FULL PAPER
Electronic Tattoos
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Self-Healable Multifunctional Electronic Tattoos Based
on Silk and Graphene
Qi Wang, Shengjie Ling, Xiaoping Liang, Huimin Wang, Haojie Lu, and Yingying Zhang*
calcium).[7–10] As skin is microscopically
rough, only ultrathin and stretchable
E-tattoos can fully match the modulus and
morphology of natural skin without additional adhesives. Such conformal adhesion enlarges the skin/electrode contact,
thereby lowering the contact impedance
and leading to a higher signal-to-noise
ratio in recording electrophysiological signals with less mechanical constraints.[11]
Currently, the development of E-tattoos
that are biocompatible and have good
adherence to biological tissues is still a
challenge to researchers.
During the operation of E-tattoos, they
are constantly exposed to external mechanical inputs (bending, twisting, pressing,
and cutting), which may cause mechanical
damage and lead to malfunction. Thus,
E-tattoo that heals after damage is highly
desired. Moreover, considering their practical applications to health monitoring,
multiple functional E-tattoos are preferred. Therefore, self-healing capability
and simultaneous sensing of multistimuli,
similar to the inherent attributes of human skin, are expected
for E-tattoos.
Silk fibroin (SF), a natural protein material produced by silkworms, has been identified as particularly suitable for wearable
electronic applications because of its mechanical durability,
tunable secondary structure, all-aqueous processing, and good
biocompatibility.[12–16] However, regenerated SF films tend to be
stiff and brittle under ambient conditions. Recently, a stretchable and soft silk membrane was obtained by the addition of
Ca2+ ions into the SF precursor solution.[17] The SF/Ca2+ system
contains abundant hydrogen and coordination bonds, which
allows self-healing. Despite the large potential of SF for being
used as wearable electronics and even E-tattoos, its applications
in wearable electronics are largely limited to its use as a substrate, other than its use as the active functional material.
Graphene, as the thinnest electrically conductive material, shows
great promise for E-tattoo applications. Graphene is mechanically
robust, electrochemically stable, and biocompatible.[18,19] Moreover,
graphene materials can be functionalized through various physical
and chemical approaches, thereby providing plenty of room for
tailoring the properties and performance of graphene-based sensors.[20–23] The integration of graphene materials and silk has been
explored, whereas the majority of studies focus on mechanical
enhanced materials.[24,25] Silk/graphene composite is a highly
promising ­material for emerging epidermal electronics.
Electronic tattoos (E-tattoos), which can be intimately mounted on human
skin for noninvasive and high-fidelity sensing, have attracted the attention of
researchers in the field of wearable electronics. However, fabricating E-tattoos
that are capable of self-healing and sensing multistimuli, similar to the
inherent attributes of human skin, is still challenging. Herein, a healable and
multifunctional E-tattoo based on a graphene/silk fibroin/Ca2+ (Gr/SF/Ca2+)
combination is reported. The highly flexible E-tattoos are prepared through
printing or writing using Gr/SF/Ca2+ suspension. The graphene flakes distributed in the matrix form an electrically conductive path that is responsive to
environmental changes, such as strain, humidity, and temperature variations,
endowing the E-tattoo with high sensitivity to multistimuli. The performance
of the E-tattoo is investigated as a strain, humidity, and temperature sensor
that shows high sensitivity, a fast response, and long-term stability. The
E-tattoo is remarkably healed after damage by water because of the reformation of hydrogen and coordination bonds at the fractured interface. The healing
efficiency is 100% in only 0.3 s. Finally, as proof of concept, its applications
for monitoring of electrocardiograms, breathing, and temperature are shown.
Based on its unique properties and superior performance, the Gr/SF/Ca2+
E-tattoo may be a promising candidate material for epidermal electronics.
1. Introduction
Electronic tattoos (E-tattoos) or epidermal electronics are
newly developed wearable electronics, which are lightweight
and as soft as human skin, thereby allowing them to be intimately mounted on human skin for noninvasive, high-fidelity
sensing.[1–6] The satisfactory flexibility and highly conformal
contact with skin make E-tattoos promising candidates for
applications, including but not limited to continuous recording
of electrophysiological signals, skin temperature, skin hydration, and metabolites in sweat (glucose, urea, sodium, and
Q. Wang, X. P. Liang, H. M. Wang, H. J. Lu, Prof. Y. Y. Zhang
Key Laboratory of Organic Optoelectronics and Molecular Engineering of
the Ministry of Education
Department of Chemistry and Center for Nano and Micro Mechanics
Tsinghua University
Beijing 100084, P. R. China
E-mail: yingyingzhang@tsinghua.edu.cn
Prof. S. J. Ling
School of Physical Science and Technology
ShanghaiTech University
Shanghai 201800, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201808695.
DOI: 10.1002/adfm.201808695
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We developed a self-healable multifunctional E-tattoo by
incorporating graphene with SF/Ca2+ films (Gr/SF/Ca2+).
We demonstrated its use as a sensor for detecting of strain,
humidity, and temperature. Our E-tattoo can be fabricated
through a simple screen printing process or a direct writing
process on an SF/Ca2+ membrane, allowing its transfer
onto human skin exactly like a temporary tattoo for further
applications. The skin-like softness of the SF/Ca2+ matrix is
the reason for the high flexibility of the Gr/SF/Ca2+-based
E-tattoo. Its skin-like softness allows it to fully conform to
the microscopic morphology of human skin and to follow
arbitrary skin deformation without mechanical failure or
delamination. The Gr/SF/Ca2+ based E-tattoo can monitor a
variety of daily life sensations, including electrocardiogram
(ECG), breathing, and temperature with high sensitivity. The
dynamic hydrogen bonds and coordination bonds in the Gr/
SF/Ca2+ composite enable the sensors to self-heal with ≈100%
healing efficiency. Mechanical damages can be repaired by
just a drop of water. A promising approach for fabricating
self-healable, biocompatible, and multifunctional epidermal
electronics is presented.
2. Results and Discussion
2.1. Fabrication Process of the Gr/SF/Ca2+ E-Tattoo
Figure 1a illustrates the fabrication process of the E-tattoos.
Silkworm cocoons were used for the preparation of SF/Ca2+
solution and graphene was added to part of the SF/Ca2+ solution
to obtain the Gr/SF/Ca2+ suspensions. The Gr/SF/Ca2+ suspensions can be patterned on SF/Ca2+ membranes through screen
printing or direct writing to obtain the designed E-­tattoos. Only
a droplet of water sprayed onto the target p
­ osition is required
to attach the Gr/SF/Ca2+ E-tattoo onto human skin. The Gr/
SF/Ca2+ E-tattoo can be well attached on the skin due to the
slight degradation of the SF/Ca2+ membrane induced by the
water. Figure 1b,c shows the stable and conformal adhesion of
the E-tattoo on human skin. The E-tattoo can retain its position without dislocation, delamination, or breakage even in the
presence of various external forces, such as stretching, compressing, and twisting. Thus, it shows its suitability for use as a
reliable E-tattoo.
2.2. Characterization of the Gr/SF/Ca2+ Films for M
­ ultifunctional
Sensing
2.2.1. Performance of the Gr/SF/Ca2+ Films for Monitoring
of Strain
The Gr/SF/Ca2+ layer can be used in the field of epidermal
electronics to detect strain, humidity, and temperature.
Figure 2a plots the relative resistance change (ΔR/R0, where
R0 refers to the resistance under 0% strain) of the Gr/SF/Ca2+
layer as a function of the applied strain, thereby showing its
possibility to serve as a strain sensor. The sensitivity of the
sensor at different strain corresponding gauge factors (GFs),
which is the slope of the curve, is also plotted in Figure 2a.
The curve exhibited two distinct regimes. When the applied
strain was below 70%, the resistance increased in a relatively
gentle slope that corresponded to a GF in the range of 4–34.
At a higher strain of 70–90%, the GF increased from 34 to
470, which is comparable with the highest GF reported in
other strain sensors.[26,27] The dependence of the resistance on
the applied strain can be explained by the relative sliding of
Figure 1. Fabrication process of the Gr/SF/Ca2+ E-tattoo and its stable adhesion on human skin. a) Schematic illustration showing the fabrication
process of Gr/SF/Ca2+ E-tattoo. b) Photographs showing a four-leaf clover E-tattoo attached to the forearm. c) Photographs of the tattoo on skin in
different states: stretched (upper), compressed (middle), and twisted (lower).
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Figure 2. Performance of the Gr/SF/Ca2+ films for monitoring of strain, humidity, and temperature. a) Relative resistance changes as a function of
applied tensile strain. b) Long-term stability of stretchable strain sensors upon exposure to 10 000 tensile strain cycles at an applied strain of 50%.
c) Real-time relative humidity sensing of the humidity sensor at several specific relative humidity (11.3%, 33.1%, 43.2%, 54.4%, 75.5%, and 85.1%).
d) Dynamic stability test of the humidity sensor under the relative humidity of 10% and 43.2%. e) Temperature sensor performance (relative resistance
change vs temperature from 20 to 50 °C). Linear fit suggests the sensitivity is ≈2.09% °C−1. f) Dynamic stability of the temperature sensor for repeated
temperature changes from 20 to 30, 40, and 50 °C, respectively.
the neighboring graphene sheets in a low strain range and the
complete separation of some neighboring graphene sheets in
a high strain range.[27–29] The durability of the strain sensor
was tested by loading–unloading tensile strain of 50% for over
10 000 cycles. The performance of each of the tested sensors
did not change even after 10 000 cycles. The good stretchability of the Gr/SF/Ca2+ layer was attributed to the high
random coil contents and the strong water absorbing capacity
of Ca2+ ions.[30,31] High-resolution scanning electron microscopy (SEM) images (Figure S1, Supporting Information) of
Gr/SF/Ca2+ film show a honeycomb-like porous structure
without gaps or cracks.
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2.2.2. Performance of the Gr/SF/Ca2+ Films for Monitoring
of Humidity
The Gr/SF/Ca2+ film can also be used as a humidity sensor
to detect relative humidity (RH) of the environment. The
­resistance of the Gr/SF/Ca2+ film under RH values of 11.3%,
33.1%, 43.2%, 54.4%, 75.5%, and 85.1% was recorded. As
shown in Figure 2c,d, the relative resistance change (ΔR/R0)
increases from 0.2% to 2.3% when the RH varies from 11.3%
to 85.1%, thereby indicating that the Gr/SF/Ca2+ film can detect
RH with high sensitivity and in a wide range. It is noted that
the R0 refers to the resistance under 10% RH since 10% is the
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ambient RH during the measurement. Figure 2d plots the realtime humidity sensing properties of the device at 43.2% and
ambient humidity (10%) in dynamic reliability tests. The sensor’s resistance increased rapidly to high values under high RH
conditions and then reverted back to its initial value after the
RH returned to ambient. The stable signal saturation behavior
was monitored for 12 cycles, thereby demonstrating the reproducible and high stable humidity-sensing performance with
fast response and recovery rate. Remarkably, the response and
recovery time (defined as the time required to reach 90% of the
steady value) are about 3 and 6 s, respectively, and these values
are comparable with most of the reported representative ultrafast humidity sensors.[32–38]
The working mechanism of the Gr/SF/Ca2+ humidity sensor
is related to the interaction of the Gr/SF/Ca2+ matrix with water
molecules. Ca2+ ions in the SF matrix can capture water from
the environment through coordination, a Ca2+ ion can coordinate six to eight water molecules via the oxygen atoms.[39] As
a result, more and more adsorbed water molecules can be captured by the matrix with increasing RH, thereby leading to a
swollen of matrix and to a decrease of graphene-conducting
paths.[40] This process is reversible. When RH reverts back to
its initial value, the sensor resistance can recover to the origin
state.
2.2.3. Performance of the Gr/SF/Ca2+ Films for Monitoring
of Temperature
Furthermore, the Gr/SF/Ca2+ film can be used as a temperature sensor based on its resistance variation with temperature.
The mechanism underlying the Gr/SF/Ca2+ resistance change
with temperature can be attributed to the variation of electron
hopping at the interface between neighboring graphene sheets.
Increasing temperature can induce more thermally activated
charge hopping, thereby further enhancing the electrical conductance of the Gr/SF/Ca2+ film.[20,41–43] Figure 2e shows the
response of a Gr/SF/Ca2+ film to temperature variation. The
resistance of the Gr/SF/Ca2+ film decreased linearly from
0.62 to 0.28 MΩ as the temperature increased from 20 to 50 °C.
A plot of relative resistance change versus temperature was linearly fitted, and the slope corresponded to the sensitivity of the
temperature sensor (2.1% °C−1), which was higher than that of
most reported carbon-material based temperature sensors.[8,20,41]
Figure 2f shows the resistance of the sensor when the temperature was cyclically changed between 20 and 30, 40, and 50 °C for
ten cycles, respectively. During each cycle, as the temperature
decreased to 20 °C, the resistance returned to its initial value
within 13 s. The high sensitivity and rapid response validated
the use of the Gr/SF/Ca2+ film as a temperature sensor.
The Gr/SF/Ca2+ film is sensitive to strain, temperature, and
humidity simultaneously. Thus, additional postprocessing was
required to achieve separation of the three signals. When the
strain and temperature sensor was covered with a waterproof
package, the influence of humidity was eliminated. The effect
of strain on temperature and humidity was eliminated by preparing the Gr/SF/Ca2+ ink on an unstretchable substrate. In
addition, temperature changes mainly affected the baseline of
the output signal, whereas strain and humidity changes mainly
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affected the shape of the output signal. So, the temperature
signal can be separated in this way.
2.3. Self-Healing Characterization of the Gr/SF/Ca2+ Films
Besides the sensing of multiple external stimuli, self-healing
ability is another highly desired feature of epidermal electronics
for mimicking human skin. The Gr/SF/Ca2+ film shows excellent self-healing ability. To demonstrate, a Gr/SF/Ca2+ film was
connected with a commercial light-emitting diode (LED) and a
constant voltage of 1.5 V was applied. The circuit schematics,
photographs, and video are shown in Figure 3a and Figure S2a–c
and Video S1 in the Supporting Information, respectively. The
film was then cut into two parts using a razor blade, and the
LED was switched off simultaneously. To repair the damaged
film and to switch on the LED again, only a droplet of water was
added to the fracture region. SEM characterization of Gr/SF/Ca2+
film before and after healing was carried out (Figure S2d–f). A
≈30 µm wide gap formed after cutting and it was mechanically
healed by adding water.
Figure 3b shows the proposed mechanism underlying the
water healing effects and it includes the swollen of SF/Ca2+
matrix and the reformation of hydrogen and coordination
bonds. Previous works showed that water molecules can lead to
swelling of the SF chains and can soften the films.[17,44] Therefore, when water was added onto the fracture parts, SF chains
swelled up, and the viscoelasticity of the film increased, thereby
leading to the physical fusing of the two separated parts. Moreover, reversible hydrogen bonds formed between the polar
groups of SF side chains with both silk coated graphene and
the other polar groups of SF side chains. However, coordination bonds between Ca2+ and COO− are plentiful in the SF/
Ca2+ film.[45,46] Owing to the dynamic nature of the intrinsic
hydrogen and coordination bonds, small cracks within the film
can be healed rapidly when broken bonds form again at the
fractured interface.[47] Thus, mechanical and electrical properties can be restored to their original levels.[48,49]
Figure 3c exhibits the recovered mechanical properties of
Gr/SF/Ca2+ after the cutting and healing process. The curves
nearly overlapped, thereby indicating that the mechanical
property was retained during the cutting–healing cycles. The
calculated values for toughness (the integral area under the
stress–strain curve) of the original sample and after one to four
cutting–healing cycles were 5.3, 5.4, 5.4, 5.3, and 5.2 MJ m−3,
respectively, thereby also proving that the mechanical property
was recovered while strain and stress were considered.
To evaluate the self-healing efficiency and time, electrical
measurement was conducted to further confirm the fast healing
ability and the high reliability of Gr/SF/Ca2+. The self-healing
efficiency (η) is the proportion of restored electrical conductivity or mechanical toughness to the original conductivity or
toughness, respectively. As shown in Figure 3d, the current of
Gr/SF/Ca2+ film recovered its original conducting level with a η
of 100% within 0.3 s for multiple cutting and healing cycles. For
practical applications of healable materials, versatile requirements should be satisfied, such as high self-healing efficiency
of electrical and mechanical properties, short healing time, and
easy healing procedures. However, few h
­ ealable materials in
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Figure 3. Self-healing properties of Gr/SF/Ca2+ films. a) Schematic illustrations showing a visible fracture and healing process of the LED circuit
interconnected with a Gr/SF/Ca2+ film. b) Schematic illustrations of the self-healing mechanism of Gr/SF/Ca2+ based on reversible hydrogen and
coordination bonds. c) Stress–strain curves of the original and 1st, 2nd, 3rd, and 4th healed Gr/SF/Ca2+ film. d) Electrical measurements of the films
during five cutting and healing cycles and the right figure shows that the films can be healed within 0.3 s. e) Comparison of the Gr/SF/Ca2+ film to
representative reported materials in self-healing efficiency, healing time, and temperature.
literature meet the abovementioned demands simultaneously.
The healing performance of the Gr/SF/Ca2+ film were compared with the reported representative self-healing materials
(Figure 3e and Table S1, Supporting Information).[50–64] The
Gr/SF/Ca2+ system exhibited superior healing ability in critical
aspects including high self-healing efficiency, rapid healing
time, and moderate processing condition (room temperature).
Thus, it is a promising candidate to be used in E-tattoos as
­healable sensing materials.
2.4. Application of Gr/SF/Ca2+-Based E-Tattoos
To prove the practicability of our healable and multifunctional
Gr/SF/Ca2+ films in tattoo electronics, the sensing ­performance
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and the healing ability of Gr/SF/Ca2+-based E-tattoos for ECG,
respiration, and temperature were investigated. Noninvasive monitoring of ECG can help detect multiple important
features of heart malfunction, such as irregular heartbeat
(arrhythmia).[65] Low contact impedance is essential to obtain
a high signal-to-noise ratio in electrophysiological measurements. Two E-tattoos were attached directly onto the two
forearms of a volunteer (Figure 4a), and ECG signal was successfully collected and recorded using a Bluetooth module. As
seen in Figure 4b, E-tattoo captured the fine patterns of ECG
with a high signal-to-noise ratio. The characteristic ECG peaks
(P, Q, R, S, and T) were clearly distinguished. As shown in
Figure S3 in the Supporting Information, nearly identical signals were well maintained during 10 h of monitoring, thereby
demonstrating long-term stability of the E-tattoos.
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Figure 4. Applications of the Gr/SF/Ca2+ E-tattoo for ECG-, respiration-, and temperature-sensing. a) Photographs of a self-healed E-tattoo directly
attached on the forearms for ECG measurements. Right panel shows that the E-tattoos were broken (upper) and healed (bottom). b) Relative resistance
changes of pristine and healed ECG sensors. Corresponding enlargement of a single signal waveform is extracted on the right. c) Photographs of a
beard tattoo directly attached beneath the nose of a volunteer for tracing his respiration. d) Relative resistance changes of pristine and healed humidity
sensors. f) Photographs of a thermometer tattoo directly attached on the hand for sensing waters with different temperature. g) Relative resistance
changes of pristine and healed temperature sensors.
Based on the high sensitivity and fast response of the Gr/SF/
Ca2+ film to humidity variation, we demonstrated the application of the E-tattoo for respiration monitoring. A beard tattoo
was attached beneath the nose of a volunteer for tracing the
moisture variation in his expiration gas (Figure 4c). Thus, respiration rate was monitored. The fast response of the sensor
to humidity variation enables the real-time and continuous
tracking of the respiration rate for a long time. Respiration
monitoring is in great demand for some cases, e.g., during
sleep to avoid sleep apnea, which is a potentially lethal condition that is difficult to diagnose.[66,67] The E-tattoo can recover
its normal working condition by adding a droplet of water even
after a fracture (Figure 4d). In addition, no skin irritation was
observed after 10 h of long-term on-skin respiration monitoring
(Figure S4, Supporting Information), thereby demonstrating
the biocompatibility of the E-tattoo. For comparison, the
recently reported E-tattoos used commercial tattoo papers,[8,9]
where the biocompatibility of these materials is unverified.
We demonstrated the epidermal E-tattoo’s capability to perceive temperature. An E-tattoo was attached on a human hand,
as shown in Figure 4f (left panel). The dynamic response of
the E-tattoo was recorded when dropping and removing warm
(50 °C) or ice (0 °C) water droplets. Before dropping water,
a thin layer of liquid bandages was coated on the E-tattoo to
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serve as a waterproof layer. Resistance changes were observed
upon loading and removing of water at different temperatures
(Figure 4g), thereby demonstrating that E-tattoo can help the
user perceive the external ambient temperature rapidly.
3. Conclusion
We reported a self-healing silk E-tattoo that shows high sensitivity to multiple stimuli, including strain, humidity, and temperature based on a unique Gr/SF/Ca2+ combination. Using a
Gr/SF/Ca2+ suspension as the precursor, customer-designed
E-tattoo patterns were prepared through screen printing or
direct writing on SF/Ca2+ membranes. The obtained E-tattoo
was soft and highly flexible, thereby allowing it to be conformally attached to human skin and to sustain arbitrary skin
deformation without mechanical failure or delamination.
The graphene flakes distributed in the SF matrix form an
electrically conductive path that is responsive to environmental
changes, such as external strain, matrix swollen induced by
high humidity, and temperature variation, thereby enabling the
E-tattoo to serve as a multifunctional sensor. The Gr/SF/Ca2+based tattoo-like sensor showed high sensitivity, fast response,
and long-term stability. The self-healing of the E-tattoo can
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induced by a droplet of water even after being fully fractured.
A healing efficiency of 100% was demonstrated within only
0.3 s after applying the water droplet. Such effect was due to
the swollen of SF/Ca2+ matrix and the reformation of hydrogen
and coordination bonds at the fractured interface. We further
demonstrated the applications of the healable E-tattoo in monitoring health signals and daily life activities, including ECG,
breathing, and temperature. We hope that the development of
Gr/SF/Ca2+ E-tattoo leads to more opportunities to fabricate
biocompatible epidermal electronics for health and environmental monitoring and for biomedical applications.
obvious difference has been observed. The ECG signal was collected and
recorded using a BMD 101 ECG Bluetooth module. The reproducibility
of each sensing function was checked by testing at least three sensor
samples.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
4. Experimental Section
Preparation of SF/Ca2+ Solution: Degummed silk fibers were
prepared from B. mori silkworm cocoons following previous reported
procedures.[68] In brief, Bombyx mori cocoon was degummed twice in
the boiling solution of 0.5 wt% NaHCO3 for 30 min. After thoroughly
washed in deionized water, degummed silk fibers were dried at room
temperature. To obtain the SF/Ca2+ solution, 3 g degummed silk fibers
were dissolved in CaCl2/formic acid solution (weight ratio of 1:20) by
vortexing at room temperature for around 5 min.
Preparation of Gr/SF/Ca2+ Suspension: Graphene ink was purchased
from Sigma-Aldrich, USA (product number: 808156). The graphene
powder was three to eight atomic layers and 5–20 µm. The commercial
graphene ink was mixed with the SF/Ca2+ solution with the graphene
weight content is 40 wt% by ultrasonication at room temperature for
5 min to obtain well-dispersed Gr/SF/Ca2+ suspensions.[30]
Fabrication of E-Tattoos: Five grams SF/Ca2+ solution was poured
into a polystyrene weighing dish and then dried under ambient
conditions for > 4 h to evaporate the solvent. Then, the obtained SF/
Ca2+ membranes were heated at 60 °C for 2 h to remove the residual
solvent in the membranes. The designed Gr/SF/Ca2+ patterns can
be generated through screen printing or direct writing. For the screen
printing method, a stencil mask of an custom-designed pattern was
prepared using a commercial adhesive tape and then attached onto the
SF/Ca2+ membrane. Next, certain amount of Gr/SF/Ca2+ solution was
applied onto the SF/Ca2+ membrane with the mask and then left for
curing at room temperature in a fume hood for 2 h. Then, the adhesive
tape mask was released. For the direct writing method, the designed
pattern can be written on SF/Ca2+ membranes using a brush with
Gr/SF/Ca2+ suspension. A stable handwritten pattern is formed after
curing at room temperature in a fume hood for 2 h. Finally, copper wires
were attached to the Gr/SF/Ca2+ pattern as electrodes using silver paste.
To attach the Gr/SF/Ca2+ E-tattoo onto human skin, only spraying a
droplet of water to the target position before attaching is needed.
Performance Test of E-Tattoos: During strain tests, tensile was applied
using a SHIMADZUAG-IS tensile tester. The electrical measurement
was carried out using Keithley 2400 with a two-probe mode at a fixed
bias voltage of 0.3 V. During humidity tests, pulsed water vapor was
introduced into the testing chamber, in which specific RH can be
precisely controlled by using different saturated salt solutions. Schematic
illustration of the test set is shown in Figure S5 in the Supporting
Information. The Gr/SF/Ca2+ film was placed in a sealed receptacle,
which was filled with vapor from a saturated salt solution carried by
the gas flow. Several specific RH (11.3%, 33.1%, 43.2%, 54.4%, 75.5%,
and 85.1%) conditions were achieved in closed desiccators containing
different saturated salt solutions (LiCl, MgCl2, K2CO3, Mg(NO3)2, NaCl,
and KCl). To perform temperature measurement, the sensor was put
on a hot plate with controllable temperature and a DC voltage of 0.3 V
was applied. To exclude the possibility that the electrical recovery results
from the conductivity of water, the healed film was placed on a hot plate
at 120 °C for several hours and the current in the Gr/SF/Ca2+ films was
monitored. There is no variation in the current was observed. Besides,
the healing experiment was carried out at different humidity and no
Adv. Funct. Mater. 2019, 29, 1808695
This work was financially supported by the National Natural Science
Foundation of China (51672153 and 51422204), the National Key Basic
Research and Development Program (2016YFA0200103), and the
National Program for Support of Top-notch Young Professionals.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
electronic tattoos, epidermal electronics, flexible sensors, graphene,
self-healing, silk fibroin
Received: December 6, 2018
Revised: February 12, 2019
Published online: February 25, 2019
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