## 2 Materials and Methods
### Developing an Integrated Wearable Cardiopulmonary Device for a Patient Monitoring System  
This report proposes a cable-less patch-type multimodal biometric system that combines photoplethysmography (PPG) for SpO\({}_{2}\), ECG for cardiac monitoring, and a respiration sensor for RR. Additionally, the system has automated data collection and a visualization interface. The proposed patient monitoring system is designed to overcome the problems of conventional clinical cardiopulmonary monitoring systems, which cause fatigue and discomfort for patients and medical staff due to the complex arrangement of cables and powerlines from various medical equipment and generate a significant workload for medical staff due to the unintegrated system that lacks compatibility with electronic medical record (EMR) systems. Using a modularized and cable-less design, the proposed system aims to alleviate these issues and provide a more efficient and user-friendly approach to the attachment, usage, and interpretation of patient monitoring systems in the ICU. A modular approach will be employed when designing the device. Here modularization refers to dividing a complex system or device into smaller units that can be independently developed, manufactured, and replaced. In this context, the proposed device will comprise two separate modules (units): the sensing module that measures the vital signal and the control module that processes and transfers the acquired data. This modularized structure allows excellent reusability, easy replacement, and personalization of the device and, thus, provides the most optimal solution for usability and cost efficiency. The modularized devices will comprise a semi-permanent control module that includes electronics on a rigid printed circuit board (PCB) and an expendable sensing module built on a flexible PCB (Figure 1a). The modular structure of the device offers cost-effectiveness by enabling the reuse of the control module, which contains high-cost electronics. Furthermore, it can prevent cross-infection among patients with skin-transmitted infectious diseases by using personalized sensing modules for each patient. These two modules will be easily combined with a mezzanine connector, and the modular junction is marked with a dashed circle in Figure 1a. The control module has dimensions of 30 \(\times\) 40 mm. It will be powered by a rechargeable Li-polymer battery with a nominal voltage of 3.7 V. For the microcontroller, we have selected the nRF52840 (Nordic Semiconductor, Norway, distributed by Mouser Electronics) built on the powerful ARM(r) Cortex(tm)-M4 CPU (Cambridge, UK). This microcontroller offers various digital interfaces and protocol support for BLE, which is essential for establishing wired and wireless communication within the system. The overall circuit configuration is demonstrated in Figure 1b. \begin{table}
\begin{tabular}{c c c c c} \hline
**Survey for ICU Medical Staff** & \
\begin{tabular}{c} **Strongly** \\ **Agree** \\ \end{tabular} & **Agree** & **Neutral** & **Disagree** &
\begin{tabular}{c} **Strongly** \\ **Disagree** \\ \end{tabular} \\ \hline \multicolumn{5}{c}{**Ease of use of the proposed device**} \\ \hline The overall system was easy to use. & & & & \\ \hline The user interface of the system was intuitive and easy to understand. & & & & \\ \hline It was easy to learn how to use the system. & & & & \\ \hline It was easy to attach/detach the device. & & & & \\ \hline \multicolumn{5}{c}{**Effectiveness and satisfaction**} \\ \hline The usage of the system reduced the workload and fatigue. & & & & \\ \hline The usage of the system was time-efficient. & & & \\ \hline I would use the system again. & & & \\ \hline I would recommend the system to other medical staff. & & & \\ \hline \end{tabular}
\end{table} 
Table 3: Questionnaire for intensive care unit medical staff regarding their experience with the new device. The sensing module will comprise a respiration sensor, printed Cu electrodes for the ECG subsystem, and an MAX86150 biosensor module.