Letter pubs.acs.org/NanoLett
Robust and Soft Elastomeric Electronics Tolerant to Our Daily Lives Atsuko Sekiguchi,†,‡,§ Fumiaki Tanaka,†,‡,§ Takeshi Saito,† Yuki Kuwahara,† Shunsuke Sakurai,† Don N. Futaba,†,‡ Takeo Yamada,†,‡ and Kenji Hata*,†,‡ †
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan S Supporting Information *
ABSTRACT: Clothes represent a unique textile, as they simultaneously provide robustness against our daily activities and comfort (i.e., softness). For electronic devices to be fully integrated into clothes, the devices themselves must be as robust and soft as the clothes themselves. However, to date, no electronic device has ever possessed these properties, because all contain components fabricated from brittle materials, such as metals. Here, we demonstrate robust and soft elastomeric devices where every component possesses elastomeric characteristics with two types of single-walled carbon nanotubes added to provide the necessary electronic properties. Our elastomeric field effect transistors could tolerate every punishment our clothes experience, such as being stretched (elasticity: ∼ 110%), bent, compressed (>4.0 MPa, by a car and heels), impacted (>6.26 kg m/s, by a hammer), and laundered. Our electronic device provides a novel design principle for electronics and wide range applications even in research fields where devices cannot be used. KEYWORDS: wearable electronics, conductive rubber, single-walled carbon nanotube, field effect transistor, flexible electronics or the past 170 000 years, textiles made of polymer fibers have been used throughout our daily life (e.g., wearing, covering, etc.).1 The widespread usage of these textiles results from their ability to withstand the impacts, pounding, and stretching associated with the diverse actions experienced in our daily lives (Supporting Information Tables S1 and S2). For example, the pressure from high heeled shoes can be as high as 2.5 MPa, and the impact from a moving car be as high as 8800 kg m/s. For clothes, comfort demands softness (Young’s modulus ∼10 MPa) to allow freedom of movement while also providing ∼20−50% reversible stretchability and flexibility for dressing and undressing. As exemplified, the mechanical stresses in our daily actions require materials to be both robust and soft. One of the ultimate goals in electronics is to realize devices that can tolerate the diverse variety of applied stress, such as tension, compression, twist, impact, and laundering similar to clothes. Hereinafter, we refer to “robustness” as the tolerance to not one but all of these applied stresses. This is a great challenge as conventional electronics have been developed on rigid and brittle semiconducting crystals, metal, and oxidized insulators. Recent progress in material engineering has afforded flexibility and stretchability to electronics by either designing new structure architectures of conventional materials or developing new materials.2−16 For example, Rogers et al. demonstrated foldable and stretchable (10%) circuits by using thin Au wrinkled structure architectures.2 Someya et al.
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© 2015 American Chemical Society
fabricated an imperceptible device (stretchable (230%) and bendable) by utilizing Au wrinkles and reduced substrate thickness.3 Lee et al. fabricated a stretchable (20%) and bendable wrinkled Al2O3 dielectric.4 The devices fabricated exhibited high performance although the thin and wrinkled architecture might hinder robustness. An alternative approach is to make electronics from robust and soft materials. For example, single-wall carbon nanotubes (SWCNTs), ion-gels, poly(3-hexythiophene) fibers, Au nanosheets, fluorinated rubber composites have been used to realize electronics with stretchability of >70%.5,6 However, for all devices fabricated thus far, in at least one component, conventional materials, such as metals, semiconducting crystals, or oxidized insulators, are still used, likely limiting the robustness. Therefore, although great progress has been shown, electronics with the robustness and softness at the level of clothes have yet to be demonstrated. Besides clothes, elastomers also possess both robustness and softness because both materials are composed of entangled and covalently bonded hydrocarbon polymer chains that can elastically deform and provide mechanical strength. Carbon nanotubes, being 1-D high aspect ratio materials, are similar in structure with polymer chain. Therefore, when long CNTs are dispersed in an elastomer matrix, the composite can retain the Received: April 15, 2015 Revised: July 15, 2015 Published: July 28, 2015 5716
DOI: 10.1021/acs.nanolett.5b01458 Nano Lett. 2015, 15, 5716−5723
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Nano Letters
Figure 1. Elastomeric FET composed from hydrocarbon polymers and SWCNTs. (a) Photograph of elastomeric FET and logic circuits. (b) Schematic cross-section of the elastomeric FET. (c) Periodic table showing the constituent elements of elastomeric FET (red), conventional LSI (green) and clothes (orange). (d) Transfer characteristic of elastomeric FET (drain voltage, 0.5 V; length and width of SWCNT channel, 50 and 700 μm, respectively). (e) Output characteristics of elastomeric FET. The gate voltage was varied from 0 to −2 V. (f) Photographs of elastomeric FET bent, folded and stretched by hand. (g) Photograph of elastomeric FET stepped on by high heels. The inset shows the transfer characteristics before (red) and after (blue) the load from high heels. (h) Photograph of elastomeric FET being run over by a car. The inset shows the transfer characteristics before (red) and after (blue) the load from the car. (i) Photograph of elastomeric FET put on a T-shirt and washed by a washing machine. The inset shows the transfer characteristics before (red) and after (blue) washing. (j) Equivalent circuit diagrams of inverter (left) and NAND gate (middle); the truth table for a NAND gate (right). The load resistances were achieved with FET without ion-gel. (k) Performance of inverter: red, output voltage vs input voltage; blue, gain. (l) Performance of NAND gate: red, voltage of input 1; blue, voltage of input 2; green, output voltage. In (k) and (l), the Vdd applied to these logic gates is 1.5 V relative to GND. 5717
DOI: 10.1021/acs.nanolett.5b01458 Nano Lett. 2015, 15, 5716−5723
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Nano Letters
Figure 2. Fabrication scheme of elastomeric FET. (a) Schematic illustration of elastomeric FET fabrication process: (1) sprayed film of SWCNT fluorinated rubber including super-growth SWCNT, fluorinated rubber, and ionic liquid; (2) lithographically patterned film served as electrodes; (3) transfer process of electrodes into a silicone rubber substrate; (4) synthesis of e-DIPS SWCNT; (5) purification of semiconducting SWCNT (black solution, before purification; red solution, metallic-rich SWCNT solution; blue solution, semiconducting-rich one); (6) inkjet printed channel of semiconducting SWCNT; (7) spin coated ion-gel; (8) laser cut ion-gel; (9) transfer of ion-gel dielectric onto a silicone rubber substrate. (b) Photograph of the sprayed film of SWCNT fluorinated rubber on 4-in. Si wafer. (c) Photograph of the lithographically patterned electrodes of SWCNT fluorinated rubber. (d) Photograph of the transfer process of the electrodes from Si wafer to a silicone rubber substrate. (e, f) Images of the patterned electrodes for integrated circuit. For fine observation, a conducting layer (aluminum, 250 nm) was sputtered on tops of elastomeric film and Si-substrate. (e) Scanning electron microscope image. Scale bar, 100 μm (inset 10 μm). (f) Laser microscope image. The inset shows the surface profile of the patterned area. (g) Optical microscope image of the elastomeric FET (scale bar, 300 μm). The inset shows an image of atomic force microscope of the SWCNT channel on a silicone rubber substrate (scale bar, 200 nm).
robustness and softness of the elastomer, yet incorporate the conducting/semiconducting features of the CNTs to act as electronic components.6−12 Here, we have fabricated elastomeric electronics where the substrate, electrodes and gate dielectric were all made from elastomeric materials with two types of CNTs added to provide the necessary electronic properties. The electronic properties were added, while retaining the elastomeric properties, by incorporating long SWCNTs in the electrodes for conductivity and a thin film of semiconducting SWCNTs for the channel. Our elastomeric field effect transistor (FET) showed mechanical properties similar to clothes, being soft (elasticity: 110%) and robust (tolerance to pressure of >4.0 MPa and impact of >6.26 kg m/s). Our device could still be operated even after exposure to the harshest environments in our daily life, such as being run over by a car, stepped on by high heeled shoes, hammer strikes, and laundering. To date, this level and diversity of robustness have never been reported for any
electronic device. These results represent one step forward toward electronics that are fully compatible with clothes. As shown in Figure 1a,b, we have realized an FET and logic circuits that are completely free from metals, semiconducting crystals, oxidized insulators, and instead assembled solely from elastomeric components composed mainly from hydrocarbon polymers and CNTs. The source, drain, and side-gate electrodes were lithographically patterned SWCNT fluorinated rubber (Daiel-G912, Daikin Industries) embedded in a silicone rubber (poly(dimethylsiloxane), PDMS) substrate (thickness: 1 mm). The channel was a micro inkjet printed thin film (∼3 nm) of semiconducting (>95% purity) e-DIPS SWCNTs.17 The gate dielectric was made from an elastomeric film of block copolymer (poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP) immersed with ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EMIM-TFSI) covering the channel and electrodes.18 The drain current, ID, versus gate voltage of the elastomeric FET demonstrated low 5718
DOI: 10.1021/acs.nanolett.5b01458 Nano Lett. 2015, 15, 5716−5723
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Nano Letters
Figure 3. Mechanical properties of elastomeric FET. (a) Plot of elastic strain limit vs inverse Young’s modulus of metals (blue rectangle), plastics (green triangle), clothes (orange diamond), components of elastomeric FET (red circle), and elastomeric FET itself (red star). (b−g) transfer characteristics (drain voltage, 0.5 V) of elastomeric FET before and after various mechanical stresses: (b) before press (red rectangle) and with compressive stress of 1 MPa (blue circle), 2 MPa (green diamond), 3 MPa (purple triangle), 4 MPa (yellow circle), and 5 MPa (orange circle) pressed as shown in the inset; (c) before impact (red rectangle) and after impact of 6.26 kg m/s (blue circle) impacted as shown in the inset; (d) before strain (red rectangle) and with strain of 10% (blue circle), 50% (green diamond), 100% (purple triangle), and 119% (yellow circle) stretched as shown in the inset; (e) before bend (red rectangle), after bend with the radius of curvature of 10.5 mm (blue circle) and 5.1 mm (green diamond), and folding (purple triangle) as shown in the inset; (f) before twist (red rectangle) and with twisting of 180 deg (blue circle) as shown in the inset; (g) before stretching (red rectangle) and with 50% stretching and releasing cycles of 1 time (blue circle), 10 times (green diamond), 100 times (purple triangle), and 1000 times (yellow circle).
operating gate voltage (
