Low Profile SpO

2

Monitoring

A Major Qualifying Project

Submitted to the Faculty

of the

WORCESTER POLYTECHNIC INSTITUTE

In partial fulfillment of the requirements for the

Degree in Bachelor of Science

By

Linnea J. Brown

Date of Submission: December 9, 2019

Stephen J Bitar, Major Advisor

Brown, Linnea J.

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Brown, Linnea J.

Abstract

This project explores the feasibility of making a pulse oximeter or SpO2 monitor in the form

factor of a standard credit card, to minimize cost and maximize simplicity. Several commercially

available pulse oximeters were reviewed to determine their operation and whether or not they could

be adapted to the desired design. It was found that creating an inexpensive, low profile devise

appears to be a viable and achievable goal, but further testing and development is required.

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Brown, Linnea J.

Acknowledgements

A big thank you to the following people:

● My loving and supportive family

● Professor Bitar

● Alex Briskman

● Timon Butler

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Brown, Linnea J.

Glossary of Terms

SpO2 - This light based monitoring system is used to detect the percentage of oxygen in the

blood.

Arduino - a common microprocessor that runs on C++

IR - InfraRed

LED - Light Emitting Diode

MCU - MicroController Unit

MSP430 - a low power microprocessor

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Table of Contents

Introduction 6

Background 7

Common Uses for SpO 7 Prior Art Teardown 9 Problems Addressed 9 Clinical Monitoring 10

Heart Rate (BPM) 10

SpO2 10

Conclusion 12

Problem Statement 14

Client Statements 14

Design Objectives & Constraints 14

Objectives 14

Constraints 14

Methodology 16

Input 16

Digital Processing 18

User Interface 18

Summary 19

Implementation/ Design 20

Input 20

Processing 20

Output 21

Analysis and Results 22

Input 22

Output 25

Recommendations 28

Conclusion 30

Bibliography 31

Appendix A 33

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Table of Figures

Figure 1- Power Regulation Board 9

Figure 2- Topside of Monitor 10

Figure 3- Main Control Board 11

Figure 4 - Absorption Coefficient of IR and Red light 13

Figure 5 - Light Absorption 13

Figure 6 - Relationship between SpO2 and R 14

Figure 7 - System Block Diagram 18

Figure 8 - Sensor and Amplification Circuit Diagram 19

Figure 9 - Prototype Sensor input system 25

Figure 10 - Prototype amplification components setup 26

Figure 11 - Steady Heart rate Output 26

Figure 12 - Schematic for the SSD1306 LED board 27

Figure 13 - Heart-rate signal with Interference 28

Figure 14 - Powered On Initial Output 29

Figure 15 - Design Concept of Final Monitor 30

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Brown, Linnea J.

Introduction

The focus of the project is designing and building a prototype commercial

(“outpatient”) SpO2 monitor that can inform future design and refinement. There are many

different devices that allow for SpO2 monitoring. This includes large scale medical devices,

and expensive smartwatches. The closest devices to the proposed design are the current

portable models available on the market. These devices range from $15-20 and all have a

similar designs. They have a clip that goes over the finger and holds it in place as the

oxygenation is read. This is then processed and output on the screen. These devices are most

often powered by two AAA batteries. This makes the device bulky and heavy. Information

provided in the teardown of similar products, background on what is expected in the market,

and what the client expected all suggested that a smaller more portable version of SpO2

monitor was feasible. This device should be easy and intuitive so anyone can use the

monitor with minimal instruction or assistance.

The design can be broken down into 3 major parts. An input which is related to

reading in the data. Second is the related analog processing. The processing involves

manipulating the data into digital information that can be analyzed and modified into a data

stream for the output. The third aspect is the output, or what the user sees from the device.

This includes any visuals like a screen. These 3 main parts can be connected together to

create the complete system. This is a fairly linear design and this benefits a straightforward

construction, with discrete (input, processing, and output) sections that can be easily

connected.

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Brown, Linnea J.

Background

Research started many years ago, before the popularization of the smartwatch. The question

posed at the time was whether an SpO2 monitor could be built with the same dimensions of

a credit card, replicating its length, width, and thickness. With the advent of the smartwatch

we have seen many devices, such as the Apple Smartwatch *** approach relative thickness,

and surpass the specifications for length and width. These devices also include much more

than just SpO2 and heart rate monitoring. This alleviates the problem for more affluent

people who might need consistent health monitoring but leaves those who struggle to use, or

afford such technology. These people are left with old and outdated systems. The project

intends to assist people who are not affluent enough to have or use the more advanced

technologies. This allows us to focus on creating a device that would fit the original

specifications but also allow innovation in systems to ensure the device is an affordable and

easy to use.

The first version of an oximeter was designed by Glenn Allan Millikan in the 1940s. It

read absolute O2 saturation by sending light through the ear. These original systems

involving light filters and photocells are the basis for the modern pulse oximeter [Millikan.

1942]. The first version of the “modern” pulse oximeter, a device that reads Infrared (IR) and

red light absorption through the finger, was designed in the 1970s at Nihan Kohden. The

device was commercialized by Biox in 1980 [Severinghaus 1987]. The general simplicity and

non-invasive nature of this device caused it to quickly become a staple in medical care. Common Uses for SpO

Continuous monitoring is used both in and outside the hospital and helps ensure the

continued health of patients. SpO2 monitoring is a common reading for doctors and nurses.

Oximeters are a necessary part of any medical monitoring device. In the hospital oximeters

are a necessary part of a standard health monitoring. This becomes extremely important

when a patients’ heart or lungs could be compromised in some way, or they would not be

breathing on their own such as under heavy anesthetics. This can include, but is not limited

to, surgery, post-surgery care, Neonatal care, and Emergency care [Phillips. 2003]. A drop in

blood Oxygen levels is an early warning sign for possible heart failure. Having early warning

signs for dropping oxygen levels, and possible heart failure can prevent a patient from

suffering permanent damage, or in the worst cases death. While this level of monitoring has

been a staple of in-patient care for decades only within the past 10 years has it become a part

of out-patient, and independent health monitoring [El-Amrawy 2015].

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SpO2 continue to be an important health monitoring tool for long-term health

problems. In modern medicine long term data collection and monitoring has become an

important part of helping doctors and patients achieve good health. The primary conditions

monitored in this way are heart and lung conditions, this is intended for people with

repeated heart strokes, irregularity in rhythm, or people with COPD or similar breathing

problems[]. The people most at risk for contracting these health problems are also those

least likely to have access to devices and technologies that could keep them healthy.

Less than 5% of Adults over the age of 50 have “ideal” cardiovascular health,

there are many factors contributing to this issue including poor eating habits, and lack of

physical exercise[CDC maps 2015]. Though using an oximeter to monitor heart rate and

oxygen levels is not a solution to heart problems it can preemptively alert those of

pulmonary abnormalities. This can be important for those who are most at risk for heart

problems, the elderly. This warning can ensure they can receive the assistance they need.

Since Pulse Oximetry measures the successful oxygenation of the blood it is also a useful

tool for monitoring the state of the lungs in problems such as COPD, Chronic Pulmonary

Disease [Mannino 2000].

COPD is an umbrella term for diseases that cause airflow blockages and breathing

problems. Similar to heart problems continual monitoring can help to alert to potential

problems before they become dangerous[cdc.gov 2019]. COPD is more prevalent in rural

areas, counties with a population below 200,000 people. Rural populations experience higher

rates of COPD prevalence, hospitalization, and death, about 8% or double that of those in

urban areas, [cdc.gov/ruralhealth 2018]. In addition, the elderly, people over 65, people who

are unemployed or unable to work are most likely to contract COPD according to a report

by the CDC in 2013[Wheaton 2013]. Prompt response to medical emergencies increases the

likelihood that the person can be saved and minimize lasting complications. Having

continual monitoring with health issues can give predictive warnings, and prevent those

suffering from contracting dangerous side effects.

Currently the easiest way to monitor and track pulse oximetry is using a modern

smart watch. These devices range from a little under $100 to well over $500 dollars. This

means it is difficult for those in highest need of this monitoring to acquire such monitoring

as they can be difficult to use, and expensive to obtain. The current low cost alternative

available is $10-15 which is a reasonable price but lacks certain amenities that could greatly

assist with use. These designs are all very similar. They are powered by two AAA batteries

which makes these products clunky and heavy. The lack of rechargeable batteries and lack of

memory storage creates a massive gaps between the low-cost alternative and modern smart

watch.

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Prior Art Teardown

We tore down two low cost, modern SpO2 monitors. Which would give a baseline of care.

During teardown it was determined that, excluding some external differences, the circuitry

of these two devices were identical. The body was divided into 2 main parts and is designed

to clamp around the finger. One side, the bottom side, holds the 2 batteries and the LEDs

(Figure 1). This is connected to the otherside of the device by a few thin wires. These AAA

batteries make the device much heavier than necessary and requires a large cost over time.

Figure 1: Power regulation board on the Underside of the Monitor

The other part holds the main circuit board, LED screen, and a multipurpose button

(Figure 2). The LED screen shows the heart rate waveform, SpO2 percentages, and general

info about the device such as battery power.

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Figure 2: Top side of monitor with cover removed

Along with regulating the main screen the circuit board has a small MCU in charge of

calculating the SpO2 value and visual appearance of the screen (see Figure 3). This one

component is the workhorse of the circuit allowing for transfer analog input from the

receivers into a digital output onto the screen and an auditory alert through a speaker.

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Figure 3: Main control Board of the monitor. Top and Bottom .

Control over the circuit uses a small button situated near the interface screen. This

button is the user’s primary way to interact with the device. Pushing this button once turns

on and prepares it to read the blood through the finger by turning on the LEDS. This button

has a couple of additional features depending on how long you hold the button for, or how

many times its pressed. Though these features aren’t completely intuitive, and instructions

provided are on a small sheet of paper that could be easily be misplaced.

Problems Addressed

The low price device has a fairly high level of accuracy even compared to hospital

standard devices. The issues with these devices don’t come from its function as an oximeter,

but rather the gap between the inexpensive device and the more modern smartwatch, which

caters to those with a larger disposable income and a high degree of technological literacy. A

key feature of smartwatches is how multifunctional they are, which is an advantage for those

who understand how to use them, and a disadvantage to those who lack the knowledge.

These devices also give highly accurate SpO2 readings along with recording the data, which

can help enhance the quality of care of those with chronic conditions.

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The population most likely to have chronic health conditions that could be helped by

continuous SpO2 monitoring are those with low income, or those with a low level of

technological literacy. The current devices exist at two extremes, devices like the Apple

smartwatch and Fitbit, which have a high level of entry, in both cost and sophistication, and

the budget option which has fallen behind, and hasn’t evolved beyond the most basic designs

for portable monitoring. These “budget” devices lack innovation that have happened over the

past 10 years and that can be seen in its lack of data tracking and continued use of disposable

batteries, along with other signs of its age. This leaves those most in need of help monitoring

potentially fatal conditions with very few options for ideal maintenance and possible

improvement to their health.

Clinical Monitoring

Heart Rate (BPM)

This is the most basic form of monitoring and measuring for the heart. This can

differentiate between proper functioning and issues that cause irregularities in the regular

and consistent beating of the heart. Currently BPM is used as a basis for increasing accuracy

in other heart monitoring systems.

SpO2

This light based monitoring system is used to detect the percentage of oxygen in the

blood. This is used to determine if the heart is pumping correctly and if the heart is properly

infusing the blood with oxygen from the lungs. It is a necessary part of complete monitoring

systems for most in-hospital patients.

Oxygenated and non-oxygenated blood absorb light differently, these absorption

differences can be read by a photodiode as the light which passes through the finger. Red and

near-IR light is used because this wavelength range penetrates tissue where higher

wavelengths like green and blue can not accurately penetrate through the tissue [Matcher

2016].

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Figure 4 - Absorption Coefficient of IR and Red light

Two separate wavelengths near-IR and Red are used (Figure 4). This is because the difference

between oxygenated (HbO2) and non-oxygenated (Hb) blood molecules can be observed in

these two wavelengths. Oxygenated blood absorbs red light and reflects IR light.

Non-Oxygenated blood does the opposite it absorbs IR light and reflects red light. By

analyzing the information received by the photodiode. These values fluctuate as blood

pumps through the arteries. This causes the artery to grow and shrink, which also affect the

volume of blood the light is being reflected through. This gives a base level non-fluctuating

Direct Current (DC) and an Alternating current, created as the volume of the blood changes

(Figure 5) [Tamura 2018].

Figure 5 - Light Absorption

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Accounting for the fluctuations in the value we can determine a modulation ratio(R)

also sometimes referred to as R/IR[Johnson 2006].

R = (ACr DCR)/(ACir DCir)/

The relationship between the calculated value and the actual SpO2 can be seen in

Figure 6. This conversion is used to turn the information provided by the light into a value

that can be used by doctors to analyze the blood. SpO2 and heart rate are important aspects

of the human body that can give insight on how well the body is functioning while using

minimally invasive and highly effective methods.

Figure 6 - Relationship between SpO2 and R

Conclusion

SpO2 monitoring has been used in clinical settings for over 40 years, and its continued use in

the medical community is a statement to its importance. In today’s medical environment any

additional data can help diagnose and treat a variety of diseases. In the case of SpO2

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monitoring it gives information on the general effectiveness and functionality of the heart

and lungs. As time has passed the focus is most often on continued innovation leaving a gap

between those who can afford new technology and those left with the remnants of past

innovations. Options are skewed toward those with the income to spare and the

technological literacy to use these devices. This means those with the largest need to monitor

health conditions with Oximetry, the elderly and low income, are not given easy access to

the devices that could help them. The goal of this project is to close this gap. To do this the

project will take advantage of the improvements in technology in the medical industry,

portable devices, and compact designs. Using these innovations an affordable, and effective

device that reflects the needs of those who currently don’t have access, a lower price range

and higher ease of use.

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Problem Statement

Create a model of a modern commercial (“outpatient”) SpO2 monitor and

communicate its functionality.

Revised statement.

Provide a proof of concept design that focuses on heart rate monitoring that can be

easily adapted into a small form factor SpO2 monitor.

Client Statements

Information provided in the teardown, background on what is expected in the

market, and what the client expectation all suggested that a smaller more portable version of

SpO2 monitor was desirable.

Two types of people need regular SpO2 monitoring. People doing high intensity

fitness, such as runners and mountain climbers, and people with persistent heart problems

such as arrhythmia and or risk of heart attack.

Design Objectives & Constraints

Objectives

Ease of Use:

This device should be easy and intuitive to use. Anybody should be able to use the

monitor with minimal instruction or assistance.

Portability/Size reduction:

The final design profile should be similar to that of a credit card. This will make the

device more portable. This would also greatly improve the thickness in comparison to other

existing portable SpO2 monitors.

Constraints

Battery Power:

To fit such a thin design the battery needs to also be extremely thin. This means no

matter which battery is chosen it will have a lower amperage than standard portable designs.

This means power draw must be a constant consideration.

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Size:

All the parts of the design must have a low profile small form factor option.

Ease of Use:

To make sure issues are minimal for users the backend should be robust and error

proof. This helps to ensure user intuitiveness is minimal.

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Methodology

The design can be broken down into 3 major parts (Figure 7). An input which is

related to reading in the data(sensor input), and the analog processing (Amplification)

related to it. The Digital Processing which involves manipulating the data from the input into

digital information that can be used and modified then turned into a data stream for the

output. The third aspect is the output, or what the user sees from the device. This includes

any visuals like a screen. These 3 main parts can be easily connected together to create the

complete system.

Figure 7 - System Block Diagram

Input

There were two input systems discussed. These two options affected all aspects of the

project and represented a major decision point as they affected the design for all other

aspects of the project. A high-tech option focused around a chip made by MAXIM that

focused on SpO2 monitoring, and a low tech option that emphasized hardware design. These

two designs focused on different aspects of heart rate and SpO2 monitoring. Each came with

different advantages and disadvantages.

The MAXIM chip, and further the MAXIM evaluation kit focuses on high level SpO2

monitoring. It is designed to minimize some of the inherent flaws in the system. On the

evaluation board all information is controlled by the Maxim chip. It controls the LED inputs

and the photodiode receiver. Along with this it includes an accelerometer. This system allows

for a high accuracy and better erroneous reading cancellation.

The complexity of the maxim system is also its largest failing. It is a more advanced,

and self contained system. This makes it more difficult to select information from the system.

The software program provides an SpO2 output waveform but no way to route this

information to a place where it could be extracted. It transfers information over encoded

bluetooth with no easy way to add in additional receivers, and the few areas that could be

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tapped into on the board didn’t have information that could be considered “readable.” Even

if the information required could be retrieved it would require a higher level of programming

language and understanding.

The “Low Tech” option is a hardware design based on bandpass filter/amplifier. This

includes a LED setup using the standard Heart rate/SpO2 set up (Figure 8).

Figure 8 - Sensor and Amplification Circuit Diagram

This simpler design allows customizability with a general focus on hardware design.

There is only one stream of data to be processed. The MCU receives a single digital signal

that is an amplified version of the outpost signal of the LED receiver. Not only does this

allow for less complicated signal lines, which helps with circuit board design, it also means

the programing is more straight-forward. Maxim contains more safety checks and signals

but these make for increased points of system failure. The other advantages to using a

system based around analog components is the increased customizability. The components

can be adjusted easily depending on power, weight, and size requirements. This is a benefit

for the final goal of the project.

The simplicity of this system could also be a disadvantage. This setup does not

regulate the errors that are prominent to Spo2/ Heart rate reading. This means the signal has

higher likelihood of inaccuracy in the readings. This could be compounded by two

prominent issues with traditional Spo2 monitoring: moment and light. These can be

minimized by proper mechanical set-up, which requires more mechanical set-up .

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Between the high tech and low tech systems the analog system was chosen. Though

the Maxin system has more features, a combination of relative complexity and programming

inexperience made it ultimately an impractical choice. The focus on creating a solid analog

system that could be a proof of concepts without unnecessary difficulty.

Digital Processing

Determining the Input was an important factor of the design, and affects how the

system is set up and the relative focus of the project. Both types of possible input were taken

into account when determining the processing unit, but focused on the use of a hardware

input, and the necessities of the output. It was determined early on that an MCU would be

needed to create the ideal output with a screen. Though the input design could be set up to

provide an analog output it would not meet the standard we had set for the output. This is a

marginally easier decision because it is dependant on the relative familiarity with, or ease of

learning, the language used, and setup of the microcontroller.

We focused on devices and systems with which we had some familiarity. We used a

version of an MSP430, because we had previous experience and it is ideally suited for low

power devices, such as the one in our design. To assist with connecting each piece of the

system, and the focus on proving the system without additional complications, we decided to

work with an MSP430 Development board. This did not restrict our options so much that a

board with a chip that would be functional for the design was not found. An

MSP-EXP430FR2355 was eventually acquired as it was nearly ideal for our design, with its

low power requirements, and had a small variety of sister chips that could be used in the

final design if changes were necessary.

Using the MSP430 Development board came with an unexpected boon: the company

had recently switched from Verilog based software, to their own personal coding shell based

on Arduino which uses C++. This is a much more widely used language, which helps in

finding coding samples and tutorial, and that we have previous experience with.

User Interface

Large OLED screen that allows for a detailed output of SpO2, BPM, and other useful

tidbits of information including the battery charge and time of day. The focus of the

prototype is to present proof that the device designed can work in practice. Knowing this, the

focus was for choosing a screen that would be similar to a final design, but also one that

allowed ease of connectability and ease of use. The ultimate goal is to find a low profile, low

power screen that can fit into the lightweight design.

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When looking into the final design, as an outline for our choice for the proof of

concept screen, we started with the largest factor, power. The final design requires a thin

battery, which is a restriction to the amount of power that can be provided. Screens are, by

their nature, huge power draws. Knowing this we focus on the practicality, importance of,

and relative power budget necessary for each factor of the screen: size, color, lighting, type.

It was quickly determined that a large screen would be impossible without

compromising our power budget, and therefore other aspects of the system that are

considered more important. For similar reasons, and the self imposed sized restriction we

also focused on smaller screens, those with a similar size to the batteries suggested in the

final design.

Unlike the prior two attributes we felt as though some level a backlight, could be low

enough of an additional power draw to be added as more of an aesthetic feature into the

design. Throughout this process aesthetic ideas such as this will be put at a low priority,

especially during proof of concept, and are subject to being removed or ignored in favor of

presenting a cohesive prototype.

Summary

The focus of the design is on creating a prototype of an Oximeter. This design will be

a thin portable design. It will have an analog input that simple and low profile in

construction. The processing will be done by a low power MSP430, and have an LED screen

output that balances power saving and aesthetics. The final design should also incorporate a

thin rechargeable battery.

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Implementation/ Design

The focus of the project is designing and building a prototype that can be used, in

future, to refine the design. This is a fairly linear design and this benefits a straightforward

construction, with discrete (input, processing, and output) sections that can be easily

connected.

Input

An SpO2 monitor is practically two heart rate signals that are read and compared.

The calculations and comparison all happen during the processing phase. This means the

hardware has two main jobs, first read the heart rate as a waveform, and amplify it to a point

that the MCU can easily read.

The amplifier is a bandpass filter centered around the range of a human heart rate

which is about 2Hz. The design uses a low pass filter that isolates signals below 10Hz. This

gives a bit of wiggle room around the signal without picking up too much extraneous data.

The first stage of the hardware input is the LEDs, during proof of concept we focused

on acquiring a heart rate with an IR sensor system. For SpO2 monitoring an IR system and

Red LED are required. They will both read blood flow rhythm, but only the IR presents an

accurate reading of the heart rate whereas the red focuses on the difference in oxygenated

blood. Proving the specific setup works for an IR system, will also in turn provide a solid

starting point for the design and setup for the red LED receiver. The basic idea is simple

power up an IR emitter and IR receiver, both with resistors to regulate power levels.

Processing

The code can be broken into two parts: signal processing and output design. Included

in signal processing is reading the analog signal, placing it into the chart integer, and the

scaling of the visual output. The output design dictates the specific placement of the output

and its visual appearance.

The input design begins by reading in the waveform from the IR LED into one of the

arduino’s analog inputs and then storing the data stream into a circular array in order to

capture the most recent 3 seconds of data. The specific period of sample retention is

adjustable by increasing or decreasing the number of milliseconds before the next data point

is read. The array is then displayed by calculating a scalar transformation from the 10 bit

analog input (0-1024) precision that the data is stored with, to the 128 pixel vertical screen

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height that it will be displayed with. The scalar is calculated so as to ensure the signal always

fills about 80% of the screen at any time in order to allow it to be examined more easily.

The input capture code can then be duplicated to read in a second waveform for the

red LED simultaneously using a second arduino input pin, this signal. Unlike the heart rate

(IR LED signal), the visual spectrum signal will not have a direct data display output.

However, these two waveforms can processed together to determine the modulation variable.

which can then be used to determine the SpO2 percent. Additionally, the IR waveform can

be used to determine heart rate by calculating the peak to peak signal duration between

heart beats and then inverting the number into beats per minute. Signal peaks can be

identified by examining local and relative maxima relative to the adjustable data collection

speed.

The output code design focuses on how to display the information and relate it to the

input signal waveforms. The display uses some premade files and codes to assist display

output. This allows us to focus on how the signal output appears and not some of the

difficulty associated with designing code for an unsupported LED display.

Output

As stated in the methodology, the screen used for this design is not entirely reflective

of what would be, theoretically, chosen and presented in the final low profile design. This

screen does not meet those requirements because it is too thick and has a higher than ideal

power draw. It is powered at the target voltage of 3.3V. Despite these issues, it serves as an

adequate model of the kind of screen that would be in the final design. The screen is

connected over 8 lines using SPI protocol. Three of these lines provide power to the screen

while the rest provide data. SPI is a commonly used protocol across all sorts of digital

peripherals and is universal enough to reasonably assume compatibility with most other

component displays that don't have their own onboard display processing capabilities in

order to use more advanced physical layer communications such as HDMI. The SPI protocol

itself is supported using an open source SPI library freely available for arduino that is more

than robust enough to perform all of the necessary display work.

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Analysis and Results

Input

Before the final design and the hardware focused setup, time was spent designing

and building a system that worked with a digital setup provided by Maxim. This was a

complete system, and development board, that allowed for advanced SpO2 readings along

with various accessory components and systems to help minimize the problems inherent to

traditional SpO2 and heart rate monitoring. The main issue we encountered with this design

was two related ideas, its general complexity, and the amount of propriety or restricted

information that related to it. This often made the device difficult to work with, and few easy

ways to gain additional information to solve these problems. Owing to this, along with all of

the subsequent possible issues that could result from these concerns, it was decided to

transfer over to a simpler hardware design. As such the results and tests done will focus on

the success of this prototype, and how that relates to the project moving forward. The Maxim

board did still provide a useful template for design decisions and proved helpful for

determining the eventual hardware system.

The first design draft just used a low pass filter, letting only values below 10hz pass to

the amplification circuit. This only showed a minor amplification that was insufficient to

allow it to be read by the MCU (2-3x). To increase the amplification, the circuit was modified

to also include a high pass filter, turning the system into a bandpass filter. This filter closed

to 1Hz as to allow through any signal about 1Hz. Creating an amplification range for signals

between 1hz and 10hz.

When the system is turned on the LEDs begin emitting light. This light is shown

through the finger into the LED receiver . Problems initially encountered with the LED

related mostly to power draw and a proper holding system for the emitter/receiver pair. If the

regulating resistors before the LEDs are too large it lowers the intensity of the light and dulls

the measurements, which increases the likelihood of error. Once the proper resistances were

achieved the next focus was on the IR emitter the original LED had too large of a focus angle

and this caused distortion in the signal. The second emitter had a much lower focus angle,

15degrees, and this made it easier for the Receiver to catch the light after it had passed

through the finger. To further help issues with the pairing of the emitter and receiver a

holding device was used to further reduce light leakage and increase stability, Figure 9.

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Figure 9 - Prototype Sensor input system

Though clunky, the light cancelling and stabilizing properties of the PVC barrel design used

in another design by Professor Bitar proved sufficient to meet the intended purpose of this

preliminary design. A more streamlined version of this was chosen to be integrated into the

final design.

Once the signal is successfully collected it is prudent to remove the DC voltage to

ensure it can be successfully amplified by the Op-Amp. To do this, a capacitor is added to the

circuit removing any DC voltage (Figure 10). A voltage divider needs to be added back at a

voltage we can control. If this is not done, only the positive side of the signal will be

amplified and presented at the visual output. To simplify the voltage divider two resistors of

equal value allowed for a DC voltage addition of half the total Vcc.

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Figure 10 - Prototype amplification components setup

Once the signal is properly cleaned it is sent through the bandpass filter described

above. Narrowing in on the signal was difficult as it required finding a filter that could

provide adequate amplification for low frequency of the adult human heart, about 2Hz. To

do this high value resistors and capacitors were used to balance the values. Once the design

of the amplifier was done it consistently provided an output that could easily be read by the

MSP430 (Figure 11).

Figure 11 - Steady Heart-rate output

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Output

The most difficult part of processing was enhancing the heart beat signal sufficiently

to gain meaningful data without overly degrading the signal quality. After boosting the signal

as much as possible via hardware amplification, the code further aids in this process by

ensuring that the signal is always displayed across as much of the screen as possible without

having to clip it. This accounts for the height of the waveform and that it exists within the

bounds, in this case the whole screen, and expands the signal to fill the area. This helps

ensure the signal is reasonable no matter the initial size of the input. It reads the relative

minimum and maximum of the signal then runs it through an if/else statement to create an

upper and lower buffer. These buffers are then used to assist the pointer for displaying the

signal. Once this is done it is set up as an SPI output that can be read by the the LED board.

To simplify the prototype an LED board is used. This board takes an LED screen and

attaches it to a larger board that does the regulation and accessory circuitry necessary to

connect to an arduino, or similar board. The schematic seen in Figure 12 is the proto board

used for the LED and can be used to help design the final product.

Figure 12 - Schematic for the SSD1306 LED board

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Brown, Linnea J.

Most of the focus was put into the code used for the visual appearance of the LED

(see appendix A). The output design without digital amplification can be seen in Figure 13.

Problems encountered with the appearance of the waveform were to be expected because of

the nature of basic SpO2. The signal becomes degraded by any sort of obstruction on the nail

such as nail polish, and any sort of movement. These problems cannot be fixed unless the

entire system is changed or advanced.

Figure 13 - Heart-rate signal with Interference

The Physical output setup itself proved to be fairly simple as the board was connected

to a small PCB which did all the necessary hardware design on board (Figure 14). Most of the

difficulty around the Output will be related to its setup when being implemented in the final

design. This includes things like power draw, specific visual appearance - such things as

including a charge level, heart rate, and SpO2%. The current prototype is restricted to a peg

board which allows for easy adjustment as improvements are made but decreases the general

aesthetic qualities and improvements that can be made related to that concept.

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Brown, Linnea J.

Figure 14 - Powered on initial Output

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Brown, Linnea J.

Recommendations

In considering the completed design, we can safely make several conclusions about

the final hardware design and mechanical setup, starting with the LEDs and receivers. Both

LEDs should have a narrow focus angle along with this the light should be as powerful as is

reasonable for such a low power system. This mechanical setup is more temporary as it is not

a space saving system, which is in direct conflict with the ultimate goal of a thin, portable,

design. Designs such as the one shown in figure 15 adhere more closely to the final goal, and

is inspired by the prototype. Though clunky, the light cancelling and stabilizing properties of

this mechanical interface was sufficient to meet the intended purpose of this preliminary

design.

Figure 15 - Design Concept of Final Monitor

The basic components, capacitors and resistors, are a matter of acquiring the smallest

reasonable form factor. Variances in resistance and capacitance have a minimal effect on the

design functionality but with the amplification system closer to the exact value was the

better. The Op-amp should be similar to the one used in the prototype. It should be a low

power chip, meaning it requires only a small supply current and low voltage source. It should

also have a rail-to-rail output which helps to further clarify the amplified signal and make it

easier to convert the amplified signal into a digital signal.

In summary, the Op-Amp should require the least amount of power and output the

cleanest amplified signal possible. In the prototype the output is directly connected to a

development board that is capable of processing the analog data into a digital signal. In the

final design an ADC, and the components necessary to support the conversion, will be

added. As with all existing components related to this design it should be a balance of low

power and high quality with the priority focused on conserving power for higher

consumption systems like the LEDs and the output screen.

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Brown, Linnea J.

The current visual output is a bare bones design. It displays the heart rate waveform

output successfully cleaned up and amplified by the code then sent over SPI to the screen.

Combining the Arduino and screen setup was easy, as they are designed to work together,

and no major problems were encountered. This was further helped that the screen purchased

came attached to a small PCB. This did not do much more than make the connections

mechanically easier. Converting this over to a final design is simply a matter of adding the

basic components necessary when dealing with such a complex connection. The most

difficult aspect of the LED screen and Output are not as apparent or as worrying since we do

not have to worry about relative power draw and battery life.

The two parts of the design in need of the most improvement is the screen and the

ultrathin battery desired. These are the least developed aspects of the prototype, since that

focused on the input and the manipulation of it, where the battery and screen focus more on

the general power draw on the system and the restrictions these decisions cause. Though

power draw was considered when analyzing designs and parts, and the parts were deemed to

fall into rough parameters, this has not been extensively analyzed. The biggest step moving

forward is to find a screen and battery team that can mesh well with the existing parts and

outline provided from the input and processing sections.

If work is continued on the project improvements will continue to be made towards

creating a final design that meets the suggested objectives. Overall this project can be seen as

a success, as it proved the input design and the ability to read and use the information it

provides. More importantly this is a big stepping stone in the success of the larger idea, and

in turn the final project.

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Brown, Linnea J.

Conclusion

Though the project presented some unexpected difficulty and unusual problems it was able

to prove the oximeter prototype. Given the prototype we were able to prove the functionality

of an analog input which used an IR emitter/receiver and a bandpass Op-Amp filter. This

created a reasonable waveform that could then be further analyzed by the MSP430/ MCU.

This basic circuit and the suggestions provided can be used to design and build a PCB that

would fit the requested specs.

This basic structure allows for further design possibilities. The code is similar, as it

provides a basic set-up for the encoding and visual output, but could also be used as the basis

for a more complex system.

This project focused on being able to design a system from the ground up, and

understanding its component parts to enhance an existing design. This produces a different

challenge from innovating an entirely new system since it forces you to establish which

building blocks of existing designs to keep and which to replace. Each building block you

replace becomes the challenge, along with what is designed that expands and innovates the

current product.

This made for a different design challenge, one that forces looking outside the box

and coming up with different ideas that still conform to how the basic system works. Given

the proof provided by the prototype it is clear the design is a solid alternative to the modern

portable SpO2 monitors.

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Brown, Linnea J.

Bibliography

Basics About COPD, Chronic Obstructive Pulmonary Disease (COPD), Center for Disease

Control, https://www.cdc.gov/copd/basics-about.html#ref2

Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention

and Health Promotion, Division for Heart Disease and Stroke Prevention. DHDSP

Data Trends & Maps [online]. 2015. [accessed Oct 24, 2019]. URL:

https://www.cdc.gov/dhdsp/maps/dtm/index.html .

COPD Burden in Rural America, Center for Disease Control.

https://www.cdc.gov/ruralhealth/COPD/burden/ Rev. August 28, 2018

El-Amrawy, F., & Nounou, M. I. (2015). Are Currently Available Wearable Devices for Activity

Tracking and Heart Rate Monitoring Accurate, Precise, and Medically Beneficial?.

Healthcare informatics research, 21(4), 315–320. doi:10.4258/hir.2015.21.4.315

How pulse oximeters work explained simply, How Equipment Works.

https://www.howequipmentworks.com/pulse_oximeter/

Johnston , William S., "Development of a Signal Processing Library for Extraction of SpO2, HR, HRV,

and RR from Photoplethysmographic Waveforms" (2006). Masters Theses (All Theses, All

Years). 919. https://digitalcommons.wpi.edu/etd-theses/919

Mannino DM, Gagnon RC, Petty TL, Lydick E. Obstructive lung disease and low lung function in

adults in the United States: data from the National Health and Nutrition Examination Survey

1988-1994. Arch Intern Med. 2000;160:1683–1689.

34

Brown, Linnea J.

Matcher, S. J., & . Signal Quantification and Localization in Tissue Near-Infrared Spectroscopy (2016).

In Handbook of Optical Biomedical Diagnostics, Second Edition, Volume 1: Light-Tissue

Interaction. doi: https://doi.org/10.1117/3.2219603.ch9

Millikan , G. A. (1942). The Oximeter, an Instrument for Measuring Continuously the Oxygen

Saturation of Arterial Blood in Man. Review of Scientific Instruments, Volume 13, Issue 10,

p.434-444, 10.1063/1.1769941

Severinghaus , J.W. & Honda, Y. J Clin Monitor Comput (1987) 3: 135.

https://doi.org/10.1007/BF00858362 Chapter 9:

Social Vulnerability Index Mapping and Dashboard. Agency for Toxic Substances and Disease

Registry, Center for Disease Control, https://svi.cdc.gov/map.aspx

Tamura T., Maeda Y. (2018) Photoplethysmogram. In: Tamura T., Chen W. (eds) Seamless Healthcare

Monitoring. Springer, Cham. doi: https://doi.org/10.1007/978-3-319-69362-0_6

Understanding Pulse Oximetry SpO2 Concepts . (2003). Understanding Pulse Oximetry SpO2

Concepts . Andover, MA.

http://incenter.medical.philips.com/doclib/enc/fetch/586262/586457/Understanding_Pulse_Ox

imetry.pdf%3Fnodeid%3D586458%26vernum%3D2

Wheaton AG, Cunningham TJ, Ford ES, Croft JB. Employment and activity limitations among adults

with chronic obstructive pulmonary disease — United States, 2013. MMWR Morb Mortal

Wkly Rep. 2015:64 (11):290–295.

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Appendix A

#include <SPI.h> #include <Wire.h> #include <Adafruit_GFX.h> #include <Adafruit_SSD1306.h> #define SCREEN_WIDTH 128 // OLED display width, in pixels #define SCREEN_HEIGHT 64 // OLED display height, in pixels // Declaration for SSD1306 display connected using software SPI (default case): #define OLED_MOSI 5 #define OLED_CLK 4 #define OLED_DC 3 #define OLED_CS 2 #define OLED_RESET 6 #define WRITE_PIN 12 #define READ_PIN A0 Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, OLED_MOSI, OLED_CLK, OLED_DC, OLED_RESET, OLED_CS); int chart[] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 }; int head = 0; void setup() { Serial.begin(9600); // SSD1306_SWITCHCAPVCC = generate display voltage from 3.3V internally if(!display.begin(SSD1306_SWITCHCAPVCC)) { Serial.println(F("SSD1306 allocation failed")); for(;;); // Don't proceed, loop forever } // Show initial display buffer contents on the screen -- // the library initializes this with an Adafruit splash screen. display.display(); delay(100); // Pause for 2 seconds // Clear the buffer display.clearDisplay(); //display.drawPixel(10, 10, WHITE); } void loop() { int val = analogRead(READ_PIN); chart[head] = val;

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head = roundAdd(head); displayChart(head, chart); } void displayChart(int head1, int chart1[]) { display.clearDisplay(); int max = Max(chart1); int min = Min(chart1); int height = max - min; int lowerbuffer = 0; int upperbuffer = 0; int middle = max - (height / 2); if (height > 64) { lowerbuffer = more(min - (height / 10), 0); upperbuffer = less(max + (height / 10), 1024); } else { if (middle < 33){ lowerbuffer = 0; upperbuffer = 64; } else { if (middle > 992) { lowerbuffer = 960; upperbuffer = 1024; } else { lowerbuffer = middle - 32; upperbuffer = middle + 32; } } } int pointer = 0; int point1 = 0; int point2 = 0; for(int i=1; i<128; i++){ pointer = roundAdd(head1 + i - 2); point1 = ((chart1[pointer] - lowerbuffer) * 64) / (lowerbuffer - upperbuffer); pointer = roundAdd(i + 1); point2 = ((chart1[pointer] - lowerbuffer) * 64) / (lowerbuffer - upperbuffer); display.drawLine(i-1, less(point1, 63), i, less (point2, 63), WHITE); } display.display(); delay(5); // !important! controls how fast the display scrolls } int roundAdd (int num) { num++; if (num >127){ num = num - 128; } return num; } int Max(int chart1[]){ int max = 0; for(int i=0; i<128; i++){

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if (chart1[i] > max){ max = chart1[i]; } } return max; } int Min(int chart1[]){ int min = 1024; for(int i=0; i<128; i++){ if (chart1[i] < min){ min = chart1[i]; } } return min; } int less (int a, int b) { if (a < b) { return a; } else { return b; } } int more (int a, int b) { if (a > b) { return a; } else { return b; } }

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