tactile sensors

Polymer 167 (2019) 154–158

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Highly stretchable ionic conducting hydrogels for strain/tactile sensors ∗

T

Ren'ai Li, Kaili Zhang, Ling Cai, Guangxue Chen , Minghui he State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, 510640, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Conducting hydrogel Ionic conductivity Strain/tactile sensors

Ionic conductors based on stretchable hydrogels are on the rise to develop wearable devices because of choosing the same signal carriers for biological areas and intrinsic stretchability. In this paper, we demonstrate ionic strain/tactile sensors based on poly(acrylamide)/poly(ethylene oxide)/LiCl hydrogel. Owing to their chemically crosslinked structures and multiple H-bonding networks, these hydrogels exhibit excellent mechanical properties, such as high stretchability (∼8.8 times, 100 mm/min), high compression strength (556.58 MPa), high stabresistant and damage-resistant ability and nearly ∼100% electrical self-healing ability. The use of salts (LiCl) as conductive ions makes the hydrogels ideal ionic conductors, imparting an ionic conductivity of ∼8 S/m. The ionic conducting hydrogels were demonstrated as strain/tactile sensors with high sensitivity to monitor human activity and external pressure. These hydrogel-based ionic sensors may find applications in sports monitoring, human/machine interfaces and soft robotics.

1. Introduction

deliquescent salt (e.g., NaCl and LiCl) and the ionic liquid have been developed for ionic conductors [17–23]. Particularly, hydrogel-based ionic conductors have recently attracted much attention in E-skins [17], energy storages [24,25], light emitting devices [26], actuators [27,28], and sensors [29] because of their intrinsic stretchability, conductivity, biocompatibility and adopting the same carrier as biosystem. Suo's group introduced a highly stretchable, transparent, and biocompatible hydrogel as “ionic skin” sensor and position sensing [17]. Ionic touch pad based on ionic conducting hydrogel was also exploited as a novel application for HMIs. The touch pad operates perfectly even at 1000% areal expansion and can be used on complex curved surfaces, such as skin, while maintaining conformal contact with the skin [18,30]. Nevertheless, despite tremendous progress, it is a challenge to the design these polymer networks integrating such a rigorous set of requirements simultaneous with high stretchability, high compression strength, stab- and damage-resistant and electrical self-healing for wearable electronics. Here, we synthesized a highly stretchable ionic conducting hydrogel with strain and pressure sensitive features, introducing a promising candidate for wearable sensors. The hydrogel was formed with poly (acrylamide)/poly(ethylene oxide)/LiCl (PAAm/PEO/LiCl) networks through a synthetically simple approach, and here linear PEO molecules are used to enhance chemically crosslinked PAAm polymer chains by multiple H-bonding network. The dynamical, rapid association and dissociation of the H-bonding dissipate energy in hydrogels, providing

The importance of Human Machine Interface (HMI) has been emphasized as a means to provide close communication between humans and devices [1,2]. Wearable electronics, as important hardware for HMIs, have been widely studied in recent years [3–8]. People expect the device as flexible as human skin, which can undergo various forms of deformation (stretching, twisting and compression), have damage-resist ability, or even sense external signals. In view of this, researchers have devoted tremendous efforts to promote its development. One approach is synthesizing intrinsically stretchable materials with conductive components such as conductive polymers, carbon nanotubes (CNTs) and metal powders to realize stretchable electronics [9–12]. Another approach like structural engineering has been used to modify non-intrinsically stretchable materials into wavy, wrinkled or serpentine configurations to create a stretchable electronic devices [13–16]. However, despite great progresses made in the fields of wearable electronics, there still exist fundamental mismatches between human beings and wearable devices. First, metals and semiconductors are nonintrinsically stretchable, and developing these devices are very complicated and costly. Second, direct communication between electronic devices and biological areas is impossible to facilitate due to the different signal carriers (electrons and ions) [2]. Unlike electronic conductors, signals are transferred in ionic conductors by ionic conduction through media [2]. Typically, the solvated



Corresponding author. E-mail addresses: [email protected] (R. Li), [email protected] (K. Zhang), [email protected] (L. Cai), [email protected] (G. Chen), [email protected] (M. he). https://doi.org/10.1016/j.polymer.2019.01.038 Received 3 December 2018; Received in revised form 10 January 2019; Accepted 16 January 2019 Available online 06 February 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Design and synthesis of high-performance PAAm/PEO/LiCl ionic conducting hydrogels. (a) The hydrogels consist of PAAm, PEO networks and inorganic ions. (b) Photographs of the ionic conducting hydrogels showing their abilities to withstand i) large stretching, ii) curly stretching, and iii) compression.

3. Results and discussion

excellent mechanical properties for the prepared hydrogels. LiCl in the hydrogel networks act as conductive ions, imparting the hydrogels conductivity of ∼8 S/m. We apply the ionic conducting hydrogel as strain/tactile sensors to monitor human motions and external pressure.

Fig. 1 illustrates the fabrication of ionic conducting hydrogels. To prepare PAAm/PEO/LiCl hydrogel, in brief, a predetermined amount of LiCl and AAm was added into the PEO solutions. Subsequently, MBAA (crosslinker), APS (initiator), and TEMED (accelerator) was added into the solution. Thermo-polymerization of the solution produced the final PAAm/PEO/LiCl ionic conducting hydrogel. In this process, we used the chemical polymerization of AAm to synthesize the network structure, and tune PEO concentration to form a new enhanced framework. The FTIR spectra of PAAm/LiCl and PAAm/PEO/LiCl hydrogels were shown in Fig. S1. The new CeO absorption bands of PAAm/PEO/LiCl at around 1107 cm−1 showed the successful introduction of PEO into PAAm/LiCl network. Noticeably, the bands of the eOH stretching of alcohol group and the eNH stretching vibration of amide group shifted to lower wavenumbers in the presence of PEO, which can be attributed to the formation of the intermolecular H-bonding between PAAm and PEO. The dynamic, rapid association and dissociation of H-bonding in hydrogels dissipate energy and improved the mechanical properties. LiCl is used as the conductive ion because of its high conductivity, ionic strength, and hygroscopic nature [31,32]. We noticed that the as-prepared ionic conducting hydrogels were able to withstand various forms of deformation including large stretching, curly stretching, knotted stretching and compression (Fig. 1b, Fig. S2). We investigated the effect of PEO concentration and the molecular weight on the mechanical properties of ionic conducting hydrogel. Hydrogel without PEO only had a ∼1.8-fold tensile deformation under high tensile speeds (100 mm/min). When a small amount of PEO (2 wt %) was introduced to the hydrogel network, the tensile properties of the hydrogel increased significantly (∼7 times). The hydrogel with 8 wt% of PEO achieved a maximum tensile deformation of ∼8.8 times. Subsequently, with the increasing PEO concentration, the tensile deformation decreased. This may be related to the optimum crosslink density between PAAm and PEO in the hydrogel network. In terms of compression performance, the hydrogels exhibited a sharp rising stress at high compression strain (∼0.8 times). The maximum Young's modulus of hydrogel is as high as 556.58 MPa (Fig. S3). The larger PEO molecular weight exhibits a decreasing tensile deformation but with an increasing tensile modulus, this maybe attribute higher PEO molecular weights give rise to a higher degree of polymer chain entanglements, which reinforces the hydrogels but suppressed the stretchability [33]. It is worth mentioning that all the hydrogels we prepared were not crushed and recovered well even under as high as ∼ 0.95-fold deformation test (Fig. S4). In addition, our hydrogels also show robust mechanical properties while in the face of external force like stab or even partial

2. Materials and methods 2.1. Materials Acrylamide (AAm, AR, 99%), LiCl (AR, 99%), N,NMethylenebisacrylamide bisacrylamide (MBAA, AR), N,N,N′,N′Tetramethylethylenediamine (TEMED, 99%), Ammonium persulfate (APS, AR, 98.5%) and polyethylene oxide (PEO, average Mv ∼100000, 300000, and 600000, powder) were purchased from Shanghai Macklin Biochemical Co., Ltd. All materials used as received. 2.2. Preparation of hydrogel The ionic conducting gels were prepared by dissolving AAm monomer powder and LiCl into PEO aqueous solution. The composition of crosslinker, initiator, and LiCl was mainly referred to Suo's report with gentle modifications [17]. Molar concentrations of AAm and LiCl were fixed as 2.67 M throughout the entire experiments. 0.1 mol% MBAA and 1 mol% APS with respect to the AAm monomer were added as a crosslinker for AAm and a thermal initiator, respectively. After stirring for 1 h under N2 environment, 0.1 mol% TEMED with respect to AAm monomer were lastly added as an accelerator. The solutions were poured into a glass mold with a vacancy (80.0 mm × 80.0 mm × 2 mm) and covered with glass plate. The hydrogels were cured under room temperature for 12 h. 2.3. Characterization Fourier transform infrared (ATR-FTIR) spectra were recorded on a Bruker Vertex 33 spectrometer. The tensile testing was performed using a tensile machine (INSTRON 5565, 100N load cell). Stretch speed is set to 100 mm/min, compress speed is set to 1%/s. The electrochemical properties of the ionic conducting hydrogel were measured by PGSTAT 302N (Princeton Applied Research) through the electrochemical impedance spectroscopy (EIS) method. The applied frequency range in the electrical tests was from 1 to 105 HZ, the relative humidity was 35%, and the temperature was 25 °C. The resistance change with time is measured on the Keithley DMM7510 module. Optical images were taken by a Nikon Digital Sight DS-Fil camera. 155

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Fig. 2. Mechanical properties of the prepared hydrogels. (a–b) the effect of PEO concentration on the tensile and compressive properties; (c–d) the effect of PEO molecular weights on the tensile and compressive properties; (e–f) Photographs of stab-resistant and damage-resistant properties of hydrogels.

and found that there was almost no resistance change after the 9 times cut-contact cycles. This fast and good contact enabled the electrical signals in the circuit to be healed timely and quickly (Fig. 3e). A battery-powered circuit was built to visually demonstrate the healing of hydrogel for electronic circuits (Fig. S6). In addition to electrical healing behavior, the hydrogel can be used as a temporary circuit repair material. Series circuit, parallel circuit and series-parallel circuit were achieved respectively by connecting three LEDs using hydrogel. The hydrogel wires could connect well to each other without the use of any glue (Fig. S7). Due to the excellent mechanical and electrical capabilities of ionic conducting hydrogels, we explored their use in strain/tactile sensors. Unlike the electronic conductor, the ionic conducting hydrogel is more suitable for human activity monitoring because of the same signal carrier as the human body and intrinsic stretchability. Here, (R eR0)/ R0 = ΔR/R0 is defined as relative resistance change, R0 is the resistance

damage (the middle portion is perforated) (Fig. 2e and f). LiCl in the hydrogel network impart the ionic conductivity. The hydrogel was in series with a battery-powered circuit, and the lighting LED indicates its great conductivity. Due to the intrinsic stretchability of the hydrogel, the LED still kept lighting and maintained circuit connectivity even under large deformation. The diming LED indicates the increasing resistance in hydrogel (Fig. 3a). We tested the electrical properties of the ionic conducting hydrogel using EIS (Fig. 3b and c). It was found that the PEO concentration or molecular weight had little effect on the conductivity of the hydrogel, all around 8 S/m (Fig. S5). In addition, the H-bonding between PAAm and PEO also endow the hydrogel self-adhesion behavior. We cut a hydrogel column into three parts and dyed different colors to distinguish them, and then reconnected them together. These three parts could be quickly reconnected together without any external stimulation (Fig. 3d). We examined the electrical healing capability of ionic conducting hydrogel

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Fig. 3. Electrical and electrical self-healing behavior of ionic conducting hydrogels. (a) Digital photograph of hydrogel in series with a lighting LED, showing its conductivity and intrinsic stretchability. (b) Electrochemical impedance spectroscopy (EIS) plots of a series of hydrogels with different PEO concentrations. (c) EIS plots of a series of hydrogels with different PEO molecular weight. (d) Photograph of a hydrogel column, constructed from three cut blocks connected together, showing its self-adhesion property. (e) The real-time electrical healing process using resistance measurements for 2–3 s healing time at room temperature. The relative humidity was ∼35% and the temperature was 25 °C.

Fig. 4. Sensitivity of the ionic conducting hydrogel-based strain/tactile sensor. (a–c) Relative resistance change of strain sensors versus time for real-time monitoring of various human motions like bending and release of a finger, an elbow and a knee, respectively. (d) The single-sided configuration of the prepared tactile sensor. (e) The assembled “interlocking structure” of the tactile sensor, (f) the relative resistance change of the tactile sensor to external pressures.

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at 0% strain/compression, R is the resistance under stretch/press. The hydrogel was encapsulated by VHB4910 and fixed in various parts of the human body such as finger, elbow and knee for human activity monitoring. Fig. 4a shows the motion detection for the index finger. The hydrogel strain sensor was repeatedly bent. It was found that the deformation can be real-time fed back to a stable and repeatable electrical signal. When the hydrogel was fixed to the elbow, the resistance change was greater due to the larger deformation caused by the elbow (Fig. 4b). The detection of knee joint bending was illustrated in Fig. 4c. The sensor was stretched and released with the bending and straightening the knee. It showed that the resistance of the sensor increased/ decreased as the bending angle changed. As a result, the strain sensor was capable of distinguishing the different bending angles of the knee. Since the ionic conducting hydrogel can undergo large deformations, therefore a pressure sensor maybe another promising application for wearable electronics. We reconstructed the configuration by template method to make the bottom of the ionic conducting hydrogel “grow” a lot of antennae (Fig. 4d), which is more deformable and sensitive to pressure. Then we assembled up and down to form an interlocking structure tactile sensor (Fig. 4e). Due to the contact between the hydrogels, the assembled tactile sensor can sensitively detect external pressure signal. When apply to a pressure, the contact between the antennas would be less tight or partially separated, resulting in a resistance change. Thereby, due to the different separations among antennas caused by various pressures, the difference in external pressure can be distinguished readily, like ∼250 Pa, ∼500 Pa and ∼1500 Pa, respectively (Fig. 4f).

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4. Conclusion

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In summary, we designed an enhanced PAAm/PEO/LiCl network for the preparation of ionic conducting hydrogels. The introduction of PEO provides multiple H-bonding to the hydrogel network, which can dissipate energy by rapid association and dissociation and significantly improve the mechanical properties. LiCl in hydrogels provided a ∼8 S/ m ionic conductivity. These hydrogels have excellent electrical selfhealing ability, which can recover electrical signals quickly and stably. Finally, we apply ionic conducting hydrogel to strain/tactile sensors for human activity monitoring and detection of external pressure. This kind of ionic conducting hydrogel, with a synthetically simple approach and outstanding mechanical and ionic conducting properties, should dramatically increase the choice for numerous applications, such as energy storage, electrochemical devices, biological signal detection, sensors and wearable ionics.

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Acknowledgements

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This research was financially supported by Science and Technology Program of Guangzhou (201607020045, 2017B090901064).

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at https:// doi.org/10.1016/j.polymer.2019.01.038.

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