Tunable wrinkled graphene foams for highly reliable piezoresistive sensor

Accepted Manuscript Title: Tunable wrinkled graphene foams for highly reliable piezoresistive sensor Authors: Yan Zhong, Xianhua Tan, Tielin Shi, Yuanyuan Huang, Siyi Cheng, Chen Chen, Guanglan Liao, Zirong Tang PII: DOI: Reference:

S0924-4247(18)31032-X https://doi.org/10.1016/j.sna.2018.09.002 SNA 10977

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

20-6-2018 1-9-2018 3-9-2018

Please cite this article as: Zhong Y, Tan X, Shi T, Huang Y, Cheng S, Chen C, Liao G, Tang Z, Tunable wrinkled graphene foams for highly reliable piezoresistive sensor, Sensors and amp; Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tunable wrinkled graphene foams for highly reliable piezoresistive sensor Yan Zhong1, Xianhua Tan1, Tielin Shi1, Yuanyuan Huang1, Siyi Cheng1,Chen Chen1, Guanglan Liao1, and Zirong Tang1* 1

State Key Laboratory of Digital Manufacturing Equipment and Technology,

IP T

Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China

SC R

E-mail: [email protected]

N

PT

ED



A



Wrinkled graphene foams is prepared with a simple and novel method. The piezoresistive pressure sensor based on wrinkled graphene foams exhibits a reliable stability (>105 cycles). The piezoresistive pressure sensor shows short response time（0.2 s） and low relaxation time (0.15 s) The sensor demonstrates good performances in various applications, such as pulse detection, voice recognition as well as the finger joints movement.

M

 

U

Highlights

The rapid development of flexible electronics and artificial intelligence brings an urgent

CC E

demand for high-performance flexible pressure sensors. In this paper, a method is proposed to prepare wrinkled graphene foam by freeze-drying and post-annealing

A

method, where zinc chloride is used to tune structure. A piezoresistive pressure sensor based on the wrinkled graphene foam is assembled. Benefiting from the unique contact interface of wrinkled microstructures, as well as the mechanical strength and the superior resilience of the foam structure, the piezoresistive pressure sensor exhibits a reliable stability (>105 cycles), short response time（150 ms）and low relaxation time

(120 ms). And it also demonstrates good performances in various applications, such as pulse detection, voice recognition as well as the finger joints movement. It is suggested that the pressure sensor based on wrinkled graphene foam has great potential in flexible electronics to achieve health monitoring and motion detection. It also paves the way for

IP T

other wrinkled materials to be involved in the fabrication of pressure sensors. Keywords: wrinkled graphene, piezoresistive sensor, reliable stability, pulse detection,

SC R

voice recognition

U

1. Introduction

N

In recent years, with the rapid development of flexible and wearable electronic

A

technology, flexible and portable devices are highly desirable1-5. Among those products,

M

light-weight, highly sensitive and flexible pressure sensors have drawn a lot of interests

ED

because they can be easily integrated into artificial skin and show great significance in the medical diagnosis 6-8.

PT

According to the sensing mechanisms, pressure sensors can be divided into several

CC E

types, including transistor sensor 9-11, piezoresistive sensor 12-16, capacitive sensor17-19, and piezoelectric sensor

20-22

. Among of these types, piezoresistive sensors are paid a

lot of attention owing to their simple device design, convenient readout mechanism as

A

well as outstanding sensing performance19, 23-25. Traditional silicon-based piezoresistive sensors have high sensitivity, but rigid substrate limits their application in the flexible devices. Recently, extensive research have been focused on the material for piezoresistive sensor to meet the demand of flexibility, such as tissue paper coated by

gold nanowires26, monolayer capped by nanoparticles27, banana leaves-modeling PDMS microstructure decorated by silver28, carbon nanotube-polymer sponge

29

and

crumpled reduced graphene oxide sheets30. Although sensing materials are different, the sensing mechanisms of the piezoresistive sensors are almost in common. When the

IP T

pressure is applied, the contact area of the sensing material becomes larger, leading to a more conductive channel and decreased resistance. However, it is still urgent to

SC R

fabricate low-cost and highly reliable sensing material to meet the requirements of the practical applications of flexible and wearable pressure sensors.

U

Recently, graphene has won widespread researches due to its superior conductivity,

N

stable physical properties and accessible structure design. There are various researches

A

about graphene-based pressure sensor. For example, Tao et al. 31 mixed the tissue paper

M

with the graphene oxide (GO) solution to obtain a GO paper followed by heat treatment

ED

reduction. A pressure sensor based on tissue paper coated by reduced graphene has wide pressure ranges of 0-20 kPa and a reliable stability over 300 cycles. Yao et al. 16 used

PT

the polyurethane (PU) sponge as the substrate and fabricated the graphene pressure

CC E

sensor through simple dip-coating and hydrothermal method. The obtained pressure sensor showed stable performance under repeated loading and unloading pressure of 2 kPa over 104 cycles. Different structures based on graphene show long-cycle stability

A

in the application of piezoresistive sensor. In view of the porous structure of graphene sponge or foam with mechanical flexibilities and superior resilience, it is expected to be a good alternative to improve the reliable stability of the pressure sensor 32-35.

Another point drawing researchers’ interest is the wrinkled structure-based piezoresistive pressure sensor. For instance, Mu et al. 36 proposed a straightforward and versatile balloon-blowing method for fabricating hierarchically wrinkled elastic transparent conductor and had a good effect as artificial muscles and sensors. Chen et

IP T

al. 37 reported a ultrathin, and transparent pressure sensor based on wrinkled graphene prepared by a facile liquid-phase shrink method, and the pressure sensor exhibited

SC R

ultrahigh operating sensitivity (6.92 kPa-1). In comparison with the film smooth, the wrinkled structures can respond to a relatively small pressure, where the contact

U

interface of microstructures is prone to change, thus leading to changed current

N

pathways. Due to the high conductivity and the enhancement of responsivity, the

A

wrinkled graphene meets the requirements of high-performance piezoresistive sensors.

M

How to integrate the wrinkled structure with graphene foam to fabricate piezoresistive

ED

pressure sensors with outstanding stability and rapid response is significant to the application of sensors.

PT

In this paper, a simple and novel method has been proposed to prepare wrinkled

CC E

graphene foams, and a piezoresistive pressure sensor is fabricated based on the wrinkled structure, where zinc chloride is used to tune structure of the graphene foam38-39. The as-synthesized wrinkled graphene foam exhibits outstanding mechanical behavior and

A

superior resilience. Benefiting from the contact interface of wrinkled microstructures and mechanical strength of the foam structure, the pressure sensor based on the above structure can demonstrate long-cycle stability. In addition, the graphene-based pressure

sensor can realize the measurement of wrist pulse, voice recognition and finger joints movement, showing great potential in the practical application of wearable devices. 2. Experimental Section 2.1. Materials

IP T

Zinc chloride (ZnCl2) and hydrazine hydrate (N2H4) was purchased from Aladdin Industrial Inc. Graphene oxide was purchased from Suzhou Tanfeng Company.

SC R

Polyimide tape was purchased from 3M Company. All the reagents were used without further purification.

U

2.2. Synthesis of pure graphene foam and wrinkled graphene foam

N

5mL graphene oxide (GO) colloidal suspension (5 mg mL-1) was under ultrasonication

A

for 2 h. ZnCl2 powders were dissolved in the DI water to form ZnCl2 (2 M) solution.

M

Then the GO aqueous solution was mixed with ZnCl2 solution (0.5mL 2 M). After

ED

stirring 15 min, the gel was injected into cylindrical mold with a syringe. The cylindrical ZnCl2/GO foam was obtained through freezing in the refrigerator and

PT

freeze-drying for 12 h. Cylindrical ZnCl2/GO aerogel with the mold was put into a

CC E

reactor, then hydrazine hydrate (150 uL) was dropped and the reactor was sealed and reacted in the drying oven at 100oC for 8h. Then the foam was thermally treated under 650 oC for 1 h with argon flow. After cooling down to room temperature, the sample

A

was repeatedly washed with HCl solution (1M) and de-ionized water, followed by drying overnight to obtain the final wrinkled graphene foam FR-5-650-W (FR-N-T-W, N represented the mass of ZnCl2, T represented the annealing temperature and W represented the washing treatment). The porosity and density of graphene foam were

precisely tuned by controlling ZnCl2 evaporation mass, which is sensitive to heating temperature and ZnCl2 concentration. The similar fabrication process with ZnCl2 solution changing to 0.2 mL and 1 mL is used to obtain the FR-2-650-W and FR-10650-W samples. The case without adding ZnCl2 and annealing produced the sample

IP T

named the RGO, and the case with annealing is named the RGO-650. 2.3. Sensor fabrication

SC R

The resistive-type pressure sensor was fabricated by sandwiching one piece of wrinkled graphene foam between two ultrathin polyimide tape, where the carbon wires serve as

U

conducting wire to connect the top and bottom of the wrinkled graphene foam.

N

2.4. Characterization instrumentation

A

The morphology of pure reduced graphene foam and wrinkled graphene foam was

M

monitored by the Transmission electron microscopy (TEM, FEI, Tecnai G2 20) and

ED

field-emission transmission electron microscopy (FETEM, FEI, Tecnai G2 F30). Crystal structure of products were characterized with X-ray diffractometer (XRD; X9

PT

Pert PRO, PANalytical B.V., the Netherlands) with radiation from a Cu target (Ka,

CC E

l50.15406 nm). The ordering of mesostructure was analyzed by a lower-angle X-ray diffractometer system (RIGAKU D/MAX-RB, Japan) with a Cu Ka radiation source generated at 40 kV, 30 mA. The home-made force supplied equipment was used to

A

carry

out

dynamic

pressure

measurements.

Displacement

platform

(SaiFan7STA03150B) which was controlled by uniaxial motion controller (SaiFan7SC301) connected to the computer was used to apply the dynamic pressure and the force gauge (ZHIQU DS2-500N) was used to record the force value in real-

time. The Autolab workstation (PGSTAT-302N) was used to measure the real-time I-t curves. 3. Result and Discussion 3.1. Preparation and Characterization

CC E

PT

ED

M

A

N

U

SC R

IP T

3.1.1. Fabrication Process and Structure of the wrinkled graphene foam

A

Fig. 1. (a) Schematic diagrams of the fabrication process of the wrinkled graphene foam. (b-e) Different magnification SEM figures of as-synthesized wrinkled graphene foam. (f) Optical images showing the wrinkled graphene foam changes in response to loading and unloading process.

Fig. 1a illustrates the fabrication process of the piezoresistive sensor based on wrinkled graphene foams. In brief, GO solution was mixed with ZnCl2, and then the mixture was

poured into a cylindrical mold and freeze-dried. Finally, the graphene foam was obtained through reduction by hydrazine, heat-treatment and washing. The different magnification SEM figures of the wrinkled graphene foam are shown in the Fig. 1b-e. Large graphene lamellae is seen in the graphene foam, where rich pores are distributed

IP T

on the rough and wrinkled graphene layers with small bulges. Digital photos of the asprepared wrinkled graphene foam are shown in the Fig. 1f. Through compression and

SC R

relaxation, the graphene foam can return to its initial state. Benefiting from the wrinkled

and interconnected graphene structure, the graphene foam have compressive

U

deformation on applied forces and also can recover its initial state on unloading,

A

great potential in the pressure sensor.

N

exhibiting outstanding mechanical behaviors and superior relisilence, which shows

M

3.1.2. Characterization of Pristine Graphene and Wrinkled Graphene Foam tuned by

A

CC E

PT

ED

ZnCl2

Fig. 2. Low and high magnification SEM figures of as-synthesized RGO-650 (a and e), FR-2-650W (b and f), FR-5-650-W (c and g), and FR-10-650-W (d and h).

To investigate the effect of ZnCl2 loading mass on the graphene foam, the scanning electron microscope (SEM) images of RGO-650 and FR-N-650-W samples are shown in Fig. 2. The pristine RGO-650 sample seems fragile and relatively smooth (Fig. 2a and e). Due to the weak connection between dispersed graphene layers, the whole

IP T

graphene foam is prone to be damaged. In comparison, more wrinkled structures are observed in the FR-N-650-W samples with the increasing mass of ZnCl2 (Fig. 2f-h).

SC R

The digital pictures of the graphene foams tuned by ZnCl2 are shown in Fig.

S1(Supporting Information). Graphene foams get smaller with increasing ZnCl2

U

loading mass. The diameter and thickness of the samples as well as the shinkage

N

thickness are shown in Table S1(Supporting Information). In order to ensure the

A

samples before carbonization in the the same size, the molds with the same size are

M

used. After carbonization, the RGO-650 samples have no shinkage, while the FR-2-

ED

650-W samples show 10-20% shinkage in thickness with the diameter shinking. FR-5650-W and FR-10-650-W samples show ~70% and ~90% shinkage in thickness,

PT

respectively. It is mainly because that ZnCl2 evaporation leads to the shrinkage of the

CC E

whole structure, and a more shrinkage degree is obtained with the increasing ZnCl2 loading mass. It is worthy pointing out that the shrinkage structure is beneficial to the interconnection between the graphene layers, and keeps the integral structure free of

A

damage. Transmission electron microscopy (TEM) is also conducted to further verify the morphology of RGO-650 and FR-N-650-W samples, shown in Fig. S2(Supporting Information). The FR-5-650-W samples have rich micropores with diameter about 1～ 2 nm while rich macropores with diameter about 500 nm was observed in FR-10-650-

W samples. From the above results, it illustrates that ZnCl2 plays an important role in

PT

ED

M

A

N

U

SC R

IP T

tuning structure of the porous wrinkled graphene foam.

CC E

Fig. 3 (a) Nitrogen adsorption isotherms of the graphene foams. (b) Pore size distributions of the graphene foams. (c) Raman spectra of the prepared foam. (d) FTIR of RGO, FR-5 and WFR-5 samples. (e) X-ray photoelectron spectra of samples. (f) XRD spectrum of the FR-2 sample under different temperature.

N2 adsorption-desorption isothermal analysis is also conducted to investigate the

A

microstructure of the samples (Fig. 3a). The pristine graphene foam samples show a hysteresis loop at P/P0 > 0.5 , indicating the existence of mesoporous structures. The specific surface areas (SSA) are calculated by using the Brunauer–Emmett–Teller (BET) model, listed in Table 1. Clearly, the RGO-650 has a SSA of 126.1 m2 g-1, while

it is 148.2 m2 g-1 for FR-2-650-W sample after ZnCl2 tuning, with the total pore volume increased from 0.275 to 0.322 cm3 g-1. Besides, the mesopore peak of FR-2-650-W shifts to 2.78 nm from the 4.06 nm of RGO-650 samples (Fig. 3b). These results illustrates that ZnCl2 tunes the mesopores of graphene foams. However, when ZnCl2

IP T

loading mass continue to increase, the SSA of FR-5-650-W and FR-10-650-W is decreased to 85.73 and 75.90 m2 g-1, respectively. It is inferred that the mesopores with

SC R

smaller size are etched to form large-size mesopores even macropores during heat

treatment. In addition, the graphene layers gather and interconnect with each other, and

U

shrink during heat treatment, leading to a decreased surface area and a wrinkled

N

structure. These results indicate ZnCl2 can tune graphene structure to obtain a wrinkled

A

foam with good mechanical behavior and superior reselience.

M

Table 1. Surface area and the pore volume of the as-prepared graphene foams

PT

FR-2-650-W FR-5-650-W

Vtotal a [cm3 g-1]

126.1

0.275

134.4

0.322

85.73

0.191

75.90

0.185

ED

RGO-650

SSA [m2 g-1]

FR-10-650-W

CC E

a Total pore volume.

Raman spectroscopy is carried out for studying the chemical composition of the

A

graphene foams before and after ZnCl2 tuning (Fig. 3c). Two characteristic peaks located at 1359 (D band) and 1596 cm-1 (G band) are observed, which are assigned to the breathing modes of C sp2 atoms in rings and the bond stretching of C sp2 atoms in both chains and rings40-41, respectively. The RGO-650 samples (1.18) after heat treatment at 650oC exhibit a considerably lower ID/IG ratio than the pure RGO foam

(1.28), indicating surface groups on graphene layer are removed after heat treatment. Slight differences between the RGO-650 and FR-5-650-W samples are shown, confirming that ZnCl2 does not alter the chemical properties of the graphene monolith. However, the graphene activated by KOH, with the same mass ratio as for ZnCl2, shows

IP T

a higher Raman spectroscopy peak intensity ratio (1.32) (ID/IG), indicating a much more disordered structure (Fig. S3, Supporting Information).

SC R

Fourier transform infrared spectroscopy (FTIR) is also conducted for the RGO, FR-5 and the samples after HCl washing (WFR-5). As shown in Fig. 3d, FR-5 gives rise to

U

strong bands at about 3576 and 1606 cm-1, which are due to the scissoring vibrations of

N

–OH– and aromatic C=C band, respectively. After removing the ZnCl2, the WFR-5

A

sample shows weaker peaks at the about 3576, 1399 and 1206 cm-1 than the RGO

M

samples, which are characteristic signals of the –OH–, C-O and C-OH bands. It

ED

illustrates the introduction of ZnCl2 can facilitate dehydrogenation, which changes the functional groups on the graphene surfaces.

PT

The bonding states and surface functionalities of the samples are also measured with

CC E

X-ray photoelectron spectroscopy (XPS) (Fig. 3e). The RGO-650 samples show a higher C/O ratio than RGO samples, further indicating heat treatment can remove surface groups on the graphene. Especially, with ZnCl2 loading mass increased, higher

A

C/O ratio is obtained, indicating the deoxygenation effect of ZnCl242-43. XRD is used to characterize the samples before and HCl washing (Fig. S4, Supporting Information). After HCl washing, no peaks of ZnO are observed in the FR-5-650-W samples, indicating that the ZnO can be removed successfully. The FR-5 samples

through heat treatment under different temperature before washing are also characterized (Fig. 3f). By heating to the temperatures of 900oC, ZnO can be simply removed, indicating no further washing, which renders the whole process even more convenient38-39.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

3.2. Characterization of Device Sensitivity, Cyclic Test and Response Time

Fig. 4. (a) Top picture is the schematic diagram of a wrinkled graphene foam sensor, and the bottom plot is the schematic diagram of the pressure sensor in response to loading and unloading. (b)

Pressure−response curves for the wrinkled graphene foam sensor FR-2-650-W, FR-5-650-W and FR-10-650-W. (c) Enlarged pressure−response curves for the FR-2-650-W sensor. (d)Timedependent variation of relative currents change over four cycles of applied various pressure on the top of the wrinkled graphene foam sensor. (e) Short response and relaxation curve of the wrinkled graphene foam sensor. (f) A magnified transient response and recovery curve of the sensor. (g)

IP T

Durability test under a pressure ranging from 1.08 kPa to 2.52 kPa at a frequency of 2 Hz.

Based on the wrinkled graphene foam, the sensor device is assembled. The wrinkled

SC R

graphene foam is sandwiched between two polyimide tapes, where two carbon wires

serve as electronic wiring material to connect the both sides of graphene foam. As shown in Fig. 4a, an external voltage is applied on the sensor. The current in the loop

N

U

is changed with the resistance of the graphene foam, which depends on the applied

A

forces. The sensing mechanism is pressing force-dependent contact between wrinkled

M

graphene layers. On loading, the pressure sensor is prone to compressive deformation, leading to more conductive pathways, which causes an increase in current (Fig. 4a).

ED

Due to the porous and wrinkled structure, the foam can recover to its initial shapes once

PT

the applied forces are removed, therefore, leading to a decrease in current. To measure the response of our pressure sensor, a home-made system containing

CC E

displacement platform and force gauge is assembled. As shown in Fig. 4b, the pressure sensor shows a response to the pressure of 9.19 kPa. The sensitivity (S) can be defined

A

as S = (∆𝐼/𝐼0)/∆P, where I0 and ΔI are corresponding to the initial current and the change of current under pressure loading, respectively 44-45. The enlarged plot of FR-2650-W pressure sensor is shown in the Fig. 4c, which displays a sensitivity of about 1.16 kPa−1 in the low pressure range (0.06–2.82 kPa) while the FR-5-650-W sample shows a sensitivity of 0.20 kPa−1 in the 0–3.7 kPa pressure range and the sensitivity of

FR-10-650-W sample is 0.11 kPa−1 in the 0–2.8 kPa pressure range, which is consistent with the BET results. Comparing with the pristine graphene foam with a sensitivity of 0.32 kPa−1 in the 0–3.3 kPa pressure range (Fig.S6, supporting information), it illustrates that the ZnCl2 can increase the sensitivity of pressure sensor when the

IP T

suitable ZnCl2 mass is used to obtain FR-2 samples. However, with the ZnCl2 loading mass continue to increase, the samples become more wrinkled and shrink a lot (up to

SC R

90%) to form a compact structure, leading to a decreased surface area and a decreased

sensitivity. To testify the performance of the as-prepared pressure sensor, dynamic

U

force measurements are also carried out with multiple loading-unloading cycles under

N

different pressures. As shown in Fig. 4d, the pressure sensor depicts good performances

A

under dynamic pressure ranging from 0.93 kPa to 3.38 kPa, indicating a reliability in

M

the practical application. Response and relaxation time are important parameters for

ED

pressure sensors. To ascertain the time parameters, an interval time of 0.01 s is adopted in the dynamic measurements under pressure from1.13 kPa to 1.33 kPa (Fig.4e). The

PT

enlarged plot is displayed in the Fig. 4f, showing the response and relaxation time of

CC E

150 ms and 120 ms, respectively. The short response time is benefit from the immediate change of the contact area of wrinkled surfaces on loading process. The relaxation time is decided by speed of the surfaces elastically recovering to its initial shapes once the

A

external pressure is removed. Cycle life is another important parameter for the practical application of a pressure sensor. Dynamic force measurements are performed with 105 loading–unloading cycles under pressures ranging from 1.38 kPa to 3.58 kPa at a frequency of 1 Hz (Fig.4g). The enlarged plots of 201th-210th cycles and 99501th-

99510th cycles are shown on the top of Fig. 4g. Note that the sensor maintains high signal-to-noise ratios and shows little changes in the current amplitude after 105 loading–unloading cycles, exhibiting a good stability. The unique structure with rich pores and wrinkled surfaces are responsible for the outstanding response performance.

U

SC R

IP T

3.3. Sensing Mechanism

N

Fig. 5. Schematic diagram of sensing mechanism in response to loading and unloading.

M

A

In order to further clarify the sensing mechanism of the wrinkled graphene foam-based pressure sensor, the schematic diagram is shown in the Fig. 5. Several randomly

ED

distributed graphene layers in the foam are established to analyze this mechanism. In

PT

the analysis, contact points between the layers are set to describe the conductivity of the whole foam, where the conductivity is dependent on the numbers of the contact

CC E

pairs of layers. In the initial state, the graphene layers show porous and wrinkled surfaces, where contact pairs are limited due to the porous structure, leading to bad

A

conductivity. When a pressure is applied, the graphene layers get more closely and even more flat due to compressive deformation, where contact pairs are increased with the increasing pressure, leading to more conductive pathways, and thus causing an increase in current. In a word, the applied pressure controls the numbers of contact pairs, and thus decides the resistance of the whole pressure sensor.

M

A

N

U

SC R

IP T

3.4. Human Physiology Monitoring and Voice Recognition

ED

Fig. 6. The current responses and optical image of the pressure sensor of dynamic loading and unloading cycles for detection of pressing forces (a, f) and bending forces (b, g). (c) Relative current

PT

change and (h) optical image of the pressure sensor for monitoring various wrist position pulse pressure. (d) Magnified waveform extracted form (c) showing three peaks (physical condition–age:

CC E

27 years; height: 168 cm; and weight: 56 kg). (e) In situ relative current change of the wrinkled graphene foam sensor when wearer spoke “Pressure”, “Sensor” and “OK”. (i) Optical images showing the wrinkled graphene foam sensor directly above the neck with various sound stimuli.

A

In consideration of the rapid response and reliable repeatability properties, the sensor are applied in a lot of applications, such as detection of mechanical forces as well as monitoring human's pulse beat. As shown in Fig. 6a, the wrinkled graphene pressure sensor can be used to detect the pressing forces. When instantaneous forces from finger

tips are applied on the pressure sensor (Fig. 6f), a high signal-to-noise ratio and significant changes in current amplitude are observed during in the measurements. In addition to pressing forces, bending forces are also been detected (Fig. 6g). As shown in Fig. 6b, the device exhibits stable responses to bending forces. These experiments

IP T

pave the way of applications of detection of instantaneous forces, such as typing forces on the keyboard. Monitoring wrist pulse in real-time is an important tool to detect

SC R

human’s health, which is meaningful to offer some clinically information about cognitive state, cardiovascular health and many other human physiological

U

parameters44, 46. A polyimide tape is used to keep the pressure sensor fixed on the wrist

N

(Fig. 6h), which applies certain pretightening forces in favor of detection. As displayed

A

in Fig. 6c, the pressure sensor shows a stable response with high signal-to-noise ratios

M

to the pulse beat. Calculating from the testing curve, the pulse rate of

ED

the measured person sitting quietly is approximately 62 times /min, which is within the normal range. The enlarged plot is in the Fig. 6d. Under normal condition, a typical

PT

radial artery pulse waveform can be separated into two clearly distinguishable peaks

CC E

(P1 and P3) and a late systolic augmentation shoulder (P2) 47, which refer to percussion, diastolic and tidal, respectively. The blood pressure from the left ventricle contracts and reflective wave from the lower body is responsible for the line shape. The arterial

A

stiffness is characterized by radial augmentation index AIr (AIr = P2/P1), which reflects the age of people 47. The value of AIr in the above experiment is estimated to be 0.8, which is in agreement with the data of a healthy 26-year-old person in the literature. The pressure sensor can also be used for the measurement of the muscle

movement during speech. As shown in Fig. 6i, the pressure sensor is fixed on the neck with the polyimide tape. When the tester speak words, such as “Pressure”, “Sensor” and “OK”, significant signature patterns are exhibited in the signal curves (Fig. 6e), which are mainly attributed to the complex muscle movements during speech. It has

devices48, 49, which gives an application in speech rehabilitation training.

SC R

4. Conclusion

IP T

been reported that the wearable sensors are served as the specific phonation recognition

In summary, a simple method has been proposed for the fabrication of wrinkled and

U

porous graphene foam through freeze-drying and post-annealing treatment. A

N

piezoresistive pressure sensor is fabricated based on the wrinkled graphene foam.

A

Benefiting from the unique contact interface of wrinkled microstructures and the

M

mechanical strength of foam structure, the fabricated pressure sensor demonstrates a

ED

good perforamance, such as a short response time and low relaxation time (150 ms and 120 ms), and a good reliability over 105 loading-unloading cycles. It also exhibits

PT

satisfactory results in various applications such as detecting pulse detection, voice

CC E

recognition as well as monitoring the finger joints movement, confirming its ability in practical application. The results demonstrates that such a pressure sensor based on wrinkled graphene foam offers sufficient feasibility for the development of various

A

wearable electronics. Acknowledgements This work is supported by the National Science Foundation of China (No. 51275195 and No.51775218), the National Basic Research Program of China (No.

2015CB057205), the Program for Changjiang Scholars and the Innovative Research Team in University (grant No. IRT13017). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for the field emission scanning electron microscopy (FESEM) testing, as well as Flexible Electronics

IP T

Research Center of HUST. Reference

A

CC E

PT

ED

M

A

N

U

SC R

[1] Q. Sun, W. Seung, B.J. Kim, S. Seo, S.W. Kim, J.H. Cho, Active Matrix Electronic Skin Strain Sensor Based on Piezopotential‐Powered Graphene Transistors, Advanced Materials, 27(2015) 3411-7. [2] J.H. Lee, H.J. Yoon, T.Y. Kim, M.K. Gupta, J.H. Lee, W. Seung, et al., Micropatterned P (VDF‐TrFE) Film‐Based Piezoelectric Nanogenerators for Highly Sensitive Self‐ Powered Pressure Sensors, Advanced Functional Materials, 25(2015) 3203-9. [3] L. Lin, Y. Xie, S. Wang, W. Wu, S. Niu, X. Wen, et al., Triboelectric active sensor array for self-powered static and dynamic pressure detection and tactile imaging, ACS Nano, 7(2013) 8266-74. [4] W. Wu, X. Wen, Z.L. Wang, Taxel-addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging, Science, 340(2013) 9527. [5] Y. Liang, F. Zhao, Z. Cheng, Q. Zhou, H. Shao, L. Jiang, et al., Self-powered wearable graphene fiber for information expression, Nano Energy, 32(2017) 329-35. [6] K.Y. Lee, M.K. Gupta, S.-W. Kim, Transparent flexible stretchable piezoelectric and triboelectric nanogenerators for powering portable electronics, Nano Energy, 14(2015) 139-60. [7] Y. Yang, H. Zhang, Z.-H. Lin, Y.S. Zhou, Q. Jing, Y. Su, et al., Human skin based triboelectric nanogenerators for harvesting biomechanical energy and as selfpowered active tactile sensor system, ACS Nano, 7(2013) 9213-22. [8] Y. Hu, Z.L. Wang, Recent progress in piezoelectric nanogenerators as a sustainable power source in self-powered systems and active sensors, Nano Energy, 14(2015) 314. [9] J. Voorthuyzen, P. Bergveld, A. Sprenkels, Semiconductor-based electret sensors for sound and pressure, IEEE transactions on electrical insulation, 24(1989) 267-76. [10] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, A large-area, flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications, Proceedings of the National Academy of Sciences of the United States of America, 101(2004) 9966-70. [11] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, et al., Conformable, flexible, large-area networks of pressure and thermal sensors with organic transistor active matrixes, Proceedings of the National Academy of Sciences of the United States of America, 102(2005) 12321-5.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

[12] X. Wang, Y. Gu, Z. Xiong, Z. Cui, T. Zhang, Silk‐molded flexible, ultrasensitive, and highly stable electronic skin for monitoring human physiological signals, Advanced Materials, 26(2014) 1336-42. [13] H. Tian, Y. Shu, X.-F. Wang, M.A. Mohammad, Z. Bie, Q.-Y. Xie, et al., A graphenebased resistive pressure sensor with record-high sensitivity in a wide pressure range, Scientific reports, 5(2015) 8603. [14] G.Y. Bae, S.W. Pak, D. Kim, G. Lee, D.H. Kim, Y. Chung, et al., Linearly and Highly Pressure‐Sensitive Electronic Skin Based on a Bioinspired Hierarchical Structural Array, Advanced Materials, 28(2016) 5300-6. [15] B. Su, S. Gong, Z. Ma, L.W. Yap, W. Cheng, Mimosa‐inspired design of a flexible pressure sensor with touch sensitivity, Small, 11(2015) 1886-91. [16] H.B. Yao, J. Ge, C.F. Wang, X. Wang, W. Hu, Z.J. Zheng, et al., A flexible and highly pressure ‐ sensitive graphene – polyurethane sponge based on fractured microstructure design, Advanced Materials, 25(2013) 6692-8. [17] J. Wang, J. Jiu, M. Nogi, T. Sugahara, S. Nagao, H. Koga, et al., A highly sensitive and flexible pressure sensor with electrodes and elastomeric interlayer containing silver nanowires, Nanoscale, 7(2015) 2926-32. [18] S.C. Mannsfeld, B.C. Tee, R.M. Stoltenberg, C.V.H. Chen, S. Barman, B.V. Muir, et al., Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers, Nature Materials, 9(2010) 859-64. [19] S. Yao, Y. Zhu, Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires, Nanoscale, 6(2014) 2345-52. [20] W. Choi, J. Lee, Y. Kyoung Yoo, S. Kang, J. Kim, J. Hoon Lee, Enhanced sensitivity of piezoelectric pressure sensor with microstructured polydimethylsiloxane layer, Applied physics letters, 104(2014) 123701. [21] C. Dagdeviren, Y. Su, P. Joe, R. Yona, Y. Liu, Y.-S. Kim, et al., Conformable amplified lead zirconate titanate sensors with enhanced piezoelectric response for cutaneous pressure monitoring, Nature communications, 5(2014) 4496. [22] A. Shirinov, W. Schomburg, Pressure sensor from a PVDF film, Sensors and Actuators A: Physical, 142(2008) 48-55. [23] Y. Ai, Z. Lou, S. Chen, D. Chen, Z.M. Wang, K. Jiang, et al., All rGO-on-PVDFnanofibers based self-powered electronic skins, Nano Energy, 35(2017) 121-7. [24] C. Luo, N. Liu, H. Zhang, W. Liu, Y. Yue, S. Wang, et al., A new approach for ultrahigh-performance piezoresistive sensor based on wrinkled PPy film with electrospun PVA nanowires as spacer, Nano Energy, 41(2017) 527-34. [25] J. Lee, H. Kwon, J. Seo, S. Shin, J.H. Koo, C. Pang, et al., Conductive Fiber‐Based Ultrasensitive Textile Pressure Sensor for Wearable Electronics, Advanced Materials, 27(2015) 2433-9. [26] S. Gong, W. Schwalb, Y. Wang, Y. Chen, Y. Tang, J. Si, et al., A wearable and highly sensitive pressure sensor with ultrathin gold nanowires, Nature communications, 5(2014) 3132. [27] M. Segevbar, A. Landman, M. Nirshapira, G. Shuster, H. Haick, Tunable touch sensor and combined sensing platform: toward nanoparticle-based electronic skin, Acs Appl Mater Interfaces, 5(2013) 5531-41.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

[28] P. Nie, R. Wang, X. Xu, Y. Cheng, X. Wang, L. Shi, et al., High-Performance Piezoresistive Electronic Skin with Bionic Hierarchical Microstructure and Microcracks, ACS applied materials & interfaces, 9(2017) 14911-9. [29] J.-W. Han, B. Kim, J. Li, M. Meyyappan, Flexible, compressible, hydrophobic, floatable, and conductive carbon nanotube-polymer sponge, Applied physics letters, 102(2013) 051903. [30] S. Kundu, R. Sriramdas, A.K. Rafsanjani, A. Bid, R. Pratap, N. Ravishankar, Crumpled sheets of reduced graphene oxide as a highly sensitive, robust and versatile strain/pressure sensor, Nanoscale, 9(2017) 9581. [31] L.-Q. Tao, K.-N. Zhang, H. Tian, Y. Liu, D.-Y. Wang, Y.-Q. Chen, et al., GraphenePaper Pressure Sensor for Detecting Human Motions, ACS Nano, 11(2017) 8790-5. [32] Y.A. Samad, Y. Li, A. Schiffer, S.M. Alhassan, K. Liao, Graphene Foam Developed with a Novel Two‐Step Technique for Low and High Strains and Pressure‐Sensing Applications, Small, 11(2015) 2380-5. [33] Y. Qin, Q. Peng, Y. Ding, Z. Lin, C. Wang, Y. Li, et al., Lightweight, superelastic, and mechanically flexible graphene/polyimide nanocomposite foam for strain sensor application, ACS Nano, 9(2015) 8933-41. [34] H. Liu, M. Dong, W. Huang, J. Gao, K. Dai, J. Guo, et al., Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing, Journal of Materials Chemistry C, 5(2017) 73-83. [35] X.-G. Yu, Y.-Q. Li, W.-B. Zhu, P. Huang, T.-T. Wang, N. Hu, et al., A wearable strain sensor based on a carbonized nano-sponge/silicone composite for human motion detection, Nanoscale, 9(2017) 6680-5. [36] J. Mu, C. Hou, G. Wang, X. Wang, Q. Zhang, Y. Li, et al., An elastic transparent conductor based on hierarchically wrinkled reduced graphene oxide for artificial muscles and sensors, Advanced Materials, 28(2016) 9491-7. [37] W. Chen, X. Gui, B. Liang, R. Yang, Y. Zheng, C. Zhao, et al., Structural Engineering for High Sensitivity, Ultrathin Pressure Sensors Based on Wrinkled Graphene and Anodic Aluminum Oxide Membrane, ACS applied materials & interfaces, 9(2017) 24111-7. [38] Z.L. Yu, G.C. Li, N. Fechler, N. Yang, Z.Y. Ma, X. Wang, et al., Polymerization under Hypersaline Conditions: A Robust Route to Phenolic Polymer ‐ Derived Carbon Aerogels, Angewandte Chemie, 128(2016) 14843-7. [39] H. Li, Y. Tao, X. Zheng, J. Luo, F. Kang, H.-M. Cheng, et al., Ultra-thick graphene bulk supercapacitor electrodes for compact energy storage, Energy & Environmental Science, 9(2016) 3135-42. [40] H.-K. Kim, S.-M. Bak, S.W. Lee, M.-S. Kim, B. Park, S.C. Lee, et al., Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications, Energy & Environmental Science, 9(2016) 1270-81. [41] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chemical Society Reviews, 39(2010) 228-40. [42] Z. Yue, C.L. Mangun, J. Economy, Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers, Carbon, 40(2002) 1181-91.

A

N

U

SC R

IP T

[43] C. Kim, B.T.N. Ngoc, K.S. Yang, M. Kojima, Y.A. Kim, Y.J. Kim, et al., Self‐Sustained Thin Webs Consisting of Porous Carbon Nanofibers for Supercapacitors via the Electrospinning of Polyacrylonitrile Solutions Containing Zinc Chloride, Advanced Materials, 19(2007) 2341-6. [44] Z. Lou, S. Chen, L. Wang, R. Shi, L. Li, K. Jiang, et al., Ultrasensitive and ultraflexible e-skins with dual functionalities for wearable electronics, Nano Energy, 38(2017) 2835. [45] S. Chen, K. Jiang, Z. Lou, D. Chen, G. Shen, Recent Developments in Graphene‐ Based Tactile Sensors and E ‐ Skins, Advanced Materials Technologies, 3(2018) 1700248. [46] G. Schwartz, B.C.-K. Tee, J. Mei, A.L. Appleton, D.H. Kim, H. Wang, et al., Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring, Nature communications, 4(2013) 1859. [47] W.W. Nichols, Clinical measurement of arterial stiffness obtained from noninvasive pressure waveforms, American journal of hypertension, 18(2005) 3-10. [48] T.Q. Trung, N.E. Lee, Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human ‐ Activity Monitoringand Personal Healthcare, Advanced Materials, 28(2016) 4338-72. [49] U. Khan, T.H. Kim, H. Ryu, W. Seung, S.W. Kim, Graphene tribotronics for electronic skin and touch screen applications, Advanced Materials, 29(2017).

M

Yan Zhong received the B.Sc. degree in mechanical engineering from North West Agriculture and Forestry University, China, in 2014.

ED

She is now working toward the Ph.D. degree in Mechanical

PT

Engineering in HUST. Her current research activities focus on fabrication of micro/nano-materials and their applications in

CC E

supercapacitors and sensors. Tielin Shi is the director of the State Key Laboratory for Digital

A

Manufacturing Equipment and Technology at HUST. He obtained his Ph.D. in Mechanical Engineering in 1999 from HUST. His interests are in micro/nanofabrication technologies and developing advanced fabrication systems for microelectronic industry.

Xianhua Tan received the B.Sc. degree in mechanical engineering from HUST, China, in 2011. He obtained his Ph.D. in Mechanical Engineering in 2016 from HUST. He is a Post-doctoral in Mechanical Engineering in HUST. His current research activities

IP T

focus on fabrication of micro/nano-materials and their applications in collecting water and pressure sensor.

SC R

Siyi Cheng received the master degree in control technology and

instrument professional from Beijing University of Chemical

U

Technology in 2014. He is now working toward the Ph.D.degree in

N

Mechatronics Engineering at HUST. His research work includes

A

synthesis of nano-materials, and the applications in lithium battery.

M

Yuanyuan Huang received the B.Sc. degree in mechanical

ED

engineering from Sichuan University in 2015. Currently she is working on her Ph.D. degree in Mechatronics Engineering at HUST.

PT

Her current research activities are in the area of synthesis of

CC E

graphene and its application in supercapacitors. Chen Chen received the B.Sc. degree in Engineering Machinery from Chang'an University in 2016. Currently She is working with

A

Professor Z. R. Tang on his master degree in Mechatronics Engineering at HUST. His current research activities are the fabrication of micro/nano-materials.

Guanglan Liao is a professor of the State Key Laboratory for Digital Manufacturing Equipment and Technology at HUST. He obtained his Ph.D. in Mechanical Engineering in 2003 from HUST. His current interests are microelectronics manufacturing and packaging.

IP T

Zirong Tang is a professor in Wuhan National Laboratory for Optoelectronics at HUST, Wuhan, China. He obtained his Ph.D. in

SC R

Material Science and Engineering in 2001 from the University of California, Irvine, USA. His current interests are in MEMS,

U

electrochemical sensors, biosensors and micro/nanofabrication

A

CC E

PT

ED

M

A

N

technologies.