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sensors
Article
Breathable Textile Rectangular Ring Microstrip Patch Antenna
at 2.45 GHz for Wearable Applications
Abdul Wahab Memon 1,2, * , Igor Lima de Paula 3 , Benny Malengier 1 , Simona Vasile 4 , Patrick Van Torre 3
and Lieva Van Langenhove 1
1
2
3
4
*
Citation: Memon, A.W.; de Paula,
I.L.; Malengier, B.; Vasile, S.;
Van Torre, P.; Van Langenhove, L.
Breathable Textile Rectangular Ring
Microstrip Patch Antenna at 2.45 GHz
for Wearable Applications. Sensors
2021, 21, 1635. https://doi.org/
10.3390/s21051635
Academic Editor: Sima Noghanian
Received: 26 November 2020
Centre for Textile Science and Engineering, Department of Materials, Textiles and Chemical Engineering,
Ghent University, 9052 Ghent, Belgium; [email protected] (B.M.);
[email protected] (L.V.L.)
Department of Textile, Mehran University of Engineering & Technology, 76020 Jamshoro, Pakistan
Department of Information Technology, Faculty of Engineering and Architecture imec-IDLab,
Ghent University, 9052 Ghent, Belgium; [email protected] (I.L.d.P.);
[email protected] (P.V.T.)
Fashion and Textiles Innovation Lab FTILab+, HOGENT University of Applied Sciences and Arts,
9051 Ghent, Belgium; [email protected]
Correspondence: [email protected] or [email protected]
Abstract: A textile patch antenna is an attractive package for wearable applications as it offers
flexibility, less weight, easy integration into the garment and better comfort to the wearer. When it
comes to wearability, above all, comfort comes ahead of the rest of the properties. The air permeability
and the water vapor permeability of textiles are linked to the thermophysiological comfort of the
wearer as they help to improve the breathability of textiles. This paper includes the construction of a
breathable textile rectangular ring microstrip patch antenna with improved water vapor permeability.
A selection of high air permeable conductive fabrics and 3-dimensional knitted spacer dielectric
substrates was made to ensure better water vapor permeability of the breathable textile rectangular
ring microstrip patch antenna. To further improve the water vapor permeability of the breathable
textile rectangular ring microstrip patch antenna, a novel approach of inserting a large number of
small-sized holes of 1 mm diameter in the conductive layers (the patch and the ground plane) of the
antenna was adopted. Besides this, the insertion of a large number of small-sized holes improved the
flexibility of the rectangular ring microstrip patch antenna. The result was a breathable perforated
(with small-sized holes) textile rectangular ring microstrip patch antenna with the water vapor
permeability as high as 5296.70 g/m2 per day, an air permeability as high as 510 mm/s, and with
radiation gains being 4.2 dBi and 5.4 dBi in the E-plane and H-plane, respectively. The antenna was
designed to resonate for the Industrial, Scientific and Medical band at a specific 2.45 GHz frequency.
Accepted: 22 February 2021
Published: 26 February 2021
Keywords: microstrip patch antenna; wearable textile antenna; wearable wireless communication;
breathable antenna; air permeability; water vapor permeability; ISM band
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affiliations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Textile architectures with embedded antennas have turned out to be an integral
part of wearable device systems, enabling body-centric wireless communication with
garments [1]. Wearable device systems can be (others are possible as well) monitoring
systems dedicated to assisted living and lifecare. If these monitoring systems are integrated
into textile clothing, they become wearable textile systems [2]. An antenna is a part of a
monitoring system responsible for wireless communication [3]. Athletes, mountaineers,
miners, military, rescuers, firefighters, and many other outdoor users need healthcare and
navigation information to be transferred wirelessly to a base station to monitor their health
conditions. In hospitals, patients whose life is at risk need to be monitored within safe
zones every time. Body-centric devices attached to the garment of patients can notify
Sensors 2021, 21, 1635. https://doi.org/10.3390/s21051635
https://www.mdpi.com/journal/sensors
Sensors 2021, 21, 1635
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doctors and nursing personnel wirelessly about their health condition, which helps them
to look after their patients more effectively and take immediate action if needed. Effective
information transmission can be achieved through antennas embedded into garments,
termed wearable antennas [3,4]. Textile patch antennas, among a wide range of different
types of antennas, are the most popular antenna topology for wearable applications. Their
planar structure, simple design, flexibility, low weight, and ease of integration into any
garment make these antennas suitable for wearable applications [5]. A Microstrip Patch
Antenna (MPA) design includes a non-conductive textile substrate, also called the dielectric
substrate, sandwiched between the conductive patch and the conductive ground plane.
Furthermore, the presence of a conductive ground plane layer negates any adverse effect
on the human skin due to the back radiation of an antenna [5,6].
Physical factors such as bending [7], stretching [8–11] and temperature [12,13] tend
to shift the resonance frequency of an antenna. Because of the hydrophilic character of
textiles, textile antennas are affected more by humidity compared to non-textile-based
antennas such as FR4, RT/Duroid and hydrophobic foam. Textile materials used as an
antenna substrate may transport humidity between the body and the atmosphere and
absorb part of it. Water has a high relative permittivity (Er = 78) and, in comparison,
the relative permittivity of dry textile substrates ranges between 1 and 2 [14]. When the
humidity is absorbed by the textile substrate, the relative permittivity of the textile substrate
increases due to the high permittivity of water together with its high conductivity, which
results in the shift of the resonance frequency of the textile antenna towards the lower
side [15,16]. The amount of absorbed humidity depends upon atmospheric temperature,
moisture content and moisture regain of the textile material used to construct the textile
antenna. To combat the adverse effect of humidity on the performance of a textile antenna
during its wearability, the selected textile substrate should be hydrophobic or has the least
moisture uptake ability and should have the ability to transport the humidity effectively to
the environment. However, the hydrophobic nature of the textile substrate may cause an
uncomfortable feeling to the wearer because no humidity will be absorbed by the textile
substrate and the humidity will remain trapped underneath the antenna. Therefore, the
high water vapor permeability (WVP) would be a solution to prevent the accumulation of
humidity underneath and inside the textile antenna.
A breathable textile antenna can be an essential substitute to conventional textile
antennas and can cope with the above-mentioned working strains more effectively. The
breathability is a factor of porosity, the higher the porosity of materials used to construct an
antenna, the more an antenna becomes breathable [17]. The high air permeability (AP) of
textiles can effectively dissipate humidity back into the environment [18,19]. A breathable
textile antenna can also be effective in providing ease to the wearer at a time when the
wearer performs any physical activity. In [20], the porous patch antenna is constructed
through screen printing of conductive silver ink on a highly absorbent engineered Evolon®
textile substrate to help the silver ink to penetrate evenly over its surface through a strong
capillary wicking force. The antenna, later on, is packaged with a porous polyurethane
web as an additional process to make the antenna durable, breathable, and water repellent,
which is an additional, delicate and expensive step to avoid the effect of humidity over the
textile antenna. Previous literature on wearable textile antennas so far has been limited
to a number of discrete properties like dual-band antenna [21–24], bending effect on the
resonance frequency of the antenna [25,26], usage of novel conductive electrotextile fabrics
to construct textile patch antennas [27], and comparison of different textile properties [28].
Another important aspect is the physical performance of the wearer of a textile antenna,
which can be affected if a textile antenna, despite its relatively small size, resists the
transportation of humidity and/or perspiration and remain trapped inside a textile antenna
can be uncomfortable even to a little extent for the wearer. This is particularly the case
when high-intensity activities are performed with large sweat production as a consequence.
The perspiration also affects the electrical properties and the dimensional stability of the
textiles [12,15]. The height of the textile substrate changes as the perspiration swells the
Sensors 2021, 21, 1635
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Sensors 2020, 20, x FOR PEER REVIEW
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the antenna
willIfno
resonate
at the desired
frequency.
Some
textile
shrink
textile
substrate.
themore
change
in the height
of the textile
substrate
occurs,
thefabrics
antenna
will
more
and
some
less
when
they
make
contact
with
water,
which
means
they
change
no more resonate at the desired frequency. Some textile fabrics shrink more and some their
less
physical
when
exposed
water.
Therefore,
a textiletheir
substrate
with
low values
when
theydimensions
make contact
with
water, to
which
means
they change
physical
dimensions
of moisture
regain
and moisture
content
may
overcome
thelow
change
of of
dimensional
stabilwhen
exposed
to water.
Therefore,
a textile
substrate
with
values
moisture regain
ity issue.
and
moisture content may overcome the change of dimensional stability issue.
Amicrostrip
microstrip patch
patch antenna
antenna (MPA)
(MPA)has
hasaarelatively
relativelysmaller
smallerbandwidth,
bandwidth,hence
hencesome
some
A
special
techniques
have
been
followed
to
increase
the
bandwidth
of
the
antenna.
One
techspecial
have been followed to increase the bandwidth of the antenna. One
nique
is
to
cut
a
rectangular
patch
from
the
center
of
the
MPA
[29,30],
which
results
technique is to cut a rectangular patch from the center of the MPA [29,30], which resultsinina
the RRMPA
RRMPAtopology
topology
arectangular
rectangularring
ringmicrostrip
microstrippatch
patchantenna
antenna(RRMPA).
(RRMPA). In this study, the
hasbeen
beenselected
selectedas
asillustrated
illustratedin
inFigure
Figure1.1.This
Thisresearch
researchdemonstrates
demonstratesaanovel
novelapproach
approach
has
ofconstructing
constructingaabreathable
breathableperforated
perforatedtextile
textileantenna
antennathrough
throughadded
addedperforations
perforationsin
inthe
the
of
conductingelements
elementsof
ofRRMPA,
RRMPA,which
whichimproves
improvesits
itsWVP
WVPso
sothat
thatthe
thehumidity
humidityunder
underand
and
conducting
inside the
the antenna
antenna substrate
substrate can
canbe
beeffectively
effectivelytransported
transported back
backto
tothe
theenvironment.
environment. To
To
inside
validate
validatethe
theapproach,
approach,this
thispaper
paperfollows
followsaanumber
numberof
ofbasic
basicsteps
stepsto
todevelop
developaabreathable
breathable
textile
3-dimensional
(3D)
knitted
spacer
fabric
as
textileantenna,
antenna,employing
employinga ahighly
highlyperforated
perforated
3-dimensional
(3D)
knitted
spacer
fabric
the
textile
substrate
(perforations
are added
during
the knitting
process)
and theand
conductive
as the
textile
substrate
(perforations
are added
during
the knitting
process)
the conlayers
(the
ground
andplane
the patch)
with
regular
perforations/holes
(added through
ductive
layers
(theplane
ground
and the
patch)
with
regular perforations/holes
(added
laser
cutting
during
theduring
construction
of the textile
antenna).
the antenna
prototypes
are
through
laser
cutting
the construction
of the
textile All
antenna).
All the
antenna prodesigned
to
resonate
at
the
Industrial,
Scientific,
and
Medical
(ISM)
frequency
band
ranges
totypes are designed to resonate at the Industrial, Scientific, and Medical (ISM) frequency
from
GHz from
to 2.4835
GHz,towhich
MHz
of bandwidth.
It bandwidth.
should be noted
that
band2.4
ranges
2.4 GHz
2.4835covers
GHz, 83.5
which
covers
83.5 MHz of
It should
an
an RRPMA
is an
that
the ringisstructure
creates
a holecreates
within athe
antenna
be extra
notedbenefit
that anofextra
benefit of
RRPMA
that the ring
structure
hole
within
patch,
allowing
more
air andmore
water
to pass
through
antenna,
helps
to
the antenna
patch,
allowing
airvapors
and water
vapors
to passthe
through
thewhich
antenna,
which
enhance
the
breathability
of
the
antenna.
helps to enhance the breathability of the antenna.
Figure 1. Microstrip rectangular ring patch antenna.
Figure 1. Microstrip rectangular ring patch antenna.
2. Materials and Methods
2.1.
D Dielectric
Substrate
2. Materials
and
Methods
The
selectedSubstrate
3D knitted spacer antenna substrate is made of polyester and nylon that
2.1. D
Dielectric
have very low moisture regain and moisture content values of 0.54% and 0.42% [31], respecThe selected 3D knitted spacer antenna substrate is made of polyester and nylon that
tively. Furthermore, the substrate is flexible and dimensionally stable enough to withstand
have very low moisture regain and moisture content values of 0.54% and 0.42% [31], rerough handling and other stresses like bending and stretching during construction and
spectively. Furthermore, the substrate is flexible and dimensionally stable enough to withharsh use of the antenna. The structural view of the 3D substrate is illustrated in Figure 2.
stand rough handling and other stresses like bending and stretching during construction
The open mesh structure or the regular perforations in the 3D spacer substrate creates
and harsh use of the antenna. The structural view of the 3D substrate is illustrated in Figplenty of air cavities within the substrate, allowing easy passage for air and water vapors
ure 2. The open mesh structure or the regular perforations in the 3D spacer substrate creto pass through that helps to make the textile antenna breathable.
ates plenty of air cavities within the substrate, allowing easy passage for air and water
The high void ratio of a regularly perforated 3D substrate in terms of porosity ensures
vapors to pass through that helps to make the textile antenna breathable.
very low dielectric losses, as most of the volume within the substrate is occupied by air. The
The high void ratio of a regularly perforated 3D substrate in terms of porosity ensures
ratio of the total volume of voids to the entire volume of solid including voids is termed
very
losses,
as most
of the
volume
within
substrate
is occupied
bythe
air.
as thelow
voiddielectric
ratio of that
solid
[17,32].
A large
number
of the
voids
or air cavities
brings
The
ratio
of
the
total
volume
of
voids
to
the
entire
volume
of
solid
including
voids
dielectric constant of the perforated substrate closer to the dielectric constant of air andis
termed
as the
void ratio
of thatlosses
solid [33].
[17,32]. A large number of voids or air cavities brings
also
results
in lower
dielectric
the dielectric constant of the perforated substrate closer to the dielectric constant of air
and also results in lower dielectric losses [33].
Sensors 2021, 21, 1635
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Sensors 2020, 20, x FOR PEER REVIEW
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Figure 2. Structural diagram of the 3D knitted spacer substrate.
Figure 2. Structural diagram of the 3D knitted spacer substrate.
2.2. Conductive Materials
2.2. Conductive
Materials
Flectron® is
a conductive copper-coated nylon fabric and other conductive tapes
® is a conductive
constituting
different
conductive
materials like
copper,
nickel,
and silver
that hastapes
beenconused
Flectron
copper-coated
nylon
fabric
and other
conductive
in
previous
research
on textilematerials
antennaslike
[34–36].
In this
study,
conductive
stituting
different
conductive
copper,
nickel,
and three
silverdifferent
that has been
used
fabrics
from research
the Shieldex
brand
(produced
by Statex
Germany)
have been
in previous
on textile
antennas
[34–36].
In thisProduktions,
study, three different
conductive
used
as
the
conductive
patch
and
conductive
ground
plane
for
the
antenna
construction.
fabrics from the Shieldex brand (produced by Statex Produktions, Germany) have been
® Prag, Shieldex® Kiel-SK-96, and Shieldex® Koln are copper-coated nylon fabrics
Shieldex
used as the
conductive patch and conductive ground plane for the antenna construction.
® Prag, Shieldex®coating.
® Koln
with
a corrosion-resistant
They
have
different
fabric
AP values,
weight,
Shieldex
Kiel-SK-96,
and
Shieldex
arestructures,
copper-coated
nylon fabrics
and
resistivity values.
withsurface
a corrosion-resistant
coating. They have different fabric structures, AP values,
® conductive metalized fabrics collected from data sheets
The and
properties
Shieldexvalues.
weight,
surface of
resistivity
provided
by Statex of
Produktions
are summarized
in Table
The selected
conductive
The properties
Shieldex® conductive
metalized
fabrics1.collected
from data
sheets
fabrics
have
low
surface resistivity
and haveindifferent
constructions
to search
provided
byvery
Statex
Produktions
are summarized
Table 1. fabric
The selected
conductive
fab® Prag
for
most
suitable
geometry
to construct
a textile
breathable
RRMPA. Shieldex
ricsthe
have
very
low surface
resistivity
and have
different
fabric constructions
to search
for
® Prag
isthe
a compact
structured
woven
conductive
fabric
with almost
no visible
gaps
between
most suitable
geometry
to construct
a textile
breathable
RRMPA.
Shieldex
is a
® Koln and
yarns
with
the lowest
AP among
the other
conductive
Shieldex
compact
structured
woven
conductive
fabrictwo
with
almost no fabrics.
visible gaps
between
yarns
®
®
®
Shieldex
Kiel-SK-96
are nonwoven
fabrics
that have
higher
air permeabilities
compared
with the lowest
AP among
the other two
conductive
fabrics.
Shieldex
Koln and Shieldex
®
®
toKiel-SK-96
Shieldex are
Prag.
Shieldex fabrics
Prag is that
a, Figure
illustrates
cloth construction
of all to
the
nonwoven
have3 higher
air the
permeabilities
compared
® Prag. Shieldex
® Prag
three
conductive
fabrics by
Shieldex
used in
this research
is a, ®Figure
3 illustrates
thework.
cloth construction of all the
Shieldex
three conductive fabrics by Shieldex® used in this research work.
Table 1. Properties of the conductive materials
Table 1. Properties of the conductive materials
* Data Given by the Manufacturer
Shieldex® Prag
Shieldex® Kiel-SK-96
Shieldex® Koln
* data given
byType
the*
Fabric
Woven Shieldex® Kiel-SK-96
Nonwoven
Nonwoven
® Koln
Shieldex
Shieldex® Prag
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