Extremely Stretchable, Stable and Durable Strain Sensors Based on

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Extremely Stretchable, Stable and Durable Strain Sensors Based on Double-Network Organogels Haoxiang Zhang, Wenbin Niu, and Shufen Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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ACS Applied Materials & Interfaces

Extremely Stretchable, Stable and Durable Strain Sensors Based on Double-Network Organogels Haoxiang Zhang, Wenbin Niu*, and Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, West Campus, 2 Linggong Rd., Dalian 116024, China Keywords: strain sensor, double-network organogel, extreme stretchability, durability, stability, graphene

ABSTRACT: Stretchable strain sensors offer great potential for diverse applications in modern electronics. However, it is still difficult to fabricate strain sensors with extreme stretchability, high stability and superior durability due to the challenge in elastic matrix. In this work, the first example of extremely stretchable and highly stable double-networks ethylene glycol (EG) organogel is developed for the fabrication of wearable strain sensors with high performances. It is shown that the formation of hybrid physically and chemically crosslinked double-networks endows the EG organogel with an extraordinarily stretchability as high as 21 000%, which is the highest value for gels reported in the literature. Meanwhile, the low vapor pressure of EG gives the organogel high ambient stability. Benefiting from the intrinsic stretchability and stability of EG organogel, the strain sensors are fabricated easily by incorporating graphene as electrically conductive filler, which display extremely wide strain-sensing range (>10 500% fracture strain)

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with a gauge factor of 2.3. More importantly, the sensor can withstand >50 000 loadingunloading cycles in air, exhibiting high stability and superior durability. It is demonstrated that these sensors can track joint movements and muscle vibrations (such as human joint motions, drinking, saying, breathing, slight cough) of human body, and even distinguish the deformations of different directions and the touches of a hair. This work not only provided a new elastic matrix platform for the fabrication of extremely stretchable, stable and durable strain sensors, but also demonstrates their applications as wearable electronic devices for tracking both large and tiny motions of human body, which could be further extended to the practical applications in electronic skin, human-machine interactions, and personalized health monitoring.

1. INTRODUCTION Stretchable, flexible and wearable electronic devices have grown into an important branch of modern electronics to meet the increasingly diverse and complex requirements. Strain sensors, as an important subfield, can transduce mechanical deformations (stretching, compression, bending, twisting, etc) into the changes of electrical characteristics, such as resistance,1-5 capacitance,6-8 current,9-11 showing promising applications in electronic skin12, personalized health monitoring,13-17 prosthesis,3,

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human-machine interaction,4,

14, 19

and soft robotics.20-21 For

realizing such applications, high-performance strain sensors including large stretchability, high stability and durability, high sensitivity, low fabrication cost, and facility are quite required. To date, various attempts have been made to construct high-performance strain sensors. The representative sensors were fabricated by incorporating sensing elements or conductive fillers such as carbon nanotubes, 22-24 graphene, 25-27 metal,22, 28-29 conductive polymer,30-31 into rubbery matrix due to its simple, scalable and cost-effective fabrication process. These devices use the changes of electrical signals caused by the variation of interfacial resistance between fillers to


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sense deformation under external mechanical strain and stress.22,

28-29

Despite the high

sensitivity,25 most of them usually show limited stretchability (low than 200%), thus restricting their applications under large mechanical strains and rigorous deformations. To achieve large stretchability, gel matrix-based sensors were paid intense attention recently due to their high elongation, flexibility and biocompability.32-35 For instance, various hydrogelbased strain sensors have been fabricated by introducing carbon and silver nanomaterials as electronic conductive fillers into polyvinyl alcohol hydrogel.28-29 These conductive hydrogel sensors exhibited high elastic deformation (1000%) and gauge factor (1.51), which can be directly attached on skin or glove to detect the bending and stretching motions of human joints such as finger, knee, elbow etc.22 However, those hydrogel-based devices still suffered from long-term environmental instability or low durability due to water evaporation, which induced the loss of stretchability of sensor and resulted in large signal fluctuations over time.32-35 Although surface encapsulation with VHB tape or PDMS (polydimethylsiloxane) could reduce water evaporation, the intrinsic viscoelasticity of these materials would lead to signal drift and detachment between layers after continuous loading-unloading operation.30-31 It therefore remains a great challenge to fabricate high-performance strain sensors with large stretchability, high stability and durability. Ethylene glycol (EG) is the simplest vicinal diol, which exhibits high boiling point (198 °C at 1 atm) and low volatility at room temperature in comparison with water. Replacing water with EG would endow the gel high ambient stability.31, 36 Meanwhile, it is known that double-network gels which consist of a brittle first network and a ductile second network exhibit high stretchability.37 The brittle first network could serve as a sacrificial bond to dissipate energy significantly via internal fracture, whereas the interpenetrated ductile second network bears a


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large deformation,38-40 thus achieving high stretchability. If the above double-networks were introduced into EG organogel, the high stretchability would be obtained, then the strain sensors with large stretchability, high stability and durability might be achieved by incorperating conductive filler. Herein, we developed a novel double-network EG organogel for the fabrication of wearable strain sensors with extreme stretchability, high stability and superior durability. To construct the double-networks in EG organogel, covalently crosslinked polymer (polyacrylamide, AAM) was introduce as the brittle first chemical network to dissipate energy during stretching. The entanglement and hydrogen bonding interaction of polymer chains was integrated as the ductile second network to allow an extreme stretchability (21 000%). By integrating graphene as electrically conductive channel because of its high flexibility and conductivity, the strain sensors were easily fabricate, which exhibit extreme extensibility (>10 500% fracture strain), high stability, excellent durability (> 50 000 loading-unloading cycles). The extraordinary sensing performances enabled successful detecting of not only large deformations (joint movement) but also tiny motions (muscle vibrations) of human body, showing potential applications in artificial skin, wearable electronics, soft robots etc.

2. RESULTS AND DISCUSSION 2.1. Structural Characterization of PAAM/PVP/EG Organogel. For the synthesis of EG organogels, acrylamide (AAM), polyvinylpyrrolidone (PVP) and polyethylene glycol (400) diacrylate (PEGDA400, crosslinker) were dissolved in EG to form a homogeneous solution, which was then casted into a mold with desired dimension and heated to


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Figure 1. Schematic preparation process of double-network PAAM/PVP/EG gels. (a) The preparation, molecular structures, and stretching mechanism of PAAM/PVP/EG organogels. (b) The preparation and sensing application of strain sensors.

45 °C for 5 h to form opaque PAAM/PVP/EG organogel (Figures 1a and 2a) after polymerization, PEGDA400 crosslinked polyacrylamide (PAAM) served as a chemically covalent network maintaining the shape of the organogel. A scanning electron microscopy (SEM) image of the freeze-dried PAAM/PVP/EG organogel exhibits its interconnected 3D porous polymeric architecture (Figure 2b, see Experimental Section for details). This porous structure is highly beneficial to the stretchability and in favor of rapid response. Fourier transform infrared (FT-IR) spectra further evidenced the formation of PAAM and the interactions between PVP and PAAM. As shown in Figure 2c and Figure S1 Supporting Information, AAM monomer exhibited the characteristic peaks at 998 cm-1 and 961cm-1 ascribed to the C-H out-of-plane wagging vibrations of terminal double bond (-CH=CH2), and the peak at


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Figure 2. Structural characterization of PAAM/PVP/EG organogel. (a) Optical images showing gelation behavior before and after precursor polymerization. (b) Cross-sectional SEM image of extremely stretchable organogel. (c) FT-IR spectra of AAM monomer, PVP, PAAM/EG gel, and as-prepared double-network organogel. (d) Frequency dependence of the storage (G′) and loss (G″) moduli of PAAM/PVP/EG organogel.

1428 cm-1 assigned to the C-H in-plane bending vibration of =C-H. After polymerization, there were no these characteristic peaks, suggesting in-situ polymerization of AAM monomer into PAAM. Correspondingly, PAAM showed the broad peaks at 1613 cm-1 and 1666 cm-1 attributed to the N-H bending and C=O stretching vibrations of amide groups, respectively.41 When PVP was introduced, the N-H bending vibration peak was absent, and the C=O peak shifted to 1646 cm-1, implying the formation of C=O⋯H-N hydrogen-bonding interactions between PVP and PAAM in PAAM/PVP/EG organogel.37, 42 This hydrogen-bonding interaction is beneficial to enhancing energy dissipation during stretching, thus increasing the elongation of the resulting


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organogel (see below).43-44 In addition, rheological measurements at room temperature showed that its storage modulus (G′) was larger than loss modulus (G″) over the entire frequency range (Figure 2d). Also, G′ increased faster with increasing angular frequency than did G″. Such features are characteristics for typically crosslinked polymer networks, suggesting a solid behavior and elastic property of the organogel.45 2.2. Mechanical Properties of PAAM/PVP/EG Organogel. By manipulating the weight ratio of crosslinker PEGDA400, a series of organogels with distinct mechanical performances were obtained. As displayed in Figure 3a, the stretchability of organogel increased with decreasing the weight ratio of PEGDA400 from 1.25 wt% to 0.25 wt%. This is due to the decreased chemically crosslinking density with decreasing the content of PEGDA400. The decrease of covalent crosslinking density increased the distance between individual PAAM chains, which enhanced the compliance of PAAM network, thus facilitating the unfolding and sliding of the PAAM and PVP chains. As a result, the stretchability increased gradually.46 A further decrease of PEGDA400 content to 0.13 wt% reduced its enlongation (Figure S2, Supporting Information). When the weight ratio of PEGDA400 was 0.25 wt%, the organogel had the highest stretchability, and could be stretched to over 21 000%. To the best of our knowledge, this is the longest elongation value of gel.39, 47-48 The corresponding Young’s modulus was calculated to be 37.12±11.92 KPa from the low-strain range (

Extremely Stretchable, Stable and Durable Strain Sensors Based on

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