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 and MSC array. Figure 2.15h presented the mechanism in response to NO2 gas and UV light of MWCNT/SnO2 hybrid sensor. The UV sensitivity was unchanged under a strain between 0% and 50%. All the results obtained in this paper suggested the fabricated stretchable multifunctional integrated sensing system can be used as next‐generation body‐attached healthcare and environmental sensor devices for continuous nondestructive monitoring.
Figure 2.15 (a, b) Schematic illustration and circuit diagram of 2D multifunctional integrated system, containing a RF power receiver, a MSC array, strain sensor, and UV/NO2 gas sensor. (c, d) Photograph of integrated system attached to human body. (e) Charge/discharge curve of integrated system powered by RF power source. (f) Carotid pulse curve of the strain sensor. (g) NO2 gas response at strain of 0%, 20%, 50%. (h) UV detection under 0% and 50% strain with exposure of 312 nm.
Source: Reproduced with permission [106]. © 2015, Wiley‐VCH.
2.3.4 Perspective
As a new member of the SC family, stretchable devices have been greatly developed in the past few years, buoyed by the portable and wearable electronics, which need a stretchable energy storage to form a complete and safe system to monitor electrical and biomedical signal generated by human activities, thus achieving the practical application of wearable electronics in the field of biomimetic E‐skins, interactive human‐machine interface, “big health” and “big data.” In this review, we systematically summarize the recent progress in stretchable SCs from the perspective of the three dimension and corresponding configuration of the stretchable device, as well as the fabrication process and strategies toward the stretchable SCs. The stretchable integrated system is also concluded in this chapter, all of them realized stable response to the physical and bio‐signal under different stretching or deformations when attached to the human body, showing potential for wearable electronics.
Despite the considerable achievements in stretchable SCs which have been achieved, there are great challenges still remaining for future practical applications. These challenges and the direction for future development can be summarized as follows: (i) Electrochemical performance of SCs under deformation still requires to be improved. Electrode material is the decisive factor of the electrochemical performance. But most of the stretchable SCs depend on the CNTs, which lead to the low specific capacitance of the devices. Discovering novel kinds of electrode materials and design of various configuration for stretchable
