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Real-world Pulse Oximeter Design Solution

An RS Components customer is currently working on a new design of pulse oximeter and has allowed us to share with you extracts of the Bill-of-Materials from their design. In this article we will work through the key sub-systems of the pulse oximeter and discuss the design decisions and components ultimately selected for the design.

Pulse Oximeters - Explained

Handheld Pulse Oximeters are typically low-cost devices which are used in patient self-monitoring and decision-making for hospitalisation. A low blood oxygen saturation can be caused by multiple medical problems and therefore a low reading is a trigger for further medical investigation to determine the root cause. One cause of a low blood oxygen saturation is inefficiency in a patient’s respiratory system, caused by poor gaseous exchange efficiency within the lungs as a result of the onset of COVID-19. This low-cost tool therefore plays an important role when healthcare systems are stretched due to the COVID-19 pandemic.

How do they work?

The arterial blood oxygen saturation (SaO2) is the metric doctors ideally want to monitor – however the methods to monitor this are invasive and can be performed only by trained medical professionals in appropriate medical facilities. An alternative is to estimate the oxygen saturation using a pulse oximetry device, these devices provide an estimate known as the peripheral oxygen saturation (SpO2) which is a good enough approximation to understand whether a patient is well or in need of support.

Pulse oximetry uses the absorption of particular wavelengths of light to determine the level of oxygen saturation. This exploits the differing absorption of infrared light in oxygenated and non-oxygenated haemoglobin. Light in the wavelengths where the biggest differences in absorption (between oxygenated and non-oxygenated haemoglobin) are transmitted through a part of the body and detected using a photodetector which is sensitive to the same wavelengths of light. It is then possible, after calibration, to determine the level of oxygen saturation in the blood by measuring the absorption of the relevant wavelengths. For the best performance and to minimise the influence of external lighting conditions multiple LEDs of different wavelengths are used.

Design Overview

Power

Pulse oximeters can be small battery-powered devices or integrated with larger and more complex health monitoring systems such as those used for continuous patient monitoring in hospitals. The design we are sharing with you is of a rechargeable battery-powered device.

The power subsystem is largely comprised of parts from STMicroelectronics. Battery charging is controlled using an STMicroelectronics Li-Ion Linear Battery Charger with LDO regulator (829-1431) , this device regulates the charge current to the Li-Ion cell whilst protecting against overcharge, over-discharge and overcurrent scenarios.

A reliable approximation of the device charge level is important to enable medical staff to be able to rely on a device and understand when the device needs to be recharged. The Open Circuit Voltage (OCV) of a cell can be used to approximate battery charge level however there are multiple issues, it cannot be measured whilst the device is operational, OCV can vary with temperature, age of cell and condition. A more reliable method of determining battery charge state is to use a ‘gas gauge’ device.

Gas gauges utilise current sensing, coulomb counting, battery voltage and temperature to calculate an accurate approximation of the cells charge state. The STMicroelectronics STC3115 gas gauge (196-1452) utilises these techniques to provide a battery state-of-charge to the device microcontroller over an I2C interface.

Voltage regulation is required to provide a stable and constant voltage from the Li-Ion cell which will vary in voltage output dependent on its state-of-charge. STMicroelectronics provide a range of voltage regulators designed to output across a range of pre-defined voltage levels such as the STLQ015 range, the 3.0V device has been used in this design (STLQ015M30R) (188-9308) .

A typical 3.6V Li-ion cell has a max charge voltage = ~4.2V and end-of-discharge voltage = ~2.8V, the selected voltage regulator requires an input voltage of at least +0.3V than that of the output voltage, in this case, 3.3V for a 3.0V output from the regulator. This reduces the usable range of the Li-ion cell from 2.7-4.2V to 3.3-4.2V, therefore reducing the possible run-time of the device. This problem can be overcome through the use of a boost converter to ensure that the voltage regulator is always supplied with a sufficiently high voltage to produce the required output for device operation. The Monolithic Power Systems MP1541DJ-LF-P boost converter (917-5822) was used in this design, this device has a configurable output voltage and can operate to input voltages as low as 2.5V enabling the full usable-charge range of a Li-Ion cell to be utilised.

Display

A method of clearly displaying the current system status and measured blood oxygen saturation is required. This requirement must be balanced with the need to minimise device size and optimise the power consumption of the overall system. A common approach in modern devices is to use a fully customisable LCD or OLED display vs. traditional fixed LCD element devices. OLED devices have increased in popularity in recent years, as cost has decreased, given the technologies better performance across different lighting conditions and greatly reduced power consumption when compared with LCD. In this design, the Midas Displays 96x64 pixel RGB OLED display (823-5973) has been used. With an active display area of 20x13mm this display provides enough flexibility to clearly display the current blood oxygen saturation, system status and support a basic menu system.

Sensing

The sensing system of the pulse oximeter comprises two LEDs of differing dominant wavelengths around the wavelengths absorbed by oxygen-carrying haemoglobin, and a photodiode sensitive across the range of wavelengths. It is likely that the LEDs of differing wavelength are pulsed alternately, enabling the absorption of both wavelengths to be measured using the same photodiode. This approach ensures that the absorption of both wavelengths is measured at the same point on the body, eliminating variance from taking measurements at different points (affected by skin, bone and muscle variation) and interference between the two emitters. An Osram 950nm Infrared LED (778-1383) and Osram 660nm Red LED (126-3152) were used in this design. The selected photodiode is an Osram visible light photodiode (652-0207) which provides >75% sensitivity across 650-950nm range enabling accurate measurement of absorption across the wavelengths of interest.   

Interconnect

Reliable connectors are an essential part of any design and crucial to ensuring the longevity of a device over time, whilst simplifying manufacture and repair processes. When selecting an appropriate connector many factors must be considered including size, cost, number of mating cycles and ease of assembly.

Pulse oximeters are typically wearable devices and therefore a key design consideration is minimising the overall size of the device. It is therefore crucial to select connectors which take up the minimum volume inside a device casing. The Molex Pico-EZmate wire-to-board connector system offers a solution with a 1.5mm mated-height, ideal for space-constrained designs. Headers (700-0824) , contacts (700-0830) and connector housings (700-1022) are available from RS Online.

Micro-USB is currently one of the most common interfaces used for device charging. Micro-USB provides a standardised connector and power level which enables consumers to use the same power supply and cables across multiple devices, improving the user experience and reducing cost vs. using a custom-designed connector. Molex provides a range of Micr- USB connectors designed for differing environments and range of mating cycles, this design utilises the popular right-angle design (702-5481) .

Protection

It is important to provide protection between the internal electronics of the device and external interface. Fuses are commonly used on USB interfaces to protect the connected charger and device in the event of an overcurrent event. These can occur as a result of a short-circuit or failure within the device. Resettable fuses protect the connected charger and enables malfunctioning circuits to be temporarily disconnected. Littelfuse provide a wide range of circuit protection devices. This design utilises the 250mA (541-3552) and 1A (541-3574) thin-film surface mount resettable fuses.

Bill-of-materials

Our customer bill-of-materials has been attached to this article for your reference - we hope this may help you in making your design decisions!

Are you involved in medical device design and manufacture?

Let us know in the comments if you are involved in the response to the COVID-19 response in pulse oximeter design.

Lead Inspiration Engineer @ RS Components and DesignSpark

7 May 2020, 8:42