Fig. 3. Application demonstration of transformative electronics that can convert between a rigid tabletop clock and a stretchable wearable sensor.

(A) Exploded-view schematic diagram that illustrates a TES that includes a gallium-silicone composite material as a transformative platform, a heater as an actuator for accelerated phase transition from a rigid to soft state, a stretchable PCB consisting of a temperature sensor, a UV sensor, an OLED screen, capacitive touch sensors, and a microcontroller for environmental and physiological sensing. (B) Illustration of the assembled device. The top inset shows a cross-sectional view. (C) IR images showing the mechanical transformation of the device triggered by body temperature upon mounting on the skin, which leads to conversion from a rigid, flat to soft, flexible form that enables conformal contact on the curved surface of the skin. (D) Temperature at the bare heater surface (red), the heater-gallium interface (green), and the device-skin interface (blue) when operating the heater with a 3.3-V coin cell battery. The operation of the heater can accelerate the phase transition of gallium while maintaining a biologically safe temperature (<45°C) at the device-skin interface. (E) Bending stiffness of the device as a function of temperature. (F) Overlay of optical images and FEA results for the device in a soft state under uniaxial strain of 0% (left) and 26% (right). (G and H) Application demonstration of the transformative electronics as a tabletop clock in the rigid mode (G) and as a wearable sensor in the soft mode (H). The sensor on the contracted arm stretched 15% because of muscle volume expansion. (I and J) Application demonstration of the device in different scenarios (I) and use of integrated sensors for the measurement of room (in a tabletop clock mode) or body temperature (in a wearable mode) and UV index for associated situations indicated by numbers (J). Photo credit: Sang-Hyuk Byun (C, F, G, and I) and Raza Qazi (H).