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rel="nofollow" href="#ulink_8f4ed243-3cb9-53f4-97a7-8823e5901d43">Figure 2.14c showed the CV curves of the fabricated SCs after several healing cycles. The specific capacitance of the self‐healing device obtained from the CV curve was 61.4 mF cm⁻² at a scan rate of 10 mV s⁻¹. We can see even after four cutting–healing cycles, 71.8% of the capacitance retention was observed, providing an excellent self‐healing performance. This work may encourage the design and fabrication of the 1D fiber shaped self‐healable energy storage and wearable electronics.

      Self‐healable 2D planar SCs are another attractive energy storage because they possess the advantage of 2D devices like small size, low weight, ease of handing in appearance as well as the ultralong lifespan of the stretchable electronics. As a typical example, Huang et.al fabricated a 2D planar self‐healable SC based on PAA dual cross linked by hydrogen bonding and vinyl hybrid silica nanoparticles (VSNPs) [31]. Figure 2.14d illustrated the schematics of the fabrication process of the highly stretchable and self‐healable SCs. As for electrode materials, CNT papers were synthesized by CVD, and then deposited with the PPy, which were attached on the both side of the self‐healable VSNPs‐PAA gel electrolyte‐based film to prepare a self‐healable planar SCs. Figure 2.14e showed the demonstration of and ionic conductivity of the self‐healed substrate. The wound in the VSNPs‐PAA film could be autonomously repaired via the intermolecular hydrogen bonds among the cross‐linked polymer chains on the VSNPs in 10 mins under the ambient condition, which has no effect on the ionic conductivity and mechanical properties of the VSNPs‐PAA film. The CV curves with different cut‐healing times were depicted in the Figure 2.14f. The fabricated self‐healable SCs exhibited a specific capacitance of 61.4 mF cm⁻² at a scan rate of 10 mV s⁻¹, which was kept unchanged even after four healing cycles.

      2.3.3 Stretchable Integrated System

Stretchable integrated systems have demonstrated a great real‐word application in wearable electronics, such as electronic skin that monitor heart rate, wrist pulse, body temperature, voice etc., robots that move as instructed, bio‐medicine carrier that reflect signal, while leaving people's normal activities of daily life unaffected [78–82]. These integrated systems that compose of the energy harvester, energy storage, and the functional sensing/detecting units are usually connected on the same deformable substrate [83–87]. In comparison with Li‐ion battery, fuel cells, and other types of battery, SCs have gained extensive attention out of security consideration. To date, several types of integrated devices toward the direction of portable and wearable electronics, like energy storage‐sensor unit and self‐powering system have been successfully fabricated. In harmony with the configuration of the stretchable SCs, the fiber substrate‐based 1D integrated device, PDMS based 2D integrated system and elastic materials‐based 3D devices have shown a rapid development momentum in the field of deformable integrated system. For integration of different functional devices with stretchability, there are three vital factors need to be considered: (i) coordinate work, each unit or functional partition could realize their unique function after connection, the parameters like size, work current etc. should be matched to each other; (ii) a low cost and facile fabrication procedure, where every component could be stably and reliably connected in an easy way; and (iii) integral encapsulation. A favorable package could make the whole integrated system work longer and even remain their function in extreme environment.

      

Figure 2.14 (a) Schematic illustration of the self‐healing process. (b) Schematic diagram of the self‐healing mechanism. (c) CV curves after several healing cycles.

      Source: Reproduced with permission [28]. © 2015, American Chemical Society.

      (d) Schematics of fabrication strategies for highly stretchable and healable SCs. (e) Demonstration and ionic conductivity of the self‐healed substrate. (f) CV curves with different cut‐healing times.

      Source: Reproduced with permission [31]. © 2015, Nature Publishing Group.

      The integrated power pack comprising either wireless power transmission or internal power generator is highly desired for wearable electronics. Nanogenerator is one of the most popular power generators used in an integrated system, which could collect the energy produced by human activities [88–95]. Researches on nanogenerators was first reported in 2006 by Prof. Zhonglin Wang, and then a series of integrated systems with nanogenerator were designed [96–99]. For example, Guo et al. provided a stretchable all‐in‐one integrated system that contains triboelectric nanogenerator, SCs, and an electric watch, which could harvest all kinds of mechanical energy from human motions (bending, stretching) and transfer to SCs for powering the wearable watch [98]. Solar cells that convert sunlight into electricity are considered as the most promising energy conversion devices, which are also introduced to integrated system [42, 100]. Most recently, Yun et.al reported on the fabrication of stretchable integrated system including solar cells, all‐solid‐state MSC, and a strain sensor [101]. In this integrated system, the PPy@CNT electrode based MSC arrays were connected on the PI substrate, resembling a serpent in form. The graphene foam base strain sensor was directly prepared on the deformable PDMS substrate. MSC array and solar cells were separately embedded onto the PDMS substrate. When the MSC array was placed into the deformable Ecoflex substrate, the PI film was removed. The obtained integrated system was attached on human's wrist to detect externally applied strains and the arterial pulse using the energy stored in MSCs, charged with SCs. Noticeably, the charge/discharge behavior maintain their value even after 1000 stretching/releasing cycles, demonstrating the outstanding cycle stability and stretchability of the fabricated devices.

Despite the successful integration with SCs and sensors, solar cells suffer from the inherent instability caused by light intensity, which constrain the practical application of solar cells in wearable electronics, especially in some integrated systems that need continuous monitoring [102, 103]. Alternatively, the wireless charging device holds a significant place in stable and reliable energy generation of the integrated system [104, 105]. Most recently, Ha's group reported a stretchable multifunctional sensing system integrated with a wireless charging unit, as shown in Figure 2.15 [106]. This stretchable integrated device could realize not only the monitoring of bio signals such as the human pulse, motion, and voice, but also the detection to environmental signal like gas, ultraviolet (UV) light. Figure 2.15a and b displayed the schematic illustration and circuit diagram of the stretchable 2D multifunctional integrated system, which operated a RF power receiver, MSC array, graphene foam‐based strain sensor and UV/NO₂ gas sensor on the same elastic PDMS substrate. The digital photographs of integrated system attached to human body were presented in Figure 2.15c and d. The stable and repeatable wireless charging of MSCs with the integrated RF power receiver was depicted in Figure 2.15e. The carotid pulse curve (Figure 2.15f) together with the resistance versus hand motion and swallowing saliva detection curve of the fragmentized graphene foam‐based strain sensor demonstrated the strain sensor has a stable response to the bio‐signals like body motion, voice, swallowing of saliva, and the carotid artery pulse. Figure 2.15g showed the I‐t curves of the MWCNT/SnO₂ nanowire‐based gas sensor using the energy supplied by integrated MSC array under stretching varying from 0% to 50%, indicating the excellent performance and stretchability of both gas sensor

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