Highly Sensitive and Large-Range Strain Sensor with a Self

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Applications of Polymer, Composite, and Coating Materials

Highly Sensitive and Large-Range Strain Sensor with a Selfcompensated Two-order Structure for Human Motion Detection Jianhua Ma, Peng Wang, Hongyu Chen, Shenjie Bao, Wei Chen, and Hongbin Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20902 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Materials & Interfaces

Highly Sensitive and Large-Range Strain Sensor with a Selfcompensated Two-order Structure for Human Motion Detection

Jianhua Ma1,† Peng Wang1,† Hongyu Chen3, Shenjie Bao2, Wei Chen2*, Hongbin Lu1*

1. State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Collaborative Innovation Center of Polymers and Polymer Composites, Fudan University, 2005 Songhu Road, Shanghai 200438, China 2. Center for Intelligent Medical Electronics, School of Information Science and Technology, Fudan University, 220 Han Dan Road, Shanghai 200433, China 3. Department of Industrial Design, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

† These authors contributed equally.

Abstract: Constructing flexible, high-sensitivity strain sensors with large working ranges is an urgent task in view of their widespread applications including human health monitoring. Herein, we propose a self-compensated two-order structure strategy to significantly enhance the sensitivity and workable range of strain sensors. Three dimensional (3D) printing was employed to construct highly stretchable, conductive polymer composite open-meshes, in which the percolation network of graphene sheets 1

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constitutes a deformable conductive path. Meanwhile, the graphene layer coated on the open-mesh provides an additional conductive path that can compensate spontaneously the conductivity loss of the percolation network at large strains, through the new conductive paths formed by the graphene sheets in the coating layer and the inner networks. At strains lower than 20%, the sliding and disconnection of graphene sheets coated on the mesh surface largely enhance the sensitivity of the sensor, a 20 times increase as opposed to that of the non-two-order structure sensor. The resulting sensor reveals high gauge factors (from 18.5 to 88443) in a strain range of 0-350%, and the exceptional capability to monitor a wide range of human motions, from subtle pulse, acoustic vibration to breathing and arm bending.

Keywords: strain sensor, 3D printing, two-order structure, graphene, human motion detection

1. Introduction

Developing next-generation flexible, highly stretchable and sensitive strain sensors is urgently needed given their great application potential in the areas from electronic skins to real-time healthcare monitoring.1,2 Unfortunately, however, traditional strain sensors based on metals3 or semiconductors4 always exhibit high rigidity and limited sensing range (< 5%) due to the intrinsic brittle structures which significantly hinder their use in flexible, wearable and highly stretchable strain sensors. To this end, considerable effort has been attempted by combining flexible, stretchable polymer with conductive 2


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nanomaterials, including metallic nanowires,5 carbon nanotubes (CNT)6 and graphene.7 In spite of some progress in improving the sensitivity8 and stretchability9 of the composite sensors, how to simultaneously achieve high sensitivity and large workable strain range is still an open issue.

Stretchability and sensitivity are two key parameters for full-range strain sensors. Two strategies have been proposed, that is, engineering active materials and choosing elastic substrates with well-designed structure.10 For the former, a variety of active materials such as 3D graphene foam,11-15 graphene paper,16,17 woven fabrics,18,19 CNT layers20,21 have been attempted. Despite the relatively high gauge factors (GF), their preparation processes are complicated and hard to control. Moreover, the stretchability inevitably suffers from the intrinsic fracture limit of polymer matrix; for instance, no polydimethylsiloxane (PDMS)-based sensors revealed workable strain ranges larger than 100%.16 For the latter, micro-sized pores10,22 and surface strain redistribution strategy23 have been employed to optimize the elastic substrates and enhance the sensitivity of strain sensors, but the problems involving complicated preparation processes and limited workable range remain unsolved. To optimize stretchability and workable strain range of sensors, well-designed elastic substrates, such as kirigami6,24,25 and bio-inspired structure designation, have been reported.26 However, these sensors still exhibited quite low GFs especially at small strain region (GF ＜ 2), difficult to sensitively detect subtle deformations such as human pulse and phonation. Therefore, it remains highly challenging how to simultaneously achieve high sensitivity and large workable strain ranges over 100%.27 3



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High sensitivity implies an ability to perceive the abrupt structure change of sensors at small strains, but this usually conflicts with whether sensors are able to preserve structural integrity at large strains. In consequence, a trade-off between sensitivity and workable strain range always has to be performed.27 Here, we propose a two-order structure concept to endow strain sensors with high sensitivity and large workable range. A PDMS open-mesh constructed by three dimensional (3D) printing affords the deformation capability up to 350% and meanwhile, the conductive graphene coating on the mesh surface enables the sensor to perceive the subtle deformation with high sensitivity. The resulting sensor reveals high GFs (from 18.5 to 88443) and a large work range (0-350%). The novelty of this work lies in resolving the critical challenge between high sensitivity and wide sensing range on the basis of open-mesh structure and self-compensation mechanism, which is important for the development of strain sensors. First, the sliding and disconnection of RGO sheets coated on the surface (crack mechanism) of the strain sensor result in the high sensitivity at small strain range. Second, the graphene layer coated on the open-mesh provides additional conductive paths that can compensate spontaneously conductivity loss of the percolation network at large strains through combination with the inner network (self-compensation mechanism), so that a large workable strain range can be achieved along with high sensitivities. Such two-order structure allows us to adjust the brightness of a LED light through deformation (0-100%), perceives a variety of human motions including subtle pulse and phonation and affords a new solution for developing flexible strain sensors with high sensitivity and large working range. 4


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2. Results and Discussion

2.1 Preparation and characterization of 3D printed strain sensors

To construct an open-mesh through 3D printing, we employed graphene sheets to tune the viscoelasticity of PDMS. Figure 1a shows the curves of viscosity as a function of shear rate for pure PDMS and graphene/PDMS mixture (G-PDMS); G-PDMS-10 and G-PDMS-15 denote 10 and 15 wt% graphene contents in graphene/PDMS mixtures, respectively. Unlike the pure PDMS, the addition of graphene sheets makes G-PDMS composites exhibit remarkable shear-thinning behavior, which is critical for extrusionbased 3D printing.28 Different from the pure PDMS and G-PDMS-15, an elasticviscous crossover located around 100 Pa appears for G-PDMS-10 (Figure 1b), which is lower than the calculated shear stress (~167 Pa, see Figure S14 in Supporting Information), indicating the ink of G-PDMS-10 can flow smoothly through the nozzle and preserve shape integrity after leaving the nozzle.29 In addition, its electrical conductivity is high enough (~0.1 S/m) due to formation of the inner percolation networks, as shown in Figure S1. Thus, we chose G-PDMS-10 as the printing ink in this study (Figure 1c-Ι), and G-PDMS is used to denote the G-PDMS-10 composite in the following description. Figures 1c-f schematically show the preparation processes of a highly stretchable strain sensor with open-mesh structure. The obtained ink (Figure 1c-ΙΙ) was used to construct the pre-designed open-mesh through 3D printing (Figure 1c-ΙΙΙ, ΙV and Supporting movie 1), and the latter subsequently was cured for 3h at 80 oC

(Figure 1d). To improve the adhesion of GO, the printed G-PDMS mesh was first 5



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treated with O2 plasma to make it hydrophilic and then immersed successively in positively charged polyethyleneimine (PEI) solution and negatively charged GO solution (Figure 1e). Finally, the GO coated G-PDMS mesh was reduced by HI to give the highly stretchable G-PDMS/RGO strain sensor (Figure 1f). More details of the preparation process are provided in Figure S2. Figures 1g, h present a structure model and the scanning electron microscope (SEM) image of two-order structure crosssection of G-PDMS/RGO sensor, in which the region A represents the inner G-PDMS part and B represents the outer RGO coating, respectively. Figure 1i shows the image of fracture surface of the sensor, indicating the uniform dispersion state of graphene sheets in the inner G-PDMS part. The SEM image of the RGO coating is presented in Figure 1j and the characteristic wrinkled morphology signifies that RGO sheets were successfully coated onto the G-PDMS surface. Through 3D printing, we can tune the physical structure and elongation capability of G-PDMS composites (Figure 1k and Figure S3, S4), in which both amplitude and half-wavelength of the open-mesh strut are 3 mm (Figure S5), revealing a deformation capability of over 400% (Figure 1l).

6


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Figure 1. (a) Viscosity vs. shear rate curves for pure PDMS and G-PDMS composites with different filler contents; (b) corresponding storage modulus (G’) and loss modulus (G’’) vs. shear stress curves; (c) digital image of pure PDMS, G-PDMS-10 (Ι) and 3D printing process (ΙΙΙ-ΙV) of the ink (ΙΙ); (d-f) curing (d), GO coating (e) and reduction processes of the printed G-PDMS composite (f); (g-h) the structure model and SEM image of the cross-section of G-PDMS/RGO sensor (Top-view), letters A and B represents the inner G-PDMS part and outer RGO coating, respectively. (i-j) SEM images of fracture surface of the sensor (i) and the RGO coating layer (j), respectively; 7



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(k) digital images of printed G-PDMS composites with different structures; (l) digital images of G-PDMS sensor at strains from 0% to 400%.

The XRD patterns of GO, RGO and graphene sheets are shown in Figure 2a, respectively. It can be seen that the graphene sheets obtained by thermal reduction of GO at 1000 oC reveal better crystal structure compared with the HI-reduced RGO. In this study, we formulated the 3D printing ink G-PDMS by solution blending to disperse graphene sheets in PDMS, and the printed G-PDMS mesh is shown in Figure 2c. Given that PDMS is intrinsically hydrophobic, we employed plasma treatment to make it hydrophilic (Figure 2b-I), which facilitates the adhesion of RGO sheets to the PDMS surface (Figure 2b-II and III). It can be seen that after G-PDMS was coated with RGO sheets

(G-PDMS/RGO)

its

surface

becomes

more

shining

(Figure

2c).

Thermogravimetric analysis (TGA) shows that the content of the coated RGO is about 1 wt% (Figure 2d). The Raman spectra of PDMS, G-PDMS and G-PDMS/RGO samples are presented in Figure 2e. For pure PDMS, four peaks at 489, 706, 2906 and 2964 cm-1 are assigned to the symmetric stretching of Si-O-Si, Si-C, as well as the asymmetric and symmetric vibrations of CH3, respectively.30 For G-PDMS, two dominant peaks at 1349 and 1598 cm-1, corresponding to the D and G bands of graphitic carbon,31 appear while the characteristic peaks of PDMS are greatly weakened. Furthermore, there are only RGO signals appeared in G-PDMS/RGO, implying that a complete RGO coating layer was formed on the G-PDMS surface. The Raman spectra of the corresponding graphene sheets, GO and RGO samples were presented in Figure S6. Due to the presence of the RGO coating, G-PDMS/RGO reveals the significantly 8


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improved electrical conductivity, compared with G-PDMS (Figure 2f).

Figure 2. (a) XRD spectra of GO, RGO and graphene sheets; (b) photograph of the GO solution on PDMS and plasma-treated PDMS (I), optical images of GO-coated (II) and RGO-coated PDMS (III); (c) photograph of G-PDMS and G-PDMS/RGO meshes, (d-f) TGA curves, Raman spectra and current-voltage curves of PDMS, G-PDMS and G-PDMS/RGO.

2.2 Electromechanical performance and sensing mechanism

The mechanical properties of the sensors were evaluated by stress-strain curves. As shown in Figure 3a, the RGO coating slightly reduces the mechanical strength of GPDMS (from 0.39 to 0.27 MPa), but the elongations at break of both exceed 400%. A large elongation is the prerequisite for strain sensors to achieve ultra-large workable 9



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ranges. Figure 3b shows the curves of resistance (R) as a function of strain for G-PDMS and G-PDMS/RGO, respectively. It is obvious that the R value of G-PDMS/RGO is always lower than that of G-PDMS. Moreover, when R increases to 106 kΩ, the corresponding strains for G-PDMS and G-PDMS/RGO are 215% and 350%, respectively. Here, we set the value of 106 kΩ as the upper limit according to the maximum value of resistance that can be detected by the test equipment. Thus, the effective strain range of G-PDMS and G-PDMS/RGO can be determined to be 215% and 350%, respectively. The inset in Figure 3b shows the resistance change of GPDMS and G-PDMS/RGO at the strain of 20%, respectively. For G-PDMS, due to the uniform dispersion of graphene sheets, the structure integrity of conductive networks was well preserved at this strain (the corresponding SEM images for the morphological structure of graphene sheets in the PDMS at strains of 0 and 20% are shown in Figure S7), and as a result, R increases from 157 to 176 kΩ (only 12% increase). Such a small change implies that the sensitivity of G-PDMS at small strain range is low, which also signifies that with the uniform dispersion of conductive fillers, strain sensors can barely achieve high sensitivity at small strain ranges.32,33 For G-PDMS/RGO, however, its initial resistance (R0) at the unstretched state (7.8 kΩ) is far lower than that of G-PDMS and sharply increases to 26.6 kΩ at strain of 20%, a ~241% increase and thus much higher sensitivity. Figure 3c presents the gauge factors (GF) of G-PDMS and GPDMS/RGO within 0-80% strain range, which reveals a significant increase in GF from 1.9 of G-PDMS to 18.5 of G-PDMS/RGO and moreover, within 20%, the GF of G-PDMS/RGO is 20 times that of G-PDMS (the inset in Figure 3c). Further increasing strain, the GF of G-PDMS/RGO gradually increases to 448.5 (80-200%, Figure 3d), 10


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6673 (200-300%, Figure 3e) and 88443 (300-350%, Figure 3e), respectively. Apparently, such two-order structure design affords a new route to achieve high sensitivity of full-range strain sensors. This is in sharp contrast with the results previously reported,34-39 most of which only exhibited a workable strain range less than 100% (Figure 3f), far lower than that of G-PDMS/RGO (350%).

The evolution of this two-order structure during deformation can be illustrated in Figure 3g. At the initial state (Figure 3g-Ι), the RGO-coated surface (the first-order conductive network of G-PDMS/RGO, blue solid line) constitutes a conductive path that results in a relatively low initial resistance. A small strain induces sliding and disconnection of RGO sheets and break the outer conductive path (the blue dot line in Figure 3g-ΙΙ and Figure S8), which increased the resistance and sensitivity even within a smaller strain range. In this phase, the inner conductive network consisting of graphene sheets (the second-order conductive network, black solid line in Figure 3g and Figure S8c) does not show obvious structural change, but it forms an additional conductive path through the deformation-induced contacts between the outer disconnected RGO sheets and the inner network, as shown in the magnified picture in Figure 3g-ΙΙ). This may compensate the loss of electrical conductivity resulted from breakdown of the outer conductive path, engendering a resistance (26.6 kΩ for GPDMS/RGO) far lower than that of G-PDMS (176 kΩ). Such a self-compensated mechanism is different from those reported sensors in which the conductive fillers were directly coated on the surface of insulated polymer matrix.22,40 For the latter, due to the insulated character of the inner polymer part, the disconnected coating layer at small 11



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strains (Figure S9) dramatically increases the resistance of sensors so that they can usually only work in a limited, small strain range (Figure S10), thus limiting their practical applications.

For the G-PDMS/RGO sensor, with the further increase of strain, the graphene sheets dispersed in the inner part start to gradually slid and disconnected (Figure 3g-ΙΙΙ). However, the electron tunneling effect (ETE) makes the inner part still conductive and more importantly, the self-compensation mechanism provides additional conductive paths through deformation-induced contact with inner and outer RGO sheets; as observed in Figure 3b, where the resistance of G-PDMS/RGO is always lower than the G-PDMS. For an ETE dominated system, the relationship between resistance (R) and tunnel distance (d) can be expressed as41-43 (1)

R  exp( Ad )

where A represents the tunnel parameter and equation (1) can be simplified as (2)

log(R)  d

d is proportional to strain (ε); thus, the relationship between R and ε can be written as log(R)  

(3)

As shown in Figure 3b, for G-PDMS, fitting for log(R) and ε reveals two different slopes, 0.41 for 0-80% strain and 2.22 for 80-200% strain, respectively. This would closely relate to the unique structure of G-PDMS, in which the elastic mesh structure can buffers the actual strain experienced by the fillers dispersed in PDMS.26,27 As a result, the tunnel distance between the fillers at the small strain range (0-80%) did not increase as fast as at the large strain range (80-200%). A small slope at 0-80% strain 12


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for G-PDMS implies that the dispersion of conductive fillers is uniform but it can barely achieve high sensitivity at small strains. By comparison, the resistance of GPDMS/RGO at 0-80% strain range is mainly dominated by the disconnection mechanism.44 During the sliding process of RGO sheets, the relative resistance vs. strain can be written as17 R

R0



ab S 0R 0

(4)

where a, b and S0 can be regarded as constants, implying a linear relationship between ΔR and . Therefore, due to this mechanism, G-PDMS/RGO reveals a higher sensitivity at strains of < 80%. On the other hand, as the strain goes beyond 80%, most of the coated RGO sheets become disconnected and at this time the relative resistance is mainly dominated by the electron tunneling mechanism and can be written as34 R(

4d 2m L 8hd )( ) exp( ) 2 2 N 3ra e h

(5)

where L, N, h, α, e are constants, d is the tunnel distance and φ is the height of potential barrier between adjacent sheets, and can be simplified as log(R)  1 

(6)

For G-PDMS, as the strain > 80%, R vs. ε can also be described with log(R)   2 

(7)

Given that the height of potential barrier between the coated RGO sheets in GPDMS/RGO is smaller than that of graphene sheets in G-PDMS, φ1 is smaller than φ2 and as a result, the slope between log (R) and ε of G-PDMS/RGO is smaller than that of G-PDMS at strains of > 80%, as shown in Figure 3b. This enables G-PDMS/RGO to work in a strain range far larger than G-PDMS, due to the presence of such self13



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compensation mechanism.

Figure 3. (a) Stress-strain curves of G-PDMS and G-PDMS/RGO; (b) electrical resistance of G-PDMS and G-PDMS/RGO as a function of strain, the inset shows the resistance value of G-PDMS and G-PDMS/RGO at 0% and 20%, respectively; (c-e) resistance vs. strain of G-PDMS and G-PDMS/RGO in 0-80% and 0-20% (inset); (de) resistance vs. strain of G-PDMS/RGO in 80-200% and 200-350%, respectively; (f) comparison of GF (0＜strain＜20%), maximum GF and workable strain range of GPDMS/RGO and the reported strain sensors; (g) illustrated structure evolution of the 14


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conductive networks in G-PDMS/RGO at 0% (Ι), 0-80% (ΙΙ) and 80-350% (ΙΙΙ), respectively.

Figure 4a shows the variation in relative resistance of G-PDMS/RGO upon stretching to 5%, 10%, 25% and 50%, and the corresponding values are 0.36, 0.85, 3.05 and 6.91, respectively, which are in good agreement with those shown in Figure 3c, indicating the good stability and repeatability of the strain sensor. Figure 4b shows the change in relative resistance as strain increases from 0 to 100%, during which stretching and releasing result in a hysteresis. This could be ascribed to the viscoelasticity of the PDMS, as observed in the loading-unloading stress-strain curves of G-PDMS/RGO (Figure S11). Due to the presence of the RGO coating layer, G-PDMS/RGO reveals an initial resistance far lower than that of G-PDMS, which can illuminate the LED lights at un-stretched state (Figure 4c-Ι). When it was stretched, the brightness of the LED gradually decreased (Figure 4c-ΙΙ, ΙΙΙ) until the strain of 100%, at which the brightness disappeared completely (Figure 4c-ΙV). The corresponding movie was also presented in the Movie 2. This is totally different from the situation of the reported stretchable conductors that had high elongation but constant resistance.24-26 Other strain sensors exhibited the ability to change the brightness of LED lights with deformation,12,18,45 but their strain ranges are usually

Highly Sensitive and Large-Range Strain Sensor with a Self

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