THE NOVAMOUSETM

PULSE OXIMETER

SHERAZAD MAHOMEDALLY,

NICOLE SERAGO, AND POOJA SHAHDEPARTMENT OF BIOMEDICAL ENGINEERING

VANDERBILT UNIVERSITY SCHOOL OF ENGINEERING

APRIL 22, 2003

ADVISORS: BOB ALLEN & BEN SCHNITZMICRONOVA TECHNOLOGY

INSTRUCTOR: DR. PAUL KINGASSOCIATE PROFESSOR OF BIOMEDICAL AND MECHANICAL

ENGINEERINGVANDERBILT UNIVERSITY SCHOOL OF ENGINEERING

TABLE OF CONTENTS

1. ABSTRACT…………………………………………………………………………....

2. INTRODUCTION

2.1. SIGNIFICANCE OF MICE……….…….……………………………...………..

2.2. MARKET POTENTIAL…...……………………………………………...………

2.3. PULSE OXIMETRY...…………………………………………….………………

2.4. PROBLEM TO BE SOLVED……………………………………………………..

2.5. GOALS…………………………………………………………………………….

2.6. CURRENT DEVICES…………………………………………………………….

3. METHODOLOGY

3.1. PULSE OXIMETRY THEORY…..………………………………………………

3.2. RATIO OF RATIOS………………………………………………………………

3.3. PROJECT SCOPE AND CIRCUITRY…………………………………………..

3.4. DESIGN SPECIFICATIONS………....………………………………………….

4. RESULTS

4.1. EXPERIMENTAL DESIGN………………………………………….………….

4.2. DESIGN PERFORMANCE……….…………………………………….……….

4.3. PROBLEMS ENCOUNTERED/SOLUTIONS………………………….……….

4.4. SAFETY ANALYSIS……………………………………………………..………

4.5. ECONOMIC ANALYSIS…………………………………………………………

5. CONCLUSIONS…………………………………………………………….…………

6. RECOMMENDATIONS………………………………………………….…………..

7. REFERENCES…………………………………………………………...……………

APPENDIX*

A. INNOVATION WORKBENCH…………..………………………………………

B. DESIGNSAFE REPORT…………………………………………………...…….

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1. ABSTRACT

This design project is the creation of the NovaMouseTM pulse oximeter, which will

be used to measure oxygen saturation and blood pressure of mice. The measurement of

the oxygen saturation level is important because an insufficient supply of oxygen can

result in death within minutes during surgery. The miniature size of mice that allow for

ease of maintenance is also what causes the largest drawback during research involving

mice. The main goal of this project is to build a pulse oximeter that is small, has the

capability to be wireless in the future, and can reach heart rates up to 800 beats per

minute. One photo sensor is used for all measurements because the two light-emitting

diodes are pulsed at different intervals. The received red and infrared signals have a non-

linear relationship and are then mathematically preprocessed and normalized so that a

ratio of the two can theoretically be a function of only the concentration of

oxyhemoglobin and reduced hemoglobin in the arterial blood. This concentration ratio

known as the ratio of ratios (R) should be constant since oxygen saturation is essentially

constant for measurements taken over such a short period of time. The circuit designed

has a frequency range of 0.1 hertz to 250 hertz and a gain of 625. From the Bode plots,

the lower cutoff for the infrared light-emitting diode is not seen. The Bode plot of the red

light-emitting diode illustrates the stair step characteristic of a Butterworth filter. These

results are seen because the bandwidth is so small (only two magnitudes). The circuit is

ready to be miniaturized and tested on mice.

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2. INTRODUCTION

2.1 SIGNIFICANCE OF MICE

The field of biomedical research on mice is rapidly growing. The popularity of

mice in research is increasing due to their ease and quickness of breeding, the ability to

breed transgenic mice, and their miniature size, which reduces their maintenance costs in

laboratories. This design project is the creation of the NovaMouseTM pulse oximeter,

which will be used to measure oxygen saturation and blood pressure of mice.

2.2 MARKET POTENTIAL

NovaMouseTM will be marketed to universities and hospitals nationwide to aid in

research and data acquisition. Approximately 50,000 devices are projected to be sold in

2004 with one million mice as potential targets. This accounts for around 10-20% of the

market share. It is difficult to pin down the exact market since the FDA and NIH do not

keep records on mice. [1]

2.3 PULSE OXIMETRY

Pulse oximetry is used to measure blood oxygen saturation. Heart rate is also

easily attainable using a pulse oximeter. The measurement of the oxygen saturation level

is important because an insufficient supply of oxygen can result in death within minutes

during surgery. Also, pulse oximetry is very important in Intensive Care post-surgery to

monitor recovery. Pulse oximetry readings are standard measurements in human and

veterinary surgical monitoring. This shows that there is a need for such measurements in

surgical procedures on mice.

These measurements would also be useful for research on free ranging mice. For

example, oxygen saturation measurements could be used on researching the effects of

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alcohol and carbon monoxide poisoning as well as comparing cardiac output to oxygen

saturation. [2] The additional heart rate measurements would also be useful in such

studies. An example of the need for this measurement can be found here at Vanderbilt.

At the Vanderbilt Medical Center, Dr. Timothy Blackwell, M.D. would like a pulse

oximeter to use on the mice in his investigation of pulmonary fibrosis. [3] Fibrosis, or

scarring of the lung tissue, results in permanent loss of the tissue’s ability to transport

oxygen. A pulse oximetry measurement would show decreased oxygen saturation if

fibrosis was present. [4]

2.4 PROBLEM TO BE SOLVED

The miniature size of mice that allow for ease of maintenance is also what causes

the largest drawback of research involving mice. Conventional methods for obtaining

physiological measurements, such as blood gases and heart rate, do not work on mice.

The NovaMouseTM pulse oximeter is a device to accurately measure blood oxygen

saturation and heart rate in mice.

Prior to this product, any time data acquisition of pulse oximetry was desired, a

mouse was killed in the process since the only way to acquire that data was via sticking

rods through the mouse’s hands and feet. This device will spare the mouse’s life allowing

for the observance of several drug administrations and the reusing of mice.

NovaMouseTM would allow for reduction of costs. Not only would the device itself be

less expensive than competitor’s products by a factor of ten, but the survival of the mice

would also allow for reuse of the expensive transgenic mice. [1]

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2.5 GOALS

The main goal of this project is to build a pulse oximeter for a mouse with the

following characteristics. First, the sensor must be small enough to accurately obtain

pulse oximetry measurements from the proximal end of the tail of a mouse as well as

monitor the heart rate of the mouse. Second, the pulse oximeter must be compatible so in

the future it can be wireless. This will help with ease of movement of the mouse during

studies that require measurements to be taken over extended periods of time in which the

mouse should be in its natural environment. Next, the pulse oximeter must be able to

accurately detect heart rates of up to 800 beats per minute. Also, the device will be

reusable and the packaging design will be built on a flexible substrate using chip and

wire. The sensor will have a cuff format that will be located in a hard to reach place so

that the mouse doesn't eat it off. The sensor will also be discrete, meaning that there

won't be any wires attached.

To achieve these goals, a variety of literature was used. An introduction to the

aim of the project and the probable components was attained through meetings with

MicroNova Technology staff, including Ben Schnitz and Bob Allen. Further research

was done through the use of biomedical handbooks, internet searches, and patent searches

to verify pulse oximetry designs and parameters. To determine the possible uses and a

definite market for the device, a personal interview was conducted with Dr. Blackwell.

Dr. Blackwell also was helpful in providing information about previous devices and their

downfalls. With this information, websites of individual companies, including Nonin

Medical, Inc. were searched.

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2.6 CURRENT DEVICES

Recent devices do not allow for movement of the mouse and are incapable of

detecting heart rates high enough to monitor that of mice. One example is a pulse

oximeter designed by Nonin Medical, Inc. This

device, shown in Figure 1, only works on large

rodents and is not useful for a mouse. The Velcro is

flimsy and it is difficult to attach to the mouse. Also,

although this oximeter can read a human and large

rodent’s blood saturation, it cannot read a mouse’s due

to a heart rate of between 450 and 800 beats per

minute. Even though this device allows for some

movement of the mouse, the wires can still hinder normal behavior of a free ranging

mouse. On the other hand, NovaMouseTM will try to overcome all these disadvantages

posed by the Nonin device. [5]

3. METHODOLOGY

3.1 PULSE OXIMETRY THEORY

In order to design NovaMouseTM, it is important to look at the theory behind pulse

oximetry. The NovaMouseTM pulse oximeter works by measuring the hemoglobin in the

arteries found in the mouse’s tail via optical sensing. The device will be secured to the

proximal end of the tail where a central artery and two veins are found.

Relative proportions of oxygenated and reduced hemoglobin in the arterial blood

determine the oxygen saturation (SpO2) of arterial blood. Reduced hemoglobin is

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Figure 1: Nonin Pulse Oximeter

hemoglobin that is not carrying oxygen, i.e. deoxygenated blood, while oxyhemoglobin

corresponds to oxygenated blood. The pulse oximeter works by measuring the difference

in the absorption spectra of these two forms of hemoglobin in order to calculate the SpO2

using two light-emitting diodes, one of wavelength 700 nm in the red band and one of

wavelength 950nm in the infrared, and a photodiode. Similar to the human’s finger, the

light-emitting diodes are used to sense the degree of oxygen saturation in blood since the

reduced hemoglobin absorbs more light in the red band than does oxyhemoglobin, while

oxyhemoglobin absorbs more light in the infrared band than does reduced hemoglobin.

Because the tissue contains arterial, capillary and venous blood, as well as

muscle, connective tissue and bone, the red and infrared signals received from the probe

contain both a DC and a pulsatile component. While the large DC component is

influenced by the absorbency of the tissue, the intensity of the light source, and the

sensitivity of the detector, the small pulsatile component corresponds to the pulsatile

arterial blood.

3.2 RATIO OF RATIOS

The amount of light transmitted through the tail is measured several hundred

times per second at both wavelengths. One photo sensor is used for all measurements

because the two light-emitting diodes are pulsed at different intervals (6). The received

red and infrared signals have a non-linear relationship and are then mathematically

preprocessed and normalized so that a ratio of the two can theoretically be a function of

only the concentration of oxyhemoglobin and reduced hemoglobin in the arterial blood.

This concentration ratio known as the ratio of ratios (R) should be constant since oxygen

saturation is essentially constant for measurements taken over such a short period of time.

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The preprocessing MicroNova plans to do such as averaging over time and planting the

device on a relatively immobile area will eliminate extra noise variables such as motion

artifact and ambient light [7].

The mathematical side of the ratio of ratios includes Equation 1 that characterizes

( ∑ Hb( λ R) - ∑ Hb( λ IR) *R) 100 Equation 1∑ Hb(λR) - ∑ HbO2(λR) + [ ∑ HbO2(λIR) - ∑ Hb(λIR)] R

the amount of light absorption at each frequency for oxygenated and deoxygenated blood

to give the non-linear relationship referred to earlier. The values in Table 1 represent the

Table 1: Saturation of hemoglobin at both wavelengths.

∑ Hb(λR) 0.81∑ Hb(λIR) 0.18

∑ HbO2(λIR) 0.29∑ HbO2(λR) 0.08

saturation of hemoglobin at each wavelength. For instance, ∑ Hb(λIR) is the saturation of

deoxygenated blood at 950 nm. Using these values Equation 2 is generated and R, the

ratio of ratios, is used to get the oxygen saturation desired. R should be between .5 and

2.5 [1].

SpO2 = (.81-.18R) 100 Equation 2 (.29-.08R)

3.3 PROJECT SCOPE AND CIRCUITRY

Figure 1, which represents the project outline, illustrates our scope of the project

as the blue parts, while the red parts represent the parts MicroNova still has to do. The

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Biosensor

Preamp A/D Conv Transmitter Receiver D/A Conv SignalProcessing Data Acquisition

Figure 2: Project outline.

first part of the project to research and design is the biosensor. Here, the light-emitting

diodes sense the blood saturation and send both the DC and pulsatile signal to the

photodiode. In the preamplifier stage, the transimpedance amplifier converts the current

coming from the photodiode to a voltage. Because there is a very large input impedance

and negligible output impedance, a very large feedback resistor is used to set the current

gain as can be seen in Figure 2. Also included is a capacitor of .1 microfarad in order to

tune out high frequency measurements and stabilize voltage spikes. Once the signal is

conditioned into a voltage output, an analog to digital conversion card is used to format

the signal into a digital signal that can be sent wirelessly.

In order to wirelessly transfer the information serial transmission is utilized since

it requires only one transmitter. Moreover, it is accomplished using amplitude shift

keying modulation. This method is preferred to other types of modulation because 1) it is

the easiest to implement, 2) it is easy to read – does a signal exist or not, and 3) it does

not require any sort of interpretation of the signal.

Although the one drawback is that there can be lots of error since zero acts as a

data point, there are ways to fix the error. For one, there can be several check points

where one can check for 5 points before and after the point recorded to make sure the

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V+D1

700 nm

-InA

+InB

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Figure 3: Schematic of pulse oximeter using two dual op-amps.

signal received includes zeros that are part of the signal and not zeros that mean the

signal is dead. Although this decreases the sampling rate, that is acceptable because the

pulse oximeter for mice doesn’t need a high sampling rate. Another advantage is that the

distance between the mouse and receiver is short; thus there is less noise [2].

After the signal is received, it is reconstructed back into an analog signal and then

sent first through an active high pass filter of gain = 6.25 to get rid of signals below .1

Hertz and then a two-pole low pass filter (8) to rid signals above 250 Hertz. This gives

the desired frequency range of .1-250 Hertz. Finally, the signal is passed through yet

another low pass filter with a gain of 100 to get amplification and get rid of all

frequencies passed the highest, important frequency before data acquisition. If this anti-

aliasing process was not done, we would use the Nyquist theorem of sampling frequency

for biological situations and sample at a frequency of 5 times the highest frequency as

opposed to five times the highest, important frequency. Once the data is collected, the

signal is processed in LabView in order to monitor the blood saturation.

3.4 DESIGN SPECIFICATIONS

In order to be able to detect a high heart rate, the frequency range must be

between 0.1 Hertz and 250 Hertz. Therefore, the circuit must eliminate all frequencies

above and below this range including random noise. Also, power to the operational

amplifier and light-emitting diodes will be provided via inductive coupling and voltage

regulators. But right now, we are modeling the dual operational amplifiers with a power

supply of 3.6 volts. This type of operational amplifier can be operated with a power

supply from 2 volts to 44 volts. However, a voltage of 1.7 volts is needed for the light-

emitting diodes, so right now the function generator is being used as the source. Also

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there is a very large gain in the transimpedance amplifier with a gain of 625 in the active

filtration. As for the device, it will be designed to fit a mouse with the following

dimensions: 3 millimeter diameter at the base with 2 centimeters before the tail begins to

taper.

4. RESULTS

4.1 EXPERIMENTAL DESIGN

Figure 1 below shows the experimental setup for our pulse oximeter design. The

function generator provides the light-emitting diode with a voltage of 1.7 volts and the

power supply provides the circuit with a voltage of 3.6 volts. The light-emitting diode

and the photodiode are put in a box so that ambient light cannot affect the signal. The

function generator is set to a frequency of 0.1 hertz to begin with and is incremented all

the way to 250 hertz. The output signal is seen on the oscilloscope and the output voltage

is recorded.

4.2 DESIGN PERFORMANCE

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Figure 2: Experimental design setup.

Currently, the characterization of the light-emitting diodes has been completed. This

means that Bode plots have been obtained using the red and the infrared light-emitting

diodes by pulsing them from the frequency range as mentioned above. The results

obtained are shown in graphs 1 and 2 below. Looking at the graphs, it can be noted that

the since the bandwidth is very small, the plot appears as a stair step for the red light-

emitting diode.

Characterization of IR LED

0

50

100

150

200

250

0.1 1 10 100 1000

Frequency (Hz)

Volta

ge (m

V)

Characterization of Red LED

174176178

180182

184

186188

190

0.1 1 10 100 1000

Frequency (Hz)

Volta

ge (m

V)

Graph 1: Bode plot for red LED. Graph 2: Bode plot for infrared LED.

4.3 PROBLEMS ENCOUNTERED/SOLUTIONS

During this project several problems were encountered. First, the red light emitting

diode had a voltage range of 1.8 volts to 4 volts and the infrared light-emitting diode had

a voltage range of 1 volt to 1.5 volts. Since there was no overlap in the voltage range,

there was no way to power our light-emitting diodes. To solve this problem, light

emitting diodes with the same or overlapping voltage ranges were found at 1.7 volts.

Thus, a voltage divider will be used to make sure that the battery voltage does not go

below 1.7 volts; otherwise the light-emitting diodes will not work.

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Another problem was that several operational amplifiers stopped working due to

static electricity and therefore operational amplifiers had to be switched and static

wristbands were carefully used. The light-emitting diodes had inconsistent performance.

Sometimes they worked and other times they didn’t. Several different types of light-

emitting diodes were used. Similarly, there were problems with the filters. Several

different techniques of trouble shooting were used.

Finally, there were problems characterizing the circuit. The reason the Bode plot

appeared incorrect is because the frequency range is really small. Also, there was a need

to increase the gain in the circuit. This was accomplished simply by changing the

resistors in the low pass filter. Once these changes were made, it was easier to see an

output signal on the oscilloscope and get the Bode plots desired, shown above in graphs 1

and 2.

4.4 SAFETY ANALYSIS

Using the program Designsafe, we were able to perform a risk assessment on the

NovaMouseTM pulse oximeter design. From the analysis, we were able to conclude that

the severity of risk is minimal and the probability of danger is unlikely and negligible for

the most part. This program also helped us determine risk reduction methods for our

design including a manual for users, warning signs, and on-the-job training for the

operator.

4.5 ECONOMIC ANALYSIS

The development cost analysis for this project includes the cost of labor which

includes 150 hours for 3 students at $15/hour. This comes out to $6750, plus $100 for

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parts. There was no cost incurred for travel, technical services, legal fees, etc. Thus, the

final cost was $6850. There will be no FDA involvement since this device is used on

animals and there is no need for animal welfare considerations since the pulse oximeter is

just cuffed on the proximal end of the mouse’s tail and thus, is not invasive. Also, the

cost of maintenance is unknown but only includes the cost to maintain the receiver/power

supply if it breaks. The pulse oximeter cuff is reusable and will need no maintenance

since they are small and cheap and can be thrown away if broken. The cost of this device

will be in the hundreds which is less by a factor of ten from other current designs such as

Nonin Medical, Inc. MicroNova Technologies predicts $5-10 million dollars in sales for

the year 2004.

5. CONCLUSIONS

During the course of this project a lot of time was spent working on the

breadboard circuit prototype and troubleshooting. Since there were problems with the

parts, the focus was on rebuilding and testing the circuit. Research was completed on

pulse oximetry circuit design and different possible parts and different light-emitting

diodes and stages of the circuit were tested specifically to obtain readable signals. The

circuit was characterized to obtain Bode plots by changing the frequency range and gain

based upon updated schematics. DesignSafe and Innovation Workbench programs were

completed to analyze problems and risks associated with the device and the design

process.

From the Bode plots, the lower cutoff for the infrared light-emitting diode is not

seen. The Bode plot of the red light-emitting diode illustrates the stair step characteristic

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of a Butterworth filter. These results are seen because the bandwidth is so small (only

two magnitudes). The circuit is ready to be miniaturized and tested on mice.

6. RECOMMENDATIONS

From this point, the completion of the project will be done by Ben Schnitz and

future MicroNova Technology interns. The analog to digital conversion, wireless

transmittance, digital to analog conversion, and data acquisition stages need to be

designed and implemented. The circuit will then be miniaturized such that the biosensor

and transimpedance amplifier will fit in the cuff on the mouse’s tail. Testing will be done

to ensure the device works on mice. The successful device will eventually be sold to

interested customers.

Issues concerning ethics do not play a large role in this project. The

NovaMouseTM device does not in any way harm the mouse since it is noninvasive and

will be miniaturized so that it will not impair the mobility of the animal. The pulse

oximetry measurements do not have any negative effects on the quality of the life of the

mouse. Any harm to the mouse being used would be the result of whatever research is

being done on it, which is outside of the scope of this project.

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7. REFERENCES

1. Schnitz, Ben. MicroNova Technology, Inc. Personal interviews. October

2002-April 2003.

2. Diab, Mohamed K. “Signal Processing Apparatus.” United States Patent

5632272. 27 May 1997.

3. Blackwell, Timothy. Personal interview. 5 December 2002.

4. Facts About Pulmonary Fibrosis and Interstital Lung Disease. 9 April 2003.

http://www.lungsusa.org/diseases/pulmfibrosis.html

5. Nonin Medical, Inc. 6 December 2002. http://www.nonin.com

6. Webster, John G. Medical Instrumentation: Application and Design. New

York: John Wiley & Sons, Inc., 1998.

7. Mortz, Margaret S. “System for Pulse Oximetry SpO2 Determination.” United

States Patent 6385471. 7 May 2002.

8. Neamen, Donald A. Electronic Circuit Analysis and Design. Chicago: Irwin,

1996.

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