Review
Artificial skin for sweating guarded
hotplates and manikins based on
weft knitted fabrics
Textile Research Journal
0(00) 1–16
! The Author(s) 2018
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DOI: 10.1177/0040517517750646
journals.sagepub.com/home/trj
Annette Mark1, Agnes Psikuta2, Boris Bauer1,
René Michel Rossi2 and Götz Theodor Gresser1
Abstract
Measurement devices such as sweating manikins, cylinders or hotplates are used for testing thermal and moisture
transfer properties of clothing or textiles. A critical feature of these measurement devices is the design of the outer
covering fabric that tightly enfolds the device like a skin. The artificial skin principally has to match individual requirements because the different sweating devices have different sweating systems and surface compositions. In this study
knitted fabrics with different fiber and yarn types are proposed to be used as an artificial skin. Thermal and moisture
properties of the fabrics were measured to obtain skin-like characteristics and a mathematical model for the quantification of thermal and moisture-management properties based on geometrical characteristics was developed. The results
show that the thermal and moisture-management properties of the fabrics do not only depend on the fiber properties
but also relevantly on their geometrical properties such as thickness, diameter and number of stitch pores. For example,
thermal resistance is significantly affected by the stitch pore diameter, and evaporative resistance by the fabric thickness.
Furthermore, water content and drying speed are determined by the capillary structure, and therefore, are more
influenced by yarn and fabric structure parameters, whereas contact angle and wettability are more influenced by the
fiber type. In conclusion, the tested fabrics satisfy all the requirements to match the anatomical properties of the human
skin; however, two fabric types, PES_19f30_SET and PES_28f48_GL, exhibited superior characteristics suitable for application as artificial skin on measurement devices.
Keywords
knitting, fiber, yarn, fabric formation, manufacture, properties, wetting, wicking artificial skin
According to international standards and scientific
literature, there is a variety of measurement devices
used for the investigation of thermodynamic processes
in textiles and clothing, such as radiation, conduction,
convection, moisture evaporation.1 As a common basic
principle, these thermodynamic measurement devices
simulate the metabolic heat loss. A critical feature of
such measurement devices is the design of the outer
covering fabric that is tightly enfolding the device like
a skin. These fabrics are not standardized and a great
variety of materials and designs are currently used.
Most common thermodynamic measurement devices
for textile and clothing are alambeta and permetest,2,3
guarded hotplates,4–6 thermal cylinders7,8 and
thermal manikins.9–11 They comprise a great variety
of materials, such as metals (copper, aluminum,
bronze)4,11,12 or polymers,13,14 flexible material such
as membranes, woven or knitted fabrics,4,15,16 paintings
or coatings12,17 as their surface finish. The analysis of
the literature shows that there are no consistent
1
Laboratory for Knitting Technique, Thermodynamics, Fabric Assembly
Technologies, German Institutes of Textile and Fiber Research, Germany
2
Laboratory for Biomimetic Membranes and Textiles, Empa Swiss Federal
Laboratories for Materials Science and Technology, Switzerland
Corresponding author:
Annette Mark, German Institutes of Textile and Fiber Research,
Koerschtalstrasse 26, 73770 Denkendorf, Germany.
Email: annette.mark@ditf.de
2
standards addressing the surface finish.4,11,18–21 Beside
their surface finish many thermal manikins are additionally covered by fabric skin.14,22 Sweating guarded
hotplates are commonly covered by a vapor permeable
PTFE-membrane combined with a hydrophobic woven
PES-fabric according to the standard DINCEN/
TR16422-2012.23
The thermodynamic properties of the fabric skins
are assessed by measurement devices such as the
Moisture Management Tester (MMT) or the heat and
mass loss method according to ASTM F 2370.22
Different knitted fabrics (cotton, polyester and hydrophilic polyester) used as artificial skin for manikins
have been investigated with regards to moisture transfer property measured on fully wetted skin.24 In some
cases such as thermo-physiological human simulators,
transient measurements with regards to temperature
and sweat rate are also applied. To be able to evaluate
the behavior of the fabrics in different phases of sweating (onset of wetting, fully developed sweating and
drying) four fabrics (cotton with elastane, polyester,
polyamide with elastane and fabric skin provided by
manikin manufacturer Thermetrics) have been investigated by Koelblen et al.25 Findings of the study showed
similarly good performance for the three tested fabrics,
however, the recommendation was to choose the type
of fabric that best meets the requirements of the sweating system. For the wetting phase, the spreading of a
homogeneous liquid on the fabric’s surface is proposed
to guarantee the cooling effect as at the human skin.
For evaporative resistance measurements, according to
standards, a high moisture content with a low drying
rate is suggested to ensure a sufficiently steady state to
take measurements.
The ability of knitted fabrics to transport heat and
mass depends on the fiber properties and the porosity
of the yarn and fabric structure. Wetting is determined
by the fiber surface and the wetting liquid, whereby the
wicking process is mostly affected by the capillaries
between the fibers and yarns, thus, by the arrangement
of the fibers in yarns and in knitted fabrics. Therefore,
the pore size and number of pores in a fabric’s structure
are crucial parameters in determining its thermodynamic properties.26,27 The fabric pore structure also
influences further physical properties such as thermal
conductivity, capillary flow by the use of intermolecular
forces between the liquid and surrounding fiber
surfaces, water adsorption within yarn pores and the
ability of hygroscopic fibers to swell.28–31
Research groups investigated how thermal resistance
strongly correlates with thickness, mass per unit area
and porosity in fabrics related to air entrapped in the
fabric structure,32 and that the amount of water wicked
from inside to the outer surface strongly correlates with
thickness and pore size.29,33
Textile Research Journal 0(00)
The geometrical properties of the fabrics used
as artificial skin have to match the anatomical properties of the human skin to simulate a human’s heat and
mass release realistically by using thermodynamic
devices.
The human skin interacts between the body and the
environment as a multilayer interface to regulate temperature and therefore heat and mass transport.34 The
human skin’s outermost hydrophobic layer, the epidermis, is made up of horny cells, and suet and sweat
glands.35 Its thickness varies between 0.05 1.2 mm36
and it is estimated to have the ability to contain a
water amount of 72% of its skin weight.37 The thermal
conductivity of the skin varies between 0.5 and
2.8 W m1 K1 depending on blood circulation and
moisture in the hydrophilic body tissue.38 The human
skin’s moisture permeability of 0.003 W m2 Pa2 is the
inverse of evaporative resistance Re.39 Moisture evaporates through sweat glands to the skin’s surface. Sweat
glands have an average diameter of 0.4 mm and an average pore number of about 120 per square centimeter
depending on the body segment.36 The insensible water
loss defines the measured quantity of water passing
from inside the body through the epidermis to the
ambient atmosphere via diffusion and evaporation processes and depends on several anatomical factors such
as anatomical site and thickness or others such as lipid
content, blood flow or skin temperature.34 The daily
insensible water loss for humans (surface area 1.8 m2)
varies between 0.6 and 2.3 l, hands and feet losing the
most (50–160 g h-1), the head and neck losing 40–
75 g h1 and all remaining other parts between 15–
60 g h1.40 The capacity for the active secretion of
sweat is very large, whole-body sweat losses can
exceed more than 2 l h1 during physical activity,41
with rates of about 3–4 l h1 during intense exercising
in the heat for short durations.42
In this study, different knitted structures were developed to get skin-like thermal properties to be used as an
artificial skin in a relaxed state for thermal measurement devices. For this purpose the designed knitted
fabrics were developed to match anatomical properties
of the human skin, such as a thickness between
0.05 1.2 mm, a pore size of 400 mm and number of
pores about 120 cm2. Different fiber and yarn types
were selected and characterized with regards to pores
in knitted fabrics and their influence on measured heat
and mass transfer in different sweating phases (wetting,
fully developed sweating and drying). Additionally, a
functional model was developed using multiple regression analysis and correlations to investigate the relationship between geometrical and heat and mass
transfer properties particularly in fabric skins.
The fabric skin as a potential artificial skin for
sweating thermal devices was recommended by
Mark et al.
3
performing systematic studies to get the best match
with human skin.
Specific parameters of fibers, yarns and knitted fabrics such as density, diameter, thickness, yarn length in
knitted fabrics, fiber or yarn fineness in knitted fabrics,
number of yarns or stitch pores form the basis for the
calculation of geometrical structure in the investigated
textile fabrics. The symbols and abbreviations for
important technical terms are compiled in a combined
list (Table 1).
Table 1. Symbols and abbreviations for the most important
technical terms
Acronym
Description
Unit
f
y
mf
Vy
Vfy
Vpy
Vpyr
lyt
t
Vt
Vyt
Vft
Vpyt
fiber density
yarn density
fiber fineness
yarn volume
fiber volume in yarn
pore volume in yarn
relative pore volume in yarn
yarn length
fabric density
knitted fabric volume
yarn volume in knitted fabric
fiber volume in knitted fabric
yarn pore volume in
knitted fabric
total pore volume in
knitted fabric
stitch pore volume in knitted fabric
total fabric volume
stitch pore diameter
yarn fineness
number of yarns
yarn end-fineness
Weight of knitted fabric
fabric thickness
number of stitch pores per
square centimeter
thermal conductivity
thermal resistance
evaporative resistance
contact angle
wetting time
wetting radius
surfacial water content
water content
drying speed
surfacial drying speed
g cm3
g cm3
dtex
cm3 10 km1
cm3 10 km1
cm3 10 km1
%
km m2
g cm3
cm3 m2
cm3 m2
cm3 m2
cm3 m2
Vpt
Vptt
Vtt
dptt
my
ny
myn
mt
ht
nptt
Rct
Ret
T
R
SWC
WC
DS
SDS
cm3 m2
cm3 m2
cm3 m2
mm
tex
–
tex
g m2
mm
cm2
W m1 K1
m2 K W1
m2 Pa W1
s
mm
g m2
g g1
g h1
g m2 h1
Methods
Pore size calculation
To determine the geometrical properties of the knitted
fabrics, such as thickness ht [mm], stitch pore diameter
(pore size) dptt [mm] and number of stitch pores nptt, the
entire pore structure in the fabric [cm3 m2] was computed based on the pore and fiber volume [cm3 m2] in
individual yarns and in knitted fabrics.43 The entire
pore structure in the fabric consists of meso-pores
occurring between the fibers in the yarn and makropores occurring between interconnected yarns in the
fabric that are known as stitch pores. Physical parameters of fibers, such as density rt [g cm3] and the
fineness mf [dtex] of yarns such as diameter dy [mm]
and fineness my [tex], and of fabrics such as mass mt
[g m2] and thickness ht [mm], were measured. The yarn
end-fineness myn [dtex] is the product of the fineness my
[tex] and number of yarns ny.
The specific volume of the fabric Vt is the reciprocal
mathematical function of the density rt which was calculated according to Equation 1 and is an important
value for the characterization of the fiber and pore
volume in the fabric:
rt ¼
mt
0:001
ht
ð1Þ
To characterize the pore structure in the fabrics, the
pore and fiber volume [cm3] were calculated for a fabric
area of 1 m2 and to a yarn length of 10 km. For determining the volume ratio from fibers to pores, the fabric
volume Vt [cm3 m2] was calculated as follows:
Vt ¼
ht
10:000
10
ð2Þ
The yarn volume Vyt in the knitted fabric [cm3 m2]
results from the product of the yarn volume Vy [cm3
10 km1] as a result of the yarn fineness my [dtex]
divided by the yarn density ry [g cm3]. The yarn
length in the knitted fabric lyt [km m2] results from
the mass of fabrics mt [g m2] and yarn end-fineness
myn [dtex]:
Vyt ¼ Vy lyt
10:000
ð3Þ
The product of the fiber volume in yarn Vfy [cm3
10 km1] and yarn length in the knitted fabric lyt
[km m2] is the fiber volume in the textile Vft [cm3 m2]:
Vft ¼ Vfy lyt
10:000
ð4Þ
4
Textile Research Journal 0(00)
The pore volume Vpyt in the fabric [cm3 m2] results
from the product of the yarn pore volume Vpy [cm3
10 km1] and yarn length in the fabric lyt [km m2].
The pore volume in yarn Vpy [cm3 10 km1] is calculated by multiplying the yarn volume Vy [cm3 10 km1]
by the relative pore volume in the yarn Vpyr [%], which
is the result of the yarn density ry [g cm3] divided by
the fiber density rf [g cm3] minus 1:
Vpyt ¼ Vpy lyt
10:000
ð5Þ
By subtracting the fiber volume Vft [cm3 m2] from
the total volume Vt [cm3 m2] the total pore volume Vpt
[cm3 m2] of the knitted fabric is calculated as follows:
Vpt ¼ Vt Vft
ð6Þ
The stitch pore volume Vptt [cm3 m2] is calculated
analogous to the total pore volume Vpt [cm3 m2]
included in the knitted fabrics:
Vptt ¼ Vpt Vpyt
ð7Þ
To summarize, the sum of the pore and fiber volume
[cm3 m2] results in the entire fabric volume balance Vtt
[cm3 m2] as,
Vtt ¼ Vft þ Vpyt þ Vptt
ð8Þ
The number of stitch pores nptt is calculated by
multiplying the stitch pores in wales by the stitch
pores in course-direction referred for a fabric area of
1 cm2.
Given the assumption that all stitch pores are cylindrical with a circular cross-section, the equivalent pore
diameter dptt [mm] can be calculated as follows:
dptt ¼ 2
Vptt 1:000 !0:5
nptt 10:000
1:000
ht
ð9Þ
Selection of yarns
As shown in Table 2, yarns made of six different fiber
polymers, such as polyamide 6 (PA6), polyamide 6.6
(PA6.6), polyester (PES), polypropylene (PP), rayon
(CV) and cotton (CO) were selected and characterized.
The chosen fiber types differed in polymeric composition, fiber shape (staple fiber or filament) and texturing
processes. A wide selection of different yarns was purposefully chosen to represent knitted fabrics with different geometrical and fiber properties in order to have
Table 2. Characterization of yarn types
Fiber type
Code
Polymers
PA6_17
PA66_23f26/2
PA66_11f34_HE*
polyamide 6
polyamide 6.6
polyamide 6.6,
HE
PA66_11f34_SET* polyamide 6.6,
SET
PA66_18f104_GL* polyamide 6.6,
GL
PA66_24f48_GL* polyamide 6.6,
GL
PES_20
Polyester
PES_19f30
Polyester
PES_19f30_SET
polyester, SET*
PES_24
Polyester
PES_28f48_GL
polyester, GL*
PP_20
polypropylene
CV_24
rayon
CO_20
cotton
Yarn
fineness
staple filament my (tex)
x
x
x
17
23
11
x
11
x
18
x
24
x
x
x
x
x
x
x
x
20
19
19
24
28
20
24
20
*GL (flat), SET (fixed), HE (highly extensible)
broad selection of properties to compare to human
skin. Fiber types such as PA, PES and CO are already
used for fabric skins; CV is known to be highly water
absorbing and was, therefore, selected as an additional
reference fiber type. PP-fiber is known to be a polymer
that does not absorb any moisture, which is most similar to the horny layer of the epidermis.
The fiber types included natural or synthetic fibers
with varying length as staple fiber and continuous
fibers as filaments. The number of filaments varied
between 30 and 104 (e.g. f30 in Table 2). The labeling
also marks the applied texturing processes. The addition GL indicates total oriented yarns with a characteristically low elastic and volumetric quality (nontextured filament yarns). The additions SET and HE
indicate drawn-textured yarns or fully drawn yarns
with a medium elastic/volumetric quality (SET) or a
high elastic/volumetric quality (HE), respectively (textured filament yarns). The yarn fineness my is specified
in the unit [tex].
Manufacturing weft knitted fabrics
Preliminary manufacturing tests were conducted on the
knitting machine for a great variety of knitting structures to meet the geometrical properties of human skin
Mark et al.
(thickness ht, stitch pore diameter dptt and number of
pores nptt) and ensure mechanical stability. Therefore,
different machine settings such as needle gauge, number
of stitches in wales and course-direction, knitting structure and sinking depth were altered and provided crucial parameters for the knitting process to achieve the
fabric geometrical properties desired for mimicking the
human skin. The selected 18 weft knitted fabrics
matched the requirements of the human skin structure
with defined thickness ht, stitch pore diameter dptt and
number of pores nptt.
The 14 different yarn types were used to manufacture the 18 weft knitted fabrics on a flatbed knitting
12-gauge machine (type: STOLL CMS330.6 TC). The
machine was set to knit 200 stitches in wales (x) and 400
stitches in course-direction (y), so all fabrics were comprised of precisely the same number of stitches or stitch
pores. The fabrics were manufactured in double knit
structure (both needle beds activated) to obtain a
1 1 rib structure. The smallest sinking depth as the
specific parameter number given by the knitting
machine manufacturer (dimensionless number 8.3)
was used on this type of flatbed knitting machine to
manipulate the loop length and stitch density and
achieve the desired geometrical values such as thickness
ht and stitch pore diameter dptt. Eleven fabrics were
knitted using the single yarns and seven fabrics were
knitted using the double yarn described as the
number of yarns ny and marked with an ending ‘2’ in
Table 4. For statistical reasons, all knitted fabrics were
reproduced three times and a sample was cut out of
each fabric.
After knitting, all fabrics received standard care by
washing (easy-care washing program, washing temperature of 40 C, 1000 U min1) according to the standard EN ISO 6330-2001.44 Human skin is hydrophobic
at the surface, however, in order to be able to measure
mass transfer, the surface has to be fully wetted, therefore the fabric skin has to be hydrophilic. To assess the
wettability of fabrics, a simple test was performed by
dripping a water droplet on the fabric’s surface from a
syringe. The eight fabrics made of the fiber types PES
and CV turned out to be highly hydrophobic and
were additionally treated by using a padding machine
(Mathis, Switzerland), an aqueous polymer solution
ARRISTAN AIR (CHT, Germany) and a lab dryer
(Mathis, Switzerland) to turn them into hydrophilic
specimens as required for the artificial skin.
Experimental design
The size in width and length of all washed and hydrophilic-treated fabrics were measured to define the stitch
pore number nptt, whereby the total number of counted
stitch pores (wales multiplied by course-direction) is
5
divided by the measured fabric area m2. The mass mt
according to EN 1212745 and thickness ht according to
ISO 508446 of the fabric were measured and the endfineness of yarns myn was calculated by multiplying the
yarn fineness my according to EN ISO 206047 by the
number of yarns ny. In addition, the pore volumes were
calculated as outlined before.
Thermal and wicking properties related to the behavior of the fabrics in different sweating phases, such as
onset of wetting, fully developed sweating and drying
were measured in the knitted fabrics. The thermal
properties, such as conductivity and thermal resistance Rct were determined to evaluate the thermal
parameters. These properties are relevant to calculate
the thermal resistance Rc of the air layer on the device
for reference measurements or of the tested fabric or
clothing sample. Since the sensors of the (sweating)
guarded hotplate or manikins are actually under the
skin, the temperature on the outer side of the skin is
lower than that on the sensors. By using thermal parameters, further measurements can be corrected. The
evaluation of onset of wetting was indicated by the
contact angle y between liquid and surface and by the
water spreading ability on fabrics investigated by the
MMT. Therefore, wetting and wicking are influenced
by the yarn structure and contact angle. Information
about the wetting and wicking properties in fabrics is
provided by measuring the time t that is needed to
moisten the fabric successively from top to the
bottom. The shape of water spot occurring on the
bottom surface, determined by measuring the diameter
of the spot r, provides information about water spreading ability. The role of evaporative resistance Ret was
to allow sufficient evaporation from within the fabric
and from its surface, since evaporation takes place in
the entire thickness of the fabric in its pores. The maximum water content was determined to evaluate fabric
behavior in the fully developed sweating phase. The
drying speed DS is a basic indication of fabric behavior
in the drying phase. For thermal devices, the surface
area is one of the crucial parameters and therefore both
properties, maximum water content WC and drying
speed DS, are provided in relation to surface named
as surfacial maximum water content SWC or surfacial
drying speed SDS. All fabric samples were conditioned
for 12 hours at the required environmental conditions
(temperature of 20 0.5 C and relative humidity of
65 5%). Tap water was used to pre-wet fabric
samples.25
Evaluation of thermal and moisture properties on
the artificial skins
Thermal conductivity. The instrument KALCOS was used
for measuring conductivity according to standard DIN
6
5261248 at an air temperature of 20 0.5 C and relative
humidity of 65 5%. Each fabric sample with the
dimensions of 20 x 20 cm was placed in-between the
measuring plates under a defined temperature gradient
of 5 C (upper plate 25 C, lower plate 20 C, normal
pressure 2 kPa). The heating power at steady state
(maintained for at least 30 min) was measured and
the sample’s thermal conductivity (W m1 K1) was
calculated. The fabric thickness ht needed for the thermal conductivity calculation was determined using a
thickness meter (Universal Micrometer, accurate to
1 mm, Frank PTI-GmbH, Germany) at the same
normal pressure of 2 kPa that was applied during
measurement.46
Thermal and evaporative resistance. The thermal and evaporative resistances (Rct and Ret) of three samples of
each of the fabrics (25 x 25 cm) were determined using a
(sweating) guarded hotplate simulating a human’s heat
production by metabolism and heat release by conduction, radiation, convection and evaporation. The tests
were performed according to ISO 110924 in a climatic
chamber at an air temperature of 20 0.5 C, relative
humidity of 65 5%, air speed parallel to the sample of
1 0.05 m s1 for the thermal resistance, and an air
temperature of 35 0.5 C, relative humidity of
40 5%, and air speed of 1 0.05 m s1 for the evaporative resistance.
Contact angle. The contact angle between the fabric surface and water droplet on the fabric was determined by
using the drop shape analyzer DSA-25 (Krüss GmbH,
Hamburg, Germany) under climate controlled conditions (air temperature of 20 0.5 C, relative humidity
of 65 5%).49 For the tests, a water drop with a
volume of 4 ml was applied on three samples measuring
1 x 1 cm with a syringe. A camera recorded the wetting
process. The angle between fabric surface and drop
contour was measured by digital picture analysis. The
maximum measurement time was set to 25 s. Surfaces
with contact angles between 0 and 90 are defined to
be wettable or hydrophilic and surfaces with contact
angles between 90 and 180 are defined to be nonwettable or hydrophobic.
Wetting and wicking. The MMT (M290MMT, SDL Atlas,
USA) was used to characterize the wetting and wicking
processes in three samples of each fabric (ø 8 cm) at an
air temperature of 20 0.5 C, relative humidity of
65 5%, and air speed below 0.1 m s1. Principally,
the time to moisten the fabric sample successively from
top to the bottom and the radius of wetted spots were
measured. The spreading time for the upper fabric surface was set to 20 s. The results were classified according
to the terms of the device manufacturer.50
Textile Research Journal 0(00)
Water content and drying speed. The maximal moisture
content was determined by weighing the dry fabric samples (10 x 10 cm) and putting them in a water bath at an
air temperature of 20 2 C and relative humidity of
65 5% for four hours. Afterwards, the saturated fabrics were fastened with a clamp on a stand, which
was placed on a scale (Mettler AE240-S, accurate to
0.02 g, Mettler-Toledo, Switzerland) and covered by
a shield to prevent weight fluctuation due to air movement in the air-conditioned laboratory. The wet fabrics
were observed until water stopped forming drops due
to gravitational moisture displacement within the
sample and then the actual measurement started. The
scale was connected to a computer to record the weight
change over time and the total drying time.
Statistical analysis
By using a multiple linear regression model, the relationship between the thermal and wicking properties to
the geometrical properties of the fabric were described
in the statistical software SPSS (IBM Microsoft,
Version 22). This method offers the possibility to
assess geometrical properties, their influence on measured heat and mass transfer in different sweating
phases and to identify the most influential properties
(predictors).
Independent variables determined experimentally
and used in the model include thickness ht, stitch pore
diameter dptt and number of stitch pores nptt (Table 3).
Dependent variables for which the regression equations
were obtained include thermal resistance Rct and evaporative resistance Ret and drying speed DS (Table 3).
In preliminary correlation analysis and regression equations it was observed that thermal conductivity , water
content WC, wetting time t and radius r did not correlate with the geometrical properties. And thus, no prediction can be made for these properties using the
geometrical properties because the mean variation
is very large and the correlation coefficient R2 less
than 0.2.
The relationship type between independent and
dependent variables (linear or others) was determined
based on scatter plots of the experimental data for individual predictors and was tested statistically:
y ¼ a1 x1 þ a2 x2 þ a3 x3 þ b
ð10Þ
where a and b are sought coefficients for the defined
dependent variables and each predictor (independent
variables). The regression coefficient indicates the
slope of the function. Unstandardized coefficients and
the standard error indicated the degree each predictor
affected the outcome if the effects of all other predictors
were held constant. The p-value showed the significance
Mark et al.
7
Table 3. Description of the variables and constants in the model
Variables
Unit
Variable types
Symbol in model
Regression coefficients
Thickness ht
Stitch pore diameter dptt
Number of stitch pore nptt
Thermal resistance Rct
Evaporative resistance Ret
Drying speed DS
Constant
mm
mm
–
m2 K W1
m2 Pa W1
g h1
–
Independent
Independent
Independent
Dependent
Dependent
Dependent
–
x1
x2
x3
y1
y2
y3
–
a1
a2
a3
–
–
–
by1, by2, by3
of contribution to predicting the outcome for each
influential predictor (p-values less than 0.05 are
significant). To assess the goodness-of-fit the rootmean-square deviation (RMSD), which represents the
average difference between the measured results of the
thermal and wicking properties and the corresponding
calculated properties based on the geometrical properties of the fabrics. It is defined as:51
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
ð ymeasured ymodel Þ2
rmsd ¼
n
ð11Þ
where y-measured is the measured value of the thermal
and wicking properties (thermal resistance Rct, evaporative Ret resistance and drying speed DS), y-model
is the calculated value using the regression equations,
and n is the number of experimental data points. The
RMSD is an indicator of model goodness-of-fit that
can be assessed by comparing RMSD values and the
standard deviation of the measured data. The average
difference between trend line and points is considered
acceptable when the RMSD-value is smaller than the
average standard deviation of the data set.
Results
Physical and thermal properties of fabrics
The results of the measurements of the end-fineness of
yarns myn, mass mt, thickness ht and density rt of the
fabrics are presented in Table 4. The calculated numbers nptt and diameter dptt of stitch pores are also presented in Table 4.
Table 4 describes the structure of the manufactured
fabrics. The end-fineness myn of the yarns varied
between 11 and 49 tex and correlates with the fabric
weight mt, which varied between 140 and 416 g m2.
The fabric thickness ht varied between 0.91 and
1.50 mm and the fabric density rt varied between 0.14
and 0.35 g cm3. The fabrics made of textured filament
yarns (marked with ‘x’ in Table 4) represented densities
rt smaller than 0.25 g cm3, which was on average
smaller than many fabrics made of non-textured filament yarns. The double-yarn processed fabrics had
higher mass mt and density rt than the single-yarn processed fabrics. The stitch pore number nptt ranged
between 190 cm2 (for PES_24) and 362 cm2 (for
PA66_11f34_HE) by a factor of about two. Fabrics
made of filament yarns tended to have more stitch
pores than fabrics made of staple fiber yarns due to
decreasing yarn fineness myn, and consequently, a smaller stitch pore diameter dptt as shown in Table 4. The
stitch pore diameter dptt ranged from 225 mm (for
PA66_11f34_SET_2) to 710 mm (for PES_24).
In order to validate the calculated equivalent diameter dptt and number of stitch pores nptt for a
fabric area of 1 cm2, a comparison between the microscopic and graphic display of these geometrical parameters are shown in Figure 1.
The
results
from
the
conductivity
and
resistance measurements to investigate thermal properties and the measurements of contact angle, moisturemanagement testing, water content and drying
speed to investigate moisture properties are shown in
Table 5.
Thermal conductivity. Fabrics consisting of staple fiber
yarns resulted in higher values for thermal conductivity
compared to fabrics consisting of filament yarns. The
fabrics consisting of filament yarn PA66_11f34 textured
as HE and SET showed only a difference of about
0.001 W m1 K1.
In the fabrics consisting of the yarn
PA66_11f34_SET the thermal conductivity differed
by about 0.004 W m1 K1, which was related to the
number of yarns ny. In comparison, for the fabrics consisting of non-textured filament yarn PA66_
18f104_GL, thermal conductivity differed by a five
times higher value of about 0.021 W m1 K1.
The thermal conductivity of human skin is
higher by more than one power of ten than that of
the fabrics.
8
Textile Research Journal 0(00)
Table 4. Physical properties of the fabrics
Code
PA6_17_2*
PA66_28f26/2
PA66_11f34_HE
PA66_11f34_SET
PA66_11f34_SET_2*
PA66_18f104_GL
PA66_18f104_GL_2*
PA66_24f48_GL
PES_20_2*
PES_19f30
PES_19f30_SET
PES_19f30_SET_2*
PES_24
PES_24_2*
PES_28f48_GL
PP_20
CV_24_2*
CO_20
Textured
x
x
x
x
x
myn (tex)
mt (g m2)
ht (mm)
t (g cm3)
nptt (cm2)
dptt (mm)
34
28
11
11
22
18
35
24
39
19
18
37
24
49
28
20
48
19
304
254
162
140
237
171
394
275
352
172
154
306
200
416
244
164
401
178
1.25
1.30
1.05
1.02
1.13
0.91
1.14
1.17
1.14
1.14
1.07
1.21
1.32
1.50
1.07
1.16
1.36
1.04
0.24
0.20
0.16
0.14
0.21
0.19
0.35
0.23
0.31
0.15
0.14
0.25
0.15
0.28
0.23
0.14
0.29
0.17
226
283
362
351
294
254
318
326
213
244
202
207
190
215
236
193
199
213
584
472
393
390
225
554
334
402
560
554
632
470
710
567
513
672
555
644
*_2 implies the number of yarns ny
Thermal and evaporative resistance. Fabrics made of staple
fiber yarns systematically showed values higher than
0.027 m2 K W1, fabrics made of filament yarns
showed values lower than 0.027 m2 K W1. The
double-yarn processed fabrics had lower values for
thermal resistance Rct than the single-yarn processed
fabrics.
The evaporative resistance Ret ranged between
about 2.5 and 6.0 m2 Pa W1. Particularly, fabrics
made of staple fiber yarns showed values higher
than about 4.0 m2 Pa W1, and fabrics made of filament
yarns showed values lower than 4.0 m2 Pa W1,
except for PA66_100f26/2, which had a value of
4.30 m2 Pa W1. The double-yarn processed fabrics
had higher values for evaporative resistance Ret than
the single-yarn processed fabrics.
Thermal resistance Rct is basically the inverse of
thermal conductivity . Human skin has different
values depending on its blood circulation and moisture
compared to the fabrics, which varied between 0.016
and 0.046 m2 K W1. In comparison to the human
skin moisture permeability of 0.003 W m2 Pa2, the
inverse of the evaporative resistance Ret of the fabric
varied in a range between 0.16 and 0.39 W m2 Pa2.
Contact angle. In Table 5 it is shown that the treated
fabrics made of PES- and CV-fiber and two PA-fiber
fabrics (PA66_11f34_HE and PA66_24f48_GL) were
highly hydrophilic with contact angles y of 0 , while
the other fabrics were hydrophobic with contact
angles y greater than 90 . Contact angles between 0
and 90 were not observed.
Wetting and wicking. In the top-to-bottom surface
wicking process in the tested knitted fabrics, the wetting
time of the bottom surface was slightly delayed
compared to the top surface (since the water droplets
were applied from the top in the MMT device). The
hydrophilic fabrics (contact angle y ¼ 0 ) mostly
showed a short top-to-bottom wetting time of less
than about 5 s. Most of the hydrophobic fabrics (contact angle y > 100 ) showed a wetting time between
about 5 and 15 s (medium). The wetting radius on
the top and bottom surface of the fabric, respectively,
was approximately the same size in both hydrophilic
and hydrophobic fabrics. The hydrophilic fabrics
(y ¼ 0 ) mostly showed a wetting radius on the
bottom between about 15 and 28 mm (large). The wetting radius on hydrophobic fabric surfaces (y > 100 )
was slightly smaller and ranged between about 10 and
15 mm
(medium).
The
hydrophobic
fabrics
PA66_23f26/2, PA66_11f34_SET and PA66_18f104_
GL showed radii smaller than 10 mm. The effect of
both transversal and lateral wetting is related to the
water affiliation properties of the fabrics (e.g. contact
angle) for initial wetting and the capillary forces within
the fabric structure for further spreading of the moisture within the fabric. The particular results obtained
Mark et al.
9
Figure 1. Comparison between the microscopic and graphic display of stitch pore diameter dptt and number of stitch pores nptt in
manufactured knitted fabrics.
for the fabrics considered in this study follow this logic
consistently.
Water content and drying speed. The maximum surfacial
water content SWC was determined and varied between
364 and 971 g m2 with high standard deviations. The
relationship between wetted and dry weight referred to
m2 was derived from determining the water content.
The single-yarn processed fabrics showed higher values
than double-yarn processed fabrics. Fabrics from
0.059 0.001
0.052 0.000
0.047 0.000
0.046 0.000
0.050 0.001
0.041 0.003
0.062 0.001
0.052 0.000
0.057 0.000
0.042 0.002
0.045 0.002
0.051 0.000
0.046 0.001
0.054 0.000
0.046 0.003
0.041 0.001
0.057 0.000
0.053 0.001
PA6_17_2
PA66_23f26/2
PA66_11f34_HE
PA66_11f34_SET
PA66_11f34_SET_2
PA66_18f104_GL
PA66_18f104_GL_2
PA66_24f48_GL
PES_20_2
PES_19f30
PES_19f30_SET
PES_19f30_SET_2
PES_24
PES_24_2
PES_28f48_GL
PP_20
CV_24_2
CO_20
0.043 0,000
0.027 0.002
0.026 0.000
0.027 0.001
0.019 0.000
0.019 0.001
0.019 0.001
0.016 0.001
0.035 0.000
0.024 0.002
0.027 0.002
0.019 0.001
0.046 0.000
0.035 0.001
0.018 0.000
0.036 0.000
0.027 0.001
0.038 0.002
Rct (m2 K W1)
5.92 0.14
4.30 0.27
2.69 0.08
2.91 0.03
3.20 0.20
2.59 0.03
4.35 0.11
3.75 0.02
4.45 0.10
3.03 0.16
2.70 0.06
3.46 0.02
4.26 0.05
5.07 0.07
2.97 0.19
4.03 0.10
4.20 0.02
3.97 0.17
Ret (m2 Pa W1)
150 26
129 6
00
144 6
180 0
138 22
180 0
00
00
00
00
00
00
00
00
138 31
00
180 2
y ( )
7.1 1.3
15.3 3.3
3.7 0.3
4.7 2.0
5.8 0.9
4.7 1.0
6.7 1.0
5.4 0.4
3.3 0.1
2.9 0.1
3.4 0.2
3.9 0.2
2.9 0.4
3.7 0.1
3.0 0.2
4.7 1.7
4.5 0.2
7.4 2.5
t (s)
14.0 2.2
8.0 2.7
12.0 2.7
9.0 4.2
15.0 5.0
9.0 4.2
12.0 2.7
16.2 2.2
21.0 2.2
24.0 2.2
19.0 2.2
20.0 0.0
27.0 2.7
20.0 0.0
27.0 2.7
13.0 2.7
15.0 0.0
11.0 4.2
r (mm)
971 29
941 28
614 18
602 18
631 19
354 11
740 22
424 13
832 25
575 17
686 21
775 23
777 23
874 26
364 11
589 18
893 27
672 20
SWC (g m2)
3.26 0.10
3.72 0.11
4.60 0.14
3.95 0.12
3.13 0.09
2.11 0.06
1.92 0.06
1.59 0.05
2.30 0.07
3.15 0.09
4.23 0.13
2.38 0.07
3.38 0.12
2.10 0.06
1.46 0.04
2.37 0.10
2.50 0.07
4.77 0.14
WC (g g1)
14.4 2.7
14.1 0.9
9.1 0.6
9.1 0.7
9.2 2.9
10.1 1.8
11.9 2.4
9.5 0.4
15.5 0.9
9.6 0.4
11.6 0.3
13.9 0.2
12.3 0.6
17.2 0.3
8.9 1.2
14.2 2.0
16.7 0.4
10.2 1.3
DS (g h1)
1593 266
1413 85
917 60
913 72
977 289
923 175
1170 244
943 38
1547 95
960 40
1150 30
1383 23
1233 57
1713 32
957 124
1193 197
1670 40
1073 127
SDS (g m2 h1)
– thermal conductivity; Rct – thermal resistance; Ret – evaporative resistance; y – contact angle; t – wetting time; r – wetting radius; SWC – surfacial water content; WC – water content; DS – drying speed;
SDS – surfacial drying speed.
(W m1 K1)
Fabric sample
Table 5. Thermal and moisture properties determined in fabrics
10
Textile Research Journal 0(00)
Mark et al.
11
Figure 2. Correlations between predictors and dependent variables and the corresponding coefficient of determination.
textured filament yarns and usually staple fiber yarns
showed water content WC values higher than 2.30 g g1
compared to fabrics from non-textured filament yarns
with, unexceptionally, only values smaller than
2.30 g g1 and thus, maximum water content WC was
about three times higher in comparison to human skin
with an average of 0.75 g g1.
The surfacial drying speed SDS of the knitted fabrics
varied from 917 to 1713 g m2 h1. The fabrics of staple
fiber yarns usually had surfacial drying speeds higher
and fabrics of filament yarns lower than 1000 g m2
h1. The surfacial drying speed SDS decreased by the
number of yarns ny.
The daily insensible water loss for humans varying
between 22 and 89 g m2 h1, exceeding up to
1000 g m2 h1 during physical performance, with
rates up to about 1600 g m2 h1 lies within the same
range as the surfacial drying speed SDS of fabrics.
Statistical analysis
The conducted correlations between predictors related
to dependent variables are presented in Figure 2,
whereby the predictor thickness ht was related to evaporative resistance Ret and drying speed DS showed
coefficients of determinations R2 of about and higher
than 0.5, also the predictor diameter of stitch pores dptt
was related to thermal resistance Rct with an R2 of
about 0.5 and RMSD values smaller than the average
standard deviation of the given data set.
12
Textile Research Journal 0(00)
By replacing the sought coefficients a and b,
the specific model for thermal resistance Rct,
evaporative resistance Ret and drying speed DS was
defined and the most influential predictors identified.
In all regression models the standard error was
very low.
The specific model for thermal resistance Rct was
defined as:
Rct ¼ 0:023 ht þ 6:50105 dptt
þ 5:49105 nptt 0:046
ð12Þ
The value a2 indicated that as the diameter of stitch
pores dptt increased by one unit, thermal resistance Rct
increased by 6:50105 units. The diameter of stitch
pores dptt was identified as the most influential predictorð p value ¼ 0:008Þ. The fit of this multiple
regression model was assessed with the coefficient
of
determination
R2 ¼ 0.60
and
root-meansquare deviation RMSD ¼ 0.002 m2 K W1 (the mean
standard deviation of measured Rct approximated
0.001 m2 K W1 for Rct values in the range of 0.016–
0.046 m2 K W1).
The specific model for evaporative resistance Ret was
defined as:
Ret ¼ 4:48 ht þ 0:001 dptt þ 0:001 nptt 2:32 ð13Þ
The value a1 indicated that as thickness ht increased
by one unit, evaporative resistance Ret increased by
4.48 units. The thickness ht was identified as the most
influential predictor (p-value ¼ 0.004). The fit of this
multiple regression model was assessed with R2 ¼ 0.62
and root-mean-square deviation RMSD ¼ 0.15 m2 Pa
W1 (the mean standard deviation of measured Ret
approximated 0.10 m2 Pa W1 for Ret values in the
range of 2.59–5.92 m2 Pa W1).
The specific model for drying speed DS was
defined as:
RDS ¼ 12:58 ht þ 0:00 dptt 0:018 nptt þ 2:09
ð14Þ
The value a1 indicated that as thickness ht increased
by one unit, drying speed DS increased by 12.58 units.
The thickness ht was identified as the most influential
predictor (p-value ¼ 0.002). The predictor number of
stitch pores nptt showed a negative value, indicating a
negative relationship to the outcome. The fit of this
multiple regression model was assessed with R2 ¼ 0.62
and root-mean-square deviation RMSD ¼ 1.3 g h1
(the mean standard deviation of measured DS approximated 1.1 g h1 for DS values in the range of 8.9–
17.2 g h1).
Discussion
The geometrical properties of the weft knitted fabrics
developed in this study are similar to the anatomical
properties of the human skin. In particular, the thickness ht (from 0.90 to 1.20 mm), stitch pore diameter dptt
(from 200 to 700 mm) and stitch pore number nptt (from
190 to 360 cm2) of the knitted fabrics are very similar
to the characteristic distribution of thickness between
about 0.05 1.2 mm, sweat gland number between
about 80 and 500 cm2 or sweat gland diameter of
about 400 mm of the human skin. The water content
WC of the fabrics varied between 354 and 971 g m2
(1.46 and 4.77 g g1), and thus, it was about three
times higher in comparison to human skin with an average of 0.75 g g1. The wetting contact angle y of the
fabrics ranged from 0 (highly hydrophilic) to 180
(highly hydrophobic), whereby the contact angle y of
human skin is averagely 104 (hydrophobic).
Interestingly, the tested hydrophobic fabrics
PA66_23f26/2 with 129 6 , PA66_11f34_SET with
144 6 , PA66_18f104_GL with 138 22 and PP_20
with 138 31 best met the hydrophobic top layer of
human skin (epidermis). Nonetheless, the main interest
of this study focuses on the functionality of fabric skin
as the imitation of human-like sweating, which implies
good moisture spreading properties for efficient skin
wetting. In this case the hydrophilic fabrics have a
clear advantage to fulfill the expected wetting and wicking performance of the fabric skin. Another unexpected
phenomenon was that the fabrics made of PA-fibers
expected to be highly hydrophilic turned out to be
highly hydrophobic. The different yarn structure and
number of yarns in the fabrics might explain the
hydrophobicity.28
The conductivity of the fabrics investigated,
which ranged from 0.041 to 0.062 W m1 K1,
increased by the increase of fabric density rt and had
a quantity between air (0.025 W m1 K1) and water
(0.598 W m1 K1)52 saying that knitted fabrics as
porous material mainly consist of pores formed by
fibers filled with air or water. The thermal resistance
Rct of the fabrics investigated ranged from 0.016 and
0.046 m2 K W1 and is basically the inverse of the thermal conductivity, however the thermal resistance values
are about five times higher because different measurement conditions such as compression and air movement
on fabric influence its surface structure. The thermal
resistance Rct of air with 0.022 m2 K W1 53 had a quantity in the middle of the fabric’s range. The thermal
conductivity and resistance R as insulation properties
are higher by more than one power of ten for the
human skin consisting of hydrophilic body tissue and
hydrophobic epidermis depending on blood circulation
and moisture compared to fabric skins. However, the
Mark et al.
knitting structure provides geometrical structure to
transfer mass for simulating the wicking process.
The thermal properties, such as conductivity and thermal resistance Rct, are relevant to calculate
the thermal resistance Rc of the air layer on the
device for reference measurements or of the tested
fabric or clothing sample. Since the sensors of the
(sweating) guarded hotplate or manikins are actually
under the skin, the temperature on the outer side of
the skin is lower than that on the sensors. By using
thermal parameters further measurements can be
corrected.
The role of evaporative resistance Ret was to allow
sufficient evaporation from within a fabric and from
its surface since evaporation takes place in the
entire thickness of the fabric in its pores. In
comparison to a human skin moisture permeability of
0.003 W m2 Pa2, the inverse of the evaporative resistance Ret of the fabric varied in a range between 0.16
and 0.39 W m2 Pa2.
The wettability of fabrics affects moisture behavior,
whereby hydrophilic fabrics (contact angle y ¼ 0 ) show
short wetting times and large wetting radii on the
bottom of measured fabrics. The cooling effect that
occurs on top of human skin is imitated on the
bottom of fabric skins where the actual cooling and
wetting take place. According to Koelblen et al.25 the
cooling effect shows a desirable characteristic for sweating simulation on fabric skins. They also discussed how
measured moisture properties might be different in horizontal and vertical orientations of the measurement
surface or skin, and the cooling effect might be smaller
when testing samples in a vertical orientation due to
water flowing downwards under the influence of gravity, and also forces and couples acting in-between the
fibers might manipulate moisture behavior. Therefore,
fabric samples with lower downward wetting ranges are
recommended due to moisture migration and the dripping of water.
Fabrics from textured filament yarns and usually
staple fiber yarns reach a water content based on dry
weight higher than 2.30 g g1, while non-textured filament yarns have a water content based on dry weight
lower than 2.30 g g1. The water content WC is about
three times higher in comparison to human skin with an
average of 0.75 g g1. That means that the critical thickness of the water layer on the skin’s surface and water
droplets start to drip off laying between 364 and
971 g m2 and are hardly comparable with human
skin with about 37.6 g m2.54 Due to water on fabrics
surface traps inside the fabric skin structure while water
on humans’ skin forms droplets and runs off the surface. Also, literature data were given from only one test
person, which is not conclusive enough for statistical
reliability.
13
Fabrics made of staple fiber yarns usually show
drying speeds higher than fabrics made of filament
yarns, and the double-yarn processed fabrics have
higher drying speeds in comparison to single-yarn processed fabrics due to the pore structure in yarns and
fabrics. The drying speed DS is related to the amount of
captured water within the fabric, as human skin is
related to the blood circulation and water loss.
The daily insensible water loss for humans varying
between 22 and 89 g m2 h1, exceeding up to 1000 g
m2 h1 during physical performance, with rates up to
about 1600 g m2 h1, lies within the same range as the
surfacial drying speed SDS of fabrics varying between
917 and 1593 g m2 h1, which indicates that the fabrics
ensure continuous wicking and moisture transfer for
sweating simulation.
The statistical analysis of measured properties in
Figure 2 shows correlations between the geometrical
properties such as thickness ht, which are related to
evaporative resistance Ret and drying speed DS with
coefficients of determinations R2 of about and higher
than 0.5, also the diameter of stitch pores dptt is related
to thermal resistance Rct with an R2 of about 0.5.
Thermal conductivity , water content WC, wetting
time t and radius r do not correlate with geometrical
properties with R2 being below 0.2. And thus, it is not
possible to accurately predict these parameters using
the geometrical properties because the mean variation
is very large. In the resultant regression equations
(equations 12–14) the thermal and evaporative resistance Rct and Ret, and drying speed DS of the knitted
fabrics are predominantly affected by the most influential geometrical properties, such as thickness ht, diameter of stitch pores dptt and stitch pore number nptt,
whereby the p-value smaller than 0.05 proved their significance. The stitch pore diameter dptt significantly
contributes to thermal resistance Rct with a coefficient
of 6:50105 , whereby the thickness ht significantly
contributed to the evaporative resistance Ret with a
coefficient of 4.48 and also the drying speed DS with
a value of 12.58. Finally, the R2 values were higher or
equal to 0.60 indicating a medium-strength correlation.
On the other hand, the RMSD values were very close to
the mean standard deviation of the measured values
(RMSD ¼ 0.002 m2 K W1, mSD ¼ 0.001 m2 K W1 for
Rct; RMSD ¼ 0.15 m2 Pa W1, mSD ¼ 0.10 m2 Pa W1
for Ret; RMSD ¼ 1.3 g h1, mSD ¼ 1.1 g h1 for DS,
respectively) and smaller by several orders than the
measured values (below 6%, 9% and 15% for Rct,
Ret and DS, respectively), which indicates relatively
strong predictive power of the developed regression
equations.
The different sweating devices have different sweating systems, e.g. water supply through horizontally
oriented porous metal plates, sweat glands distributed
14
on vertically oriented cylindrical shapes or water supply
on artificial skin draped on measurement surface.
Therefore, the artificial skin properties need to be
chosen individually to match the requirements of the
specific devices. Furthermore, pre-wetted skins used on
non-sweating devices require high water content WC
and a low drying speed DS to provide a longer steady
state for measuring, whereas a fast drying speed DS
and low water content WC are beneficial to mimic artificial skin used on devices including continuous water
supply. The requirements are related to thermal properties (conductivity, thermal and evaporative resistance)
and moisture behavior (water content, drying speed)
investigated on the fabrics, and are influenced by
geometrical characteristics and the wettability of the
fabrics.
The fabric sample PES_19f30_SET made of fiber
polymer PES and textured yarn (FDY) provides
highly desirable thermal and moisture-management
properties to mimic human skin to be used as prewetted artificial skin on measurement devices. The
hydrophilic fabric presents low wetting times and a
large wetting radius in comparison to a high water
content of about 21 g g1 and a relatively low surfacial
drying speed of 1150 g m2 h1. However, to mimic
human skin for use as artificial skin on measurement
devices with a continuous water supply, the fabric
type made of non-textured yarn (TOY) is preferred.
Therefore, the hydrophilic fabric sample PES_
28f48_GL made of fiber polymer PES meets the
requirements with low wetting times, a large
wetting radius and even a low water content of
about 11 g g1 and related fast surfacial drying speed
of 957 g m2 h1.
A comparison of the results on proposed fabric skins
with existing fabric skins (25) will be subjected to
further studies. However, the focus of this study was
to create fabrics with specific properties based on the
developed theoretical model and in this way, the fabrics’ performance can be improved by changing production parameters.
The authors Postle and Munden55,56 investigated
how the fiber and pore structure in relaxed plainknitted fabrics are influenced by flexural and torsional
strains in fibers, which implies a function of force distribution that varies the effective yarn diameter and
mesh pore diameter. That approach has not been considered in this study yet because the equivalent pore
diameter dptt was calculated from the relevant geometric modeling of knitting structures. Additionally,
further investigations are necessary on artificial
skins based on weft knitted fabrics fashioned as tightfitted garments dressed on cylindrically shaped and
upright positioned devices, e.g torso or sweating manikin in stretched conditions. Therefore, a redistribution
Textile Research Journal 0(00)
of geometrical characteristics such as stitch pore diameter dptt and thickness ht might occur and affect the
thermal and moisture behavior.
Conclusion
The tested fabrics satisfy all the requirements to match
the anatomical properties of the human skin, such as
thickness, pore size and number of pores, used as artificial skin on measurement devices measuring thermal
and moisture properties.
The regression analysis offers the possibility to assess
the impact of geometrical properties thickness ht, stitch
pore diameter dptt and number of stitch pores nptt on
measured heat and mass transfer in different sweating
phases and to identify the most influential predictors.
The stitch pore diameter dptt significantly increased
thermal resistance Rct although with an extremely low
value, whereby the thickness ht significantly increased
the evaporative resistance Ret and also the drying speed
DS. The knitting structure provides geometrical structure to transfer mass for simulating the wicking
process.
Two fabric samples are proposed for different types
of sweating systems. The fabric sample PES_
19f30_SET is considered for the use as artificial skin
on measurement devices without any water supply due
to its high wettability and potential to absorb and
store high amounts of water rapidly in combination
with its low drying speed. The fabric sample
PES_28f48_GL is recommended for artificial skin on
measurement devices with a water supply due to its
high wettability and the ability to absorb water
rapidly in combination with low water content and
related fast drying speed.
Those two ideal fabric samples provide possible
alternatives to previously used artificial skin for measurement devices such as guarded hotplates and thermal manikins to determine thermodynamic properties
of clothing textiles.
Acknowledgements
The authors wish to thank scientists from BioMemTex at
Empa, Dr Emel Mert and Barbara Koelblen for the introduction to and quality assurance of the fabric tests, Dr Agnieszka
Dabrowska for consultation on the properties of hydrated
human skin. We further acknowledge the support rendered
by the scientist from Laboratory Knitting Technology at
DITF, Uwe Röder, for the technical development of the knitting fabrics.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Mark et al.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this
article: This work was supported by Fundamental Research
Funds of the DITF German Institutes of Textile and Fiber
Research and Empa Swiss Federal Laboratories for Materials
Science and Technology.
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