Perspective—Supercapacitor-Powered Flexible Wearable Strain Sensors

Currently and also in future, the flexible and wearable strain sensor would be in high demand due to its direct applications in biomedical health monitoring and other engineering applications. The challenge is to make the flexible and wearable sensor to function continuously with no additional wired powered supply system. In line with this, there has been considerable research works towards the integration of supercapacitor into flexible and wearable strain sensors, to make them self-powered and more efficient. This perspective provides more insights on design and fabrication of flexible and wearable strain sensor, working, components, and materials used, integration with supercapacitor, challenges and future scope.

Strain sensors in wearable, flexible and stretchable forms have attracted lots of attention from researchers in recent years. 1 These sensors are used in a wide variety of applications like soft robotics, human-machine interaction, health monitoring systems, and the detection of human motion. 2,3 Nowadays, strain sensors have caught the interest of many researchers because they can be made into flexible and wearable sensors 4,5 by using nanomaterials. They help in the sensor exhibiting good properties like flexibility, low cost and ease of fabrication, better sensitivity, and light weight. 6 Over the years, carbon materials like graphene, carbon nanotubes, MXenes, carbon black particles, metal oxides, metal chalcogenides, polymer nanofibers, and hybrid nanomaterials have been well-known to show desired piezoelectric properties required for a strain sensor. 7,8 Conventionally, strain sensors have always been integrated with an external power supply, making the device very bulky. There has been a lot of improvements in the field of flexible all-solid-state supercapacitors and micro-supercapacitors owing to its applications in portable sensors with developments in novel materials. Integration of micro-supercapacitors with the strain sensor will make the overall device less bulky and able to apply onto curved surfaces like skin. This has inspired many researchers to look into the integration of flexible all-solid-state supercapacitors with wearable stretchable/ flexible strain sensors. 9,10 The supercapacitor powered strain sensors, their general specifications, most commonly used materials used for fabrication and applications are representatively shown in Fig. 1a. As shown in Fig. 1b, there has been increase in the publications and patent numbers from 2018 to 2022. The data was analyzed based on google scholar articles searched using the keywords such as "supercapacitors and sensors," "supercapacitors and strain sensors." The trends in publications of papers and patents have clearly reflected the consistent advancement in the field of supercapacitors for strain sensors over the past few years. (Google scholar website accessed on 10th December 2022). This trend is an evidence for the advances in supercapacitor powered stain sensors.
Currently, the pressure sensor market is projected at approximately USD 14.8 billion and growing. By the year 2026, the market is projected to reach USD 22 billion at a CAGR of 8.2%. The pressure sensor market has experienced growth due to increased demand for energy-efficient vehicles, portable healthcare devices, and consumer electronics. These applications require the use of pressure sensors to monitor and regulate various systems and processes.
The pressure sensor market is experiencing growth due to the increasing urbanization, digitalization, adoption of the Internet of Things (IoT), and the emergence of Industry 4.0. Similarly, the supercapacitor market is looking at a huge growth from USD 472 million in 2022 to USD 912 million by 2027 with a CAGR (Compound Annual Growth Rate) of 14.1% during the forecast period. 17 The increased demand for energy-efficient solutions and high storage capacity in consumer electronics, driven by the growing use of supercapacitors in smart wearable sensors, is a major factor driving the integration and growth of both the supercapacitor and pressure sensor markets. 17 In this perspective article, the recent advances in materials used for both supercapacitor and strain sensor, fabrication challenges, future scope and perspectives are discussed.

Current Research Updates
It is very important to improve the efficiency of health monitoring electronic devices, which are specifically designed to monitor signals due to strain in the body, like human skin. In this direction, Junyeong Yun et al., 18 have fabricated a stretchable sensor devices consisting of a strain sensor, Si solar cells (energy conversion) and micro-supercapacitors (energy storage) devices. The strain sensor device is made to work continuously by powering with the help of micro-supercapacitors (MSCs), which are charged by a solar cell (SC). Figure 2a represents the fragmentized graphene foam (FGF) strain sensor integrated with MSC and SC, designed to sense a wrist motion with no connections to an external power supply. As depicted in Fig. 2a, as an elastomer substrate of the device is elongated due to applied strain, results in linear increased contact resistance (134 to 194 kΩ) up to 30% strain even after 1000 stretching/releasing cycles. Figure 2b represents the photograph of the wrist attached with an integrated MSC-SC-FGF sensor. Upon bending the wrist, the tensile strain induced in the sensor due to the change in resistance is recorded as shown in Fig. 2c. It is observed that the resistance increases from 134 to 145 Ω, upon bending, and returned to the same upon relaxation. The change in resistance of the MSC-SC-FGF sensor powered by MSC, which is charged by three different sources is represented in Fig. 2c. The resistance response of the sensor in which the MSC system is charged by external power (red), solar simulator (green), and Sunlight (blue), exhibit similar resistance variations. The sharp resistance peaks, during the relaxation of the wrist, are attributed to the viscoelasticity of the PDMS/ z E-mail: manjunathac@rvce.edu.in; manju.chem20@gmail.com; sudhaka-math@rvce.edu.in Ecoflex composite of the sensor. The relative strain response from P1 to P9 (Fig. 2c), was found to vary from 0.058 to 0.077. This value confirms the stability of the sensor exposed to Sunlight. The photocharge/discharge characteristics and electrochemical performance of the MSC-SC-FGF sensor powered by MSC could be a stretchable, wearable self-charging power-sensor system, that finds practical applications in skin-related health-watching devices.
It is highly suggested that integrated high-performance energy storage systems that are also versatile, should be used to power devices such as bio-signal sensor devices in wearable electronics. 22,23 Moreover, conformal attachment of the entire system to the rough skin surface and the use of high-performance sensing devices are strongly recommended for even more reliable and accurate sensing of bio-signals, as shown in Fig. 2d. Planar micro-supercapacitors (MSCs) have the unique features of a simple fabrication, fast ion diffusion due to the short range between electrodes without the requirement for a separator, and ease of integration into circuit design over tightly packed supercapacitors, as shown in Fig. 2e. 24,25 Hyojin Park et al. 26 fabricated a highly engineered versatile MSC to power a skin-attachable sensing system. They reported that the synergistic utilization of mixed manganese/vanadium oxide grown on MWCNT electrode with the  fitted to the wrist, neck, and glabellar to measure arterial heartbeat, swallowing, and brow facial expressions. Swallowing signal and frowning pulse was evaluated by squeezing the brow, as shown in Figs. 2f and 2g. The integrated device was then connected to the neck and glabellar and the bio-signals have been tested.
Wide-ranging applications of portable sensor technologies have previously been demonstrated in the following areas like human movement sensing, health management, electronic skin interconnection, and many others. 19,27 It has been reported that some efforts have been made to integrate energy sources and sensors into one system. 28,29 A straightforward electrodepositing and nitriding methodology has been used to prepare the helix structure of MoN on nitrogen-doped carbon (CN) and carbon cloth (CC) (CC@CN@ MoN). Furthermore, the flexible all-solid-state asymmetrical SCs (ASCs) of CC@CN@MoN/CC@NiCo 2 O 4 as-fabricated, as shown in Fig. 2h, showed exceptional electrochemical performance after 10,000 cycles and over 90% retention, and the value of areal capacitance could reach 90.8 mFcm −2 at 10 mAcm −2 . ASCs can be used as a self-powered energy system for strain sensing devices for human movement when integrated with solar energy, as shown in Fig. 2j. Finger movements could also be remotely surveilled in actual using a smart device, as shown in Fig. 2i. The response of the sensor to strains developed by bending the finger through 30°and 90°are measured, as shown in Fig. 2k. The advancement of structured materials for energy backup, compact self-powering, and strain or chemical/biochemical smart sensors may be energized by portable helical MoN solid-state SCs for self-powered strain smart sensors.
Weigu Li et al. 20 have developed the solid-state flexible supercapacitor employing a multilevel porous graphite foam doped with manganese oxide (MPGM) incorporated in a wearable strain sensor, as shown in Fig. 2l. 30 The top layer of PDMS (polydimethylsiloxane) of the sensor was used as a flexible substrate to lay the supercapacitor composite material (MPGM). The supercapacitor having low capacitance variance and under mechanical deformation, displays a self-powering capability. The sensor conforms to the human skin and detects both coarse and fine motions. Oscillating electric signals are synchronously shown relative to the bending and flexing of a finger when the sensor is applied onto the finger, as shown in Fig. 2m. The operation requires lesser voltage and current because the sensor detects the change in electrical resistance. 31 When incorporated within the sensor, the supercapacitor displays a capacitance retention of 80% after 1000 mechanical bending cycles. It was recorded that the sensor was able to pick up human vital signs with 98 min −1 heart beat rate when applied near to a carotid artery, as shown in Fig. 2n. Therefore, understanding the strategical design and fabrication of sensor device is vitally necessary and could be helpful to achieve fully integrated flexible energy storage devices for high-performance wearable flexible sensors. 32 Yu Song et al. 21 fabricated a compressible system made from carbon nanotube polydimethylsiloxane (CNT-PDMS) sponge as a piezoelectric sensor for monitoring small-scale human motions, and as a compressible electrochemical supercapacitor. The piezoelectric sensor (PRS) applied at the neck was able to catch signals during the pronunciation of "Hello," "Hi," and "Bye," as shown in Fig. 2r. The resistance was also measured while drinking water, as shown in Fig. 2s. Figure 2o shows the PRS behind the knee, which measured the current response while walking, jogging, and running and the intensity can be seen increasing with the intensity of the activity, as shown in Fig. 2p. Therefore, the integration of the CNT-PDMS sponge CSC with the PRS shows very good potential in distinguishing the motions of human physiology and helps in monitoring real-time health. 33

Current Challenges
The design and synthesis of materials and optimization of fabrication techniques are crucial in order to meet the unique characteristics and specifications of flexible supercapacitor-powered stress sensing devices. 34 There are many great opportunities for the viable use and continuous improvement of stress sensing devices due to the fast and effective advancement of nanomaterials and nanoscience. In order to ensure an effective and rapid ionic reaction mechanism under various flexible parameters, the required solid electrolytes should, on the one hand, be remarkably ion conductive and structurally flexible. 35 On the contrary side, the effectiveness and adaptability of stress sensing devices depend primarily on the electrode material properties such as flexibility, electroactivity, ionic responsiveness, and rapid charge transfer. 36 In order to achieve wearable and flexible applications of devices, it is critical to develop high-performance materials and simple fabrication methods. Stretchability in supercapacitors can be accomplished in two distinct ways: by using multiple materials and compositional configurations. In order to generate flexible electrodes and electrolytes, adaptable and structurally durable materials are needs to be developed first. Flexible active material can be loaded on flexible substrates or coupled with thermoplastics to generate flexible composites. From an architectural standpoint, flexible substrates or a matrix can be employed to fabricate supercapacitors with a specifically designed geometrical structure. [37][38][39] Nano -structured conductive polymers can be used as electrode material in various shapes, from linear to three -dimensional structures, without any need for conductive dopant and have flexible, enhanced mechanical strength under stress, a higher surface area, and shortened ion or charge transfer paths than their bulk form. 40 Additionally, conductive polymers can be combined with carbon based components or oxides of transition metals to generate enhanced electrodes for flexible and versatile supercapacitors. 41,42 The electrolyte is an additional major element of a supercapacitor. High electrical conductivity, ionic mobility, diffusion coefficient, ionic radius, dissociation, thermal and electrical stability are eventually determined by the characteristics of the electrolyte. In order to develop electrolytes for flexible supercapacitors, it is necessary to further modify their characteristics into stretchable solids such that the electrolyte would not drip under elastic deformation. Polymer matrix such as poly(vinyl alcohol), poly (methyl methacrylate), poly(acrylic acid), poly(ethylene oxide), and poly(vinylidene fluoride) are commonly used to develop electrolytes that retain a solid or jelly structure and can ensure to withstand mechanical stress. 43 Design changes to the structural analysis can also significantly enhance the mechanical properties. With an architectural twist in the structure, stretchable and flexible supercapacitors can be made using common electrode preparation methods like dip-coating, layer-by-layer assembly, spray-coating, and electrodeposition. Structural engineering, which is used to generate wavy or buckled 44 coil shaped 45 serpentine, 46 provides elastic properties to otherwise non-stretchable elements. Standard stress sensing devices assembly methods essentially incorporate the electrolyte layer and electrode layers together, depending on the physical bonding of the composite material. The poor interface will lead in an unsteady cyclic stability, speedily diminishing power capacity, and poor flexible strength. Pressure and temperature specifications should be monitored simultaneously based on various composite materials to achieve the optimal device interface. The interface as-built offers the device with long cycling reliability under electro -chemical environments, displaying a great strategy for efficient assembly of devices instead of simple integration technique. 47,48 Paper-based SCs are especially promising for future "ecofriendly" and easily disposable devices because they are inexpensive and environment friendly nature, along with their simple fabrication methods. Especially considering the fact that much research has been carried on the fabrication and use of flexible strain sensors for medical and biological and health monitoring applications, there are still a few shortfalls that must be filled. Although these kinds of sensors enable for widespread detection, long-term testing of the production models has been restricted. All-in-one implementations seem to be more beneficial to start realizing the key benchmarks like size reduction, low energy consumption, and ergonomically designed than flexible substrate-based assimilation. However, flexible strain sensors are presently the only easy integration models that have been widely developed. As a consequence, the signals produced are inadequate for complicated health care system. Therefore, a major research orientation could be established for developing fully -featured sensing devices considering properties such as temperature, sweat, gas, and so on. Furthermore, utilizing sophisticated micro -scale and nanofabrication techniques, to develop an all-in-one integration with various configurations. Single -layer and co -axial fibres architectures have been the only two main types of all-in-one assimilation that have been published thus far. Incorporating such design ideas to multiple sensor environments is still intriguing. Many efforts are required to take full advantage of their implementation for the all-in-one integration given the excellent compatibility of certain advanced 3D printing screen-printing, inkjet printing, and laser writing with the assembly of energy devices and sensors. With all these innovations, it would be possible to reproduce existing portable template designs for compact, comfortable, and energy-efficient sensor applications. Even though the hot-pressing method is a substitute method for fabricating reliable stress sensing device interfaces, the mechanical and redox reactions characteristics of the interfaces cannot sustain operation of the device under tough flexible situations such as curl, wrapping, and bending states. To further enhance the properties, a new assembly technique must be developed, but materials with distinct characteristics are difficult to incorporate together. Although several studies on integrated sensor devices have been performed, the large percentage of them really focused at power source or sensor effectiveness. No study has thoroughly evaluated the performance measures of such integrated sensing systems, such as sensing precision, repeatability, energy consumption configuration, and other variables, till date. To undertake the necessary studies for a more unified assessment of integrated sensor devices should be focused. Ultimately, to enhance the wearable technical knowledge, such integrated detection systems with power sources and sensor systems have to be modelled with the intricate performance -based design, consistency, viability, compact size, energy-efficiency, and user friendliness.
From the commercial aspects, the pressure sensors and supercapacitors can present several challenges for industry. One of the main challenge is the high cost of production, which can make these products less attractive to consumers and limit their adoption. Another challenge is the level of competition in the market. Many companies produce pressure sensors and supercapacitors, which can make it difficult for individual firms to differentiate their products and gain a competitive advantage. Regulatory compliance is also a challenge for companies in this industry. Both of them must meet certain standards and requirements to be sold and used in certain markets, and companies must ensure that their products meet these standards. Intellectual property protection is another issue that companies may face. This can include patenting new technologies and defending against IP infringement. Finally, the demand for both can vary depending on market conditions and the specific applications they are used in. Companies may face challenges in predicting and meeting changing demand. 49,50 Summary and Future Scope To summarize, we have discussed the recent developments of supercapacitor powered wearable flexible strain sensor. The requirements and challenges in developing the various materials for electrodes, electrolytes, piezo active materials, substrates etc., required for both flexible supercapacitor and strain sensor is summarized. The current supercapacitor performance measurement technique being used is not satisfactory approach. For example, the specific capacitance obtained for the given electrode material in three electrode system (say 1100 F g −1 ) is almost 3 to 4 times less than the specific capacitance value (say 350 F g −1 ) obtained for the same material in two electrode system. Therefore, it is very much needed to develop the standard technique which can give the exact values. So that it will be easy for choosing the supercapacitor to power the strain sensor. The sustainability of the supercapacitor integrated stain sensor depends on the selection of various components like separators, electrolytes, current collectors, counter electrodes, etc. Currently, these components are ambiguously chosen to design the supercapacitor, which intern leads to error/failure of the integrated strain sensor. Therefore, there is more scope for developing the approaches/techniques/specifications in choosing/designing the components of supercapacitor. It is very important to focus on developing a flexible supercapacitor of different shapes like thin films, wires, rods, which helps to integrate with stain sensors. There is large scope in developing biocompatible, low cost flexible supercapacitor, because its role is to power the strain sensors which are largely used in wearable electronics for biomedical applications. The microminiaturization research in developing a supercapacitor having high power and energy density, high flexibility, shapeconformability, light weight and compatible to strain sensors is less explored.
It is very challenging to develop the multifunctional integrated self-powered strain sensor system using various electronic components on soft tiny substrate. There is more scope for future research in developing the simple and low cost fabrication techniques towards this system. The strain sensor powered by supercapacitor, is expected to generate the large amount of data, as it is being continuously active upon using (say electronic skin). Therefore, the more efficient and promising system to be developed to collect, process, manage, maintain, and understand the data and provide correct output from the sensor signals. Currently a soft lithography, nanoparticle deposition, plasma etching, self-assembly, and chemical-treatment methods in combination with others are being used to develop the supercapacitor powered strain sensor, which are quite expensive and complicated. Therefore, the commercial scale mass production of multifunctional integrated self-powered, electrically stable, mechanically robust, flexible, inexpensive strain sensor system using above methods is still a challenging.