Authors

Jack Burd, Joon Yang, Ada Poon

Abstract

The Maxim Integrated MAXREFDES117# infrared and red light sensor chip has been shown to be viable for measuring SpO₂ in a forehead-mounted sensor array setup. This was demonstrated through the creation of a forehead-mounted pad prototype which successfully measured SpO₂ with >99% accuracy as compared to much larger conventional finger measurement apparatuses. This study’s results show that it is essential that the sensor patch be applied to thoroughly cleaned skin and placed in a dark environment in order to produce accurate measurements. The results also illustrate that this chip is sufficiently accurate to be employed in a sleep-monitoring fully-integrated system implemented on a patch that would replace the plethora of wires and sensors in current clinical sleep studies.

Introduction

Current sleep studies require an abundance of bulky equipment, including jumbles of wires, a finger pulse oximeter, and various electrode pads placed on the skull. As a result of this, it is more difficult for researchers to conduct sleep studies on children and the elderly. In addition, the bulky setup means that participants have greater difficulty falling asleep and that many participants who already went through a sleep study do not want to return for a second trial. 

The solution to this is to consolidate all of the sensors required for a sleep study into a single forehead-mounted sensor array pad, which is a project being developed by my mentor. This one single system adhered to the forehead would house four electrodes to measure brain waves via an electroencephalogram (EEG) as well as two chips, one to measure the participant’s pulse oximetry and another to process data from the electrodes and transmit all data from the sticker’s various sensors wirelessly to a remote computer for processing and analysis. In addition, there would be a thin-film battery providing power to the patch’s integrated circuits. This work focuses on the commercially-available Maxim Integrated MAXREFDES117# infrared and red light sensor chip [1] to be used for the pulse oximetry measurement. During this project, this chip was mounted in a prototype forehead patch to evaluate that it is accurate enough to be used in a future stand-alone pad monitoring system. 

Background

Most doctors and scientists today use the finger pulse oximeter, which is a device that clamps onto the finger to read a person’s heart rate and blood oxygen level, or saturation (SpO₂). However, this project uses the MAXREFDES117# chip, mounted on a patch that can be applied to a subject’s forehead to implement the same functionality. In the prototype pulse oximetry measurement system, an Arduino Uno is used to stream out the data to be analyzed on a computer, but this would be replaced by an integrated processing chip in a future forehead pad system. 

Both technologies work because of a technology called photoplethysmogram technology [2]. This method involves emitting weak infrared or red light radiation, and this light is absorbed by many different tissues in the body but is absorbed most by the blood. Oxygenated blood absorbs a different amount of light than deoxygenated blood because of the difference in light absorptions between oxyhemoglobin and deoxyhemoglobin, respectively. Thus, the sensor can determine what percent of one’s blood is oxygenated, or the SpO₂. While the finger pulse oximeters emit light from the top and detect how much has passed through the finger from the bottom, the Maxim Integrated chip emits light into the skin and detects how much is reflected. 

Pulse oximetry is critical in sleep studies. Pulse oximetry data can be used to diagnose in excess of 109 different sleep disorders [3], and information about these disorders is essential for sleep scientists and patients to know about. One of the most common sleep disorders is Obstructive Sleep Apnea (OSA), which may cause snoring and is fairly common. OSA occurs when the throat muscles relax and a person’s airway narrows, disrupting sleep. In addition to causing severe sleep deprivation, OSA can increase a person’s risk for cardiovascular disease. However, prior work by Chiang [4] demonstrates that SpO₂ measurements were incredibly effective for diagnosing sleep apnea in primary care hospital patients. His work further illustrates the importance of pulse oximetry measurements in sleep studies. 

Previous work by Longmore [5] looked into which parts of the body would give the best heart rate, SpO₂, and respiration rate readings when using reflective photoplethysmography, which is the same technology that the Maxim Integrated chip in this study uses. The study found that the best place to measure both heart rate and SpO₂was the forehead, but the forehead was one of the worst spots to measure respiration rate. Because this project is aimed at recording SpO₂ data for sleep studies, Longmore’s work served as support for the fact that a forehead setup could be a viable alternative to commercial finger pulse oximeters. 

Test Setup

Materials:

• Black cardstock paper –   1 3m Tegaderm Transparent Film Dressing
• 5 elegoo Dupont wires –   1 Arduino Uno
• Superglue –   Green tape
• MAXREFDES117#   1 Maxim integrated HR and SPO2 measurement chip

Physical Construction:

A 1×1 cm square was cut in the center of a Tegaderm forehead sticker. Then, the Maxim Integrated chip was inserted into this hole and secured using small strips of green tape. Next, two sheets of black cardstock were cut in the shape of the forehead sticker and super glued together and then onto the sticker. Five Dupont wires connected the chip up to an Arduino Uno, which then connected a computer via USB. A slight curvature was added to the forehead sticker system. 

The chip itself provides the best data when operating in dark conditions with a good point of contact between the glass sensor box and the skin. The Tegaderm sticker adequately provided a strong seal between the forehead and the sticker system, and pressed the sensor surface firmly against the skin, for optimal positioning. In addition, the black paper backing reduced the amount of light shining through the semi-transparent sticker in order to maintain a dark environment for the chip for best operation. Bending the sticker slightly created a better seal with the forehead.

Images of my prototype:

Front view: Side view:

Back view: Connected to Arduino and Computer:

Programming:

The Arduino Uno was programmed to take in a constant stream of infrared and red light data from the chip via an I2C input pin, and to calculate a new heart rate and SpO₂ value every second. The data values were then sent to the serial port in a specific format so that it could be read into excel for data analysis, utilizing the PLX-DAQ Microsoft Excel Macro [6].

Results 

Two and a half thousand data points were collected in order to adequately test the accuracy of this forehead mounted Maxim Integrated pulse oximetry chip. Below are some of the graphs of the accumulated data.

Two-hundred fifty seconds worth of forehead pulse oximetry data and finger pulse oximetry data were collected from three subjects while they were at rest. The forehead setup streamed the data from this forehead sticker, while the finger pulse oximetry data was manually recorded. Using the conventional finger pulse oximeter readings as the accurate baseline reference, the measurements of the chip in this work was compared against the baseline to quantify the error.

Subject 1 (13 year old male): 

    Maximum error:  9.2% 

        Average error: 0.67%

Standard deviation: 1.7%.

Subject 2 (50 year old female):

    Maximum error:  3.1% 

        Average error: 0.36%

Standard deviation: 1.7%.

Subject 3 (50 year old male):

    Maximum error:  5.2% 

        Average error: 1.3%

Standard deviation: 1.5%.

Across all three subjects, the average percent error was 0.78%, with a standard deviation of 1.63%, as shown the in summary table below:

	Subject 1	Subject 2	Subject 3	Total
Mean Error	0.67%	0.36%	1.3%	0.78%
Std. Deviation	1.7%	1.7%	1.5%	1.6%

Conclusion

Given the average percent error of 0.78%, and 1.6% standard deviation, this work has demonstrated that a forehead measurement of SpO₂ based on the commercial Maxim MAXREFDES117 chip infrared sensor is sufficiently accurate for clinical studies, and removes the need for the inconvenient finger sensor typically used for SpO₂ measurements. While the prototype setup used an Arduino Uno to interface to the chip and provide the measurement readout, this functionality can be readily miniaturized to achieve the ultimate goal of replacing all the wired telemetry sensors in a sleep study with a single patch applied to the forehead. The goal of this work was achieved to demonstrate the viability of the infrared and red light SpO₂ sensor.

Throughout the testing phase, several limitations with the prototype were uncovered. The chip’s readings became more inconsistent and less accurate if light leaked through the sticker and into the environment around the chip. In addition, participants needed to scrub their skin thoroughly in order to produce consistent, accurate data with the prototype. Lastly, the chip requires firm placement against the skin but it cannot be pressed into the forehead; one participant applied too much force to the sticker and pressed the chip into his forehead, which resulted in a number of data points where the chip did not take a usable reading. 

Beyond this forehead-mounted sensor array project, this methodology could be employed in other applications where SpO₂ is a salient data point, such as devices enabling the early and efficient diagnosis of COVID-19 [7], health-sensing wearables, etc.

Acknowledgements

I’d like to thank my mentor Joon Yang and Professor Ada Poon for their guidance and support and for this opportunity to explore the field of bioengineering and meaningfully contribute to an ongoing research. I’d also like to thank Professor Tsachy Weissman and Cindy Nguyen of the Stanford STEM to SHTEM Program for an enlightening and engaging experience this summer. 

References

[1] Maxim MAXREFDES117#: Heart-Rate and Pulse-Oximetry Monitor.  Retrieved July 10, 2020, from https://www.maximintegrated.com/en/design/reference-design-center/system-board/6300.html

[2] How Does PPG Technology Works? – SoulFitBlog. (2018, April 14). Retrieved July 10, 2020, from https://soulfit.io/blog/how-does-ppg-technology-works/

[3] Www.facebook.com/randy.clare.1. (2018, March 19). Here is what no one tells you about Pulse Oximetry for Sleep. Retrieved August 08, 2020, from https://thesleepandrespiratoryscholar.com/pulse-oximetry-for-sleep/

[4] Chiang, L. (2018). Overnight pulse oximetry for obstructive sleep apnea screening among patients with snoring in primary care setting: Clinical case report. Retrieved July 10, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6259496/

[5] Longmore, S., Lui, G., Naik, G., Breen, P., Jalaludin, B., & Gargiulo, G. (2019, April 19). A Comparison of Reflective Photoplethysmography for Detection of Heart Rate, Blood Oxygen Saturation, and Respiration Rate at Various Anatomical Locations. Retrieved July 10, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6514840/

[6] Parallax Data Acquisition tool (PLX-DAQ) software add-in for Microsoft Excel. Retrieved Jul 10, 2020, from https://www.parallax.com/downloads/plx-daq

[7] Pathak, N. (2020, April 28). What Is a Pulse Oximeter and Can It Help Against COVID-19? Retrieved August 07, 2020, from https://blogs.webmd.com/webmd-doctors/20200428/what-is-a-pulse-oximeter-and-can-it-help-against-covid19