Pattern design of a liquid metal-based wearable heater for constant heat generation under biaxial strain.

As the wearable heater is increasingly popular due to its versatile applications, there is a growing need to improve the tensile stability of the wearable heater. However, maintaining the stability and precise control of heating in resistive heaters for wearable electronics remains challenging due to multiaxial dynamic deformation with human motion. Here, we propose a pattern study for a circuit control system without complex structure or deep learning of the liquid metal (LM)-based wearable heater. The LM direct ink writing (DIW) method was used to fabricate the wearable heaters in various designs. Through the study about the pattern, the significance of input power per unit area for steady average temperature with tension was proven, and the directionality of the pattern was shown to be a factor that makes feedback control difficult due to the difference in resistance change according to strain direction. For this issue, a wearable heater with the same minimal resistance change regardless of the tension direction was developed using Peano curves and sinuous pattern structure. Lastly, by attaching to a human body model, the wearable heater with the circuit control system shows stable heating (52.64°C, with a standard deviation of 0.91°C) in actual motion.

Consistent and Reproducible Direct Ink Writing of Eutectic Gallium-Indium for High-Quality Soft Sensors.

Given the need for stretchable sensors, many studies have been conducted on eutectic gallium-indium, which has superior properties as a conductive ink. However, it has remained a challenge to manufacture sensors in a consistent and reproducible manner because conventional mold-based fabrication still depends highly on manual techniques. To overcome this limitation, the direct ink writing was used in this study, focusing on improving the stability of writing by exploring issues related to failure and ensuring the consistency of the microchannel by selecting appropriate process variables, including the syringe material. As a result, multiple sensors produced under the same manufacturing conditions had similar behaviors. This fabrication technique improved the accuracy of manufacturing a microchannel, and its behavior was predicted successfully by a simple mathematical model, which was confirmed by nondestructive inspections of the microchannel. In developing a one-piece glove-type sensor without an assembly process, the efficiency of the fabrication technique was also emphasized.