Sensors & Actuators: B. Chemical 354 (2022) 131152

Available online 29 November 2021

0925-4005/© 2021 The Authors.

Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(

http://creativecommons.org/licenses/by-nc-nd/4.0/

).

3-Dimensional

electrical

impedance

spectroscopy

for

in situ

endoluminal

mapping

of metabolically

active

plaques

Parinaz

Abiri

a

,

b

,

1

, Yuan

Luo

c

,

g

,

1

, Zi-Yu

Huang

d

,

1

, Qingyu

Cui

a

,

1

, Sandra

Duarte-Vogel

e

,

Mehrdad

Roustaei

b

, Chih-Chiang

Chang

b

, Xiao

Xiao

b

, Rene

Packard

a

, Susana

Cavallero

a

,

Ramin

Ebrahimi

a

, Peyman

Benharash

f

, Jun Chen

b

, Yu-Chong

Tai

d

, Tzung

K. Hsiai

a

,

b

,

d

,

*

a

Division

of Cardiology,

Department

of Medicine,

David

Geffen

School

of Medicine,

University

of California,

Los Angeles,

Los Angeles,

CA 90095,

USA

b

Department

of Bioengineering,

University

of California,

Los Angeles,

Los Angeles,

CA 90095,

USA

c

State

Key Laboratory

of Transducer

Technology,

Shanghai

Institute

of Microsystem

and Information

Technology,

Chinese

Academy

of Sciences,

Shanghai

200050,

China

d

Department

of Medical

Engineering,

California

Institute

of Technology,

Pasadena,

CA 91125,

USA

e

Division

of Laboratory

Animal

Medicine,

University

of California,

Los Angeles,

Los Angeles,

CA 90095,

USA

f

Division

of Cardiac

Surgery,

Department

of Surgery,

David

Geffen

School

of Medicine,

University

of California,

Los Angeles,

Los Angeles,

CA 90095,

USA

g

Center

of Materials

Science

and Optoelectronics

Engineering,

University

of Chinese

Academy

of Sciences,

Beijing

100049,

China

ARTICLE

INFO

Keywords:

Electrical

impedance

spectroscopy

(EIS)

Electrical

impedance

tomography

(EIT)

Intravascular

microelectrode

array

3-D histology

for conductivity

modeling

Metabolically

active

plaque

ABSTRACT

Electrical

impedance

spectroscopy

(EIS)

has been

recognized

to characterize

oxidized

low-density

lipoprotein

(oxLDL)

in the metabolically

active

plaque.

However,

intravascular

deployment

of 3-D EIS-derived

electrical

impedance

tomography

(EIT)

for endoluminal

mapping

of oxLDL-laden

arterial

walls

remains

an unmet

clinical

challenge.

To this end, we designed

the 6-point

microelectrode

arrays

that were

circumferentially

configurated

onto the balloon

catheter

for 15 intravascular

EIS permutations.

In parallel,

we created

the metabolically

active

plaques

by performing

partial

ligation

of right

carotid

artery

in Yorkshire

mini-pigs

(n

=

6 males),

followed

by

demonstrating

the plaque

progression

at baseline,

8 weeks,

and 16 weeks

of high-fat

diet via computed

to

-

mography

(CT) angiogram.

Next,

we deployed

the 3-D EIS sensors

to the right

and left carotid

arteries,

and we

demonstrated

3-D EIS mapping

of metabolically

active

endolumen

in the right

but not left carotid

arteries

as

evidenced

by the positive

E06 immunostaining

for oxLDL-laden

regions.

By considering

electrical

conductivity

(

σ

) and permittivity

(

ε

) properties

of collagen,

lipid,

and smooth

muscle

presence

in the arterial

wall,

we further

validated

the 3-D EIS-derived

EIT by reconstructing

the histology

of right

and left carotid

arteries

for the finite

element

modeling

of the oxLDL-laden

endolumen,

and we accurately

predicted

3-D EIS mapping.

Thus,

we

establish

the capability

of 3-D EIS-derived

EIT to detect

oxLDL-laden

arterial

walls

with translational

implication

to predict

metabolically

active

plaques

prone

to acute

coronary

syndromes

or stroke.

1. Introduction

Electrical

impedance

spectroscopy

(EIS)

detects

oxidized

low-density

lipoprotein

(oxLDL)-laden

plaques

[1

–

3], and oxLDL

promotes

meta

-

bolically

active

plaque

to rupture,

leading

to acute

coronary

syndromes

or stroke

[4

–

8]. Diagnostic

results

obtained

from

histology,

intravas

-

cular

ultrasound

(IVUS),

and optical

coherence

tomography

(OCT)

support

that high-oxLDL

content

predicts

the metabolically

active

pla

-

ques

[9

–

15]. Despite

the advances

in computed

tomographic

(CT)

angiography

[16], high

resolution

MRI [17], IVUS

[18], near-infrared

fluorescence

(NIRF)

[19], and time-resolved

laser-induced

fluores

-

cence

spectroscopy

(LIF)

[20], endoluminal

mapping

of oxLDL-laden

arteries

remains

an unmet

clinical

need

to predict

metabolically

unsta

-

ble plaque.

We and others

have

previously

demonstrated

that an increase

in the

frequency-dependent

impedance

develops

in the oxLDL-rich

aortic

walls

[2,3,21

–

23]. Lipid-free

arterial

wall is an efficient

electrical

conductor

due to its high

water

(approximately

73%)

and electrolyte

(ions

and

* Correspondence

to: Department

of Bioengineering

and Division

of Cardiology,

Center

for Health

Sciences,

CHS 37-2000G,

65 Charles

Young

Dr, University

of

California,

Los Angeles,

Los Angeles,

CA 90073,

USA.

E-mail

address:

THsiai@mednet.ucla.edu

(T.K.

Hsiai).

1

These

authors

contributed

equally

to this work.

Contents

lists available

at ScienceDirect

Sensors

and Actuators:

B. Chemical

journal

homepag

e:

www.el

sevier.com/loc

ate/snb

https://doi.org/10.1016/j.snb.2021.131152

Received

21 September

2021;

Received

in revised

form

17 November

2021;

Accepted

23 November

2021

Sensors and Actuators: B. Chemical 354 (2022) 131152

2

proteins)

content,

whereas

lipid-rich

wall is anhydrous

and thus,

a poor

conductor.

When

an alternative

current

(AC)

stimulation

is applied

to

the arterial

plaque,

electric

impedance

(

Z

) is acquired

as a function

of

frequency.

Z

is defined

as the summation

of the resistance

(

R

) and

reactance

(

X

) multiplied

by a complex

number,

i

(

Z

=

R

+

iX

). By

recording

the endoluminal

impedance

from

10 to 1000

kHz,

the elec

-

trical

and dielectrical

properties

can be determined

[2]. Thus,

the

frequency-dependent

EIS of the oxLDL-laden

plaque

is significantly

elevated

as compared

to the oxLDL-free

arterial

walls

between

10 kHz to

100 kHz [1].

To this end,

we sought

to demonstrate

3-D EIS-derived

electrical

impedance

tomography

(EIT)

for

in situ

endoluminal

mapping

of carotid

arterial

walls

in a pre-clinical

swine

model.

We designed

3-D EIS sensors

by configurating

6-point

microelectrode

array

on a balloon

catheter,

allowing

for 15 permutations

of intravascular

EIS measurements.

Next,

we developed

oxLDL-rich

plaques

in the Yucatan

mini-pigs

via partial

ligations

of right

carotid

arteries.

After

16 weeks

of high-fat

diet,

we

deployed

the EIS catheter

to the carotid

arteries

for 3-D EIS mapping.

The low conductivity

regions

in the arterial

walls

corresponded

to the

positive

immunostaining

(E06)

for oxLDL

in the right

but not left carotid

arteries.

We further

reconstructed

the 3-D histologic

features

of the ca

-

rotid

arteries

for finite

element

modeling

of impedance

by considering

electrical

conductivity

(

σ

) and permittivity

(

ε

) properties

of collagen,

lipid,

and smooth

muscle

presence

in the arterial

wall.

We demonstrated

the capability

of our 3-D histology-derived

finite

element

modeling

to

predict

and validate

3-D EIS mapping.

Thus,

we establish

3-D EIS-

derived

EIT mapping

to detect

oxLDL-laden

arterial

walls

that holds

promises

for early

detection

of metabolically

active

plaques

prone

to

develop

acute

coronary

syndromes

or stroke.

2. Materials

and methods

2.1.

Development

and integration

of flexible

3-D microelectrode

array

A catheter-based

electrical

impedance

spectroscopy

(EIS)

sensor

(7F

diameter)

was developed

for intravascular

delivery

and endoluminal

mapping

(Fig. 1A and B). Custom-designed

flexible

polyimide

electrodes

(600

μ

m x 300

μ

m) (FPCexpress,

Canada)

were

affixed

onto the balloon

catheter

via silicon

adhesive

for EIS measurements.

Six of these

elec

-

trodes

were

positioned

in two rows

(3 by 3 electrodes)

at 1.4 mm apart

along

the circumference

of an inflatable

balloon

of 9 mm in length

(low-

durometer

urethane

Ventiona

Medical,

NH),

and the inflatable

di

-

ameters

of the balloon

ranged

from

2 to 10 mm (Fig. 1C). The electrodes

in contact

with

the endolumen

generated

electrical

field

for intravas

-

cular

EIS measurements.

The equivalent

circuit

reveals

the 6-point

electrodes

for 15 permutations

(Fig. 1D).

The balloon

was coaxially

inserted

into the distal

end of a poly

-

ethylene

catheter

(Vention

Medical,

NH),

and was anchored

with

the

epoxy

glue.

Micro

holes

were

created

on the catheter

for balloon

infla

-

tion (Figs.

2A and 2B). The catheters

were

insulated

with

heat-shrink

tubing

(Vention

Medical,

NH).

A pair

of tantalum

foils

(Advanced

Research

Materials,

UK) was added

to both

ends

of the balloon

as

radiopaque

markers

for visualization

during

fluoroscope.

The electrical

conduction

to the impedance

analyzer

was connected

by soldering

a

joint

between

the copper

wires

(26 AWG)

and contact

pads

at the ter

-

minal

end of the flexible

electrodes.

The electrodes

were

electroplated

with

platinum

black

(Sigma-Aldrich)

to increase

the junction

capaci

-

tance

and to enhance

the accuracy

of two-point

electrode

measurements

(see Supplementary

Figure

S-4).

2.2.

Creating

a swine

model

of metabolically

active

plaque

A combination

of high-fat

diet and partial

carotid

arterial

ligation

was previously

demonstrated

to promote

atherosclerotic

plaques

in a

swine

model

[24]. We performed

partial

ligation

to the right

carotid

arteries

(Fig. 3). Detail

surgical

procedures

for partial

carotid

ligation

are available

in Supplementary

(SI-1).

The animal

study

was approved

by the UCLA

Animal

Research

Committee

in compliance

with

the

institutional

IACUC

protocols.

The surgical

procedures

and the post

-

operative

care were

performed

by experienced

veterinarians

from

the

Division

of Laboratory

Animal

Medicine

at UCLA

School

of Medicine.

The surgical

wound

was closed

layer

by layer

to avoid

manipulation

to

Fig. 1.

6-Point

Microelectrode

Configuration

to Interrogate

the Plaque.

(A) The flexible

6-point

electrodes

were

affixed

on the balloon.

(B) A cross-sectional

view

shows

the inflated

balloon

in relation

to the eccentric

atherosclerotic

plaque.

The electrodes

in contact

with

the endolumen

generated

electrical

field

from

EIS

measurements.

(C) The flexible

polyimide

and electrodes

were

configurated

for the 6-point

arrangement,

generating

15 permutations.

(D) The equivalent

circuit

reveals

the 6-point

electrodes

and the 15 pairs

of electrodes

for EIS measurements.

P. Abiri

et al.

Sensors and Actuators: B. Chemical 354 (2022) 131152

3

the adjacent

tissues.

The animals

were

allowed

for recovering

after

surgery,

and they were

resumed

to the high-fat

diet for 16 weeks

(Test

Diet;

Purina,

St. Louis,

MO).

Serial

aortic

CT angiograms

were

performed

to demonstrate

the reduction

in the diameters

of the carotid

arteries

following

iodinated

contrast

injection

to the tail vein at baseline,

8 weeks,

and 16 weeks.

Next,

we deployed

the 3-D microelectrodes

to

Fig. 2.

Intravascular

deployment

of the 3-D 6-point

microelectrodes.

(A) Schematic

demonstrates

the balloon

catheter-based

microelectrode

array

affixed

onto

the

balloon

catheter.

The flexible

polyimide

electrodes

(600

μ

m x 300

μ

m) connects

with the 3 polyimide

PCB bars to the external

impedance

analyzer.

Micro

holes

were

opened

on the catheter

for balloon

inflation.

(B) A photo

of the balloon

catheter

reveals

the microelectrodes

connected

to the polyimide.

A pair of tantalum

foils was

added

to both ends of the balloon

as radiopaque

markers.

(C) An anatomic

depiction

of the heart

and the 3-D sensor

deployment

to the right

carotid

artery

in a swine

model.

(D) A 3-D CT scan depicts

the right

carotid

artery

following

partial

ligation

in comparison

to the left. (E) An enlargement

of the 3-D CT scan reveals

the

narrowing

of the right

carotid

artery

(dotted

circle)

following

16 weeks

of high-fat

diet.

Fig. 3.

Development

oxLDL-Laden

Right

Carotid

Arteries

in the Yucatan

mini-pigs.

(A) Cross-sectional

(terminal

axial)

CT scan at base line reveals

right

and left

carotid

arteries

in relation

to the spine,

trachea,

and esophagus

of the Yucatan

mini-pig.

(B) Partial

ligation

of right

carotid

artery

promotes

disturbed

flow

downstream

from

the narrowing

or stenosis.

Disturbed

flow

is well-recognized

to prime

the development

of plaque

formation

or atherosclerosis.

(C) Doppler

ul

-

trasound

provides

the inlet and outlet

velocity

to perform

computational

fluid

dynamics

(CFD)

simulation

across

the stenosis.

(D) CT scan at 16 weeks

of high-fat

diet

indicates

narrowing

or stenosis

of the right

carotid

arteries

in comparison

to the left. (E) Comparison

between

the mean

right

and left carotid

diameters

at 0 weeks

(baseline),

8 weeks

post-surgery

(intermediate),

and 16 weeks

post-surgery

(terminal)

demonstrates

development

of significant

stenosis

at the intermediate

and

terminal

time

(

p

<

0.05,

n

=

6).

P. Abiri

et al.