A multifunctional highly sensitive multiscale stretchable strain sensor

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Applications of Polymer, Composite, and Coating Materials

A multifunctional highly sensitive multiscale stretchable strain sensor based on a graphene/glycerol–KCl synergistic conductive network Chunrui Liu, Songjia Han, Huihua Xu, Jin Wu, and Chuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12674 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

A multifunctional highly sensitive multiscale stretchable strain sensor based on a graphene/glycerol–KCl synergistic conductive network Chunrui Liu, Songjia Han, Huihua Xu, Jin Wu*, Chuan Liu* State Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China

Email of corresponding authors: [email protected], [email protected]

Abstract

Stretchable strain sensors have promising applications in health monitoring and human motion detection. However, only a few of the strain sensors reported to date have exhibited a multiscale strain range and a high gauge factor simultaneously. As such, most strain sensors cannot be used in applications that require both high sensitivity and a multiscale strain range. In this work, we develop a wearable multifunctional strain sensor, using graphene and a new ionic conductor as the sensing material and Ecoflex as the encapsulant. In the ionic conductor, KCl and glycerol are

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used as the electrolyte and solvent, respectively. This deformable ionic conductor connects cracked graphene sheets electronically, enabling the strain sensor to be stretched to 300% of its original length with a moderate gauge factor of 25.2. The sensor can respond to various mechanical deformations including stretching, bending, and pressing. When attached to human body, the sensor can monitor large-scale strains (>50%) for joint movement and small-scale strains (50%), the ionic conductor provides electrical percolation paths that connect the cracked graphene fragments, thereby enhancing the stretchability and durability of the sensor. In addition to the dynamic response of its strain sensing, the sensor also exhibits a static thermal response based on the thermally activated process of ion transport, making it also a stretchable and sensitive temperature sensor. Moreover, the strain sensor can be fixed on human skin for human motion detection, being wearable and


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biocompatible. This high-performance multifunctional strain sensor meets the requirements of multiscale strain and/or temperature sensing in the human body and those of sensing units in soft robotics.

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Experimental section 2.1. Synthesis of ionic conductor. 6.25 g of glycerol (≥99%; Sigma, China) and 0.5 g of KCl

(≥99.0%; Sigma, China) were placed in a beaker and stirred at 800 rpm for 12 h. The product, a colorless transparent liquid, was stored in a reagent bottle. 2.2. Fabrication of strain sensor. First, a graphene suspension (1 mg/mL; XFNANO, China) was drop cast onto a glass slide patterned with polyimide (PI) tape (with a rectangular pattern size of roughly 15 mm × 4 mm). The sample was exposed to an oxygen plasma (PT-5S; Sanhoptt, China) at 100 W for 180 s. After the drop casting, we heat the glass slide at 60°C for 5 min to dry the graphene solution. Once the solvent had evaporated, graphene solution was deposited onto the glass slide for a second time. Parts A and B of the Ecoflex silicone elastomer (Ecoflex 0050; Smooth-On, Inc., USA) were then mixed with equal volume, stirred vigorously, degassed, and poured onto the graphene pattern on the glass substrate, followed by curing at room temperature for 2 h. The Ecoflex film was then peeled off from the glass substrate to form the microchannels. Next, silver wires were attached to the ends of the embedded graphene using a silver paste for subsequent mechanical and electrical tests. After injecting a mixture of glycerol and KCl (8 wt%) into the small holes of the microchannels using a syringe, the injection holes were sealed using Ecoflex. 2.3. Electrical characterization. Mechanical tests were conducted using a motorized moving stage (SC300-3a; Zolix, China). To demonstrate the ability to monitor human body motions in


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real time, the sensors were mounted on the forefinger, cheek, elbow, or radial artery with Tegaderm film (1624W; 3M, USA). The current changes in the strain sensors were measured under a bias voltage of 1 V using a Keithley 2400 SourceMeter.

3.

Results and discussion The scheme for fabricating the sandwich-structure strain sensor is shown in Figure 1. First, PI

tape was attached to a glass substrate to obtain a patterned hard substrate. Plasma surface pretreatment was applied to the PI and glass substrate, making it hydrophilic. The graphene suspension solution was then deposited on the PI tape to form a conductive thin film. For a flexible substrate, Ecoflex prepolymer was poured on the glass substrate to cover the conductive thin film, followed by thermal curing. The Ecoflex film was then peeled off from the glass to form a microchannel, and the graphene conductive film was partly embedded in the Ecoflex. Silver wires were bonded to both ends of the microchannel with silver paste, generating the contact electrodes. Another Ecoflex slab was used to cover the entire device. Subsequently, the ionic conductor, namely a mixture of glycerol and KCl (8 wt%), was injected slowly through the microchannel using a syringe, followed by sealing of the holes using liquid Ecoflex. After curing the polymer, a composite strain sensor with a graphene conductive film and ionic conductor was generated. Photographs of our composite strain sensor under applied strains of zero and roughly 300% are shown in Figure 1g, h.


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Figure 1. Schematic of process for fabricating the strain sensor. (a) Graphene suspension solution is deposited on the patterned glass substrate. (b) Uncured Ecoflex is poured onto the conductive thin film. (c) The Ecoflex film is peeled off after curing. (d) Electrodes are deposited, silver wires are connected, and the entire sensor is covered with an Ecoflex slab. (e) Ionic conductor is injected into the microchannel; Gly–KCl denotes a mixture of glycerol and KCl (8 wt%) solution. (f) The channel is sealed with uncured Ecoflex. (g, h) Photographs of the composite strain sensor with applied strains of zero and roughly 300%. To evaluate the properties of the proposed composite structure, we fabricated three types of strain sensor made of different conductive materials but using Ecoflex as the common stretchable substrate. The different materials were (i) a pure graphene film, (ii) pure glycerol with 8-wt% KCl solution, and (iii) a composite film combining graphene with the ionic conductor (8-wt% KCl in glycerol solution). The three types of strain sensor exhibited linear current–voltage


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curves, indicating ohmic contact behavior (Figure 2a). The resistance of the graphene film (~67.9 kΩ) was lower than that of glycerol with 8-wt% KCl solution (~250 kΩ) but higher than that of the composite conductor (graphene/Gly–KCl) (~42 kΩ). We attribute the electrical conductivity of the pure glycerol with 8-wt% KCl solution to the directional movement of the ions (K+ and Cl−) in the glycerol under the bias voltage.49 The resistance of the composite strain sensor depends on those of both the graphene and the ionic conductor. We tested these strain sensors under different strains. Their electrical response is defined as the relative resistance change, namely ∆R/R0 = (R−R0)/R0, where R0 is the original resistance in the relaxed state and R is the real-time resistance when the sensor is stretched. The strain is defined as ε = (L−L0)/L0, where L0 and L are the lengths of the strain sensor without and with applied tension strain, respectively. The response–strain curves of the three types of sensor are compared in Figure 2b. The strain sensor based on the pure graphene film (blue line) clearly shows the lowest stretchability of ε = 10%. When the strain exceeded 10%, the relative change of resistance for the pure-graphene sensor increased suddenly to a relatively high value of 1,000, indicating that the graphene film was damaged. By contrast, the other two types of strain sensor exhibited the same stretchability up to 300%, at which strain they broke down. Additionally, our sensor based on graphene and Gly–KCl had a lifetime of at least two months. Because of a tiny leak of ionic liquid at the joint between the silver wires and the Ecoflex, the resistance of the composite conductor increased from 42.5 kΩ to 63.7 kΩ (Figure S1) over two months and the response to strain weakened (Figure 2c). At the largest strain, the resistance of our composite strain sensor (orange line) rose to as high as 16 times the initial resistance at zero strain. We evaluate the sensitivity of the strain sensor by calculating its figure of merit (i.e., the GF), which is defined as GF = (∆R/R0)/(∆L/L0), in which


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R0 and L0 are the original resistance and length of the sensor, respectively, and ∆R and ∆L are the changes in resistance and length, respectively. As such, the slopes of the curves in Figure 2b represents the GF of the sensors. The strain sensors based on pure graphene film, pure Gly–KCl, and the composite film displayed GF values of 1,000, 2, and 25.2, respectively. In particular, even at the smallest strain that our set-up could afford (4%), a sensor with four different prestretched strains (zero, 20%, 50%, and 100%) showed almost the same the response curves (Figure 2d). Therefore, the sensor resolution should be less than 4%. Moreover, according to the International Union of Pure and Applied Chemistry, the expected resolution of a sensor is obtained when the signal is three times higher than the noise level.55,56 By using the GF (sensitivity) and the root-mean-square deviation at the baseline, we estimated the theoretical resolution of our composite strain sensor to be a strain of 10−3 (Figure S2 and Table S1).55 The GF depends on the properties of the sensing material layers and the substrates.2,16,17 Typically, brittle and poorly stretchable materials possess higher GF, such as graphene film and carbon nanotube film.24 However, these materials either cannot be stretched or can withstand only very limited stretchability. Our graphene/Gly–KCl composite strain sensor displayed a large strain range of 300% (with a GF of 25.2), exceeding the strain ranges (

A multifunctional highly sensitive multiscale stretchable strain sensor

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