ALLPCB
Background
A joint report by China's Center for Disease Control and the Ministry of Health indicated that dyslipidemia, diabetes, hypertension, and other chronic diseases have seriously affected public health and safety. Attention to health management and blood pressure monitoring is increasing, and measurement methods and accuracy are becoming key concerns. Compared with traditional mercury sphygmomanometers, electronic blood pressure monitors are easier to operate, more portable, and suitable for single-person use, so they are widely applied for home blood pressure monitoring.
1. Measurement principle
The device uses the oscillometric method. Blood pressure refers to the lateral pressure exerted by pulsatile blood flow on the arterial wall. The peak of the pressure perpendicular to the aortic wall is systolic pressure and the trough is diastolic pressure. The oscillometric measurement process is consistent with the Korotkoff sound method. First, the cuff is inflated by a pump to occlude the brachial artery. After the pulse wave disappears, the cuff is slowly deflated and the pulse reappears. When cuff pressure reduces from above systolic pressure toward systolic, the pulse oscillation suddenly increases. When cuff pressure reaches mean arterial pressure, the oscillation amplitude reaches a maximum, then decays as cuff pressure further decreases. The oscillometric method determines blood pressure from the relationship between cuff pressure and pulse oscillations: the maximum pulse amplitude corresponds to mean arterial pressure, while systolic and diastolic pressures are computed from proportional coefficients relative to that maximum.
2. System architecture
The designed electronic blood pressure monitor uses Bluetooth 4.0 for data transmission, with an Android app and a bedside data acquisition unit (pressure acquisition box and cuff). A manifold connects the pressure sensor, solenoid valve, pump, and cuff via rubber tubing to ensure airtightness. The solenoid valve and pump connect to respective driver circuits, which interface with the control chip I/O ports. The control chip manipulates valve switching and pump inflation/deflation through the drivers. Cuff pressure and derived pulse oscillation data are acquired by a pressure sensor and separated by a signal conditioning stage. After computations inside the control chip, calculated blood pressure values are transmitted to the Android app for display, analysis, and storage.
3. Hardware design
3.1 Pump and solenoid valve
The device selects a 370 DC air pump (max pressure 450 mmHg), rated current 250 mA, operating voltage DC 3–6 V, noise <65 dB. The solenoid valve selected is model CJ13-A06A1 with leak rate 3 mmHg/min, rated current 60 mA, rated voltage DC 3–6 V, minimum operating voltage 3.5 V, DC resistance 100 Ω. This combination supports both fast and slow deflation modes.
3.2 Pressure sensor
Pulse oscillation signals are weak, prone to interference, and difficult to acquire. This design uses the MPS20N0040D-D piezoresistive pulse sensor (MPS), which offers accurate measurement, small size, stability, and good repeatability.
3.3 Signal conditioning module
The signal conditioning module amplifies and filters the sensor outputs: a slowly decreasing DC baseline representing cuff pressure and a superimposed oscillatory pulse signal. The conditioned signals are fed to the microcontroller. The design uses millivolt-level amplifier chips featuring low power, low input bias, low noise, and adequate precision, which are common in blood pressure measurement equipment.
3.4 Bluetooth communication module
The device uses an HC-08 Bluetooth serial module based on BLE 4.0. Compared with Bluetooth 2.0 modules, it has lower power consumption and faster transmission. In low-power MODE3, sending a single byte to the serial port can wake the module. The module supports multiple roles (broadcaster, observer, peripheral, central) and can reach up to 80 m range in open environments.
4. Bedside unit software
The bedside software receives commands from the Android app and executes corresponding operations. After signal conditioning and control-chip processing, the data are uploaded to the Android app via Bluetooth 4.0.
The system uses an STM32F103ZET6 microcontroller and Keil MDK as the development environment. Processed signals can be debugged via a serial terminal.
5. Android app
The Android app includes data processing, waveform plotting, and Bluetooth modules to handle data conversion, display, and transmission. The interface includes a splash animation, main measurement screen with controls (connect Bluetooth, start/stop), display area for heart rate, cuff pressure, systolic and diastolic pressures, an analysis view for historical waveform review, and dialogs for status prompts (reconnect, Bluetooth not connected, cuff not fastened correctly).
6. Measurement results and analysis
Stability and accuracy were evaluated for the Bluetooth 4.0 oscillometric monitor. Compared with mercury sphygmomanometer measurements, results were similar with no significant differences. The device met AAMI standards: 90% of diastolic measurements had a difference ≤5 mmHg, and 80% of systolic measurements had a difference ≤5 mmHg. The design is suitable for routine home blood pressure monitoring due to portability, simple operation, and stable performance.
