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Sensors and Actuators B 181 (2013) 172– 178

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o me pa ge: www.elsev ier .com/ locate /snb

miniature fiber optic blood pressure sensor and its application in in vivo bloodressure measurements of a swine model

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

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

r t i c l e i n f o

rticle history:eceived 17 September 2012ccepted 3 February 2013vailable online 17 February 2013

a b s t r a c t

Fractional flow reserve (FFR) is a promising technique in diagnosis of coronary artery stenosis. Thetechnique is applied in coronary catheterization to measure the blood pressure (BP) difference acrossa 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

eywords:lood pressureptical fiberressure sensorractional flow reserve

model in vivo. A 25–50 kg Yorkshire swine model was used as the test target. A guiding catheter wasintroduced into the coronary artery, and blood pressure signals in aortic arch and right coronary arterywere 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 wasrecorded by the fiber optic sensor. The experiment demonstrates that the reported fiber optic sensor has

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the capability of measuri

. Introduction

Coronary artery disease (CAD), which is caused by the accumu-ation of atheromatous plaques within the walls of the coronaryrteries that supply the oxygen and nutrients to the myocardium1], is the leading cause of the death. Most CAD patients are notware of the disease for decades as the disease progressing untilhe first symptoms, often an acute heart attack, finally show up.he disease is the most common cause of sudden death [2] as wells the most common reason for the death of people over 20 yearsld [3]. Moreover, half of healthy 40-year-old males will probablyevelop CAD in the future, and one in three healthy 40-year-oldomen [3].

Percutaneous coronary intervention (PCI) is a common ther-py directed toward alleviating CAD. It is important to assess theeverity of the lesion and its impact on blood flow before, duringnd after the angioplasty procedure. Assisted by this information,ardiologists can determine whether a PCI is necessary. Tradition-lly, angiography is the standard method to assess the severityf the lesion but is of limited value when lesions of intermediateeverity are identified, because it cannot provide adequate infor-

ation regarding whether the blood flow can be impacted by such

n intermediate lesion. In order to determine the lesion’s impactn blood flow, additional information is required. By interrogating

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

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.02.002

od pressure in vivo and can be used for FFR technique.© 2013 Elsevier B.V. All rights reserved.

the frequency shift between the sound wave emanated from thesource and that reflected from the moving blood cells, a Dopplerultrasound 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-wireanemometry, uses thermistors to monitor a tiny thermal gradientin a fluid flow stream [5]. However, this method may damage bloodcells or tissues because of the heat.

Fractional flow reserve (FFR) is an alternative method toevaluate the stenosis in coronary artery [6–12]. The severity ofthe narrowing is determined by measuring the blood pressuredifference across a coronary artery stenosis in the blood flowthrough coronary catheterization. This method has been eventuallyaccepted by doctors since 1990s. In order to achieve accurate FFRdiagnosis, in vivo blood pressure measurement is critical. Variousstudies have been conducted on how FFR benefits to the diagno-sis of the coronary artery stenosis by conducting the intravascularblood pressure measurement [12]. Most of the sensors used in thesestudies are electrical sensors which may generate electrical noisesto interfere with other electrical equipments in the operating room,where the electromagnetic interference is risky to patients. On theother hand, fiber optic pressure sensor is a potential substitutionto the current electrical pressure sensors. The fiber optic pressuresensor can be easily packaged in a guide wire due to its compact

size which is generally 125 �m in diameter. Due to its all opticaloperating principle, the fiber optic sensor cannot interfere withother electrical equipments. Samba Sensors released a fiber opticpressure sensor for intravascular blood pressure measurement. The

ctuators B 181 (2013) 172– 178 173

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ensor is made by attaching a 0.36–0.42 mm diameter silicon sen-or head to the tip of a 0.25 mm diameter optical fiber [13]. Theulky head of the sensor prevents it from further reducing the size.

This paper presents a fiber optic blood pressure sensor with uni-orm diameter of 125 �m for the purpose of in vivo blood pressure

easurement. The sensor was tested in a swine model at Universityf Massachusetts Medical School in Worchester, MA. The stenosis ofhe coronary artery was simulated by inflating a 2.25 mm balloon inhe catheter. A reference manometer was placed side by side withhe fiber optic sensor for the comparison purpose. In order to com-ensate the bending loss introduced by the big curvature of aorticrch, a special interrogation system was designed. Experimentalesults demonstrated that the fiber optic sensor has the capability ofonitoring the blood pressure profile and the blood pressure drop

aused by the inflated balloon. Moreover, the reported fiber opticlood pressure sensor proves its capability of being utilized in FFRpplications to determine the location of stenosis in the coronaryrtery.

. Fiber optic blood pressure sensor

.1. Design and principle

The fiber optic pressure sensor is designed based on Fabry–PerotFP) interferometer [14–18], as shown in Fig. 1. Three elements cane observed: a single-mode fiber, a multi-mode fiber, and a sil-

con dioxide (SiO2) diaphragm. The multi-mode fiber is appliedo fabricate an air cavity which is formed by wet etching usingydrofluoric acid (HF). The diameter of the cavity is determinedy the diameter of the multi-mode fiber core and the depth of theavity is determined by the etching duration. The SiO2 diaphragms attached by the end of the air cavity by thermal bonding tech-ique. Therefore, the multi-mode fiber core/air cavity interface andhe air cavity/diaphragm interface form an FP cavity on which theeflection lights will generate an interference pattern. The single-ode fiber guides the interrogation light exciting on the FP cavity

nd collects the reflected lights.According to the principle of the FP interferometer, the optical

hase � of the interference pattern is governed by

= 4�Ln

�0, (1)

here L is the length of the FP cavity; n is the refractive index ofhe cavity, which is 1 (air) in this case; �0 is the wavelength of thenterrogation light. The optical phase changes when the length ofhe FP cavity changes due to an external pressure applying on theiaphragm. The relationship between the FP cavity length changesnd the external pressure can be expressed by [19]:

c = 3(1 − v2)Pr4

16Eh3, (2)

here Yc is the center deformation of the diaphragm; v is theiaphragm’s Poisson’s ratio; P is the external pressure applied to

he diaphragm; r is the radius of the diaphragm; h is the thicknessf the diaphragm; E is the Young’s modulus of the diaphragm.

The external pressure can be determined by interrogating theptical 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, thereflected optical intensity can be approximated by a sinusoidalfunction [20]:

I = I0[1 + V cos(� − �0)], (3)

where I0 is the mean optical intensity; V is the visibility of the FPinterferometer; �0 is the initial optical phase. Therefore, the pres-sure applied to the diaphragm can be determined by interrogatingthe 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 deepreaction ion etching (RIE). The thickness of the diaphragm wasdetermined by the thickness of the grown oxide layer on the siliconsubstrate. This method promises that a diaphragm with control-lable and uniform thickness can be achieved. The fiber was preparedby splicing a piece of MMF with the SMF followed by cleaving theMMF so that approximately 30–40 �m length of the MMF was left.The FP cavity was formed by immersing the fiber with the MMF endin a 49% HF. Finally, the silicon diaphragm was thermally bondedonto 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 thenecessity of the bulky sensor head introduced by the support-ing structure [20–22]. The robustness of this structure has beendemonstrated in various pressure measurement applications [15].The uniform diameter of the sensor head keeps the sensor to itsminimized dimension and allows the following package procedure.

2.3. Package

As shown in Fig. 2, the pressure sensor is fabricated on a barefiber, which is about the size of a human hair without any outsideprotection such as buffer or jacket. The bare fiber is made of pureglass (silica) with special doping and its performance and long-termdurability can be affected by environmental conditions. Directlyexposed to the complex blood vessel circumstance, the fiber, espe-cially the fragile diaphragm, is prone to be broken and the opticalsignal will be not accurate. Therefore, utilization of the fiber opticsensor as a medical device requires a delicate protection. In addi-tion, a steerable tip section and a biocompatible coverage are alsorequired in in vivo blood pressure measurement applications.

The schematic of a fully packaged fiber optic sensor is shown inFig. 3, and the photograph of the tip section of a packaged sensoris illustrated in Fig. 4. There is a Kapton tubing covered around the

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

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.

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

o interact with the fiber sensing area. A stainless steel coil with aolymer head is bonded on the tip area for flexible steering. All ofhe covering materials are either biocompatible polyimide or witholytetrafluoroethylene (PTFE) coating. The mechanical propertiesf different sections of the fully packaged device vary for steer-ble tip, flexible middle and stiff extension. The packaging length isonger than 1.5 m for swine model usage. The final outer diameterOD) is around 360 �m which is close to commercial guide wiresor medical applications.

. Sensor verification

.1. Experimental setup

Prior to the animal test, static experiments were performedo investigate the sensor’s static performance. Fig. 5 shows thechematic diagram of the experimental setup. The fiber optic sen-or was placed in a sealed chamber in which the pressure wasontrolled by a pressure controller (NetScanner Model 9034, Pres-ure Systems Inc.). The sealed chamber was filled with water toimulate the internal environment of the swine artery. In order toompensate the bending loss occurred when the fiber optic sen-or travels through the coronary artery during the animal test, annterrogation system with the capability of detecting the bendingoss was introduced [23]. A wideband light source (OEBLS-200, O/Eand Inc.) was used to excite the fiber optic sensor through a cir-

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

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

rs B 181 (2013) 172– 178

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 thebroadband channel.

In the narrowband channel, the reflected interference spectrumfrom the fiber optic sensor can be observed because that the coher-ence length was much longer than the FP cavity length in thesensor. On the contrary, there is no interference can be observedin the broadband channel because the coherence length was muchshorter than the FP cavity length in the sensor. Therefore, the pho-todetector in the narrowband channel is used to interrogate thesensor while the photodetector in the broadband channel is usedto 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 whenthe peak wavelength of the tunable filter was set to 1547.5 nmare shown in Fig. 6b. The pressure in the chamber was increasedfrom 0 mmHg to 200 mmHg with steps of 50 mmHg and then wasdecreased from 200 mmHg to 0 mmHg with the same steps. Theresults indicate that the sensor has a low hysteresis and a highrepeatability according to low standard deviations. The sensitivitywas 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) afterwhich endotracheal intubation was performed. Anesthesia wasmaintained with inhalational 2–3% isoflurane. Next, femoral arte-rial access was obtained via cutdown, and a 6 French introducersheath 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 pressuremeasurements were obtained with standard invasive manometry.Fiber optic blood pressure measurements were similarly obtainedfor comparison offline.

The guiding catheter was advanced to the aortic arch. In orderto demonstrate the capability of capturing heart beat signals, theblood 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 fiberoptic sensor was outside the catheter. The blood pressure at theright coronary artery was measured when the fiber optic sensor wasinside 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 whenthe 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 aorticarch. 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 pressuresensor demonstrated its capability of capturing heart beat signalsin this measurement.

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

1520 154 0 156 0 158 0-50

-45

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10

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16

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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 at1547.5 nm.

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ig. 7. (a) Blood pressure measurements at aortic arch outside the catheter by fiber o

.3. Blood pressure measurements in the right coronary artery

In order to reach the right coronary artery, the fiber optic bloodressure sensor had to pass through the aortic arch where theptical fiber suffered a huge optical intensity loss due to the big cur-ature of the aortic arch. Therefore, it is critical to identify when theptical fiber suffers the optical intensity loss. The interrogation sys-

em of the fiber optic sensor has the capability to identify the opticalber bending loss. The broadband channel in Fig. 5 was used to cap-ure the optical intensity drop caused by the bending loss because its insensitive to the blood pressure variation. Therefore, the signal

220 230 240 250 260 270 280 290 300

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Narrowband channel

Volt

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coronary artery

Broadband channel

Volt

age

(mV

)

ig. 8. (a) Signals from the photodetectors in both channels when the fiber optic sensoompensation.

essure sensor. (b) Blood pressure measurements taken by the reference manometer.

from the broadband channel was used to compensate the bend-ing loss of the optical fiber. The electrical voltage signals from bothbroadband and narrowband channels are shown in Fig. 8a. At about249 s, the fiber optic sensor was advanced to the right coronaryartery and was suffered from the bending loss. It can be seen fromboth channels that there were huge voltage drops. The differencemagnitude of the voltage drops was due to the different gain set-

tings of photodetectors. After identified that the signal drops werecaused by the bending loss, the signals from the broadband chan-nel was used to compensate the signal drops in the narrowbandchannel, which is shown in Fig. 8b.

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13

14

10

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(b)

Narrowband channel

after compensation

Time (s)

Narrowband channel

r was subjected to a severe bending. (b) Signals in narrowband channel after the

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 referencem

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anometer.

The blood pressure measured by the fiber optic sensor is shownn Fig. 9a. The signals from the fiber optic sensor and the reference

anometer (Fig. 9b) are not very consistent. The inconsistence wasue to the different locations of the fiber optic sensor and the ref-rence manometer. The fiber optic sensor was inside the catheterhile the manometer was on the tip of the catheter. The bloodressure inside the catheter may be different from the outside pres-ure due to the presence of the catheter. However, the pressureange obtained from the fiber optic sensor was from 60 mmHg to00 mmHg, which is consistence with the reference manometerfrom 60 mmHg to 96 mmHg). It can be observed that the systolicnd diastolic pressures at the right coronary artery taken by bothber optic sensor and the reference manometer are higher thanhose taken at the aortic arch.

.4. Blood pressure measurements with balloon

After measuring the blood pressure at different locations, forhe purpose of demonstrating the fiber optic blood pressure sen-or in FFR applications, a 2.25 mm balloon was inserted into theatheter to mimic stenosis. The blood pressure will drop whenhe balloon inflates while the blood pressure will resume whenhe balloon deflates. The inflation/deflation cycle was repeated 3

imes. Fig. 10 shows electrical voltage signals from both broad-and and narrowband channels. According to the voltage shiftrom the broadband channel, the curvature of the blood ves-el was changed when the balloon was inflated. Therefore, huge

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ig. 10. Electrical voltage signals from both broadband channel and narrowbandhannel.

changes of the baseline from the narrowband channel can beobserved.

Fig. 11 shows blood pressure readings from the fiber optic sen-sor for the whole inflation/deflation procedure after compensationby the broadband channel. It can be clearly seen that there are 3cycles of balloon inflation and deflation. When the balloon wasinflated, the peak to peak amplitude of the blood pressure wasdecreased. After the balloon was deflated, the peak to peak ampli-tude of the blood pressure was resumed. The detailed informationof the blood pressure at each transient period are illustrated inTable 1.

A big increase of the blood pressure can be observed when theballoon was inflated for the first time. The same blood pressureraise was shown to the reference manometer as well. The bloodpressure was out of the reading range in the reference manometer.It might be because the pressure was built up when the ballooninflated. However, in practical FFR applications, the peak to peakamplitude of blood pressure signals is more important and thereis no pressure accumulation in FFR applications. In the second andthe third inflation/deflation cycle, the peak to peak value of theblood pressure variation can be observed. Readings from the fiberoptic sensor and the reference manometer are consistent. Duringthe inflation of the balloon, the amplitude of the blood pressure

dropped while the amplitude of the blood pressure increased whenthe 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

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pre

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mm

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Time (s)

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

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. Conclusions

In this paper, a miniature fiber optic blood pressure sensor forFR applications was designed, fabricated and tested in a swineodel. Static experiments were performed to verify the sensor’s

erformances. In vivo experiments were performed by using awine as the animal target. In order to compensate the bending lossrom the optical fiber caused by the big curvature at the coronaryrtery, a special interrogation system was designed.

Blood pressure was measured at different locations in the coro-ary artery to demonstrate the capability of the fiber optic sensor

or capturing heart beat signals. The bending loss caused by the cur-ature of the coronary artery was compensated by the interrogationystem very well. In order to demonstrate the sensor’s usage in FFRpplications, the drop and recovery of the peak to peak BP ampli-ude caused by the balloon-mimic stenosis were recorded by theber optic sensor successfully. Due to its compact size and all opti-al operation principle, such fiber optic sensors have wide potentialpplications in medical area.

cknowledgment

The authors would like to thank University of Massachusettsedical School for providing swine and proceeding the surgical

rocedure.

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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.