In a cold night, a clear window that will become opaque while retaining the indoor heat is highly desirable for both privacy and energy efficiency. A thermally responsive material that controls both the transmittance of solar radiance (predominantly in the visible and near-infrared wavelengths) and blackbody radiation (mainly in the mid-infrared) can realize such windows with minimal energy consumption. Here, we report a smart coating made from polyampholyte hydrogel (PAH) that transforms from a transparency state to opacity to visible radiation and strengthens opacity to mid-infrared when lowering the temperature as a result of phase separation between the water-rich and polymer-rich phases. To match a typical temperature fluctuation during the day, we fine-tune the phase transition temperature between 25 and 55 °C by introducing a small amount of relatively hydrophobic monomers (0.1 to 0.5 wt % to PAH). To further demonstrate an actively controlled, highly flexible, and high-contrast smart window, we build in an array of electric heaters made of printed elastomeric composite. The multipixelated window offers rapid switching, ∼70 s per cycle, whereas the device can withstand high strain (up to 80%) during operations.
The emergence of the Internet of Things (IoT) necessitates the development of electronic components with various form factors and mechanical properties. 3D printing is an effective tool to realize objects with arbitrary form factors. Various 3D printable materials have recently been commercialized; among them, stretchable materials are particularly useful in the IoT because they enable adaptability in the dimensional change of the electronics. Most of these stretchable materials are, however, not electrically conductive; conductive coating can enable the functionality. Here, we propose a selfreinforcing conductive coating strategy, which reduced graphene oxide (RGO) self-assembles to wrap graphene nanoflakes (GNF) as a conductive binder that can also achieve mechanical integrity. The conductivity of the GNF-RGO coating reaches 4.47 × 10 4 S m −1. To demonstrate the potential applications of the GNF-RGO coating, applying the coating on 3D printed porous elastomers enabled flexible radio frequency (RF) antennas and strain sensors. The RF antenna shows high radiation efficiency and maintains excellent performance under bending conditions. The coating also produces a strain sensor with a gauge factor of ∼13 up to 40% of strain. We foresee that the electrically conductive GNF-RGO composite coating can provide versatile functionalization strategy in flexible electronics and in wearable biomedical devices.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.