## 3 LIG-Based Wearable Systems for Multifunctional and Intelligent HCI
### Humidity-Based HCI System for Health Care and Non-Contact Interaction  
Currently, HCI systems based on LIG are mainly based on piezoresistive sensors, self-powered sensors, visual sensors, and acoustic sensors. While these sensors are effective in various HCI applications, they are not as reliable for monitoring respiration [38; 39; 40; 41; 42; 43]. On the other hand, humidity sensors have a high level of reliability for respiratory monitoring, but there is limited research on HCI systems using humidity sensors.  
To address this, [PERSON] et al. proposed a humidity-based HCI system that consisted of a LIG-based humidity sensor and LIG-based acoustic device [29]. As shown in Figure 15, the humidity sensor and the acoustic device were made on two sides of a Nomex fire-resistive paper. The acoustic device was made of LIG obtained by laser writing directly on the surface of the Nomex fire resistive paper, while the capacitive humidity sensor was made by a two-step process. The first step was to obtain a LIG-based interdigital electrode by laser writing directly on the paper. The second step was to apply a graphene oxide (GO) solution to electrodes to form a GO film.  
The flexible humidity-based HCI system can be attached to a medical mask and operates in the following manner: the humidity sensor side of the system is aimed toward the wearer for continuous monitoring of respiratory signals, and the acoustic device side of the system is the outer side to ensure the sound emitting effect. In the case of abnormal breathing patterns such as respiratory arrest or rapid breathing, the system will trigger an alarm through the acoustic device. Furthermore, the system can also achieve functions similar to the artificial throat mentioned in Section 3.5, the difference being that the sound emitted is controlled by the wearer's breathing rhythm and intensity. Along with these two features, this system can realize non-contact HCI functions. Based on the high sensitivity of the humidity sensor, the system can sense whether a finger is near the system by detecting the water vapor emitted by the finger. As the finger approaches, the capacitance of the sensor increases. The capacitance readings differed significantly between the wet and dry fingers.  
Figure 15: The humidity-based HCI system for health care and non-contact interaction. Reproduced with permission from [29]. Copyright 2022 American Chemical Society.  
Based on the above observation, the research team assigned the capacitance peaks from wet fingers as lines in Morse code and those from dry fingers as dots in Morse code. By approaching the system with fingers of varying levels of humidity following Morse code, a non-contact HCI was successfully implemented. Moreover, the research team realized the control of a smart car through the system. By matching different breathing rhythms with different control commands (such as rapid breathing corresponding to moving forward), the system could control the car in response to the breathing of the wearer.  
In summary, this humidity-based non-contact HCI system has a wide range of applications including the detection and alerting of respiratory illnesses, an artificial throat to help aphasia patients to speak again, and controlling robots without physical contact (which is particularly valuable during pandemics) and so on. With a broad range of applications in health care and intelligent HCI, this wearable system has enormous potential for future development.  
The above HCI systems focused on receiving and analyzing instructions but less on executing them. To further construct a closed-loop HCI wearable system with a complete instruction collection-instruction transmission-information processing-instruction execution (feedback) function, in the next section, a closed-loop HCI system with the motion instruction execution function will be introduced and discussed.