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Article
An All Silk-Derived Dual-Mode E-skin for
Simultaneous Temperature-Pressure Detection
Chunya Wang, Kailun Xia, Mingchao Zhang, Muqiang Jian, and Yingying Zhang
ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13356 • Publication Date (Web): 24 Oct 2017
Downloaded from http://pubs.acs.org on October 25, 2017
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ACS Applied Materials & Interfaces
An All Silk-Derived Dual-Mode E-skin for
Simultaneous Temperature-Pressure Detection
Chunya Wang a,b, Kailun Xia a,b, Mingchao Zhang a,b, Muqiang Jian a,b, Yingying Zhang a,b,*
a
Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China
b
Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, PR
China
*Address correspondence to yingyingzhang@tsinghua.edu.cn (Y. Y. Zhang)
Keywords: silk nanofibers, combo temperature-pressure sensors, strain sensors, electronic skins,
flexible electronics
ABSTRACT: Flexible skin-mimicking electronics are highly desired for development of smart
human–machine interfaces and wearable human-health monitors. Human skins are able to
simultaneously detect different information, such as touch, friction, temperature, and humidity.
However, due to the mutual interferences of sensors with different functions, it is still a big
challenge to fabricate multi-functional electronic skins (E-skins). Herein, a combo temperaturepressure E-skin is reported through assembling a temperature sensor and a strain sensor in both
of which flexible and transparent silk-nanofiber-derived carbon fiber membranes (SilkCFM) are
used as the active material. The temperature sensor presents high temperature sensitivity of
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0.81% per centigrade. The strain sensor shows an extremely high sensitivity with a gauge factor
of ~8,350 at 50% strain, enabling the detection of subtle pressure stimuli which induce local
strain. Importantly, the structure of the SilkCFM in each sensor is designed to be passive to other
stimuli, enabling the integrated E-skin to precisely detect temperature and pressure at the same
time. It is demonstrated that the E-skin can detect and distinguish exhaling, finger pressing, and
spatial distribution of temperature and pressure, which cannot be realized using single mode
sensors. The remarkable performance of the silk-based combo temperature-pressure sensor,
together with its green and large-scalable fabrication process, promising its applications in
human–machine interfaces and soft electronics.
1. INTRODUCTION
The Electronic skins (E-skins), which are flexible circuitry matrices composed of sensing pixels
for monitoring various external stimuli, have been vigorously developed for their great potential
of applications in advanced artificial intelligence, such as smart skins for robots and human–
machine interaction.1-3 E-skins have been extensively studied with the development of specific
sensors, such as pressure sensors,4,
5
strain sensors,6,
7
temperature sensors,8-11 and humidity
sensors.12 Specifically, multi-functional E-skins that mimic human skin and simultaneously
detect multiple stimuli including temperature, strain/pressure, and humidity, are highly desired
for practical applications.13-15 Pioneer works have tried to develop multi-functional E-skins
through integration of multiple sensors into one pixel,16,
17
which generally confronts with
limitations, such as the interplay of different sensors, the sophisticated structural design, as well
as the complicated and high-cost fabrication process. The fabrication of multi-functional E-skins
through cost-effective and large-scalable strategy is still challenging.
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Carbon materials have attracted extensive research attention for their potential applications in
flexible electronics owing to their superior electrical and mechanical properties.13,
18-21
Particularly, carbon materials derived from natural biomaterials have been arousing intense
interest for applications in flexible electronics due to their good electrical conductivity,
mechanical flexibility, low-cost and large-scale production capability, as well as environmental
and human benignity.22-27 Silk, as a kind of well-known and widely utilized natural materials, has
been developed as promising building blocks for sustainable high technology applications owing
to its distinguished features of biocompatibility, robust mechanical property, designable material
formats (such as fibers, films and foams), unique optical and electrical property, as well as large
production.28,
29
Furthermore, silk protein can be converted into conductive pseudographitic
pyroprotein through a simple thermal treatment, which endows it with potentiality in energy and
sensing applications.27,30-32
In this study, we developed a silk based combo temperature-pressure E-skin which can
simultaneously detect temperature and pressure with high sensitivity. Carbon nanofiber
membranes with localized graphitic structures that were transformed from silk nanofiber
membranes were utilized as the active sensing materials for both temperature and strain sensors.
Continuous nanofiber membranes were used directly as the active materials of temperature
sensors and the temperature dependence of the membrane resistance enables the sensors to
monitor temperature. In contrast, fractured nanofiber membranes were used as the active
materials of strain sensors, where the contact points between fractured nanofibers enable the
sensors to detect external strain. The temperature sensor exhibits high sensitivity, fast response,
and shows stable performance under deformation. The strain sensor also shows ultrahigh
sensitivity, low detection limit, outstanding stability and durability, and can be utilized to detect
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subtle pressure stimulus. As demonstration, the combo temperature-pressure E-skin was applied
in monitoring daily life sensations, such as exhaling and finger touching. The utilization of silkderived graphitic-like carbon nanofiber membranes (SilkCFM) as active sensing materials in Eskins paves a new way to achieve multimodal sensors through an easy fabrication process.
2. RESULTS AND DISCUSSION
Figure 1a illustrates the fabrication process of the combo temperature-pressure E-skin (Details
can be found in Experimental Section). Silk nanofibers were deposited on a copper foil using
silkworm cocoons as the raw materials through electrospinning and then transformed into
electrically conductive carbon nanofibers with graphitic local structure after thermal treatment.
The obtained SilkCFM can be transferred to flexible polymer films (polyethylene terephthalate
(PET) for temperature sensors and polydimethylsiloxane (PDMS) for strain sensors) through an
elastomeric stamp strategy, which is a general transfer method for graphene grown on copper
foils onto flexible substrates.33 Noted that recently reported cost- and time-effective roll-to-roll
transfer process for graphene can also be borrowed to the transfer of large-area SilkCFM onto
flexible substrates because of the flexibility of SilkCFM and copper foils, which is favorable for
the fabrication of sensor arrays and the potential large-scale production.34A training process
through prestretching is necessary for the SilkCFM/PDMS strain sensor to perform ultrasensitively and stably, which would be discussed in detail in the following section. The PET
substrate with a silk-based temperature sensor array was then laminated onto the PDMS substrate
with a silk-based strain senor array to integrate into a flexible combo temperature-pressure Eskin sensor matrix. The relative simple and controllable fabrication procedure, which includes
electrospinning, thermal treatment, large-scalable transfer, and lamination steps, promises the
integration of different sensors at high yield and low cost.
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Figure 1. Fabrication and characterization of the silk-based combo temperature-pressure E-skin
sensor. (a) Schematic illustration showing the fabrication process of the all-silk-derived
temperature-pressure combo E-skin. (b) SEM image of a silk-derived carbon fiber membrane. (c)
TEM image of the silk-derived carbon, in which red arrows indicate the randomly oriented
graphitic local structure. (d) Raman spectra of the pristine silk nanofibers and a silk-derived
carbon fiber membrane. (e, f) Photographs of a temperature sensor (e) and a strain sensor (f),
showing good flexibility. (g) Transmittance spectrum of the silk based combo temperaturepressure sensor in the visible wavelength ranging from 350 to 800 nm. The inset is a photograph
of the sensor, showing its transparency.
Figure 1b shows a typical scanning electron microscope (SEM) image of the SilkCFM, which
displays good uniformity of the carbon nanofibers derived from the pristine silk nanofiber
membrane(see SEM image of the pristine silk nanofiber membrane in Figure S1, Supporting
Information). The typical transmission electron microscope (TEM) image (Figure 1c) of the silk-
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derived carbon exhibits randomly oriented localized graphitic structure, which accounts for the
electrical conductivity of the SilkCFM. The Raman spectrum of the SilkCFM (Figure 1d)
presents two distinct carbon characteristic peaks at ~1328 cm-1 (D-band, related to defects) and
~1594 cm-1 (G-band, related to crystalline sp2 carbon), revealing a graphitic crystalline structure.
In contrast, the Raman spectrum of the pristine silk nanofiber (Figure 1d) shows the feature peak
of silk fibroin at 1667 cm-1, indicating the high content of β-sheet crystallites,35 which is
favorable for the transformation of fibroin into graphitic nanocarbon in the thermal treatment
process.30 As seen in Figures 1e and 1f, both the silk-based temperature sensor and strain sensor
show good flexibility, which is important for the fabrication of flexible E-skins. Besides, the
integrated temperature-pressure combo E-skin exhibits a transmittance over 70% in the
wavelength ranging from 350 nm to 800 nm (Figure 1g). The obtained integrated E-skin can
allow clear visualization of a picture underneath the device (inset in Figure 1g).
The silk-based temperature sensor exhibited remarkable sensing performance with high
sensitivity of 0.81% resistance change per °C. The response of the silk-based temperature sensor
to the change of temperature was investigated by measuring its resistance variation along with
temperature change. Figure 2a presents the I-V curves of a temperature sensor over the
temperature range of 35 °C to 80 °C with the measurement step of 5 °C. At a fixed voltage of 4.8
V, the current of the temperature sensor increased from 1.51 µA at 35 °C to 2.33 µA at 80 °C,
indicating a notable negative temperature coefficient. The relative change of resistance with
temperature is plotted in Figure 2b, which shows nearly linear increase in resistance with
temperature and the corresponding sensitivity of 0.81% per centigrade (extracted from the linear
fitting). The sensitivity was higher than recently reported carbon nanotube-based flexible
thermistors.36,
37
A temperature change of 1 °C (the minimum temperature change can be
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obtained by the test instrument) can be detected by the silk-based temperature sensor (Figure
S2a).
Figure 2. Sensing Performance of the silk-based temperature sensor. (a) I-V curves of the
temperature sensor in the temperature range of 25 to 80 oC. (b) Relative resistance change of the
temperature sensor versus temperature from 25 to 80 oC. (c) Response/recovery curve of the
temperature sensor between room temperature (RT) and 40 °C. (d) Resistance response of the
temperature sensor to cyclic test of temperature between RT to 40 oC. (e) Relative resistance
change versus temperature from 25 oC to 80 oC with the temperature sensor under relaxed and
bent states. (f) Resistance responses of the temperature sensor to cold water (~0 oC) (blue curve)
and hot water (~55 oC) (red curve); the inset is a schematic diagram of a cup attached with a
temperature sensor.
Furthermore, the temperature sensor showed fast response, good stability and flexibility. It
showed a response time of less than 1.8 s, as shown in Figure 2c. Notably, a stable cyclic
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resistance change of the temperature sensor under a cyclic temperature change between room
temperature and 40 °C was observed, demonstrating the high stability and reliability of the
sensor (Figure 2d). Furthermore, the silk-based temperature sensor exhibited good flexibility, as
demonstrated by the almost unchanged I-V curves (Figure S2b) and the nearly constant response
to the external temperature change (Figure 2e) under relaxed and bended states, which is
significant for practical applications as E-skin. The exciting flexibility of the temperature sensor
can be attributed to the large length–diameter ratio of the silk-derived carbon nanofibers and the
macrostructure of SilkCFM induced intrinsic flexibility.
The sensitivity of the temperature sensor decreased with the increment of the thickness of
SilkCFM (Figure S3a), which is in accordance with other carbon nanomaterial based thermal
sensors.19 The transmittance and the SEM images of the SilkCFM with different electrospun
times are shown in Figures S3b–S3e to manifest the different thickness. The charge transport in
the SilkCFM occurred through two mechanisms: intra-nanofiber transport (such as carrier
hopping) within each carbon nanofiber and inter-nanofiber transport (tunneling conduction). An
elevated external temperature stimuli would induce the increased carrier hopping and tunneling
conduction, leading to the increased conductance of the SilkCFM, which is proposed to be the
working mechanism of the temperature sensor. To demonstrate the possibility of the silk-based
temperature sensor to detect the temperature change of objects, a temperature sensor was
attached to a cup (the inset in Figure 2f). The resistance of the temperature sensor increased and
decreased when cold or hot water was poured into the cup, respectively, manifesting that the
silk-based temperature sensor can quickly and precisely measure the temperature change of
objects (Figure 2f).
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Figure 3. Sensing performance of the silk-based strain sensor. (a) Schematic illustration of the
training process and the working mechanism of the strain sensor. (b) Relative resistance change
of the strain sensor versus applied strain; the inset shows the corresponding gauge factor versus
applied strain. (c) Resistance response to gradually enhanced strain from 0.1% to 5%; the inset
shows the resistance variation versus repeated cycles of strain between 0–0.1%. (d) Resistance
change of the strain sensor under repeated loading and unloading of 50% strain for 5,000 cycles.
(e) Relative resistance change of the strain sensor versus applied pressure. (f) Resistance
response of the strain sensor to cyclic loading of different pressure of 100 Pa, 1 kPa, 5 kPa and
10 kPa.
The silk-based strain/pressure sensor fabricated with elastic PDMS as substrate exhibited
outstanding sensing performance, including ultrahigh sensitivity, large sensing range, low
detection limit, and high stability and durability. As illustrated in Figure 3a, the strain sensor
should be first trained through a cyclic stretching-releasing process (90% strain) to induce the
fracture of nanofibers throughout the SilkCFM. After the release of the pre-strain, the fractured
nanofibers could reconnected with each other to form electrically conductive paths. The contact
points of the fractured nanofibers would decrease with the applied subsequent strain, leading to
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the corresponding increment of resistance. Figure 3b shows the relative resistance change and the
corresponding gauge factor (GF) of a transparent silk-based strain sensor (with the
electrospinning time of about 5 min and the light transmittance of over 80% in the wavelength
range of 350–800 nm, as shown in Figure S4) versus the applied strain. The strain sensor
exhibited a tolerable strain of over 50% and an exponentially increased resistance response to the
applied strain with an incremental GF. Notably, the GF of the strain sensor was approximately
26.3 at 1% strain and dramatically enhanced to approximately 8350 at 50% strain, which is much
higher than recently reported strain sensors with comparable stretchability.38, 39 The resistance
change of the strain sensor under gradually increased strain from 0.1% to 5% was displayed in
Figure 3c, further indicating the ultrahigh sensitivity and detection limit as low as 0.1% strain.
Besides, the silk-based strain sensor exhibited highly stable electrical response during 5,000
loading–unloading cycles of 50% strain, indicating the high stability and durability (Figure 3d).
The overshoot of the incipient cycles can be observed, which is attributed to the viscoelasticity of
PDMS and is common for resistance-typed strain sensors encapsulated with elastic polymers.27
Besides, we investigated the influence of the thickness of SilkCFM on the performance of the
strain sensors (Figure S5). Strain sensors based on thinner SilkCFM showed higher sensitivity
but smaller tolerable strain. In contrast, strain sensors based on thicker SilkCFM exhibited larger
tolerable strain but lower sensitivity.
Based on the ultrasensitive sensing performance of the silk-based strain sensor, it can be utilized
as a pressure sensor to detect subtle pressure stimuli, such as touch. The pressure sensing
performance was measured by attaching the strain sensor onto an elastic silicone (Ecoflex)
substrate to mimic the lamellar structure of human skin. The relative resistance change of the
sensor was monitored during the loading of continuously increased pressure, which revealed
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wide sensing range and satisfying sensitivity of 0.097 kPa-1 within 0–20 kPa, 0.66 kPa-1 within
20–30 kPa, and 3.01 kPa-1 within 30–35 kPa (Figure 3e). The response of the sensor to dynamic
mechanical pressures (pressure input frequency of 1 Hz) were further characterized, as shown in
Figure 3f, from which the noise-free, stable, and continuous real-time resistance variation to
different cyclic pressures can be observed. Besides, the sensor can be used to detect finger press
(Figure S6), demonstrating its potentiality for practical application of sensing a touch. Noted that
although the strain sensor can sustain large strains up to 50%, the local strains induced by
external pressures generally correspond to a small strain.
Figure 4. Combo E-skin sensor matrix with integrated temperature and strain sensors. (a)
Performance of the silk-based temperature sensor under external pressure stimuli, proving
external pressure stimuli does not influence its output signals. (b) Strain sensing of the silk-based
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strain sensor under different temperatures (20 oC, 40 oC, 60 oC and 80 oC), proving temperature
change does not influence its performance. (c, d) Schematic illustration showing the fabrication
of an integrated combo temperature-pressure E-skin sensor (c) and a combo E-skin sensor matrix
(d).
The integration of temperature sensors and strain sensors into E-skin sensor matrix requires that
each sensor responds to corresponding specific stimulus while does not response to other
stimulus. To test whether the silk-based temperature sensor would show response to pressure
stimuli, the resistance change of the temperature sensor was measured when different pressure
stimuli were loaded on the sensor by placing different weights (inset in Figure 4a) on it. As
shown in Figure 4a, the temperature sensor showed almost no resistance change under different
pressures, indicating no response to the external pressure stimuli, which can be ascribed to the
intrinsic flexibility of the SilkCFM. Accordingly, the sensing performance of the silk-based
strain sensor under different temperatures was also tested. The relative resistance change of the
strain sensor versus temperature under 6.5% strain was similar with that under 0% strain (Figure
4b), indicating the performance of the strain sensor was almost not affected by temperature.
Noted that the absolute resistance of the strain sensor will increase with the increment of
temperature induced by the thermal expansion of the PDMS. Based on the above results, two
individual silk-based temperature and strain sensor or their array can be integrated into
transparent and flexible combo E-skin sensor or sensor matrix through a lamination scheme to
imitate the simultaneous sensing of temperature and pressure of human skin (Figures 4c and 4d).
Pioneer works have reported multifunctional E-skins based on other functional materials, such as
ultrathin single crystalline silicon nanoribbons40 and hydrogels41. In comparison, the
characteristics and merits of the SilkCFM based dual-mode E-skin can be summarized as the
following: i) The active sensing materials (SilkCFM) is made from silk, which is biocompatible,
human and environment benign and commercially available at low cost. ii) The carbonized
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nanofiber membranes are lightweight, and have high chemical stability and thermal stability,
benefiting the wearable applications in various environment. iii) The fabrication process of the
dual-mode E-skins does not involve complicated manufacturing techniques, which is important
for the practical applications of E-skins.
The obtained combo E-skin can simultaneously detect temperature and pressure under various
stimuli, which were demonstrated by monitoring human activities, such as exhaling and finger
pressing. The output signals of each sensor in the obtained combo temperature-pressure sensor
were measured when the E-skin was under exhaling stimulus (Figure 5 a,b) and under finger
pressing (Figure 5 c,d). When a volunteer exhaled to the combo sensor, the resistance of the
temperature sensor first decreased because of the halitus-induced rising surrounding temperature
and then increased because of the decreased surrounding temperature induced by the evaporation
of condensed water. Therefore, the silk-based temperature sensor in the combo sensor exhibited
cyclic and stable signals to the cyclic exhaling stimulus (the red curve in Figure 5b). At the same
time, no obvious resistance variation of the silk-based strain sensor was observed (the blue curve
in Figure 5b). In contrast, the finger pressing resulted in the resistance change of both sensors in
the combo sensor (Figure 5d). The resistance of the temperature sensor decreased when a human
finger touched the combo sensor and tardily increased to the initial level (red curve in Figure 5d).
The strain sensor responded rapidly to the finger pressing, as indicated by the immediately
increased and decreased resistance (blue curve in Figure 5d). The slower sensing response of the
temperature sensor than the pressure sensor can be attributed to the slow heat transfer.
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Figure 5. Schematic illustration and simultaneous sensing performance of the combo E-skin
sensor under various stimuli. (a-d) exhaling stimulus (a, b) and finger pressing stimulus (c, d).
Red and blue curves in (b) and (d) refer to the electrical response of the silk-based temperature
and strain sensor, respectively. (e) Schematic illustration of the combo sensor matrix with a cup
of ice water placed onto it. (f, g) Distribution of the relative resistance change of the temperature
(f) and pressure (g) sensor arrays during the loading of a cup of ice water on the combo sensor
matrix.
Furthermore, a proof-of-concept combo E-skin sensor matrix with a 3 x 3 array, as illustrated in
Figure 4d, was fabricated and was tested for its ability to resolve the spatial distribution of the
external stimuli. A photograph of an actual E-skin sensor matrix with a 3 x 3 array is shown in
Figure S7 for easier understanding the integration structure. Temperature and pressure stimuli
were simultaneously loaded on the sensor by placing a cup of ice water on the combo sensor
matrix (Figure 5e). The electrical signals of the silk-based temperature and pressure sensor arrays
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were monitored. As shown in Figures 5f and 5g, the output electrical signals from the
temperature and strain sensors clearly demonstrated the loading of the cold object. The
temperature and pressure sensors under the cup showed obviously increased resistance because
of the cold object induced decreased temperature and certain pressure stimuli, whereas sensors
far from the contact regions exhibited almost no response. The above results clearly manifested
the ability of the silk-based combo E-skin sensor to mimic human skin to sense temperature and
pressure simultaneously, indicating its high potential for applications in human–machine
interface and human activity detection.
3. CONCLUSIONS
In summary, the fabrication of silk-based transparent and flexible combo temperature-pressure
sensors were demonstrated for the first time. Electrically conductive silk-derived carbon
nanofibers were utilized as the active sensing materials for both temperature and strain sensors.
The silk-based temperature sensor exhibited a high sensitivity of approximately 0.81% per
centigrade and showed nearly no response to external pressure stimulus. The silk-based strain
sensor displayed combined features of large strain sensing range (over 50% strain), ultrahigh
sensitivity with a GF of approximately 8350 at 50% strain as well as excellent stability and
durability. The sensing performance of the strain sensor will not be influenced by external
temperature stimulus. Based on the outstanding performance of both sensors, a flexible
temperature-pressure E-skin sensor can be prepared by integrating silk-based temperature and
pressure sensors through a lamination strategy. The obtained combo E-skin sensor can
monitordaily life sensations, such as exhaling and finger pressing, and detect the spatial
distribution of external stimuli, showing great potential for applications in human–machine
interface and human activity monitoring. The concept of utilizing silk-derived graphitic-like
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carbon nanofibers as active sensing materials in E-skin is innovative and possesses obvious
superiority, such as the good compatibility and relative low-cost of the natural silk material, as
well as the facile and large-scalable fabrication process for active materials and devices, paving a
practical avenue for the fabrication of high performance multi-mode E-skin.
4. EXPERIMENTAL SECTION
Preparation of SilkCFM. The silk nanofiber membrane deposited on copper foil was prepared
through an electrospinning process as reported in our previous work.42 Silk fibroin (extracted
from Bombyx mori silkworm cocoons) solution in formic acid (15% w/v) was used to prepare
silk nanofiber membrane with a solution flow rate of 0.6 ml/h and an electrical intensity of 1
kV/cm. To achieve the conductive SilkCFM, the electrospun silk nanofiber membrane on the
copper foil was thermally treated at 1050 oC under a mixed atmosphere of high-purity argon (100
sccm) and hydrogen (10 sccm) through a heating schedule reported in our previous work27
except for that the silk nanofiber membrane was treated at 220 oC for 1 hour to induce the
formation of β-sheet crystallites43 to avail the formation of graphitic nanocarbon.
Fabrication of silk-based sensors and integrated E-skin. For the fabrication of the silk-based
temperature sensor and strain sensor, the SilkCFM was transferred onto the flexible PET
substrate and PDMS substrate (10:1 of the weight ratio of the base prepolymer and the
crosslinking agent), respectively, through an elastomeric stamp strategy. For both of the sensors,
copper wires were connected to the SilkCFM at two ends with silver paste for the convenience of
measurements. Afterwards, liquid PDMS was dropped onto the surface of SilkCFM on PET
substrate or PDMS substrate to encapsulate the sensors. Noted that the silk-based strain sensors
should be first trained by several stretching-releasing cycles to induce the fracture of nanofibers
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throughout the SilkCFM and the contact points between fractured nanofibers to achieve the
sensory capacity for strain. As results, the SilkCFM in the temperature sensor maintains the
intact continuous nanofiber structure (Figure S8a) and the SilkCFM in the strain sensor shows
fractured nanofibers (Figure S8b). Then, air plasma was used to treat the contact surface of the
temperature sensor and the strain sensor before putting them face-to-face. To obtain the final
combo temperature-pressure E-skin, a temperature sensor was put on the top of a pressure sensor
which was supported by an Ecoflex substrate. For better understanding the structure, a schematic
illustration of the side view of the combo temperature-pressure E-skin is shown in Figure S9.
Characterization of SilkCFM and Sensing Performance of silk-based devices. A field
emission SEM (Quanta 650, FEI) and TEM (JEM2010F, JEOL) was used to characterize the
morphology and microstructure of the SilkCFM, respectively. The Raman spectra of the pristine
electrospun silk nanofiber and SilkCFM were obtained using a Raman spectroscope with a laser
excitation wavelength of 532 nm (HR800, HORIBA). An UV-visible light spectrophotomer
(UV-2600, SHIMADZU) was utilized to evaluate the optical transmittance of the silk-based
devices. A digital meter (Keithley 2400) with a constant voltage of 5 V was used to measure the
electrical signals of the silk-based temperature and strain sensors. The temperature stimulus
applied for measurement of temperature sensing performance was controlled by a hot plate. The
loading of tensile strain and pressure was performed by using a universal testing machine (AGSX, SHIMADZU).
ASSOCIATED CONTENT
Supporting Information Available: Supporting figures (Figure S1-S6). This material is
available free of charge via the Internet at http://pubs.acs.org
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ACKNOWLEDGMENT
This work was supported by the NSF of China (51672153, 51422204, and 51372132) and the
National
Key
Basic
Research
and
Development
Program
(No.2016YFA0200103,
No.2013CB228506).
Author Contributions
Y. Y. Zhang and C. Y. Wang. conceived the project and designed the experiments. Y. Y. Zhang
supervised the project. C. Y. Wang performed most of the experiment. K. L. Xia, M. C. Zhang
and M. Q. Jian contributed to part of the measurement of sensing performance. All authors
discussed the results and commented on the manuscript.
Notes
The authors declare no competing financial interests.
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Table of Contents
A combo temperature–pressure E-skin sensor is fabricated with silk-derived carbon
nanofibers as the active material through a facile lamination strategy, which can simultaneously
sense temperature and pressure stimuli with high sensitivity. The combo sensor shows sensitivity
of 0.81% per centigrade to temperature and a gauge factor of 8350 to strain with high stable
performance.
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