Final Design

     The final design features three subcomponents effectively integrated under one housing structure. The integrated housing positions the three sensor subsystems based on a single device body, which was mechanically attached with clips to the BP cuff to be worn around the user’s wrist during operation. The internal electronics were organized using a printed circuit board (PCB) with an embedded Raspberry Pi Pico 2W microcontroller responsible for all subsystem control and signal processing, including the algorithms used to convert raw sensor data to interpretable medical results. An ePaper screen was fixed to the top surface of the device that displays these results to the user in real time, along with the two buttons that provide the inputs of the user interface. Data was simultaneously streamed to a WiFi-hosted webpage for wireless display of results on a personal device, allowing a medical professional or individual user to save the results for future analysis. 

     The heart rate monitor is an electrocardiogram (ECG) module located inside the housing with electrode leads fixed to a strap worn around the user’s chest, providing the most accurate and centralized readings for heart rate variability. This chest strap is wired to the main device body, constrained by a connector port, and features three contact points for the electrode leads that directly touch the user’s skin. The module records and outputs a voltage difference (typically on the scale of 0.5-5 mV) across two electrodes located on the front of the body, with the third electrode placed on the user’s back serving as a reference point. 

     The spirometer measures breathing volume using a single static pressure tap that reads the pressure difference between the pipe flow inside the spirometer and atmospheric pressure. As a user exhales through the device, the air moves through a flow-straightening section used to reduce turbulence and make the pressure response more repeatable between trials. That airflow creates a pressure change compared to atmospheric, which a piezo-resistive pressure sensor measures through a port mounted flush on the cylinder wall. The measured pressure difference was experimentally calibrated against known flow rates to create a relationship between pressure and airflow. During the spirometer trial, the device converts the measured pressure difference into flow rate, then integrates the flow over time to estimate exhaled volume. A second calibration is applied between the measured volume and the true volume. This correction was applied because the experimental calibration relied on maximum flow measurements; as a result, an additional adjustment was needed to better align the integrated spirometer output with the actual delivered volume. After both calibrations, the spirometer can determine breath volume from breathing pressure measurements in an accurate and repeatable way, serving the overall purpose of the device to prompt intentional breathing as a stimulus for the user’s autonomic nervous system while also reporting the breath volume as an important respiratory health indicator.

     The BP cuff is worn around the user’s wrist, and its mounting connections to the main body enable the device to be fully wearable. The cuff is inflated and deflated by a small brushed motor pump and solenoid valve, while the pressure of the cuff is measured with an identical pressure sensor to the ones used in the spirometer. These devices are pneumatically connected to the cuff via air-line tubing routed through ports in the housing. The sensor reads the pressure inside the BP cuff, to which a program on the microcontroller applies the oscillatory method and identifies the user’s resting systolic and diastolic blood pressure through analysis of arterial oscillation amplitudes. This system also has the functionality to inflate the cuff to systolic blood pressure and hold for five minutes as a stimulus for the user’s autonomic nervous system.