## 2 Materials and Methods
### Developing an Integrated Wearable Cardiopulmonary Device for a Patient Monitoring System
As a sensing material, the respiration sensor will utilize Laser-Induced Graphene (LIG), a 3D porous graphene that was found to exhibit high sensitivity and biocompatibility in our previous study [5]. LIG is an attractive option for the practical fabrication and application of wearable sensors owing to its simple synthesis process, controllable design and morphology, and excellent electrical and mechanical properties [38; 39; 40]. Compared with conventional graphene synthesis processes, which include complex chemical reactions and multi-stage mechanical mechanisms [41; 42], direct lasing of polymer substrates such as polyimide (PI) converts the layer into graphene through a photothermal effect without the requirement of additional chemicals or high-cost equipment. As shown in Figure 2a, laser irradiation on the exposed PI layer of FPCB makes a stable electrical connection between copper traces and fabricated LIG sensing area, and another LIG will be formed on a single PI film stacked on the top of the LIG area on FPCB (Figure 2b,c). As the final step of the fabrication process, the LIG  
Figure 1: Modularized device for the simplified patient monitoring system. (**a**) A 3D drawing of the modularized device for the proposed patient monitoring system. (**b**) Configuration of the overall circuit components. sensor with a double-layer structure will be covered with an additional layer of elastomeric silicon to enhance its biocompatibility and protect it from external damage. The fabricated sensor will measure the respiratory signal by pressure sensing. It will be able to measure the patient's respiratory motion, detecting the subtle changes in pressure caused by the movement of the chest during inhalation or exhalation, which provides reliable data for respiratory monitoring. One of the challenges facing wearable sensor development is the issue of the reliable connection between the sensor and its front-end circuit [43; 44]. In this study, we propose a solution to overcome this challenge by utilizing a silk-screened area of FPCB and direct lasing of the exposed PI layer during sensor fabrication. This approach allows for a straightforward and stable connection between the sensor and its front-end circuit, ensuring reliable and accurate data acquisition. MAX86150, which operates on a 1.8 V supply voltage and includes two internal LEDs with different wavelengths, a photodetector, and an ECG subsystem optimized for dry electrode operation, will be placed on a platform to record PPG and ECG signals. The dry electrodes will be integrated into FPCB as part of the Cu signal traces, eliminating the need for separate electrode attachments. Conventional disposable gel electrodes, a type of wet electrode, are most commonly used in the ICU and show excellent adhesion and signal quality; however, their long-term usage may cause skin irritation (e.g., tickle, skin redness, and allergic reaction) [45] and discomfort from the long cables that come out of the electrodes. Moreover, the long-term use of gel electrodes triggers the problem of signal degradation caused by the dehydration of the electrolyte gel, which may lead to inaccurate measurements and reduced monitoring system reliability [46]. Conventionally, the high electrode-skin impedance of dry electrodes makes the clinical usage of dry electrodes challenging. The absence of conductive gel makes it difficult to form a stable interface between the skin and the electrode, increasing impedance and inhibiting conformality between the electrode and skin [47; 48]. To address this issue, the proposed  
Figure 2: Fabrication process of the LIG-based mechano-acoustic sensor. (**a**) laser irradiation on the exposed PI layer of FPCB to make a stable electrical connection between copper traces and fabricated LIG sensing area; (**b**,**c**) another LIG will be formed on a single PI film stacked on the top of the LIG area on FPCB; (**d**) Integrated version of the proposed sensor.