## 2 LIG-Based Wearable Systems for Multifunctional and Intelligent Health Care
### Integrated Multi-Modal Wireless System with Feedback Functions  
For those who have limited ability to care for themselves such as the elderly, disabled, and infants, health monitoring systems that not only provide long-term daily use but also feedback and alarm functions are especially crucial.  
Considering the high rate of sudden infant death during sleep, [PERSON] et al. developed a wearable system suitable for real-time health monitoring and the alerting of abnormal physical conditions of infants during sleep or in the intensive care unit [21]. The system can monitor the sleeping postures, respiration rate, and diaper moisture wirelessly, and immediately provide an alarm if there are any abnormal situations. To continuously track the baby's sleep status with high comfort, the whole flexible system (in addition to mobile signal analysis software) was integrated into the diaper using a customized layout. As shown in Figure 8, the system consisted of three components: flexible sensor sheets, a flexible processing circuit with a Bluetooth module, and a mobile phone with a signal analysis and alarm module. The flexible sensor sheets contained three kinds of sensors including a tilt sensor, a respiration sensor, and a humidity sensor. The tilt sensor was used to record sleeping postures. This encapsulates a non-toxic gallium-based liquid metal droplet in a laser etching hollow space with PDMS and bare LIG electrodes and uses its slide to feel the baby's postures. To sense the sliding posture of the liquid metal droplet, eight pairs of LIG-based electrodes were realized using a two-step laser direct writing process, where the rough surface of LIG allows the liquid metal droplet to slide freely on it. The attachment and separation between the electrode and the liquid metal droplet caused by the different tilt directions of the sensor belonged to the \"on\" and \"off\" states, respectively. Eight pairs of electrodes could distinguish the eighteen tilt angles. Based on the tilt sensor, the system could distinguish whether the baby was in a good sleeping position. The respiration sensor was realized using a strain sensor made by infiltrating a PDMS solution into a porous LIG layer. The respiration rate was sensed using the skin deformation of the baby's chest and abdomen. The humidity sensor used stacked ZnIn\({}_{2}\)S\({}_{4}\) as the active sensing material and porous interdigital LIG as electrodes to promote the absorption and desorption of water vapors. In addition, there was a moisture-proof film on the surface of the sensor to avoid skin damage or material contamination caused by direct contact between the water and ZnIn\({}_{2}\)S\({}_{4}\). The humidity sensor could also tell whether the diaper needed to be changed.  
The whole system can monitor the baby's sleep position, breathing, body surface humidity, and other key information without affecting the baby's normal sleep, and transmit the information wirelessly to the neonatal nurses' mobile phone in real-time. When there are abnormal conditions such as abnormal infant posture, suffocation, and excretion, the system can sound a timely alarm that alerts the personnel to intervene, reducing the probability of sudden infant death and the pressure on the workers. In addition, the system is also suitable for the elderly and disabled auxiliary care, and more sensors can be added based on the wearer's disease characteristics to achieve targeted health monitoring.  
It can be seen from this research that long-term wearing comfort, long-term signal acquisition capability, and closed-loop feedback capability are the key conditions for an integrated wearable health care system suitable for daily applications.