Researchers from the BEBC Research Institute of Bionics Engineering and Biomechanics at Xi'an Jiaotong University published a review article titled "The New Generation of Soft and Wearable Electronics for Health Monitoring in Varying Environment: From Normal to Extreme Conditions". The article briefly describes the three environmental challenges (Figure 1) faced by flexible electronic devices during long-term monitoring, namely mechanical force, temperature, and humidity, and summarizes the research achievements in ensuring the functional realization of flexible electronic devices under multiple changing environmental factors through material innovation and structural design at home and abroad in recent years.

Figure 1. Various environments faced by human activities: from normal to extreme mechanical, temperature, and humidity environments

1. Flexible Electronic Components withstanding Large-amplitude Mechanical Deformation

Human activities are often accompanied by mechanical deformations such as stretching, compression, shearing, torsion, and bending (like limb movements, heartbeats, and blood vessel expansions). The parameter range of these mechanical deformations puts specific requirements on the design of flexible wearable electronic materials and structures. For example, the two-dimensional planar structure and material design used to improve the extensibility of wearable electronics, and the three-dimensional structure that enhances the sensitivity of wearable electronics.

2. Flexible Wearable Electronics withstanding Large-amplitude Temperature Changes

In human activities, we face constantly changing ambient temperatures, from hot to cold, from normal room temperature to extreme cold and severe cold, with a temperature change range reaching -50°C to 60°C. Such constantly changing temperature conditions bring challenges to the durable use of materials (such as high-temperature resistance and low-temperature resistance). Taking hydrogel flexible electronics as an example, in terms of materials, the introduction of organic salts and deep eutectic solvents can improve the low-temperature resistance of materials; in terms of structure, the addition of elastic polymers can have a positive impact on the low-temperature resistance and high-temperature resistance of hydrogels; in addition, combining the two design ideas can significantly improve the performance of hydrogels at different temperatures. Based on these design schemes, various anti-freezing and high-temperature-resistant flexible wearable electronics have been widely used in human motion monitoring and energy supply fields.

3. Flexible Wearable Electronics withstanding Large-amplitude Humidity Changes

Changing humidity environments bring challenges to the use of flexible wearable electronics. For example, dry environments can cause hydrogel flexible electronics to lose water and shrink, thereby losing stretchability and functionality, and humid environments may cause hydrogel electronic devices to swell, leading to uncontrollable shape changes and surface structure damage. Moreover, as an important guarantee for the reliability of signal collection, the effective adhesion between devices and human skin will also be affected by the accumulation of water molecules at the adhesive interface in humid environments (such as sweating). The article introduces the various humidity environments faced by human activities, including internal environments such as human respiration, wound exudation, and sweating, as well as external environments such as sandstorms, swimming, showers, and rainy days; and introduces their design strategies respectively for the impact of humidity environments on the integrity and functionality of flexible electronic devices; finally, it shows the prospects of flexible wearable electronics applied in fields such as electronic skin, intelligent dressings, wearable sensors, and artificial muscles under multiple humidity environments.

Currently, the main imported instrument manufacturers that can meet the needs of comprehensive working condition testing for vacuum, mechanics, thermology, and electricity during the research and development of flexible materials include Linkam and Instec. Domestic instrument manufacturers mainly include GoGo Instruments, etc. Various heating/cooling stages provided by several companies can realize multiple environments (atmosphere, vacuum, humidity, etc.), and can be used in conjunction with source meters, microscopes, XRD, DIC, Raman, etc. to test the electrical, optical, and mechanical properties of materials and components. The maximum control temperature range is -190~600℃, and the temperature control precision can reach ±0.1℃.

Among them, GoGo Instruments also integrates the heating/cooling in-situ tensile stage with the visual measurement DIC to launch the heating/cooling in-situ tensile microscopic testing system. It can realize simultaneous testing of stress-strain, electrical, and DIC data of flexible materials in multiple environments (atmosphere, vacuum, humidity, etc.). This system can realize two loading modes: constant speed or constant force, realize the dynamic stress-strain characteristics of materials at temperatures from -190~600℃, and can also quantitatively analyze processes such as phase transition behavior, crack initiation and propagation, fracture, and bending of materials during the testing process.

With the continuous development of China's scientific and technological strength, domestic instruments are gradually improving their original image, and are catching up in some fields. Relying on advantages such as short supply cycles, high cost performance, excellent after-sales service, and acceptance of customization, they are gradually replacing imported instruments. In the future, China's scientific and technological industry will gain greater development space and ultimately eliminate the "foreign brand" phenomenon.

Variable Temperature Stress-strain Testing of Electronic Components