P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
181–190
MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL
STIMULATION OF THE HUMAN EXTERNAL EAR
SKOZIKO@NA STIMULACIJA ZUNANJEGA ^LOVE[KEGA U[ESA
Z VE^ ELEKTRODAMI
Polona Pe~lin
1
, Janez Rozman
2,3*
, Renata Jane`
3
, Anja Emri
4
, Samo Ribari~
3
,
Tomislav Mirkovi}
5
1
University Medical Centre Ljubljana, Division of Gynaecology and Obstetrics, [lajmerjeva 3 and Zalo{ka 7, 1000 Ljubljana, Slovenia
2
Center for Implantable Technology and Sensors, ITIS d. o. o. Ljubljana, Lepi pot 11, 1000 Ljubljana, Slovenia
3
University of Ljubljana, Institute of Pathophysiology, Faculty of Medicine, Zalo{ka 4, 1000 Ljubljana, Slovenia
4
Medical Chamber of Slovenia, Dunajska cesta 162, 1000 Ljubljana, Slovenia
5
University Medical Centre Ljubljana, Department of Anaesthesiology and Surgical Intensive Therapy, Zalo{ka 2, 1000 Ljubljana, Slovenia
Prejem rokopisa – received: 2023-01-31; sprejem za objavo – accepted for publication: 2023-02-21
doi:10.17222/mit.2023.779
This article reviews the development of a new concept for crafting and testing a multi-electrode set-up for selective
transcutaneous auricular nerve stimulation (tANS) using particular superficial regions of the external ear (EE). The purpose of
the work is to assess the mechanical properties of the set-up to ensure the optimum conditions for the users, and to test the abil-
ity to affect vital physiological functions. The set-up consisted of eight cap-like platinum simulating cathodes (S = 14.58 mm
2
)
that were embedded in the left and right silicone ear plugs. The plugs were mounted onto the frame of dummy headphones and
inserted into the EE, while a common anode (anode) (S = 4500 mm
2
) was placed at the nape of the neck. The mechanical perfor-
mance was assessed by measuring the axial force F x acting on each cathode against the bending of the dummy headphone frame
while simulating the conditions of the set-up being mounted onto the head. The functionality was tested by applying stimuli
onto four predefined sites at the EE to modify particular vital physiological functions of female volunteers. The preliminary re-
sults show that during the tANS, the bi-nasal respiration became shorter but more frequent with steeper inspiration and a lower
airflow. The results also show that during the tANS, the heart rate was slightly diminished along with the respiratory sinus ar-
rhythmia. Finally, a dicrotic notch within the toe photoplethysmogram was masked by the waves with a frequency of approxi-
mately 300 min
–1
. The presented work has implications for the multi-electrode set-up design and the prediction of efficient
tANS used to modify physiological functions.
Keywords: platinum stimulating electrode, spot welding, transcutaneous auricular nerve stimulation, physiological function
^lanek opisuje razvoj novega koncepta izdelave in testiranja ve~elektrodnega sistema za skoziko`no stimulacijo avrikularnega
`ivca (tANS) preko posameznih povr{inskih podro~ij zunanjega u{esa (EE). Namen raziskave je bil izmeriti mehanske lastnosti
sistema, da bi zagotovili optimalne pogoje za uporabnike ter testirati zmo`nosti sistema vplivanja na fiziolo{ke `ivljenjske
funkcije. Sistem je vseboval osem platinastih stimulacijskih katod v obliki kapice (S = 14,58 mm
2
), ki so bile vgrajene v levi in
desni u{eni vstavek iz silikona. Vstavka sta bila vgrajena na okvir slu{alk in vstavljena v EE, skupna anoda (anode)
(S = 4500 mm
2
) pa je bila nalepljena na tilnik. Mehanske lastnosti so bile pridobljene z merjenjem aksialne sile F x,kijeob
ukrivljanju okvirja slu{alk delovala na vsako od stimulacijskih katod pri ~emer so bile mehanske razmere, ko je bil sistem
nataknjen na glavo, simulirane. Funkcionalnost je bila preizku{ena z dovajanjem stimulusov na {tiri prednastavljena mesta na
EE prostovoljk pri ~emer so s stimulacijo teh mest vplivali na fiziolo{ke funkcije. Predhodni rezultati so pokazali, da je bilo
predihavanje skozi obe nosnici med tANS kraj{e in pogostej{e ter bolj strmo s plitvej{imi vdihi. Rezultati so tudi pokazali, da
sta bila med tANS rahlo zmanj{ana tako ritem srca kakor tudi predihovalna sinusna aritmija. Dikroti~ni prevoj v pletizmogramu
izmerjenem na palcu leve noge je bil prekrit z valovi frekvence pribli`no 300 min
–1
. Predstavljeno delo ima posledi~no vpliv na
razvoj ve~elektrodnih sistemov in na pri~akovano u~inkovitost vplivanja na fizolo{ke funkcije.
Klju~ne beside: platinasta stimulacijska elektroda, to~kovno varjenje, skoziko`na stimulacija avrikularnega `ivca, fiziolo{ka
funkcija
1 INTRODUCTION
Vagus nerve stimulation (VNS) is a stimulation
method that is used to treat various diseases.
1
VNS refers
to any technique used to stimulate the parasympathetic
vagus nerve.
2–4
Invasive VNS, which was approved in the early
1990s, was established as a safe and effective technique,
particularly for epilepsy. However, because invasive
VNS requires anaesthesia, it may cause surgical risks
during device implantation. With regard to this factor, a
myriad of models and methods for a non-invasive inter-
face between the human nervous system and electronic
devices have been developed. An example of this method
is transcutaneous vagus nerve stimulation (tVNS) which
can elicit similar therapeutic effects. The tVNS is used in
the application of electrical currents via surface elec-
trodes at selected locations such as tragus, concha and
cymba concha with cutaneous afferent vagus nerve dis-
tributions.
5
However, the optimal location and the stimu-
lation parameters that provide the desired therapeutic ef-
fects for a specific condition are still speculative.
6,7
Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190 181
UDK 621.791:544.076.32:611.85 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(2)181(2023)
*Corresponding author's e-mail:
jnzrzmn6@gmail.com (Janez Rozman)

The tVNS method has been recently suggested as an
interesting alternative for generating effects for a specific
condition.
1,6,7
Recent studies have revealed that the tANS
can modulate vagal afferents within the auricular branch
of the vagus nerve that may be accessed through particu-
lar sites of the external ear (EE).
8
However, the existing set-up designs do not have the
ability to selectively modulate the function of a particu-
lar internal organ via the tANS of particular populations
of nerve fibres, located in the EE. A disadvantage of
such multi-electrode tANS set-ups may be related to the
design, composition of the electrode material, and sur-
face properties while being in contact with the human
skin.
9
All the tANS applications require electrodes that
ensure an appropriate electrode-tissue interface and ap-
propriate interactions with the skin to avoid any unpleas-
ant feelings or pain. This interface mainly depends on
the quality of the physical contact made by a particular
stimulating electrode, established by the pressure applied
during the tANS. Since the stimulation intensity provides
some control of the variation in the electrode-tissue im-
pedance, the intensity of the tANS is usually set accord-
ing to the patient-specific pain threshold. As it is neces-
sary that the materials used for tANS electrodes enable
the functioning in electrochemically harsh environments,
an understanding of the electrode-tissue interface and its
interactions is necessary.
10
Therefore, the material used
and the modification techniques that may be used to opti-
mize the surface and properties of the materials should
be identified.
10–14
The basic motivation for the work was the desire to
generate therapeutic effects on chronic disease patients
in a selective way and with the aim of re-establishing the
diminished function of particular internal organs using
the single-part multi-electrode tANS set-up.
6
Precisely,
the motivation for the proposed work was the necessity
to overcome the disadvantage of the existing tANS
set-ups that were designed to have up to three electrodes
in different configurations to enable tANS of an EE at
one or two particular sites. We also intended to increase
the understanding of the electrode-tissue interface.
The significance of our work is related specifically to
the desire to affect the respiratory and cardiovascular
function using the selective tANS of afferent nerve fibres
located at particular sites on the EE. This indicates the
potential for therapeutic effects on cardiovascular disease
patients.
Most research groups generally report on the use of
tANS set-ups designed in such a way that they have, at
the most, one monopolar or two bipolar electrodes to
stimulate one or two particular sites on the EE.
7,8
In most
cases, their technical specifications are not provided.
9
Some studies do not provide sufficient information
about the tANS electrodes such as the material and size
of the electrodes, especially when they are integrated in
custom-made set-ups.
7–9
As a result, an adequate descrip-
tion of the electrode-tissue interface and its interactions
is not provided.
Among the metallic materials for stimulating elec-
trodes, titanium and silver are the most commonly used
materials.
6,12,13
As the reported tANS set-ups generally include only
a few stimulating electrodes to stimulate the EE, sin-
gle-part systems with a larger number of electrodes are
required.
To address these needs, an entire conceptual model
for crafting a multi-electrode set-up for the selective
tANS of afferent nerve fibres within particular sites of
the EE was developed based on the theoretical back-
ground.
Pure platinum (99.93 w/%) with superior elec-
tro-chemical properties may be used as the material to
make stimulating electrodes.
10
Except for the low
strength, pure platinum exhibits several physical proper-
ties of great significance, allowing its use in the tANS
electrode crafting. A cold-rolled thin platinum ribbon
that underwent different thermal treatments may be pro-
posed for the crafting process
11–14
to overcome the inade-
quate properties. For the crafting process, plastic defor-
mation of a thin platinum ribbon with a custom-designed
tool and pressing device may be envisaged.
The resistance spot welding and custom-designed
tool were proposed to connect the electrodes to the lead
wires.
15–17
For this purpose, a custom-designed capaci-
tive-discharge welding device would be used. This ap-
proach would significantly reduce the time required for
welding, and thus, the melting spots would be small, and
only a small number of heat-affected zones would exist.
For the set-up, reusable earplugs made of soft and
sterilisable silicone should be installed onto a frame un-
mounted from audio headphones. Plugs used by swim-
mers should be used as the basis for mounting the stimu-
lating electrodes onto predefined sites.
According to the motivation for the work, three aims
were set as mentioned below.
The first aim is to develop a set-up that exhibits ap-
propriate mechanical features, ensuring that the system
can generate an optimum pressure onto the stimulated
sites of the EE during tANS.
The second aim is to develop a set-up that can be me-
chanically tested.
The third aim is to assess the efficiency of the set-up
using a human model. For this purpose, tANS should be
performed on healthy female volunteers aiming at elicit-
ing a measurable effect on the respiratory and cardiovas-
cular functions.
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
182 Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190

2 EXPERIMENTAL PART
2.1 Development of the set-up
2.1.1 Selection of the materials
Commercially available and reusable silicone ear
plugs that are used by swimmers (ear plugs, product
code: 885037, Slazenger, Shirebrook, United Kingdom)
were used as the basis for the development of the ear
plugs.
The mechanical and electrochemical characteristics
were the criteria for choosing the material for the stimu-
lation (cathodes) of the electrical contact with the skin.
With regard to the electrochemical characteristics,
pure platinum was selected as the cathode material be-
cause it can activate nerve fibres with predominantly ca-
pacitive and faradaic mechanisms.
10,18,20
However, the
properties of a cold-rolled platinum ribbon were consid-
ered.
19
Accordingly, a cold-rolled platinum ribbon with a
thickness of 0.2 mm and purity of 99.99 w/% was used
(Zlatarna Celje d. d., Kersnikova 19, 3000 Celje, Repub-
lic of Slovenia).
13,14
2.1.2 Crafting of cathodes
The crafting of cathodes comprises several handcraft
steps.
19,20
The first step involves cutting discs with a di-
ameter of 4 mm from the 0.2-mm thick platinum ribbon
using a special tool and press. The second step involves
forging the discs into 0.8-mm high caps. When com-
pleted, the geometric surface of the cathode shown in
Figure 1a that should be deployed during tANS, also
shown in Figure 1a, is 14.58 mm
2
. The third step in-
volves welding lead wires onto the bottom of the
spherical cavity of the cathode. As the lead wires of the
set-up require high flexibility, a finely stranded, supe-
rior-quality, silver-plated, copper conductor wire (type
AS155-30-1SJ, Cooner Wire, Chatsworth, CA, USA)
was used. The specifications of the wire are as follows:
– Number of conductors – 1#,
– North American – 30 AWG,
– Cable insulation – silicon rubber,
– Cable conductor – silver-plated copper.
For a reliable electrical and mechanical connection
between a particular cathode and conduction wire, a ca-
pacitive discharge spot welder was used. The spot
welder, shown in Figure 1c, provided a single high-cur-
rent pulse. For this purpose, a thyristor powered by a
large capacitor discharge was deployed. We used a ca-
pacitor of 3 F, which produced a voltage as high as 35 V
and could store energy up to 1837.5 W. In the discharg-
ing process, all the stored capacitor energy was dumped
at once using a thyristor. To ensure appropriate and re-
producible welds, however, the thicknesses of the cable
and the cap cathode, the type of the welding electrode,
and the amount of energy delivered were chosen by the
welder on a subjective basis.
With regard to the aforementioned aims, the melting
point and electrical resistivity of platinum and pure cop-
per, which were 1769 °C and 10.6 μ cm (at 25 °C), and
1083.4 °C and 16.78 n m (at 20 °C) were the optimum
welding conditions.
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190 183
Figure 1: Technology of crafting cathodes: a) cup cathode, b) cup cathode in contact with skin and an electric field generated by a radially dis-
persed stimulating current (a – distance, d – silicone insulation thickness, h – cap height, r – cap base radius, R – sphere radius,  – cap angle,
t – skin thickness) and c) micro-spot-welding device

The monopolar technique was used for welding, and
in this process, the electrical current flowed from the
positive welding electrode, within the penholder-like
handle through the lead wire and spherical cavity, to the
negative bulk copper base. To obtain quality welds, wol-
fram welding electrodes (with a diameter of 3 mm) were
customized to obtain a round edge at the tip. During
welding, special attention was paid to the pressure ap-
plied to the positive electrode. The quality of the weld
depended on the internal resistance, achieved by the
welder during pressing.
Finally, both silicone plugs were upgraded with four
platinum cathodes that were situated at the predefined
sites on the tANS model. The cathodes were fixed using
a silicone adhesive (ASC, Applied Silicone Corporation,
part No.: 40064, MED RTV adhesive, implant grade,
Santa Paula, California, USA) that was deposited into
the spherical cavity on the back side of the cathode to fill
in the cavity and the feedthrough at each predefined site.
The manufactured tANS set-up containing the left
and right multi-electrode plugs with four cathodes each
is shown in Figure 2.
2.2 Mechanical testing
Figure 3 shows a schematic diagram of the assess-
ment of the set-up mechanical performance. Figure 3a
shows the assessment of axial force F
x
versus the bend-
ing of the dummy headphone frame and 3b shows the re-
sulting force F
r
acting onto the plug inserted into the EE.
To assess the axial force F
x
, a digital force gauge
(AccuForce Cadet, Ametek, Mansfield & Green Divi-
sion, Largo, Florida, U.S.A.) was used. Once F
x
was
measured, the resulting force F
r
was calculated using a
simple trigonometric function based on the angle of 60°
between forces F
x
and F
r
. This angle was defined by the
inclination of the silicone plug where the electrodes were
situated. Further, the pressure acting onto each cathode,
dependent on the bending of the dummy headphone
frame when the set-up is mounted onto the head, was
calculated.
2.3 Testing the functionality
The primary aim of the testing was to measure the
potential impact of multi-electrode tANS of the auricular
fibres of the vagus nerve at predefined sites on both EEs
on some vital functions of female subjects within an age
range of 23–25 years. All subjects were healthy and in
ideal physical and psychical conditions. The study was
approved by the National Medical Ethics Committee,
Ministry of Health, Republic of Slovenia (Tel: +386 01
478 69 13, http://www.kme-nmec.si/kontakt/, Unique
Identifier No. 0120-297/2018/6).
To protect the subjects against a potential electrical
shock, all devices that were in galvanic contact with the
subjects were galvanically decoupled from the 220 V
power line using a high-quality vintage 500 VA isolation
transformer (Mechanikai Laboratórium (ML), Budapest,
Hungary).
For the tANS, a certified microprocessor-controlled
stimulator (Model SM9079, Shenzhen L-Domas Tech-
nology Ltd., Shenzhen, Guangdong, China) (FDA, Medi-
cal CE, FCC, ISO13485), was used. The stimuli for the
selected tANS were cathodic-first, rectangular cur-
rent-regulated biphasic stimulating pulse pairs (cathodic
and anodic phase). The temporal parameters selected
were as follows:
– Frequency – f = 45.5 Hz,
– Stimulating phase width – t
c
= 200 μs,
– Anodic phase width – t
a
= 200 μs,
– On time (pulse train duration) – 3.32 s,
– Time gap between pulse trains – 2.8 s.
The plugs of the tANS set-up with cathodes at the
predefined sites were placed into EEs in such a way that
each cathode was in contact with the corresponding site
at the EE. The stimuli were then delivered to specific
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
184 Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190
Figure 3: Assessment of the set-up mechanical performance: a) as-
sessment of axial force F
x
versus bending of the headphone frame; and
b) force F
r
resulting from force F
x
acting onto the plug at an angle of
60° when inserted into an external ear
Figure 2: Manufactured tANS set-up; a) left plug; and b) right plug

preselected sites at the EE using a custom-developed
switching unit situated between the stimulator output and
the wires leading to the cathodes.
A common anode (CA), however, was placed at the
nape of the neck and connected to another output of the
stimulator. As the CA, a reusable and self-adhering
neurostimulation electrode (Model: 895240, rectangle of
2" × 3.5", (5 × 9 cm), Axelgaard Manufacturing Co.,
Ltd., Fallbrook, CA 92028, USA) with a geometric sur-
face of about 4500 mm
2
was used.
To establish a low-resistance contact between an
electrode the and sites at the EE, a thin layer of transpar-
ent conductive hypo-allergic water-soluble gel (GEL
G008, FIAB Spa, Vicchio–Firenze, Italy) was deposited
onto the EE.
In our previous study,
20
the highest stimulating inten-
sity i
c
that produced still acceptable discomfort of the
subjects was 50 mA. Accordingly, the i
c
of the same in-
tensity (50 mA) was used to evaluate the electrochemical
performance of the cathode. The stimulating charge Q
c
injected by the test cathode into a site at the EE was ob-
tained by calculating the surface under the cathodic cur-
rent phase i
c
of the stimulating pulse pair:
Q
c
= i
c
× t
c
=50mA×200μs=10μC, (1)
where i
c
is the stimulating intensity and t
c
is the pulse
duration.
The spherical cathode surface area A
c
calculated as
the average of all eight spheres in the right and left plug
is 14.58 mm
2
. The cathodic charge density C
d
based on
the aforementioned calculations is as follows:
C
d
= Q
c
/A
c
= 10 μC ÷ 14.58 mm
2
= 0,68 μC/mm
2
(2)
The charge transferred by a particular cathode was
the same but opposite to the charge transferred by the
CA, while the charge densities differed vastly. Accord-
ingly, the charge density Ad transferred by the CA is as
follows:
A
d
= Q
c
/A
a
= 10 μC ÷ 4,500 mm
2
= 2.22 nC/mm
2
(3)
To characterize the interface between the four cath-
odes located at the left and right EEs, ensemble vari-
ables, such as capacitance C
e
and parallel resistance R
e
,
were measured using an LCR meter (AT2816A Precision
Digital LCR Meter, Changzhou Applent Instruments,
Ltd., Jiangsu, China). The measuring conditions were as
follows:
– Source resistance – 30 ,
– Testing excitation voltage–1V ,
– Testing frequency – 1.2 kHz.
The testing frequency of 1.2 kHz was extracted from
the power spectrum density of the stimulation pulse (not
shown in this paper).
Electroimpedance measurements (single cathode vs.
large CA) were attempted while the electrodes were
worn by the subjects. To protect the subjects against a
potential electrical shock, the LCR meter was galvani-
cally decoupled from the 220 V power line using the
aforementioned vintage isolation transformer.
Figure 4 shows a schematic diagram of testing the
set-up functionality. Figure 4 also shows the sites on the
right EE that were predefined for the tANS. They were
indicated in the following order: pole position, red (R);
below pole position, yellow (Y); above bottom position,
black (B); and bottom position, white (W).
For the measuring set-up, various custom-designed
and commercially available devices were used so that the
following physiological quantities could be measured:
– tANS intensity – i
c
,
– Phonothyrographic signal – PTG,
– Phonocardiographic signal – PCG,
– Heart rate – HR,
– Airflow – AF,
– Toe photoplethysmographic signal – PPG,
– Blood pressure – BP,
– Korotkoff sounds – KS.
2.3.1 Measurement of the airflow in nasal ventilation
An airflow (AF) measuring system was used to as-
sess the variations in the pressure difference elicited by
the bi-directional total nasal AF,
21
changes in the rhythm,
and the changes in the character of inhalation and exha-
lation produced by the selective tANS. The system com-
prised a full face mask (iVolve Full Face Mask, BMC
Medical Co., Ltd., Shijingshan, Beijing, China) and a
mass AF meter (FS6122-100F100-5P40-TH1,
NONCON, Hubei, China) designed for personal ventila-
tors for continuous positive airway pressure. The specifi-
cations of this system were as follows:
– Airflow range – –100 + 100 SLPM,
– Accuracy – ± (2.0 + 0.5 FS) %,
– Output mode – Linear analogue VDC,
– Output range – 0.5 ~ 4.5 VDC,
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190 185
Figure 4: Testing the functionality: a) right plug; b) predefined stimu-
lating sites on the right EE

– Response time – 1.8 ms.
In the resulting bi-direction AF curve, the horizontal
line denotes zero airflow, positive AF values correspond
to periods of inhalation, and negative AF values corre-
spond to periods of exhalation.
2.3.2 Capturing heart rate and the toe plethysmographic
signal
To study the impact of the tANS on a subject’s heart
metrics, photo-plethysmography was used. Accordingly,
to capture the toe photo-plethysmographic waveforms
(PPG) during the selective tANS, a pulse oximeter
(Nellcor N-595, SpO2 accuracy of ±2 % (90–100 %),
±3 % (70–89 %), pulse rate accuracy of ±1 bpm
(30–90 bpm), 2 bpm (60–149 bpm), 3 bpm
(150–245 bpm), Tyco Healthcare Group LP, Nellcor Pu-
ritan Bennett Division, Pleasanton, CA, U.S.A.), instru-
mented with a custom-crafted PPG toe clip, was used.
An IR LED pair and photodiode taken from a commer-
cially available SpO2 sensor (Model Number: CSL032H,
Nellcor DOC-10, Oximax Animal Ear Clip SpO2 Sensor
for Nellcor N-595, Certificates CE and ISO13485,
Shenzhen YKD Technology Co., Ltd., Brand Name:
YONGKANGDA, Guangdong, China) were attached to
two halves of a tube, made of carbon fibre so that they
face the lower and upper side of the toe, respectively.
The toe clip was used to capture both the heart rate (HR)
and PPG form the cardiac cycle, referring to the contrac-
tion and relaxation of the ventricles while attached onto
the left toe. To reduce the noise from the motion, and ob-
tain an optimum signal representing a distinguished
dicrotic notch, a no low-pass filter was applied. The opti-
mum filter for the PPG processing was already applied to
the pulse oximeter. The average of the HR was calcu-
lated from fifteen peaks of KS within the BP intervals
that were recorded before, during and after tANS.
2.3.3 Capture of Korotkoff sounds and phonothyreo-
graphic signal
The system used for capturing the phonothyreogram
(PTG) and Korotkoff sounds (KSs) included a precision
active contact acoustic transducer (CM-01B, Shenzhen
Aimin Intelligent Technology Co., Ltd., Shenzhen,
China) and a signal amplifier module (CM-01, Shenzhen
Aimin Intelligent Technology Co., Ltd., Shenzhen,
China). The specifications of the system were as follows:
– Sensitivity – 40 mV/mm,
– Lower limit frequency (–3dB)–8Hz,
– Upper frequency (+3dB) – 2.2 kHz,
– Output voltage range – 0–1.215 mV.
The two aforementioned combinations were attached
onto the custom-designed frame pieces; one is designed
as a necklace and the other is designed as an upper arm
cuff. The transducer was calibrated at quiet normal room
sound conditions, with an estimated peak-to-peak sound
of approximately 25 dB.
The KSs were captured using the transducer placed
over the brachial artery simultaneously with the mea-
surement of BP using the cuff
22–25
while the PTG repre-
senting a bruit coming out from the thyroid vasculature
was captured using the transducer placed above the thy-
roid isthmus.
2.3.4 Measurement of blood pressure
The oscillometric method was used to measure BP,
and with this method, the cuff was wrapped around the
left upper arm. The brachial artery, which was con-
stricted by an inflated cuff, was used as the primary ves-
sel. The BP value needed as the calibration reference in
the BP measurement was measured manually with a
commercially available appliance (Oberarm-Blutdruck-
messgerät MD 12450, salvatec, Essen, Germany) and a
proprietary cuff, which were attached to the area around
the left upper arm. The specifications of the device were
as follows:
– BP range – 30–250 mmHg,
– Pulse range – 40–180 beats/minute,
– BP accuracy – ±3 mmHg,
– Pulse accuracy – ±5 % of value displayed.
The pressure within the BP cuff was measured using
a pressure transducer (type 4-327-C, range: 0–400 mmHg,
Beckman Instruments Inc., Fullerton, CA, U.S.A.),
which was situated between the aforementioned cuff and
the appliance.
To calibrate the pressure transducer, a digital non-in-
vasive sphygmomanometer calibrator (SLK-BXY-250,
Shelok, Shaanxi, China), situated between the cuff man-
ual pump and pressure transducer, was used. The specifi-
cations of the device were as follows:
– Pressure range – 0–40 kPa,
– Accuracy – 0.16 %.
2.3.5 Capturing the phonocardiographic signal
The phonocardiographic signal (PCG) associated
with heart sounds
23
was captured in real-time using a
custom-modified stethoscope comprising an auscultation
headset
24
with a diameter of 41 mm. The headset con-
sisted of a diaphragm, which was the sensing part of the
stethoscope, and a head, onto which an electret con-
denser microphone (CMA-4544PF-W, CUIINC,
Tualatin, Oregon, USA) was fitted. To add an "ear" to the
headset, an assembled low-noise microphone amplifier
module (GY-MAX4466, MINGYUANDINGYE, China)
instrumented with a Maxim MAX4466 operational am-
plifier, specifically designed for this delicate task, was
used. The gain could be adjusted between 25× and 125×
using a small trimmer potentiometer. The device specifi-
cations were as follows:
– Frequency range – 20–20,000 Hz,
– Analogue signal output – 0.2–1 V,
– Output signal bias – VCC/2 DC.
This module actually came with a wide bandwidth
omnidirectional electret condenser. Its specifications in-
clude the following:
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
186 Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190

– Sensitivity – –44±2dB,f = 1 kHz,
1Pa0dB=1V/Pa,
– Operating frequency – 20~20,000 Hz,
– Signal-to-noise ratio – 60 dBA, f =1k H z ,1P
a
A-weighted.
To capture the two main heart sounds for the PCG
analysis, namely S1 and S2,
24
a stethoscope is placed at
the fifth auscultation area, known as the mitral area or
apex area (M) above the cardiac apex.
24
2.3.6 Assessment of respiratory sinus arrhythmia
The respiratory sinus arrhythmia (RSA) was assessed
during bi-nasal and bi-directional breathing before, dur-
ing, and after the tANS at a relaxed non-forced rate. Pre-
cisely, the sinus arrhythmia ratio (E/I) was calculated by
dividing the longest with the shortest interval, extracted
from the fifteen peaks of KS within the BP intervals re-
corded before, during, and after the tANS.
During the 20-min test period, tANS was applied us-
ing each of the four cathodes of the left and right plugs.
Each 20-min test period started with a 5-min sham seg-
ment when stimulating pulses were not delivered (sham
I), proceeded with a 10-min tANS segment, and ended
with another 5-min sham segment (sham II). During the
testing, the stimulating intensity i
c
was pre-set by the
subject using the dial on the electrical stimulator until
minimum discomfort at the particular site below the de-
ployed cathode was detected.
20
2.3.7 Ending the experiment
The first step in ending the experiment was minimiz-
ing the i
c
and turning the stimulator off. Then, the subject
remained lying supine on the chair for additional 10 min
so that the sensory and stimulating components could be
removed. The first and the most delicate component re-
moved was the tANS set-up. At the same time, the CA
placed at the nape of the neck was removed. The second
component removed was the full face mask with the
mass AF meter. The third component removed was the
BP cuff wrapped around the subject’s left arm. The
fourth component removed was the SpO2 sensor placed
on the left toe. The components removed last were the
PCG stethoscope and PTG active contact acoustic trans-
ducer.
Finally, the skin at the EE sites and the skin at the
back cervical neck was degreased with 70 % isopropyl
alcohol and let to dry.
2.3.8 Signal pre-processing and acquisition
All the analogue signals coming from the sensors
were prepared for the identification of important signal
features, whilst avoiding false identification. In this pro-
cedure, the type and order of the applied filter were se-
lected. Each of the eight signals coming from the condi-
tioning circuits was digitized at 50 kHz with a 24-bit
resolution using a high-performance I/O data-acquisition
system (DEWE-43, DEWESOFT d. o. o., Republic of
Slovenia) and the acquisition software (DEWESoft
7.0.2) Finally, the data were stored on a portable com-
puter (Lenovo W541, Lenovo, Beijing, China) for further
signal processing.
In an offline analysis, the average of the quantities
that were extracted from the waveforms within sham I,
during the tANS and within sham II always contained
the proprietary BP waveform.
Finally, Figure 7 was constructed to represent groups
of columns/quantities describing: experimental condi-
tions, respiratory function, cardiovascular function, and
the effect.
3 RESULTS
The ensemble variables, namely capacitance C
e
and
parallel resistance R
e
, representing an interface between
each of the four cathodes when located at the left and
right EE and the CA, are shown in Table 1.
Table 1: Average of the interfacial ensemble variables
C
e
(nF) R
e
(k )
Average 3.9 37.86
The results of the mechanical testing depicted in Fig-
ure 5 show that the pressure on each stimulating elec-
trode is almost linearly dependent on the bending of the
headphone frame.
The results of testing the functionality are depicted in
Figure 6.
These results are represented through waveforms of
the quantities recorded for one of the participants during
the selective tANS of the site at the bottom (see Fig-
ure 4) of the right EE marked with white colour. The
waveforms of the quantities are indicated in the follow-
ing order (from top to bottom): i
c
, PTG, PCG, HR, AF,
PPG, BP and KS.
Figure 7 shows an example of the most apparent ef-
fects of the selective tANS of the RW site on the two vi-
tal physiological functions. Precisely, Figure 7 contains
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190 187
Figure 5: Pressure acting on the stimulating electrode according to the
bending headphone frame when the set-up is mounted onto the head

averaged values of the effects elicited onto the respira-
tory and cardiovascular functions during sham I, tANS
of the RW site, and sham II.
4 DISCUSSION
The findings of the study are consistent with the pub-
lished results of other investigators
25–27
. However, com-
pared to other systems, the developed four-electrode
set-up is more favourable because of its ability to pro-
vide tANS using any combination of the four stimulating
electrodes at the plug without the need to change the lo-
cation of the physical electrode.
The spatial selectivity, however, is strongly depend-
ent on the position of the electrodes above the nerve end-
ings located within a particular site at the EE as well as
on stimulating parameters.
The set-up was developed in such a way that it en-
ables several possibilities for determining better stimu-
lating sites, using different electrode arrangements, and
using a variety of stimulating parameters. There is a vast
stimulating parameter space, which is closely related to
the temporal conditions and stimulating conditions.
A major weakness of the developed set-up is the fact
that the tANS efficiency is significantly dependent on the
pressure applied to the electrodes and generated by the
headphone frame.
The factor that contributed the most toward under-
standing the problem with tANS is that tANS was deliv-
ered monopolarly to four particular sites on the EE that
are innervated predominantly by the fibres of the auricu-
lar nerve.
27
Namely, no studies have addressed the effects
of the tANS of the EE using a single-part set-up contain-
ing multiple platinum-stimulating electrodes.
The results obtained with mechanical testing showed
that the pressure exerted by a particular electrode onto
the EE during tANS, while simulating the head fitment,
was within a range of 0–9.8 N/cm
2
(see Figure 5).
This pressure was deemed appropriate for the tANS.
During the short-term tANS, the set-up was rated good
by all subjects, and no side effects were noticed.
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
188 Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190
Figure 7: Average of the most indicative effects of selective tANS of
the right white site: a) sham I (blue); b) tANS at i
c
= 57.3 mA (red); c)
sham II (grey)
Figure 6: Quantities recorded during selective tANS of RW (from top to bottom): i
c
(train of pulses and detailed shape of pulses); PTG; PCG;
HR; AF; PPG; BP and KS; a) sham I; b) tANS; c) sham II; d) low amplitude masking waves

In Table 2, preliminary results obtained when testing
functionality are presented. To interpret these results,
short descriptions of the effects of the tANS on the quan-
tities related to the two assessed vital physiological func-
tions during the 10-min tANS and sham II are provided.
It was shown that the results obtained with mechani-
cal testing correlated to those obtained with functionality
testing.
However, due to a low number of subjects tested and
trials performed, the results representing the changes in
physiological variables shown in Table 2 should be con-
sidered as the basis for further research activities and not
as the basis for credible conclusions. Further research,
necessary for addressing the issues highlighted in these
preliminary results, could proceed in the following direc-
tions:
– to introduce computational models incorporating
theoretical and experimental insights,
– to accomplish a more detailed analysis of tANS ef-
fects,
– to develop a set-up with even more stimulating cath-
odes, thus providing the tANS at more sites on the
EE,
– to systematically study the mechanism of actions
and optimal tANS modalities,
– to improve the stimulating electrode/skin contact.
The next step in our study could comprise the follow-
ing tasks:
– to assess other vital physiological functions,
– to evaluate the interaction between different physio-
logical functions upon tANS,
– to perform more experiments to show group statis-
tics.
Our aim was to develop a set-up with appropriate me-
chanical features so that it is capable of generating an
optimum pressure for the stimulating sites at the EE dur-
ing tANS.
During tANS, an adequate cathode/skin contact was
made, ensuring that a high i
c
density, potentially eliciting
skin irritation, was avoided.
31
The maximum i
c
of 50 mA
applied to a preselected site is rather high. However, to
obtain a tANS effect on a particular internal organ, one
needs to activate the nerve endings located at a distance
from the skin surface. Precisely, the dV/dt ratio needs to
be high enough at particular nerve endings so that the re-
sulting function is capable to depolarize the nerve end-
ings. Subthreshold currents, however, do not depolarize
nerve endings. Besides, a relatively large amount of volt-
age is needed to overcome electroporation, so the i
c
must
elicit a voltage drop high enough to overcome both of the
aforementioned voltages.
Most importantly, the selective tANS of the prede-
fined sites on the EE elicited a measurable effect on both
the cardiovascular and respiratory functions of female
subjects.
We believe that the present study contributes to the
further development of multi-electrode set-ups that could
help transfer the tANS technique into clinical practice as
a relevant therapy.
5 CONCLUSIONS
The presented work has implications for the design of
a multi-electrode set-up, and the prediction of a
long-term tANS efficacy. Furthermore, an increased
number of channels may extend the application of tANS,
potentially enhancing its performance and autonomic
function and modifying some other physiological func-
tions.
26,27
P. PE^LIN et al.: MULTI-ELECTRODE TRANSCUTANEOUS ELECTRICAL STIMULATION OF THE HUMAN EXTERNAL EAR
Materiali in tehnologije / Materials and technology 57 (2023) 2, 181–190 189
Table 2: Potential effects of the tANS on the quantities related to the two vital physiological functions
Physiological function Quantity Description of tANS effect on the quantity
Respiratory function
LBR
The length of binasal respirations shortened by 27 % during the tANS and changed
to 16.5 % below the initial value during sham II.
SBR
The slope of binasal respirations was enlarged by 24.21 % during the tANS and re-
mained at this value during sham II.
AFBR
The airflow of binasal respirations diminished by 8.95 % during the tANS and re-
turned to the initial value during sham II.
BR
The breathing rate was enlarged by 38.31 % during the tANS and returned to
19.46 % above the initial value during sham II.
Cardiovascular func-
tion
HR
The heart rate diminished by 8.7 % during the tANS and returned to 4.6 % below the
initial value during placebo II.
E/I
The respiratory sinus arrhythmia ratio diminished by 6.87 % during the tANS and in-
creased to 3.91 % above an initial value during sham II.
PPG
PPG exhibited two features, namely peak systolic perfusion and the start of the next
systole. However, unlike the normal PPG waveform observed within the time periods
without tANS, an underdamped like PPG waveform was observed within the time pe-
riods with tANS. Namely, a slightly narrow peak systolic PPG, deep dicrotic notch
and non-physiological oscillations with a frequency of approximately 300 min
–1
dur-
ing the diastolic phase were observed.
28–30
This effect may be attributed to
tachydysrhythmia.
30
This assumption was confirmed with the PPG trace in Figure 6,
which showed a slightly narrow peak systolic PPG, deep dicrotic notch and
non-physiological oscillations during the tANS solely.

Acknowledgement
The authors acknowledge the financial support from
the Slovenian Research Agency (Research core funding
No. P3-0171).
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