Abstract - This paper describes the design of a tri-axial
microelectromechanical force sensor (FS) that can be mounted
on the tip of the guidewire. Piezoresistive silicon nanowires
(SiNW) are embedded into a cross cantilever design with a
manoeuvrable stylus to allow the detection of force in all
directions. The electrical resistance changes in the four SiNWs
are used to decode an arbitrary force applied onto the FS. The
sensitivity of the device can be improved by two orders of
magnitude compared to bulk Si thanks to the giant
piezoresistive effects offered by the SiNW. Robustness of the FS
is improved due to the novel design by incorporating a
mechanical stopper at the tip of the stylus. Finite element
analysis (FEM) analysis was used in designing the FS.
Index Terms— Minimally invasive surgery, sensorized
guidewire, tri-axial force sensor, tactile sensor, MEMS, finite
element analysis
I. INTRODUCTION
INIMALLY invasive procedures are preferred due to
small incisions (leaving small tissue scar after healing),
less hospitalization time and quick recovery from incision
trauma. For many cardio vascular and thoracic interventional
procedures, passing a guidewire through vascular vessel is the
first step followed by the surgical procedures such as stenting
or so as shown in Figure 1. The ability to successfully treat a
vascular lesion via endovascular methods (wires, catheters
and angioplasty balloons) is dependent on the ability to pass a
guidewire across the lesion (usually a stenosis or occlusion).
Blockage of the vessel lumen in the range from 50% to 100%
makes passage of the guidewire a challenging affair.
Currently, passage of the guidewire is primarily through
the haptic feeling of the surgeon (accompanied with eye hand
coordination for on screen x-ray imaging) and the force
feedback of the passing guidewire is extremely difficult to
quantify. Quantitative information of force feedback of the
passing guidewire can be used in facilitating robotic aided
surgeries, training the residents etc in the future. Tactile
feedback of the guidewire while it is passing is very critical.
Manuscript received 30th March, 2010. This work was supported in part
by A*Star science and research council under Grant 0921480070.
Kotlanka Ramakrishna, Liang Lou, Lichun Shao, Woo-Tae Park, Daquan Yu, Lishiah Lim, Yongjun Wee, Vaidyanathan Kripesh, Dim-Lee
Kwong are with the A*Star Institute of microelectronics, Science park 11,
Singapore 117685, (corresponding author : +65-67705615; e-mail: [email protected]; [email protected])
Liang Lou, Chengkuo Lee, are with Department of Electrical &
Computer Engineering, National University of Singapore, Singapore 117576 Benjamin S Y Chua is with National University Hospital, 5 Lower Kent
Ridge Road, Singapore 119074.
Figure.1 Passage of the guidewires © 2004 IEEE. Inset shows the passage of guidewire under radiation.
Microelectrormechanical systems (MEMS) have enabled
the possibility of making sensorized guidewires by placing a
pressure sensor at the tip of the guidewire [1]. This helped on
the information pertaining to exact the location of the stenosis
by obtaining the difference in the pressure at the lesion.
Rebello et al [2] reported that there is a change of about 3oC in
temperature at the location of the stenosis and hence the
temperature sensor be used at the tip of the guidewire.
Bonanomi et al [3] mentioned that the hardness of the calcified
tissue at stenosis location is higher than the healthy vascular
vessel and the force sensor be used to identify stenosis.
Haga et al [4] have reported the assembly of three pressure
sensors at an angle of 120o on the guidewire to obtain the
contact information of the guidewire while it makes a touch to
the vascular vessel. However, with the use of three pressure
sensors, the dimension of the guidewire becomes about five
French thus making it non useful for smaller vascular vessels.
Valdastri et al [5,6] have used tri-axial force sensor on the
surgical knife (nine French diameter) for obtaining force
applied by the surgeon for making incisions. The sensor
robustness is improved by the packaging technique.
A comprehensive review of FS and their applications are
available in the cited papers [7-10]. In the field of FS, the
most common transduction methods are piezoresistive,
capacitive and piezoelectric. We have implemented
piezoresistive transduction method for force sensing. The
piezoresistive effect of SiNW has been shown to be
increasable up to gauge factors of 5000 from 50 in bulk by
shrinking cross-sectional dimensions. Furthermore, this
effect can be tuned during operation by applying back
gate-bias [11].
Sensorized guidewires with MEMS tri-axial force sensor for
minimally invasive surgical applications
Liang Lou, Kotlanka Ramakrishna, Lichun Shao, Woo-Tae Park, Daquan Yu, Lishiah Lim, Yongjun
Wee, Vaidyanathan Kripesh, Hanhua Feng, Benjamin S Y Chua, Chengkuo Lee, Dim-Lee Kwong
M
32nd Annual International Conference of the IEEE EMBSBuenos Aires, Argentina, August 31 - September 4, 2010
978-1-4244-4124-2/10/$25.00 ©2010 IEEE 6461
II. SENSOR DESIGN
A. FS structure and working principle
The sensing mechanism of a piezoresistive FS lies in
silicon‟s ability to change carrier mobility under strain
loading. The proportional change in electrical resistance can
then be measured using a Wheatstone bridge through
common-mode measurement.
ttll
R
R
(1)
where, R= resistance, π= piezo-resistive coefficient, σ=stress,
l and t subscripts refer to longitudinal and transverse
components.
In this work, a tri-axial FS similar to that of Beccai et al. [8]
was used. The major difference is that we have used
embedded SiNW as the piezoresistive transducer and a novel
mechanical stopper, as depicted in Figure 2. An array of four
cantilevers with embedded SiNW (along <110> directions to
get maximum coefficient) forms the transducer assembly.
Figure 2 (a) shows the model of the FS prior to assembly to
the ASIC. The mechanical stopper is attached to the end of
the stylus, which will be in contact with the lumen during
guidewire‟s passage and thus transfers the force to the SiNW
accordingly. After certain threshold of force, it acts as a
stopper protecting the MEMS features (the cross-cantilevers).
Figure 2(b) depicts the proposed tactile sensor assembly on
the guidewire. The active FS is of 350µm as we targeted to
assemble FS for one French diameter guidewire.
Figure. 2(a) MEMS without assembly depicting the structural features (b) FS after assembly on guidewire.
B. Analytical model of MEMS FS under normal &
transverse load
Any force applied on the mechanical stopper can be
decomposed into normal and shear components, which are
analyzed separately. When normal load is applied to the
mechanical stopper, the cantilever will deform in the
clamped-guided form shown in Figure 3a with one cantilever
as illustration. The clamped end at the frame serves as the
support, and the other end with stylus will slide vertically.
The dimension of the piezoresistor is sufficiently small
compared to the cantilever thickness. Hence we can assume
that the SiNWs have insignificant effect on the mechanical
properties of the cantilevers. The axial stress distribution on
the cantilever surface along its length can be derived as
0
3
3( 2 )
4z
zf
f hl x
bt (2)
where fz is the vertical force, ho is the distance from the SiNW
to the neutral axis, l, b and t are the beam length, width and
thickness respectively. x is measured from the beam end at the
support as shown in Figure 3.
Figure.3. Clamped-guided cantilever model (a) normal force (b) transverse
force.
When transverse force is applied in parallel with two of the
cantilevers, these cantilevers will undergo a combination of
axial, bending and torsion at the junction to the bottom of the
stylus. The cantilever end can be approximately considered as
a hinge shown in Figure 3b. The axial stress now becomes
0
3 3
6 3(1 )
(1 ) 2 (1 )x
xf
km kf
M d fx
bt l bt
(3)
where fz is the transverse force Mo=fxd is the torque
introduced by the lateral force, and d is the distance from the
force to the cantilever neutral axis. 2
2
4kf
b
l
,
2
28 (1 )km
t
b
and is the Poisson‟s ratio [13].
III. MODELING, FABRICATION &DISCUSSION
The FS has to perform under fluidic environment and
against continuous blood flow. Hence, it is important to
understand the amount of force that the FS would be
subjected to due to blood flow. Thus, the blood vessel is
modeled as a cylinder with 6mm diameter and 2.4mm length
while the distal portion of the FS and guidewire assembly is
modeled as a hemisphere tip with 350µm in diameter and
1.2mm in length. The fluidic behavior (blood flow) was
simulated using ANSYS Fluent (as shown in Figure 4).
Figure.4. Fluidic simulation for obtaining the force that the FS was subjected.
Insert shows the blood flow velocity vector due to the presence of sensorized
guidewire.
In Figure 4, the blue face and the red face represent the blood
flow inlet and outlet respectively. The blood density and
viscosity assigned in the model are 1025kg/m3 and 0.0035
6462
kg/m.s respectively. The inlet flow velocity assigned is
80cm/s. Insert shows the fluidic trajectory (velocity vector) of
the zoom-in region near the FS. The force that the FS is
subjected to, is a maximum of 0.025mN at 80cm/s and such
force needs to be offset for the final sensory reading.
The responses of FS to normal and transverse loadings
were simulated using ANSYS and ABAQUS. Figure 5(a)
shows that under normal loading, the strain in the four SiNWs
are of the same magnitude and sign. On the other hand, if a
transverse load is applied as shown in Figure 5(b), the two
SiNWs in parallel with the force experience strain of the same
magnitude but opposite sign. The remaining two cantilevers
perpendicular to the loading undergo torsion. Torsion induces
shear on the SiNW but does not change much of the SiNW
resistance value due to the low torsion gauge factor.
Figure.5. Strain along the cantilever under (a) normal and (b) transverse
loading for the applied force of 10mN. Strain is being reported as it has direct
relationship to the gauge factor of the SiNW (Gauge factor is the ratio of relative change in electrical resistance to the mechanical strain).
Critical structural parameters such as the cantilever length,
cantilever thickness, stylus length, and mechanical stopper
diameter are all linked together for optimal design of the FS.
A detailed finite-element modeling (FEM) was carried out to
investigate this link and was shown in Figure 6. Specifically,
whatever normal or transverse load is applied, the strain in the
SiNW increases for longer cantilever designs but decreases
for thicker cantilevers. On the other hand, longer stylus
enhances the transverse sensitivity, but merely has any effects
on the normal sensitivity as anticipated. Figure 6 can be taken
as a design chart for fabrication of the FS for the appropriate
structural dimensions and the maximum strain that we require
on SiNW.
It was observed that as the mechanical stopper gets bigger,
the allowable displacement and force range is reduced, which
ensures a more robust FS. Hence, a proper compromise needs
to be made between the working range and robustness of the
sensor according to the specific application requirements.
Fabricated sensor has cantilever length of 50µm, width of
10µm, thickness of 30µm, stylus length of 400µm and
mechanical stopper dimensioning 300µm.
Figure.6. Structural interdependence of the internal components such as
cantilever length, thickness and stylus length for the strain on silicon nanowire for (a) normal and (b) transverse loadings (c) Von Mises stress
contours on the Si cantilevers subjected to normal (left) and transverse (right)
force for the maximum permissible displacement allowed due to mechanical stopper. Stresses are within the fracture toughness of the Si beams.
Figure.7. Process integration of the FS
Si
SiO2
30µm
400µm
Step 1: SiNW definition on SOI wafer
Step 2: Thermal oxidation followed by ion
implantation
Step 3: Deposit 4kA USG PMD layer
Step 4: Contact formation followed by
metal line definition
Step 5: Deposit 5kA USG passivation layer
followed by bond pad opening
Step 6: Cantilever definition by front side
DRIE followed by stylus formation and
structural release by backside DRIE
Top view of fabricated force sensor
SiNW
(a)
(b)
(a)
(b) (c)
6463
We leverage on the large piezoresistance of SiNWs, acting
as the electromechanical elements for strain sensing. The key
process steps are illustrated in Figure 7. Firstly, SiNWs are
defined on a SOI wafer by standard lithography and etch
process. Next, thermal oxidation is performed to further
shrink down the dimensions of NWs to ~100nm wide and
thick. After pre-metal dielectric (PMD), metallization and
passivation layer formation, Deep Reactive Ion Etching
(DRIE) process is carried out to define the four cantilever
structures on the front side of the wafer. Both etch rate and
etch time of this DRIE step are critical in defining the final
thickness of the cantilever beam as per design requirements.
Upon completion of the front side DRIE, the wafer is flipped
over and another DRIE process is applied to form of the stylus
structure and release the cantilevers of the FS. SEM of the
fabricated FS from front side is shown in Figure 7.
After sensor fabrication, the next step is to form a
mechanical stopper to enhance the robustness of the structure.
To achieve this, flip chip packaging technique is utilized to
attach the ball shape stopper to the stylus. Figure 8a
summarizes the stopper attachment process. The sensor
temperature is set at 100 degree C and stopper holder is set at
room temperature. The force applied during flip chip process
is 5 gram with 30 sec bonding time. Figure 8b presents the
cross section view of a fabricated device. As shown, the
spatula is fixed on the tip of the stylus and the horizontal
misalignment between them is sufficiently small. Future
work is ongoing to test the robustness of the mechanical
stopper.
Figure.8. (a) Epoxy bonding method: Stopper (solder ball) is placed on the Si holder and adhesive epoxy is dispensed on the top of the stopper. After
alignment and bonding process in FC150 flip chip bonder, baking step is
needed to cure the epoxy, (b) The SEM picture of the fabricated sensor showing the mechanical stopper at the tip of the stylus.
IV. CONCLUSION
In this paper, we report the design, and fabrication of FS for
guidewire applications. Highly sensitive piezoresistive
SiNWs are used for force detection. We provided detailed
relationship between the applied normal/transverse force and
induced strain on SiNWs wherein SiNWs were embedded
within the cantilevers as a transducer. In the later portion of
the paper, FEM was carried out to investigate the structural
interdependence (among stylus diameter, cantilever length,
cantilever width, cantilever thickness and stylus length) for
normal and transverse force loadings. The FEM results agree
well with the theoretical predictions. Furthermore, to improve
the robustness of the force sensor in the presence of large
force, a mechanical structure is proposed and fabricated. The
results of this paper provide useful guidelines for optimal
design of the tri-axial force sensor while the sensor is robust
for large force yet highly sensitive due to the giant gauge
factor of the SiNW. The fabrication and assembly of the force
sensor with readout ASIC is still the focus of our ongoing
research.
ACKNOWLEDGMENT
This work was funded by A*STAR science and research
council under Grant 0921480070.
REFERENCES
[1] Haga, Y. Mineta, T. Esashi, M.Yamagata. “Active catheter,
active guidewire and related sensor systems” Automation Congress,
2002 Proceedings of the 5th Biannual World pp. 291-296, 2002 .
[2] Keith J. Rebello, “Applications of MEMS in Surgery”.
Proceedings of the IEEE,Vol.92, pp.43-55, 2004.
[3] Gianluca Bonanomi, Keith Rebello, Kyle Lebouitz, Cameron
Riviere, Elena Di Martino, David Vorp, Marco A. Zenati.
“Microelectromechanical systems for endoscopic cardiac Surgery”,
J. Thorac Cardiovasc Surg, Vol.126, pp.851-852,2003 .
[4] Yoichi Haga, Masayoshi Esashi, “Biomedical Microsystems for
Minimally Invasive Diagnosis and Treatment”, Proceedings of the
IEEE, Vol.92, pp. 98-114, 2004.
[5] Pietro Valdastri, Kanako Harada, Arianna Menciassi, Lucia
Beccai,Cesare Stefanini, Masakatsu Fujie, and Paolo Dario,
“Integration of a Miniaturised Triaxial Force Sensor in a minimally
Invasive Surgical Tool”, IEEE transactions on bio-medical
engineering, Vol.53, pp.2397-2400, 2006.
[6] Pietro Valdastri, Keith Houston, Arianna Menciassi, and Paolo
Dario,“Miniaturized cutting tool with triaxial force sensing
capabilities for minimally invasive surgery”, J. Med. Devices ,Vol.1,
pp.206-211, 2007.
[7] Fahlbusch, S, S. Fatikov, and K. Santa, “Force sensing in
microrobotic systems: an overview”, IEEE International Conference
on Electronics, Circuits and Systems. Vol.3, pp.259-262, 1998.
[8] Lucia Beccai, Stefano Roccella, Alberto Arena, Francesco Valvo,
Pietro Valdastri, Arianna Menciassi, Maria Chiara Carrozza and
Paolo Dario, “Design and Fabrication of a hybrid silicon three-axial
force sensor for biomechanical applications”. Sensors and
Actuators(A),Vol.120,pp. 370-382, 2005.
[9] Tibrewala, A., A. Phataralaoha, and S. Buttgenbach, “Analysis
of full and cross-shaped boss membranes with piezoresistors in
transversal strain conFigureuration”, J. Micromech. And Microeng.,
Vol.18, pp.1-6, 2008.
[10] Doelle, M, Peters, C, Ruther, P and O. Paul, „„Piezo-FET
stress-sensor arrays for wire-bonding characterization,‟‟ Journal of
Microelectromechanical System , Vol. 15, pp. 120–130, 2006.
[11] Pavel Neuzil, C.C Wong, and J Rebound “Electrically
Controlled Giant Piezoresistance in Silicon Nanowires”, Nano
Letter, Vol.10 (4), pp1248-1252, 2010.
[12] Reck, K, Richter, J, Hansen, O and Thomsen, E .V,
„„Piezoresistive effect in top-down fabricated silicon nanowires”,
21st IEEE Conference on MicroelectromechanicalSystems, pp.
717–720, 2008.
[13] W.L. Jin, C.D. Mote, Jr., “Development and calibration of a
sub-millimeter three-component force sensor”, Sensors and
Actuators(A), Vol.65, pp.89-94, 1998.
(a)
(b)
Mechanical
stopper
Epoxy
Ball
Holder
(a)
(b)
6464
Top Related