## 3 LIG-Based Wearable Systems for Multifunctional and Intelligent HCI
### LIG and Liquid Metal Integrated Multifunctional Wearable HCI System  
The development of high-sensitivity motion detection is crucial for monitoring human movement, robot activity, and human-machine interaction. In particular, the development  
Figure 8: The integrated multi-modal wireless system with feedback functions. Reproduced with permission from [21]. Copyright 2021 [PERSON]. of HCI systems that can achieve highly sensitive motion interaction is of great significance for robot control, which can realize high-precision remote surgery, remote robot control, and so on. [PERSON] et al. developed an integrated multifunctional wearable HCI system for motion monitoring and human-machine interfacing through the clever combination of LIG and liquid metal [12]. Similar to the work mentioned in Section 2.8, this work also determined motion by detecting the contact between a sliding liquid metal droplet and the LIG electrode. However, this work improved the sensitivity of motion monitoring by using the principle of inertial sensing and improving the sliding ability of the liquid metal droplet on the PI-based flexible substrate. The LIG particles provided a 3D interpenetrated shell around the liquid metal droplet to prevent infiltration to laser-treated PDMS, PI, and LIG surfaces, which allowed the liquid metal droplet to slide in the sensing channel freely. Moreover, by designing the arc structure composed of the PI substrate, PDMS package, and LIG interfinger electrode, the liquid metal droplet could slide freely under the influence of acceleration and quickly return to the initial point thanks to gravity. This ensures accurate real-time acceleration monitoring and direct reading. Figure 9 displays a graphical representation of the wearable system. A wireless PSoC (programmable system on chip) was used to realize multi-channel signal processing and wireless transmission. The LIG-based humidity sensor, temperature sensor, and strain sensor were used to record the temperature and humidity information of the human body, robot, or environment, and the strain information of the human or robot during movement. A pair of liquid metal-based inertial sensors was used to monitor the acceleration and the force applied to the system when the acceleration was applied. The sensors, electrodes in inertial sensors, and interconnects in this wearable HCI system were all made by laser direct writing technology and were further encapsulated by PDMS if necessary. Ag paste was used on the contact pads to realize the electrical connection between the PSoC and the other external modules. The inertial sensor demonstrated a high sensitivity of 6.52% m\({}^{-1}\) s\({}^{2}\) in an acceleration range from 1 to \(\sim\)30 m/s\({}^{2}\) at the 0.167 Hz low frequency. During the demonstration, the multifunctional HCI system realized a highly sensitive human/robot motion monitoring, and the human wearer realized the precise control of a robotic arm. The system can not only be used to collect physiological information such as human movement to realize health and safety monitoring but can also be used to accurately record the motion state of the robot, determine if their functions are normal, and provide real-time feedback. One of the most notable features of this system is its ability to establish a closed-loop remote interaction between humans and robots. After further integration with motion execution units and realizing 3D high sensitivity inertial sensing, this wearable  
Figure 9: The LIG and liquid metal integrated multifunctional wearable system. (**a**) Conceptual illustration of the multifunctional wireless platform showing an exploded view of the different layers composing the platform containing the inertial, temperature, humidity, and breathing sensors. (**b**) Optical photograph of the fabricated multifunctional platform; the inset shows the LM placed inside the curved patterned PDMS channel. Reproduced with permission from [12]. Copyright 2022 American Chemical Society.