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Simultaneously Optimize the Response Speed and Sensitivity of Low Dimension Conductive Polymers for Epidermal Temperature Sensing Applications


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Low dimension poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS) has been applied as resistor-type devices for temperature sensing applications. However, their response speed and thermal sensitivity is still not good enough for practical application. In this work, we proposed a new strategy to improve the thermal sensing performance of PEDOT: PSS by combined micro/nano confinement and materials doping. The dimension effect is carefully studied by fabricating different sized micro/nanowires through a low-cost printing approach. It was found that response speed can be regulated by adjusting the surface/volume (S/V) ratio of PEDOT: PSS. The fastest response (<3.5 s) was achieved by using nanowires with a maximum S/V ratio. Besides, by doping PEDOT: PSS nanowires with Graphene oxide (GO), its thermo-sensitivity can be maximized at specific doping ratio. The optimized nanowires-based temperature sensor was further integrated as a flexible epidermal electronic system (FEES) by connecting with wireless communication components. Benefited by its flexibility, fast and sensitive response, the FEES was demonstrated as a facile tool for different mobile healthcare applications.
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published: 19 March 2020
doi: 10.3389/fchem.2020.00194
Frontiers in Chemistry | 1March 2020 | Volume 8 | Article 194
Edited by:
Weiwei Wu,
Xidian University, China
Reviewed by:
Soong Ju Oh,
Korea University, South Korea
Han Jin,
Shanghai Jiao Tong University, China
Xuexin Duan
Specialty section:
This article was submitted to
a section of the journal
Frontiers in Chemistry
Received: 27 September 2019
Accepted: 02 March 2020
Published: 19 March 2020
Zhou C, Tang N, Zhang X, Fang Y,
Jiang Y, Zhang H and Duan X (2020)
Simultaneously Optimize the
Response Speed and Sensitivity of
Low Dimension Conductive Polymers
for Epidermal Temperature Sensing
Applications. Front. Chem. 8:194.
doi: 10.3389/fchem.2020.00194
Simultaneously Optimize the
Response Speed and Sensitivity of
Low Dimension Conductive Polymers
for Epidermal Temperature Sensing
Cheng Zhou, Ning Tang, Xiaoshuang Zhang, Ye Fang, Yang Jiang, Hainan Zhang and
Xuexin Duan*
State Key Laboratory of Precision Measuring Technology & Instruments, Tianjin University, Tianjin, China
Low dimension poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT: PSS)
has been applied as resistor-type devices for temperature sensing applications. However,
their response speed and thermal sensitivity is still not good enough for practical
application. In this work, we proposed a new strategy to improve the thermal
sensing performance of PEDOT: PSS by combined micro/nano confinement and
materials doping. The dimension effect is carefully studied by fabricating different sized
micro/nanowires through a low-cost printing approach. It was found that response speed
can be regulated by adjusting the surface/volume (S/V) ratio of PEDOT: PSS. The fastest
response (<3.5 s) was achieved by using nanowires with a maximum S/V ratio. Besides,
by doping PEDOT: PSS nanowires with Graphene oxide (GO), its thermo-sensitivity can
be maximized at specific doping ratio. The optimized nanowires-based temperature
sensor was further integrated as a flexible epidermal electronic system (FEES) by
connecting with wireless communication components. Benefited by its flexibility, fast
and sensitive response, the FEES was demonstrated as a facile tool for different mobile
healthcare applications.
Keywords: temperature sensor, nanowires, fast response, healthcare, tumor, smartphone
Real-time and continuous measurement of human body local epidermal temperature enables a
better tracking of personal health status such as local wounds infection (Celeste et al., 2013),
subcutaneous tumor (Sudharsan et al., 1999), as well as monitoring of body activities, since many
diseases and physiological behaviors will cause local changes in body epidermal temperature (Deng
and Liu, 2004; Helmy and Rizkalla, 2008; Ng, 2009; Li et al., 2017). Recently, wearable epidermal
electronic systems (EESs) based on flexible devices have opened new frontiers in the measurement
of body local temperature (Gao et al., 2014; Takei et al., 2015). Due to their soft and flexible nature,
they can be directly attached to the human skin and conform to the body, local temperature can
Zhou et al. Skin-Attachable Temperature Sensor
be detected anytime and anywhere (Webb et al., 2013; Zhang
et al., 2016). Besides, the use of soft substrate enables high
mechanical durability in different bending conditions, thus
their responses will not be influenced by the movement of
the body (Wu and Haick, 2018; Chang et al., 2019). However,
due to the materials limitation, current EESs based thermal
meters still suffer from the issues of responding slowly or not
sensitive enough to body temperature change, which in fact
cannot provide real-time precise temperature tracking. Hence the
development of fast and sensitive response wearable temperature
sensors which can track personal health status is still required.
Among all EES temperature sensor, PEDOT: PSS based
resistor-type electronics have been largely applied since the
PEDOT: PSS itself is very sensitive to temperature changes
(Culebras et al., 2016) and the soft nature of such organic
electronics ensures its excellent performance as flexible sensors
(Lipomi et al., 2012). The thermosensitive mechanism of PEDOT:
PSS can be explained by the structural change of PEDOT: PSS
induced by the temperature change which eventually alters the
conductivity of PEDOT: PSS (Takano et al., 2012; Zhou et al.,
2014; Vuorinen et al., 2016). Previous studies on PEDOT: PSS
based temperature sensors were focused on doping the PEDOT:
PSS with other materials to enhance the device’s response to
temperature (Honda et al., 2014; Oh et al., 2018). However,
understanding the control parameters regarding their response
time to temperature change is less covered.
In this work, in order to simultaneously optimize the
response speed and sensitivity of conductive polymers for
epidermal temperature sensing applications such as quick
temperature detecting or real-time precise temperature tracking.
we proposed a strategy to combine the micro/nano confinement
with the materials doping to simultaneously optimize the
response time and sensitivity of PEDOT: PSS based resistor-
type temperature sensor. Low dimension PEDOT: PSS wires
from a few micrometers down to sub-100 nm in diameters were
fabricated using a low-cost micro/nanoscale printing approach
(Gates et al., 2005; Massi et al., 2006; Duan et al., 2010;
Tang et al., 2019). Their sensitivity and response time to
temperature were compared (Scheme 1). Then, graphene oxide
(GO) was selected to dope the PEDOT: PSS and the doping
ratios were carefully optimized to enhance their temperature
sensing performance. Hence the fabricated sensor under such
strategy is responding both fast and sensitive enough to detective
minute temperature change and track temperature in real-time.
The sensor shows ultra-fast response and sensitive to body
temperature change. Furthermore, the sensor is also used to
achieve temperature monitoring of subcutaneous tumors in
mice and by detecting the minute change of body temperature
in mice to test the effect of drugs. Given the prominent
mechanical and sensing properties, a homemade wearable system
based on the temperature sensor was further developed to
achieve a live and wireless transmission of the signals to a
smartphone using Bluetooth assisted communication. These
results demonstrate that this fast response skin-attachable
nanowires-based temperature sensor has great potential as a
wearable bioelectronic for application in medical diagnosis and
mobile healthcare.
Reagents and Materials
mr-I T85 was purchased from Micro-Resist. PEDOT: PSS
aqueous suspension (1.3 wt%) with a conductivity of 1 S cm1
was purchased from Sigma-Aldrich. GO was purchased from
Chengdu Organic Chemical company and further sonicated
for 30 min to form a uniform and stable dispersion with
a concentration of 7 mg/ml. The compound solution was
obtained by stirring PEDOT: PSS aqueous suspension and GO
aqueous suspension for 10 min. Polydimethylsiloxane (PDMS)
was purchased from Dow corning.
Fabrication of Flat PDMS Molds With
Micro/Nano Grooves
First, casting the liquid prepolymer of PDMS base and the curing
agent in a 10:1 (w/w) ratio onto a silicon wafer with two kinds
of silicon micro-ridges (3 µm in width and 1 µm in height and
spacing 5 µm, 5 µm in width and 1 µm in height and spacing
10 µm, respectively). After the bubbles generated during the
stirring process disappear, the wafer with the uncured PDMS was
put into hot oven. After curing at 80C for 40 min, the PDMS
with microgrooves was cooled to room temperature and peeled
off from the silicon wafer. Then, the PDMS with microgrooves
was cut into pieces with the boundary of the microgrooves. The
fabrication of the molds with nanogroove can be seen on previous
case articles performed in our lab (Tang et al., 2019).
Formation of Micro/Nano Channels
The molds with micro/nano groove were first treated with oxygen
plasma (10 mTorr;10 sccm O2; 10 W; 15 s) to facilitate the contact
between the molds and PET substrates. The molds were then
bonded to the PET substrates which were also treated with
oxygen plasma (20 mTorr;20 sccm O2; 20W; 15s). The molds
can conformal contact with the PET substrates because of the
oxygen plasma treated procedure and soft properties of the molds
and PET substrates. Hence the micro/nano grooves changed into
micro/nano channels after contacting.
Electrodes Fabrication
The Au electrodes were then prepared by thermal evaporation
on the PET substrate with nanowires under self-made copper
shadow masks to promote the electrical connection. The
electrodes were with pad length in 7 mm, width in 300 µm
and spacing in 300 µm. The electrodes of microwires-based
devices were silver paste at an interval of 3 mm approximately.
Silver paste and silver wires were used to have further
electrical contacts.
Atomic force microscope (AFM), Scanning electron microscope
(SEM) and the confocal laser scanning microscope (CLSM)
images were carried out with Dimension Icon (Bruker), FEI
FP 2031/12 Inspect F50 and OLYMPUS OLS5000, respectively.
Electrical measurements with a constant voltage of +1 V
were performed with a semiconductor analyzer (Agilent
Technologies, B1500A).
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Zhou et al. Skin-Attachable Temperature Sensor
SCHEME 1 | Morphological model shows the different S/V ratio of PEDOT: PSS (A) microwires and (B) nanowire. Morphological model of the (C) thermosensitive and
(D) doping mechanism of PEDOT: PSS. (E) Carton shows the fabrication process of micro/nanowires.
Animal Tumor Model
Liver cancer tumor cells was injected into KM mice to establish
an animal tumor model.
Temperature Monitoring Based on
The homemade wearable watch-type system was
fabricated by integrating the AD5933/STM32/HC-05
and their peripheral circuits on a flexible circuit board.
AD5933/STM32/HC-05 control resistance measurement,
communication, and data transmission, respectively.
An Android application program was developed on
the smartphone to receive real-time data from the
wearable watch-type system and plot the responses on the
screen. Comparing the response change to its resistance
calibration at standard temperature, the temperature was
successfully measured.
Micro/nanowires Fabrication and
To study the structure effects on the thermal sensing performance
of PEDOT: PSS, here we design three different sized PEDOT:
PSS wires, where their widths vary from 4 µm to 70 nm
(Scheme 1). Hence different S/V ratios are achieved. The
micro/nanowires were fabricated by an adapted soft lithography
which does not require any cleanroom facilities (Gates et al.,
2005; Massi et al., 2006; Duan et al., 2010; Tang et al., 2019).
Scheme 1E shows the schematic of fabricating procedure
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Zhou et al. Skin-Attachable Temperature Sensor
FIGURE 1 | (A) Optical microscope, (B) SEM, (C) AFM, and (D) CLSM, image of microwires @ 4µm. (E) Optical microscope, (F) SEM, (G) AFM, and (H) CLSM,
image of microwires @ 2 µm. (I) Optical microscope, (J) SEM, (K) AFM, and (L) CLSM, image of nanowires.
of micro/nanowires. After the micro/nanochannels (The
preparation process is shown in the Experimental Methods)
have completely formed, the PEDOT: PSS aqueous solution
were dropped to the margins of channels. The solution filled
the channels spontaneously by traction of the Laplace pressure.
After the solvent has completely evaporated, removed the
mold from PET and leaved the micro/nanowires on the PET
substrate. Then, metal electrodes were fabricated on the side
of the micro/nanowires through evaporation as the contact
electrodes, thus resistor-type sensors have been achieved. The
comparison between our method and current available methods
for fabricating nanowires in the format of a table is shown in
Supplementary Information.
Figure 1 shows the optical microscope, Scanning Electron
Microscope (SEM), Atomic Force Microscope (AFM) and
Confocal Laser Scanning Microscope (CLSM) images of the
micro/nanowires with different sizes, respectively. These results
show that the microwires have the same height (0.7 µm)
but with different width (4 µm, 2 µm). The nanowires have
an average width of 70 nm and an average height of 59 nm.
Thus, the S/V ratio of microwires @ 4 µm, microwires @
2µm and nanowires can be calculated as follow: 1.928, 2.429,
and 45.52. As can be seen in the AFM images, the height
of the microwires sides are higher than the middle part,
which is due to the microchannels are treated with O2plasma
hence are more hydrophilic, thus the aqueous solution tends
to stick to the wall before evaporates, which resulting in
this phenomenon.
Temperature Sensing
To study the structure effects on their temperature sensing
behavior, we compared the response value and response speed
of the three PEDOT: PSS based resistor-type temperature sensors
with different S/V ratio. Their resistance were measured under
a temperature range from 30 to 80C with an interval of 10C
(Figures 2A–C). The response value is defined as:
where R0and R are the resistance values at 30C
and the set temperature. And TCR is defined as
TCR =((RR0)/R0)/1T=δR/1T. The results show
that the resistance of the three devices decrease linearly as
the temperature rises and their sensitivity (slope of the liner
fitting) are rather similar (Figure 2D). The δR of PEDOT: PSS
microwires @ 4 µm, microwires @ 2 µm and nanowires-based
temperature sensor between 30 and 80C (303 and 353 K)
are 0.377, 0.378, and 0.379, so the TCR of them can
be calculated as 0.007545, 0.007579, and 0.007599 K1.
We then tested their response speed to temperature change
by attaching or removing these devices to human arm which
has a constant temperature. As shown in Figures 2E–G, the
currents show increase or decrease after touching or removing
the devices from the human skin. Compare with their thermal
sensitivity, very differently, the devices with different S/V ratio
show quite different temperature response speed (defined as the
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Zhou et al. Skin-Attachable Temperature Sensor
FIGURE 2 | Relative resistance changes depending on the temperature with increments of 10C of the (A) microwires @ 4µm, (B) microwires @ 2 µm and (C)
nanowires, based temperature sensor. (D) Sensitivity of the sensors with different S/V ratio. Real-time response of (E) microwires @ 4µm, (F) microwires @ 2 µm and
(G) nanowires, based temperature sensor. (H) Total response time of the sensors with different S/V ratio.
time required to reach 90% of the zenith response). The response
and recovery time of the microwires @ 4 µm, microwires @
2µm and nanowires-based temperature sensor were 6.9 and
25.2 s, 5.8 and 23.8 s, 3.5 and 13.4 s, respectively. As compared
in Figure 2H, a clear trend can be observed that the response
speed increases with the increase of the S/V ratio, especially
the nanowires with a maximum S/V ratio shows an ultra-fast
response speed. This behavior can be explained by the electron
transportation and thermal sensing mechanism of PEDOT:
PSS which is very related to its microstructure. As shown in
Scheme 1C, PEDOT: PSS has a typical core-shell structure
in which the core is PEDOT nanocrystal and surrounded by
PSS-rich shell. The insulating PSS boundaries with a strong
hygroscopic ability have a major effect to the overall conductance
of the PEDOT: PSS (Takano et al., 2012; Zhou et al., 2014). When
temperature rising, the water molecules absorbed in the PSS
boundaries will be partially released, which leads to the shrinkage
of PSS boundaries and results in a decrease of the distance
between adjacent PEDOT (Scheme 1C). This will facilitate the
electron transportation between the PEDOT domains and results
the decrease of the resistance of PEDOT: PSS (Zhou et al., 2014;
Vuorinen et al., 2016). The response of PEDOT: PSS film are
shown in Figure S1 in Supplementary Information. By confining
the PEDOT: PSS from 2D thin film to 1D nanowire (increase
of its S/V ratio), PSS boundaries can be fully exposed to the
external environment, thus increase its thermal conduction
and facilitates the water evaporation, which will increase its
response speed to temperature change. Besides, the electron
transport efficiency within the 1D nanowire is improved as well.
Other nanomaterials-based resistive temperature sensors also
have showed such advantages of nanomaterials in temperature
detecting (Joh et al., 2018; Sehrawat et al., 2018; Bang et al., 2019;
Cui et al., 2019). However, reducing the materials dimension
will not influence the deformation ratio of the PSS boundaries,
which is related to the amount of the water absorbed, thus the
change of S/V ratio will not influence the thermal sensitivity of
Doping Effect
As discussed above, nanoscale confinement strategy can enhance
the response speed of the PEDOT: PSS to temperature and
PEDOT: PSS nanowires with the highest S/V ratio show the
fattest response to temperature change which is rather important
to develop as temperature sensor. Next, we focus on improving
its thermal sensitivity. As reported before, doping of PEDOT:
PSS with other temperature-sensitive materials such as graphene
(Trung et al., 2014, 2016, 2018) can further increase its response
to temperature (Honda et al., 2014; Oh et al., 2018). Here,
we applied the graphene doping strategy to further adjust the
temperature sensitivity of PEDOT: PSS nanowires. Since the
mixtures are required to be dispersed in aqueous solution for
capillary filling, the oxidation form of graphene (GO) was used,
which contains many oxygen-containing functional groups, such
as hydroxyl, epoxide and carboxyl groups and ensures its good
solubility in aqueous solution (Wang et al., 2009; Chen and
Li, 2012; Lee et al., 2012; Dwandaru et al., 2019). The doped
mixtures were characterized by FT-IR (Figure S2 in Supporting
Information), the mixtures have the same characteristics as
PEDOT: PSS and contain the specific bands of GO, which proves
that the two materials have been fully mixed and the chemical
structures of PEDOT: PSS and GO are well-maintained. We then
optimized the GO doping ratio to understand its thermal sensing
effect. I–V curves of the nanowires constructed by mixtures with
different mixing ratios (PEDOT: PSS/GO, V/V) were measured
under temperature range from 30 to 80C with an interval of
10C. As shown in Figure 3A, the GO doping has a conspicuous
influence on the thermal sensitivity of the PEDOT: PSS. The
nanowire device with mixing ratio of 13:1 shows the highest
temperature response. This can be explained by the fact that GO
is a kind of relatively hydrophilic material, thus GO should be
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Zhou et al. Skin-Attachable Temperature Sensor
FIGURE 3 | (A) Relative resistance changes of the sensors with different mixing ratios at different temperatures. (B) Schematic diagram of PEDOT: PSS nanowires
doped with opportune, insufficient and excessive amount of GO; Real-time response of the nanowire sensors with a mixing ratio of (C) 13:1 and (D) 20:1, after
attached to and removal off the human arm.
mostly adsorbed on the hydrophilic PSS when it is mixed with
PEDOT: PSS. Hence a little bit of GO will not affect the overall
conductivity of PEDOT: PSS due to PSS does not participate in
conductivity of PEDOT: PSS. However, GO has excellent thermal
conductivity (Teng et al., 2011; Yao et al., 2016), hence doping
GO can affect the thermal sensitivity of PEDOT: PSS. When the
mixing ratio is 13:1 (The upper schematic of Figure 3B), the
GO flakes are fully filled in the PSS and the gap between the
adjacent PEDOT: PSS nanoparticles. Such composite materials
show higher temperature sensitivity compared to the less, more
or no GO filling. If the mixing ratio is higher than 13:1, such as
15:1, which means the amount of GO flakes is less than that of
13:1, hence the GO flakes are not adequately filled in the PSS and
the gap between adjacent PEDOT: PSS nanoparticles(The middle
schematic of Figure 3B), leading to the temperature sensitivity
of this ratio lower than that of 13:1 but higher than non-doped
pure PEDOT: PSS. If the mixing ratio is lower than 13:1, such
as 10:1, under such mixing ratio, the amount of GO is larger
than that of 13:1, hence the connection between adjacent PEDOT:
PSS nanoparticles will be affected (The bottom schematic of
Figure 3B), leading to the response of this ratio lower than that of
13:1. Furthermore, when the ratio is further reduced, such as 1:1,
which means the proportion of GO is far more than that of 13:1,
hence the connection between the PEDOT: PSS nanoparticles
will be affected severely, leading to the response of this ratio
is even lower than that of non-doped pure PEDOT: PSS. We
also explored whether the GO doping would affect the response
speed of PEDOT: PSS nanowires. As shown in Figures 3C,D,
compared with the pure PEDOT: PSS nanowires (Figure 2E), the
response time and recovery time of these GO doped nanowires
have no obvious difference. It proves that the GO doping did
not affect the response speed of PEDOT: PSS nanowires but
only enhance the thermal sensitivity. Thus, by combining the
nanoconfinement and GO doping strategy, the response speed
and thermal sensitivity can be simultaneously optimized. The
13:1 GO doped nanowires are used to prepare the temperature
sensor for rest of the applications.
Repeatability and Stability
After optimization the structure and the materials, we then
fabricated the optimized Go doped PEDOT: PSS nanowires on
a flexible PET substrate to facilitate their epidermal sensing
applications. The repeatability and stability of the nanoscale
flexible sensors were tested, especially for their hysteretic
behavior (Han et al., 2018). Figure 4A shows the hysteresis of the
nanowires response to temperature with a heating and cooling
cycle, which indicates a very minor hysteresis (0.0046). Next,
we evaluated the repeatability of the nanowires-based sensor by
applying the sensor for multiple tests of touch to and removal
from the skin (Figure 4B). The stable performance demonstrates
that the devices have good repeatability and stability. The sensors
were then kept under ambient air for 1 months and their
response to temperature change were measured again. The
results in Figure S3 in the Supporting Information show that
their temperature response after 1 months is slightly changed
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Zhou et al. Skin-Attachable Temperature Sensor
FIGURE 4 | (A) Relative resistance changes depending on the temperature with increments/decrements of 1C. (B) Real-time response of a nanowire sensor after
attached to and removal off the human arm for three times.
FIGURE 5 | (A) IR thermograms and picture(middle) of the temperature sensor attached on the head of five volunteers. Resistance of the device and measured skin
temperature of (B) five volunteers and (C) a volunteer before and after 30 push-ups.
in comparison with the as-fabricated sensors, which prove the
long-term stability of the PEDOT: PSS nanowires in ambient air.
Skin Temperature Detection
We then tested the nanowires-based temperature sensor for
skin temperature detection. Five volunteers were chosen to
use the sensors for monitoring their local skin temperature by
attaching these devices on their heads (Figure 5A). The good
mechanical property and ultra-thin size of the flexible sensor
facilitate its attachment to human skin and hence ensure the
reliable temperature measurement. According to the results of
the previous fitting, the corresponding skin temperature of five
volunteers can be calculated. As shown in Figure 5B, the skin
temperature of the five volunteers is 35.12C, 34.86C, 34.71C,
35.05C, 35.36C, respectively. The results are consistent with
the results obtained by thermocouple which proves the accuracy
of the device (35.1C, 34.7C, 34.7C, 35.0C, 35.2C). Since
the skin temperature will change during strenuous or long-term
exercise, we apply the temperature sensor to track the volunteer
exercise status. The sensor was attached to the head of a volunteer
to monitor the subtle temperature changes during exercise. The
normal skin temperature of the volunteer’s temple was measured
to be approximately 34.93C. It increased to approximately
35.37C after doing 30 push-ups (see Figure 5C). These results
demonstrate that the nanowire-based temperature sensors can be
effectively applied for rapidly and accurately tracking of human
skin temperature.
Mobile Healthcare Based on Nanoscale
Many diseases and physiological behaviors will cause local
changes in body epidermal temperature, hence real-time and
continuous measurement of the local skin temperature could
enable a better tracking of personal health status (Deng and
Liu, 2004; Helmy and Rizkalla, 2008; Ng, 2009; Li et al.,
2017). Based on the nanoscale flexible temperature sensor,
a wearable FEES was further developed by integrating the
nanowires with commercial electronic components to enable
the wireless communication. An Android application program
was further developed and installed on a smartphone to
directly receive and process the sensing signals in real-
time. As shown in Figure S4, the FEES can be worn on
the wrist for continuously monitoring the skin temperature.
Figure 6A shows the screenshot of the smartphone interacting
with the FEES system by touching or removing the system
from the skin, which demonstrates their rapid response to
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Zhou et al. Skin-Attachable Temperature Sensor
FIGURE 6 | A screenshot of the smartphone when the sensor was attached on (A) a human arm, (B) a mouse after intraperitoneal injection of adrenaline and a
mouse after hypodermic injection of adrenaline. (C) Picture of the mouse with a subcutaneous tumor. (D) Skin temperature distribution of subcutaneous tumor and
eight points near it measured by the nanowires-based EES.
human skin temperature. The system was further applied to
compare the administration route of drugs by monitoring
the subtle skin temperature changes after intraperitoneal and
hypodermic injection of adrenaline to a mouse (Figure 6B). It
clearly shows that after intraperitoneal injection of adrenaline,
the resistance of the device did not change significantly,
indicating that body temperature of the mouse was constant.
While after hypodermic injection, the resistance of the sensor
was slowly and continuously decreased, indicating that body
temperature of the mouse was slowly increasing, which are
consistent with the existing studies (Maling et al., 1979). These
results demonstrate the feasibility of the FEES system in real
temperature monitoring applications.
Subcutaneous tumors, as a kind of common tumor, will
change the metabolic activity of the lesion area and in turn
change the normal temperature distribution on the skin surface
(Sudharsan et al., 1999). Benefited by the fast response and
high thermal sensitivity, the EES could provide a neat tool to
monitoring skin temperature outside the subcutaneous tumors
in a new way. Figure 6D shows the skin temperature distribution
around the tumor area detected by attaching the FEES on
different place of the skin (Figure 6C). The results indicate that
the temperature in the tumor area is significantly lower than
the surrounding area, which is consistent with the results of
existing studies (Konerding and Steinberg, 1988). Meanwhile,
the results detected by the FEES are more accurate than IR
thermograms (inset in Figure 6C), and this method provides
the possibility of real-time monitoring. It also shows that the
sensor has great potential to be made into temperature sensor
arrays, which can detect spatial mapping of skin temperature
so that the arrays could provide a feasible method to judge
the diffusion area and monitor the deterioration status of
the subcutaneous tumors efficiently and conveniently. All
these results demonstrate that this EES has great potential
as a wearable bioelectronic for mobile medical diagnosis and
healthcare applications.
In this work, we developed a strategy by combining the
micro/nano confinement with materials doping to enable
simultaneous optimization of the response speed and sensitivity
of low dimension conductive polymers. By confining the PEDOT:
PSS into nanowires and doping with GO at optimized doping
ratio, ultra-fast response to temperature change (<3.5 s) and
maximized thermal-sensitivity is achieved. Well-defined sub-
100 nm nanowires were fabricated on flexible substrates using
a low-cost nanoscale printing approach which were further
integrated as a functional skin-attachable flexible epidermal
electronic system (FEES) to enable a live and wireless
temperature sensing. The developed FEES were applied for
different physiological behaviors and diseases monitoring by
recording the real-time skin temperature changes.
All datasets generated for this study are included in the
article/Supplementary Material.
Frontiers in Chemistry | 8March 2020 | Volume 8 | Article 194
Zhou et al. Skin-Attachable Temperature Sensor
The animal study was reviewed and approved by the ethics
committee of the Institute of Radiation Medicine Chinese
Academy of Medical Sciences.
CZ developed the ideas of the study, carried out most of the
experiments, and also wrote most parts of the manuscript. NT
and XZ also developed some ideas of the study and participated in
some experiments, and also reviewed the manuscript. YF, YJ, and
HZ participated in some experiments of the study and reviewed
the manuscript. XD developed some ideas and improved the
ideas other authors developed and also wrote parts of the
manuscript. All authors read and approved the final manuscript.
This work was supported by National Natural Science
Foundation of China (NSFC Nos. 61674114, 91743110,
21861132001), National Key R&D Program of China
(2017YFF0204604, 2018YFE0118700), Tianjin Applied Basic
Research and Advanced Technology (17JCJQJC43600), the
Foundation for Talent Scientists of Nanchang Institute
for Microtechnology of Tianjin University, and the 111
Project (B07014).
The Supplementary Material for this article can be found
online at:
Bang, J., Lee, W. S., Park, B., Joh, H., Woo, H. K., Jeon, S., et al. (2019).
Highly sensitive temperature sensor: ligand-treated ag nanocrystal thin films
on PDMS with thermal expansion strategy. Adv. Functional Mater. 29:1903047.
doi: 10.1002/adfm.201903047
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Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Copyright © 2020 Zhou, Tang, Zhang, Fang, Jiang, Zhang and Duan. This is an
open-access article distributed under the terms of the Creative Commons Attribution
License (CC BY). The use, distribution or reproduction in other forums is permitted,
provided the original author(s) and the copyright owner(s) are credited and that the
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with these terms.
Frontiers in Chemistry | 10 March 2020 | Volume 8 | Article 194
... The E-tattoos have great healing efficiency and can 100% healed in just 0.3 s, and there are no significant changes in the sensing performance before and after self-healing. To improve the response speed, Zhou et al., proposed a new strategy to improve the thermal sensing performance of PEDOT: PSS by combining micro/nano confinement and materials doping through fabricating different sized micro/nanowires [95]. They found that the response speed can be regulated by adjusting the surface/volume (S/V) ratio of PEDOT:PSS. ...
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Wound healing is a complex tissue regeneration process involving many changes in multiple physiological parameters. The pH and temperature of a wound site have long been recognized as important biomarkers for assessing wound healing status. For effective wound management, wound dressings integrated with wearable sensors and systems used for continuous monitoring of pH and temperature have received much attention in recent years. Herein, recent advances in the development of wearable pH and temperature sensors and systems based on different sensing mechanisms for wound status monitoring and treatment are comprehensively summarized. Challenges in the areas of sensing performance, infection identification threshold, large-area 3-dimensional detection, and long-term reliable monitoring in current wearable sensors/systems and emerging solutions are emphasized, providing critical insights into the development of wearable sensors and systems for wound healing monitoring and management.
... Due to progress in materials science and manufacturing capacity, wearable devices [46][47][48], a type of portable electronic equipment that integrates sensors, have received considerable interest and displayed a great increase in both research and commercialization, including those integrated on watches, bracelets, glasses, clothes, shoes, socks, necklaces, and other accessories, as well as directly attached to the skin of the human body [49,50]. Wearable devices can monitor personal physiological statuses and environmental conditions through detecting health-related physical, chemical, and biological signals [51][52][53][54], such as respiration, temperature, heartbeat, blood pressure, exhaled breath, blood glucose [55][56][57][58][59][60]. Among various wearable sensors, the wearable gas sensor has become an emerging area of critical importance because there is an increasing demand for monitoring exhaled and surrounding air in real-time to achieve breath diagnosis and identify potential environmental hazards, respectively. ...
With the progress of intelligent and digital healthcare, wearable sensors are attracting considerable attention due to their portable and real-time monitoring capabilities. Among them, wearable gas sensors, which can detect both gas markers from the human body and hazardous gas from the environment, are particularly gaining tremendous interest. To ensure the gas sensors can be worn and carried easily, most of them were fabricated on flexible substrates. However, some traditional fabrication techniques of gas sensors such as lithography and chemical vapor deposited, are incompatible with most flexible substrates due to the flexible substrates cannot endure the harsh fabricated conditions, for instance, high temperature. Therefore, fabrication techniques for wearable gas sensors are extremely limited, thus a summary of which is necessary. Here, recent advances in the fabrication techniques of wearable gas sensors are presented. Fabrication techniques included coating techniques, printing techniques, spinning techniques, and transferring techniques are discussed in detail, respectively.
Flexible bifunctional sensors that mimic the function of human skin are essential as they provide critical interacting information with human and environment for intelligentization. However, the problem of interference between sensing signals from different stimuli and the deficiency of comfortability after long-time skin-attaching wearing still exist. Herein, we present an ultrathin and flexible bifunctional sensor based on two measurable parameters of pressure-induced supercapacitance and temperature-induced resistance with neglectable crosstalk between the two. The sensor consists of a planar iontronic supercapacitor underneath a serpentine resistor with assembly of total seven layers integrated on an electrospun thermoplastic polyurethane (TPU)-based nanofiber platform. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based sensing electrodes are patterned by direct ink writing with addition of graphene nanoflakes and Co3O4 nanoparticles to enhance the sensing performance. A high pressure sensitivity (147.19 kPa⁻¹, 0-7 kPa; 4.41 kPa⁻¹, 25-85 kPa) and a high temperature sensitivity (0.040 ℃⁻¹, 25-50 ℃; 0.002 ℃⁻¹, 50-100 ℃) are simultaneously achieved. The sensor also has the advantages of humidity inertness, waterproof ability and air permeability, which is breathable to obtain a wearing comfortability. Decoupled pressure-temperature sensing applications of detecting various subtle pressures with objects of different temperatures are comparatively demonstrated in a wireless and real-time mode. The proposed bifunctional sensor is potential in wearable healthcare monitoring and human-machine interaction.
Poly(3,4-ethylenedioxythiophene) (PEDOT), a very stable and biocompatible conducting polymer, and alginate (Alg), a natural water-soluble polysaccharide mainly found in the cell wall of various species of brown algae, exhibit very different but at the same complementary properties. In the last few years, the remarkable capacity of Alg to form hydrogels and the electro-responsive properties of PEDOT have been combined to form not only layered composites (PEDOT-Alg) but also interpenetrated multi-responsive PEDOT/Alg hydrogels. These materials have been found to display outstanding properties, such as electrical conductivity, piezoelectricity, biocompatibility, self-healing and re-usability properties, pH and thermoelectric responsiveness, among others. Consequently, a wide number of applications are being proposed for PEDOT-Alg composites and, especially, PEDOT/Alg hydrogels, which should be considered as a new kind of hybrid material because of the very different chemical nature of the two polymeric components. This review summarizes the applications of PEDOT-Alg and PEDOT/Alg in tissue interfaces and regeneration, drug delivery, sensors, microfluidics, energy storage and evaporators for desalination. Special attention has been given to the discussion of multi-tasking applications, while the new challenges to be tackled based on aspects not yet considered in either of the two polymers have also been highlighted.
HIGHLIGHTS • The morphology of PEDOT:PSS, including in the forms of aqueous dispersions, solid films, and hydrogels, is outlined, and the application potential of PEDOT:PSS hydrogels is described. • Fabrication techniques for PEDOT:PSS-based devices are introduced, including coating, printing, conventional lithography, and soft lithography. • The latest developments in four main categories of PEDOT:PSS-based physical sensors, for humidity, temperature, pressure, and strain, respectively, are introduced. • The development prospects for PEDOT:PSS, from materials to fabrication techniques to physical sensors, are outlined.
The rapid development of portable/wearable electronics proposes new demands for energy storage devices, which are flexibility, smart functions and long-time outdoor operation. Supercapacitors (SCs) show great potential in portable/wearable applications, and the recently developed flexible, smart and self-sustainable supercapacitors greatly meet the above demands. In these supercapacitors, conductive polymers (CPs) are widely applied due to their high flexibility, conductivity, pseudo-capacitance, smart characteristics and moderate preparation conditions. Herein, we'd like to introduce the CP-based flexible, smart and self-sustainable supercapacitors for portable/wearable electronics. This review first summarizes the flexible SCs based on CPs and their composites with carbon materials and metal compounds. The smart supercapacitors, i.e., electrochromic, electrochemical actuated, stretchable, self-healing and stimuli-sensitive ones, are then presented. The self-sustainable SCs which integrate SC units with energy-harvesting units in one compact configuration are also introduced. The last section highlights some current challenges and future perspectives of this thriving field.
A wound dressing is a sterile pad or compress that is used in direct contact with a wound to help it heal and prevent further issues or complications. Though wound healing is an intricate dynamic process that involves multiple biomolecular species, conventional wound dressings have a limited ability to provide timely information of abnormal conditions, missing the best time for early treatment. The current perspective presents and discusses the design and development of smart wound dressings that are integrated with multifunctional materials, wearable sensors and drug delivery systems as well as their application ranging from wound monitoring to timely application of therapeutics. The perspective also discusses the ongoing challenges and exciting opportunities associated with the development of wearable sensor‐based smart wound dressing and provide critical insights into wound healing monitoring and management. Wound management has become an important part of healthcare systems, as improper monitoring and/or treatment might cause health complications. The current perspective presents and discusses the design and development of smart wound dressings as well as their application ranging from wound monitoring to timely application of therapeutics.
Real‐time and accurate detection of localized temperature on curved human bodies is always highly demanded for health and medical applications. In this work, a bioinspired temperature‐sensitive yarn is designed and fabricated with different coverage degrees, in which pure metal fiber of platinum instead of conductive polymers is incorporated. The temperature sensitive yarn is observed with excellent temperature–resistance linearity (>0.99), high sensitivity (≈3.00 × 10⁻³ °C⁻¹), high repeatability (>1000 cycles), fast response, and low hysteresis (<3%). Most importantly and thanks to the bioinspired structure, this superelastic yarn supports extremely large strain of up to 300%, yet can still maintain original electrical integrity. It is further observed that the temperature coefficient of resistance and thermal response velocity are improved by the coverage degrees, which are attributed to the bioinspired helical structure. A bracelet based on the bioinspired yarn is then fabricated to detect temporal skin temperature around the wrist. Not only mechanical advantages, but also chemical stability in human sweat is observed for the bracelet. It is suggested that the bioinspired temperature‐sensitive yarn can be implemented for continuous monitoring of skin temperature for healthcare or medical applications.
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Small-sized, low-cost, and high-sensitivity sensors are required for pressure-sensing applications because of their critical role in consumer electronics, automotive applications, and industrial environments. Thus, micro/nanoscale pressure sensors based on micro/nanofabrication and micro/nanoelectromechanical system technologies have emerged as a promising class of pressure sensors on account of their remarkable miniaturization and performance. These sensors have recently been developed to feature multifunctionality and applicability to novel scenarios, such as smart wearable devices and health monitoring systems. In this review, we summarize the major sensing principles used in micro/nanoscale pressure sensors and discuss recent progress in the development of four major categories of these sensors, namely, novel material-based, flexible, implantable, and self-powered pressure sensors. Keywords: M/NEMS, Pressure sensor, Flexible sensor, Piezoresistive sensor, Capacitive sensor, Piezoelectric sensor, Resonant sensor, 2D material
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Highly sensitive temperature sensors are designed by exploiting the interparticle distance–dependent transport mechanism in nanocrystal (NC) thin films based on a thermal expansion strategy. The effect of ligands on the electronic, thermal, mechanical, and charge transport properties of silver (Ag) NC thin films on thermal expandable substrates of poly(dimethylsiloxane) (PDMS) is investigated. While inorganic ligand‐treated Ag NC thin films exhibit a low temperature coefficient of resistance (TCR), organic ligand‐treated films exhibit extremely high TCR up to 0.5 K−1, which is the highest TCR exhibited among nanomaterial‐based temperature sensors to the best of the authors' knowledge. Structural and electronic characterizations, as well as finite element method simulation and transport modeling are conducted to determine the origin of this behavior. Finally, an all‐solution based fabrication process is established to build Ag NC‐based sensors and electrodes on PDMS to demonstrate their suitability as low‐cost, high‐performance attachable temperature sensors. All‐nanocrystal based, solution processed, highly sensitive temperature sensors are fabricated. The combinational strategy of an interparticle‐dependent transport mechanism and strain‐concentrated thermal expansion achieves a high temperature coefficient of resistance of up to 0.5 K−1, which is the highest among nanomaterial‐based temperature sensors.
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This study aims to determine the effect of audiosonic sonication in normal modes on the formation of graphene oxide (GO) from carbon rods of zinc-carbon (ZnC) battery wastes. The method used in this study was sonication with an audiosonic frequency in normal modes, assisted by a surfactant solution derived from a commercial detergent. A graphite-detergent solution was exposed to audiosonic waves using a frequency of 170 Hz for 3 h with a pattern on the surface of the solution. The graphite solution was a mixture of 0.8 g of graphite powder and 100 ml of distilled water that was mixed using a blender for 2 min. 25 ml of the solution was then taken and dripped with two drops of detergent solution containing 0.2 g detergent powder dissolved into 100 ml distilled water, so that a graphite-detergent solution was obtained. The tools used in this study included UV–Visible spectroscopy (UV–Vis), Fourier Transform InfraRed spectroscopy (FTIR), and a Scanning Electron Microscope (SEM). The solution that was audiosonicated showed a strong visible nodal pattern on its surface. The UV–Vis spectroscopy produced absorbance peaks at wavelengths of 225 nm and 270 nm, and the FTIR indicated the presence of OH and CC functional groups, which suggested the existence of GO. The SEM images showed GO in the form of coral-like materials. Keywords: Audiosonic sonication, Commercial detergent, Graphene oxide, ZnC battery
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Wearable devices are gaining considerable attention owing to the ease with which they can collect crucial information in real-time, both continuously and noninvasively, regarding a wearer's health. A concise summary is given of the three main elements that enable autonomous detection and monitoring of the likelihood or the existence of a health-risk state in continuous and real-time modes, with an emphasis on emerging materials and fabrication techniques in the relevant fields. The first element is the sensing technology used in the noninvasive detection of physiological markers relevant to the state of health. The second element is self-powered devices for longer periods of use by drawing energy from bodily movement and temperature. The third element is the self-healing properties of the materials used in the wearable devices to extended usage if they become scratched or cut. Promises and challenges of the separately reviewed parts and the combined parts are presented and discussed. Ideas regarding further improvement of skin-based wearable devices are also presented and discussed.
Flexible ammonia (NH3) sensors based on one-dimensional nanostructures have attracted great attention due to their high flexibility and low-power consumption. However, it is still challenging to reliably and cost-effectively fabricate ordered nanostructure-based flexible sensors. Herein, a smartphone-enabled fully integrated system based on a flexible nanowire sensor was developed for real-time NH3 monitoring. Highly aligned, sub-100 nm nanowires on a flexible substrate fabricated by facile and low-cost soft lithography were used as sensitive elements to produce impedance response. The detection signals were sent to a smartphone and displayed on the screen in real time. This nanowire-based sensor exhibited robust flexibility and mechanical durability. Moreover, the integrated NH3 sensing system presented enhanced performance with a detection limit of 100 ppb, as well as high selectivity and reproducibility. The power consumption of the flexible nanowire sensor was as low as 3 μW. By using this system, measurements were carried out to obtain reliable information about the spoilage of foods. This smartphone-enabled integrated system based on a flexible nanowire sensor provided a portable and efficient way to detect NH3 in daily life.
An E-type high-precision temperature sensor, which is adopted for upper air meteorology, was proposed in this paper. A computational fluid dynamics (CFD) method was implemented to analyze temperature rise induced by solar radiation at different altitudes and solar radiation intensities. A temperature rise correction equation was obtained by fitting the CFD results using a Broyden-Fletcher-Goldfarb-Shanno (BFGS) method. To verify the performance of the temperature sensor, an experimental platform was constructed. Through simulations and experiments, the relationship among the altitude, solar radiation intensity and radiation temperature rise was obtaned. The root-mean-square error (RMSE) between the temperature rise derived from the correction equation and that derived from the experiments is 0.013 K. The sample determination coefficient r² of the solar radiation error correction equation is 0.9975 © 2018, Editorial Office of Nanotechnology and Precision Engineering. All right reserved.
Multifunctional temperature-strain sensors that can simultaneously detect temperature and strain are fabricated through all-solution processes using colloidal Ag nanocrystals (NCs). Material and architecture design are introduced to efficiently distinguish signals, allowing accurate measurement from one sensor device. For material design, a ligand-exchange and reduction process is developed to increase the sensitivity. As a result, higher temperature coefficients of resistance, lower resistivity, and lower gauge factor values are observed. Furthermore, a partial oxidation process is used to widen the sensing range above 673 K, which overcomes the most challenging issue of nanomaterial based sensors. For architecture design, three-dimensional mirror-stacked layer structures are fabricated at the top and bottom layers of the neutral mechanical plane for effective strain decoupling. Our two-fold strategy provides a low cost, simple, single-material-based method to achieve highly metallic thin films constructed on flexible substrates. Our sensor platforms can be fabricated on numerous substrates with a high pixel density for high spatial resolution, and we expect that they can be used for a variety of applications, such as bioelectronics and robotics.
Fiber‐based sensors integrated on textiles or clothing systems are required for the next generation of wearable electronic platforms. Fiber‐based physical sensors are developed, but the development of fiber‐based temperature sensors is still limited. Herein, a new approach to develop wearable temperature sensors that use freestanding single reduction graphene oxide (rGO) fiber is proposed. A freestanding and wearable temperature‐responsive rGO fiber with tunable thermal index is obtained using simple wet spinning and a controlled graphene oxide reduction time. The freestanding fiber‐based temperature sensor shows high responsivity, fast response time (7 s), and good recovery time (20 s) to temperature. It also maintains its response under an applied mechanical deformation. The fiber device fabricated by means of a simple process is easily integrated into fabric such as socks or undershirts and can be worn by a person to monitor the temperature of the environment and skin temperature without interference during movement and various activities. These results demonstrate that the freestanding fiber‐based temperature sensor has great potential for fiber‐based wearable electronic platforms. It is also promising for applications in healthcare and biomedical monitoring. A freestanding, wearable, fiber‐based temperature sensor is presented, which can easily be integrated into clothing such as socks and undershirts and can be worn by a person to monitor the temperature of the environment around the person and the temperature of the person without interference from movement and activity. This sensing device has great potential for next‐generation wearable electronics.
In this study, we demonstrate a fabrication of highly sensitive flexible temperature sensor with a bioinspired octopus-mimicking adhesive. A resistor-type temperature sensor consisting of a composite of poly(N isopropylacrylamide) (pNIPAM)-temperature sensitive hydrogel, poly(3,4 ethylenedioxythiophene) polystyrene sulfonate, and carbon nanotubes, exhibits a very high thermal sensitivity of 2.6%•ºC⁻¹ between 25 and 40 °C so that the change in skin temperature of 0.5 °C can be accurately detected. At the same time, the polydimethylsiloxane adhesive layer of octopus-mimicking rim structure coated with pNIPAM, is fabricated through the formation of a single mold by utilizing undercut phenomenon in photolithography. The fabricated sensor shows stable and reproducible detection of skin-temperature under repeated attachment/detachment cycles onto skin without any skin irritation for a long time. This work suggests a high potential application of our skin-attachable temperature sensor to wearable devices for medical and healthcare monitoring.