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Electric Field Actuation of Tough Electroactive Hydrogels Crosslinked by Functional Triblock Copolymer Micelles Yufen Li, Yuanna Sun, Ying Xiao, Guorong Gao, Shuhui Liu, Jianfeng Zhang, and Jun Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08841 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Electric Field Actuation of Tough Electroactive Hydrogels Crosslinked by Functional Triblock Copolymer Micelles Yufen Li,a,b,† Yuanna Sun,a,† Ying Xiao,a Guorong Gao,a Shuhui Liu,a Jianfeng Zhang,b and Jun Fua,*

a. 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

b. Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo, 315211, China

ACS Paragon Plus Environment


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ABSTRACT: Multi-responsive

polyelectrolyte

hydrogels

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with

extraordinary

toughness have great potential in soft device applications. Previously we have demonstrated a series of tough and multi-responsive hydrogels by using multi-functional triblock copolymer (Pluronic F127 diacrylate, F127DA) micelles to crosslink cationic polyelectrolyte chains into 3D network. Herein, we further synthesize negatively charged hydrogels comprised of 2-acrylamido-2-methyl propylsulfonic acid (AMPS) monomers by using F127DA micelles as crosslinkers. Similar to the positive nanomicelle (NM) hydrogels, the negative NM hydrogels exhibited a compressive strength up to 59 MPa with a fracture strain re to 98%, and tensile fracture strain higher than 2000%. These charged hydrogels were actuated by electric field when immersed in salt solutions. The effects of electrolyte concentration, electric field strength and ionic monomer content on the electric actuation behavior of these electroactive hydrogels (EAHs) have been systematically investigated. It is concluded that the electroactive hydrogel show a fast actuation rate with a bending angle up to 87° at 120 s and the bending angle was cyclically reversed upon changing bias direction without large decrease. This study demonstrates that such tough and multi-responsive electroactive hydrogels may have great potentials in sensors, actuators, switches, and artificial muscles.

KEYWORDS：Micelles; tough; hydrogels; electric- field; electroactive


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INTRODUCTION Actuators are materials and devices which can be triggered by environmental stimuli and perform mechanical work on the nano-,1 micro-,2-3 and macroscales.4-5 There are many kinds of materials that can be used to fabricate actuators, especially, polymers.6-7 Stimulus-responsive hydrogels have recently attracted much attention as smart materials for soft actuators 8 because of their capability to respond to external stimuli such as temperature,9-10 pH,11 salt,12 solvent composition,13 electric field

14

and

light.15 Among these, electroactive hydrogel (EAH) actuators are promosing for soft robotic applications that require low voltages for actuation.16-17 EAH actuators are capable of experiencing deformations when subject to an electrical stimuli.18 Unlike traditional electrostatic, electromagnetic, and piezoelectric actuation technologies, EAH actuators show much differences in functional and structural properties, such as large deformation in response to an electric field, lightweight, high flexibility, high energy density, high mechanical compliance, and generally low costs. 14, 19-22 The response mechanism of the gels in applied electric fields remains debatable. Shiga et al.23 prepared electroactive hydrogels by blending polyvinyl alcohol (PVA) and polyacrylic acid (PAA), which was cyclically freeze–thawed to generate hydrogels with high strength. When the gels carrying negative charges were immersed in buffer solution, they were actuated under an electric field and bent to the cathode. During actuation, the local pH decreased at the anode due to water electrolysis, which may protonate the carboxylic acid group in the gel and thus the PVA/PAAc network deswelled. Kim et al.

24

studied the gel bending in a direct-current electric field and



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introduced a depletion polarization theory to explain the behavior. By assembling hydrogel legs carrying opposite charges into a single walker, Velev et al.25 demonstrated a controlled unidirectional walking due to a subtle difference in the crosslink density and thus the friction coefficients of the gel legs under reversing electric field. Based on the controllable and reversible bending /stretching behaviors of an arched poly (2-acrylamido-2-methylpropanesulfonic acid-co-acrylamide) hydrogel walker with “two legs” by repeated “on/off” electro-triggers in electrolyte solution, Chu et al.26 achieved uni-directional walking motion on a rough surface. Tommaso et al.16 fabricated an electro-responsive hydrogel based on sodium 4-vinylbenzenesulfonate (Na-4-VBS), hydroxyethyl methacrylate (HEMA) and acrylonitrile

(AN)

(PSS-co-PHEMA-co-PAN),

which

can

operate

an

electro-mechanic actuation at low voltage at macroscale in NaCl aqueous solutions. However, it has been challenging for hydrogel actuators to experience fast and cyclic actuations due to the limits of conventional fragile polyelectrolyte hydrogels. It is desirable to develop novel electroactive hydrogels with outstanding mechanical properties and fast responsiveness. Previously, we have used multifunctional micelles of Pluronic F127 diacrylate (F127DA) to in situ copolymerize with neutral and cationic monomers, generating tough and ultrastretchable charged hydrogels with excellent fatigue resistance, strength, and responsiveness to environmental stimuli including changes in pH and ion strength.

27

Moreover, such F127DA micelle crosslinkers could be used to

crosslink various monomers, including ionic monomers. This merit enables the


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synthesis of tough polyelectrolyte hydrogels with excellent fatigue resistance, strength and toughness. In the present study, we further demonstrate that such electroactive hydrogels could be cyclically actuated by electric field. We study the mechanical properties and swelling properties of such tough polyelectrolyte hydrogels and investigate the electric actuation details of the charged nanomicelle hydrogels.

2. EXPERIMENTAL SECTION 2.1 Materials. Acrylamide (AAm), 2-acryloylamino-2-methyl-1-propanesulfonic acid (AMPS) and potassium peroxydisulfate (KPS) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The AAm was purified by twice recrystallization from acetone and dried under vacuum at 30 °C for 24 h before use. Pluronic F127 (PEO99-PPO65-PEO99) was provided by Sigma-Aldrich, and used as received. Dichloromethane, diethyl ether, triethylamine, and acryloyl chloride were all purchased from Aladdin Reagent Co., Ltd., China. Methyl chloride quarternized N, N-dimethylamino ethylacrylate (DMAEA-Q, 80 wt. % in aqueous solution) was purchased from Wanduo Fu Co., Ltd., China. Both ends of the Pluronic F127 triblock copolymer were acrylated according to a well-established literature procedure to yield F127DA.28 Typically, 10 g F127 and 1.673 g triethylamine were dissolved in 100 mL anhydrous dichloromethane under nitrogen atmosphere, and then 1.496 g acryloyl chloride in a dropping funnel were introduced to the three-neck flask. The mixture was stirred at 25 °C under nitrogen for 24h, then the precipitated triethylammoniumchloride was filtered away, and the



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filtrate was precipitated by adding into anhydrous diethyl ether. The resulting product was dried under vacuum at 30 °C for 24 h. Then, the formation of F127DA was confirmed by 1H NMR. 27, 29 2.2 Synthesis of the anionic and cationic nanomicelle hydrogels. Anionic hydrogels were synthesized by using F127DA micelles as macrocrosslinkers for in situ copolymerization of AAm and AMPS. Firstly, AAm, AMPS and F127DA were mixed in water. After bubbling under a nitrogen atmosphere for at least 30 min, 0.1 mol% of the initiator potassium persulfate (KPS) with respect to the total molar number of AAm and AMPS were added into the mixture. The solution was injected by using a syringe into molds made of two parallel glass plates with a silicone rubber spacer, with the spacings of 4 mm and 1.5 mm. Free radical polymerization was conducted in these molds 60 °C for 24 h in a water bath. Here, we use the SxMy hydrogels to stand for anionic hydrogels, with S for AMPS and M for AAm. Meanwhile, cationic hydrogels were synthesized by using F127DA micelles for in situ copolymerization of AAm and DMAEA-Q. The polymerization procedures were almost the same as anionic hydrogels, with the same concentration of AAm, initiator, and cross-linker under the same condition. The concentration of DMAEA-Q was the same as that of AMPS. Here, we use the QxMy hydrogels to stand for anionic hydrogels, where Q stands for DMAEA-Q. 2.3 Mechanical tests. The hydrogel specimens were coated with a thin layer of silicon oil to prevent water loss during mechanical testing by using an Instron 5567 Instrument (Instron Inc, MA). Five specimens were tested for each hydrogel.


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For the compression tests, the as-prepared cylinder hydrogels specimens with 9 mm diameter and 4 mm thickness were conducted at a crosshead speed of 0.167% strain per s. The engineering stress and strain in the strain range from 0.1 and 0.3 were used to calculate the initial Young’s modulus (E). For uniaxial tensile tests, the as-prepared hydrogels with thickness of 1.5 mm were cut into dumbbell-shaped bars with 20 mm long and 2 mm wide, and the dumbbell-shaped bars with 10 mm gauge length were tested at a crosshead speed of 1.33mm/s. The fracture energy was calculated by the area under the engineering stress-stain curves. 2.4 Swelling measurements. The swelling measurements were conducted by immersing the as-prepared hydrogels in salt (NaCl) solutions with different concentrations at room temperature. The swelling ratio (SR) was calculated based on the following equation: SR = (Ws –Wp)/Wp, where Ws and Wp are the weights of the swollen hydrogel and the as-prepared hydrogel, respectively. 2.5 Hydrogel actuation under an electric field. A watch glass with two parallel carbon electrodes with 70 mm apart was filled with Na2SO4 aqueous solution for the electric field actuation of hydrogels. The as-prepared SxMy and QxMy hydrogels were cut into a strip with dimension of 50 mm × 2 mm × 1.5 mm, and the SxMy and QxMy hydrogels strips were stained with methylene blue, and methyl orange, respectively. A hydrogel strip 50 mm long, with one end fixed with a clip, was put in the center of the watch glass. A voltage was then applied between the electrodes, and the degree of bending of the hydrogel strip was recorded by using a digital camera



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every five seconds. When subjected to an applied voltage, the SxMy hydrogels bent toward the cathode, while the QxMy gels bent toward the anode. The bending angle is defined as the degree that hydrogels deviate from the original location (the bending angle of the hydrogels at the original location is defined as 0°). The bending angle and time in the time range from 5 and 10 were used to calculate the actuation rate. The average value of bending angle was obtained by repeating each measurement for three times.

3. RESULTS AND DISUSSIONS Scheme 1 shows the synthesis procedure of ionic nanomicelle hydrogels. Herein, the vinyl functionalized F127 chains self-assembled into micelles in aqueous solutions and were stable at the presence of ionic monomers.27, 29 Free radical polymerization of F127DA micelles with acrylamide (AAm) and ionic monomers generated tough and stimuli-responsive hydrogels. By using DMAEMA-Q, positively charged hydrogels have been synthesized.

27, 29

In this work, negatively charged hydrogels were also

synthesized by using AMPS. In order to study the influence of ionic monomer content on the strength, toughness, and responsiveness, different formulations were used for the hydrogel synthesis (Table 1). Here, S stands for AMPS, Q for DMAEA-Q, and M for AAm. SxMy refers to a nanomicelle hydrogel with the AMPS/AAm molar ratio of x/y. QxMy hydrogels were prepared according to previous work.27 For the SxMy and QxMy hydrogels, the monomers (CS+M, CQ+M) and F127DA (CF127DA) concentrations, were 5 mol /L, 5 mol /L and 6×10-3 mol/L, respectively, unless otherwise specified.


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3.1 Mechanical properities of the SxMy hydrogels. The as-prepared hydrogels were subjected to mechanical tests. The hydrogels sustained a compressive strain up to 98% without fracture, showing a compressive stress up to 59 MPa (Figure 1a). With the increasing AMPS content (CS), the compressive stress at 98% strain (εc,0.98) decreased from 59 MPa for S1M11 to 36 MPa for S1M5, and the compressive Young’s modulus was reduced from about 127 kPa to 100 kPa (Figure 1b). These results suggest that the anionic monomers may cause a reduction in the hydrogel strength, which may be related to the less chain entanglements caused by the increased osmotic pressure and electrostatic repulsion. Meanwhile, the decreased content of AAm monomers may decrease the hydrogen bonding between the polymer chains, which may partially account for the lower strength and modulus of the gels. On the other hand, these hydrogels showed extraordinary stretchability upon uniaxial tensile tests. The S1M11 gel exhibited a tensile strength of 254 kPa and fracture strain of about 2400% (Figure 1c). With increasing CS, the fracture strength was gradually decreased to 132 kPa, the fracture strain was decreased to about 1900%, and the fracture energy (or the area under the stress-strain curve) was decreased from 2.91MJ/m3 to 1.40MJ/m3 (Figure 1d). These results are in accordance with our previous study of the cationic nanomicelle hydrogels, where the cationic monomer leads to the decreased toughness and strength of hydrogels. Furthermore, with the same ionic content (molar percentage), the anionic nanomicelle hydrogels showed a higher strength than those with positive charges.



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3.2 Swelling in ionic solution. In addition to excellent mechanical properties, these anionic nanomicelle hydrogels also show swelling behaviors dependent on the ion strength of solutions. We investigate the swelling of these anionic hydrogels in solutions with different ion strengths. Buffer solutions with pH = 7 and different ion strengths were used to swell/deswell the gels. The swelling ratio was decreased with increasing ion strength. As for the S1M8 hydrogel, for instance, the equilibrium swelling ratio (ESR) was 15 at I = 0.10, 13 at I = 0.20 and 27 at 0.05 (Figure 2). The variation of the swelling ratio was dependent on the electrostatic repulsion of the charged groups in the gels and the donnan potential of mobile counterions. In buffer solutions with a high ion strength (I = 0.20), the osmotic pressure between internal and external gels was low and the electrostatic repulsion was shield by the counterions, so, the water uptake was adverse, causing a low ESR value. 3.3 Actuation of the anionic (SxMy) and cationic (QxMy) nanomicelle hydrogels by electric field. According to Shiga type bending theory30 of the electric drive gel, the bending degree of the gel Y: Y = [RCp ht L2 (1 - ht)] / DE

(1)

where E is Young's modulus, D is the thickness, and L is the length of a hydrogel strip before bending, Cp is the concentration of the counterions of the polyions, h is the counterions migration rate, t is the exposed time to an electric field. The bending behaviors of the hydrogels are the results of the movement of the counterions of the polyions inside the hydrogels and the anions and cations in the solution. Because the motion of polyions were bound by the network and the


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counterions migrated to opposite electrodes, the ionic strength on each side of the hydrogel is different, and thus causing an osmotic pressure that drives gel bending. As for the polyanion gel, the counterions migrated to the cathode, causing the differences between inside and outside the hydrogel in the ionic strength, which further resulted in the osmotic pressure of the hydrogel close to the anode greater than the osmotic pressure of the hydrogel near the cathode. Thus, the gel near the anode is swelled, to bend the hydrogel toward the cathode. This behavior is opposite to that of the polycation gel (Figure 3). When a strip of anionic (or cationic) hydrogels was placed in an electric field in the Na2SO4 aqueous solution, the hydrogels show remarkable and fast bending toward cathode (or anode). When the bias direction is changed, the hydrogels bend reversely. Figure 4 shows the reversible bending behavior of the SxMy and QxMy hydrogels under cyclically reversed bias. The voltage between the electrodes was 20 V and the concentration of Na2SO4 aqueous solution (CNa2SO4) was 0.05 M. When the voltage was 20 V, the S1M5 hydrogel bent toward cathode (the bending angle was positive), and S1M5 hydrogel bent toward the opposite direction when the applied voltage was -20 V due to the change of the migration direction of the counterions and electrolyte ions of the solution (Figure 4a). For the Q1M5 hydrogel, it bent toward positive electrode when the applied voltage was 20 V and bent toward negative electrode when change the polarity of electric field (Figure 4b). Then, we investigate the effects of electrolyte concentration on the bending behaviors of the S1M5 and Q1M5 hydrogels. The applied voltage was 20 V and the



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concentration of Na2SO4 aqueous solution (C Na2SO4) was 0.01 M, 0.05 M, 0.10 M and 0.15 M (Figure 5). For the S1M5 hydrogel, the bending angle was 87° at 120s with C Na2SO4

= 0.05 M, 59° for C

Na2SO4

= 0.01M and 30° for C

Na2SO4

= 0.15 M. By

comparing the actuation rate of the bending behavior, we found that the actuation rate peaked at 0.05 M and it increased with the increasing concentration of Na2SO4 aqueous solution until the concentration was 0.05 M, then decreased with the increasing C

Na2SO4

(Figure 5c). With C

Na2SO4

< 0.05 M, the counterions going into

the inside of the gel increase with the increase of electrolyte concentration, leading to an increase of the osmotic pressure on both sides of the gel . As for C Na2SO4 > 0.05 M, with the increase of concentration of electrolyte, the counterion concentration was gradually increased, causing the number of charge within the gel gradually being blocked. Thus, the response rate of the gel was slower, and the bending degree decreased. Q1M5 has a similar bending behaviors to S1M5 in these electrolyte concentrations, that is, the actuation rate peaked at 0.05 M and it increased with the increasing concentration of Na2SO4 aqueous solution until the concentration was 0.05 M, and then decreased with the increasing C

Na2SO4

above 0.05 M. However, at the

same actuating time, the S1M5 hydrogel has a higher bending degree and it can be more easily actuated than the Q1M5 hydrogel, which may be caused by the different mechanical properties (the S1M5 hydrogel had better toughness and strength than the Q1M5 hydrogel). We also investigate the effect of electric field strength on the actuation of the polyelectrolyte QxMy and SxMy nanomicelle hydrogels. The relationship between


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the bending angles of these nanomicelle hydrogels and the strength of the applied electric field in a 0.05 M Na2SO4 aqueous solution at room temperature were investigated (Figure 6). The electric field strengths were 143 V/m, 214 V/m, 286 V/m, and 357 V/m. Applying an electric field provided energy for bending of the gel, and the bending angle of gel changed significantly with electric field strength. For a given distance between anode and cathode, the bending angle was significantly dependent upon the variations in voltage (or electric field). The gradient slope in the plot of the bending angle versus time becomes steep for a short time and the bending angle increases remarkable with an increase in the applied field strength. The bending degree of the gel (Y) relates with the counterions migration rate (h), with the increase of voltage, the counterions migration rate increases. As a result, the gel bending angle and the actuation rate increased (Figure 7). The actuation rate in the electric field decreased with the decrease of the AMPS concentrations (CS) or DMAEA-Q (CQ) except when the field strength was 357 V/m (Figure 8). The bending degree of the gel correlate with the concentrations of counterions of polyion (Cp) and the Young’s modulus (E) of the gels, with the decrease of the AMPS concentrations or DMAEA-Q, the Young’s modulus increases, and thus the gel bending angle and the actuation rate decreased. However, when the field strength was 357V/m, the actuation rate increased with the decrease of the AMPS concentrations or DMAEA-Q perhaps due to the influence of the counterions migration rate (h) was higher at the low concentrations of counterions of polyion than the Young’s modulus (E) of the gels when the field strength was 357V/m.



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4. CONCLUSIONS In this study, we successfully synthesized a series multi-responsive polyelectrolyte hydrogels

by

in

situ

copolymerization

of

acrylamide

(AAm)

and

2-acryloylamino-2-methyl-1-propanesulfonic acid (AMPS) with the F127DA nanomicelles as soft macro-crosslinkers. These hydrogels presented high compression and tensile properties due to the energy dissipation of physical crosslinking instead of chemical crosslinking. Moreover, such nanomicelles hydrogels exhibited significant electric field sensitivity when subjected to an electric field in Na2SO4 aqueous solution. The hydrogels bent in electric field, and the bending angle was cyclically reversed upon changing bias direction. The effect of electrolyte concentration, electric field strength of the applied voltage and ionic monomers content on the electric actuation behavior of these electroactive hydrogels have been systematically investigated. Such strong and tough nanomicelle-crosslinked hydrogels may have great potentials in sensors, actuators, switches, and artificial muscles.

AUTHOR INFORMATION Corresponding Author *J. Fu. E-mail: [email protected]. † These authors contributed equally to this work. Funding Sources This work was supported by the Ministry of Science and Technology of China


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(2016YFC1101902), the Natural Science Foundation of China (21574145), the Zhejiang Natural Science Foundation of China (LR13B040001) and the Ningbo Natural Science Foundation (2015A610012, 2015A610025). Notes The authors declare no competing financial interest.



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Scheme 1. (a) The chemical structures of F127DA, AMPS, DMAEA-Q, AAm, and SxMy/QxMy hydrogel. (b) Schematic illustration of the synthesis of the SxMy and QxMy hydrogels with the micelles as macro-crosslinkers.


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Figure 1. (a) Compressive stress–strain curves of the nanomicelle hydrogels with different AMPS concentrations (CS). (b) Young’s modulus of the SxMy hydrogels. (c) Tensile stress–strain curves of the nanomicelle hydrogels with different CS. (d) The dependence of tensile stress and fracture toughness of hydrogels with different CS.



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Figure 2. Swelling of S1M8 hydrogel in solutions with different ion strength.


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Figure 3. (a) Schematic description of the electric field actuation setup for the electroactive hydrogels immersed in salt solution. The bending angle (θ) is defined as the angle deviated from the initial position of the gel. (b) Bending behavior of the S1M5 hydrogel. (c) Bending behavior of the Q1M5 hydrogel.



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Figure 4. Reversible bending behavior of the S1M5 (a) and Q1M5 (b) hydrogels in 0.05 M Na2SO4 aqueous solution with a voltage of 20 V.


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Figure 5. Bending behavior of the S1M5 (a) and Q1M5 (b) hydrogels with different Na2SO4 concentrations: 0.01 M, 0.05 M, 0.10 M, and 0.15 M. The applied voltage is 20 V. (c) The effect of ion strength on actuation rate.



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Figure 6. Bending kinetics of the Q1M8 (a), and S1M8 (b) hydrogels as a function of the strength of electric field in 0.05 M Na2SO4 aqueous solution.


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Figure 7. Effect of electric field strength on actuation rate: (a) the SxMy hydrogels, (b) the QxMy hydrogels.



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Figure 8. Effect of charge density on actuation rate: (a) the SxMy hydrogels, (b) the QxMy hydrogels.


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Table 1. Formulations for the synthesis of F127DA micelle-crosslinked hydrogels (SxMy) through in situ copolymerization of AAm and AMPS.

Sample

AMPS(g)

AAm(g)

F127DA (mol/l)

AMPS: AAm

Water content (%)

S1M5

1.727

2.962

6×10-3

1:5

68.10%

S1M8

1.151

3.159

6×10-3

1:8

69.90%

S1M11

0.863

3.258

6×10-3

1:11

70.80%



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