Self-Cooling Transformative Electronics with a Radiative Cooler for Reliable Stiffness Tuning in Outdoor Use

Traditionally, electronics are designed to have invariant mechanical properties to serve specific purposes. For example, rigid electronics provide convenient and robust handling, optimized for handheld or tabletop setup, and soft electronics are capable of deforming their shapes and geometries dynamically, enabling comfortable wearing on the body. Unfortunately, however, current electronics cannot provide advantageous features of both rigid and soft electronics due to inherent limitations in their form factors. To leverage key features of both rigid and soft electronics, we have developed a transformative electronics system (TES), which can change their shape and stiffness to realize both rigid handheld and soft wearable configurations as needed [1]. The TES integrates flexible, stretchable electronics onto a thermally responsive transformative platform which is composed of gallium (<i>T_melt</i> = 29.76 °C, where <i>T_melt</i> represents a melting point of gallium) to implement stiffness tuning by changing its phase between solid and liquid. However, the TES have difficulties converting into a rigid form outdoors because excessive heat is accumulated from themselves or the environments such as the sun. Integrating a flexible thermoelectric device with TES [2] can address this issue, but it requires a huge amount of power to maintain rigid operation mode outdoors.<br/>Here, we present a design of transformative electronics with the integration of a radiative cooler (TER-RC), enabling reliable bidirectional stiffness tuning in both indoor and outdoor environments. Specifically, rigid-mode outdoor operation under the sunlight has been substantially improved by a radiative cooler. We fabricated a flexible and stretchable radiative cooler (FSRC) that achieves ~97% of reflectivity in the solar spectrum and ~90% of emissivity in an atmospheric window so that it can effectively block solar energy and radiate internal thermal energy [3]. The radiative cooler can maintain its temperature 3.9 °C below the melting temperature of gallium under 1000 W/m² of direct sunlight and an additional 4469 mW/m² of external heat input. Based on this, we have demonstrated a transformative optoelectronic device that can convert between rigid handheld and stretchable wearable forms. The integrated radiative cooler successfully reduces the temperature of electronics and enables stable rigid mode operation even in the hot outdoor environment, thus making transformative electronics highly reliable both in rigid handheld setup and soft wearable form.<br/>In conclusion, the transformative electronics with radiative cooler (TER-RC) enables desired stiffness tuning between rigid and soft modes without dependence on ambient temperatures and environments. The integrated FSRC can effectively reflect solar energy and radiate thermal energy so that it cools down the TES to allow rigid mode under sunlight. Finally, we verified the design concept through the demonstration of an optoelectronic device that can convert between rigid handheld and stretchable wearable forms to overcome the limited applications of the existing TES.<br/><br/>[1] Byun, S.-H. et al. Mechanically transformative electronics, sensors, and implantable devices. <i>Sci. Adv.</i> <b>5</b>, eaay0418 (2019)<br/>[2] Byun, S.-H. et al. Design strategy for transformative electronic system toward rapid, bidirectional stiffness tuning using graphene and flexible thermoelectric device interfaces. <i>Adv. Mat. </i><b>33</b>, 2007239 (2021)<br/>[3] Kang, M. H. et al. Outdoor-useable, wireless/battery-free patch-type tissue oximeter with radiative cooling. <i>Adv. Sci.</i> <b>8</b>, 2004885. https://doi.org/10.1002/advs.202004885 (2021)