Developing a Wearable OximeterFollow project
|1||LPT 80A Osram Opto, 70 ° IR + Visible Light Phototransistor, Through Hole 2-Pin Side Looker package||6547993|
|1||2.25 V Red LED 5mm Through Hole, Kingbright L-53HD||2285988|
|1||Breadboard Prototyping Board 80 x 60 x 10mm||102-9147|
|2||220 ohm resistors|
Inspiration and Objectives:
Wearable devices are changing the way we control our health and manage the available resources. Under the possibility of developing a certain sickness due to genetics, injury or other external factors, the severity of the disease and consequent risks can be lowered if symptoms are caught at an early stage. Wearable devices allow patients to monitor their health in a consistent manner such that the patient can realize any abnormalities present on the biological parameter being monitored as soon as they appear, without the need of a sign of discomfort present (e.g., pain, fever, etc).
Simultaneously, hospitals and care centres can discharge patients early on using these devices and use this space for other patients who may be more vulnerable and require more medical attention.
In fact, during the ongoing COVID-19 pandemic, some healthcare systems decided to provide recovered patients or individuals who are more vulnerable if they were to face the virus, with oximeters and a paper-based schedule, asking them to record their oxygen levels 3 times a day. Nevertheless, this methodology is highly inconvenient. Not only demands that the elderly remember to take the measurements and the time to take them, but also requires to carry the bulky oximeter structure to obtain the measurements, which may be also an inconvenience.
For this reason, the development of a wearable oximeter seems a more adequate system for oxygen concentration monitoring and it is the main objective of this project.
This is an ongoing project, to be finished by June 2021 as part of my final year project at the University. New updates coming soon!
Design and Implementation
The development of the project has been divided into 2 stages, the first being the ring oximeter development and the second: the wireless system development in the shape of a bracelet.
An oximeter measures the amount of oxygen dissolved in the blood known as Oxygen Saturation or SpO2. It is possible to obtain this concentration through Photoplethysmography (PPG), a technique that implies shining the skin with a certain wavelength of light and retrieving the waveform of the light reflected. The waveform is then divided into an 'AC' or 'pulsatile' component and a 'DC' or 'non-pulsatile' component.
The oxygen in the blood is carried by a protein called haemoglobin. When shining Infrared light towards blood, it will be absorbed by proteins carrying great amounts of haemoglobin, while the ones which do not, will absorb red light.
When using the waveforms resulting from both light sources, we can create a ratio called 'R' which can then be used to determine SpO2.
SpO2= -19 * R + 112
Thus, to design the sensor, the main circuit is simple, the two light sources in combination with a photodetector capable of detecting both wavelengths. As shown below.
The components are tested on a prototyping breadboard to test the system and later on placed into a ringed DIY structure, which, at this stage, is made out of cardboard and elastic bands to attach the sensor to the finger:
Why a ring? Aside from the considerable size and comfort difference with commercially available oximeters, the effect the shape would have on the signal has also been considered. The different shape aims to allow the user to perform daily activities as well as wearing gloves while using the device. The fact that gloves can be worn, would help the PPG signal to avoid being influenced by environmental light, while simultaneously enhancing blood circulation through the user’s hand which allow more accurate measurements. The latter benefit is not covered by conventional oximeters, it may be of special interest for people with circulatory health disorders such as Raynaud’s disease.
The waveform output for this system is shown by the serial plotter from the Arduino
Case 1when not wearing gloves (uncovered) :
AC and DC components are calculated through the serial monitor in the Arduino as
DC=minimumof the waveform
AC= maximum-minimum of the waveform
for each waveform, i.e.:
The DC signal can be further stabilized when gloves or any other covering that block environmental light, in particular, helps to determine an accurate baseline for the DC component calculation as seen below.
The values of SpO2 obtained were between 97 and 103. Future work would include a filtering system which perhaps may aid to make the device closer to our gold standard.
The bracelet is still in progress. It supposes to be the carrier of larger electronic platforms or interfaces that will send the data over a more complex database, for instance in the computer or mobile phone.
As a wearable device, this interface must allow communication exchange wirelessly. 3 different modules are currently being studied with this purpose: WI-FI, Bluetooth and NFC technologies. However, the size of each module, battery consumption and configuration is different and challenging when interfacing the sensor with it.