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: parkwt@ime.a-star.edu.sg; krkntu@gmail.com)

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