Nanocellulose-Mediated Electroconductive Self-Healing Hydrogels

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Functional Nanostructured Materials (including low-D carbon)

Nanocellulose-mediated electroconductive self-healing hydrogels with high strength, plasticity, viscoelasticity, stretchability, and biocompatibility toward multifunctional applications Qinqin Ding, Xinwu Xu, Yiying Yue, Changtong Mei, Chaobo Huang, Shaohua Jiang, Qinglin Wu, and Jingquan Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09656 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Applied Materials & Interfaces

Nanocellulose-mediated

electroconductive

self-

healing hydrogels with high strength, plasticity, viscoelasticity, stretchability, and biocompatibility toward multifunctional applications Qinqin Ding,a Xinwu Xu,a Yiying Yue,b Changtong Mei,a Chaobo Huang,c Shaohua Jiang,a Qinglin Wu,d Jingquan Hana,e*

a

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037,

China b

c

College of Biology and Environment, Nanjing Forestry University, Nanjing 210037, China

College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China

d

School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA 70803,

USA e

Dehua TB New Decoration Material CO., LTD., Zhejiang, 313200,China

KEYWORDS: cellulose nanofibers, polypyrrole, polyvinyl alcohol, hydrogels, conductivity ABSTRACT: Conducting polymer hydrogels (CPHs) emerge as a fascinating class of smart soft matters important for various advanced applications. However, achieving the synergistic

ACS Paragon Plus Environment

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characteristics of conductivity, self-healing ability, biocompatibility, viscoelasticity and high mechanical performance still remains a critical challenge. Here we develop for the first time a type of multifunctional hybrid CPHs based on a viscoelastic polyvinyl alcohol-borax (PB) gel matrix and nanostructured CNF-PPy (cellulose nanofibers-polypyrrole) complexes that synergizes the biotemplate role of CNFs and the conductive nature of PPy. The CNF-PPy complexes are synthesized through in situ oxidative polymerization of pyrrole on the surface of CNF templates, which are further well-dispersed into PB matrix to synthesize homogeneous CNF-PPy/PB hybrid hydrogels. The CNF-PPy complexes not only tangle with PVA chains though hydrogen bonds, but also form reversibly crosslinked complexes with borate ions. The multi-complexation between each component leads to the formation of hierarchical 3D network. The CNF-PPy/PB-3 hydrogel prepared by 2.0 wt% of PVA, 0.4 wt% of borax and CNF-PPy complexes with a mass ratio of 3.75/1 exhibits the highest viscoelasticity and mechanical strength. Due to a combined reinforcing and conductive network inside the hydrogel, its maximum storage modulus (~0.1 MPa) and nominal compression stress (~22 MPa) are 60 and 2240 times higher than those of pure CNF/PB hydrogel, respectively. The CNF-PPy/PB-3 electrode with a conductivity of 3.65±0.08 S m-1 has a maximum specific capacitance of 236.9 F g-1, and its specific capacitance degradation is less than 14% after 1500 cycles. The CNF-PPy/PB hybrid hydrogels also demonstrate attractive characteristics, including high water content (~94%), low density (~1.2 g cm-3), excellent biocompatibility, plasticity, pH sensitivity and rapid self-healing ability without additional external stimuli. Taken together, the combination of such unique properties endows the newly developed CPHs with potential applications in flexible bioelectronics and provides a practical platform to design multifunctional smart soft materials.


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1. INTRODUCTION Nowadays, the rapid development of soft, stretchable and flexible electronic components expands the applications of electronic devices.1 Multifunctional electroconductive components are FHQWUDOO\ LPSRUWDQW LQ GHYHORSLQJ DGYDQFHG ³VPDUW´ GHYLFHV VXFK DV ZHDUDEOH Hlectronics, implantable biomedical devices and portable electronic equipment.2-5 To realize practical soft electronics, the fixed shape and inflexibility of the conductive component is a particular limiting factor. Therefore, the adoption of moldable, flexible and elastic materials would be a promising solution.6 Hydrogels are stretchable, compressible and cross-linked three-dimensional networks of hydrophilic polymer with tunable amounts of water and switchable mechanics, while maintaining their structural integrity during deformation.7-8 Due to their high biocompatibility and hydrophilic property, hydrogels can adapt to complex environments and have been explored for various DSSOLFDWLRQV ZKHUH ³VPDUW´ VRIW PDWHULDOV DUH UHTXLUHG 9 Because the water contained in hydrogels can dissolve ions, hydrogels are also considered as ideal ionic conductors.10 As a special class of intelligent hydrogels, conducting polymer hydrogels (CPHs) synergize the advantageous features of organic conductors and hydrogels, allowing both materials to retain their respective unique properties.11 CPHs with continuous conductive backbones and intrinsic 3D network micro/nano structures can provide an electrically conductive yet mechanically robust framework, thus facilitating the transport and diffusion of electrons, charges, ions and small molecules.12 Due to their excellent conductivity, biocompatibility, mechanical flexibility and tissue-like softness, CPHs are regarded as a powerful material platform and provide a new route to design soft bioelectronics and flexible energy storage/conversion devices,13-14 such as wearable and implantable devices,

flexible supercapacitors,16 biosensors,17 electro-stimulated drug

release devices and electronic skin.18


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As a class of functional polymers that have a delocalized p-system backbone along the polymeric chains, conducting polymers are emerging as a mechanically soft and electrically conducive platform for cell integration, making the CPHs suitable candidates as flexible bioelectronics for improving both the interface between biological and synthetic systems and the performance of medical devices.19 Among conductive polymers, polypyrrole (PPy), the intrinsically FRQGXFWLYH SRO\PHU ZLWK KLJKO\ Œ-conjugated polymeric chains, has attracted significant attention due to the ability to efficiently switch between the redox states, distinguished in vitro/vivo cytocompatibility, excellent ion-exchange property, controllable electrical conductivity (varying from 10-4 to 102 S cm-1), thermal and environmental stability.20-21 Recently, great efforts have been dedicated to developing functionalized CPHs using PPy. Wallace et al. developed a cytocompatible PPy/PEDOT hydrogel for use in an Mg biobattery. By introducing a two-step crosslinking process, PPy was electrochemically deposited onto PEDOT gel, forming a robust CPH with good electrochemical properties and biocompatibility.22 Takeuchi et al. demonstrated a tunable 3D nanostructured conductive gel electrode based on PPy gel and Fe3O4 nanoparticles. The multifunctional PPy gel framework was adopted as both binder and 3D conductive network, significantly improving the performance of active inorganic nanoparticlebased electrodes in the next-generation high-capacity lithium ion batteries.23 +RZHYHU WKH GHORFDOL]HG Œ-electron system along the backbone of conjugated polymers causes rigid polymeric chains of PPy. When incorporated into hydrogel individually, PPy is generally difficult to disperse and even tend to form separate domains of aggregations, probably due to the inaccessibility, rigidity and dense layers of PPy.24 Not surprisingly, most previously reported CPHs based on PPy suffer from mechanical brittleness and rigidity, thus they cannot bear large deformation, which limits their practical application in soft electronics.6,

14

In addition, the


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noncontinuous conductive PPy network within gel matrix severely degrades the electrochemical performance of CPHs.20, 25 Therefore, it remains a challenge to fabricate homogeneous CPHs with high conductive, elastic and flexible nature using PPy. Fortunately, direct polymerization of pyrrole monomers on flexible template materials can overcome above-mentioned problems.20 Among various flexible substrates, cellulose nanofibers (CNFs), derived from the most abundant and renewable biomass on earth-cellulose, are particularly promising and desirable because of their appealing properties, such as intrinsic flexibility, high strength (modulus in the range of 134-140 GPa), high aspect ratio and excellent biocompatibility.26-30 These distinguished characteristics of CNFs endow them as ideally suitable templates to enhance the flexibility, toughness and electrochemical properties of the brittle PPy.24 The CNF carriers are expected to serve the dual function of mechanically toughening the fragile PPy and increasing the conductivity through providing a continuous 3D scaffold of high porosity.31 Based on the issues above, it is reasonable to assume that CNFs functionalised with PPy could be applied for reinforcing and supporting 3D conductive network of hydrogel to create stable, tough and flexible CPHs with properties that can be optimised both with respect to their electrochemical property and mechanical integrity.32 In our previous study, we have prepared hybrid hydrogels with enhanced viscoelasticity, strength and toughness by incorporated well-dispersed nanocellulose to the polyvinyl alcohol (PVA)-borax gel matrix via cross-linking between nanocellulose and PVA-borax (PB) system in an aqueous medium.33-34 Inspired by their appealing features, we believe that PB hydrogels with hierarchically 3D framework can serve as an ideal matrix for the introduction of a second conductive network, thus potentially providing continuous pathways for electron transport and forming multifunctional CPHs by introducing CNF-carried PPy.23 To the best of our knowledge,


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we are unaware of any reports on the multifunctional CPHs based on PB system and CNFtemplated PPy. In the present work, we report a novel type of multifunctional smart CPHs that possess synergistic characteristics, including low density, decent viscoelasticity, inherent plasticity, adequate conductivity, fast self-healing ability, intrinsic biocompatibility and pH sensitivity. Specifically, PPy was firstly in situ polymerized onto the surface of CNF biotemplates to synthesize conductive CNF-PPy complexes, which were further uniformly dispersed into PB gel matrix to form homogeneous CNF-PPy/PB hybrid hydrogels with hierarchical 3D network structure.

Through

microscopic

observation,

mechanical,

dynamic

rheological

and

electrochemical measurements, a successful in situ polymerization of pyrrole monomers onto CNFs was verified, and the regulating effects of CNF-PPy complexes on the viscoelasticity, conductivity, mechanical properties and electrochemical performances of hybrid CPHs were investigated in detail. The multi-complexation of each component within hydrogel, the relationships between hydrogel microstructure and performances, as well as the plausible mechanism for hierarchical 3D network formation of hydrogel were also described. By combining desirable properties of conductive CNF-PPy complexes and PB hydrogel matrix, we demonstrated that this kind of CPHs could be practically used to fabricate flexible, biocompatible and fast self-healable bioelectrodes or strain sensors with excellent electrochemical performances. 2. EXPERIMENTAL SECTION Materials. Bleached wood pulp (BWP, W-50 grade of KC Flock, Tokyo, Japan) was dried for 12h at 60ºC, and sulfuric acid (98 wt%, Nanjing Chemical Reagent Co., Ltd., China) was diluted to a concentration of 48 wt% before use. Pyrrole monomers (C4H5N, Mw=67.09 g mol-1) and Iron (III) chloride (FeCl3, Mw=162.2 g mol-1) were purchased from Aladdin Industrial Co. Polyvinyl


6

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alcohol (PVA, Mw=146,000-186,000 g mol-1, 99.0% hydrolyzed) and sodium tetraborate decahydrate (borax, Na2B4O7·10H22 •

0w=381.37 g mol-1) were obtained from Aldrich

Chemical Co. Deionized water was used in the preparation of all solutions. All solvents and reagents used were of analytical grade. Preparation of CNFs and CNF-PPy Complexes. CNFs were extracted from BWP using acid hydrolysis followed by an ultrasonication as previously described in our work (detailed preparation process is given in Supporting information).

CNF-PPy complexes were

synthesized via an in situ oxidative polymerization of pyrrole monomers using CNFs as biotemplate and FeCl3 as oxidant in an acid aqueous PHGLXP. Before use, the pyrrole monomers were distilled under reduced pressure to prevent their oxidization in air. Based on the preliminary experiments, the feeding mass ratio of pyrrole monomers to CNFs were 1.25/1, 2.50/1, 3.75/1, 5.00/1 and 6.25/1 w/w, and the corresponding CNF-PPy complexes were labeled as CNF-PPy-1, CNF-PPy-2, CNF-PPy-3, CNF-PPy-4 and CNF-PPy-5, respectively. Typically, CNF-PPy-1 was synthesized as follows. Pyrrole (2.5 g, 0.036 mol) and virgin HCl (0.050 mol, 1.0 M) were added into the CNF aqueous suspension (200 g, 1.0 wt%). After magnetic stirring in ice-water for 1h, a solution of FeCl3 (2.9 g, 0.018 mol) in 50 mL deionized water, used as oxidant catalyst, was added into the suspension, and the polymerization proceeded in an ice-water bath for 2h. The obtained product was filtered and rinsed with water several times to a neutral pH value. TKH ILQDO &1) 33\ FRQGXFWLYH FRPSOH[HV ZHUH UHGLVSHUVHG LQ ZDWHU WR IRUP D VXVSHQVLRQ ZLWK D WRWDO ZHLJKW RI J ZKLFK ZDV WKHQ XOWUDVRQLFDWHG IRU

PLQ WR GLVSHUVH WKH DJJUHJDWLRQ The solid contents

of the &1) 33\ FRPSOH[ suspension from No.1 to No.5 were measured 4 times, and the average values were 1.3±0.04, 2.1±0.06, 2.9±0.02, 3.5±0.04, 4.2±0.03 wt%, respectively. Neat PPy was


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prepared via an oxidative synthetic approach without CNFs, and all other processing conditions were in accordance with CNF-PPy complexes. Preparation of CNF-PPy/PB Hybrid Hydrogels. The hybrid hydrogels with 2.0 wt% of PVA, 0.4 wt% of borax and different CNF-PPy complexes were prepared as follows. Firstly, borax powder (0.8 g) was dissolved in CNF-PPy aqueous suspension (195.2 g) with continuous stirring at room temperature for 20 min. Subsequently, PVA powder (4.0 g) was slowly added into the stirred aqueous solutions. After a complete swelling of PVA powder, the solutions were heated to 90°C and stirred for 2h. As the temperature increased, the PVA powder began to dissolve and the mixture gradually became viscous. After the PVA was completely dissolved, homogeneous PVA-borax (PB) solutions with well-dispersed CNF-PPy complexes were formed. As the temperature decreased, the solutions started to present viscoelastic performance. The solutions were further cooled slowly to room temperature to form the final CNF-PPy/PB hydrogels. A series of hybrid hydrogels with different CNF-PPy complexes were designated as CNF-PPy/PB1, CNF-PPy/PB-2, CNF-PPy/PB-3, CNF-PPy/PB-4 and CNF-PPy/PB-5, respectively. As a reference, the hydrogel synthesized in the absence of PPy was marked as CNF/PB. The weight ratio of CNF in all the hydrogels were kept at 1.0 wt%. Characterizations. Fourier transform infrared spectrometry (FTIR) was conducted using a VERTEX 80v spectrophotometer (Thermo Fisher Scientific inc. USA) with the samples mixed with KBr. Transmission electron microscopy (TEM) was performed using a transmission electron microscope (JEM-1400, Japan) with an accelerating voltage of 80 kV. Scanning electron microscopy (SEM) was carried out on JSM-7600F field emission scanning electron microscopy (FE-SEM, JEOL, Japan) at 5.0 kV. The densities (!, g cm-3) of the hydrogels were determined from the dimensions and weights


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of the samples. Each sample (initial weight=Wi) was dried in a vacuum oven at 50°C until a constant weight (Wd) was reached. The water contents (Wc) of the hydrogels were calculated from eq 1. 9a L

9g F 9b H srr¨ 9g

(1)

Dynamic Oscillation Measurement. The dynamic rheological behaviors, including dynamic strain sweep, dynamic frequency sweep, continuous step strain and dynamic temperature sweep, were investigated with an HAAKE RheoStress 600 (Thermo Fisher Science Inc. USA) using a plate-and-SODWH JHRPHWU\ GLDPHWHU

PP JDS

P

Dynamic Strain Sweep. Before the dynamic viscoelastic measurements, the dynamic strain sweep from 0.01 to 100% at &=1.0 Hz was performed, and the storage modulus was recorded to define the linear viscoelastic region in which the storage modulus was independent to the strain amplitude. A strain ( ) of 1.0% was selected in the subsequent oscillation tests to ensure that the dynamic oscillatory deformation of each sample was within the linear viscoelastic region. Dynamic Frequency Sweep. The viscoelastic parameters (log mode), including shear storage modulus (G') and loss modulus (G'') as functions of angular frequency (&) were measured over the & range of 0.1-100 rad s-1 at = 1% at 25°C. The complex modulus (G*) and complex viscosity ( *) were calculated based on the values of G' and G'' (G*=¾*

* ,

*

=G*/&).

Continuous Step Strain. To investigate the self-healing properties of the hydrogels in response to applied shear forces (expressed in terms of strain), a 1h-programmed procedure was XVHG DV IROORZV VWUDLQ V :

V :

DQG GXUDWLRQ LQ SDUHQWKHVHV

V :

V :

V DQd the G' and G'' dependence of time was recorded in

continuous step strain measurements at & = 1.0 Hz and at 25°C. For the test, CNF-PPy/PB-3 was selected as a representative hydrogel.


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Mechanical Measurements. Compression stress (1)-strain (0) measurements were performed on the cylindrical hydrogel samples (35 mm in diameter and 10 mm in height) using a universal mechanical testing machine (CMT4304, SANS Test Machine Co. Ltd., Shenzhen, China) at a crosshead speed of 10 mm/min. The compression for all samples ended when the normal force reached the maximum loading capacity (30 kN). The energy absorption (Ea) under uniaxial compression was defined as the area below the stress-strain curve. The specific compressive stress (1s) was calculated by dividing densities. The compression results were averaged over three specimens per group. Tensile stress-strain tests were carried out with a universal testing machine (TY-8000B, China) at a pulling rate of 25 mm/min. The samples in a size of 30mm × 15mm × 5mm were nipped to the tensile machine while testing. Then the hydrogels were cut into halves, and the two separate parts were contacted in air and underwater without any external stimulus during the healing process. The tensile tests were repeated to calculate the healing efficiency. Conductivity. In a two-electrode system, the hydrogel samples with a size of 1.0cm × 0.5cm × 0.5cm were sandwiched between two platinum plate electrodes. The measurements were carried out on an electrochemical workstation (CHI760E, China). The resistance was calculated from the current-time curve by eq 2. 4 L 7¤+

(2)

where R ZDV WKH UHVLVWDQFH

U was the open circuit potential of the conductive hydrogel

(viewed in the open circuit voltage-time curve) (V), and I was the current corresponding to the open circuit potential (A). The conductivity was calculated by eq 3. PL

s. 45

(3)

where L, S and R were the hydrogel thickness in cm, the electrode area in cm2, and the hydrogel UHVLVWDQFH LQ

UHVSHFWLYHO\


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Electrochemical Properties. The electrochemical measurements were carried out on a threeelectrode system using an electrochemical workstation (CHI760E, China). The hydrogel sample (12 mg), a Pt wire and a saturated calomel reference electrode were used as the working electrode, counter electrode and reference electrode, respectively. All measurements were performed in a 6.0 M KOH electrolyte. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), cycling performance and electrochemical impedance spectroscopy (EIS) were conducted to examine electrochemical performance of hybrid hydrogels. CV tests were performed with 20, 50, 80 and 100 mV s-1 scan rates in a potential window of -0.6±0.4 V. The GCD experiment was carried out in the potential range from 0 to 0.4 V with applied current density of 0.5, 1.0 and 2.0 A g-1. The EIS tests were conducted over the frequency range from 0.01 Hz to 100 kHz using a sine wave with alternate signal amplitude of 5 mV. The cell specific capacitance (Cs) was calculated from the slope of the linear region of GCD curves, using the following eq 4. %q L

+ ® ¿P I ® ¿8

(4)

where I, ûW and û9 was charge/discharge current in A, discharge time in s, potential drop during discharge in V, respectively, and m was the mass of active material within electrode in g. In Vitro Cell Culture. L-929 cells (mouse fibroblast cells) were selected as the testing cell model. L-929 cells were routinely cultured in culture flasks with High Glucose-Dulbecco´s 0RGLILHG (DJOH¶V 0HGLXP +*-DMEM), supplemented with 10% fetal bovine serum. The cells were then incubated at 37°C in 5% CO2 and 95% air. Once the cells became nearly confluent, they were washed three times with phosphate buffer saline (PBS). Subsequently, the cells were released by treating with trypsin-EDTA solution for 3 min at 37°C, and then they were resuspended in fresh complete culture media with a final concentration of 1.5×105 cells mL-1. In Vitro Cytotoxicity Assays. The effects of the hybrid hydrogel on cell viability were


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assessed by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] colorimetric assays (detailed assay process is given in Supporting information). The optical density values of the blank group, control group and experimental group were measured at 490 nm using Fullwavelength microplate reader and marked as ODb, ODc, and ODe, respectively. The cell viability was calculated using the eq 5. %AHH RE=>EHEPU >¨? L

1&c F 1&` H srr¨ 1&a F 1&`

(5)

SEM of Cell Adhesion and Morphology. The morphology and cellular state of CNF-PPy/PB hydrogels were examined using SEM (Quanta 200). Before observation, the hydrogels were sterilized by UV irradiation for 30 min, and then the cells were seeded on the hydrogels surface in 24-well tissue-culture plates. After 24h in culture, the hydrogels were rinsed three times using PBS to remove non-adherent cells. The adherent cells were fixed with 2.5% glutaraldehyde for 2d at 4°C, followed by a dehydration in graded ethanol series (50, 70, 80, 90, and 100%). After a vacuum drying, the samples were sputter-coated with a thin Au layer and then observed under SEM. Fluorescent Staining. Cell apoptosis was observed by fluorescence staining with fluorescence microscopy (OLYMPUS IX53). After 1d of culture with extract liquid, a certain amount of Hoechst 33342 live cell staining solution (100X) (Shanghai Beyotime Biotechnology Co., Ltd) was added and adjusted to the final concentration of cell staining solution 1X. Then the sample was incubated for 10 min at 37°C, followed by a removal of the mixed solution. Afterwards, the cell was washed with PBS three times and observed.


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3. RESULTS AND DISCUSSION 3.1. Synthesis Process and Formation Mechanism of CNF-PPy/PB Hybrid Hydrogels

Figure 1. (a) Schematic illustration of preparation and synthesis process of CNF-PPy/PB hydrogels. (b) CNF-PPy/PB hydrogels molded into various shapes. (c) Formation mechanism of integrated 3D network within CNF-PPy/PB hydrogels. (d) Multi-complexation between PVA, CNF-PPy and borax. The facile two-step synthesis process of CNF-PPy/PB hydrogels is schematically illustrated in Figure 1a. A stable aqueous suspension of CNFs was firstly prepared through acid hydrolysis and ultrasonication. Because of their excellent dispersibility and nanoscale dimension, CNFs were applied as a stable and robust nanocarrier to disperse PPy in water. CNF-PPy complexes were then synthesized via an in situ oxidative chemical polymerization of pyrrole monomers using FeCl3 as oxidant and CNFs as biotemplate in an acid aqueous medium. The water-dispersed


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CNF-PPy complexes were successfully achieved by the grown of PPy coating on the surface of individual CNF. Subsequently, the well-dispersed CNF-PPy complexes were incorporated into PB matrix to form CNF-PPy/PB hybrid hydrogels. CNFs acted as a biotemplate and guide the growth of PPy into compact CNF-PPy complexes with ideal dispersity, which enabled the formation of a hierarchical 3D conductive network structure in PB gel matrix. Interestingly, the obtained hybrid hydrogels exhibited high water content (~94%) and low density (~1.2 g cm-3). These homogeneous, stable and moldable hydrogels could be readily and repeatedly remolded into various 3D shapes after gelation (Figure 1b). A schematic illustration of hierarchical 3D network formation of CNF-PPy/PB hybrid hydrogels is proposed in Figure 1c. In a dilute solution, borax (Na2B4O7‡

+2O) dissociated into

equal quantities of trigonal planar B(OH)3 and tetrahedral B(OH)4í that quickly interchanged in water, and the authentic concentration of B(OH)4í was nearly twice the initial borax concentration. More importantly, the B(OH)4í ion was shaped like a tetrahedron (four sides, each side an equilateral triangle) with the boron in the center and the four -OH groups at each corner, which could form various complexes with cis-diol sites of PVA chains and/or CNF-PPy hybrids (Figure 1d).27 In this way, borax acted as a cross-linker between the PVA chains and CNF-PPy complexes, resulting in strong interactions between these components and H2O molecules. As a result, the coexistence of multi-complexation, H2O interpenetration, chain entanglement of CNF-PPy hybrids and PVA, as well as inter- and intramolecular hydrogen bonding systems between them led to the formation of an integrated 3D network within the hybrid hydrogels. 3.2. Synthesis Process and Dispersion State of CNF-PPy Complexes in Aqueous Suspensions 7KH )7,5 VSHFWUD RI &1)V 33\ &1) 33\

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FRPSOH[HV DQG &1) 33\ 3%

E 3KRWRJUDSKV RI SXUH 33\ DQG &1) 33\ DTXHRXV VXVSHQVLRQV DIWHU VWDQGLQJ IRU

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FRPSOH[HV $V VKRZQ LQ )LJXUH E WKH QHDW 33\ DQG &1) 33\ FRPSOH[HV ZHUH GLVSHUVHG ZHOO DIWHU D PLQ VWLUULQJ $IWHU

K WKH SXUH 33\ SDUWLFOHV ZHUH DJJUHJDWHG WRJHWKHU LQ DTXHRXV VXVSHQVLRQ

GXH WR WKHLU LQWULQVLF LQWHUPROHFXODU LQWHUDFWLRQV ZKLOH WKH DTXHRXV VXVSHQVLRQ RI &1) 33\ FRPSOH[HV VWLOO UHPDLQHG VWDEOH ZLWKRXW DQ\ SUHFLSLWDWHV LQGLFDWLQJ WKDW &1) WHPSODWHV ZHUH LGHDOO\ VXLWDEOH GLVSHUVDQW IRU 33\ The interaction between PPy and CNFs might ascribe to the hydrogen bonding between N-H in the pyrrole ring and the O-H functional groups of CNFs (Figure 2c). In addition, WKH 89 9LV VSHFWUD RI 33\ DQG &1) 33\

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3.3. Linear Viscoelastic Region Determined by Dynamic Strain Sweep

Figure 3. Dynamic viscoelasticity performance at 25°C and thermo-reversibility of the hydrogels at =1%: (a) strain dependence of G' (&=1.0 Hz); (b) frequency dependence of G' and G'' (&=1.0100 Hz); (c) frequency dependence of G* and CNF-PPy/PB-3 hydrogel (PRUH WKDQ

*

(&=1.0 Hz); (d) Stretching demonstration of

RI WKH RULJLQDO OHQJWK).

Figure 3a shows the curves for G' as a function of based on the results from dynamic strain sweep tests, demonstrating a typical elastic response of hybrid hydrogels. The strain sweep tests were conducted to confirm the linear viscoelastic region RI hydrogels, where G' and G'' were independent of the applied

(from 0.01 to 100%). The G' value of all samples was relatively

stable within = 1.0%, so = 1.0% was selected in the subsequent oscillation tests to ensure that


18

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the following dynamic oscillatory deformation of each hydrogel was within linear viscoelastic region. As expected, the addition of a small amount of &1) 33\ FRPSOH[HV pronouncedly increased the viscoelasticity and toughness of hydrogel. All CNF-PPy/PB hybrid hydrogels exhibited considerably higher * values than CNF/PB hydrogel, indicating a more significant reinforcing effect of CNF-PPy complexes than CNFs. Compared with the other hydrogels, CNFPPy/PB-3 present the highest G'max value (~105 Pa), which was almost 60 times higher than that of CNF/PB hydrogel (G'max § PB hydrogel (G'max §

3D DQG three orders of magnitude greater than that of pure

3D

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19


Page 20 of 50

only showed a UXEEHU\-like behavior over the accessible frequency window (& = 0.1-100 rad s1

), revealing that their intersection point located out of & window and the gelation occurred at a

lower &, much earlier than PB and CNF/PB hydrogels. This result demonstrated the IRUPDWLRQ RI WKH HODVWLF ' QHWZRUN ZLWK ERWK HQWDQJOHG FKDLQV DQG UHYHUVLEOH FURVV OLQNV ZDV DFKLHYHG As noted from the graph, the G' values of all CNF-PPy/PB hydrogels gradually increased with the increasing content of PPy. Compared with CNF/PB hydrogel, an increase of almost 100-fold in G'’ (high-frequency plateau of G') was generated for CNF-PPy/PB-3. However, excessive PPy would hinder the formation of 3D network within hydrogel, thus leading to the decline of G' values. 7KHUHIRUH &1) 33\ 3%

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*

within the whole & range. As shown in Figure 3d, a block of elastic CNF-

PPy/PB-3 hydrogel could be stretched beyond 600% strain into a thin film without damage, exhibiting its excellent stretchability, efficient energy dissipation behavior and damage-tolerant ability. Its elastomeric nature was probably attributed to the entanglements of PVA chains and flexible CNF-PPy complexes through physical junctions and hydrogen bonds with the presence of borax. In this view, the homogeneously dispersed CNF-PPy complexes in PB matrix could be considered as multifunctional cross-linkers and reinforcing nanofillers. 3.5. Self-healing Behavior of the Hydrogels under Continuous Step Strain %HFDXVH DOO K\GURJHOV ZHUH VHOI KHDODEOH DW URRP WHPSHUDWXUH &1) 33\ 3%

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Water Content

Density !

1s

[MPa]

[kJ m-3]

Wc [%]

[g cm-3]

[MPa·cm3 g-1]

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10.8 ± 0.3

73.5 ± 0.1

95.4 ± 0.1

1.17 ± 0.12

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CNF-PPy/PB-2

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116.3 ± 0.3

94.6 ± 0.2

1.19 ± 0.09

~11.26

CNF-PPy/PB-3

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262.9 ± 0.3

93.9 ± 0.1

1.21 ± 0.05

~18.51

CNF-PPy/PB-4

7.6 ± 0.4

42.1 ± 0.5

93.2 ± 0.2

1.24 ± 0.08

~6.13

CNF-PPy/PB-5

1.1 ± 0.3

11.2 ± 0.6

92.5 ± 0.2

1.27 ± 0.11

~0.87

0.5 ± 0.1

95.8 ± 0.1

1.15 ± 0.07

~0.01

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hydrogel extract liquid. SEM of the cell adhered to the CNF-PPy/PB-3 hydrogel after 24h in the culture at low (c) and high (d) magnifications. 'XH WR WKH excellent biocompatibility of CNF, PVA and PPy, the biocompatibility of CNFPPy/PB hydrogels were highly expected. Therefore, the cytotoxicity of representative CNFPPy/PB-3 hydrogel was HYDOXDWHG XVLQJ / VKRZQ LQ )LJXUH

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4. CONCLUSIONS In summary, we demonstrated a novel strategy to design and develop a type of PXOWLIXQFWLRQDO CPHs based on a viscoelastic PB gel matrix and nanostructured CNF-PPy complexes that synergized the biotemplate role of CNFs and conductive nature of PPy. CNFs VHUYHG DV D ELRWHPSODWH DQG JXLGHG WKH JURZWK RI 33\ LQWR FRPSDFW CNF-PPy complexes ZLWK LGHDO GLVSHUVLW\ ZKLOH ERUD[ DFWHG DV D G\QDPLF FURVV OLQNHU EHWZHHQ WKH 39$ FKDLQV DQG &1) 33\ FRPSOH[HV Following the two-step design strategy, the CNF-PPy complexes, which not only imparted high conductivity to the CNF-PPy/PB hydrogel but also reinforced the hydrogel network, were homogeneously incorporated into the hydrogel to establish a combined reinforcing and conductive network. 7KH FRH[LVWHQFH RI multi-complexation, + 2 interpenetration FKDLQ HQWDQJOHPHQW RI &1) 33\ FRPSOH[HV DQG 39$ DV ZHOO DV LQWHU DQG LQWUDPROHFXODU K\GURJHQ ERQGLQJ V\VWHPV DPRQJ WKHP OHG WR WKH IRUPDWLRQ RI DQ LQWHJUDWHG ' QHWZRUN ZLWKLQ WKH CNFPPy/PB K\EULG K\GURJHOV The final homogeneous, stable and moldable hydrogels with high water content (~94%) and a low density (~1.2 g cm-3) exhibited high strength (1s XS WR FP J

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F\FOHV In conclusion, this study LOOXVWUDWHG D QHZ SDWKZD\ for the

fabrication of CPHs with multifunctionality, opening up a number of future research directions and applications, SDUWLFXODUO\ LQ intelligent bioelectronics devices, such as bioelectrodes ELRVHQVRUV DQG LPSODQWDEOH GHYLFHV ASSOCIATED CONTENT Supporting Information. The preparation of CNFs and characterization of in vitro cytotoxicity assays, UV-Vis of neat PPy and CNF-PPy-3 complex suspensions, pH-responsive ability of hydrogels, tensile stress-strain curves of the original and healed hydrogels, and the specific capacitance variation of the hydrogel supercapacitors were supplied as Supporting Information. AUTHOR INFORMATION Corresponding Author *Prof. H. J. Quan; E-mail: [email protected] (J.H.). Author Contributions Q.D. and X.X. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS 7KLV ZRUN ZDV VXSSRUWHG E\ 1DWLRQDO 1DWXUDO 6FLHQFH )RXQGDWLRQ RI &KLQD 1LQWK &KLQD 6SHFLDO 3RVWGRFWRUDO 6FLHQFH )RXQGDWLRQ 6FLHQFH 5HVHDUFK 3URMHFW RI -LDQJVX 3URYLQFH 1R 3URYLQFH

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