Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Flexible, Degradable, and Cost-Effective Strain Sensor Fabricated by a Scalable Papermaking Procedure Hanbin Liu,*,†,‡,§ Huacui Xiang,† Yubo Ma,† Zhijian Li,*,†,‡ Qingjun Meng,†,‡ Huie Jiang,†,‡ Haiwei Wu,†,‡ Peng Li,§ Hongwei Zhou,*,⊥ and Wei Huang§
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Shaanxi Provincal Key Laboratory of Papermaking Technology and Specialty Paper Development, College of Bioresource Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi’an 710021, P.R. China ‡ National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Xi’an 710021, P.R. China § Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University, Xi’an 710072, P. R. China ⊥ School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, P. R. China S Supporting Information *
ABSTRACT: Flexible strain sensors (FSSs) are essential components in intelligent systems, especially in soft robots, human sport monitoring, ect., but their scalable preparation remains a challenge. In this work, we first proposed and demonstrated a strategy to prepare FSS with a scalable and cost-effective papermaking procedure. Cellulose fibers from waste papers and conductive graphite were mixed and subject to a paper former (papermaking machine in laboratory), producing a strain sensitive paper with diameter of 20 cm in 10 min. With the scrips from the strain sensitive paper, the strain sensor was assembled showing good sensing performance for both bending (gauge factor (GF) = 27, response time of 360 ms) and twisting (GF = 26.5, response time of 440 ms) strains. It can be used in movement detections of soft matters (such as a plastic ruler), elbow joints of a puppet, and human fingers. The cost of the sensor was calculated as low as $0.00013, and the strain sensitive paper can be degraded in around 1 min in water under stirring. Furthermore, the strategy can be expanded to the sensor based on carbon black (CB), indicating a universality, which may pave a way for developing more intelligent materials and devices. KEYWORDS: Flexible electronics, Strain sensor, Papermaking, Waste paper, Graphite, Carbon black
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techniques of printing, spraying, or sputtering.6−9,15,25 The substrate can be plastics, papers, or rubbers. The conductive materials usually construct micro-cracks or wrinkles to give resistance changes under deformations and thus generate current signals under a constant voltage. The second strategy is encapsulating conductive materials, including carbonized fiber, conductive polymers, etc., into elastomers or hydrogels.10−12,14,16,26 The conductive networks can be distorted, stretched, or pressed under strains and produce electric signals under an identical voltage. These methods can successfully produce FSSs with good sensing performance. Nevertheless, the preparation of a strain sensor on a large-scale still remains a challenge due to the multistep operations, expensive raw materials, or complex equipment. The technology of papermaking was invented at around 2000 years ago, which supplies us with abundant paper products every day. Over the centuries, it has transformed from
INTRODUCTION Flexible strain sensors (FSSs), which transform deformation into electric signals that could be readout, have drawn great attentions in recent years,1−17 because they are widely demanded in wearable devices, soft robots, sport and health monitoring, protection of buildings, and so on. Furthermore, plenty of FSSs must be used in even one smart system to collect all possible deformation signals in different parts. Therefore, massive efforts have been made to develop various FSSs. Nevertheless, there is a long way to their large-scale applications.18,19 A scalable and cost-effective strategy for the preparation is necessary for their wide utilization. In different mechanisms of strain sensing involving capacitance,20,21 field-effect transistor (FET),22,23 piezoelectricity,24 and triboelectricity,5 the resistive strain sensors6−16,25 have a simple device structure and a relatively stable signal output, thus showing much broader prospects.9 To fabricate this kind of sensor, researchers have raised and demonstrated various strategies. One method is coating conductive materials, like carbon black (CB), graphite, graphene, silver, gold, metallic oxide, etc., onto various flexible substrates using © XXXX American Chemical Society
Received: August 28, 2018 Revised: October 13, 2018 Published: October 15, 2018 A
DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic of the preparation procedure (a). Photograph (b) and SEM images (c, d) of the strain sensitive papers. The preview image (e) during the Raman test and the corresponding Raman spectrum (f, g) of the sensitive paper at point A and B, respectively. The paper for testing contains 55 wt % graphite.
a manual skill into a modern industry with super highefficiency. A modern papermaking machine can produce 300 000−450 000 tons paper in one year (up to 1400 ton/d, data comes from the Web site of Voith GmbH & Co. KGaA, Germany). If a strain sensor is prepared using the papermaking techniques, its scalable production will be easily realized, which must be also a cost-effective strategy considering the highefficiency of the papermaking industry. Furthermore, the paper related products are usually recyclable and degradable due to the utilization of cellulose fibers from plants as raw materials. The electronic device on the basis of this technique may be sustainable and free of pollutants.15 To prove our concept, we herein attempt to fabricate strain sensors though a papermaking procedure in laboratory with a paper former (papermaking machine in a laboratory). Graphite powder and cellulose fibers from waste printing papers were totally mixed (Figure 1a) and then transferred into the paper former to generate a strain sensitive paper with the assistance of cationic polyacrylamide (CPAM). In the strain sensitive paper, the fibers construct the physical frameworks, while the graphite serves as the conductive component. The CPAM was used to increase the retention ratio of graphite. After clipping into scrips and connecting with copper wires at the two ends by conductive silver paste, a strain sensor can be assembled with the strain sensitive paper. We studied the sensing performances under deformations and demonstrated the viability of this strategy. Furthermore, the degradability of the strain sensor was also proven, and the strategy can be
expanded to strain sensors containing CB, suggesting a good universality.
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EXPERIMENTAL PART
Materials. Waste papers (A4) after printing without deinking were collected from an office. The powders of graphite (Tianjing Dengke Chemical Co., Ltd., China), CB (Cabot Corporation, USA), and CPAM (Mn = 1.2 × 107, Zhengzhou Guangya Sterilization Supplies Co., Ltd., China) were used without further treatment. Deionized (DI) water was produced by a Milli-Q Integral Water Purification System in the laboratory (15 MΩ·cm). Preparation of Strain Sensitive Papers. The waste papers were cut into small pieces and soaked in DI water for 12 h, followed by defibering with a disintegrator (L260-5005, ADEV, Italy) for 10 000 revolutions. The yield pulps (suspensions of cellulose fibers) were squeezed in a filter gauze to remove water and stored for the next step. The water content was subsequently determined as 68 wt %. The CPAM was dissolved into DI water with a concentration of 2 wt % for further use. The typical steps for preparation of the strain sensitive papers were as follows, taking the paper containing 45 wt % graphite as an example. In a beaker, 1.71 g of graphite powder was dispersed into 150 mL of DI water with magnetic stirring for 10 min, and ultrasonic treatments were 5 min. Then, 2.09 g of cellulose fibers (equal to 6.54 g of wet pulps with 68 wt % water), 100 mL of water, and 20 mL of the solution of CPAM (2 wt% in water) were successively added into the graphite suspension under intensive stirring. The above mixed pulp was transferred into a paper former (Xianyang Tongda Light Industry Equipment Co., Ltd., China) to generate paper sheets with a diameter of 20 cm, which were the strain sensitive papers. B
DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Resistance of strain sensitive paper as a function of graphite content (a) and the length of scrips with a width of 10 mm (b). Schematic deformations of the sensor under bending and twisting strains (c). Resistance of the sensor as a function of sagitta (d) and corresponding resistance change (e) under various bending strains. Resistance of the sensor as a function of the twisting angle (f) and its resistance change under various twisting strains (g). The sensor was assembled with a 40 mm × 10 mm paper scrip and copper wires at the two ends with silver paste in all performance tests, including (d), (e), (f), and (g). Assembly of the Strain Sensor. The strain sensitive paper was cut into 40 mm × 10 mm scrips, which were connected with copper wires at the two ends by a conductive silver paste, yielding strain sensors. The sensitive part can be sealed with scotch tape for protection. Characterizations. The surface of the strain sensitive paper was visualized by a Vega-3 (TESCAN, Czech Republic) scanning electron microscope (SEM) after being spray-coated with gold. The chemical structure of the strain sensitive paper was analyzed with a DXRxi Raman imaging microscope using a 532 nm laser. The resistance of the paper and sensor was tested with a VC97 multimeter (Victor, China). The current signals at a voltage of 3.0 V during motion monitoring were recorded by a PARSTAT 4000+ Potentiostat Galvanostat EIS analyzer system.
1f, appearing with chemical shifts at 1576 (G peak) and 2733 cm−1 (D peak) of graphite. While at point B, the typical Raman shifts of cellulose at 1086 and 2895 cm−1 were detected.15 These results proved the coexistence of graphite and cellulose fibers in the paper. To target the good conductivity, papers with differing content of graphite were produced and their resistances were recorded with 40 mm × 10 mm scrips. As shown in Figure 2a, the resistance jumped from 112.7 to 5.8 kΩ, indicating the formation of the conducting network until the graphite occupied 55 wt % of the paper. Then the resistance wandered from 5.8 to 1.2 kΩ under the graphite content increasing from 55 to 65 wt %, suggesting a conductive ceiling of the papers limited by the intrinsic conductivity of graphite. Therefore, the paper with a graphite content of 55 wt % was chosen for further characterizations and utilizations. Besides, the resistance of the scrips increased linearly against the length under a consistent width of 10 mm (Figure 2b). Considering the operability of the performance tests, we chose 40 mm × 10 mm scrips to assemble the strain sensors in the following study. After the strain sensor was assembled with the scrips and copper wires, the sensitivity was investigated. As shown in Figure 2c, two kind of strains, bending and twisting, were applied on the sensor. The resistance against the sagitta and twist angle were studied. Under bending strains, the resistance of the sensor decreased from 8.45 to 7.07 kΩ, corresponding to the sagitta increasing from 0 to 16 mm (Figure 2d), indicating a higher bending might generate more contact points in the conducting networks formed by graphite. When the sensor recovered from the bending, its resistance regained as well (Figure S5a). To quantify the resistance changes against bending strains, the gauge factor (GF) of the sensor was calculated following the reported procedures.6,8 Briefly, the GF was defined as GF = (ΔR/R0)/Δε, in which the ΔR, R0, and ε are the resistance variations, original resistance, and bending strains applied, respectively. The ε have relationships with the thickness (h), curvature radius (r), arc length (l), and chord length (c) of the sensor (Figure 2c) under bending strains as equations of ε= ± h/2r and c = 2rsin(l/2r).6,15 The calculation results are depicted into Figure 2e, and GF1 = 10.5 only if the bending strain is lower than 0.24%, while GF2 = 27.0 when a
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RESULTS AND DISCUSSION The paper former, also called a sheet former, is a papermaking machine in laboratory scale which makes a paper sheet by simply draining a very dilute suspension of cellulose fibers though a fine-mesh wire screen followed by vacuum drying in the machine. The main procedures include draining, pressing, and drying (see details in Figure S1), which produces one paper sheet (diameter 20 cm, thickness 150 μm, Figure S2) in around 10 min. The papermaking machine in industry is an amplifying instrument with the same protocol involving draining, pressing, and drying (Figure S3). The papers that can be prepared with the lab-scale equipment imply the feasibility of their continuous production in large scale with a papermaking machine in industry, suggesting a low cost. Herein, the strain sensitive papers were prepared with the paper former (Figure 1a) in laboratory. The obtained papers appear as dark gray with good flexibility (Figure 1b). Mechanical performance testing gives a tensile strength of 0.81 MPa with an elongation of around 0.35% (Figure S4). From SEM visualization, it was found that the graphites were inset into networks formed by cellulose fibers (Figure 1c,d). The polymer CPAM, which increases the retention of graphites, cannot be observed in the SEM images due to its small dimension and low concentration in the paper. Furthermore, the Raman spectra were recorded to confirm the chemical structure of the two components. The Raman spectra scanned at point A (Figure 1e) are depicted in Figure C
DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
results show a slight increase from 8.25 to 8.41 kΩ (Figure 3e), which might be ascribed to the defects produced by the repeating deformations. Nevertheless, no significant attenuations of the sensing performance were observed. After 1000 cycles, the sensor gives regular signals under the bending strain as previously shown (inset image of Figure 3e), suggesting good reliability of the sensor. Potential applications of the strain sensor for monitoring various movements were further explored. As shown in Figure 4a, the bending of a plastic ruler was detected by sticking the
0.24−0.73% bending strain was applied, which is comparable with the reported results (around 4−25 in literatures).10,15,22,27 Similar to the bending strain, the resistance of the sensor decreased from 8.21 to 0.81 kΩ when the twisting angle was increased from 0 to 90°, as shown in Figure 2f. This might be caused by the squeezing of the conducting network under twisting, which pushed more graphite contact with each other and produced more conductive paths. Similar to bending strain, the unloading of the twist strain also makes the resistance recover (Figure S5b). Furthermore, the twisting strain was quantified according to the definitions ψ = Rθ/L, where the ψ, R, θ, and L are the twisting strain, twisting radius, twisting angle, and the length of the sensor (Figure S6). The calculations give GFa = 3.0 and GFb = 12.5, as shown in Figure 2g, implying the sensor can be used for detecting twist deformations. The ability to detect both the bending and twisting strains is rarely reported,19 which may hold more potential in complex systems. To investigate their flexibility, repeated bending and twisting deformations were applied to the sensors (Figure 3a,c).
Figure 4. Photographs and corresponding current signals of the sensor for detecting movements of a ruler (a, b), elbow joint of a puppet (c, d), and the finger motion of a human (e, f) at a consistent voltage of 3.0 V.
sensor on the surface of the ruler, which produced regular current signals at a consistent voltage of 3.0 V (Figure 4b). Every current peak is different from each other because the bending of the ruler was controlled by hand rather than a machine, indicating the sensor is sensitive for even slight changes of stains. Furthermore, the sensor was used to detect the motions of the elbow joint of a puppet (Figure 4c), which worked very well and generated rhythmic signals, as shown in Figure 4d. These results indicate the sensor hold potentials in the safety control of architectures, such as the bridge deformation detection as well as robotic-related applications. On the other hand, for human motions, the sensor also showed good sensitivity for finger bending and stretching (Figure 4e,f, Movie S3), suggesting the sensor can be used in the sport monitoring of humans. In the current signals (Figure 4f), the intensity of the highest peak at 10 s is almost three times that of the lowest peak at 26 s caused by two different movements of the finger, indicating the sensor can identify the amplitude of human motions, which is an important parameter for sport detections. To examine the universality of this strategy, strain sensitive papers containing CB were prepared in a similar way. Keeping the preparation protocol, we only replaced the graphite with
Figure 3. Photographs and corresponding current signals of the sensor under repeated bending (a, b) and twisting (c, d). Resistance during 1000 bending−unbending cycles (e) and current signals under a bending strain after 1000 cycles (inset of e). All current signals were collected under a consistent voltage of 3.0 V.
Simultaneously, the current of the sensor was monitored under a consistent voltage of 3.0 V with an EIS system. The sensor generated regular signals under bending (Figure 3b, Movie S1) and twisting (Figure 3d, Movie S2). The response time of bending and twisting are 360 (inset curve in Figure 3b) and 440 ms (inset curve of Figure 3d), respectively, which are comparable with the reported results (110−300 ms).6,15,28−30 To prove the robustness, the resistance of the sensor was monitored during 1000 bending−unbending cycles. The D
DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 5. Photograph (a) and SEM images (b, c) of strain sensitive paper containing CB. Resistance of the paper as a function of CB content (d). Resistance changes under various bending strains (e). Photographs of finger motions (f, g) and corresponding current signals (h) produced by the sensor at a consistent voltage of 3.0 V. The CB content of the samples are 44 wt % for surface visualization (a, b, c) and a sensing performance test (e, h). The CB-papers for assembling strain sensors have a dimension of 40 mm × 10 mm.
Figure 6. Degradation of strain sensitive paper containing graphite in water over time.
CB as the conductive material, yielding a flexible paper with dark black colors, as shown in Figure 5a. SEM images show that the cellulose fibers from waste paper interweaved into networks, and grainy CB particles surrounded the fibers and were imbedded into pores (Figure 5b,c). In the Raman spectrum of the CB-paper, typical shifts at 1344 and 1594 cm−1 of CB appear (Figure S7b).15 However, we cannot find the peaks of the celluloses, even though the cellulose fibers are obvious in the preview images (Figure S7a). This may be ascribed to the strong Raman shift of CB particles, which were absorbed and completely covered the cellulose surface. Similar with graphite papers, the resistance of the CB paper decreased with the increase of CB content and became stable until it reached up to 44 wt % (Figure 5d). Therefore, we focus on the paper with this composition and use it for the sensor assembly with dimensions of 40 mm × 10 mm. By studying the resistance change against the bending strain applied, the GF of the CB-paper-based sensor was determined as GFα = 3.7, below the bending strains of 1.6%, and GFβ = 7.8, for the strain
range of 1.6−2.1% (Figure 5e), lower than the graphite-paperbased sensor, which may be caused by the different morphology and conductivity of the two carbon allotropes. Furthermore, the CB-paper-based sensor was also used to monitor the joint motion of fingers (Figure 5f,g) and to produce regular current signals with a response time of approximately 210 ms (Figure 5h), which is comparable with the graphite-paper-based strain sensor. These results suggest the feasibility and universality of the papermaking strategy for strain sensor preparation. More intelligent materials may be developed through this method in the future. Moreover, the strain sensor is also cost-effective and degradable. As estimated, one piece of strain sensitive paper from the paper former with a diameter of 20 cm was about $0.0069 US dollars. Supposing two-thirds of the paper can be used for sensors (about 52, if 40 mm × 10 mm for one sensor), the cost of the sensitive component of one sensor is about $0.00013. Given the consumption of water and energy in preparation, this value can be doubled. However, it is still E
DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering ORCID
much lower than the reported paper-based strain sensor which is prepared by coating, drawing, or sol−gel growing on paper substrate.6,15,31 Except for cost, the electric pollution is another concern, especially in modern society inundated by electronic devices,32−34 for which the sustainability of devices is desired. A decomposition experiment was performed to verify the degradability of the strain sensitive papers. As shown in Figure 6, a scrip from the paper containing graphite was put into water with stirring and rubbing, taking photographs at different times (see Movie S4). From 8 to 37 s, the scrip was broken into several small pieces while stirring. From 37 to 64 s, slight rubbing was applied, and the paper quickly transferred into fibers and graphite particles, which could be used to prepare another sensitive paper. This experiment demonstrated that the strain sensitive paper can be degraded and recovered, which is free of electric pollutants and meets the requirements of sustainable developments. It should be pointed out, that the degradation in water does not mean the sensor cannot work under high humidity. In fact, the monitoring of the finger bending with this sensor at a low humidity (42%) and high humidity gives similar output signals (Figure S8). Besides, the sensor can be also sealed with other materials, such as scotch tape, which endow its capability of being used in more severe environments.
Hanbin Liu: 0000-0002-2141-796X Hongwei Zhou: 0000-0002-7518-9464 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Natural Science Foundation of China (21805178 and 51603164), the Natural Science Basic Research Plan in Shaanxi Province (2018JQ5104 and 2017JQ2031), the Foundation for Selected Oversea Chinese Scholar in Shaanxi Province (2017016), and the advanced research fund of SUST (2016GBJ-14).
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CONCLUSIONS In summary, a scalable papermaking procedure to prepare a strain sensor was proposed and demonstrated in this work. Conductive materials (graphite or CB) and cellulose fibers from waste papers can be easily transferred into a strain sensitive paper with a paper former in laboratory. By being assembled with copper wires, the yield sensors can detect both bending and twisting strains with a fast response and good sensitivity and can be used in deformation detections of soft matters (such as a plastic ruler) and monitoring of human motions, indicating they may also find applications in robotics, building safety control, and sports monitoring of humans. Furthermore, they are cost-effective and degradable, being free of electronic pollutions. This strategy shows universality and might be used in developing more intelligent materials in the future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04298. Papermaking procedure in lab, photographs of strain sensitive paper, industry papermaking procedure, mechanical strength, resistance under recovery, definition of twisting strains, additional Raman spectrum, and current signals under high humidity (PDF) Video of the bending test (AVI) Video of the twisting test (AVI) Video of the finger motion (AVI) Video of the degradation (AVI)
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REFERENCES
AUTHOR INFORMATION
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DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acssuschemeng.8b04298 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
