Sensors and Actuators B 181 (2013) 172– 178

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A miniature fiber optic blood pressure sensor and its application in in vivo blood

pressure measurements of a swine model

Nan Wua, Ye Tiana, Xiaotian Zoub, Yao Zhaib, Kurt Barringhausc, Xingwei Wanga,∗

a Electrical and Computer Engineering Department, University of Massachusetts Lowell, 1 University Ave., Lowell, MA 01854, USAb Biomedical Engineering and Biotechnology Department, University of Massachusetts Lowell, 1 University Ave., Lowell, MA 01854, USAc University of Massachusetts Memorial Medical Center, University of Massachusetts Medical School, 55 Lake Ave., North Worcester, MA 01655, USA

a r t i c l e i n f o

Article history:Received 17 September 2012

Accepted 3 February 2013

Available online 17 February 2013

Keywords:Blood pressure

Optical fiber

Pressure sensor

Fractional flow reserve

a b s t r a c t

Fractional flow reserve (FFR) is a promising technique in diagnosis of coronary artery stenosis. The

technique is applied in coronary catheterization to measure the blood pressure (BP) difference across

a coronary artery stenosis in the blood flow. In vivo BP measurement is the key element in FFR diagnosis.

This paper describes the utilizing of a novel miniature fiber optic sensor to measure the BP of a swine

model in vivo. A 25–50 kg Yorkshire swine model was used as the test target. A guiding catheter was

introduced into the coronary artery, and blood pressure signals in aortic arch and right coronary artery

were measured by the fiber optic sensor. A standard invasive manometry was used as the reference.

Finally, a 2.25 mm balloon was inflated in the catheter to simulate the stenosis and the BP drop was

recorded by the fiber optic sensor. The experiment demonstrates that the reported fiber optic sensor has

the capability of measuring blood pressure in vivo and can be used for FFR technique.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Coronary artery disease (CAD), which is caused by the accumu-

lation of atheromatous plaques within the walls of the coronary

arteries that supply the oxygen and nutrients to the myocardium

[1], is the leading cause of the death. Most CAD patients are not

aware of the disease for decades as the disease progressing until

the first symptoms, often an acute heart attack, finally show up.

The disease is the most common cause of sudden death [2] as well

as the most common reason for the death of people over 20 years

old [3]. Moreover, half of healthy 40-year-old males will probably

develop CAD in the future, and one in three healthy 40-year-old

women [3].

Percutaneous coronary intervention (PCI) is a common ther-

apy directed toward alleviating CAD. It is important to assess the

severity of the lesion and its impact on blood flow before, during

and after the angioplasty procedure. Assisted by this information,

cardiologists can determine whether a PCI is necessary. Tradition-

ally, angiography is the standard method to assess the severity

of the lesion but is of limited value when lesions of intermediate

severity are identified, because it cannot provide adequate infor-

mation regarding whether the blood flow can be impacted by such

an intermediate lesion. In order to determine the lesion’s impact

on blood flow, additional information is required. By interrogating

∗ Corresponding author.

E-mail address: xingwei wang@uml.edu (X. Wang).

the frequency shift between the sound wave emanated from the

source and that reflected from the moving blood cells, a Doppler

ultrasound guidewire can be used to measure the blood flow rate

[4]. However, technical limitations have prevented this technol-

ogy from becoming clinically useful. Another method, hot-wire

anemometry, uses thermistors to monitor a tiny thermal gradient

in a fluid flow stream [5]. However, this method may damage blood

cells or tissues because of the heat.

Fractional flow reserve (FFR) is an alternative method to

evaluate the stenosis in coronary artery [6–12]. The severity of

the narrowing is determined by measuring the blood pressure

difference across a coronary artery stenosis in the blood flow

through coronary catheterization. This method has been eventually

accepted by doctors since 1990s. In order to achieve accurate FFR

diagnosis, in vivo blood pressure measurement is critical. Various

studies have been conducted on how FFR benefits to the diagno-

sis of the coronary artery stenosis by conducting the intravascular

blood pressure measurement [12]. Most of the sensors used in these

studies are electrical sensors which may generate electrical noises

to interfere with other electrical equipments in the operating room,

where the electromagnetic interference is risky to patients. On the

other hand, fiber optic pressure sensor is a potential substitution

to the current electrical pressure sensors. The fiber optic pressure

sensor can be easily packaged in a guide wire due to its compact

size which is generally 125 �m in diameter. Due to its all optical

operating principle, the fiber optic sensor cannot interfere with

other electrical equipments. Samba Sensors released a fiber optic

pressure sensor for intravascular blood pressure measurement. The

0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.snb.2013.02.002

N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178 173

sensor is made by attaching a 0.36–0.42 mm diameter silicon sen-

sor head to the tip of a 0.25 mm diameter optical fiber [13]. The

bulky head of the sensor prevents it from further reducing the size.

This paper presents a fiber optic blood pressure sensor with uni-

form diameter of 125 �m for the purpose of in vivo blood pressure

measurement. The sensor was tested in a swine model at University

of Massachusetts Medical School in Worchester, MA. The stenosis of

the coronary artery was simulated by inflating a 2.25 mm balloon in

the catheter. A reference manometer was placed side by side with

the fiber optic sensor for the comparison purpose. In order to com-

pensate the bending loss introduced by the big curvature of aortic

arch, a special interrogation system was designed. Experimental

results demonstrated that the fiber optic sensor has the capability of

monitoring the blood pressure profile and the blood pressure drop

caused by the inflated balloon. Moreover, the reported fiber optic

blood pressure sensor proves its capability of being utilized in FFR

applications to determine the location of stenosis in the coronary

artery.

2. Fiber optic blood pressure sensor

2.1. Design and principle

The fiber optic pressure sensor is designed based on Fabry–Perot

(FP) interferometer [14–18], as shown in Fig. 1. Three elements can

be observed: a single-mode fiber, a multi-mode fiber, and a sil-

icon dioxide (SiO2) diaphragm. The multi-mode fiber is applied

to fabricate an air cavity which is formed by wet etching using

hydrofluoric acid (HF). The diameter of the cavity is determined

by the diameter of the multi-mode fiber core and the depth of the

cavity is determined by the etching duration. The SiO2 diaphragm

is attached by the end of the air cavity by thermal bonding tech-

nique. Therefore, the multi-mode fiber core/air cavity interface and

the air cavity/diaphragm interface form an FP cavity on which the

reflection lights will generate an interference pattern. The single-

mode fiber guides the interrogation light exciting on the FP cavity

and collects the reflected lights.

According to the principle of the FP interferometer, the optical

phase � of the interference pattern is governed by

� = 4�Ln

�0, (1)

where L is the length of the FP cavity; n is the refractive index of

the cavity, which is 1 (air) in this case; �0 is the wavelength of the

interrogation light. The optical phase changes when the length of

the FP cavity changes due to an external pressure applying on the

diaphragm. The relationship between the FP cavity length changes

and the external pressure can be expressed by [19]:

Yc = 3(1 − v2)Pr4

16Eh3, (2)

where Yc is the center deformation of the diaphragm; v is the

diaphragm’s Poisson’s ratio; P is the external pressure applied to

the diaphragm; r is the radius of the diaphragm; h is the thickness

of the diaphragm; E is the Young’s modulus of the diaphragm.

The external pressure can be determined by interrogating the

optical phase changes. In a low finesse FP cavity case, which is

Single-mode fiber Multi-mode fiber

Fiber core Air cavity

SiO2 diaphragm

Fig. 1. The schematic structure diagram of the fiber optic blood pressure sensor.

Fig. 2. Microscopic photograph of a fabricated BP sensor. (a) Sideview and (b) end-

face.

caused by the low reflectivity of each reflection interface, the

reflected optical intensity can be approximated by a sinusoidal

function [20]:

I = I0[1 + V cos(� − �0)], (3)

where I0 is the mean optical intensity; V is the visibility of the FP

interferometer; �0 is the initial optical phase. Therefore, the pres-

sure applied to the diaphragm can be determined by interrogating

the reflected optical intensity.

2.2. Fabrication

The fabrication method was published elsewhere [14]. Briefly,

the silicon dioxide diaphragm was released by back etching sili-

con away the silicon substrate with an oxide layer through deep

reaction ion etching (RIE). The thickness of the diaphragm was

determined by the thickness of the grown oxide layer on the silicon

substrate. This method promises that a diaphragm with control-

lable and uniform thickness can be achieved. The fiber was prepared

by splicing a piece of MMF with the SMF followed by cleaving the

MMF so that approximately 30–40 �m length of the MMF was left.

The FP cavity was formed by immersing the fiber with the MMF end

in a 49% HF. Finally, the silicon diaphragm was thermally bonded

onto the end of the etched fiber by a torch. The microscopic pho-

tographs of a fabricated sensor are illustrated in Fig. 2.

The independent fabrication of the diaphragm eliminates the

necessity of the bulky sensor head introduced by the support-

ing structure [20–22]. The robustness of this structure has been

demonstrated in various pressure measurement applications [15].

The uniform diameter of the sensor head keeps the sensor to its

minimized dimension and allows the following package procedure.

2.3. Package

As shown in Fig. 2, the pressure sensor is fabricated on a bare

fiber, which is about the size of a human hair without any outside

protection such as buffer or jacket. The bare fiber is made of pure

glass (silica) with special doping and its performance and long-term

durability can be affected by environmental conditions. Directly

exposed to the complex blood vessel circumstance, the fiber, espe-

cially the fragile diaphragm, is prone to be broken and the optical

signal will be not accurate. Therefore, utilization of the fiber optic

sensor as a medical device requires a delicate protection. In addi-

tion, a steerable tip section and a biocompatible coverage are also

required in in vivo blood pressure measurement applications.

The schematic of a fully packaged fiber optic sensor is shown in

Fig. 3, and the photograph of the tip section of a packaged sensor

is illustrated in Fig. 4. There is a Kapton tubing covered around the

stripped bare fiber to protect it from the external force. The Kapton

tubing and coil reinforced Kapton tubing work together as an enclo-

sure around the delicate fiber tip to prevent surrounding touch.

The Kapton tubing has openings which allow the outside media

174 N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178

Sensor tip

(125 μm)Polymer bead

Pun ched Kapton tubing Coil re inforce d Kapton tubingCoil

Fiber (250 μm) Stainless stee l tubing

Fig. 3. Schematic of the packaging design.

Fig. 4. Photograph of the tip section of a packaged sensor.

to interact with the fiber sensing area. A stainless steel coil with a

polymer head is bonded on the tip area for flexible steering. All of

the covering materials are either biocompatible polyimide or with

polytetrafluoroethylene (PTFE) coating. The mechanical properties

of different sections of the fully packaged device vary for steer-

able tip, flexible middle and stiff extension. The packaging length is

longer than 1.5 m for swine model usage. The final outer diameter

(OD) is around 360 �m which is close to commercial guide wires

for medical applications.

3. Sensor verification

3.1. Experimental setup

Prior to the animal test, static experiments were performed

to investigate the sensor’s static performance. Fig. 5 shows the

schematic diagram of the experimental setup. The fiber optic sen-

sor was placed in a sealed chamber in which the pressure was

controlled by a pressure controller (NetScanner Model 9034, Pres-

sure Systems Inc.). The sealed chamber was filled with water to

simulate the internal environment of the swine artery. In order to

compensate the bending loss occurred when the fiber optic sen-

sor travels through the coronary artery during the animal test, an

interrogation system with the capability of detecting the bending

loss was introduced [23]. A wideband light source (OEBLS-200, O/E

Land Inc.) was used to excite the fiber optic sensor through a cir-

culator. The reflected light was split into two optical fibers through

a splitter. One of the optical fibers was connected to a photodetec-

tor (PDA10CS, Thorlabs) through a tunable filter (FOTF-025121333,

Sealed chamber

Pressure

controller

Fiber optic BP sensor

Wideband light

source

Photodetectors

Circulator

Tunable filter

Splitter

Broadband

channel

Narrowband

channel

Fig. 5. The schematic diagram of the static experimental setup.

Agiltron). This channel was referred as the narrowband channel.

The other optical fiber was directly connected to another pho-

todetector (PDA10CS, Thorlabs). This channel was referred as the

broadband channel.

In the narrowband channel, the reflected interference spectrum

from the fiber optic sensor can be observed because that the coher-

ence length was much longer than the FP cavity length in the

sensor. On the contrary, there is no interference can be observed

in the broadband channel because the coherence length was much

shorter than the FP cavity length in the sensor. Therefore, the pho-

todetector in the narrowband channel is used to interrogate the

sensor while the photodetector in the broadband channel is used

to detect the bending loss [23].

3.2. Sensor calibration

A typical reflection spectrum of a fiber optic blood pressure sen-

sor is shown in Fig. 6a and the calibration results of the sensor when

the peak wavelength of the tunable filter was set to 1547.5 nm

are shown in Fig. 6b. The pressure in the chamber was increased

from 0 mmHg to 200 mmHg with steps of 50 mmHg and then was

decreased from 200 mmHg to 0 mmHg with the same steps. The

results indicate that the sensor has a low hysteresis and a high

repeatability according to low standard deviations. The sensitivity

was calculated as 0.035 mV/mmHg.

4. Intravascular blood pressure measurements

4.1. Protocol of animal test

A 25–50 kg Yorkshire swine was premedicated with intramus-

cular Glycopirrolate B (0.01 mg/kg) and an anesthetic cocktail

(5 mg/kg Telazol; 2.5 mg/kg Ketamine; 2.5 mg/kg Xylazine) after

which endotracheal intubation was performed. Anesthesia was

maintained with inhalational 2–3% isoflurane. Next, femoral arte-

rial access was obtained via cutdown, and a 6 French introducer

sheath was inserted. Heparin was administered intravenously

(50 units/kg), and a 6 French JR-4 guide catheter (Medtronic; Min-

neapolis, MN) was guided to the aortic arch. Baseline blood pressure

measurements were obtained with standard invasive manometry.

Fiber optic blood pressure measurements were similarly obtained

for comparison offline.

The guiding catheter was advanced to the aortic arch. In order

to demonstrate the capability of capturing heart beat signals, the

blood pressure was measured by the fiber optic blood pressure sen-

sor at two points: the aortic arch and the right coronary artery.

The blood pressure at the aortic arch was measured when the fiber

optic sensor was outside the catheter. The blood pressure at the

right coronary artery was measured when the fiber optic sensor was

inside the catheter. Finally, a 2.25 mm balloon (Quantum Maverick,

Boston Scientific) was inserted into the catheter to mimic stenosis.

The blood pressure was measured by the fiber optic sensor when

the balloon was inflated and deflated.

4.2. Blood pressure measurements in aortic arch

Fig. 7a shows results of a period of the blood pressure measure-

ment taken by the fiber optic blood pressure sensor in the aortic

arch. The results from the fiber optic sensor are consistent to the ref-

erence manometer, which are shown in Fig. 7b. The pressure range

was from 54 mmHg to 88 mmHg and the heart beat was approxi-

mately 83 beats per minute (bpm). The fiber optic blood pressure

sensor demonstrated its capability of capturing heart beat signals

in this measurement.

N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178 175

1520 154 0 156 0 158 0-50

-45

-40

-35

-30

-25

-20

(a)

Inte

nsi

ty (

dB

)

Wavelength (nm)

1547.5 nm

0 50 100 150 200

9

10

11

12

13

14

15

16

17

(b)

Volt

age

(mV

)

Pressure (mmHg)

Increase

Decrease

Fig. 6. (a) The typical reflection spectrum from a fiber optic blood pressure sensor. (b) The calibration results when the peak wavelength of the tunable filter was chosen at

1547.5 nm.

Fig. 7. (a) Blood pressure measurements at aortic arch outside the catheter by fiber optic pressure sensor. (b) Blood pressure measurements taken by the reference manometer.

4.3. Blood pressure measurements in the right coronary artery

In order to reach the right coronary artery, the fiber optic blood

pressure sensor had to pass through the aortic arch where the

optical fiber suffered a huge optical intensity loss due to the big cur-

vature of the aortic arch. Therefore, it is critical to identify when the

optical fiber suffers the optical intensity loss. The interrogation sys-

tem of the fiber optic sensor has the capability to identify the optical

fiber bending loss. The broadband channel in Fig. 5 was used to cap-

ture the optical intensity drop caused by the bending loss because it

is insensitive to the blood pressure variation. Therefore, the signal

from the broadband channel was used to compensate the bend-

ing loss of the optical fiber. The electrical voltage signals from both

broadband and narrowband channels are shown in Fig. 8a. At about

249 s, the fiber optic sensor was advanced to the right coronary

artery and was suffered from the bending loss. It can be seen from

both channels that there were huge voltage drops. The difference

magnitude of the voltage drops was due to the different gain set-

tings of photodetectors. After identified that the signal drops were

caused by the bending loss, the signals from the broadband chan-

nel was used to compensate the signal drops in the narrowband

channel, which is shown in Fig. 8b.

220 230 240 250 260 270 280 290 300

10

11

12

13

14

10

11

12

13

14220 230 240 250 260 270 280 290 300

Narrowband channel

Volt

age

(mV

)

Time (s)

(a)

Advanced to right

coronary artery

Broadband channel

220 230 240 250 260 270 280 290 300

10

11

12

13

14

10

11

12

13

14220 230 240 250 260 270 280 290 300

(b)

Narrowband channel

after compensation

Volt

age

(mV

)

Time (s)

Narrowband channel

Fig. 8. (a) Signals from the photodetectors in both channels when the fiber optic sensor was subjected to a severe bending. (b) Signals in narrowband channel after the

compensation.

176 N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178

Fig. 9. (a) Blood pressure measurements at the right coronary artery in the catheter by fiber optic pressure sensor. (b) Blood pressure measurements taken by the reference

manometer.

The blood pressure measured by the fiber optic sensor is shown

in Fig. 9a. The signals from the fiber optic sensor and the reference

manometer (Fig. 9b) are not very consistent. The inconsistence was

due to the different locations of the fiber optic sensor and the ref-

erence manometer. The fiber optic sensor was inside the catheter

while the manometer was on the tip of the catheter. The blood

pressure inside the catheter may be different from the outside pres-

sure due to the presence of the catheter. However, the pressure

range obtained from the fiber optic sensor was from 60 mmHg to

100 mmHg, which is consistence with the reference manometer

(from 60 mmHg to 96 mmHg). It can be observed that the systolic

and diastolic pressures at the right coronary artery taken by both

fiber optic sensor and the reference manometer are higher than

those taken at the aortic arch.

4.4. Blood pressure measurements with balloon

After measuring the blood pressure at different locations, for

the purpose of demonstrating the fiber optic blood pressure sen-

sor in FFR applications, a 2.25 mm balloon was inserted into the

catheter to mimic stenosis. The blood pressure will drop when

the balloon inflates while the blood pressure will resume when

the balloon deflates. The inflation/deflation cycle was repeated 3

times. Fig. 10 shows electrical voltage signals from both broad-

band and narrowband channels. According to the voltage shift

from the broadband channel, the curvature of the blood ves-

sel was changed when the balloon was inflated. Therefore, huge

0 20 40 60 80 100 120 140 16013.8

13.9

14.0

14.1

14.219

20

21

22

230 20 40 60 80 100 120 140 160

Broadband channel

Volt

age

(mV

)

Time (s)

Narrowband channel

Fig. 10. Electrical voltage signals from both broadband channel and narrowband

channel.

changes of the baseline from the narrowband channel can be

observed.

Fig. 11 shows blood pressure readings from the fiber optic sen-

sor for the whole inflation/deflation procedure after compensation

by the broadband channel. It can be clearly seen that there are 3

cycles of balloon inflation and deflation. When the balloon was

inflated, the peak to peak amplitude of the blood pressure was

decreased. After the balloon was deflated, the peak to peak ampli-

tude of the blood pressure was resumed. The detailed information

of the blood pressure at each transient period are illustrated in

Table 1.

A big increase of the blood pressure can be observed when the

balloon was inflated for the first time. The same blood pressure

raise was shown to the reference manometer as well. The blood

pressure was out of the reading range in the reference manometer.

It might be because the pressure was built up when the balloon

inflated. However, in practical FFR applications, the peak to peak

amplitude of blood pressure signals is more important and there

is no pressure accumulation in FFR applications. In the second and

the third inflation/deflation cycle, the peak to peak value of the

blood pressure variation can be observed. Readings from the fiber

optic sensor and the reference manometer are consistent. During

the inflation of the balloon, the amplitude of the blood pressure

dropped while the amplitude of the blood pressure increased when

the balloon was deflated.

0 50 10 0 15 00

20

40

60

80

100

120

140

160

180

200

220

Inflated the b alloon

(blood pressur e drops)

Blo

od

pre

ssu

re (

mm

Hg

)

Time (s)

Fig. 11. Blood pressure readings of the whole inflation/deflation procedure from

the fiber optic sensor after compensation.

N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178 177

Table 1Transient blood pressure readings from the fiber optic sensor and the reference manometer for all three inflation/deflation cycles.

Transient period Fiber optic pressure sensor readings Reference manometer readings

First inflation

First deflation

Second inflation

Second deflation

Third inflation

Third deflation

178 N. Wu et al. / Sensors and Actuators B 181 (2013) 172– 178

5. Conclusions

In this paper, a miniature fiber optic blood pressure sensor for

FFR applications was designed, fabricated and tested in a swine

model. Static experiments were performed to verify the sensor’s

performances. In vivo experiments were performed by using a

swine as the animal target. In order to compensate the bending loss

from the optical fiber caused by the big curvature at the coronary

artery, a special interrogation system was designed.

Blood pressure was measured at different locations in the coro-

nary artery to demonstrate the capability of the fiber optic sensor

for capturing heart beat signals. The bending loss caused by the cur-

vature of the coronary artery was compensated by the interrogation

system very well. In order to demonstrate the sensor’s usage in FFR

applications, the drop and recovery of the peak to peak BP ampli-

tude caused by the balloon-mimic stenosis were recorded by the

fiber optic sensor successfully. Due to its compact size and all opti-

cal operation principle, such fiber optic sensors have wide potential

applications in medical area.

Acknowledgment

The authors would like to thank University of Massachusetts

Medical School for providing swine and proceeding the surgical

procedure.

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Biographies

Nan Wu completed his PhD degree in Department of Electrical and Computer Engi-neering at University of Massachusetts Lowell (2012). He is now working as thepostdoctoral researcher at UMass Lowell. His research interests include fiber opticpressure sensors, fiber optic photoacoustic generators, and photoacoustic imaging.

Ye Tian received his MS degree from the Institute of Modern Optics of Nankai Uni-versity in 2008. He is now pursuing his PhD degree in the Department of Electricaland Computer Engineering, UMass Lowell. His research interests are biosensors,fiber optic sensors, MEMS, and FIB.

Xiaotian Zou received his MS degree in Mechanical Engineering at the Universityof Connecticut in 2010. He is a PhD candidate in the Department of BiomedicalEngineering and Biotechnology, UMass Lowell. His research interests are controland data analysis algorithms for bio-systems and optical biosensors.

Yao Zhai received his MS degree from Institute of Semiconductors, Chinese Aca-demic of Sciences (2010), BS degree from Tianjin University (2007). He is a PhDcandidate in the Department of Electrical and Computer Engineering at UMassLowell. His research interests are quantum dot photodetectors.

Kurt Barringhaus is an assistant professor of Medicine at the University of Mas-sachusetts Medical School. His research interest is in coronary artery physiology.

Xingwei Wang is an associate professor in Department of Department of Electri-cal and Computer Engineering at UMass Lowell. Her expertise are optical sensorsfor medical, chemical and industrial applications; assistive technology program;nanoprobe design and fabrication; self-assembled nanostructures; optical biosens-ing and biomedical devices; optical imaging; MEMS technology and electromagneticwave propagation.