SIMULTANEOUS MEASUREMENT OF

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Nov 14, 2013 (3 years and 11 months ago)

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MEMS TACTILE SENSOR ARRAYS FOR
SIMULTANEOUS MEASUREMENT OF
FORCES, TISSUE DISPLACEMENT AND
TISSUE STIFFNESS



P. Peng, A. S. Sezen, R. Rajamani, A. G. Erdman

University of Minnesota


MIMTeC
-
CONFIDENTIAL

OBJECTIVE


To

develop

a

novel

technology

for

tactile

sensors

that

will

provide



tissue

stiffness

measurement




force




tissue

displacement

readings



by

utilizing

a

single

sensor
.




The

concept

is

currently

being

evaluated

by

designing

and

testing

a

MEMS

capacitive

membrane

array

for

minimally

invasive

applications
.




Once

tested

and

confirmed

the

idea

is

expandable

to

other

types

of

sensors

such

as

compliant

thin
-
film

transducers

and

to

both

shear

and

normal

tactile

measurements
.

MIMTeC
-
CONFIDENTIAL

MOTIVATION


Tactile

force

and

displacement

measurements

are

extremely

important

in

telerobotics

and

minimally

invasive

surgery



Knowledge

of

tissue

stiffness

is

a

valuable

tool

in

many

biomedical

applications
:

Figure
:

University

of

Washington

School

of

Medicine

(
www
.
orthop
.
washington
.
edu
)


-
Providing

real
-
time

feedback

on

the

type

of

tissue

to

the

surgeon

during

minimally

invasive

surgery

-
Ligament

tension

measurement

during

knee

implant

surgery

-
Early

detection

of

compartment

syndrome

-
Cartilage

hardness

measurements





and

many

other

applications

MIMTeC
-
CONFIDENTIAL

EXISTING TECHNOLOGY


Several

examples

of

tactile

sensors

measuring

contact

forces
.



Stiffness

measurement



a

predetermined

known

displacement

must

be

applied

at

the

time

of

force

measurement
.



Indentation

type

tissue

stiffness

tests

have

been

performed

on

cancerous

breast

tissue
.

“highly

nonlinear

and

quite

stiff

cancerous

and

nearly

linear

and

extremely

soft

fat

tissue”

[
1
]


A

laparoscopic

grasper

attached

to

a

robot

arm

[
2
]

has

been

designed

to

provide

force

and

vision

feedback

[1] Wellman et al., “Breast Tissue Stiffness in Compression is Correlated to Histological Diagnosis”,


Harvard BioRobotics Laboratory Technical Report
, 1999

[2] Tholey et al,
Annals of Surgery
, vol. 241, pp. 102
-
109, 2005

MIMTeC
-
CONFIDENTIAL

EXISTING TECHNOLOGY


Research

has

been

performed

in

detecting

bulk

tissue

compliance

such

as

soft

tissue

compliance

measurements

in

stumps

of

amputated

lower

limbs[
3
]




Piezoelectric

cantilevers

have

been

used

to

investigate

the

force
-
deformation

response

of

soft

tissues[
4
]



Non
-
invasive

methods

such

as

MRI[
5
]

and

ultrasound

have

also

been

shown

to

work

for

measuring

tissue

stiffness

in

many

works

presented

in

literature
.


[3] Vannah et al
, Journal of Rehabilitation Research and Development, vol.36, no 1, pp. 1
-
7, 1999

[4] Szewczyk et. al.
IEEE 29th Bioengineering Conference
, pp.146
-
147, 2003

[5] McKnight et al.,
American Journal of Roentgenology
, vol.178, no.6, pp.1411
-
1417, 2002

MIMTeC
-
CONFIDENTIAL

CHALLENGES


Tissue

material

properties

can

be

easily

measured

in
-
vitro,



Estimation

of

stiffness

is

challenging

since

it

is

defined

by

both

the

material

properties

and

boundary

conditions
.



Traditional

stiffness

measurement
:



known

displacement

applied



force

measured



stiffness

estimated




Cannot

be

used

for

real
-
time

applications

where

it

is

not

possible

to

apply

a

known

and

controlled

displacement
.


Tissue Type

Elasticity Mod. (MPa)

Healthy

cardiac

tissue

0.03

Liver

0.05 to 0.1

Human

small

artery

0.1
-

4

Elastin

0.6

Collagen

sponge

0.017 to 0.028

Collagen

fibers

1,000

Skin

0.1 to 2


Stiffness,

force

and

tissue

displacement

all

have

to

be

measured

simultaneously

and

continuously

MIMTeC
-
CONFIDENTIAL

CHALLENGES


Other challenges include:


the need for flexible sheets of tactile sensor arrays in many
applications,


the need to measure both shear and normal compliance


wiring challenges for arrays of sensors.

Figure : Colorado Advanced Photonics Technology Center

A typical wire bond

MIMTeC
-
CONFIDENTIAL

PROPOSED APPROACH



2
1
x
x


t
s
t
s
t
h
t
h
k
k
k
k
F
k
k
k
k
F
)
(
)
(
2
1



)
(
)
(
2
1
t
h
s
t
s
h
k
k
k
k
k
k
F
F



t
h
t
s
s
h
k
k
k
k
x
x





Membrane

parameters

and

deflection

measurements

from

sensors

are

known


Tissue

stiffness

and

total

force

on

the

tissue

can

both

be

calculated
.


Two

or

more

sensing

elements

with

different

spring

constants
.



The

sensing

element

(
k
h

or

k
s
)

and

the

tissue

(
k
t
)

viewed

as

two

springs

connected

in

series



deflections

are

different


Total

deflection

of

the

tissue

and

sensor

spring

are

equal

for

both

sensor

elements

or


MIMTeC
-
CONFIDENTIAL

CAPACITIVE MEMS SENSOR ARRAY

Measurement

of

capacitance

between

electrodes

provides

a

measure

of

membrane

displacement

The capacitive MEMS sensor and
simplified electrical model

An

array

of

MEMS

capacitive

membranes

with

varying

compliance




MIMTeC
-
CONFIDENTIAL

CAPACITIVE MEMS SENSOR ARRAY


Relative displacement of sensor elements provides tissue compliance.


Total force is equal to the sum of forces at all sensor elements

MIMTeC
-
CONFIDENTIAL

TISSUE STIFFNESS MODEL


where



shear modulus



deflection of the tissue



radius of the punch



Poisson ratio of tissue



a

2
2
1
)
1
(
2
)
(








a
p


The pressure distribution model is









1
4
a
dA
p
L


The total load under punch is

MIMTeC
-
CONFIDENTIAL

MEMBRANE DEFLECTION MODELS

a.
Simple

circular

membrane

with

clamped

edges
:

a

straight
-
forward

expression

for

membrane

deflection
.


b.
Central

boss

model
:

Top

electrode

will

have

an

effect

on

membrane

response
.




Membrane

center

will

have

less

deflection
.




Membrane

model

approximation

with

a

rigid

center
.

MIMTeC
-
CONFIDENTIAL

MEMBRANE DEFLECTION MODELS

ANSYS simulation results for the membrane models

Simple circular model with clamped edges

Central boss model


ANSYS

simulations

for

the

deflection

of

a

300

μm

diameter

membrane

have

been

performed
.



The

effect

of

having

etch

holes

which

are

a

necessary

part

of

the

surface

micromachining

microfabrication

process

was

also

investigated
.

MIMTeC
-
CONFIDENTIAL

0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0.089
0.09
0.091
0.092
0.093
0.094
0.095
pressure (pascal)
capacitance (pF)
capacitance vs. pressure


200um
COMPARISON OF THE MODELS

0
0.5
1
1.5
2
2.5
3
3.5
4
x 10
4
0
5
10
15
20
25
30
pressure (pascal)
membrane deflection (um)
deflection vs. pressure


clamped plate
considering rigid center
experimental estimation

The

central

boss

model

is

much

more

effective

in

approximating

the

response
.



Therefore

this

model

was

utilized

during

the

initial

simulations

to

determine

the

ideal

membrane

geometry

for

our

design
.


Membrane simulation versus
preliminary exp. results

Load vs. Capacitance curve for the
200 µm membrane

MIMTeC
-
CONFIDENTIAL

SENSOR LAYOUT

0
1
2
3
4
5
6
7
8
9
10
x 10
4
0
0.5
1
1.5
2
2.5
3
pressure (pascal)
membrane deflection (um)
deflection vs. pressure


200um
300um
400um

To

minimize

the

number

of

microfabrication

layers,

membrane

stiffness

is

determined

by

membrane

diameter

rather

than

thickness



Three

membrane

diameters

have

been

chosen

as

200
,

300

and

400

μm

Simulation results for membrane
deflection

0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0
0.2
0.4
0.6
0.8
1
1.2
1.4
pressure (pascal)
capacitance (pF)
capacitance vs. pressure


200um
300um
400um
Load vs. capacitance curves for
three different membrane sizes

MIMTeC
-
CONFIDENTIAL

SENSOR MEMBRANE LAYOUT


MASK DESIGN

a.

b.


Layouts

with

combinations

of

membrane

distribution

and

numbers


Mask

design

by

using

ICED

TM

MIMTeC
-
CONFIDENTIAL

MEMS SENSOR FABRICATION

MIMTeC
-
CONFIDENTIAL

FABRICATED SENSORS

Actuation

of

a

membrane

Fabricated

Sensors

MIMTeC
-
CONFIDENTIAL

CURRENT STATUS AND FUTURE DIRECTIONS

The capacitive MEMS sensor arrays have been successfully
designed and the microfabrication has recently been completed.

Designed

PCB

for

capacitive

readout

MIMTeC
-
CONFIDENTIAL

CURRENT STATUS AND FUTURE DIRECTIONS



Laboratory

tests

to

evaluate

the

ability

of

the

tactile

sensors

to

measure

both

forces

and

tissue

compliance

will

be

conducted

in

the

coming

months
.



Future

goals

include

the

creation

of

a

flexible

array

of

tactile

sensors,

measurement

of

both

shear

and

normal

forces

and

compliance,

electronics

for

sequential

readings

of

an

array

of

sensors

using

minimal

wires

and

study

of

a

specific

medical

application
.


Some

challenges

related

to

microfabrication,

such

as

the

sealing

of

the

membrane

etch

holes,

will

be

addressed

after

the

preliminary

experiments

to

validate

the

proposed

sensing

method

are

completed
.


MIMTeC
-
CONFIDENTIAL

MEASUREMENT OF SHEAR FORCES AND COMPLIANCE

Quad
-
configuration
capacitive sensing unit


Normal

force

on

the

substrate



uniform

distance

reduction

for

all

four

capacitors
.



Shear

force

is

applied

on

the

silicon

mass



capacitance

change

for

the

four

capacitive

pairs

will

be

different
.


MIMTeC
-
CONFIDENTIAL

MEASUREMENT OF SHEAR FORCES AND COMPLIANCE

If

the

idea

that

was

presented

for

stiffness

measurement

is

applied

here

by

utilizing

“softer”

and

“harder”

membranes

under

a

shear

load,

a

sensor

for

stiffness

measurement

both

for

normal

and

shear

loadings

is

created

MIMTeC
-
CONFIDENTIAL

FLEXIBLE ARRAY OF TACTILE SENSORS


Compliant

tactile

sensors

that

conform

to

a

surface

are

desirable

in

a

variety

of

applications
.




Once

fabricated,

the

array

of

MEMS

sensors

developed

herein

can

be

diced

directly

on

a

flexible

substrate
.




The

gap

created

during

dicing

would

enable

the

sensors

to

move

with

respect

to

each

other
.




The

sensors

can

be

further

covered

with

a

protective

medium

such

as

silicone

rubber

which

will

both

protect

the

sensors

and

enable

transduction

of

forces

to

the

sensor

membranes
.

MIMTeC
-
CONFIDENTIAL

CONCLUDING REMARKS


A

novel

technology

that

provides

tissue

stiffness

measurements

along

with

force

and

displacement

readings

with

a

single

sensor

for

a

variety

of

biomedical

applications

has

been

presented
.



A

surface

micromachining

method

developed

in

the

ACML

has

been

successfully

used

to

fabricate

MEMS

sensors

which

will

be

used

in

experiments

to

confirm

the

applicability

of

the

technology

in

minimally

invasive

applications



Once

tested

and

confirmed

the

idea

is

expandable

to

other

types

of

sensors

such

as

compliant

thin
-
film

transducers

and

to

both

shear

and

normal

tactile

measurements
.