Printable elastic conductors with a high conductivity for electronic textile applications

Abstract

The development of advanced flexible large-area electronics such as flexible displays and sensors will thrive on engineered functional ink formulations for printed electronics where the spontaneous arrangement of molecules aids the printing processes. Here we report a printable elastic conductor with a high initial conductivity of 738 S cm(-1) and a record high conductivity of 182 S cm(-1) when stretched to 215% strain. The elastic conductor ink is comprised of Ag flakes, a fluorine rubber and a fluorine surfactant. The fluorine surfactant constitutes a key component which directs the formation of surface-localized conductive networks in the printed elastic conductor, leading to a high conductivity and stretchability. We demonstrate the feasibility of our inks by fabricating a stretchable organic transistor active matrix on a rubbery stretchability-gradient substrate with unimpaired functionality when stretched to 110%, and a wearable electromyogram sensor printed onto a textile garment.

Figure 1. Highly stretchable elastic conductors.

(a) Fabrication process of elastic conductor ink. Upper picture, elastic conductor ink. Scale bar, 10 mm. Lower picture, printed elastic conductor with high resolution. Scale bar, 100 μm. (b) Printed elastic conductor and demonstration of the stretchability. Scale bar, 10 mm. (c) Conductivity dependence on tensile strain of printed elastic conductor with and without surfactant. The maximum stretchability of elastic conductor with surfactant is limited by the strain limit of the substrate. (d) A comparison of this work to recent work in elastic conductors. Data points are extracted from the following papers: light blue filled triangle, Ag nanowires (Ag NW)—the study by Xu and Zhu (calculated from resistance change under the assumption that the total volume does not change); orange open square, Au nanoparticles (Au NP)—the study by Kim et al.; blue open triangle, Ag nanoparticles (Ag NP)—the study by Park et al.; purple open circle, multi walled carbon nanotubes (MWCNT)—the study by Chun et al.; black filled square, single walled carbon nanotubes (SWCNT)—the study by Sekitani et al.; light purple filled diamond, polyaniline (PANI)—the study by Stoyanov et al.; red filled circle, this study (corresponds to c). (e) Initial conductivity and stretchability dependence on surfactant content. The weight ratio of Ag flakes, fluorine rubber and 4-methyl-2-pentanone was fixed at 3:1:2 (volume fraction, 1:1.94:8.74). Red circles, initial conductivity; blue squares, stretchability. (f) Initial conductivity and stretchability dependence on the Ag flakes content. The weight ratio of fluorine rubber, 4-methyl-2-pentanone and surfactant solution was fixed to 1:2:1 (volume fraction, 1:4.5:1.64). Error bars in e,f represent standard error.

Figure 2. Elastic conductor self-assembly by phase separation.

(a–f) Addition of surfactant/water solution results in a phase-separated morphology consisting of an elastic core topped by an Ag-dense surface layer. Optical microscopy and SEM images. In SEM images, bright areas correspond to Ag-rich phases, and the dark areas to non-conductive elastomeric regions. Upper row (a–c) without surfactant. Lower row (d–f) with surfactant. Left column (a,d) optical micrographs. Scale bars, 200 μm. Middle column (b,e) top-surface SEM images. Scale bars, 10 μm. Right column (c,f) cross-sectional SEM image. Scale bars, 10 μm. (g–l) Cross-sectional ToF-SIMS images. Upper row (g–i) without surfactant. Lower row (j–l) with surfactant. Left column (g,j) optical micrographs corresponding the ToF-SIMS images. Middle column (h,k) ToF-SIMS images of Ag. Right column (i,l) ToF-SIMS images of surfactant. Scale bars, 200 μm. Addition of surfactant reduces Ag signal as the surfactant binds to the Ag surface and is responsible for increasing the affinity between Ag flakes and fluorine rubber matrix. (m–o) Top-surface SEM images of stretched elastic conductors. (m–o) are elastic conductors stretched by 0%, 100% and 200%, respectively. Scale bars, 20 μm.

Figure 3. Soft and stretchable organic thin-film-transistor active matrix.

(a) Illustrations of active matrix. Upper left, whole structure. Upper right, structure of an OTFT cell on a rigid island with elastic conductor wiring. Lower, schematic structure of an OTFT cell on the rigid island, the light green box corresponds to the entire OTFT device. (b) Pictures of OTFT-active matrix. Left, 12 × 12 active matrix. Scale bar, 10 mm. Centre, magnified OTFT cells. Scale bar, 5 mm; Right, optical micrograph of the embedded OTFT. Scale bar, 500 μm. (c) Active matrix is relaxed (left) and stretched to about 60% (right). Scale bars, 20 mm.

Figure 4. Stretchable organic transistors with stretchability-gradient substrate.

(a) Device structure of a stretchable single transistor. Top, schematic from top surface. Centre, schematic from side, the green box corresponds to an entire OTFT device. Bottom, device picture. Scale bar, 5 mm. (b) Robustness evaluation with different concentrations of curing agent in PDMS 1. (PDMS 1 is patterned on stiff polyimide during the fabrication process of this substrate). Red circles, normalized strain-to-failure; blue squares, normalized failure stress. Error bars represent s.e. (c) Single stretchable transistor mobility dependence on tensile strain. Red circles represent normalized mobility. Scale bars, 3 cm. The inset shows the transistor relaxed (upper) and stretched (100%, lower). (d) Transfer characteristics of a relaxed and stretched single organic transistor corresponding to (c,e) mobility dependence on tensile strain of four transistors in a 2 × 2 stretchable active matrix. Scale bars, 1 cm. The inset shows the device in relaxed (upper) and stretched (110%, lower) configurations. (f) Transfer characteristics of one transistor in a stretchable active matrix corresponding to e.

Figure 5. Measurement of arm EMG signals with an elastic conductor electronic textile.

(a) Pictures of the EMG measurement system, using elastic conductor. Scale bars, 25 mm. (b) Organic amplifier circuit. Above, picture of organic amplifier circuit. Scale bar, 1 mm. Below, circuit diagram. (c) Performance of the organic pseudo-complementary metal–oxide–semiconductor inverter. Above, V_OUT-V_IN curve. Below, inverter gain. (d) Frequency dependence of the amplifier gain. (e) EMG signals obtained through the elastic conductor attached to a forearm while opening (I) and closing a hand (II). Upper black line is the unamplified signal and lower red line is the signal amplified with the organic amplifier. Signals from muscle activity are observed when the hand is closed.