Mechanically Transformative Electronics Integrated with Soft, Stretchable Radiative Cooler for Reliable Rigidity- and Shape-Tuning in Hot Outdoors

The integration of a mechanically tunable gallium-based platform with soft electronics, referred to as a transformative electronic system (TES), provides an optimized mechanical interface of electronic devices intended for specific use. The stiff and flat interface of rigid electronics, such as smartphones and laptops, allows robust and convenient handling, while the compliant and deformable nature of soft electronics favors wearable and implantable applications. However, the mechanical properties of each electronics are fixed, restricting their applications beyond the original target use. The stiffness-tuning ability of TES can overcome the inherent constraints of both rigid and soft electronics owing to their fixed rigidity. Through the liquid-solid phase transition of gallium, the TES can exploit the key features of both rigid and soft electronics. While the low melting point of gallium (<i>T_melt</i>; 29.76 °C) makes the softened TES appealing for wearable and implantable applications, however, it can also inhibit the stable rigid-mode operation of TES, especially in a hot outdoor environment due to unwanted liquefaction of gallium. Therefore, proper thermal management is required for the gallium-based TES to prevent undesired softening during the rigid-mode operation. Previous approaches to managing excessive heat of electronics involve either enhancement of the device’s thermal conductivity for effective heat dissipation or integration of an active cooling system (e.g., coolant circulator and thermoelectric device). However, these strategies are not suitable for TES in preventing unwanted softening due to insufficient cooling capability and high power consumption, respectively.<br/>Here, we present an advanced design of TES that incorporates a stretchable radiative cooling system for a reliable rigid-mode operation in the hot outdoors. The stretchable radiative cooler provides enhanced reflectivity in the solar spectrum and thermal emissivity in the atmospheric window for zero-power cooling of TES. The outstanding optical characteristics are attributed to our unique design of the radiative cooler, which is a multi-layered structure of porous styrene-ethylene-butylene-styrene (SEBS) to maximize multiple Mie scattering, and provide sufficient cooling capability even under strong sunlight. Therefore, the integration of the radiative cooler with TES effectively prevents the undesired rigid-to-soft mode conversion in the hot outdoors. The radiative cooler shows sub-ambient cooling of ~4.5 °C under peak solar power of 927 W/m², maintaining the desired high rigidity of TES in a hot environment under sunlight. When the radiative-cooler-integrated TES is mounted on the skin (33–35 °C), the system becomes soft and available for use as a wearable device. Due to the flexible and stretchable properties of the radiative cooler, our proposed TES design allows 3-dimensional conformal integration on the skin with natural deformations. Furthermore, the proof-of-concept demonstration of the radiative-cooler-integrated TES, which can convert between a rigid handheld form and a soft wearable form, verifies the reliable rigid-mode operation in a hot outdoor environment when desired, suggesting that the radiative cooler is a highly favorable option for thermal management of a gallium-based TES owing to its zero-powered sub-ambient cooling capability.