Proceedings of BioMed2007
2nd Frontiers in Biomedical Devices Conference
June 7-8, 2007, Irvine, California, USA

BioMed2007-38089
POLYMER-BASED CARDIOVASCULAR SHEAR STRESS SENSORS
Hongyu Yu, Lisong Ai, Mahsa Rouhanizadeh, Ryan Hamilton, Juliana Hwang,
Ellis Meng, Eun Sok Kim+ and Tzung K. Hsiai
Departments of Biomedical Engineering and Cardiovascular Medicine and Electrical Engineering+
USC Viterbi School of Engineering
University of Southern California,
Los Angeles, California 90089, USA
hongyuyu@usc.edu
ABSTRACTThis paper describes a polymer-based cardiovascular shear stresssensor built on catheter for atherosis diagnosis. This flexible sensor
detects small temperature perturbation as fluid past the sensing
elements leading to changes in the resistance, from which shear stressis inferred. MicroElectroMechanical System (MEMS) surface
manufacture technology is utilized for fabrication of the devices with
biocompatible materials, such as parylene C, Titanium (Ti) andPlatinum (Pt). The temperature coefficient of resistance (TCR) of thesensor is 0.11%/oC. When a catheter-based sensor is positioned near
the wall of the rabbit aorta, our 3-D computational fluid dynamic
model demonstrates that flow disturbance is negligible under steadystate in a straight segment. The sensor has been packaged with acatheter and will be deployed into the aorta of NZW rabbits for realtime shear stress measurement

INTRODUCTION
Coronary artery disease remains the leading cause of death in
the US and is the emergent global health issue within the next 15years. Hemodynamic forces, specifically, fluid shear stress, play animportant role in the biological activities of cardiovascular endothelial
cells. At arterial bifurcations where inflammatory processes prevail,the fluid mechanical environment is distinct from the laminar pulsatileenvironment present in the long and straight regions of the vessel orthe medial wall within bifurcations. At the lateral walls of arterial
bifurcations, disturbed flow, including oscillatory flow (bidirectional
net zero forward flow), is considered to be an inducer of vascularoxidative shear stress that promotes the initiation and progression ofatherosclerosis.
Measuring the vessel wall shear stress precisely remains as achallenging issue for engineers, although several methods have beendeveloped for wall shear stress measurement, including, micro force
sensing element or ultrasonic Doppler measurement. However, thesemethods are either too difficult to fabricate or lack the resolution.
There is another method which is thermal anemometry based on theheat transfer principle to be discussed in the ensuing paragraph. The
advantages of this technique are simplicity in fabrication, avoidance ofmoving elements, and high sensitivity
FLOW

plaqueVelocity profile Temperature profile
Tv

yx

Flow boundary layer Thermal boundary layer(a)Catheter(b)Fig. 1. (a) A polymer-based shear stress sensor in a model ofrabbit aorta. (b) Packaging of sensors on the catheter in relation
to thermal boundary layer.
When an electric current passes through the heated element, theheat convection from a resistively heated element to the flowing fluid
is measured, from which a value for shear stress is inferred. Over past
decade Dr. Ho and Tai [1] have been developed polysilicon based theMEMS surface micromachined sensors for shear stress sensing in airand liquid.
In previous research [2,3], our labs have developed and tested
bulk micromachined silicon-based MEMS sensors that have enabled(b)us to measure intravascular physical parameters; specifically,real-timeshear stress, temperature, pressure and velocity with high spatial andtemporal resolution. This abstract describes our recently developed
and tested bio-compatible, flexible polymer-based MEMS sensors forreal-time measurements of shear stress and velocity with catheter (Fig.
1)

DEVICE AND RESULTS
The micro polymer-based vascular sensors are composed of resistive
heating and sensing elements that are encapsulated by biocompatiblepolymer (parylene C). The sensor detects small temperature
perturbation as fluid past the sensing elements leading to changes inthe resistance, from which shear stress is inferred. Biocompatible

1Copyright © 2007 by ASME

Titanium (Ti) and Platinum (Pt) are used as the heating and sensing
elements to be exposed to blood flow. The Ti/Pt sensing element offers
low resistance drift, large range of thermal stability, low 1/f noise
without piezoresistive effect, and biocompatibility and resistance tocorrosion/oxidation. Moreover, Ti/Pt is deposited at room temperature,
allowing for integration with flexible parylene fabrication process.

(2)

Platinum/Titanium

Parylene C(3)

Gold/Chromium

(4)

(5)

(6)

Fig. 2. Fabrication processof
polymer-based
microvascular sensor.

971.5
971-6
2

The fabrication process of thesensor is as follows (Fig. 2): (1)thermal growth of SiO2 on
Silicon wafer and deposition(or coating) of a thin layer ofSilicon (1 m); (2) deposition
andpatterningof
SiO2(0.3μm)andPt/Tilayers(0.1μm) for the heating elementon the surface of the Silicon;(3) deposition of Parylene C
and etching of a hole to form
electrode contact (4) depositionand patterning of a metal layerof Au/Cr for electrode leads(2μm); (5) deposition andpatterning of a thick layer ofParylene C to form the device
structure; and (6) etching theunderneath Silicon sacrificial
layer leading to the final
device-a flexible parylene Cbased sensor.

y = -8.1027x + 976.25
R2 = 0.9904

972
V /R (10 VA)

Silicon Dioxide

(1)

972.5

970.5
970
969.5
969
0.5

0.6

0.7
1/3

0.8

0.9

1/3τ (pa )Fig. 4 Circuit signal vs. shear stress
A 3-D CFD model using the FLUENT software demonstrates
that flow disturbance is negligible under steady flow in a straightsegment if a catheter integrated with the sensor is placed near the wallof the rabbit aorta (Fig. 5). The sensor has been bonded with catheter
guide wire with biocompatible conductive epoxy for electrodeconnection and insulating epoxy for protecting the sensor body, which
will be deployed into the aorta of NZW rabbit.

(a)

(b)
Fig. 5 The catheter is placed (a) attaching the blood vessel wall
for accurate wall shear stress with minor disturbance to the flow,(b) in the middle of blood vessel with totally changed flow profile
from its disturbance.

SUMMARY
(a)

(b)

(c)

Fig. 3. (a) The sensor is flexible, assuming linear or zigzag
fashions. (b) The sensor is folded by a tweezers without
structural or functional damage. (c) The sensing element ismade of 2μm wide Pt/Ti thin film line
The fabricated sensors are shown in fig. 3, where the dimension of
the sensing element of the flexible parylene-based sensor isproximally 160 μm in length and 80μm in width, while whole sensor
has 4 cm in length, 320 μm in width and 21 μm in thickness. Given
that parylene offers the structural stiffness and sturdiness to
encapsulate the electrodes, the sensor is easily conformed to various
anatomic curvatures with excellent mechanical strength
The resistance of the sensing element is about 1.0 kOhm, and thetemperature coefficient of resistance (TCR) is at 0.11%/oC, compatible
for blood Rheology.
Under a fully developed laminar flow, the boundary layer
velocity profile determines the rate of heat transfer from a heatedresistive element to the surrounding fluid field. Thus, a linear
relationship is established between V2/R and τ1/3 (Fig. 4).

A biocompatible polymer-based shear stress sensor has beendeveloped for precisely measurement of cardiovascular wall shear
stress. Owing to its flexibility and working principle, it can also be
utilized to measure other intravascular physical parameters, such as
flow velocity and temperature with high spatial and temporal
resolution.

ACKNOWLEDGMENT
These studies were supported by AHA GIA 0655051Y (TKH), NIH
HL068689 (TKH), NIH HL083015 (TKH), AHA Pre-Doctoral
Fellowship 0615063Y (MR).

REFERENCES
1. Ho, C-M. and Tai, Y-C, “Microelectromechanical Systems (MEMS)
and Fluid Flows,” Annu. Rev. Fluid Mech.,Vol. 30, 1998, 579-612.
2. Soundararajan G, Rouhanizadeh M, Yu H, DeMaio L, Kim ES,Hsiai TK. MEMS shear stress sensors for microcirculation. Sensor
Actuat a-Phys. 2005;118(1):25-32.
3.Rouhanizadeh M, Lin TC, Arcas D, Hwang J, Hsiai TK. Spatial
variations in shear stress in a 3-D bifurcation model at low Reynolds
numbers. Ann Biomed Eng. 2005;33(10):1360-1374.

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Copyright © 2007 by ASME