Poster Q15 - Open source platform to measure physiological signals in a classroom setting

Introduction: To complete their education, biomedical engineering undergraduate students must measure and interpret data from living systems[1]. One approach to achieve this learning goal, is to record data from normal volunteers in a classroom setting. Inexpensive equipment is needed that can safely record common physiological signals. While there are commercial systems available [2] [3], there is no approach that uses an open hardware design and gives the option to expand functionality at later time. We present an example solution that can be produced in most academic settings using commonly available electronic and additive manufacturing techniques. An advantage of such a solution is that its data recording and computing components support programming courses and the measurement modules can also serve academic research projects.

Materials and

Methods: The system consists of several independent modules, a charging station and a base station. The modules as well as the base station contain an ESP32 micro controller. The modules transmit data to the base station using the ESPNow protocol. All modules are battery operated using lithium ion battery. Common ESP32 platforms connect to custom electronics boards. The boards contain analog front ends and interface with standard biomedical sensors. The components are mounted in a 3d-printed housing.

The system uses micro controllers to communicate wirelessly with a data recorder. It uses common protocols to control and record data. All systems are battery operated to electrically isolate the sensor modules from building ground. Open source software is used to develop micro controller instructions (Arduino IDE [4] and display and record data (Python [5]). To develop custom electronic circuits we used Eagle CAD (Autodesk) which is available freely when designing two layer printed circuit boards. For mechanical designs, Solidworks (Dassault Systems) and OnShape (PTC Inc) was used. Solidworks is available through academic licensing and OnSahpe offers free education accounts.

Custom electronic boards were manufactured by PCBWay (PCB assembly service).

Most components were designed as part of a senior capstone project in the college of engineering at the University of Arizona.

For manufacturing, soldering equipment and a 3d-pinter is needed. Electronic wiring and component assembly requires soldering of through hole components. Programming requires a regular desktop or notebook computer.

Results, Conclusions, and Discussions: All design files and software for this project are available on Github [6].

Analog front ends and analog measurement electronics were selected to record the following physiologic signals:

- Temperature
- Electric Potential (e.g. electro cardiogram or electro myogram)
- Electric Impedance
- Optical Transmission (e.g. plethysmogram or blood oxygenation)
- Physical activity (e.g. inertial measurement)
- Sound (e.g. heart or lung sounds)
- Pressure

A module to measure air quality parameters was also developed. Further modules are under development such as an air flow sensor module to measure lung functions.
For temperature measurements a medical grade temperature sensor (Amphenol, NTC Therrmistor) is connected to Wheatstone bridge. The micro controller’s internal analog to digital unit converts the voltage differential to resistance. The Steinhart-Hart equation is used to produce temperature readings. An error of less than 0.1 degrees C was achieved in the 0-50 centigrade range.
Sounds are recorded with a modified stethoscope that is connected to a microphone. The frequency range and amplification of an amplifier was adjusted to match the expected sound levels. An audio codec (ES8366) produces the digital readings. Reducing the sampling rate and dynamic range allows transmitting the data with the ESPNow protocol. Arduino-audio-driver and Arduino-audio-tools [7] allowed for efficient programming.
To create a blood pressure module the sound sensor was combined with an air pressure sensor (Honeywell, MPR Series) that can be connected to an arm cuff.
For bio potential and and bio impedance recordings a custom circuit for the MAX30001G (Maxim Integrated) was created. It allows monitoring ECG and respiration.
For pulse oximetry the AFE4490 (Texas Instruments) was used, allowing to connect reusable finger clip sensors that use a DB9 connector.
For activity measurements, the inertial measurement unit ICM20948 (TDK Invensense) and the barometric pressure sensor BMP581 (Bosch) were integrated into a sensor module.
The cost for each module varies and depends on the quantity ordered through the assembly service. An estimated cost for the assembled PCBs, housing and auxiliary parts for six different senor modules is in the range of $500-$1000.

Acknowledgements (Optional): [1] ABET Accreditation Criteria, Program Criteria for Bioengineering and Biomedical and Similarly Named Engineering Programs, Citerion d, "Making measurements on and interpreting data from living systems", Biomedical Engineering Society.
[2]: Bioradio, https://staging.glneurotech.com/product-category/bioradio-all
[3]: Protocentral https://protocentral.com
[4]: https://www.arduino.cc/en/software
[5] https://www.python.org/
[6] https://github.com/uutzinger/BioMedicalSensorBoard
[7] https://github.com/pschatzmann