWNT elastic conductor having a net-shaped structure was formed with a mechanical punching system and then coated with silicone rubbers. (C) Schematic illustrations of the stretchable active matrix.

Taking full advantage of the SWNT elastic conductor and paste, we manufactured a 19-by-37–cell organic transistor-based stretchable active matrix (Fig. 3) by combining printing, vacuum evaporation, and mechanical processes [supporting online material (SOM) and figs. S10 to S12] (23). We fabricated an array of organic transistors that were connected with each other by elastic conductors (bit lines for source electrodes or word lines for gate electrodes). The transistor characteristics did not change after the formation of the interconnections with the fabricated elastic conductors (fig. S11). The SWNT elastic conductors were connected to the contact pads for gate, source, and drain electrodes by the SWNT paste.

The sheet was stretched by increasing the tensile strain, and the transistor characteristics on the stretched sheet were examined (Fig. 4). In order to apply accurate strains uniformly to the transistor active matrix, we used a small active matrix comprising 10 by 10 cells. Furthermore, we measured the characteristics of the transistor positioned at the center of the matrix, because that transistor is stretched to a greater extent than those at the corners. When the sheet was stretched by 70% or less, the change in the transistor characteristics was negligibly small. Subsequently, the extension distortion was released, and recovery tests for the measurement of the transistor characteristics without strain were performed (fig. S13). After it was stretched by 70% or less, the transistor characteristics without strain did not change as compared with those measured before the stretching tests. Furthermore, no changes in electrical characteristics or mechanical damages were observed after 30 cycles at 70% stretching, indicating the excellent electrical functionalities and stability of the active matrix under stretching. However, stretching by 70% or more resulted in irreversible degradation. This is mainly due to the exfoliation of printed Ag electrodes from plastic substrates. The disconnection of organic transistors from the SWNT elastic conductor is quite rare because of the good adhesion of SWNT pastes.

Fig. 4.

Stretching tests of the active matrix. (A) Images captured in the initial state before stretching and under uniaxial and biaxial stretching. (B) Transistor characteristics and (C) channel currents measured under uniaxial stretching. The channel currents (IDS) were normalized, with IDS being measured before the experiments as the initial state. (D) Transistor characteristics and (E) channel currents measured under biaxial stretching. Recovery performance after stretching is shown in fig. S13.

The present approach in which semi-rigid active components are connected to each other through stretchable wirings is complementary to fully elastic circuits reported by Khang et al., Sun et al., and Kim et al. (1416). Their circuits exhibit good mechanical durability under stretching; large deformations of stretchable active components may lead to changes in their electrical characteristics. In our approach, the electrical characteristics of active components do not change during stretching because they are not deformed. Therefore, the two approaches should be adopted according to the purpose of the application.

The conductivity of the SWNT-based elastic conductor is sufficiently high for application to high-performance and large-area electronic circuits. In the case of commercial conducting rubber using carbon particles (∼0.1 S/cm), however, the resistance of electrical wiring between transistors (word and bit lines) is higher than the resistance of transistors in one state (fig. S14). This causes a decrease in channel currents, delay in circuits, and other detrimental effects.

Although we demonstrated the feasibility of SWNT-based elastic conductors and paste by fabricating an active matrix, the materials and integration technology can be applied to various types of electronic functionalities. When the stretchable active matrix is integrated with a two-dimensional array of pressure sensors, a rubber-like artificial skin can be obtained. Such sensor and actuator applications (912) do not always require components with dimensions of several tens or hundreds of micrometers. To realize this integration, it is necessary to develop mechanically robust, via interconnections. Furthermore, if the stretchable active matrix is integrated with an array of actuators and mounted on a curved surface, the touch feeling on the surface will be changed electrically. In this manner, the elastic conductor developed in this work enables electronic circuits to be mounted at locations where to date we have been unable to provide electrical functionalities. This is an important step toward producing intelligent surfaces as friendly human/electronics interfaces; in the future, such intelligent surfaces will be able to interact with people, objects, and the environment in new ways.

Supporting Online Material

www.sciencemag.org/cgi/content/full/1160309/DC1

Materials and Methods

Figs. S1 to S14

References

References and Notes

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