Biaxial stretchable light-emitting device using kirigami-elastomer structure

Flexible and stretchable electronic devices are becoming increasingly important. These devices can conform to non-planar surfaces and have various applications in healthcare, energy harvesting, and artificial intelligence. Conventional electronic devices are fabricated by laminating and patterning non-stretchable functional materials such as metals and oxides. Consequently, the resulting devices are not stretchable, even though flexible devices could be manufactured by laminating thin-film materials on flexible sheets. Therefore, the stretchability of electronic devices remains a challenge for scientists and engineers. Among the various strategies for fabrication of stretchable devices, material synthesis is one of the primary approaches. For example, the use of polymers has enabled the fabrication of stretchable and functional materials for conductive wires,^1,2 light-emitting devices,³ and energy harvesters.⁴ Nevertheless, there is often a trade-off relationship between stretchability and electronic functions, and a one-by-one investigation of stretchable materials for a wide variety of functions is inevitably time consuming. Another application of stretchable devices is the design of stretchable structures. In contrast to the approach of material synthesis, the approach of structural design could essentially make an entire structure stretchable, and potentially a wide range of electronic devices could be made stretchable. The use of wavy structures is one way to fabricate stretchable devices. By flattening or straightening the wavy structures under stress, fracturing can be avoided. For example, a wavy structure can be obtained by depositing a thin film onto a pre-stretched elastomer.⁵ By releasing the pre-stretch, the thin film periodically buckles out of the plane and forms a wavy structure to compensate for the length mismatch between the thin film and elastomer. Such a wavy structure resists stretching up to pre-stretch. As the buckling of thin films is a ubiquitous phenomenon, stretchable electronic devices can be fabricated by attaching thin-film devices onto elastomers.⁶ However, for the use of wavy structures, complicated fabrication steps, such as handling of thin-film electronic devices and precise control of the adhesion between the thin film and the elastomer, are required. For the easy fabrication of stretchable devices, kirigami, a traditional paper-cutting art, has recently attracted attention for use case scenarios in stretchable electronics. A sheet with periodic cutting patterns can resist stretching by opening its cutting pattern and/or buckling it. Utilizing the stretchability of kirigami sheets, a wide variety of stretchable devices, such as strain sensors,⁷ breath sensors,⁸ batteries,⁹ and bioprobes,¹⁰ have been developed. The use of kirigami is advantageous compared to other methods of fabricating stretchable devices, such as material synthesis and wavy structures. Owing to their flat form, conventional fabrication techniques such as vacuum processing, lithography, and printing can be applied for the deposition, lamination, and patterning of functional materials on kirigami sheets. In addition to their stretchability and compatibility with conventional fabrication techniques, kirigami sheets are also useful for fabrication of three-dimensional structures, as reported previously. When stretched, the kirigami sheets buckle out-of-plane, resulting in the formation of three-dimensional structures. Three-dimensional structures of kirigami are available from micrometer¹¹ to centimeter dimensions^12,13 and are used for a wide range of applications, such as solar tracking,¹⁴ refractive grating,¹⁵ and chirality control.¹⁶ The interaction between liquids and three-dimensional structures of kirigami has also been investigated recently, and the interaction is used for liquid manipulation, such as water harvesting¹⁷ and anisotropic wetting surfaces.¹⁸ 

Motivated by the increasing demand for stretchable electronic devices and the recent progress in kirigami, in this study, we present a new method for fabricating a stretchable device by integrating a kirigami sheet and an elastomer. Using the three-dimensional structure of kirigami, a liquid elastomer can be trapped on a kirigami sheet. The kirigami-elastomer structure can be stretched biaxially, even though the cutting pattern of kirigami is a parallel arrangement of slits, which normally allows only one-dimensional stretching. To present the concept of kirigami–elastomer integration for stretchable devices, a light-emitting elastomer is placed on a kirigami sheet with electrodes patterned on it, and a flexible and biaxially stretchable light-emitting device is fabricated, as shown in Fig. 1. This study reports the fabrication method, fabricated device on activation, and stretchability.

The fabrication steps of the proposed stretchable device are as follows: First, conductive wires and electrodes were patterned on a PET sheet of 14 cm length, 7 cm width, and 25 µm thickness using screen printing. To induce an electric field for the light-emitting elastomer, electrodes were patterned in a comb shape (the gap between electrodes was 300 µm) with a screen printer (NT-15TVA, Neo Techno Japan, Tokyo, Japan) and a screen mask of stainless steel mesh with 15 μm-diameter wire (640 threads per inch). The emulsion layer on the mesh was 10 µm thick. The ink used for screen printing was silver paste (REXALPHA RA FS 059, Toyo Ink Co. Ltd., Tokyo, Japan), which was cured for 30 min at 130 °C after printing. To control the light-emitting area, eight sections of comb electrodes were patterned, as shown in Fig. 2(a). The flatness of the kirigami sheet in the initial state allows for screen printing. This advantage can potentially lead to a large area and mass production. After patterning the electrodes, a cutting pattern parallel to the long direction of the sheet was formed using a laser cutter. It is known that kirigami with a parallel arrangement of slits can randomly buckle upward or downward at the stretch owing to its symmetry.¹⁹ To avoid this random undesired disordering, a groove pattern of ∼10 µm depth was formed with a laser cutter in addition to the cutting pattern [the design of the electrodes, the cutting pattern, and the groove pattern is presented in Fig. 2(b)]. Then, an organic film, Parylene (Parylene C, Specialty Coating Systems, Indianapolis, USA), ∼2 µm thick, was coated onto the entire surface of the sheet via chemical vapor deposition (PDS 2010, Specialty Coating Systems, Indianapolis, USA). Parylene is an insulating material that ensures electrical insulation between the electrodes. The sheet was then stretched by 50% in the length direction. For the groove pattern, by applying the stretch, the sheet buckled downward around the groove pattern and formed concave saucers at the stretch [Fig. 2(c)]. Consequently, two pairs of electrodes were situated on each saucer (the relationship between the electrode pattern and light emission is discussed later in this paper). Subsequently, a light-emitting elastomer was prepared by mixing polydimethylsiloxane (PDMS) and inorganic electroluminescent (EL) powder (728 green electroluminescent phosphor, OSRAM SYLVANIA, Pennsylvania, USA), which emits light by applying an alternating electric field. PDMS (the weight ratio of the base to catalyst is 5:1) was prepared using a commercially available elastomer kit (Sylgard 184, Dow Corning Corp., Michigan, USA). The prepared PDMS was mixed with inorganic EL powder (the weight ratio of the PDMS to the powder was 6:1) using a centrifugal mixer (Thinky mixer, THINKY Inc., California, USA). After stretching, the concave saucers on the sheet were filled with the prepared light-emitting elastomer using a syringe. Generally, to minimize the surface free energy, liquid droplets tend to unify and form one large droplet rather than the average size of the droplets. However, in contrast to the ordinary cases, each droplet of the light-emitting elastomer on the concave saucers is in contact, and the volume of the light-emitting elastomer on the concave saucers is averaged. After placing the droplets while keeping the stretch, the kirigami sheet with the liquid light-emitting elastomer was cured at 75 °C for 2 h to solidify the elastomer. For solidification of the elastomer, the length of the kirigami sheet remains elongated. The magnified images of the fabricated device are shown in Fig. 2(d). As explained later in this paper, the resulting structure is biaxially stretchable, even though the cutting pattern is a parallel arrangement of slits.

The light emission of the fabricated device was examined, as shown in Fig. 3. In this experiment, an alternating current voltage of 800 V at 40 kHz was applied. By applying a voltage, the fabricated device emits light both on a flat plane [Fig. 3(a)] and on a cylinder with a diameter of 5 cm [Fig. 3(b)]. As eight sections of the comb electrodes are patterned, the light-emitting sections can be controlled by selecting the voltage-applied electrodes (the movies are available in the supplementary material). As shown in the movie, voltage-applied electrodes were switched every 0.5 s using relays controlled by a microcontroller unit (RL78/G13, Renesas Electronics Corp., Tokyo, Japan). The light-emitting elastomer emitted light at 0.1 cd/m. In the literature,²⁰ the luminosity is increased by applying a higher voltage and increasing the weight ratio of the inorganic EL powder to PDMS. However, the high density of the inorganic EL powder also increases the viscosity of the light-emitting elastomer, and there is a trade-off relationship between the luminosity and ease of handling of elastomer droplets. To investigate another method of increasing the luminosity, the electric field between the electrodes was estimated using finite element method simulation (COMSOL Multiphysics, COMSOL Inc., Massachusetts, USA) in a simplified two-dimensional model. For comparison, cases with a single pair, two pairs, or four pairs of electrodes were simulated. The simulation results for the energy density of the electric field are shown in Fig. 3(c) (the unit of the energy density is J/m³). The high energy density area was situated at the gaps between the electrodes, and a larger number of electrode pairs led to a broader high energy density area. From a practical viewpoint, the design of the electrodes is determined by the precision of the patterning and cutting processes. Because of the cutting pattern, the space for wiring was limited, and two pairs of electrodes on a saucer were patterned in our case. As mentioned above, metal deposition (i.e., vacuum vapor deposition and sputtering) and lithography, which enable comb electrodes with narrow spacing, are also applicable for electrode patterning. Depending on the desired precision and dimensions of the devices, the fabrication method can be further tuned.

The stretchability of the kirigami sheet and elastomer enables the resulting structure to be stretchable. To investigate the stretchability of the kirigami-elastomer structure, strain–stress (s–s) curves were measured in both the length [Fig. 4(a)] and width [Fig. 4(b)] directions with a tensile tester (AGS-X, Shimadzu Corp., Kyoto, Japan). For simplicity, a kirigami-elastomer structure composed of an electrode-patterned kirigami sheet and a light-emitting elastomer was cut into a strip for the measurement of s–s curves. In both the measurements, the strip resists ∼7% and 12% stretch, respectively, as presented in Figs. 4(a) and 4(b). In the case of measurements in the length direction, the openings are enlarged with the stretch. The elastomer was delaminated from the kirigami sheet at ∼7% stretch, and the kirigami sheet was fractured at ∼14% stretch. For the measurement in the width direction, the openings were closed with the stretch. The elastomer was delaminated from the sheet at ∼12% stretch and fractured at ∼22% stretch. The transitions of the strips under stretching are shown in the insets of Figs. 4(a) and 4(b). This result implies that the improvement in the adhesion between the elastomer and the sheet could enhance the stretchability of the fabricated device because the elastomer and the kirigami sheet themselves are stretchable at a magnitude of 50%. Nonetheless, biaxial stretchability permits the fabricated device to be attached to a three-dimensional structure, as shown in Fig. 4(c). Through the experiment, it was verified that the kirigami-elastomer structure can be stretched biaxially and is attachable onto three-dimensional surfaces. In general, biaxial stretchability is achieved using only a two-dimensional cutting pattern on a sheet.²¹ However, even though the cutting pattern of the kirigami sheet is a parallel arrangement of the slits, biaxial stretchability is achieved with the kirigami–elastomer structure. Biaxial stretchability is induced by pre-stretching the kirigami sheet during integration with the elastomer. With the pre-stretch, the kirigami sheet shrinks in the width direction and elongates in the length direction. As a result, shrinkage from the initial state enables stretchability in the width direction. In general, for biaxial stretchability, two-dimensional cutting patterns are required. However, such complicated cutting patterns have a drawback in electrode patterning. In addition, the formation of three-dimensional structure from a kirigami sheet with a two-dimensional cutting pattern is an ongoing issue. In our case, for electrode patterning and the simplicity of the formation of a three-dimensional structure, a parallel arrangement of slits was used as the cutting pattern.

In this study, we demonstrate a biaxially stretchable light-emitting device composed of a kirigami sheet and an elastomer. Owing to the flatness of the kirigami sheet in the initial state, conventional fabrication methods for a flat plane (i.e., vacuum process, lithography, and printing techniques) are still applicable. In our study, the electrodes were patterned on a kirigami sheet using screen printing. Compatibility with the conventional fabrication method is advantageous, while advanced fabrication processes such as patterning on tissue, precise stamping transfer, and use of liquid metal are often required for the fabrication of stretchable devices. For the fabrication of the device, the formation of a three-dimensional structure of the kirigami sheet was used to place the elastomer. The fabrication steps would help future progress in the study of the interaction between kirigami and liquids. Our device is composed of a light-emitting elastomer and a kirigami sheet. With conventional fabrication methods, such as vacuum processing and printed electronics techniques, another function can be integrated on a kirigami sheet. Biaxial stretchability and compatibility with conventional fabrication methods enable the fabrication of a wide range of stretchable devices.

See the supplementary material for the movies of the light-emitting device on activation.

This research was partially supported by the JSPS KAKENHI, Grant No. 16K18088. The authors are grateful to Toshifumi Satoh for fruitful discussions and the preparation of inorganic EL phosphor powder.