With the advancement of medicine and our deepening understanding of early symptoms of diseases, the demand for new technologies dedicated to healthcare has been increasing in recent years. These technologies can perceive and monitor the functions of complex biological systems without causing unnecessary reactions. Similarly, materials that can effectively transmit remote sensing signals are needed to connect and synchronize human activities with the environment through new digital technologies. However, traditional health monitoring technologies often rely on a large number of machines and qualified medical personnel, which requires expensive and complex monitoring. The introduction of wearable health monitoring technology effectively addresses these issues, as it can collect real-time body data from users, assess their health status, and provide personalized medical advice based on data analysis. Wearable monitoring systems are crucial for healthcare, disease prevention, and control. Signal conversion and recording play a crucial role in the health monitoring process as they determine the accuracy of the data received by the user. Therefore, an area of great interest in the academic community is the manufacturing of sensors with consistent performance. Consumer wearable health monitoring devices, such as smartwatches and smart glasses, include internal sensors formed by semiconductor methods based on rigid substrates. This makes it difficult for the sensor to adapt to the surface of the human body, thereby reducing the comfort of wearing.

The ongoing research on synthesis methods that combine the flexibility and diversity of polymer matrix and gel with the characteristics of photonic, conductive and semiconductor materials is driven by the growing popularity of flexible electronic products. Many of them, such as biomimetic and biomimetic materials, imply the complex hierarchical structures and tissues observed in biological structures. Once the main functions of the devices are completed, creating biocompatible materials with a controllable lifecycle for these devices is one of their biggest obstacles. Their ability to collect vital signs is limited, especially blood oxygen levels, body temperature, and pulse, which makes it difficult for them to adapt to more complex and customized medical environments. In addition, there is a significant difference in Young's modulus between the surface of human skin and these rigid sensors. When a person engages in significant physical activity, rigid sensors often detach from the surface of the body, which can lead to erroneous data collection. Therefore, creating a fully flexible and resilient sensor is crucial for building a wearable health monitoring system. Despite the latest developments, there are still some technological barriers in the preparation process of stretchable devices. So far, two different methods have been proposed to achieve the stretchability of supercapacitors: (I) from a material perspective, active materials can be deposited or combined with elastic polymer substrates and conductive fillers to form stretchable electrodes. (II) From the perspective of structural design, electrodes can be made using specially formed geometric structures such as spirals, springs, wrinkles, and honeycomb geometries. Due to the additional inert components, the energy density at the device level will decrease and the internal resistance will increase. Activated carbon and synthetic pseudocapacitive materials are examples of traditional particulate materials, but their applicability is limited due to their large aspect ratio, which requires them to be transformed into ultra-thin self-supporting thin films. Another issue is that the electrode material itself is very hard. The mechanical mismatch between the electrode and the gel electrolyte may lead to a significant concentration of stress on the contact surface, which may lead to sliding and delamination during tension.

Due to the rapid development of flexible electronic technology, including soft robots, artificial electronic skin, wearable devices, and health monitoring systems, there is a high demand for stretchable conductor materials. Researchers pay more and more attention to conductive hydrogels because of their remarkable biocompatibility, low cost, excellent conductivity and adjustable mechanical properties. These hydrogels also have great potential in the biomedical field. Conductive hydrogels are now used in actuators, sensors and flexible energy storage devices. However, when hydrogels contain a large amount of water, they will lose flexibility and conductivity, because this will cause them to evaporate at high temperature or even room temperature, and condense at low temperature. Researchers have studied the addition of organic solvents such as glycerol and ethylene glycol to hydrogels. These solvents have good antifreeze and moisture retention properties and can solve the problem of temperature stability. This method has successfully extended the service life of the hydrogel. For example, polyacrylic acid hydrogels exhibit excellent mechanical and electrical conductivity when dissolved in a mixture of glycerol and water. The gel continued to show nearly 1000% of the cracking strain at -50 ° C. After being placed at ambient temperature for 7 days, the stability of the gel was maintained due to the excellent water holding capacity of glycerol molecules. However, after 30 days of testing, the gel gradually lost water molecules, which reduced its conductivity and brittleness.

Ionic liquids are organic salts in liquid form composed of cations and anions. They have strong thermal stability, good conductivity, and non volatility. Ionic gel have higher conductivity and temperature resistance than volatile hydrogels. They are interesting candidates to replace them in flexible batteries, soft robots and energy storage devices. They can be manufactured by fixing them in a three-dimensional gel network. However, the expensive price and potential cytotoxicity of ionic liquids limit their use. To address the drawbacks of ionic liquids, researchers are focusing on developing new environmentally friendly solvent alternatives. Deep eutectic solvents (DESs) are a novel liquid combination that combines hydrogen bond donors and acceptors to produce; Abbott et al. (2003) discovered it in 2003. Similar to ionic liquids, DES has strong conductivity, low volatility, and thermal stability. However, it also has non toxicity, affordability, simplicity, and biodegradability, making it a "green" ionic liquid. Researchers have used DES to create a unique gel called eutectoid gel. But most of the gel under study are not very tough or strong; Therefore, if high ductility is not required, they can only be used as solid electrolytes. Although some studies have replaced the solvents in eutectoid gel to improve their mechanical properties, the conductivity of gel has significantly decreased. Therefore, it is still difficult to manufacture eutectoid gel with excellent mechanical and electrical properties, especially for applications using flexible sensors.