Highly Flexible Strain Sensor from Tissue Paper for Wearable

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Highly flexible strain sensor from tissue paper for wearable electronics Yuanqing Li, Yarjan Abdul Samad, Tarek Taha, Guowei Cai, Shao-Yun Fu, and Kin Liao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00783 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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ACS Sustainable Chemistry & Engineering

Highly flexible strain sensor from tissue paper for wearable electronics Yuanqing Li1*, Yarjan Abdul Samad2, Tarek Taha3, Guowei Cai3, Shao-Yun Fu1, Kin Liao2* 1. College of Aerospace Engineering, Chongqing University, No. 174 Shazhengjie Road, Chongqing 400044, People's Republic of China 2. Department of Mechanical Engineering, Khalifa University of Science, Technology, & Research, Abu Dhabi 127788, United Arab Emirates 3. Robotics Institute, Khalifa University of Science, Technology & Research, Abu Dhabi 127788, United Arab Emirates *Address correspondence to [email protected]

ABSTRACT: We introduce a simple method to fabricate a highly flexible resistive-type strain sensor composed of carbon paper (CP) and polydimethylsiloxane (PDMS) elastomer. The key resistance sensitive material of the sensor, carbon paper, is prepared from tissue paper by a simple high-temperature pyrolysis process. At the same time, the as-fabricated CP/PDMS strain senor is highly sensitive to applied strain with a gauge factor (GF) of 25.3, almost 10 times higher than that of conventional metallic strain gauge. Furthermore, the response of CP/PDMS strain sensor under cyclic tensile strain with a peak strain of 3% was also investigated, which

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exhibits fast and steady response with excellent durability within the frequency range of 0.01-10 Hz. Finally, we demonstrate the successful utilization of the CP/PDMS strain sensor as wearable electronics in breath monitoring and robot controlling. The eminent performance, low material cost, and facile fabrication process make the CP/PDMS strain sensor exceptionally promising in flexible, stretchable and wearable electronics.

KEYWORDS: Strain sensor, Flexible, Resistive, Biomass, Wearable electronics

INTRODUCTION Strain sensors are devices which convert mechanical deformations into output signals based on the change of electrical characteristics.[1] In recent years, the demand for flexible strain senor is rapidly growing because of considerable potential applications in sports, personal health monitoring, soft robots, and prosthetic devices.[1-4] Although conventional strain sensors based on thin metal-wires and semiconductors are well developed, their fragile and rigid nature impose limitation as flexible/wearable devices.[1, 5] Many attempts have been made to enhance the flexibility of strain sensor with stretchable materials.

Most widely employed strategy in

preparation of flexible strain sensor is to use resistive-type sensor due to their relatively simple structure and fabrication process, as well as low energy consumption in operation.[6-21] Flexible resistive-type strain sensors can be fabricated by embedding conducting fillers into soft elastomeric materials, and the strain can be monitored by simply measuring the change in the resistance of the sensor.[16, 22]

Constructing conductive networks in two or three

dimensions within elastomeric matrix is essential to achieve the high sensitivity required in efficient strain sensors. One or two dimensional materials with high conductivity and aspect


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ratio, such as gold and silver nanowire,[7-11] carbon nanotube,[12, 13] and graphene,[14-20] are particularly suited to construct such conductive networks, which have been widely used for designing novel strain and pressure sensors.

However, cost of raw-materials, complicated

procedures, and/or complex equipment involved in the preparation of those nanomaterials drastically hamper their large-scale production for mass market penetration. Although high performance strain sensor based on micro-cracks instead of expensive nanomaterials have been demonstrated recently,[23, 24] it remains significant to develop low-cost flexible strain sensor based on easy-processing materials and simple device structures. From economic and environmental point of view, materials derived from biomass are prominently attractive due to their low cost, easy extraction, sustainability, and ecofriendliness.[25] Recently, conversion of biomass, such as cotton, sugarcane, water melon, and winter melon, into conductive three-dimensional carbon materials have been reported by our group, highlighting great potentials of such material in various applications, such as: sensor technology, energy conversion and storage, electromagnetic interference shielding, and in water treatment.[26-29] It is well known that cellulose is the most abundant natural polymer on earth, widely available at low cost.[30, 31]

Other advantages of cellulose include: renewability,

biodegradability, nontoxicity, high aspect ratio, etc.[32] Tissue paper is essentially a network of cellulose microfibers, by converting these fibers into carbonous material it is possible to construct two-dimensional conductive network. In this work, commonly available tissue paper is first converted into carbon paper (CP) with tunable resistivity by high-temperature pyrolysis. Then a resistive-type, highly flexible, and sensitive strain sensor is fabricated using the converted carbon paper as the resistance sensitive material, and polydimethylsiloxane (PDMS) elastomer as the flexible matrix material.

The mechanical


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behaviors of the CP/PDMS sensor under tension, compression, and bending show fast and stable response, and excellent durability within the frequency range of 0.01-10 Hz. To demonstrate the capability of CP/PDMS strain sensor as a wearable device, real time breathing of an adult was monitored by a “smart waist belt” integrated with the CP/PDMS sensor. Finally, the motion controlling of a robot was successfully demonstrated by stretching and relaxing the CP/PDMS sensor.

RESULTS AND DISCUSSION Tissue paper is inherently composed of a network formed by cellulose microfibers.[33] It is known that cellulose fiber can be converted into carbon fiber via high temperature treatment.[34] As illustrated in Figure 1, carbon papers are obtained by high temperature pyrolysis of the tissue paper in N2 atmosphere. After pyrolysis, there is a ~50% volume shrinkage of the tissue paper, owing to decomposition of cellulose and evaporation of organic components. As shown in Figure 2, the as-prepared carbon paper is an interconnected network of long, belt-shaped carbon fibers. The formation of belt-like fiber is attributed to the collapse of biomass fiber first in the pulping process and later in the paper making process. It is revealed by high-magnification scanning electron microscopy (SEM) image that the width of fiber is around 5-10 µm, and the length of fiber is in the scale of millimeters. In order to probe into the chemical composition of the carbon paper formed, energy-dispersive X-ray spectroscopy (EDS) was performed on these samples. As presented in Figure S1, the element composition of carbon paper are 95.31, 4.45, 0.09, 0.08, and 0.08 atomic% of C, O, Mg, Si, and Ca, respectively. It is obvious that carbon is the dominant element. In addition, the atomic content of Mg, Si, and Ca constitutes only 0.25% of the total material, which is normal for carbon from biomass.


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Figure 1. Schematic of the fabrication process of the CP/PDMS strain sensor.

(A)

(B)

Figure 2. (A) Low and (B) high magnification SEM images of the carbon paper. The scale bar of (A) and (B) is 100 and 10 µm respectively. For resistive-type sensors, the electrical conductivity of the resistance sensitive materials is vital to the performance of the devices. Generally, high conductivity of resistance sensitive material means high device current, which leads to high device energy consumption. Whereas, if the conductivity of a resistance sensitive material is too low, measuring its electrical signal could


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be problematic. Therefore, it would be ideal if the electrical conductivity of the resistance sensitive material could be tuned to balance energy consumption and signal sensitivity. It is well known that temperature has a great influence on the structure and properties of pyrolyzed products. To obtain carbon paper with ideal electrical conductivity, tissue papers were pyrolyzed separately in the range of 400-800°C. The effect of pyrolysis temperature on the conductivity of carbon paper is shown in Figure 3. The original tissue paper is almost an insulator with a conductivity of 3.5×10-10 S/m. When the pyrolysis temperature is 400°C or below, the conductivity of the black-colored carbon paper obtained is close to that of the original tissue paper. By further increasing the pyrolysis temperature to 500°C, the conductivity of carbon papers is significantly improved. The conductivity of carbon papers prepared at 600, 700, and 800 °C is 9.6×10-9, 6.10×10-5, 0.57 and 12.4 S/m, respectively. It is know that the carbon materials converted from biomass at low pyrolysis temperature is dominated by amorphous structure with low electrical conductivity. With an increase of the pyrolysis temperature, a portion of the amorphous carbon is converted into graphite, which lead to an increase of the conductivity. Compared with original tissue sample, the conductivity of carbon paper prepared at 800°C has increased by 11 orders of magnitude, reaching a few kilo ohms, which is ideal for the resistance sensitive sensor. Therefore, in this work, carbon papers prepared at 800°C were used to fabricate CP/PDMS strain sensor.


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Conductivity (S/m)

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Figure 3. Electrical conductivity of carbon papers prepared at various pyrolysis temperatures. Error bars indicate the standard deviation (SD), lines connecting data points only indicate data trend. To fabricate the flexible strain sensor, carbon paper with electrodes was encapsulated by PDMS resin via a simple vacuum infusion process. As indicated in Figure 4(A), the CP/PDMS composite fabricated has excellent flexibility, which can be easily bent or folded multiple times without any visible damage. The mechanical behavior of CP/PDMS composite was studied using an Instron Micro force tester.

From the typical tensile strain-stress curve of CP/PDMS

composite (Figure 4(A)), it is clear that the CP/PDMS composite fabricated is highly flexible with an average elongation at break of 228%. The elastic modulus of the CP/PDMS composite under tension, compression, and 3-point-bending are 1.1, 21.4, and 3.0 MPa, respectively. Their low elastic modulus implies that the material will exhibit high strain under low stress, a preferred trait as flexible strain sensor. More importantly, resistance change was observed when an external stimulus such as strain or pressure is applied on the sensor. Figure 4(B) shows the current-voltage (I-V) curve of CP/PDMS sensor under a static strain from 0 to 20%. It is clear that the I-V curves of CP/PDMS composite under different strains are linear, which indicates the


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ohmic behavior of CP/PDMS composite with constant resistance. At the same time, the current passing through a sensor at a larger strain is lower than that with smaller strain, indicating that an increase in applied strain leads to an increase in the sensor’s resistance.

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Current (mA)

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0.2 0.0 -0.2 -0.4 -0.6 -0.8 -2.0

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Figure 4. (A) A typical tensile strain-stress curve of CP/PDMS sensor, the inset shows the flexibility of CP/PDMS strain sensor; (B) current-voltage curves of CP/PDMS sensor under various applied strains. In order to fully characterize the performance of the CP/PDMS sensor, real-time resistance and strain of the sensor were monitored while deforming the sensor, and the relative change of resistance (RCR, ∆R/R0= (Rp-R0)/R0, where R0 and Rp are the resistance without and with applied strain, respectively) were calculated.

The relationships of RCR with applied tensile,

compressive, and bending strain are presented in Figure 5 (A), (B), and (C), respectively. Under tension, the RCR of sensor increases continuously with the applied strain up to 20%, and two linear regions with different slopes are seen in a typical RCR-strain curve. The slope of the RCR


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vs. strain curve reflects the gauge factor (GF, the sensitivity of the sensor to strain), which is defined as δ(∆R/R0 )/δS, where S denotes the applied strain. As indicated in Figure 5(A), when the strain is less than 3%, the GF calculated is 25.3, which is almost 10 times higher than that of the conventional metallic strain gauge (GF around 2). When the applied strain is larger than 3%, the GF calculated is 4.73, which is still comparable to the metallic strain gauges.

For

compression, when the applied strain is less than 5%, the CP/PDMS sensor exhibits a low GF of 0.131. Beyond which the GF increases with an increase of strain, and reaches up to 0.731 in the strain range of 15~20%. The RCR behavior in bending is similar to that of compression, where two different GFs of 0.343 and 0.917 were observed at low (

Highly Flexible Strain Sensor from Tissue Paper for Wearable

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