Three-dimensional out-of-plane geometric engineering of thin films for stretchable electronics: a brief review

Thin Solid Films 688 (2019) 137435

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Three-dimensional out-of-plane geometric engineering of thin films for stretchable electronics: a brief review Dong-Bin Moona, Jaedeuk Leea, Eun Rohb, Nae-Eung Leea,b,c,

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a

School of Advanced Materials Science & Engineering, Sungkyunkwan University, 2006 Seobu-ro, Jangan-gu, Suwan, Gyeonggi-do 16419, Republic of Korea SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Republic of Korea c Samsung Advanced Institute for Health Sciences & Technology (SAIHST), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: Stretchable electronics Stretchability Structural engineering Stretchable substrate

Recent progress in engineering approaches for stretchable electronics and electronic components, including strategies focused on materials science or structural engineering, offer high signal-to-noise detection of vital signs via systems that provide conformal, noninvasive contact to curvilinear skin and are unobtrusive during human activity, such as general motion, exercise, and respiration. Structural engineering strategies with flexible thin films, whose deformation can be categorized as two-dimensional (2D) in-plane or three-dimensional (3D) out-of-plane, provide a release of stress created by stretching, bending, or twisting. Beyond 2D in-plane structural engineering techniques, 3D out-of-plane structural engineering techniques effectively distribute nonlinear and multidirectional 3D strain. Here, we review recent advances in 3D out-of-plane engineering techniques, including wavy and wrinkled structures, pop-up structures, kirigami and origami structures, and nature-inspired structures, and describe the strain distribution mechanisms, fabrication processes, applications, and characteristics of these approaches. We conclude with perspectives on applications of stretchable electronic devices with multidirectional stretchability and the existing challenges for future research.

1. Introduction Wearable smart electronics are expected to play an important role in collecting and handling massive and diverse data from humans and their environment in the era of the Internet of Things (IoT) and artificial intelligence (AI). The extension of wearable smart electronics from accessory types to stretchable platforms has been fulfilled through the engineering of materials and their structures. Therefore, mechanical stretchability at the level of components and systems will increase the potential for future smart healthcare and human monitoring applications because stretchable electronics are well-matched with skin-attachable and implantable systems [1,2]. These devices could measure physical factors (strain, pressure, temperature, etc.) and physiological signals (electroencephalography (EEG), electrocardiogram (ECG), electromyography (EMG), mechanomyogram (MMG), electrooculography (EOG), etc.). Additionally, stretchable electronic systems can be used in a much larger variety of situations than devices based on a fixed form factor. However, the current state-of-the-art commercialized wearable accessory-type devices such as smart watches, glasses, or

bands are difficult to be stretched or conformably attached to curvilinear surfaces, such as human skin. An air gap existing between a device and human skin causes signal noise and interruption during physiological and biological monitoring. In addition, a lack of stretchability makes the layers in the devices unstable and fatigued after repetitive strain during bodily motion, and eventually the device fails [1–5]. In particular, nonlinear and multidirectional three-dimensional (3D) stretching, twisting, and bending of these devices on human skin induces strain in the devices, which comprise passive and active thin films. Therefore, engineering of thin films for stretchability is required to secure the stability and durability of the thin films. Numerous studies to develop engineering approaches for stretchable electronics and electronic components have been carried out so far, which can be mainly categorized into two types of approaches: materials science and structural engineering. Advances in material strategies for improvement of intrinsic stretchability include liquid metals (Hg, EGaIn, gallium-based alloys, etc.) [6–9], conducting polymers (PEDOT:PSS (poly(3,4-

⁎ Corresponding author at: School of Advanced Materials Science & Engineering, Sungkyunkwan University, 2006 Seobu-ro, Jangan-gu, Suwan, Gyeonggi-do 16419, Republic of Korea. E-mail address: [email protected] (N.-E. Lee).

https://doi.org/10.1016/j.tsf.2019.137435 Received 29 April 2019; Received in revised form 17 July 2019; Accepted 19 July 2019 Available online 29 July 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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of the wavy Ag electrode was negligible. The Ag electrode on a wavy substrate showed durability over 600 stretching cycles of 20% strain. Ultra-flexible and stretchable transistors were fabricated onto prestretched elastomers. Kaltenbrunner et al. demonstrated an ultrathin, light-weight (3 g m −2), ultra-flexible, virtually unbreakable, and imperceptible transistor that can be stretched up to 233% by simply dropping the electronic film on a pre-stretched elastomer (Fig. 1(b)) [68]. After releasing the elastomer, the electronic film formed out-ofplane wrinkles.

ethylenedioxythiophene) polystyrene sulfonate), polyaniline, polypyrrole, etc.) [10–19], stretchable hydrogels [20–24], and nanocomposites based on elastomeric polymers (PDMS (polydimethylsiloxane), Ecoflex, polyurethane, etc.) embedded with conductive nanofillers (carbon materials such as graphene, carbon nanotubes, and carbon black, and metal materials such as metallic flakes, metal nanoparticles, and metal nanowires) [14,25–40] and nanofibrils of semiconducting fillers [41–43]. In this regard, intrinsically stretchable conductive and semiconducting nanocomposites consist of percolation networks of distributed nanoscale conductive and semiconducting fillers in an elastomeric medium [44]. Studies of structural design strategies of flexible thin films for mechanical stretchability, on the other hand, can be sorted as two-dimensional (2D) in-plane structural engineering designs (serpentine, coil-like shapes, nanoribbons, etc.) [45–55] and three-dimensional (3D) out-of-plane structural engineering designs (wavy and wrinkled structures, helical structures, Kirgami and origami designs, nature-inspired structures, etc.) [56–95]. Adopting the engineering of 2D in-plane or 3D out-of-plane structures can minimize the strain applied by stretching, bending, or twisting. However, 2D in-plane structural design has challenges in preventing the occurrence of nonlinear and multidirectional straining not only in inplane direction but also in out-of-plane direction at large stretching. Beyond in-plane structural engineering techniques, therefore, out-ofplane structural engineering techniques have been studied to effectively distribute the applied strain from multidirectional axes by 3D geometrical deformation. Herein, we review recent progress toward the 3D out-of-plane engineering techniques for stretchable electronics in applications such as displays, human healthcare monitoring devices, human-machine interfaces, communications, and wireless networks. This review covers the mechanism of strain distribution using 3D out-of-plane structural designs, fabrication processes, applications to stretchable electronics, and perspectives on this engineering approach.

2.2. Pop-up structure Wavy and wrinkled structures are improved by bonding the mesh sheet to PDMS only at nodes (Fig. 2(a)) [69]. As compressive and tensile strains are applied to the structure, the unbonded part can move freely out of plane to accomplish 5–10 times higher stretchability than the wavy and wrinkled patterns. The pop-up structure consists of an island part in which the device is placed and a bridge part which plays a role as an interconnection and gives stretchability. The bridge part absorbs the stress applied on the island part by being flat like a wavy or wrinkled structure when it is stretched. To attach the active islands selectively, a thin adhesion layer was deposited onto biaxially pre-stretched PDMS. After treating both the substrate and the film with ozone, the film was aligned on top of the elastomer [69]. Releasing the pre-stretched PDMS led to a pop-up film structure. The pop-up interconnects could be straight [70] or serpentine lines [69]. By controlling the design of the bridge and the pre-strain of the substrate, a pop-up structure could become a helical structure [71]. To protect the pop-up bridge, an encapsulating layer of PDMS was applied. However, the encapsulation layer degraded the stretchability and conductivity. The strategy of encapsulation has involved softer elastomer encapsulants [72–74], liquid encapsulation [75,76], and pre-stretched encapsulation [77]. Jang at el. fabricated a 3D network designed ECG sensor that could be stretched 70% and 109–111% radially and uniaxially, respectively (Fig. 2(b)) [78]. An adhesion layer was selectively deposited on the elastomer substrate and a 2D serpentine network was transferred to the prestretched substrate. After releasing the substrate, the 2D serpentine network became a 3D helical coil structure. Nan at el. applied a pop-up structure as a stretchable thermoelectric coil for an energy harvester that could be biaxially stretched 60% with a strategy similar to that of Jang et al.'s [79].

2. Out-of-plane engineering techniques 2.1. Wavy and wrinkled structures Wavy and wrinkled forms can be produced due to differences in Young's moduli between a film and a substrate [56–59]. This structure involves deposition or transfer of thin layers on a uniaxially or biaxially pre-stretched substrate [60–64]. Releasing the pre-stretched substrate creates a compressive stress in the film, leading to buckling and structural corrugation into an out-of-plane wavy or wrinkled form (Fig. 1(a)) [65]. After coating of the films, the length of the substrate with a wavy structure becomes shorter than that of the initially prestretched substrate. To avoid global buckling, the initially stretched length should be less than a critical length. In the range where the prestretching strain is less than a critical point, strain-stress relation of stretchable substrate with metal film is similar to that of the stretchable substrate under stretching and then wavy structure is formed. In the range where the pre-stretching strain is higher than a critical point, however, the strain-stress relation of the substrate with metal film becomes different from that of the stretchable substrate and then global buckling occurs. Increasing the strain on a substrate with a wrinkled film structure causes the surface to become flat and have a high tangent modulus with an extremely sharp transition point, thereby resulting in stiffness. Below the transition point, the wrinkled film is mechanically stretchable. Metal interconnects with wavy or wrinkled structures often have been formed using this pre-stretching and releasing method. For example, Görrn et al. demonstrated the design of stretchable circuits combined with pre-stretched gold interconnects [66]. It showed a stretchability of 50% strain in the uniaxial direction and 40% strain in the biaxial direction with electrical stability. Ko at el. fabricated a semitransparent, stretchable Ag film by depositing Ag on a wavy-patterned PDMS substrate [67]. At 30% elongation, the change in sheet resistance

2.3. Kirigami and origami structures Kirigami and origami structures are ancient paper craft techniques to form 3D sculptures from a 2D planar structure, and can be easily restored to their initial 2D form. While origami involves folding of a single sheet of paper, kirigami involves cutting and folding. By placing cuts in a 2D planar structure, the paths of cutting are protruded when folded or stretched (Fig. 3(a)) [80]. A simple kirigami structure arranges straight lines at regular intervals to impart stretchability to the material. The edge of pattern moves in out-of-plane direction and is twisted during stretching of kirgami structure [81]. Deformations by kirigami patterns progresses through three stages. First, the kirigami structure deforms into in-plane buckling under a small amount of strain. Next, as the applied strain is increased, out-of-plane buckling starts to occur at the edge of the cuts. Finally, as the structure gets straightened while narrowing the gap of cuts, the structure stiffens until it breaks. As kirigami and origami techniques have a limitless variety of patterns with variable size scales, even in the micro- and nano-meter scale, they can be easily applied from traditional materials such as paper [82] or fabric [83] to advanced materials including graphene [81] or carbon nanotubes [84] and also have potential in several applications including cell/tissue engineering, energy devices, solar panels, 3D inductors, and healthcare devices [85–88]. Various design patterns could be chosen depending on purpose, such as twistable, 2

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Fig. 1. (a) Wavy and wrinkled structures for stretchable electronics [65] and (b) transparent plastic electronics based on a wavy and wrinkled structure [68]. (Copyright 2007 American Chemical Society [65]. Copyright 2013 Springer Nature [68].)

Fig. 2. (a) Pop-up structure for stretchable electronics [69] and (b) an electrocardiography sensor based on a pop-up structure [78]. (Copyright 2008 National Academy of Sciences [69]. Copyright 2017 Springer Nature Publishing AG [78].)

bendable, flexible, and stretchable designs. For example, Noy et al. proved that origami can achieve + 90o and -90o folding angles for different hinge lengths with rigid materials (Cu, Cr). Jang et al. showed a wearable thermotherapy application with a simple, computer-controlled kirigami process (Fig. 3(b)). The wearable device could be stretched up to 400% and was durable at 300% strain without electrical failure [89].

substrates. Guo et al. proposed elastomeric petals (E-petals) directly replicated from natural rose petals [91]. Using natural rose petals as a mold, the surface of the rose petals was directly patterned on the surface of an elastomeric PDMS substrate (Fig. 4(a)). The surface structure of the Epetal substrate was made of numerous microcraters that allowed the film on the substrate to be stretched with only small cracks (Fig. 4(b)). The electroless deposition of a thin film of Cu on the E-petal substrate showed uni-directional stretchability at up to 90% strain and durability over 1000 cycles of stretching with 40% strain. Additionally, the E-petal substrate showed the possibility of omni-directional stretching over 1000 cycles of stretching with 13% strain. They also fabricated a strain sensor on an E-petal substrate using inkjet printing and spin-coating. The strain sensor, made with an Ag/Cu electrode and reduced graphene oxide, showed durability over 1000 cycles of stretching with 20% strain. Lee et al. proposed a stretchable elastomeric substrate inspired by a mogul run on a ski slope, which has a repetitive pattern of bumps and valleys [90]. The mogul-patterned elastomeric substrate was a three-

2.4. Nature-inspired structures Nature-inspired structures are artificially created structures inspired by shapes or characteristics of structures found in nature. 3D out-ofplane engineering designs of elastomeric substrates inspired from a variety of natural structures have been made by molding a substrate directly from a natural object (for example, a rose petal) or periodically arranging a specific shape (for example, mogul patterns) [90]. The substrate with 3D out-of-plane patterns can enable the stress in the thin films formed on it to be relieved in a stretched condition. We examine the rose-petal and mogul patterns as two examples of 3D engineered 3

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Fig. 3. (a) Kirigami and origami structures for stretchable electronics [80] and (b) stretchable heaters based on a kirigami structure [89]. (Copyright 2015 Springer Nature [80]. Copyright 2017 American Chemical Society [89].)

was confirmed that various materials, such as metals, polymers, networked 2D nanomaterials, fibers, and nanorods, could be stably stretched on the mogul-patterned substrate and, therefore, a range of devices, including chemical sensors [90], photodetectors [93], piezoelectric nanogenerators [94], and pressure sensors [95], could be fabricated with minimal change in their electrical properties caused by stretching. Resistive chemical sensors fabricated with a deposited Cr/ Au electrode and drop-casted reduced graphene oxide on a mogul PDMS substrate showed a stable sensing performance to NO2 gas when under 30% strain, with response characteristics similar to that of the device under no strain [90]. This chemical sensor exhibited stable performance even after 2000 cycles at 30% stretching. An omni-directionally stretchable photodetector fabricated by depositing a Cr/Au electrode, spin-coating (PEDOT:PSS), and using the hydrothermal

dimensionally patterned substrate for imparting mechanical stretchability to thin films placed on the substrate [90,92]. Bumps and valleys on the mogul-patterned substrate were regularly arranged in a hexagonal closed packed structure for multidirectional stretchability (Fig. 4(c)), allowing stretchable skin-patch devices to follow the deformation of human skin during bodily motions. The cross section of this substrate showed a wavy form (Fig. 4(c)). As shown in (Fig. 4(c)), the mogul-patterned substrate effectively absorbed an applied stress in the thin film by straightening out the wavy form across the bumps and valleys, which minimized damage to the films as well as variations in the performance of electronic devices. In addition to the multidirectionality of the substrate, the mogul-patterned substrate enabled thin layers formed directly on the substrate using conventional thin film processes including evaporation, sputtering, and spin coating. Also, it

Fig. 4. Nature-inspired three-dimensional out-of-plane structural engineering designs for stretchable electronics. (a, b) Natural rose petal mold and surface SEM image of e-petals molded from rose petal [91] (Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA.) and (c) Mogul-patterned structure [90]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4

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Fig. 5. Applications using the mogul structure. (a) Omni-directionally stretchable photodetector [93]; (b) Omni-directionally stretchable piezoelectric nanogenerator [94], and (c) Omni-directionally stretchable piezoresistive pressure sensor [95].

developing a variety of devices that can be conformally attached on human skin. However, there are still challenges that need to be solved for applying these approaches to fully stretchable electronic devices in which all components are omni-directional stretchability. First, the durability problem of thin films on 3D out-of-plane engineered substrates that experience more than the expected number of repetitive deformations remains a major problem due to mechanical mismatch and poor adhesion between dissimilar materials. Therefore, more compliant materials and adhesion control between layers are required for the development of practical stretchable electronic devices. In recent years, research has combined intrinsically stretchable materials with structural design strategies [94,95] and 2D in-plane geometric engineering with 3D out-of-plane geometric engineering [96] to solve some of the issues that still exist for the development of stretchable electronic devices. Further reduction of the stress subjected to thin films on 3D engineered elastomeric substrates during bodily motion is required to mitigate signal interference created by the motion. Second, the limitation in the omni-directional stretching of electronic devices is also an issue. Although some successful examples [91,93–95] of omni-directional stretchability have been reported, most of the works were limited to uniaxial or biaxial stretchability or occupied a large area to achieve multiaxial stretchability. Third, conformal contact between the devices and organisms is an important issue for high signal-to-noise ratio. Although much research has been done on this issue [97–99], this issue can be more critical for the 3D out-of-plane geometric engineering approach due to the structural limitations in which the device thickness can become thicker. Therefore, further research is needed about conformal contact of 3D-engineered stretchable electronic devices to skin for practical skin-attachable electronics. Finally, fabrication processes need to be simplified for practical applications of 3D engineered substrates due to their complicated substrate structure or required transfer processes. Although successful examples of solution and deposition processes on various 3D out-of-plane engineered substrates have been

growth of ZnO nanorods on a mogul-patterned PDMS substrate (Fig. 5(a)) showed a similar response to UV light at an omni-directional stretching condition of 30% strain and high mechanical durability after 15,000 cycles of stretching at 30% strain. These results suggest that this device could be attached to a human wrist to detect UV light continuously even during wrist movement [93]. An omni-directionally stretchable piezoelectric nanogenerator fabricated by spin-coating a graphite electrode and placing multi-layered, electrospun nanofibers of a barium titanate nanoparticle/polyurethane (BT NPs-PU) nanocomposite (Fig. 5(b)) showed high stretchability with 40% strain and high mechanical durability after 9000 stretching cycles stretching with 30% strain [94]. An omni-directionally stretchable piezoresistive pressure sensor fabricated by spin-coating a composite of PEDOT:PSS and single-walled carbon nanotubes and a spin-coated Ag paste electrode (Fig. 5(c)) showed almost the same responses to dynamic and static pressure in the unstretched condition and in an omni-directionally stretched condition with 30% strain. This pressure sensor also had high mechanical durability after 2000 cycles of stretching with 30% strain [95]. These results indicate that the mogul pattern on the elastomeric substrate contributed to minimal signal interference of a stretchable piezoresistive pressure sensor under static or dynamic stretching conditions. 3. Conclusion and perspectives This review focused on methods of imparting mechanical stretchability to non-stretchable but flexible thin layers formed on an elastomeric substrate with a 3D out-of-plane structure. Use of various 3D outof-plane engineering techniques, such as wavy and wrinkled structures, kirigami and origami structures, and nature-inspired structures, enables the materials and processing technologies for rigid devices to be applied to stretchable electronic devices. Unlike 2D in-plane structural engineering, examples of multidirectional stretchability in in-plane directions beyond unidirectional stretching indicate the possibility of 5

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reported [56–95], it is still difficult to perform nano-patterning and form uniform thin films compared to doing so on a planar rigid substrate. The difficulty causes a relatively poor performance of the electronic device. Continuing research on 3D out-of-plane engineering of substrates is necessary for high performance, high efficiency, miniaturization, and high-density integration in future stretchable electronics. These various and exciting research possibilities present a bright future for this area.

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Acknowledgment

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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2019R1A6A1A03033215) and by the GRRC program of Gyeonggi province (GRRC Sungkyunkwan 2017-B04, Development of Wearable Chemical Sensor).

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