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Highly Stretchable and Compressible Cellulose Ionic Hydrogels for Flexible Strain Sensors Ruiping Tong, Guangxue Chen, Danhong Pan, Haisong Qi, Ren'ai Li, Junfei Tian, Fachuang Lu, and Minghui He Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00322 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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Highly Stretchable and Compressible Cellulose Ionic Hydrogels for Flexible Strain Sensors Ruiping Tong,† Guangxue Chen,† Danhong Pan,† Haisong Qi,†,§ Ren’ai Li,† Junfei Tian,† Fachuang Lu,†,§ and Minghui He* †,§ †State

Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, Guangzhou 510640, China. §Guangdong

Engineering Research Center for Green Fine Chemicals, Guangzhou 510640,

China. KEYWORDS: cellulose ionic hydrogels, high stretchability, high compressibility, ionic conductivity, strain sensors

ABSTRACT: Stretchable and compressible hydrogels based on natural polymers have received immense considerations for electronics. The feasibility of using pure natural polymer-based hydrogels could be improved, if their mechanical behaviors satisfy the requirements of practical applications. Herein, we report highly stretchable (tensile strain ~126%) and compressible (compression strain ~80%) cellulose ionic hydrogels (CIHs) among pure natural polymer-based hydrogels including cellulose, chitin and chitosan, via chemical

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crosslinking based on free radical polymerization of allyl cellulose in NaOH/urea aqueous solution. In addition, the hydrogels have good transparency (transmittance of ~89% at 550 nm), ionic conductivity (~0.16 mS cm-1), and can be worked at -20 °C without freezing and visual loss of transparency. Moreover, the CIHs can serve as reliable and stable strain sensors and have been successfully used to monitor human activities. Significantly, the various properties of hydrogel can be controlled through rationally adjusting the chemically crosslinked density. Our methodology will prove useful in developing the satisfied mechanical and transparent CIHs for a myriad of applications in flexible electronics.

INTRODUCTION Modern electronics, such as human wearable sensors and feedback sensors in soft robotics, more demand development of soft electrolytes with high stretchability, compressibility and ion transport.1, 2 Noticeably, the sensors with low-modulus will more easily perceive micro strains of skin composed of the dermis (modulus, 2 KPa to 80 KPa) and epidermis (modulus, 140 KPa to 600 KPa).3, 4 However, the combination of sensory and mechanical properties is still a profound challenge for imitating human skin.5 Particularly, hydrogels, 3D polymeric materials with abundant water, are very stretchable and compressible in striking contrast to the widely used polyvinyl alcohol-based electrolytes.1 The aqueous phase in hydrogels enables fast diffusion, and endows the hydrogels with liquid-like transport properties for conductivity.6 Additionally, the hydrogels can transfer electrons, when they are treated with carbon nanotubes,7 conductive polymers,8,

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graphene,10,

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etc.12 Whereas, these


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electroconductive hydrogels are limited by optical transparency nevertheless maintain high conductivity.13-15 In contrast, ionic conductive hydrogels, swollen with electrolytes solution,12, 16 meet most of these demands readily and have attracted an increasing interest.17 Significantly, hydrogels based on natural polymers have received immense considerations for electronics, because of their safe nature, biocompatibility, inexhaustibility, and so on.12 Natural polymer-based hydrogels that are made by blending synthetic polymers or inorganic contents with natural polymer are the recent attractive type hydrogel, for having advantages over pure natural polymer-based hydrogels, such as adjustable mechanical property and high functionality in the electric field.18-20 Notably, these hydrogels can’t give full play to the advantages of natural polymers. Thus, the development of pure natural polymer-based hydrogels was more attractive and an irresistible trend. Unfortunately, pure natural polymerbased hydrogels usually have low stretchability (< 100%)21-23 and compressibility (< 75%),2325

and water-based conductive hydrogels inevitably freeze and become non-conductive at

subzero temperatures,26 which severely limit the practical applications. Therefore, the development of pure natural polymer-based ionic hydrogels with high stretchability, compressibility and anti-freezing remain a considerable challenge. Cellulose, a nearly inexhaustible natural polymer, is a strong candidate for the fabrication environmentally friendly and biocompatible products.27 Nevertheless, the crystalline structure in cellulose prevents it from dissolving in water and even in common organic solvents.22 At present, most studies focus on the improvements in functional performance of cellulose, while using it to make highly stretchable and compressible hydrogel remains a challenge.23


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Herein, we first report cellulose ionic hydrogels (CIHs) via chemical crosslinking based on free radical polymerization of allyl cellulose (AC) in NaOH/urea aqueous solution. The acquired hydrogels display high stretchability (tensile strain ~126%) and compressibility (compression strain ~80%) among pure natural polymer-based hydrogels including cellulose, chitin and chitosan, as shown in Figure 1. In addition, the hydrogels have good transparency (transmittance of ~89% at 550 nm), ionic conductivity (~0.16 mS cm-1), and can be worked at -20 °C without freezing and visual loss of transparency. This technique also had great potential for applications in strain sensors created by encapsulating the hydrogels in commercial tapes,3,

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showing useful in developing the ionic conductors for a myriad of

applications in flexible electronics. The benefits of this strategy are as follows: a) The NaOH/urea aqueous solution is a green solvent for cellulose, and a suitable homogenous reaction medium for the etherification of cellulose;29 b) the cellulose can be completely dissolved in a pre-cooled to -5 °C to -20 °C NaOH/urea solution,30 which endows the hydrogel with anti-freezing properties; c) the NaOH parts of NaOH/urea aqueous systems provide ionic conductivity for chemically crosslinked CIHs, due to the Na+ and OH- existed in softer liquid materials; d) various properties of hydrogel can be controlled via rationally adjusting the amount of allyl glycidyl ether (AGE) or ammonium persulphate (APS) for chemically crosslinked density. This novel strategy greatly enriches the function and applications of CIHs.


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Figure 1. Schematic illustration of synthesis of cellulose hydrogels, and A), B) and C) good properties include compressible, stretchable, ionic conductive, and transparent. EXPERIMENTAL SECTION Chemicals and Materials. The cellulose (power: 50 μm), allyl glycidyl ether (AGE, 99%), and ammonium persulphate (APS, 99.99% metals basis) were provided by Shanghai Aladdin Bio-Chem Technology Co., Ltd. (China). Sodium hydroxide (NaOH), ether, and acetone were purchased from Guangzhou Chemical Reagent Factory (China), and urea was purchased from Guangdong Guanghua Sci-Tech Co., Ltd. (China), which are analytical grade. Those chemicals and materials were used without further purification.


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Synthesis of Allyl Cellulose (AC). The cellulose was added to an aqueous 7 wt% NaOH/12 wt% urea solution, and then stirred extensively at -12.5 °C in the thermostatic reaction tank to obtain a completely dissolved and transparent 6 wt% cellulose solution. AGE was added dropwise into the stirred cellulose solution until the desired molar ratios (5, 6, 7, 8 and 9) of AGE to anhydroglucose unit (AGU) of cellulose were reached, at 30 °C and under N2 for 24h.31 Subsequently, the mixture was thoroughly washed with ether, and rotary evaporation at 30 °C to remove the residual ether for preparing the hydrogels. For structural characteristics of AC, the mixture was precipitated from acetone and then thoroughly washed with acetone to remove the residual AGE. Next, the crude product was dissolved in deionized water and dialyzed for 3 days. The purified AC was for analysis purposes after freeze drying, and denoted as ACx, which represents x molar ratio of AGE to AGU of cellulose. Fabrication of Cellulose Ionic Hydrogels (CIHs). For preparing the hydrogel, various dosages (0.6 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%) of APS were added in AC solution and stirred extensively for few minutes. After the air bubbles removed via centrifugation, the mixture was rapidly spread into two glass plates with a 2 mm rubber spacer or poured into a 5 ml beaker, and maintained at 30 °C for 24h, for chemical crosslinking based on free radical polymerization. The CIH was obtained after removing from the mold. The final products were denoted as CIHx-y, which represents x molar ratio of AGE to AGU of cellulose, and y wt% APS.


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Characterization. 1H-NMR spectra were recorded on a 400 MHz Bruker AVANCE III NMR spectrometer (Karlsruhe, Germany). The samples of 50 mg were completely dissolved in 1 ml 7 wt% NaOH/ 12 wt% urea/ 81 wt% D2O solution. The proton spectra at 4.73 ppm was used as an internal standard for 1H-NMR spectra. The ATR-FTIR spectra was characterized by attenuated total reflectance Fourier transform infrared spectrometer (VERTEX 70, Bruker, Germany), and recorded in the wavenumber range of 3600 cm-1 to 600 cm-1. X-ray diffraction (XRD) patterns were recorded on a polycrystall X-raydiffraction (D8 ADVANCE, Bruker, Germany) equipped with a Cu radiation and collected in the 10° to 50° range of 2θ. The CIHs were washed with deionized water, and freeze-dried for ATR-FTIR and XRD characterizations. The transmittance spectrum was measured with a UV-vis Spectrometer Cary60 (Agilent, USA.). The surface morphology of cellulose hydrogels was done by the scanning electron microscopy (EVO 18, Germany). Swollen ratios of the hydrogels were evaluated by immersing the hydrogels in deionized water at room temperature for 3 days, and then the samples were dried in vacuum over P2O5 at 50 °C for 48h. The swollen ratio was calculated as follows:32 (1)

The Wwet and Wdry represent the weights of swelling equilibrium hydrogel and dried hydrogel, respectively.


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Differential scanning calorimetry (DSC) was performed on 214 polyma NETZSCH tester, over the temperature range of -150 °C to 30 °C, with a heating rate of 10 °C min-1 under a nitrogen atmosphere. Compressive and tensile tests were performed on the hydrogels using a tensile-compressive tester (INSTRON5565, America). The cylindrical hydrogels (17.2 mm diameter × 15 mm height) were put on the lower plate and compressed via the upper plate. The strain rate of compression was fixed at 5 mm/ min. For tensile tests, the samples were cut into rectangular shape (50 mm length × 8 mm width) and stretched at a tension speed of 10 mm/ min. The initial linear region of the stress-strain curves was used for calculation of modulus.33 The conductivity of samples was recorded using a PARSTAT 2273 Electrochemical System (Princeton Applied Research), through an alternating current impedance method. The frequency range was from 0.01 Hz to 105 Hz, at room temperature 25 °C. The sample was cut into pieces 10 mm (width) × 10 mm (length) × 2 mm (thickness), sandwiched by copper tapes, and then fixed with the transparent tape. The resistance variations of hydrogel were tested by DMM7510 Digital Graphical Sampling Multimeter (DMM7510, Keithley, USA). The subzero experiments were carried out in the refrigerator (BCD-445WDCA, Haier, China). Nikon Digital Sight DS-Fil camera was used for optical images. RESULTS AND DISCUSSION Synthesis of Cellulose Ionic Hydrogels (CIHs). Simply, the highly stretchable and compressible CIHs were fabricated by three steps, as shown in Figure 1. Firstly, transparent cellulose solution was prepared by dissolving cellulose in an aqueous NaOH/urea solution,


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and then the desired molar ratios of AGE to AGU of cellulose were introduced to endow cellulose with the double bonds. Finally, after thoroughly washing the allyl cellulose (AC) solution with ether, the required dosages of APS as the initiator were added for chemical crosslinking based on free radical polymerization. The weak physically crosslinked domains including hydrogen bonds caused by residual hydroxy groups on the cellulose chains, and crystallite hydrates of cellulose II also existed in the CIHs.23 The acquired hydrogels displayed good properties include compressible, stretchable, ionic conductive, and transparent. Figure 1Aa-d shows that CIHs were quickly almost recovered their original shape and maintained, demonstrating their excellent shape-recovery property. The hydrogel could be highly stretchable (Figure 1Ba-b), twisted stretchable (Figure S1a), and readily adapted to different fish shapes (Figure S1b-c). Additionally, the hydrogels had ionic conductivity (Figure 1C), due to the Na+ and OH- existed in softer liquid materials. When heating, the reaction of NaOH with urea was generated,34 and persulfate initiation decomposed into sulfate ion-radicals in aqueous solution.35 Thus, a part of CO32-, SO42-, and NH4+ might also exist in the solution, for the exothermic reaction of the chemical crosslinking based on free radical polymerization. Moreover, the sheet hydrogels (2 mm thickness) also showed good transparency, because the thinner hydrogels could lead to higher transparency,16 as shown in Figure 1C and Figure 3A-B.


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Figure 2. A) 1H-NMR spectra of cellulose, and AC with molar ratios of AGE to AGU of cellulose from 5 to 9. B) Statistics of substitution degree of AC. C) ATR- FTIR spectrum and D) XRD profiles of cellulose, AC, and CIH8-1.0. Structural Characterization. 1H-NMR spectra was performed to verify the existence of double bonds in AC (Figure 2A). The peaks at 5.63 and 4.96 ppm can be assigned to the ethylenic protons H-4 and H-5, respectively.31 This confirmed that AC was successfully synthesized. In addition, the signal at 4.14 ppm belongs to the alpha anomeric proton of glucose residues (H-1), and the peak at 3.73 ppm is attributed to the protons H-3. The substitution degree was also determined by digital integration of the signals arising from the alpha anomeric proton of glucose residues (H-1), and the ethylenic protons of H-4 and H-5


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from 1H-NMR spectra (Figure S2), just as Hachet et al. described.36 As shown in Figure 2B and Table S1, the substitution degree exhibits an increased trend following an increase of the AGE amount, which indicated that more and more alpha anomeric proton of glucose reacted with AGE epoxy moiety and induced high double bond conversion. Further evidence for successful synthesis of AC and CIHs, ATR-FTIR spectra was introduced (Figure 2C). The characteristic peaks at 3600 cm-1 to 3200 cm−1 (typical sharpening at 3430 cm-1) correspond to O-H stretching vibration of hydroxy groups, and the peak at 1570 cm-1 is attributed to the C=C bond.31, 37 In the spectra of the AC and CIH8-1.0, the intensity and width of O-H band (3600 cm-1 to 3200 cm-1) decreased comparing with that of O-H band in the spectra of cellulose, especially with the increase of the AGE amount. This result implied a part of the hydroxyl groups on the cellulose chains reacted with AGE, and a decrease of hydrogen bonding between cellulose chains. Remarkably, the peak at 1570 cm-1 disappeared in CIH8-1.0, which sufficiently demonstrated the chemical crosslinking based on free radical polymerization of AC occurred and high double bond conversion was obtained. To understand the nature of CIH, and its structural changes during preparation, XRD patterns of cellulose, AC and CIH8-1.0 are shown in Figure 2D. The XRD pattern of cellulose shows several typical cellulose I crystalline peaks at 14.9°, 16.4°, 22.4° and 34.4°.38,

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In

contrast, the XRD pattern of AC exhibited one peak at 20.6°, which corresponds to the (200) reflection of cellulose II crystallite. NaOH/urea solution can destroy the intra- and intermolecular hydrogen bonding between the cellulose molecules, resulting the transition of


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cellulose I to II in the dissolution process.30 In addition, XRD patterns of CIH8-1.0 exhibit a new peak at 12.3° that is ascribed to the (110) reflection of cellulose II crystallite.40 Moreover, after the chemical crosslinking based on free radical polymerization, the intensity of the peak at 20.6° of the cellulose II crystallite hydrates in CIH8-1.0 was higher than that in AC.

Figure 3. Optical property (2 mm thickness, this work), and swelling ratios of CIHs. A) Digital photograph of CIH8 with 0.6 wt% to 3.0 wt% APS. B) UV spectrum of CIH8. C) Swelling ratios of CIH8. D) Swelling ratios of CIH-2.0 with molar ratios of AGE to AGU of cellulose from 6 to 9.


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Transparent and Swelling Properties. The sensory sheet with high transparency can transmit electrical signals without impeding optical sign.13, 41 The property of transparency of hydrogels is determined as their ability to transmit light with minor deflection or scattering.16 Figure 3A presents the visual appearance of CIH8 with 0.6 wt% to 3.0 wt% APS. The pattern closely contacts with the hydrogels for minimizing the light scattering, which can be clearly observed despite of the varying APS dosages, revealing that the hydrogels with high transparency. The colors of hydrogels were gradually deepened with the increase of APS dosage. This result is more likely attributed to the increase of denser network structures of CIHs as shown in scanning electron microscopy images (Figure S3), which reduced the pore diameter of hydrogels and improved the light reflection. To further verify the high transparency, a UV-vis spectrometer was used to measure the transmittance of CIH8 in a wavelength range of 200 nm to 800 nm (Figure 3B). The UV-vis spectrum at 550 nm exhibits a reduced trend of the transparency with an increase of APS dosage, of which the values were 91%, 89%, 86%, 85%, 83%, and 82%, respectively. The reduction of the transparency was ascribed to the denser network structures of CIHs with the increase of the APS dosage, which would prohibit the light passing.16 To further reveal the structural features of the hydrogels, the swelling behaviors were investigated (Figure 3C-D). As shown in Figure 3C, the swelling ratios exhibit a reduced trend with an increase of APS dosage, which indicated that more complex molecular networks caused, for the swelling behaviors depend on the cross-linked density of the gel network.20,

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Swelling ratios of CIH-2.0 with different molar ratios of AGE to AGU of


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cellulose were also studied (Figure 3D). The 2.0 wt% APS was the boundary dosage between the gel and the non-gel of AC6, while the AC5 couldn’t be gelled even increasing the dosage of APS to 3.0 wt%. This implied the amount of double bonds in AC5 were too low to initiate a clear polymerization for gel. With the increase of AGE amount, the swelling ratios also exhibit a reduced trend, which are attributed to more replacement of hydrophilic hydroxyl groups, and the increase in polymerized density. According to this analysis, it is possible to regulate swelling behavior by rationally adjusting the amount of AGE or APS. Mechanical Performance. The cellulose hydrogel’s mechanical properties are strongly affected by chemically crosslinked procedure. To evaluate the general performance of hydrogels, we performed tensile and compression tests as shown in Figure 4A-F and Table S2. Under compression, when the dosage of APS increased, the fracture strain of CIH8 decreased from 80.05% to 61.13% except the soft CIH8-0.6, and the modulus of CIH8 increased from 1.7 KPa to 36 KPa as shown in Figure 4A and Figure 4C. The large dosage of APS made a significant contribution to high chemically crosslinked density in the cellulose network, which led to stiffness hydrogels (the high modulus) and damaged their mechanical performance.16, 43 With the increase of AGE amount, the compression properties exhibit a similar trend with CIH8 (Figure 4B-C), which implied that increasing the amount of APS or AGE caused the fragile property of hydrogels. Moreover, the work of fracture, a parameter for toughness of hydrogels, can be predicted from the area under stress- strain curves.44 Downtrend of work of fracture also occurred at a high amount of APS and AGE, as shown in Figure 4C. The tension behavior of CIH also demonstrated that strongly affected by


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chemically crosslinked procedure (Figure 4D-F). For chemically crosslinked hydrogels, a crack caused by breaking polymer chains.45 As the amount of APS or AGE increased, the elongation at break increased at first and then decreased, and the highest elongation at break was 129% of CIH8-2.0. This implied appropriate chemically crosslinked density endows good elongation of hydrogels. Moreover, CIHs have low Young’s modulus of strain below 21 KPa, which are beneficial for soft strain sensors (Figure 4F).3, 5, 46


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Figure 4. Mechanical properties of CIHs. A) Compressive stress-strain curves of CIH8 with different dosages of APS. B) Compressive stress-strain curves of CIH-2.0 with different molar ratios of AGE to AGU of cellulose. C) Modulus and work of fracture of various CIHs under compression mode. D) Tensile stress-strain curves of CIH8 with different dosages of APS. E) Tensile stress-strain curves of CIH-2.0 with different molar ratios of AGE to AGU of cellulose. F) Young’s modulus and work of fracture of various CIHs under tension mode. Comparison of G) compression and H) tensile properties of this study with that of pure natural polymer-based hydrogels including cellulose (five-pointed star), chitin (regular pentagon), and chitosan (solid circle) reported in literatures, respectively. The -C, -P, and -D represented the hydrogels preparation methods of chemical cross-linking, physical crosslinking, and double-cross-linking, respectively. The highly stretchable and compressible hydrogels can serve as wide range strain sensors. Noticeably, the sensors with low-modulus, especially for soft skin and wearable sensors, will more easily perceive micro strains of skin (2 KPa to 80 KPa for dermis, and 140 KPa to 600 KPa for epidermis).3, 5 Therefore, the low-modulus CIH8-1.0 with high stretchability (tensile strain ~126%) and compressibility (compression strain ~80%) was judged as the remarkable mechanical performance. The chitin and chitosan (producing by deacetylation of chitin) are abundant on earth and widely used in preparation of hydrogels.47,

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Thus, our CIH with

transparent properties was compared with pure natural polymer-based hydrogels including cellulose, chitin, and chitosan, as shown in Figure 4G-H, and the detailed information was


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listed in Table S3 and Table S4. The compression strain is estimated to be the highest compared with that of pure cellulose9,

23-25, 38, 49, 50

(come from the green plants unless

otherwise specified) based hydrogels. Remarkably, the tensile strain outperforms that of pure nature polymer-based hydrogels including cellulose,22,

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chitin21 and chitosan.52,

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Additionally, the compression and tensile stress of our hydrogel were lower, which endowed the hydrogel with low-modulus. The combination of high compressibility, stretchability and low-modulus will bring good adaptability to the hydrogel as strain sensors. Obviously, the remarkable mechanical performance of this study was mainly derived from moderate chemically crosslinked density. Conductive Performance. The NaOH parts of NaOH/urea aqueous systems mainly take charge in conductivity of CIH, and conductive properties of hydrogels are also investigated, as shown in Figure 5. Electrochemical impedance spectroscopy was used to measure the conductivity of the hydrogels (Figure 5A). The equivalent series resistance in the high frequency region and the radius of the semicircle increased with an increase of APS dosage, revealing the decreasing of the conductivity and an increasing of interfacial resistance (Figure 5A-B). Meanwhile, with an increase in the dosage of APS, the conductivities of CIH8 decreased from 0.22 mS cm-1 (0.6 wt%) to 0.1 mS cm-1 (3.0 wt%), seen in Figure 5C. This result mainly ascribed to larger dosage of APS causing more complex molecular networks, in which positive/ negative ions might be blocked from the free movement.16 Considering optical, mechanical and conductive properties played an important role in electronics sensors, CIH8-1.0 was judged as the favorable hydrogel for further use unless otherwise specified.


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Figure 5. Conductive properties of CIHs. A) Electrochemical impedance spectroscopy plots of a series of CIH8. B) Details in the high-frequency region of electrochemical impedance spectroscopy. C) The calculated conductivity values for CIH8. D) The DSC trace of CIH8-1.0. The inserted image of the hydrogel worked at -20 °C and 25 °C respectively, and the voltage was 5V. The logo is used with permission from South China University of Technology. Owing to the freezing point of solvents, the conductivity was inevitably influenced by the temperature. The DSC from -150 °C to 30 °C was performed to obtain the boundaries of the various phase regions (Figure 5D). The melting point (Tm) of CIH8-1.0 was around -16 °C. Significantly, the hydrogel could work at -20 °C without freezing and visual loss of


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transparency compared with that at 25 °C (inset picture). Moreover, the hydrogel displayed good stretchability (tensile strain ~80%) under a uniaxial force stretch at -20 °C (Figure S4a). The electrochemical impedance spectroscopy plots and calculated conductivity values for CIH8-1.0 at various low temperatures were also studied (Figure S4b-c). The resistivity of the hydrogel increases as the temperature decreases due to the reduction of charge carriers, while the brightness of light emitting diode dimmed slightly at -20 °C, which will prove useful in extending its application under a broad range of temperature.16, 26

Figure 6. The reliability and stability of CIH sensors. A) and B) The dependence of resistance change ratio variation of CIH8-1.0 on applied tensile and compression strain, respectively. The insert pictures were the luminance variations with the increase strain of tensile and


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compression loading. C) The resistance change curve was recorded after 1100 cycles, strain of 30% under tensile mode, at a tension speed of 15 mm/ s. D) Response of strain sensor in monitoring human activities of wrist bending. The logo is used with permission from South China University of Technology. The Integration to Strain Sensor. The dependence of resistance change ratio variation of CIH8-1.0 on diverse tensile and compression strain has been investigated (Figure 6A-B). The points of Figure 6A show the almost linear relationship (R0-90%2= 0.963), and the gauge factor (representative of sensitivity) is 0.30, which was calculated by the ratio of the resistance change ratio to strain, as Guan et al. escribed.54 This result indicated the CIH can serve as a reliable tensile sensor in the strain region of 0-90%. The resistance variation of the hydrogel was also visually observed using light emitting diodes, which gradually dimmed with increasing tensile strain. Moreover, the resistance change ratio increased from 0% to 61.1% with compression strain from 0% to 65%, and the light emitting diodes become brighter (Figure 6B). Obviously, the plots of CIH8-1.0 should be divided into three regions based on the differences in gauge factor. In the strain region of 20%- 50%, the gauge factor (1.74) is higher than that of 0%- 20%, and 20%- 65%, which might attribute to the lagged effect of the begin compression strain, and with the increase of compression strain, positive/negative ions might be blocked from the free movement by the increased density of the hydrogel. The strain sensor also showed good stability after being tested under tensile strain between 0%– 30% and subjected 1100 cycles, as shown in Figure 6C. In the experiment, the fixtures


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were covered with rubber pads and the hydrogel was wrapped in elastomeric VHB tapes (VHB 4905, 3M) as shown in the inset picture. Thus, the ionic conductive hydrogel with high stretchability and compressibility, could be used as wearable strain sensor to monitor human activities, such as wrist bending (Figure 6D), and finger bending (Figure S5). The elastomeric VHB tapes were used as encapsulant for ionic conductive hydrogels, which was not only preventing the water evaporation of the hydrogel but also protecting the human body when the hydrogel served as wearable sensors.28 We put the hydrogel encapsulated with the VHB tapes on the joint. The resistance change ratio variations of strain sensor, corresponding to wrist and finger bending are different and distinguishable by comparing the intensity and shape change. Meanwhile, the strain sensor switched rapidly at bending and unbending, where the resistance change ratio values remained nearly identified corresponding to the same motion. CONCLUSION Herein, we first report CIHs via chemical crosslinking based on free radical polymerization of AC in NaOH/urea aqueous solution. The acquired hydrogels display high stretchability (tensile strain ~126%) and compressibility (compression strain ~80%), among pure natural polymer-based hydrogels including cellulose, chitin, and chitosan. In addition, the hydrogels have good transparency (transmittance of ~89% at 550 nm), ionic conductivity (~0.16 mS cm1),

and can be worked at -20 °C without freezing and visual loss of transparency, which will

prove useful in extending its application under a broad range of temperature. Moreover, the CIHs encapsulated with commercial tapes can serve as reliable and stable strain sensors, and


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have been successfully used to monitor human activities. The benefits of this strategy are attributed to the green NaOH/urea aqueous systems, and various properties of hydrogel can be controlled via rationally adjusting the chemically crosslinked density. ASSOCIATED CONTENT Supporting Information Photographs of cellulose ionic hydrogels (CIHs), 1H NMR spectra of allyl cellulose, conditions and results of synthesis of allyl cellulose, scanning electron microscopy images of CIH8-0.6 and CIH8-2.0, conditions and physical properties of CIHs, summary of compression properties of pure cellulose based hydrogels reported in literatures, summary of tensile properties of pure natural polymer-based hydrogels, mechanical properties and conductivity of the CIH8-1.0 at low temperature, response of CIH8-1.0 as strain sensor in monitoring human activities of finger bending. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.H.). Author Contributions All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Science and Technology Program of Guangzhou (201607020045, 2017B090901064), Guangdong Province Science Foundation for Cultivating National Engineering Research Center for Efficient Utilization of Plant Fibers (2017B090903003), State Key Laboratory of Pulp and Paper Engineering (201827), and the Fundamental Research Funds for the Central Universities (2018ZD36). ABBREVIATIONS CIHs, cellulose ionic hydrogels; AC, allyl cellulose; AGE, allyl glycidyl ether; APS, ammonium persulphate; AGU, anhydroglucose unit; XRD, X-ray diffraction; DSC, differential scanning calorimetry; Tm, melting point. REFERENCES (1) Huang, Y.; Zhong, M.; Shi, F.; Liu, X.; Tang, Z.; Wang, Y.; Huang, Y.; Hou, H.; Xie, X.; Zhi, C. An Intrinsically Stretchable and Compressible Supercapacitor Containing a Polyacrylamide Hydrogel Electrolyte. Angew. Chem. Int. Ed. 2017, 129 (31), 9141-9145. (2) Tian, K.; Bae, J.; Bakarich, S. E.; Yang, C.; Gately, R. D.; Spinks, G. M.; In Het Panhuis, M.; Suo, Z.; Vlassak, J. J. 3D Printing of Transparent and Conductive Heterogeneous Hydrogel-Elastomer Systems. Adv. Mater. 2017, 29 (10), 1604827.


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