## 2 LIG-Based Wearable Systems for Multifunctional and Intelligent Health Care
### Gas-Permeable and Multifunctional On-Skin System  
The LIG-based wearable system must be designed to accommodate the daily wear and tear it will face without hindering the wearer's daily activities while also minimizing the impact of the wearer's activities on the functionality of the system. If the on-skin wearable system is gas-permeable, it can avoid interfering with the perspiration of the wearer, which may cause redness or inflammation, and can improve comfort during daily long-term wearing.  
To make the LIG-based wearable system gas-permeable, [PERSON] et al. proposed a gas-permeable and multifunctional on-skin wearable system based on a sugar-templated porous substrate [10]. The fabrication process of the on-skin wearable system is shown in Figure 1. A variety of LIG-based devices including electrophysiological signal electrodes, hydration sensors, temperature sensors, and joule-heating elements were realized by CO\({}_{2}\) laser writing on polyimide (PI). By designing open-mesh patterns and serpentine layouts, the sensors adapt to the stretching of the human skin during wearing. To achieve the gas-permeability, they used a soft silicone elastomer (Young's modulus \(\sim\)170 kPa) and cane sugar powders to make the porous elastomer substrate. The silicone elastomer was mixed with the sugar powder at a 1:2 weight ratio. When the mixture was partially cured, the LIG-based devices on the PI substrate were transferred onto the elastomer-sugar composite substrate through direct transfer printing with added pressure. Finally, the composite substrate with LIG devices was cured at room temperature and the sugar template was dissolved with deionized water to form a porous sponge-like elastomer substrate. Since the on-skin system consists of a porous LIG layer and porous elastomer substrate, the whole system has good gas permeability (water-vapor permeability \(\sim\)18 mg cm\({}^{-2}\) h\({}^{-1}\)).  
LIG-based devices in this LIG-based wearable system prepared using the same method can enable the wearable system to achieve multiple functions. The electrophysiological electrodes can detect high-quality signals from EEG (electroencephalogram), ECG (electrocardiograph), and EMG (electromyography). The hydration sensor consists of a LIG-based coaxial dot-ring structure and can be used to detect the skin impedance and reflect the skin hydration level. The multi-modal temperature device can be used for both temperature sensing and joule-heating.  
The realized wearable system enables the monitoring of a variety of key physiological signals and can realize health interventions (such as heat packs). These wearable systems have scalable functionalities where manufacturing new devices, sensors, or actuators can easily be added to meet the changing needs. Most significantly, the gas-permeability makes wearable systems more suitable for daily long-term wear. This will be an important requirement for the practical development of wearable systems, which is of great significance for improving the comfort of wearers. In the next section, we will introduce another important development direction of wearable systems, namely, the development of degradable wearable systems with low cost to meet the requirements of green applications and large-scale applications.