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chemosensors
Article
Low-Cost Inkjet-Printed Temperature Sensors on Paper
Substrate for the Integration into Natural Fiber-Reinforced
Lightweight Components
Johanna Zikulnig 1, * , Mohammed Khalifa 2, Lukas Rauter 1, Herfried Lammer 2and Jürgen Kosel 1
Citation: Zikulnig, J.; Khalifa, M.;
Rauter, L.; Lammer, H.; Kosel, J.
Low-Cost Inkjet-Printed Temperature
Sensors on Paper Substrate for the
Integration into Natural
Fiber-Reinforced Lightweight
Components. Chemosensors 2021,9, 95.
https://doi.org/10.3390/
chemosensors9050095
Academic Editors: Emilia
Witkowska Nery and
Martin Jonsson-Niedziolka
Received: 24 March 2021
Accepted: 22 April 2021
Published: 27 April 2021
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1Silicon Austria Labs GmbH, Europastraße 12, 9524 Villach, Austria; lukas.rauter@silicon-austria.com (L.R.);
juergen.kosel@silicon-austria.com (J.K.)
2Wood Kplus—Kompetenzzentrum Holz GmbH, Klagenfurter Straße 87, 9300 St. Veit an der Glan, Austria;
mohammed.khalifa89@gmail.com (M.K.); h.lammer@wood-kplus.at (H.L.)
*Correspondence: johanna.zikulnig@silicon-austria.com
Abstract:
In a unique approach to develop a “green” solution for in-situ monitoring, low-cost inkjet-
printed temperature sensors on paper substrate were fully integrated into natural fiber-reinforced
lightweight components for which structural health monitoring is becoming increasingly impor-
tant. The results showed that the sensors remained functional after the vacuum infusion process;
furthermore, the integration of the sensors improved the mechanical integrity and stability of the
lightweight parts, as demonstrated by tensile testing. To verify the qualification of the printed sensors
for the target application, the samples were exposed to varying temperature and humidity conditions
inside of a climate chamber. The sensors showed linear temperature dependence in the temperature
range of interest (
−
20 to 60
◦
C) with a TCR ranging from 1.576
×
10
−3
K
−1
to 1.713
×
10
−3
K
−1
.
Furthermore, the results from the tests in humid environments indicated that the used paper-based
sensors could be made almost insensitive to changes in ambient humidity by embedding them into
fiber-reinforced lightweight materials. This study demonstrates the feasibility of fully integrating
paper-based printed sensors into lightweight components, which paves the way towards integration
of other highly relevant sensing devices, such as strain and humidity sensors, for structural health
monitoring of smart, sustainable, and environmentally compatible lightweight composite materials.
Keywords:
inkjet printing; temperature sensor; structural health monitoring; paper electronics;
sustainable sensors
1. Introduction
Current social and ecological challenges demand innovative solutions for the efficient
use of resources associated with the increased use of renewable raw materials. In the field
of lightweight construction, natural fiber-reinforced biopolymers are a promising candidate
for fulfilling the requirements for ecological compatibility while providing advanced mate-
rial performance [
1
–
3
]. Natural fibers, such as flax, hemp, or cotton, have been extensively
studied throughout recent years to evaluate their qualifications for mechanical reinforce-
ment of lightweight parts [
4
–
6
]. In order to further improve the performance and reliability
of the respective composites, structural health monitoring in the fields can be a powerful
tool for future material development [
7
,
8
]. Printed sensors on biocompatible substrates,
such as paper, are well-suited for the integration into the respective lightweight parts
during manufacturing, as paper is a low-cost, easily available, sustainable, and biologically
degradable material, and its application for printed electronics has been well-studied [
9
,
10
].
Speaking of sustainability, digital additive electronics manufacturing technologies, such
as inkjet printing, are considered to have a low environmental impact provided that eco-
friendly materials are used [
11
]. As illustrated in Figure 1, a sustainability cycle can be
established: paper-based printed sensors and natural fiber-reinforced lightweight compos-
ites consist of sustainable materials and are manufactured in an ecologically friendly way.
Chemosensors 2021,9, 95. https://doi.org/10.3390/chemosensors9050095 https://www.mdpi.com/journal/chemosensors
Chemosensors 2021,9, 95 2 of 14
By employing the sensors for structural health monitoring of the respective lightweight
parts, valuable performance data from the fields will be collected, generating new knowl-
edge, which can be used as a basis for improved material development to further enhance
the mechanical properties and the longevity of sustainable lightweight composites.
Chemosensors 2021, 9, x FOR PEER REVIEW 2 of 14
manufacturing technologies, such as inkjet printing, are considered to have a low envi-
ronmental impact provided that eco-friendly materials are used [11]. As illustrated in Fig-
ure 1, a sustainability cycle can be established: paper-based printed sensors and natural
fiber-reinforced lightweight composites consist of sustainable materials and are manufac-
tured in an ecologically friendly way. By employing the sensors for structural health mon-
itoring of the respective lightweight parts, valuable performance data from the fields will
be collected, generating new knowledge, which can be used as a basis for improved ma-
terial development to further enhance the mechanical properties and the longevity of sus-
tainable lightweight composites.
Figure 1. Cycle for sustainable material development enabled by low-cost resource-efficient sen-
sors for structural health monitoring.
Printed sensors have been presented as promising candidates for structural health
monitoring of large structures due to their low manufacturing costs, which consequently
enable the cost-efficient integration of a large number of sensors over an extended area.
As an example, Zymelka et al. [12] developed a screen-printed, temperature-compen-
sated, graphite-based strain sensor array for the structural health monitoring of large ar-
eas. They impressively demonstrated the functionality of their devices by installing them
on a highway bridge. Even 7 months and 1 year later, the sensors remained fully func-
tional and were capable of detecting and localizing cracks in the monitored bridge accu-
rately. In a different approach, Cook et al. [13] developed a passive inkjet-printed patch
antenna sensor, which enables the detection of crack formation, orientation, and shape by
means of resonant frequency shifts in the two resonant modes of the antenna. Zhang et al.
[14] presented the development and manufacturing of inkjet and screen-printed strain
sensors on polyethylene-terephthalate (PET) flexible substrates for the purpose of struc-
tural health monitoring in aircraft. While printed sensors have been proposed for struc-
tural health monitoring before [12–14], a full integration making the sensor an inherent
part of the composite material to monitor, as presented in this work, is highly innovative
and distinctive of previous works in this field.
The current case study builds upon the results presented in [15], where inkjet-printed
resistive temperature sensors on commercial uncoated paper substrates have already been
fabricated and characterized while evaluating their suitability for the proposed task of
structural health monitoring of natural fiber-reinforced lightweight composites. The un-
coated paper substrate does not contain any synthetic layers, which makes it fully ecolog-
ical while being low-cost. However, considering the inkjet printing process, several chal-
lenges arise due to the highly porous, rough, and fibrous nature of uncoated paper com-
pared to, e.g., most polymer-based substrates, resulting in lower conductivity, as well as
reproducibility, and inhomogeneous layers [16,17]. On the other hand, the inkjet printer
ink is partially absorbed, due to its low viscosity, increasing the adhesion and, conse-
quently, the stability and durability of the printed films [18]. Results from [15] indicated
that the bare sensors would be applicable for the task of structural health monitoring of
Figure 1.
Cycle for sustainable material development enabled by low-cost resource-efficient sensors
for structural health monitoring.
Printed sensors have been presented as promising candidates for structural health
monitoring of large structures due to their low manufacturing costs, which consequently
enable the cost-efficient integration of a large number of sensors over an extended area. As
an example, Zymelka et al. [
12
] developed a screen-printed, temperature-compensated,
graphite-based strain sensor array for the structural health monitoring of large areas. They
impressively demonstrated the functionality of their devices by installing them on a high-
way bridge. Even 7 months and 1 year later, the sensors remained fully functional and
were capable of detecting and localizing cracks in the monitored bridge accurately. In
a different approach, Cook et al. [
13
] developed a passive inkjet-printed patch antenna
sensor, which enables the detection of crack formation, orientation, and shape by means
of resonant frequency shifts in the two resonant modes of the antenna. Zhang et al. [
14
]
presented the development and manufacturing of inkjet and screen-printed strain sensors
on polyethylene-terephthalate (PET) flexible substrates for the purpose of structural health
monitoring in aircraft. While printed sensors have been proposed for structural health
monitoring before [
12
–
14
], a full integration making the sensor an inherent part of the com-
posite material to monitor, as presented in this work, is highly innovative and distinctive
of previous works in this field.
The current case study builds upon the results presented in [
15
], where inkjet-printed
resistive temperature sensors on commercial uncoated paper substrates have already
been fabricated and characterized while evaluating their suitability for the proposed task
of structural health monitoring of natural fiber-reinforced lightweight composites. The
uncoated paper substrate does not contain any synthetic layers, which makes it fully
ecological while being low-cost. However, considering the inkjet printing process, several
challenges arise due to the highly porous, rough, and fibrous nature of uncoated paper
compared to, e.g., most polymer-based substrates, resulting in lower conductivity, as well
as reproducibility, and inhomogeneous layers [
16
,
17
]. On the other hand, the inkjet printer
ink is partially absorbed, due to its low viscosity, increasing the adhesion and, consequently,
the stability and durability of the printed films [
18
]. Results from [
15
] indicated that the bare
sensors would be applicable for the task of structural health monitoring of natural fiber-
reinforced lightweight materials, in particular rotor blades of small wind turbines. In the
temperature range of interest for manufacturing of the lightweight parts (vacuum infusion
process and thermal post-curing 20 to 80
◦
C), the sensors showed good linear temperature
dependence, minimal hysteresis, and low baseline drift. In an extended temperature range
(
−
25 to 150
◦
C) and when being exposed to humid environments (20 to 80% rH), the sensor
performance worsened, which can mainly be attributed to fiber swelling, as the porous
Chemosensors 2021,9, 95 3 of 14
paper substrate absorbs a large amount of ambient humidity, leading to a mechanical
deterioration (cracking) of the printed structure [19].
Building on the results presented in [15], two paper-based temperature sensors were
integrated into natural fiber-reinforced composites as part of the present work. After the
integration, the samples were exposed to varying relevant temperature and humidity
conditions inside of a climate chamber.
2. Materials and Methods
2.1. Inkjet-Printed Temperature Sensor
The sensors were fabricated in accordance with [
15
] using inkjet printing of Ag-
nanoparticle ink (Sicrys 150-TM119, PVNanocell, Migdal Ha’Emek, Israel) on uncoated
paper substrate. The sensors were designed as meander-line structures based on other
resistive sensor designs reported in the literature [
20
–
25
] with a total sensing area of
45 ×25.5 mm
as well as line width and spacing of 0.5 mm. A PIXDRO LP50 (Süss Microtec
SE, Garching, Germany) system with a Spectra SE-128 AA 128 (Fujifilm Dimatix Inc., Santa
Clara, CA, USA) 30 pL print head assembly at a resolution of 500
×
500 dpi was used.
Two layers of ink had to be applied, as the first layer is largely absorbed by the paper
fibers, which results in poor electrical conductivity, while the second layer on top of the
first can be considered as comparatively homogeneous, as illustrated in Figure 2a,b for
one and two printed layers, respectively. Between the two printing passes, no sintering
was performed, as intermediate sintering can lead to interface structures in multilayer
printing, which reduces the homogeneity of the conductive path [
26
]. After drying the
two layers, the sensor structures were sintered. Due to the low thermal tolerance of the
used paper substrate, photonic curing (Pulse Forge 1200, NovaCentrix, Austin, Texas.
USA) at overall energy of 2.1 J/cm
2
was employed. As part of previous works, the sheet
resistance of the same substrate-ink combination (2-layers) was evaluated using Van-der-
Pauw’s method [
17
,
27
]. According to this study, the median sheet resistance of the printed
temperature sensors was expected to be around 60 m
Ω
/
, which equals to a specific
resistivity
ρ
= 12
µΩ×
cm (7.6
×
bulk silver) at a layer thickness of 2
µ
m (as obtained
by SEM imaging, see Figure 2c). However, this applied only in small regions, where the
layer was rather homogeneous. As soon as a larger area was observed, the influence of the
surface roughness and porosity of the paper substrate became obvious, which reduced the
integrity of the printed Ag layer, as illustrated in Figure 2d.
Amongst other things, the electrical resistance of electrical conductors strongly de-
pends on the temperature. For many materials, including silver, and in a specified tem-
perature range, the relationship between the temperature T and the resistance R(T) can be
approximated linearly using the following Equation (1) [28]:
R(T)=R0×[1+α×(T−T0)] (1)
R
0
equals the resistance at a defined temperature T
0
(as part of the present work, it was
defined as T
0
= 20
◦
C). The material-dependent constant
α
corresponds to the temperature
coefficient of resistivity (TCR), which is a positive value for metals (e.g., silver), implying
that the sensor’s resistance increases with increasing temperature.
2.2. Sensor Integration and Characterization
To determine the influence of the paper sensor on the mechanical properties of the
fiber-reinforced lightweight structure, a blank paper was integrated into the samples.
Tensile test was carried out using a universal testing machine (Zwick Roell, Z020, Ulm,
Germany) according to EN ISO 527-5:1997 standard [
29
]. The specimens were prepared in
the form of rectangular strips of size 250 mm
×
25 mm
×
2 mm (L
×
b
×
h). At least five
specimens were tested for each configuration.
Chemosensors 2021,9, 95 4 of 14
Chemosensors 2021, 9, x FOR PEER REVIEW 4 of 14
(a) (b)
(c) (d)
Figure 2. Optical microscopy image (20×) of (a) one and (b) two sintered inkjet-printed Ag-layers on the paper substrate;
(c) SEM image of a crack in the printed layer, which reveals a layer thickness of approximately 2 µm; (d) optical microscopy
image: cross-section of printed layer on paper substrate.
2.2. Sensor Integration and Characterization
To determine the influence of the paper sensor on the mechanical properties of the
fiber-reinforced lightweight structure, a blank paper was integrated into the samples. Ten-
sile test was carried out using a universal testing machine (Zwick Roell, Z020, Ulm, Ger-
many) according to EN ISO 527-5:1997 standard [29]. The specimens were prepared in the
form of rectangular strips of size 250 mm × 25 mm × 2 mm (L × b × h). At least five speci-
mens were tested for each configuration.
For the sensor integration, silver wires were attached to the sensors using fast set
epoxy adhesive (Araldite, Huntsman International LLC, Houston, Texas, USA). To ensure
a good electrical connection between the sensor and wire, silver paste (Oegussa, Vienna,
Austria) was applied prior to the application of the adhesive.
Figure 3a,b. Therefore, a composite laminate was reinforced with six layers of unidi-
rectional natural fibers, i.e., flax fibers (Eco-technilin, Valliquerville, France) of 180 g/cm2
each. Bio-based epoxy (Sicomin, SR info green 810, Châteauneuf-les-Martigues, France)
was used along with the hardening agent in the ratio of 100:33 (based on wt% of resin). In
brief, all the materials, including natural fibers and the sensor, were arranged into the
mold. Subsequently, the mold was covered with vacuum foil. Once the vacuum was cre-
ated, the resin was introduced into the mold through inlets using tubing. As soon as suf-
ficient resin was sucked into the mold, the inlets were closed, and the resin was allowed
to cure at room temperature, followed by post-curing it for 16 h at 80 °C in an oven. The
electrical resistance of the sensor (before and after the infusion) was monitored to study
Figure 2.
Optical microscopy image (20
×
) of (
a
) one and (
b
) two sintered inkjet-printed Ag-layers on the paper substrate;
(
c
) SEM image of a crack in the printed layer, which reveals a layer thickness of approximately 2
µ
m; (
d
) optical microscopy
image: cross-section of printed layer on paper substrate.
For the sensor integration, silver wires were attached to the sensors using fast set
epoxy adhesive (Araldite, Huntsman International LLC, Houston, Texas, USA). To ensure
a good electrical connection between the sensor and wire, silver paste (Oegussa, Vienna,
Austria) was applied prior to the application of the adhesive.
Figure 3a,b. Therefore, a composite laminate was reinforced with six layers of unidi-
rectional natural fibers, i.e., flax fibers (Eco-technilin, Valliquerville, France) of 180 g/cm
2
each. Bio-based epoxy (Sicomin, SR info green 810, Châteauneuf-les-Martigues, France)
was used along with the hardening agent in the ratio of 100:33 (based on wt% of resin). In
brief, all the materials, including natural fibers and the sensor, were arranged into the mold.
Subsequently, the mold was covered with vacuum foil. Once the vacuum was created,
the resin was introduced into the mold through inlets using tubing. As soon as sufficient
resin was sucked into the mold, the inlets were closed, and the resin was allowed to cure at
room temperature, followed by post-curing it for 16 h at 80
◦
C in an oven. The electrical
resistance of the sensor (before and after the infusion) was monitored to study the effect of
resin on the electrical properties of the sensor. Figure 4a shows a bare paper sensor before
the electrical connection and the integration, while an integrated sensor sample as used for
the tests is illustrated in Figure 4b.
Chemosensors 2021,9, 95 5 of 14
Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 14
the effect of resin on the electrical properties of the sensor. Figure 4a shows a bare paper
sensor before the electrical connection and the integration, while an integrated sensor
sample as used for the tests is illustrated in Figure 4b.
Figure 3. Vacuum infusion process: (a) schematic representation; (b) digital photograph.
(a) (b)
Figure 4. (a) Printed temperature sensor on the paper substrate; (b) bottom view of integrated sen-
sor sample.
The sensors were then simultaneously exposed to varying environmental conditions
inside of a climatic chamber while recording the associated resistances. The sensor perfor-
mances were studied in the temperature range of interest (−20 to 60 °C), which covers the
range that the lightweight composites are expected to be exposed to during their useful
life in the fields when being integrated into, e.g., rotor blades for small wind turbines. Due
to the integration into the lightweight material, the sensor response time is expected to be
significantly larger than for the bare sensor. Therefore, in the first step, the response times
of the individual embedded sensors were determined by employing step response func-
tions with an increasing step size of 20 °C in the range between −20 °C and 60 °C and vice
versa.
In a subsequent test, the samples were exposed to ambient humidity levels between
20%rH and 90%rH at temperatures ranging from −20 to 60 °C in steps of 20 °C and the
performance compared to a non-encapsulated paper temperature sensor. The sensors’ re-
sistances were measured every 30 s using a Keithley Digital multimeter with a measure-
ment current of 1 mA at a pulse length of 0.002 s.
3. Results
3.1. Sensor Integration
Figure 3. Vacuum infusion process: (a) schematic representation; (b) digital photograph.
Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 14
the effect of resin on the electrical properties of the sensor. Figure 4a shows a bare paper
sensor before the electrical connection and the integration, while an integrated sensor
sample as used for the tests is illustrated in Figure 4b.
Figure 3. Vacuum infusion process: (a) schematic representation; (b) digital photograph.
(a) (b)
Figure 4. (a) Printed temperature sensor on the paper substrate; (b) bottom view of integrated sen-
sor sample.
The sensors were then simultaneously exposed to varying environmental conditions
inside of a climatic chamber while recording the associated resistances. The sensor perfor-
mances were studied in the temperature range of interest (−20 to 60 °C), which covers the
range that the lightweight composites are expected to be exposed to during their useful
life in the fields when being integrated into, e.g., rotor blades for small wind turbines. Due
to the integration into the lightweight material, the sensor response time is expected to be
significantly larger than for the bare sensor. Therefore, in the first step, the response times
of the individual embedded sensors were determined by employing step response func-
tions with an increasing step size of 20 °C in the range between −20 °C and 60 °C and vice
versa.
In a subsequent test, the samples were exposed to ambient humidity levels between
20%rH and 90%rH at temperatures ranging from −20 to 60 °C in steps of 20 °C and the
performance compared to a non-encapsulated paper temperature sensor. The sensors’ re-
sistances were measured every 30 s using a Keithley Digital multimeter with a measure-
ment current of 1 mA at a pulse length of 0.002 s.
3. Results
3.1. Sensor Integration
Figure 4. (a) Printed temperature sensor on the paper substrate; (b) bottom view of integrated sensor sample.
The sensors were then simultaneously exposed to varying environmental conditions
inside of a climatic chamber while recording the associated resistances. The sensor perfor-
mances were studied in the temperature range of interest (
−
20 to 60
◦
C), which covers the
range that the lightweight composites are expected to be exposed to during their useful life
in the fields when being integrated into, e.g., rotor blades for small wind turbines. Due
to the integration into the lightweight material, the sensor response time is expected to
be significantly larger than for the bare sensor. Therefore, in the first step, the response
times of the individual embedded sensors were determined by employing step response
functions with an increasing step size of 20
◦
C in the range between
−
20
◦
C and 60
◦
C and
vice versa.
In a subsequent test, the samples were exposed to ambient humidity levels between
20%rH and 90%rH at temperatures ranging from
−
20 to 60
◦
C in steps of 20
◦
C and the
performance compared to a non-encapsulated paper temperature sensor. The sensors’ resis-
tances were measured every 30 s using a Keithley Digital multimeter with a measurement
current of 1 mA at a pulse length of 0.002 s.
3. Results
3.1. Sensor Integration
Figure 5a shows the optical microscopy (VHX-7000 series, Keyence International,
Mechelen, Belgium) images illustrating the cross-section epoxy/flax fiber composite im-
pregnated with paper. Prior to the investigation, the surface of the composite was polished
using an automated polishing machine. The thickness of the laminate was ~2 mm, and
the paper thickness was 100
µ
m (Figure 5b). The images also indicate that the fibers and
the impregnated paper fused well with the epoxy matrix. The stress-strain curves of
epoxy-natural fiber composites with and without the integrated paper sheet are illustrated
in Figure 6a. The ultimate tensile strength of the composite without the integrated paper
was 172 MPa, while it increased to 240 MPa upon the integration of paper. The increase in
Chemosensors 2021,9, 95 6 of 14
the tensile properties indicates that the impregnation of paper contributed to the reduction
of fiber and surface defects, which might be due to the resin filling rich zones and by capti-
vating voids. In addition, the interfacial interactions between the matrix and paper might
have resulted in better stress transfer, consequently contributing to the augmentation of the
tensile strength. This indicates that the impregnation of paper does not reduce the integrity
and mechanical stability of the lightweight structure but that it even strengthens it [
30
,
31
].
Chemosensors 2021, 9, x FOR PEER REVIEW 6 of 14
Figure 5a shows the optical microscopy (VHX-7000 series, Keyence International,
Mechelen, Belgium) images illustrating the cross-section epoxy/flax fiber composite im-
pregnated with paper. Prior to the investigation, the surface of the composite was polished
using an automated polishing machine. The thickness of the laminate was ~2 mm, and the
paper thickness was 100 µm (Figure 5b). The images also indicate that the fibers and the
impregnated paper fused well with the epoxy matrix. The stress-strain curves of epoxy-
natural fiber composites with and without the integrated paper sheet are illustrated in
Figure 6a. The ultimate tensile strength of the composite without the integrated paper was
172 MPa, while it increased to 240 MPa upon the integration of paper. The increase in the
tensile properties indicates that the impregnation of paper contributed to the reduction of
fiber and surface defects, which might be due to the resin filling rich zones and by capti-
vating voids. In addition, the interfacial interactions between the matrix and paper might
have resulted in better stress transfer, consequently contributing to the augmentation of
the tensile strength. This indicates that the impregnation of paper does not reduce the
integrity and mechanical stability of the lightweight structure but that it even strengthens
it [30,31].
(a) (b)
Figure 5. (a) Optical microscopy image showing the cross-section of the epoxy/natural fiber composite laminate with pa-
per; (b) enlarged view of impregnated paper in the composite laminate.
Figure 6b shows the resistance change of one exemplary sensor during the fabrication
of the composite. To affirm, a J-type thermocouple was placed next to the sensor to probe
the temperature change during the process. When the uncured resin flowed over the sen-
sors, the electrical resistances of the sensors remained unchanged. After 60 min, the sen-
sors’ resistances increased slightly, which might be attributed to the increase in the tem-
perature of the resin because of the initiation of the exothermic curing reaction [32]. Sub-
sequently, the sensors’ resistances increased further; even after completion of the curing
process, the nominal resistance values irreversibly changed, which could be attributed to
fiber swelling due to absorption as well as thermal expansion and chemical shrinkage of
the epoxy during curing, leading to cracks in the printed structure.
Figure 5.
(
a
) Optical microscopy image showing the cross-section of the epoxy/natural fiber composite laminate with paper;
(b) enlarged view of impregnated paper in the composite laminate.
Chemosensors 2021, 9, x FOR PEER REVIEW 7 of 14
(a) (b)
Figure 6. (a) Tensile stress-strain curves of epoxy-natural fiber composite integrated with and without paper; (b) response
of two sensors during the vacuum infusion process of epoxy-natural fiber composite.
When measuring the temperature-dependent resistance, the obtained value can be
significantly affected by a local rise in temperature inside of the conductive traces due to
the current flow, commonly referred to as the Joule heating effect. For the given measure-
ment setup, the influence of Joule heating was empirically studied on the integrated sen-
sor sample. Figure 7 illustrates this effect at a constant ambient temperature of 25 °C, em-
ploying a measurement current of 1 mA and a signal length of 2 ms at a measurement
frequency of 4 Hz (Test 1); the resulting rise in resistance amounted to 0.3% of the nominal
resistance R
0
at 25 °C. In contrast to that, Joule heating did not affect the measurement
signal when the frequency was kept low (1 measurement every 30 s); therefore, in the
following tests, the resistance was measured at a frequency of 0.03 Hz.
Figure7. Resistance measurement over time at constant ambient temperature (25 °C) with a meas-
urement current of 1 mA and a signal length of 2 ms at a measurement frequency of 4 Hz (Test 1)
and 0.03 Hz (Test 2).
3.2. Thermal Sensor Characteristics
Two sensors were exposed to a temperature profile following a step response func-
tion with step sizes of 20 °C in the application-relevant range between −20 and 60 °C, as
illustrated in Figure 8c. All temperature responses followed the same exponential func-
tion, regardless of the absolute temperatures and whether they were rising or falling, as
Figure 6.
(
a
) Tensile stress-strain curves of epoxy-natural fiber composite integrated with and without paper; (
b
) response
of two sensors during the vacuum infusion process of epoxy-natural fiber composite.
Figure 6b shows the resistance change of one exemplary sensor during the fabrication
of the composite. To affirm, a J-type thermocouple was placed next to the sensor to probe
the temperature change during the process. When the uncured resin flowed over the
sensors, the electrical resistances of the sensors remained unchanged. After 60 min, the
sensors’ resistances increased slightly, which might be attributed to the increase in the
temperature of the resin because of the initiation of the exothermic curing reaction [
32
].
Subsequently, the sensors’ resistances increased further; even after completion of the curing
process, the nominal resistance values irreversibly changed, which could be attributed to
Chemosensors 2021,9, 95 7 of 14
fiber swelling due to absorption as well as thermal expansion and chemical shrinkage of
the epoxy during curing, leading to cracks in the printed structure.
When measuring the temperature-dependent resistance, the obtained value can be
significantly affected by a local rise in temperature inside of the conductive traces due
to the current flow, commonly referred to as the Joule heating effect. For the given mea-
surement setup, the influence of Joule heating was empirically studied on the integrated
sensor sample. Figure 7illustrates this effect at a constant ambient temperature of 25
◦
C,
employing a measurement current of 1 mA and a signal length of 2 ms at a measurement
frequency of 4 Hz (Test 1); the resulting rise in resistance amounted to 0.3% of the nominal
resistance R
0
at 25
◦
C. In contrast to that, Joule heating did not affect the measurement
signal when the frequency was kept low (1 measurement every 30 s); therefore, in the
following tests, the resistance was measured at a frequency of 0.03 Hz.
Chemosensors 2021, 9, x FOR PEER REVIEW 7 of 14
(a) (b)
Figure 6. (a) Tensile stress-strain curves of epoxy-natural fiber composite integrated with and without paper; (b) response
of two sensors during the vacuum infusion process of epoxy-natural fiber composite.
When measuring the temperature-dependent resistance, the obtained value can be
significantly affected by a local rise in temperature inside of the conductive traces due to
the current flow, commonly referred to as the Joule heating effect. For the given measure-
ment setup, the influence of Joule heating was empirically studied on the integrated sen-
sor sample. Figure 7 illustrates this effect at a constant ambient temperature of 25 °C, em-
ploying a measurement current of 1 mA and a signal length of 2 ms at a measurement
frequency of 4 Hz (Test 1); the resulting rise in resistance amounted to 0.3% of the nominal
resistance R
0
at 25 °C. In contrast to that, Joule heating did not affect the measurement
signal when the frequency was kept low (1 measurement every 30 s); therefore, in the
following tests, the resistance was measured at a frequency of 0.03 Hz.
Figure7. Resistance measurement over time at constant ambient temperature (25 °C) with a meas-
urement current of 1 mA and a signal length of 2 ms at a measurement frequency of 4 Hz (Test 1)
and 0.03 Hz (Test 2).
3.2. Thermal Sensor Characteristics
Two sensors were exposed to a temperature profile following a step response func-
tion with step sizes of 20 °C in the application-relevant range between −20 and 60 °C, as
illustrated in Figure 8c. All temperature responses followed the same exponential func-
tion, regardless of the absolute temperatures and whether they were rising or falling, as
Figure 7.
Resistance measurement over time at constant ambient temperature (25
◦
C) with a mea-
surement current of 1 mA and a signal length of 2 ms at a measurement frequency of 4 Hz (Test 1)
and 0.03 Hz (Test 2).
3.2. Thermal Sensor Characteristics
Two sensors were exposed to a temperature profile following a step response func-
tion with step sizes of 20
◦
C in the application-relevant range between
−
20 and 60
◦
C,
as illustrated in Figure 8c. All temperature responses followed the same exponential
function, regardless of the absolute temperatures and whether they were rising or falling,
as exemplarily illustrated for sensor 1 in Figure 8a,b. Consequently, the time-dependent
temperature T(t) can be modeled as:
T(t)=T0×1−e−t
τ(2)
And:
T(t)=T0×e−t
τ(3)
For increasing and decreasing temperature, respectively. The response time
τ
is a time
constant that defines the time that a sensor needs to reach 63.2% (
T(t)
T0=
0.632
)
of a sudden
temperature change under specified conditions. The sensors reached the actual temperature
(in the following, referred to as steady-state condition) within five times this time constant.
Both sensors showed a similar response time around
τ
= 11 min for a temperature change
of 20 ◦C, which means that the steady-state was reached after 5τ= 55 min.
Figure 8d shows the temperature-dependent changes in resistances of the three sensors
after having reached a steady state. From the graphic representation, a linear behavior was
observable without any hysteresis. From this data, the TCR could be calculated according
to Equation (1), as presented in Table 1.
Chemosensors 2021,9, 95 8 of 14
Chemosensors 2021, 9, x FOR PEER REVIEW 8 of 14
exemplarily illustrated for sensor 1 in Figure 8a,b. Consequently, the time-dependent tem-
perature T(t) can be modeled as:
T(t) = 𝑇×(1−𝑒
) (2)
And:
T(t) = 𝑇×(𝑒
) (3)
For increasing and decreasing temperature, respectively. The response time τ is a
time constant that defines the time that a sensor needs to reach 63.2% (()
= 0.632) of a
(a) (b)
(c) (d)
Figure 8. Response of sensor 1 to the step function: (a) rising temperature; (b) falling temperature; (c) sensor responses to
temperature step response function ranging from −20 to 60 °C with a step-size of 20 °C; (d) temperature response at steady
state (5τ).
Figure 8d shows the temperature-dependent changes in resistances of the three sen-
sors after having reached a steady state. From the graphic representation, a linear behavior
was observable without any hysteresis. From this data, the TCR could be calculated ac-
cording to equation 1, as presented in Table 1.
Table 1. Calculated temperature coefficients of resistance (TCR) of the sensors under test and the
nominal resistances R
0
at a temperature of 20 °C.
Sensor TCR in K
−1
R
0
(20 °C) in Ω
Sensor 1 1.576 × 10
−3
200.0
Figure 8.
Response of sensor 1 to the step function: (
a
) rising temperature; (
b
) falling temperature; (
c
) sensor responses to
temperature step response function ranging from
−
20 to 60
◦
C with a step-size of 20
◦
C; (
d
) temperature response at steady
state (5τ).
Table 1.
Calculated temperature coefficients of resistance (TCR) of the sensors under test and the
nominal resistances R0at a temperature of 20 ◦C.
Sensor TCR in K−1R0(20 ◦C) in Ω
Sensor 1 1.576 ×10−3200.0
Sensor 2 1.713 ×10−3140.6
3.3. Sensor Characteristics in Humid Environments (20%rH to 90%rH)
The sensor samples were then exposed to different humidity levels. First, the tem-
perature was kept constant at 20
◦
C, while the ambient humidity level was increased
from 40%rH to 90%rH (40%rH–60%rH–80%rH–90%rH) and vice versa. In the following
step, the temperature was increased from 20 to 60
◦
C in steps of 20
◦
C and subsequently
decreased down to 0
◦
C. At each temperature level, the relative humidity was increased
from 20%rH to 90%rH and then back to 20%rH, as illustrated in Figure 9a. As a compar-
ison, one bare paper temperature sensor was exposed to the same cycle (Figure 9b). At
20
◦
C, the laminated samples did not show any remarkable response to the changes in
ambient humidity, as illustrated in Figure 9c. For sensor 1, the change in average resistance
due to an increase in ambient humidity from 20% to 90% at a constant temperature of
20
◦
C amounted to
∆
(R/R
0
) = 0.001 (1
‰
), which equaled to a calculated temperature
measurement error of T
err
= 0.69
◦
C (Equation (1). At a temperature of 60
◦
C and when
increasing the humidity level from 20%rH to 90%rH, the mean resistance change of the
same sensor became larger. It equaled to
∆
(R/R
0
) = 0.0026 (2.6
‰
), which accordingly
resulted in a temperature measurement error of T
err
= 1.64
◦
C. However, this effect was
Chemosensors 2021,9, 95 9 of 14
reversible; after the humidity level was lowered again, the sensors recovered. To put those
results into relation, the standard deviation of the sensor data at different temperature
levels and humidity levels were calculated (Table 2for sensor 1). This calculation revealed
that even at 20 ◦C and 20%rH, the standard deviation equaled to 0.42‰ (Terr = 0.27 ◦C).
Chemosensors 2021, 9, x FOR PEER REVIEW 9 of 14
Sensor 2 1.713 × 10
−3
140.6
3.3. Sensor Characteristics in Humid Environments (20%rH to 90%rH)
The sensor samples were then exposed to different humidity levels. First, the tem-
perature was kept constant at 20 °C, while the ambient humidity level was increased from
40%rH to 90%rH (40%rH–60%rH–80%rH–90%rH) and vice versa. In the following step,
the temperature was increased from 20 to 60 °C in steps of 20 °C and subsequently de-
creased down to 0 °C. At each temperature level, the relative humidity was increased from
20%rH to 90%rH and then back to 20%rH, as illustrated in Figure 9a. As a comparison,
one bare paper temperature sensor was exposed to the same cycle (Figure 9b). At 20 °C,
the laminated samples did not show any remarkable response to the changes in ambient
humidity, as illustrated in Figure 9c. For sensor 1, the change in average resistance due to
an increase in ambient humidity from 20% to 90% at a constant temperature of 20 °C
amounted to ∆(R/R
0
) = 0.001 (1‰), which equaled to a calculated temperature measure-
ment error of T
err
= 0.69 °C (Equation (1). At a temperature of 60 °C and when increasing
the humidity level from 20%rH to 90%rH, the mean resistance change of the same sensor
became larger. It equaled to ∆(R/R
0
) = 0.0026 (2.6‰), which accordingly resulted in a tem-
perature measurement error of T
err
= 1.64 °C. However, this effect was reversible; after the
humidity level was lowered again, the sensors recovered. To put those results into rela-
tion, the standard deviation of the sensor data at different temperature levels and humid-
ity levels were calculated (Table 2 for sensor 1). This calculation revealed that even at 20
°C and 20%rH, the standard deviation equaled to 0.42‰ (T
err
= 0.27 °C).
(a) (b)
(c) (d)
Figure 9. (a) Sensor responses of encapsulated sensors to different humidity levels at 0 °C, 20 °C, 40 °C, and 60 °C (overall
test record); (b) sensor response of the non-encapsulated paper sensor to different humidity levels at 0 °C, 20 °C, 40 °C,
and 60 °C (overall test record); (c) sensor responses of encapsulated sensors at humidity levels changing from 20%rH to
90%rH and vice versa at a constant ambient temperature of 20 °C; (d) temperature-dependent change in resistance of
sensor 1 at 20%rH and 90%rH.
Figure 9.
(
a
) Sensor responses of encapsulated sensors to different humidity levels at 0
◦
C, 20
◦
C, 40
◦
C, and 60
◦
C (overall
test record); (
b
) sensor response of the non-encapsulated paper sensor to different humidity levels at 0
◦
C, 20
◦
C, 40
◦
C, and
60
◦
C (overall test record); (
c
) sensor responses of encapsulated sensors at humidity levels changing from 20%rH to 90%rH
and vice versa at a constant ambient temperature of 20
◦
C; (
d
) temperature-dependent change in resistance of sensor 1 at
20%rH and 90%rH.
Table 2.
Temperature-dependent change in resistance
∆
R at different temperature and humidity
levels and calculated standard deviation s of sensor 1.
Temperature in ◦C∆R at 20%rH s(∆R) at 20%rH ∆R at 90%rH s(∆R) at 90%rH
60 1.0634 0.00045 1.0660 0.00010
40 1.0312 0.00037 1.0330 0.00021
20 0.9990 0.00042 1.0001 0.00008
In addition, when comparing the sensor data after temperature equilibrium was
reached (5
τ
= 55 min) at 20%rH and 90%rH, both sensors showed a linear temperature
dependence, as exemplarily illustrated for sensor 1 in Figure 9d. Compared to that, the
non-laminated paper reference temperature sensor heavily responded to ambient humidity
levels, as observed in previous works [
15
]. After increasing the relative humidity from
20%rH to 90%rH and decreasing it back to 20%rH at a constant temperature of 20
◦
C, the
nominal resistance irreversibly increased by 10%, as illustrated in Figure 9b. The sensor
Chemosensors 2021,9, 95 10 of 14
degraded even more during the course of the test, resulting in a change in resistance of
around 40% by the end of the cycle.
To further study the dependence of the sensor signal on the ambient humidity, strain
gauges were applied to the sample surface to monitor potential warping and deformation
at a constant ambient temperature of 60
◦
C and humidity levels jumping from 20%rH to
90%rH. As illustrated in Figure 10, the signal of the strain gauge revealed that the sample
experienced deformation due to changes in the ambient humidity level, which equally
influenced the signal of the embedded temperature sensor due to geometrical deformation
of the silver sensor structure (piezoresistivity).
Chemosensors 2021, 9, x FOR PEER REVIEW 10 of 14
Table 2. Temperature-dependent change in resistance ∆R at different temperature and humidity
levels and calculated standard deviation s of sensor 1.
Temperature in °C ∆R at 20%rH s(∆R) at 20%rH ∆R at 90%rH s(∆R) at 90%rH
60 1.0634 0.00045 1.0660 0.00010
40 1.0312 0.00037 1.0330 0.00021
20 0.9990 0.00042 1.0001 0.00008
In addition, when comparing the sensor data after temperature equilibrium was
reached (5τ = 55 min) at 20%rH and 90%rH, both sensors showed a linear temperature
dependence, as exemplarily illustrated for sensor 1 in Figure 9d. Compared to that, the
non-laminated paper reference temperature sensor heavily responded to ambient humid-
ity levels, as observed in previous works [15]. After increasing the relative humidity from
20%rH to 90%rH and decreasing it back to 20%rH at a constant temperature of 20 °C, the
nominal resistance irreversibly increased by 10%, as illustrated in Figure 9b. The sensor
degraded even more during the course of the test, resulting in a change in resistance of
around 40% by the end of the cycle.
To further study the dependence of the sensor signal on the ambient humidity, strain
gauges were applied to the sample surface to monitor potential warping and deformation
at a constant ambient temperature of 60 °C and humidity levels jumping from 20%rH to
90%rH. As illustrated in Figure 10, the signal of the strain gauge revealed that the sample
experienced deformation due to changes in the ambient humidity level, which equally
influenced the signal of the embedded temperature sensor due to geometrical defor-
mation of the silver sensor structure (piezoresistivity).
Figure 10. Strain measurement on the sample surface; the sample experiences deformation due to
the ambient humidity, which also influences the signal of the embedded temperature sensor.
4. Discussion
The ultimate tensile strength of the composite increases noticeably upon the integration
of paper, as illustrated in Figure 6a. This could be explained by a strong interfacial bonding
between the cellulose paper and the epoxy groups. Further, the resin might have held tightly
between the intervenes of the fiber, which could have provided better stress transfer and
interfacial bonding. This result suggests that the integrated sensor not only monitors the
condition of the composite but also improves the mechanical properties [33–35].
However, the resistance of the sensor irreversibly increases during the integration
into the lightweight parts (see Figure 6b), which could be due to the resin trapped between
intervenes of the paper. During the vacuum infusion process, the liquid binder is first
absorbed by the paper substrate, which leads to fiber expansion and, consequently, might
damage the printed structure. In addition, a piezoresistive change in resistance can be
induced due to the deformation of the sample. When the curing is induced, the epoxy
Figure 10.
Strain measurement on the sample surface; the sample experiences deformation due to
the ambient humidity, which also influences the signal of the embedded temperature sensor.
4. Discussion
The ultimate tensile strength of the composite increases noticeably upon the integra-
tion of paper, as illustrated in Figure 6a. This could be explained by a strong interfacial
bonding between the cellulose paper and the epoxy groups. Further, the resin might
have held tightly between the intervenes of the fiber, which could have provided bet-
ter stress transfer and interfacial bonding. This result suggests that the integrated sen-
sor not only monitors the condition of the composite but also improves the mechanical
properties [33–35].
However, the resistance of the sensor irreversibly increases during the integration into
the lightweight parts (see Figure 6b), which could be due to the resin trapped between
intervenes of the paper. During the vacuum infusion process, the liquid binder is first
absorbed by the paper substrate, which leads to fiber expansion and, consequently, might
damage the printed structure. In addition, a piezoresistive change in resistance can be
induced due to the deformation of the sample. When the curing is induced, the epoxy
expands due to an exothermic reaction. During the transformation of the resin from
liquid to solid, it experiences chemical shrinkage again, which is a direct consequence
of crosslinking of epoxy, which leads to residual stresses and warpage. As a result, the
printed sensor structure might have deteriorated to some extent, resulting in an irreversible
increase of resistance [36,37].
As illustrated in Figure 7, the resistance measurement value can be significantly af-
fected by a local rise in temperature inside of the conductive traces due to the measurement
current flow, commonly referred to as the Joule heating effect. At a current of 1 mA with
2 ms pulses and a frequency of 4 Hz, the resulting rise in resistance amounts to 0.3% of
the nominal resistance R
0
at 25
◦
C, which would potentially lead to a measurement error
of about 2
◦
C. This effect can be neglected at the low measurement frequency used in
the present work (1 measurement every 30 s). Therefore, depending on the application
requirements, the Joule heating effect has to be taken into account, and, if necessary, dif-
ferent measurement parameters (current, pulse duration, or frequency) might need to be
employed. Compared to the results from [
15
], the samples show a large response time
of
τ= 11 min
for a temperature change of
∆
T = 20
◦
C, as illustrated in Figure 8a,b, which
Chemosensors 2021,9, 95 11 of 14
can be attributed to the thermally insulating properties of the fiber-reinforced lightweight
embedding material. This offers novel opportunities for employing the printed sensors as a
tool to determine the thermal properties of the embedding material as part of future mate-
rial optimizations. In addition, the response time of the sensor can neither be considered as
a limitation for the practical application task of structural health monitoring, as proposed
in the present work, as ultimately the temperature inside of the material is supposed to
be monitored. This can be exploited for conducting highly targeted measurements of the
actual material temperature at defined device positions and material depths.
The calculated TCR of the sensors under test ranges from 1.576
×
10
−3
K
−1
to
1.713 ×10−3K−1
for sensor 1 and sensor 2, respectively. This is due to the differences in
the nominal resistances (140
Ω
to 200
Ω
), which can be explained by manufacturing-related
variations. Although both sensors are manufactured using the same process, they are not
fabricated as one single batch under the exact same conditions. Changes in the ink com-
position over time, such as nanoparticle agglomeration and evaporation of solvents, can
have an impact on the printing results. Furthermore, the printhead is prone to degradation
over time, especially when not being operated continuously, leading to partial clogging
of nozzles, resulting in different drop sizes and, consequently, affecting the amount of
conductive ink that is deposited. Last but not least, the high porosity and fibrousness of
the used paper substrate lead to poor reproducibility, as discussed for the used paper type
in [
17
]. Already in previous works, the bare temperature sensors on the same substrate
show large variations of the nominal resistances of around 15% [
15
]. Furthermore, the
sensors experience increases in resistance due to the absorption and thermal deformation
of the epoxy during curing. This implies that each sensor would have to be calibrated
individually to gain absolute measurement results for structural health monitoring in the
fields. While this would not always be needed, the printing processes used for prototyping
in research cannot be compared to well-controlled industrial manufacturing conditions
that are achievable in high throughput production lines. Furthermore, industrial vacuum
infusion processes for the fabrication of lightweight parts also provide a higher level of
reproducibility regarding the amount of binder used and the processing temperature. To
put the results of this work into perspective, previous publications reported TCR values
for printed silver lines ranging from 6.52
×
10
−4
K
−1
[
23
] to 2.19
×
10
−3
K
−1
[
20
], which
indicates that
α
is rather dependent on the individual processing conditions than on the
metallic sensing material used. However, the results from the present work are well aligned
with the TCRs obtained in [
15
], which lie between 1.630
×
10
−3
K
−1
and 1.705
×
10
−3
K
−1
.
The results from the tests in a humid environment indicate that the paper-based
sensors are almost insensitive to changes in ambient humidity due to the embedding into
fiber-reinforced lightweight materials, as illustrated in Figure 9a. The tests are conducted
at temperatures above freezing point, as only very little water is dissolved in the air below
0
◦
C, for which an impact on the sensor performance is not expected. Furthermore, this
would be outside the operating range of the used climatic chamber. The results for sensor
1 reveal that a change in the ambient humidity level from 20%rH to 90%rH at 20
◦
C and
60
◦
C leads to temperature errors of T
err
= 0.69
◦
C and T
err
= 1.64
◦
C, respectively. Since
this effect is reversible, it might be explained by piezoresistivity. Strain measurements
using commercial strain gauges on the sample surface indeed indicate that the sample
mechanically deforms in humid environments (Figure 10). However, the standard deviation
at steady-state and a constant humid environment (20%rH) and temperature (20
◦
C)
results as well in a temperature measurement error of T
err
= 0.27
◦
C. To put those results
into relation, a commercial Pt100 temperature sensing element of class C might have a
temperature tolerance of around 1.2
◦
C at 60
◦
C, as specified by DIN EN 60751:2009-05 [
38
].
It can be assumed that those limitations in measurement accuracy are negligible for the
proposed application of structural health monitoring of natural fiber-reinforced lightweight
materials during their useful life.
The bare paper-based temperature sensor is shown to be extremely sensitive towards
humidity, as already reported in [
15
] and illustrated in Figure 9b. Since this effect is irre-
Chemosensors 2021,9, 95 12 of 14
versible, it might be explained by fiber expansion due to the absorption of liquids, leading
to cracks in the printed layer. Hence, the results of this work indicate that embedding the
sensors can be a highly effective method to avoid these degradation mechanisms, paving
the way towards extremely low-cost robust paper-based sensors for humid environments.
5. Conclusions
In this work, low-cost inkjet-printed temperature sensors on the paper substrate
were fully integrated into natural fiber-reinforced lightweight components. Subsequently,
the samples were exposed to varying relevant temperature (
−
20 to 60
◦
C) and humidity
(20%rH to 90%rH) conditions inside of a climate chamber.
The integration of the sensors had the purpose of providing a sustainable solution
for structural health monitoring. In addition, it improved the mechanical integrity and
stability of the lightweight part, as indicated from the stress-strain curves of epoxy-natural
fiber composite integrated with and without paper. The results also showed that all
sensors under test remained functional after the vacuum infusion process; however, the
nominal resistance increased irreversibly, which might be attributed to fiber swelling due
to absorption, as well as thermal expansion and chemical shrinkage of the epoxy, which
caused deterioration of the printed structures.
When being exposed to varying temperature and humidity conditions inside of a cli-
mate chamber, both sensors showed a linear temperature dependence and no hysteresis in
the temperature range of interest (
−
20 to 60
◦
C) with a TCR ranging from
1.576 ×10−3K−1
to 1.705
×
10
−3
K
−1
. The results from the tests in a humid environment indicated that the
paper-based sensors had become almost insensitive to changes in ambient humidity after
embedding them into fiber-reinforced lightweight composites.
The paper-based sensors were shown to be suitable for the integration into natural
fiber-reinforced biopolymer-based lightweight composites, creating a potential platform for
sustainable structural health monitoring. The complete integration of the devices, making
them an inherent part of the composite material, to monitor could be considered as a highly
innovative approach. Nonetheless, the monitoring of temperature alone did not provide
sufficient information on the structural health status of the components; still, invaluable
sensing data for a deeper understanding of degradation modes due to environmental
influences could be obtained. In addition, the use of wired external devices for the readout
of sensor data could not be considered as convenient for the proposed application. This
drawback becomes particularly relevant when the monitoring of mobile components,
such as rotor blades for small wind turbines, etc., is desired. Therefore, as part of future
works, wireless readout options will be studied, as well as the applicability of different
additively manufactured sensors; for e.g., humidity and strain sensing to obtain further
valuable performance parameters of smart, sustainable, and environmentally compatible
lightweight composite materials for the future.
Author Contributions:
Conceptualization, J.Z. and L.R.; methodology, J.Z., M.K., and L.R.; formal
analysis, J.Z., M.K., L.R., H.L., and J.K.; data curation, J.Z. and M.K.; writing—original draft prepara-
tion, J.Z., M.K., and L.R.; writing—review and editing, J.Z., M.K., L.R., H.L., and J.K.; supervision,
H.L. and J.K.; project administration, L.R. All authors have read and agreed to the published version
of the manuscript.
Funding:
This work has been conducted as part of the research project Smarter Leichtbau 4.1 funded
by the European Regional Development Fund (ERDF).
Chemosensors 2021, 9, x FOR PEER REVIEW 13 of 14
further valuable performance parameters of smart, sustainable, and environmentally
compatible lightweight composite materials for the future.
Author Contributions: Conceptualization, J.Z. and L.R.; methodology, J.Z., M.K., and L.R.; formal
analysis, J.Z., M.K., L.R., H.L., and J.K.; data curation, J.Z. and M.K.; writing—original draft prepa-
ration, J.Z., M.K., and L.R.; writing—review and editing, J.Z., M.K., L.R., H.L., and J.K.; supervision,
H.L. and J.K.; project administration, L.R. All authors have read and agreed to the published version
of the manuscript.
Funding: This work has been conducted as part of the research project Smarter Leichtbau 4.1 funded
by the European Regional Development Fund (ERDF).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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15. Zikulnig, J.; Hirschl, C.; Rauter, L.; Krivec, M.; Lammer, H.; Riemelmoser, F.; Roshanghias, A. Inkjet printing and
characterisation of a resistive temperature sensor on paper substrate. Flex. Print. Electron. 2019, 4, 015008.
16. Siegel, A.C.; Phillips, S.T.; Dickey, M.D.; Lu, N.; Suo, Z.; Whitesides, G.M. Printable Electronics: Foldable Printed Circuit Boards
on Paper Substrates. Adv. Funct. Mater. 2009, 20, 28–35.
17. Zikulnig, J.; Roshanghias, A.; Rauter, L.; Hirschl, C. Evaluation of the Sheet Resistance of Inkjet-Printed Ag-Layers on Flexible,
Uncoated Paper Substrates Using Van-der-Pauw’s Method. Sensors 2020, 20, 2398.
18. Öhlund, T.; Örtegren, J.; Forsberg, S.; Nilsson, H.-E. Paper surfaces for metal nanoparticle inkjet printing. Appl. Surf. Sci. 2012,
259, 731–739.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Chemosensors 2021,9, 95 13 of 14
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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