## 2 Materials and Methods
### Utilization of the Proposed Apparatus  
The proposed apparatus is intended to acquire biometric data during the writing activity. In addition to the gathering of handwritten signatures, this study also includes the collection of handwritten phrases, sentences, and upper- and lowercase letters. A tablet computer is used in conjunction with its corresponding stylus to gather handwritten data. The apparatus is currently implemented using Samsung Galaxy Tab S6, Galaxy S9+, and Gear S3 (Samsung Electronics Co., Ltd., Suwon, Republic of Korea) primarily because these are the devices currently available to us, which we typically conduct experiments with and are equipped with the necessary sensors. The apparatus uses standard Wi-Fi and Bluetooth communication and runs on Android/Tizen, making it adaptable to other brands of Android devices with similar hardware. Regarding cross-platform considerations, our system is adaptable to different devices and is not limited to Samsung models only. Android was initially selected as the primary test platform due to its open and flexible nature. However, modern tools for cross-platform development can be utilized to extend compatibility to systems such as iOS, thus expanding the use of the proposed solution across different devices and operating systems. In this case, some software adjustments may be necessary to accommodate varying sensor specifications and data handling. Due to manufacturer restrictions, a direct connection between the smartwatch and tablet was not possible, so a smartphone was used as an intermediary. Communication between the smartphone and smartwatch was handled via the Galaxy Wearable app over Bluetooth. Commands to control the smartwatch sensors were sent from the tablet to the smartphone, which then relayed the messages to the smartwatch. To synchronize sensor measurements and manage battery consumption, a WebSocket mechanism enabled real-time communication between the devices, also handling commands for the piezoelectric sensor connected to the smartphone. This approach ensures low-latency, bi-directional communication, allowing efficient data transfer between Android devices. The scenario shown in Figure 6 demonstrates an experiment participant interacting with the system. As seen in the image, every component of the apparatus is mounted on a perforated plate measuring 120 by 60 cm. In addition to the utilization of screws, a variety of metal brackets, holders, and plates were employed to securely fasten the devices in their designated positions. The purpose of this practice is to provide equitable circumstances for all users, maintaining the stationary position of the gadgets during their use. To ensure data collection is conducted under controlled conditions, the same guidelines are applied to all participants in the study. The user starts the experiment by putting a smartwatch on their dominant hand at the beginning of the procedure. The tablet computer is positioned in a stationary position above the two piezo sensors on the board. The first piezoelectric sensor establishes a connection with the smartphone via the use of the 3.5 mm connector present on the device. However, a USB sound card and a USB-C to USB-A converter are both required in order to properly link the second piezo sensor to the tablet. To partake in writing activities on the tablet, the user employs either a stylus or a finger. Depending on the modality, in the instance of stylus writing, the user employs a stylus equipped with magnets and an ArUco marker, as seen in Figure 3. When using finger writing, the user will position the 3D-printed ring with the magnet on the index finger of the writing hand. Furthermore, the smartphone device is mounted on the custom-made holder next to the tablet. Its function is to capture the complete procedure with its camera sensor. Also, it is important to point out that a direct connection between the smartwatch and the tablet device was unfeasible. This is a consequence of the restriction that was specified by the manufacturer of the devices.