Tough, Adhesive, Self-Healable, and Transparent Ionically Conductive

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Applications of Polymer, Composite, and Coating Materials

Tough, adhesive, self-healable and transparent ionically conductive zwitterionic nanocomposite hydrogels as skin strain sensors Liufang Wang, Guorong Gao, Yang Zhou, Ting Xu, Jing Chen, Rong Wang, Rui Zhang, and Jun Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20755 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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ACS Applied Materials & Interfaces

Tough, Adhesive, Self-Healable and Transparent Ionically Conductive Zwitterionic Nanocomposite Hydrogels as Skin Strain Sensors Liufang Wanga,b, Guorong Gaob, Yang Zhoub, Ting Xub, Jing Chenb, Rong Wangb, Rui Zhang*a, and Jun Fu*b a

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, 130 Meilong Road, Shanghai 200237, China b

Cixi Institute of Biomedical Engineering & Polymers and Composites Division, Ningbo

Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China.

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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KEYWORDS: tough hydrogels, adhesion, zwitterionic polymer, self-healing, strain sensors

ABSTRACT: It is desired to create skin strain sensors composed of multifunctional conductive hydrogels with excellent toughness and adhesion properties to sustain cyclic loadings during use and facilitate the electrical signal transmission. Herein, we prepared transparent, compliant and adhesive zwitterionic nanocomposite hydrogels with excellent mechanical properties. The incorporated zwitterionic polymers can form interchain dipole-dipole associations to offer additional physical crosslinking of the network. The hydrogels show a high fracture elongation up to 2000%, a fracture strength up to 0.27 MPa and a fracture toughness up to 2.45 MJ m-3. Moreover, the reversible physical interaction imparts the hydrogels with rapid self-healing without any stimuli. The hydrogels are adhesive to many surfaces including polyelectrolyte hydrogels, skin, glasses, silicone rubbers, and nitrile rubbers. The presence of abundant zwitterionic groups facilitates ionic conductivity in the hydrogels. The combination of these properties enables the hydrogels to act as strain sensors with high sensitivity (gauge factor = 1.8). The strategy to design the tough, adhesive, self-healable, and conductive hydrogels as skin strain sensors by the zwitterionic nanocomposite hydrogels is promising for practical applications.


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1. INTRODUCTION

Polymer hydrogels with excellent performances including toughness,1-6 responsiveness,7-9 selfhealing10-11 and adhesiveness,12-13 have great potentials for applications in electronic skin, biomedical sensors, soft robotics and so on, and have attracted great research interests. However, it is yet a challenge to achieve ideal adhesion between hydrogels and most surfaces. For example, conformal contacts of the sensors to the skin are required to maximally convert the skin deformation into electrical signals. However, many skin strain sensors are taped to the skin, which may form gaps at the interface14 and is uncomfortable. Enormous efforts have been made to prepare integrative hydrogels with both conformal adhesion and sensitive sensing. Sitti et al.15 prepared skin adhesive strain sensors composed of microfibrillar films and silver nanoparticle thin film-based strain sensors. The adhesion was enhanced by crosslinking the viscous mushroom-shaped vinylsiloxane tips directly on the skin surface. But the reusability was limited: the adhesion strength decreased by 48% after three cycles. Zhang et al.16 prepared adhesive and conductive hydrogels by combining the mussel-inspired polydopamine and conductive functionalized single-wall carbon nanotubes. However, the improved adhesion is obtained at the expense of the mechanical properties because the free movement of molecular chains may facilitate interactions between hydrogels and adherends but reduce the stability of cross-linking in the hydrogels. Gao et al.17 prepared adhesive hydrogels by introducing the adenine and thymine into the polyacrylamide backbone, which had excellent adhesive properties for various solid materials but low fracture strength (25 kPa). Liu et al.18 reported the epidermal strain sensors with the addition of polydopamine into polyvinyl alcohol hydrogels, whose modulus of compression was 4.6 kPa. Practically, excellent mechanical properties of adhesive hydrogels are required for skin strain sensors, which can ensure high and stable performance during



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applications. Meanwhile, the mechanical performance matching the hydrogel and the skin can better avoid cohesive fracture. Therefore, it is desired to prepare a hydrogel strain sensor with reversible and compliant adhesion, toughness and high sensitivity. It is well known that the positive and negative charges of the overall neutral zwitterionic molecules make a high dipole moment.19 Such strong dipolarity entitles excellent adhesion of zwitterionic hydrogels to many surfaces through ion-dipole or dipole-dipole interactions.20-21 In addition, the associations of zwitterionic polymers can provide physical crosslinking to enhance the mechanical properties of hydrogels. The zwitterions can assist the ion transportation along the highly dipolarized skeleton to promote ion conduction, which endows zwitterion hydrogels with good conductive properties.21 Hydrogels of zwitterionic betaine polymers are potential for integrating multifunctionality on a single hydrogel such as adhesiveness, conductivity, and so on. However, most of the reported zwitterionic hydrogels22-23 are mechanically weak. Haraguchi et al.24 prepared nanocomposite hydrogels comprised of zwitterionic polymers or copolymers. An effective polymer-clay network was formed by ionic interactions between the sulfobetaines polymer and clay platelets and interchain associations of the zwitterionic group charges. The hydrogels showed superior mechanical properties and well-controlled thermosensitivities. Here we present a facile method to prepare zwitterionic nanocomposite hydrogels physically crosslinked by exfoliated Laponite XLG® nanosheets with excellent mechanical properties, compliant adhesion, reliable self-healing, and high strain sensitivity. The transparent hydrogels are comprised of zwitterionic [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) and 2-hydroxyethyl methacrylate (HEMA) copolymer. They can be stretched to 2000% at a fracture strength up to 0.27 MPa and can adhere to various substrates including hydrogels, skin, glasses, silicone rubbers, and nitrile rubbers. The reversible interaction between


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the zwitterionic nanocomposite hydrogels and the adherends enables repeated adhesion and peeling. Moreover, the hydrogels form conformal interface between the zwitterionic nanocomposite hydrogels and the skins with complex surface topography and dynamic motions. The reversible physical interactions make the hydrogels self-healable. Moreover, the hydrogels have high conductivity and high sensitivity (GF = 1.8) with increasing zwitterion content. These outstanding properties make the zwitterionic nanocomposite hydrogels promising for skin strain sensors to monitor the movement of body.

2. EXPERIMENTAL SECTION

2.1. Materials. A synthetic hectorite “Laponite XLG®” was provided by Zhejiang Fenghong New Materials Co. Ltd. [2-(Methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA, 97%), potassium persulfate (KPS, 99.5%), 2-hydroxyethyl methacrylate (HEMA,

96%),

2-acrylamide-2-methylpropanesulfonic

(acryloyloxy)ethyl]trimethyl-ammonium

chloride

acid

solution

(AMPS, (DAC,

98%), 80%),

[2N,N'-

methylenebis(acrylamide) (MBAA, 99%), and N,N,N',N'-tetramethylethylenediamine (TEMED, 99%) were purchased from Aladdin (Shanghai, China). All these reagents were used without further purification. Deionized water (18.2 MΩ at 25 C) was prepared before use. 2.2. Preparation of zwitterionic nanocomposite hydrogels. Zwitterionic nanocomposite hydrogels were synthesized by free radical polymerization with the presence of exfoliated clay platelets in aqueous media. First, a uniform aqueous suspension of Laponite XLG was prepared at ice-water temperature. Then HEMA, SBMA, and initiator KPS were added to the clay suspension and stirred to generate a homogeneous solution at ice-water temperature. Subsequently, TEMED was added into the solution and stirred for 5 min. The solution was



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transferred into molds comprised of a 1.5 mm silicone rubber spacer between two glass plates. Free radical polymerization was carried out at 25 °C for 24 h. A series of synthesis formulations are shown in Table S1. The zwitterionic nanocomposite hydrogels are denoted as LmSn hydrogels, with L for Laponite XLG, S for SBMA, m for the concentration of clay (102 mol/L), and n for the mol% of SBMA in the total monomers. For comparison, we also prepared the hydrogels with NaCl. During the preparation process of the hydrogels, only the aqueous solution was replaced by 1 mol/L NaCl solution. 2.3. Preparation of polyelectrolyte hydrogels. Polyelectrolyte hydrogels were synthesized by using MBAA as the cross-linker for in situ copolymerization of HEMA and AMPS (or DAC). As described in previous studies,25 a solution of HEMA (2.3 g), MBAA (0.003 g), KPS (0.0035 g) and AMPS or DAC in 3 mL deionized water was prepared before TEMED was added into the mixture and stirred for 5 min. Finally, the solution was injected into molds made of two parallel glass plates and a 1.5 mm silicone rubber spacer. Free radical polymerization was conducted at 25 °C for 24 h. The synthesis formulations are shown in Tables S2 and S3. We denote the Cx hydrogels and Ay hydrogels as cationic and anionic hydrogels, respectively. Herein, C refers to DAC and A refers to AMPS, while x and y refer to the molar concentrations of ionic monomers (mol/L). 2.4. Tensile tests. Mechanical performance was investigated by using a Universal Testing Machine (Instron 5567, USA) at room temperature on dumbbell specimen (35 mm  2 mm  1.5 mm) at 100 mm/min. The fracture toughness of the hydrogel was calculated by integrating the area under the stress-strain curve. Measurements on five specimens were conducted for each material and the results were reported as mean ± standard deviation.


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2.5. Lap shear tests. A Universal Testing Machine was used to test the adhesion performance of the LmSn hydrogels by the lap shear tests. A zwitterionic nanocomposite hydrogel (15 mm  15 mm  1.5 mm) was placed between two polyeletrolyte hydrogels, glass slides, silicone rubbers, or nitrile rubbers (17 mm  30 mm  1.5 mm) to form a sandwiched area of 15  15 mm2. A contact pressure (100 g) was applied for 10 min to form a good contact before the tests. All tests were performed at a shear rate of 50 mm/min at room temperature. Measurements of each assembly were repeated at least five times and the results were reported as mean ± standard deviation. For the repeated adhesion, the zwitterionic nanocomposite hydrogel sample was also used for the lap shear tests, where the waiting time attached to the surface of substrates was 10 min. The next test was conducted immediately after each test. 2.6. Measurement of electric properties. The conductivity of the LmSn hydrogels was measured by using a digital four-probe tester (ST2258C, China). Besides, the electrochemical impedance tests were also performed by using a Solartron 1470E multi-channel potentiostat electrochemical workstation (Solartron Public Corp., Ltd). The impedance data were analyzed by using the Software ZView according to the Randles equivalent circuit. The resistance changes of the zwitterionic nanocomposite hydrogels were obtained by using an electrochemical analyzer/workstation (CHI600E, China). The gauge factor is defined as GF = [(R - R0)/R0]/ε = (ΔR/R0)/ε, where R0 and R are the resistances of the original and stretched hydrogels, respectively, and ε is the strain of the hydrogel.

3. RESULTS AND DISCUSSION

3.1. Toughness and Adhesiveness of Zwitterionic Nanocomposite Hydrogels



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z nanocompoosite hydroggels Schemee 1. Schemaatic illustratiion to the syynthesis of zwitterionic

The transparent zwitterionnic nanocom mposite hyydrogels w were preparred by freee radical SBMA andd HEMA wiith the preseence of exfo foliated clayy (Scheme 11). During polymerrization of S the prepparation proocess, the introductionn of the zw witterionic monomer S SBMA intoo the clay solutionn significanttly decreaseed the viscoosity and supppressed the formationn of a clay ggel, which indicatees the interacction betweeen the clay platelets annd the SBMA A monomerr in aqueouss solution. The zw witterionic naanocomposiite hydrogeels have exccellent mechhanical properties. As shown in Figure 11a-b, the L44S90 hydroogel can be stretched too 1000% strrain withouut fracture. A After load release, the hydrogel can recovver to the orriginal state in 4.5 min (Video S1). The repeatted stretch and release were performed.. No signifficant channges in thee gel were observed by visual witterionic nnanocompoosite hydroggels show excellent inspection, which implies thhat the zw mes. toughneess and could be reusedd multiple tim


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S90 hydrogeel. (c) The Figure 1. Photograaphs of (a) the as-prepaared and (bb) highly strretched L4S transparrent L4S90 gels are useed to adheree glass, polyyanionic (purple) and ppolycationicc (yellow) hydrogeels togetherr. (d) The lap shear test of thhe C0.25-L44S90-C0.255 assembly.. (e) The compliaant adhesionn of L4S90 hydrogel too fingers. Sccale bars, 100 mm. (f) T The possiblee adhesion mechannisms includding ion-dippole interacction and diipole-dipolee interactionn between tthe LmSn hydrogeels and the aadherends. Moreoover, the frree zwitterioonic groupss of the LS S hydrogels can endow w the hydroogels with unique adhesion too substrates with chargged groups or polar grroups. Figuure 1c show ws that the mbled togetther by usinng L4S90 glass, ppolycation hhydrogel, annd polyanioon hydrogeel are assem hydrogeels as the inntermediate layer. In coontrast, the ppolycationicc or polyaniionic hydroggel cannot directly adhere to the glass. The interfa faces of thee assembliees are robust and could sustain



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stretching. As the cationic C0.25 gel attached with a blue polyzwitterionic nanocomposite hydrogel is stretched, the zwitterionic nanocomposite hydrogel (blue) is deformed along with the cationic gel substrate without delamination until the fracture of the cationic gel (Figure 1d). More interestingly, the zwitterionic nanocomposite hydrogels are able to form stable and conformal adhesion to the finger skin (Figure 1e). The L4S90 hydrogel well conform with the finger surface during the cyclic bending or stretching of the finger, without delamination or forming gaps from the finger surface (Video S2). Besides, the hydrogel can be easily and completely peeled off from the skin without residues (Figure S3). This result indicates an outstanding compliant adhesiveness that is important for applications. This outstanding adhesion of the hydrogels on the polyelectrolyte hydrogels could be attributed to the dipole-dipole interactions between the zwitterion moieties with the ionic groups (Figure 1f). Moreover, the OH, C≡N, COOH, and NH2 groups on many other substrates may also adhere with the polyzwitterionic hydrogels through dipole-dipole interactions with the N+(CH3)3 and SO3　 groups (Figure 1f). 3.2. Tensile Properties In contrast to many other reported adhesive hydrogels with weak mechanical strength and deformability,18,

26

the zwitterionic nanocomposite hydrogels have outstanding mechanical

properties. The zwitterionic nanocomposite hydrogels exhibited high stretchability and toughness, which may be related to the unique composition comprised of the zwitterionic SBMA and the clay platelets. Figure 2 shows the representative tensile stress-strain curves of hydrogels. The changes in SBMA and Laponite® XLG contents obviously influence the stretchability, strength, and toughness of the hydrogels.


0.20

1.0 2000

Fracture strain (%)

0.25

Stress (MPa)

(b)

L5S50 L5S60 L5S70 L5S80 L5S90

0.15 0.10 0.05

1500

0.6

1000

0.4

1000

1500

Strain (%)

Fracture strain (%)

0.3 0.2 0.1 0.0 0

-3

1000

1500

2.0 1.5 1.0 0.5 0.0

50

60

70

80

SBMA content (%)

90

90

0.8 0.6 0.4

1400 1200

0.2

1000 800

0.0 1.0

1600

(f)

2.5

80

1800

2000

Strain (%)

70

2000

-3

(e)

500

60

SBMA content (%)

(d)2200

L4S90 L5S90 L6S90

0.4

50

2000

Fracture toughness (MJ m )

Stress (MPa)

(c)

500

0.2

500

0.00 0

0.8

4

5

-2

Fracture strength (MPa)

(a) 0.30

Fracture toughness (MJ m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

0.0

Fracture strength (MPa)

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XLG content (10 mol/L) 3.0 2.5 2.0 1.5 1.0 0.5 0.0

4

5

-2

6

XLG content (10 mol/L)

Figure 2. The mechanical properties of LmSn hydrogels. (a) The tensile stress–strain curves and (b) fracture strain and fracture strength of L5S hydrogels. (c) The tensile stress–strain curves and (d) fracture strain and fracture strength of LS90 hydrogels. The fracture toughness of (e) L5S hydrogels and (f) LS90 hydrogels.



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Interestingly, both the fracture strain and strength increased monotonously from 470% to 2000% and from 0.15 to 0.27 MPa, respectively, as the SBMA content was increased from 50 to 90% (Figure 2a,b). Meanwhile, the fracture toughness of the L5S hydrogels increased with the SBMA content, from 0.32 to 2.45 MJ m3 (Figure 2e). As for the L4S and L6S hydrogels, a similar dependence was observed (Figure S1). The fracture strength of the LS90 hydrogels increases from 0.21 to 0.27, and 0.38 MPa as the clay concentration was increased to 4, 5, and 6×102 mol/L (Figure 2c,d). Correspondingly, the fracture toughness of the LS90 hydrogels increased from 1.60 to 2.44, and 2.88 MJ m3 (Figure 2f). However, the fracture strain increased first and then decreased slightly. The excellent mechanical properties of the zwitterionic nanocomposite hydrogels can be attributed to the ionic interactions24 between the zwitterion and clay platelets and the interchain associations27 of the zwitterionic groups to form the physically cross-linked networks in the LmSn hydrogels (Scheme 1). The increase of the SBMA and Laponite XLG content facilitates the formation of denser networks, which can dissipate more energy to toughen the hydrogels. Therefore, one could tune the mechanical performance of the hydrogels by varying the SBMA and clay content. 3.3. Adhesion Performance and Possible Mechanisms To quantitatively characterize the adhesion performance of the hydrogels, the lap shear tests have been conducted. For this purpose, the zwitterionic nanocomposite hydrogel is sandwiched between polyelectrolyte hydrogels, glasses, or other materials (Figure 3a). The assemblies are stretched on both ends until separation (adhesion failure) or fracture (cohesion failure). Representative curves of adhesion shear force vs. displacement for each model are presented in Figures 3b, 3c and S4 (Supporting Information). The maximum force is taken as the interface


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adhesionn force wheen the interfface rupturees. In Figurre 3d, the addhesion forcce of the L44Sn to the C0.15 hhydrogels w was significcantly increaased with iincreasing S SBMA conntent. In conntrast, the adhesionn force of tthe L4Sn hyydrogels to the A0.15 hydrogels first increassed and subbsequently descendded with inncreasing SB BMA conteent. At higgh SBMA ccontent from m 70% to 90%, the adhesionn force was almost uncchanged.

Figure 3. (a) Scheematic illusttration of laap shear tesst. Represenntative curvves of adhession shear ment for (bb) the L4S hydrogels to the C0.15 hydroggels and (c)) the L4S force vs. displacem force of L4S S hydrogels with differeent SBMA hydrogeels to the A00.15 hydroggels. (d) Thee adhesion fo content to C0.15 hyydrogels annd A0.15 hyydrogels. Thhe adhesion force of L44S90 hydroggels to (e) i monom mer contentt. polycatiion hydrogeels and (f) poolyanion hyydrogels witth different ionic The charge densiity in the poolyelectrolytte hydrogelss has a significant influuence on thee adhesion Figures 3e annd 3f show the effect of o DAC and AMPS conntent on the adhesion foorce of the force. F L4S90 hhydrogels too polyelectrrolyte hydroogels. The addhesion forcce of the L44S90 hydroggels to the



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polycatiionic hydrogels increassed with thee DAC conntent. As thee DAC conntent was 0.25 mol/L, the adheesion of thee L4S90 hyddrogel to thee C0.25 hyddrogel was so s strong thhat the polyeelectrolyte hydrogeel fractured instead of interface ffailure. Interrestingly, thhe adhesionn force of the t L4S90 hydrogeels to the A hydrogels w was almost uunaffected despite d of thhe change off the AMPS S content. The aadhesive prooperties of zwitterionic z nanocompoosite hydroggels are attrributed to thhe charged groups of zwitteriionic polym mers. The zwitterionic polymerss with the cationic qquaternary ammoniium groups and anionicc sulfonate groups on tthe same moonomeric unnit exhibit nneutral net charge, but have higgh dipole m moment. Thee charged grroups can innteract with other chargged groups or polarr groups thrrough ion-diipole and diipole-dipolee interactionns28-29 (Figuure 1f). Therrefore, the zwitterioonic nanoccomposite hhydrogels ccan attach to t other m materials thrrough the iinteraction betweenn the cationiic groups annd anionic ggroups on Lm mSn hydroggels surfacees and chargged groups on otherr substrates.. Schemee 2. Schemaatic illustratiion of the suurface of thee zwitterionic nanocom mposite hydrogels.

mposite gelss to polyannionic gels Intereestingly, thee adhesion bbehaviors oof zwitterionnic nanocom are signnificantly disstinct from the t polycatiionic hydroggels (Figuree 3d, e, f). Thhis can be aascribed to the uniqque molecuular structurre of the S SBMA monnomer. In thhe SBMA ppolymer, thhe anionic


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groups are outreaching the dangling zwitterionic side groups. Therefore, most anionic groups are pointing out the surface of the LmSn hydrogels. This explains the observation that the adhesion force between the zwitterionic nanocomposite hydrogels and the polycationic hydrogels is larger than that for the zwitterionic-polyanionic hydrogel pairs. The SBMA dipoles may adopt two different conformations on the gel surface, depending on the crowdness of the PSBMA chains. Scheme 2 illustrates the flat-on conformation at low PSBMA density and the standing conformation of the zwitterion dipoles at overcrowded PSBMA surface. At the low SBMA content regime, less zwitterionic side groups are available to interact with cationic polymers. The cationic groups on the surface of the LmSn hydrogels interact more easily with the anionic groups on the A0.15 hydrogels surface. Increasing the SBMA content contributes to the increase in the cationic groups on the gel surface, which leads to an increase in the adhesion force of the L4S hydrogels to the A0.15 hydrogels. In contrast, at the high SBMA content regime, the zwitterionic groups may form a saturated and tight structure, which may make the interface adhesion insensitive to the ionic monomer concentration in the polyelectrolyte gels. The adhesion force of the L4S hydrogels to the A0.15 hydrogels is roughly the same when the SBMA content changes from 70% to 90%. In addition, the adhesion force of the L4S90 hydrogels to polyanionic hydrogels with different AMPS content varies slightly. When the SBMA content is 90%, the cationic groups on the surface of the gel are limited and can equally interact with the anionic groups in different polyanionic hydrogels. This explains the observation that the change in the anionic monomer content has little effect on the adhesion force.



(a)

(b)

2.0

1.8

1.5

Force (N)

Force (N)

1.7 1.6 1.5

1.0 0.5

1.4 4

(c)

5

-2

XLG content (10 mol/L)

6

2.0

0.0

0

1

NaCl content (mol/L)

(d) 6

Silica rubbers Nitrile rubbers Glasses

5

Force (N)

1.5

Force (N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

4 3 2 1

0.5

0 0.0

1

2

3

4

5

The number of adhesion cycles

0

2

4

6

8

10

Displacement (mm)

12

14

Figure 4. (a) The adhesion force of LS90 hydrogels with different XLG content to C0.15 hydrogels. (b) The adhesion force of L4S90 hydrogels with or without NaCl to C0.2 hydrogels. (c) The repeated adhesion between L4S90 hydrogels and C0.2 hydrogels. (d) The adhesion force of L4S90 hydrogels to various materials. The mechanical properties of the LS hydrogels can influence the adhesion properties of the LS hydrogels to the polyelectrolyte hydrogels. Figure 4a shows that the adhesion force of the LmS90 hydrogels to the C0.15 hydrogels significantly decreased with the increase in the clay content. Meanwhile, the elastic modulus of the LmS90 hydrogels dramatically increased with the clay content (Figure 2c). It has been reported that the elastic modulus of the hydrogels has a significant effect on the deformability and thus the adhesion behavior of the hydrogel surface.30


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Here, with a high clay content, the polymer chains are adsorbed to the clay nanosheets, leading to a high crosslink density and high modulus. Since the zwitterionic contents in these gels are almost the same, the interface adhesion is mainly affected by the compliance, or the reciprocal of the modulus of the gels. This explains the decrease of adhesion force with the increasing clay content. To further illustrate the adhesion properties of the zwitterionic nanocomposite hydrogels through ion-dipole or dipole-dipole interactions, NaCl was introduced into the LmSn hydrogels, only to obtain the same mechanical properties as the hydrogels without NaCl (Figure S1). With the presence of NaCl, the adhesion force of the L4S90 hydrogel to the C0.2 hydrogels significantly decreases by 43% in comparison with that of the L4S90 to C0.2 hydrogel pair free of salt (Figure 4b). The Na+ and Cl ions could interact with the charged groups of the zwitterions and thus shield the interaction between the SBMA moieties and the charged groups in the polyelectrolyte hydrogels. The zwitterionic nanocomposite hydrogels can repeatedly adhere to the adherends through reversible physical interactions. Repeated lap shear tests were carried out immediately after each test and the results are shown in Figure 4c. After 5 cycles, the adhesion force of the L4S90 hydrogel to the C0.2 hydrogels slightly decreased. It may be attributed to the contamination or slight damage of the sample surface after each cycle, which may reduce the extent of the interactions. The adhesion force after multiple cycles was almost unchanged, which allows the gel to be reused multiple times. Moreover, the zwitterionic nanocomposite hydrogels can also adhere to silica rubber, glasses, and nitrile rubber, as shown in Figure 4d. The polar groups (OH of glasses, C ≡ N of nitrile rubber) in these materials may offer interactions with the zwitterionic nocomposite hydrogels.



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S hydrogels with diffferent SBM MA content. (b) The Figure 5. (a) Thee conductivvity of L4S s . (c) The seelf-healing resistancce change oof L4S90 hyydrogel beffore facture and after self-healing. process.. (d) Repressentative tennsile stress––strain curvees of the as--prepared annd self-healled L4S90 hydrogeels with diffferent healinng times. 3.4. C Conductivity and Straiin Sensitiviity The zzwitterionic nanocompoosite hydroggels have goood conducttivity and hhigh strain ssensitivity. The connductivity ooriginates frrom the free ions from m clay in thhe hydrogel network annd thus is featuredd as ionic coonductivity. In contrast to the well known elecctronic condductive hydrrogels, the ion connductive hyydrogels maay behave differently d under direcct current ((DC) and aalternating current (AC). Theerefore, thee conductivvity or resistance of tthe zwitteriionic nanoccomposite


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hydrogels is measured by using a four-point probe method and the electrochemical impedance spectroscopy (EIS). By using the four-point probe tester with DC, the conductivity of the L4S hydrogels significantly increases from 0.0066 to 0.24 S m−1 as the SBMA content increases from 50 to 90 mol% (Figure 5a). On the other hand, the conductivity of the L4S hydrogels by EIS under AC shows increases from 0.076 to 0.099 S m−1 with increasing SBMA content (Figure S5). The conductivity changes obtained by the two test methods are consistent. The zwitterionic nature makes the cationic and anionic counterions of polyzwitterions easily separated during ion migration, ensuring a high ionic conductivity.31-32 A previous study33 also discovered that the ion movement could be enhanced by the weak interactions between the mobile ions and the zwitterionic polymers. The zwitterionic moieties can assist the ion conduction and thus the hydrogels behave conductive with mobile ions.34 On the other hand, the zwitterionic nanocomposite hydrogels have the self-healing capability due to the reversible physical cross-linking in the hydrogels. Figure 5b compares the resistance of the L4S90 hydrogel before facture and after self-healing. When the L4S90 hydrogel was cut with a razor blade, the resistance increased immediately and sharply. Subsequently, the resistance recovered to the original level by contacting the two fractured parts together in 11s (Figure 5c). On the other hand, the mechanical self-healing is also systematically investigated. Tensile test results of the as-prepared and self-healed hydrogels are showed in Figure 5d. After 1h healing, the hydrogels can be stretched to 350%. After 24 h, the healed hydrogels can be stretched to 0.17 MPa, and 1700%, with a healing efficiency about 74% in terms of the fracture strength. Therefore, the zwitterionic nanocomposite hydrogels have self-healing properties in both mechanical properties and conductivity.



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r raatio–strain ccurve of the L4S90 hyddrogel. (b) T The resistancce ratio of Figure 6. (a) The resistance nbending of a finger. the L4S90 hydrogel strain senssor with revversible bendding and unb The zzwitterionic nanocompoosite hydroggels can servve as wearabble soft straain sensors tto monitor a A As shown in Figure 6a, tthe resistancce of the hyydrogel increased with iincreasing human activities. strain. W When the L4S90 L hydrrogel was stretched s to 100%, thee resistance increased to around 280%, w which exhibbited high strain s sensittivity (GF = 1.8). The L4S90 hyddrogels can adhere on the indeex finger witthout tapes because of tthe compliaant adhesionn of the hydrrogels to skkins. When bendingg the indexx finger, thhe relative resistance of the hyydrogel sennsor increasses. After straighteening the inndex finger,, the relativee resistancee rapidly retturns to initiial value (F Figure 6b). The relationship bbetween thee relative reesistance off the stretcched hydroggel and thee strain is consisteent with the result in Fiigure 6a. Moreover, thee determined strain of bbending fingger (about 40%) iss similar wiith that in oother reporteed sensors ((35–45%).355-36 It is proomising to uutilize the zwitterioonic nanoccomposite hydrogels h too preciselyy monitor the t real-tim me change oof human motion. 3.5. D Discussions


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Despite of the increasing progresses in conductive hydrogels in literature, it remains an open issue to combine the adhesion and conduction in hydrogels. For practical applications, conductive hydrogels attaching firmly to other surfaces are in favor of the electrical signal transmission. We synthesized transparent, compliant and adhesive zwitterionic nanocomposite hydrogels that can conformally adhere to skin without the tape. Hydrogels with robust adhesion may sacrifice mechanical properties and are prone to damage at large strains. There are few examples of comformally adhesive hydrogels that can also possess excellent mechanical properties. The polyvinyl alcohol hydrogel containing polydopamine for the epidermal strain sensor18 can be easily reformed to various shapes and the modulus of compression is 4.6 kPa. The bioinspired mineral hydrogels are composed of amorphous calcium carbonate nanoparticles physically crosslinked by polyacrylic acid and alginate chains for ionic skin.37 The maximum fracture stress of the hydrogel is 6 kPa, although the ionic skin is compliant with curved or dynamic surfaces. We prepare the zwitterionic nanocomposite hydrogels that show a large strain to 2000% and the fracture strength up to 0.27 MPa. Moreover, the hydrogels can rapidly recover to its original state after the release of force. These properties allow the hydrogels to achieve high and stable performance and match with the modulus of skin (0 to 600 kPa).38 More interestingly, the transparent, tough and adhesive zwitterionic nanocomposite hydrogels can also conduct through the mobile ions and possess good conductivity (up to 0.099 S m−1), which is comparable with that of some electronically conductive hydrogels.39 In addition, the zwitterionic nanocomposite hydrogels have high sensitivity and is superior to previously reported ionically conductive hydrogels. Wan et al.14 reported multifunctional hydrogels with synergistic soft and hard, showing a gauge factor of 0.478. Yang et al.40 designed mussel-inspired cellulose nanocomposite tough hydrogels with a gauge factor of 1 (0% < strain < 80%). In short, the



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zwitterionic nanocomposite hydrogels are advantageous beyond most ionic conductive hydrogels in literature. The outstanding mechanical properties, reversible and compliant adhesion, self-healing capability, and good conductivity and sensitivity are primarily attributed to the introduction zwitterion into the nanocomposite hydrogels. The zwitterion polymer can interact with the clay platelets by ionic interaction and form self-association, which can provide more physical crosslinking to dissipate energy. Moreover, the free zwitterionic groups can interact with other charged or polar groups by the ion-dipole interaction or dipole-dipole interaction so that the zwitterionic nanocomposite hydrogels can attach to certain surfaces including polyelectrolyte hydrogels, glasses, nitrile rubber and skins. The physical interactions in hydrogels can endow the hydrogels with self-healing properties and reversible adhesive properties. Moreover, the zwitterionic groups can act as carriers to promote ion conduction and the hydrogels have high sensitivity in strain conduction. With comprehensive properties, the transparent zwittterionic nanocomposite hydrogels can serve as skin strain sensors to monitor the movement of body. The excellent mechanical properties and self-healing capability can extend the life of the sensors. Moreover, the reversible and compliant adhesion can enhance the intimate contact between the sensors and skins to amplify the transmission of signals. Further, the zwitterionic nanocomposite hydrogels can be used solid electrolytes because the hydrogels are mechanically stable and safe, and can simultaneously play multiple roles of electrolyte, separator, and binder. Despite of the abovementioned merits, it is challenge for fast and effective self-healing of hydrogels that are critical for the lifespan of electronic devices and improve stability. The zwitterionic nanocomposite hydrogels possess self-healing properties due to the existence of


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physical interactions in hydrogels. However, the fast self-healing is limited and further efforts should be devoted to prepare fast and effective self-healing of hydrogels for practical applications.

4. CONCLUSIONS

In summary, we demonstrated a facile strategy to prepare tough and adhesive hydrogels by introducing the zwitterionic monomer into the nanocomposite hydrogels. The transparent zwitterionic nanocomposite hydrogels simultaneously possess satisfying mechanical properties, compliant adhesion, reliable self-healing capability, and high sensitivity in conductivity, which can make the LmSn hydrogels ideal candidates for wearable skin strain sensors. In addition, the zwitterionic nanocomposite hydrogels with excellent mechanical properties and the compliant adhesion can be applied to more aspects, such as wound dressing. In brief, the strategy to synthesize the zwitterionic nanocomposite hydrogels by the free radical polymerization of SBMA and HEMA in the exfoliated clay has shed new light on preparing tough and adhesive hydrogels. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ×××××××××. The formulations of zwitterionic nanocomposite hydrogels, polycation hydrogels and polyanion hydrogels; typical tensile stress–strain curves of L4S hydrogels, L5S hydrogels and L4S90 hydrogels with or without 1 mol/L NaCl; typical tensile stress–strain curves and fracture



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strength of polycation hydrogels and polyanion hydrogels; the stripping process of the L4S90 hydrogel from the skin; adhesion shear force vs displacement curves of

the zwitterionic

nanocomposite hydrogels to polyelectrolyte hydrogels; the electrochemical impedance spectroscopy (EIS) and the conductivity of the L4S hydrogels with different SBMA content (PDF). The repeated stretching and releasing of the L4S90 hydrogel (AVI) The compliant adhesion between L4S90 hydrogel and skins and the easy stripping process (AVI) AUTHOR INFORMATION Corresponding Authors E-mail: Prof. R.Zhang, [email protected], Prof. J. Fu, [email protected] ORCID Jun Fu: 0000-0002-8723-1439 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21574145, 51873224, 21374029), Natural Science Foundation of Zhejiang Province (LQ16E030002, LY17E030011), and Natural Science Foundation of Ningbo (2017A610057, 2016A610255). We


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TOC Graphic


Tough, Adhesive, Self-Healable, and Transparent Ionically Conductive

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