Part I: SUB Pulse Oximeter Construction

A ruggedized and universal platform for recording of physiological, physical, or chemical parameters in challenging environments (Figure 1A) was developed within a corporation of Austrian, Croatian, Swedish, and United Kingdom teams.

(A) The SUB pulse oximeter; (B) Close of the pulse oximetry sensor, distance between LED and photodiodes (3 mm) is marked with yellow arrow; (C) Freediver in the pool with the two sensor heads on the temples with the data storage unit attached to the arm.

The core component of the device is a ST Microelectronics STM32L452 microprocessor (STMicroelectronics International N.V. Amsterdam, The Netherlands), which is based on a 32 bit ARM Cortex M4 core. The integrated floating-point unit allows fast calculation of advanced algorithms. This processor is especially designed for low power consumption making it perfectly suitable for battery powered instrumentation. A 32 GB micro-SD card is integrated for data storage. A basic 4 × 20 characters liquid crystal display (LCD) is used to show status information of the device. The device can be connected to a USB port to charge the internal Li Ion battery and to download the recordings. The device is operated with one magnetic switch. Prototypes of the housing of the device were 3D printed. The electronics are encapsulated in silicone gel (Wacker Sil Gel 612, Wacker Chemical Corporation, MI, United States) making it dust, water, pressure, shock, and vibration proof. The microcontroller features several interfaces, which can be used to connect various sensors. A schematic overview is provided in Figure 2.

Layout of the technical platform with sensors.

All data can be transmitted to a PC in real time with the use of either a WIFI interface based on the ESP8266 chip (Espressif Inc., Shanghai, China) or an optical fiber output. The advantage of the latter is that it can be used in water and does not require special sealing. WIFI is not suitable for underwater applications (Hollinger et al., 2011). A small screen enables real time display of the different raw variables.

The device is designed as a datalogger platform. Several prototypes were developed for recording of different parameters, but for the current study focusing on SpO₂, the prototype was equipped with two SpO₂ probes based on the MAXIM MAX30102 chip (max 50 Hz sampling rate), temperature sensors, and an ambient pressure sensor.

The rapid development of the smartphone and fitness tracker industry has led to new low-cost electronic chipsets for measurements of physiological parameters in battery powered instruments. The MAX30102 (Maxim Integrated, CA, United States) sensor frontend was chosen for recording of plethysmograms. It includes red and infra-red emitters, diode drivers, photodiode, photodiode amplifier, analog to digital converter, and controller. Up to two sensor frontends can be connected to the platform, which allows synchronous recording of plethysmograms (Figures 1B,​,CC).

The firmware of the platform was developed in the programming language C and Eclipse (Eclipse Foundation, Inc., Ontario, Canada) was chosen as development environment. A graphical user interface was developed in Labview (National Instruments Corporation, Austin, United States), which can be used to show all parameters in real time and display the recordings (Figure 3). Sample algorithms for SpO₂ calculations in low noise environments were supplied by the manufacturer (Maxim Integrated, CA, United States), however, artifacts of any kind may lead to incorrect SpO₂ calculations. The algorithm was therefore optimized and includes an auto-correlation algorithm to filter motion artifacts. More specifically, the SpO₂ algorithm is based on calculation of the Root Mean Square (RMS) value of alternating current (AC) and direct current (DC) of the red and infrared channel as described by the manufacturer (Application note 6,845, Maxim Integrated, CA, United States). An improved version of this algorithm was also performing a correlation between infrared and red signal to calculate a measure of signal quality. In an undisturbed signal, infrared and red signal correlate well, and in case of bad correlation, the calculated values are discarded.

Example from the recorded plethysmograms (two signals for red light and two signals for infra-red light). The pulsatile signals show a large waveform with a sharp peak and a clear dicrotic notch, indicating good signal quality.