Deformable devices with integrated functional nanomaterials for wearable electronics
- Jaemin Kim†1, 2,
- Jongsu Lee†1, 2,
- Donghee Son1, 2,
- Moon Kee Choi1, 2 and
- Dae-Hyeong Kim1, 2Email author
© Kim et al. 2016
Received: 6 August 2015
Accepted: 4 November 2015
Published: 15 March 2016
As the market and related industry for wearable electronics dramatically expands, there are continuous and strong demands for flexible and stretchable devices to be seamlessly integrated with soft and curvilinear human skin or clothes. However, the mechanical mismatch between the rigid conventional electronics and the soft human body causes many problems. Therefore, various prospective nanomaterials that possess a much lower flexural rigidity than their bulk counterparts have rapidly established themselves as promising electronic materials replacing rigid silicon and/or compound semiconductors in next-generation wearable devices. Many hybrid structures of multiple nanomaterials have been also developed to pursue both high performance and multifunctionality. Here, we provide an overview of state-of-the-art wearable devices based on one- or two-dimensional nanomaterials (e.g., carbon nanotubes, graphene, single-crystal silicon and oxide nanomembranes, organic nanomaterials and their hybrids) in combination with zero-dimensional functional nanomaterials (e.g., metal/oxide nanoparticles and quantum dots). Starting from an introduction of materials strategies, we describe device designs and the roles of individual ones in integrated systems. Detailed application examples of wearable sensors/actuators, memories, energy devices, and displays are also presented.
KeywordsSilicon nanomembrane Functional nanomaterials Flexible electronics Stretchable electronics Wearable electronics
In the rapid technology development of low-power silicon electronics, light-emitting diodes (LEDs) fabricated on unconventionally shaped substrates, high-capacity lithium-ion batteries, and various wearable electronic devices such as smart glasses, watches, and lenses have been unveiled both in academic journals and on the market. In spite of their superb performance, wearable form factors, and compact sizes, challenges remain mainly owing to their large thickness and mechanical rigidity, which result in a mechanical mismatch between the device and the skin, and thereby discomfort, a low signal-to-noise ratio, and measurement errors . In this regard, achieving mechanical deformability of the wearable electronic/optoelectronic devices while maintaining high performances has been a major research goal [2–6].
One promising strategy is to replace the rigid electronic materials (e.g., silicon wafer) with flexible nanomaterials (e.g., silicon nanomembrane (SiNM) [7–11], carbon nanotubes (CNTs) [12–14], graphene (GP) [1, 15, 16], and organic nanomaterials [17, 18]). The electronic properties of the SiNM (down to tens of nanometers) remain the same as the bulk silicon wafer , but its bendability dramatically increases owing to the reduced thickness . SiNM-based devices outperform their competitors including low-temperature polycrystalline silicon (LTPS) and organic devices by virtue of their high electron mobility . However, SiNM based device might have issues in the high cost and complicated fabrication processes. Meanwhile, carbon nanomaterials (e.g., CNTs and GP) [21, 22] have been getting attentions as next-generation semiconducting nanomaterials. The mobility of single-walled CNTs (SWNTs) and exfoliated GP have been reported up to 100,000  and 230,000 cm2 V−1 s−1 , respectively, which are higher than that of single-crystal silicon. Their ultrathin thickness enables them to be seamlessly integrated in wearable devices while achieving the transparency [23–25]. The mass production, device performance, and fabrication processes of these carbon nanomaterials, however, have many remaining challenges for commercial device applications . Organic nanomaterials such as organic nanowires/nanofibers also have recently utilized as electric materials for fabricating complementary metal–oxide–semiconductor (CMOS) circuits  and wearable power generators [28, 29]. Intrinsic deformability of organic nanomaterials, solution processability, and low cost make them promising for wearable devices . However, their low electrical performances should be resolved for its widespread applications .
2.1 Wearable sensors/actuators
Wearable sensors/actuators have recently attracted considerable interest because of their mobile healthcare  and virtual reality applications . Sensors/actuators worn on the body, in particular, have drawn attention for the continuous and accurate monitoring of physiological (e.g., motion [1, 47] and temperature [56, 57]) and electrophysiological (e.g., electrocardiograms [58, 59] and electromyograms [60, 61]) signals and appropriate instant feedback on them , which are important for point-of-care medical diagnostics and therapy. This section describes representative wearable sensors/actuators based on functional nanomaterials and their application examples in healthcare and human–machine interfaces.
2.1.1 SiNM-based sensors
Deformability, which is one of the key characteristics of wearable electronics, can be achieved by making inorganic materials (i.e., silicon) as thin as possible, down to the nanometer scale (i.e., nanomembrane) . SiNM can be fabricated through several processes. One easy fabrication method is to remove the buried oxide of a silicon-on-insulator (SOI) wafer and pick the top part up or to etch the bottom silicon of the SOI wafer and use the remaining top part . The obtained SiNM can be located in the desired position of the designed layout by using previously reported transfer printing techniques. SiNM maintains the high carrier mobility  and intrinsic piezoresistivity  of the bulk monocrystalline silicon, while having a high flexibility, which enables diverse wearable electronics applications.
By combining the SiNM strain gauge, pressure sensor, and temperature sensor array in a single platform, a skin-like device conformally mounted onto a prosthetic arm is demonstrated. The SiNM strain gauge array monitors the change in the strain distribution according to the clenching motion of the prosthetic hand (Fig. 2d). Similarly, the SiNM pressure sensor measures the applied pressure when typing with a keyboard (Fig. 2e, top) and grasping a baseball (Fig. 2e, bottom). The SiNM temperature sensor mounted on the prosthetic skin distinguishes different surface temperatures (Fig. 2f). Although these SiNM sensors exhibit a high potential for various wearable sensing applications, there are cost issues to be addressed for the development of commercial products.
2.1.2 CNT-based wearable sensors
CNTs are also excellent nanoscale filler materials owing to their small size with good dispersion and exceptional electrical and physical properties [63, 64]. In this regard, electrically conductive rubber (ECR), which is a composite of CNTs and elastomeric polymers, is prepared and used for a wearable mechanical sensor . To enhance the sensitivity, nanopores and micropores are introduced into the ECR, thereby increasing its piezoresistivity and maximizing the locally induced strain when deformed . Figure 3g shows a representative method for introducing pores with a uniform size and distribution in the ECR. The key idea of this method is to use a reverse micelle solution (RMS) comprising an emulsifier, deionized (DI) water, and an organic solvent. In accordance with careful sequential heat treatments, the migration and merging of the reverse micelles and subsequent pore generations occur (Fig. 3h). A larger porosity and lower elastic modulus are achieved if a larger amount of solvent is included in the RMS, thereby resulting in a higher pressure sensitivity (Fig. 3i). An ECR-based strain gauge fabricated on a medical bandage by using ink-jet printing forms a conformal contact with the human wrist and successfully monitors wrist motions. Although sensors based on CNT networks/composites are relatively cheap, especially those that are solution-processed, and mechanically compatible when worn on the human body, several issues such as a slow response time, a large area uniformity, and the hysteresis and drift of signals still need to be solved.
2.1.3 Wearable sensors/actuators based on nanomaterial hybrids
Figure 4f shows an illustration and optical image (inset) of a semitransparent piezoelectric strain sensor and resistive temperature sensor for measuring wrist motions and body-temperature changes for wheelchair control and hypothermia diagnosis, respectively. The strain sensor consists of a ZnO nanomembrane as the piezoelectric material and SWNT networks as the performance enhancer (Fig. 4g). The temperature sensor consists of silver nanoparticles (AgNPs) embedded in the ZnO:Al (AZO) nanomembrane for improving its sensitivity (Fig. 4h). For the strain sensor, co-deposited Cr and SWNTs layers improve the crystallinity of ZnO and passivate intrinsic defects, respectively (Fig. 4i). These modifications dramatically amplify the piezoelectric voltage output of the intrinsic ZnO nanomembrane (Fig. 4j). For the temperature sensor, EC – EF (EC, minimum energy of the conduction band; EF, Fermi energy level) is proportional to the concentration of AgNPs inside the ZnO nanomembrane (Fig. 4k). The high concentration of AgNPs increases the carrier density and therefore improves the sensitivity of the temperature sensor (Fig. 4l). A more in-depth study of functional hybrid nanomaterials would provide new opportunities for high-performance wearable devices.
2.2 Wearable memories
Data recorded by wearable sensors should be either transferred or stored for the analysis. Usually, the data are stored in memory devices and retrieved when needed. In this section, two types of ultrathin deformable nonvolatile memory devices—charge-trap floating-gate memory (CTFM)  and resistive random access memory (RRAM) —are described.
2.2.1 Deformable charge-trap floating-gate memory
The floating gate of a continuous metal film has a critical limitation for the retention time . Instead, metal nanoparticles (NPs) are a promising candidate as the floating gate to realize a fast program/erase speed and long retention time . Figure 5e shows an optical image of a fabricated flexible CTFM using poly(4-vinylphenol) (PVP), pentacene, and gold nanoparticles (AuNPs) as the dielectric, semiconductor, and charge-trap layer, respectively. AuNPs are electrostatically adsorbed onto the PVP blocking oxide, thereby forming a monolayer of AuNPs (Fig. 5f). A large on/off window (>10 V) is obtained owing to the high density of AuNPs (Fig. 5g). Repetitive bending up to 1000 cycles with a bending radius of 20 mm does not diminish the performance of the CTFM.
2.2.2 Nanoparticle-embedded wearable RRAM
RRAM is another promising candidate for future nonvolatile memory devices [75–77]. By integrating RRAM with wearable sensors, a low power consumption and mechanical deformability are important for long-term use in mobile environments . Figure 5i shows wearable RRAM consisting of AuNP charge-trap layers that reduce its operation current. Serpentine interconnections make the wearable RRAM stretchable up to 25 % strain (Fig. 5j–l). AuNPs embedded between TiO2 nanomembranes by Langmuir–Blodgett assembly form a uniform layer over a large area (Fig. 5m–o). The operation current of the wearable RRAM with one AuNP layer is decreased by one order of magnitude compared to that without AuNPs (Fig. 5p). Three layers of AuNPs exhibit a larger current decrease (by almost a factor of three).
2.3 Wearable displays
To construct user-interactive wearable electronic systems, deformable displays that visualize measured or stored data are indispensable for users. Recently, several breakthroughs in deformable LED technologies, including deformable inorganic/organic LEDs [51, 78–81], polymer LEDs [82–84], and quantum-dot LEDs (QLEDs) [85–87], have been reported.
2.4 Wearable energy devices
Energy storage devices and power generators that supply power to wearable electronics need flexibility and biocompatibility. An all-solid-state supercapacitor (SC) [45, 49, 90, 91] is a suitable energy storage device with regard to this point. Moreover, SCs have a high power density, a fast charging/discharging speed, and cycle durability . In case of the wearable power generators, flexible and soft fiber-based materials are suitable owing to the requirement of high deformability . In this section, carbon-nanomaterial-based flexible SCs and organic nanofiber-based power generators are reviewed.
2.4.1 CNT-based wearable energy devices
Instead of coating fabrics, carbon fibers are used to make a woven fabric, which can be applied to flexible textile electrodes . To maximize the surface area, vertically-aligned CNTs are additionally synthesized on the carbon fabric. Electroplating vertical CNTs with RuO2 NPs further increases the capacitance (Fig. 7f), and an all-solid-state wearable SC is fabricated by sandwiching a poly(vinyl alcohol) (PVA)-H3PO4 gel electrolyte between two modified carbon fabric electrodes (Fig. 7g). The resulting SC exhibits high performance up to 135-degree bending and 4000 charge–discharge cycles (Fig. 7h–j).
2.4.2 GP-based wearable energy devices
Multiple chemically converted GP sheets are beneficial for fast ion transport . GP flakes and/or reduced GP oxides are densely packed by capillary pressure to fabricate flexible carbon electrodes (Fig. 7k). The packing density (ρ) of the GP sheets can be controlled by changing the ratio of the volatile and nonvolatile liquids in the gel (Fig. 7l, m). Figure 7n and o show the specific capacitance and volumetric capacitance, respectively, of SCs using stacked GP electrodes for different values of ρ. The specific capacitance slightly decreases as ρ increases, whereas the volumetric capacitance is nearly proportional to ρ. Although most SCs made of activated carbon exhibit a volumetric energy density of 5–8 Wh/L, SCs made of the GP assembly exhibit a volumetric energy density of 60 Wh/L, which is similar to that of lead-acid batteries (50–90 Wh/L).
2.4.3 Organic nanofiber-based wearable power generators
To harvest electrical energy from body movements, piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have been used [28, 96]. Organic nanofibers such as polyvinylidene fluoride (PVDF) formed by using electrospinning processes have shown superb deformability as well as high piezoresistivity, facilitating its use in wearable applications [18, 28]. Piezoelectric power generation using a single PVDF nanofiber , aligned multiple PVDF nanofibers [98, 99], and randomly distributed nanofiber networks  have been demonstrated. Parallel and series connection of PVDF nanofibers increase the generated voltage and current . However, relatively low output power of PENGs has limited the application for wearable devices with high power consumption . In contrast, TENGs have shown much higher output power than PENGs . Electrospun PVDF nanofibers are also suitable for fabrication of the TENG because of their strong electronegativity and high porous morphology offering large contact area to increase the output power [28, 101]. The PVDF nanofiber-based TENG has been recently demonstrated as wearable forms . Seamless integration of the organic nanofiber-based wearable power generators with energy storage devices and control circuits is another important future research topic.
The mechanical, electrical, and optical properties of bulk materials change as their size is reduced and/or nanoscale structure engineering is introduced. By using the unique properties of such nanomaterials or their hybrids, many breakthroughs in wearable devices have been accomplished. In this paper, we reviewed the current status of wearable devices including sensors/actuators, memory devices, displays, and energy storage devices. We particularly focused on the use of functional nanomaterials to enhancing the deformability and performance of these devices. Continuous research and development of new nanomaterials/hybrids and their integration into variety of electronic/optoelectronic devices would provide new opportunities for next-generation wearable electronics.
JK and JL contributed equally. JK, JL, and D-HK wrote the manuscript. JK, JL, DS, MKC and D-HK designed the figures. All authors read and approved the final manuscript.
This research was supported by IBS-R006-D1. This work was supported by a Seoul National University Research Grant.
The authors declare that they have no competing interests.
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