Programmable and Bidirectional Bending of Soft Actuators Based on

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Programmable and Bidirectional Bending of Soft Actuators Based on Janus Structure with Sticky Tough PAA-clay Hydrogel Lei Zhao, Jiahe Huang, Yuancheng Zhang, Tao Wang, Weixiang Sun, and Zhen Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00138 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

Programmable and Bidirectional Bending of Soft Actuators Based on Janus Structure with Sticky Tough PAA-clay Hydrogel

Lei Zhao,† Jiahe Huang,† Yuancheng Zhang,† Tao Wang,*,† Weixiang Sun,† and Zhen Tong*,†,‡

†Research Institute of Materials Science and ‡State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China.

KEYWORDS:

Tough PAA-clay hydrogel; soft actuator; bilayer; Janus structure;

bidirectional actuation

1

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ABSTRACT:

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Facile preparing, rapid actuating, and versatile actions are great challenges

in exploring new kinds of hydrogel actuators.

In this paper, we presented a facile sticking

method to prepare Janus bilayer and multilayer hydrogel actuators benefited from a special tough and adhesive PAA-clay hydrogel.

Combining physical and chemical cross-linking

reagents, we endowed the PAA gel with both toughness and adhesion. reinforced by further cross-linked with Fe3+.

This PAA gel was

These two hydrogels with different

cross-linking densities exhibited different swelling capabilities and moduli in the media manipulated by pH and ionic strength, thus acted as promising candidates for soft actuators. Based on these gels, we designed hydrogel actuators of rapid response in several minutes and precisely controlled actuating direction by sticking two hydrogel layers together.

Elaborate

soft actuators such as bidirectional bending flytrap, gel hand with grasp, open, and gesturing actions, as well as word-writing actuator were prepared.

This method could be generalized

by using other stimuli-responsive hydrogels combined with the adhesive PAA gel, which would open a new way to programmable and versatile soft actuators.

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INTRODUCTION Hydrogel actuators are new soft devices that are able to spontaneously perform designed actions in response to the environmental stimulus (e.g., pH, heat, light, chemicals, electric, magnetic, etc.).1-2

Because of the versatile stimuli-response properties and similar water

content to biological tissues, hydrogel actuators find broad potential applications in the biomedical and soft robot fields.3

The actions and movements of stimuli-response hydrogel

actuators are usually achieved on the asymmetrical deformation of inhomogeneous hierarchical hydrogel systems.2, 4 Up to now, several strategies have been developed to prepare smart hydrogel actuators by constructing a gradient distribution of responsive components across the hydrogel or designing bilayer, multilayer or other anisotropic structures.5-13

Aida et al. presented a

thermo-responsive L-shaped hydrogel with an oblique titanate nanosheet configuration, which could walk on a flat horizontal upon alternate heating and cooling due to the anisotropic shrink and extension in different directions of the hydrogel.6

Wang and co-workers

prepared inhomogeneous hydrogels with different surface compositions by using a mold made with a hydrophilic glass plate and a hydrophobic Teflon plate, and the different swelling rates of these two surfaces caused the bending actuation.7

Chu et al. reported a

thermo-responsive hydrogel with an asymmetrical distribution of nanoclays in the cross-section by a two-step photo-polymerization, which acted as a temperature-controlled manipulator.8

These strategies endow hydrogels with inhomogeneous structure, but the

fabrication processes are inconvenient and the gradient distributions are hardly gained precisely. Another effective strategy to establish hydrogel actuators with inhomogeneous structure is to build a Janus bilayer or multilayer structure through chemical or physical interactions. Fu et al. assembled two oppositely charged nanocomposite hydrogels together through

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electrostatic attraction to prepare a bilayer actuator responsive to the electric field, but the tensile strength of the hydrogels was lower than 100 kPa.9 infrared

driving

bilayer

hydrogel

actuator

We previously prepared an

based

on

the

graphene

oxide-poly(N-isopropylacrylamide) (GO-PNIPAm) hydrogel through a two-step synthesis with the second layer formed on the already formed first layer.14

Chen and co-workers also

reported a thermo-responsive bilayer Al-alginate/PNIPAm hydrogel actuator prepared by the method similar to ours.10

Chu et al. showed a bilayer hydrogel actuator with different

monomer contents in each, which was constructed by stacking two layers together, dried under atmospheric condition to create new hydrogen bonding interaction between two layers, and re-swollen.15

The above bilayer hydrogel actuators actuation, though complicatedly

prepared, showed only one direction bending.

Recently, Zhao et al. gave an example of

bilayer actuators with reversible bidirectional bending actuation through host-guest supramolecular interaction between β-cyclodextrin (β-CD) and ferrocene.16

Their actuator

showed versatile actuating motions, but the monomer synthesis was also complicated and the actuations usually took tens of minutes.

Thus, facile preparing method, fast actuation, as

well as bidirectional deformation are still of great challenges in exploring new kinds of hydrogel actuators. In this paper, we report a new method for preparing Janus bilayer hydrogel actuators by sticking two layers with different cross-linking densities with one layer acting as adhesive. The combination of physical cross-linker clay and chemical cross-linker in the poly(acrylic acid) (PAA) hydrogel produces balanced mechanical and adhesive properties.

As the

results, this Janus bilayer hydrogel actuator shows rapid reversible bidirectional bending actions triggered by change in pH or ionic strength.

We are expecting to provide a

convenient way to produce versatile soft actuators.

4


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EXPERIMENTAL SECTION Monomer acrylic acid (AA, stabilized with 200 ppm MEHQ, Aladdin

Materials. Industrial

Inc.),

hectorite

clay

of

gel-forming

grade

(LAPONITE®

XLG,

Mg5.34Li0.66Si8O20(OH)4, Rockwood Ltd.), N,N'-methylenebisacrylamide (BIS, Alfa Aesar), potassium peroxydisulfate (KPS, K2S2O8, 99%, J&K), methyl blue (C37H27N3Na2O9S3, Tianjin Tensing Fine Chemical Research Develop Centre), ferric chloride hexahydrate (FeCl3⋅6H2O), and hydrochloric acid (HCl) (Guangzhou Chemical Reagent Factory) were used as received.

All other chemicals were analytical grade reagents.

Pure water

(resistivity 18.2 MΩ cm) was produced by deionization and filtration using a Millipore purification apparatus and bubbled with argon gas for 3 hours to remove O2 prior to use.

Hydrogel Synthesis.

Poly(acrylic acid)-clay hydrogels (PAA gels) were synthesized by in

situ free radical polymerization of AA in a clay suspension as shown in Figure 1.

Firstly,

desired amount of clay, monomer AA, chemical cross-linker BIS, and initiator KPS were dissolved in deionized water to obtain a homogeneous and transparent solution.

Then, the

solution was transferred into a cylindrical mold with diameter of 3 mm and length of 120 mm or a laboratory made mold of 80 mm × 80 mm × 1 mm, and placed in an oven purged with argon gas.

The polymerization was conducted at 60 °C for 4 h.

The synthesized

hydrogels (Figure S1 in Supporting Information) are named as AxCyBz, where x, y and z stand for the AA in mol/L, clay in wt% of water, and BIS in 100 × wt% of AA.

For

example, the A4C2B2 gel contains 4 mol/L of AA, 2 wt% of clay in the solution, and 0.02 wt% of BIS in AA.

For comparison, hydrogel cross-linked only by either clay or BIS was also

synthesized under the same conditions, which is named as AxCy or AxBz.

Methyl blue

was added to the reaction solution at a concentration of 1.5 mg/mL to dye the hydrogel for easy observation whenever needed.

The PAA-Fe3+ gel (F-PAA gel, named as F-AxCyBz)

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was prepared by immersing the PAA gel in an aqueous solution of 0.06 M FeCl3-0.2 M HCl for 5 hours (Figure 1) resulting in reinforcement of the hydrogel by accessional Fe3+ cross-linking, similar to our previous work.17

Figure 1.

Schematic illustration of preparation of PAA-clay hydrogels, reinforcement by

Fe3+ cross-linking, and sticky assembly to a bilayer hydrogel actuator.

The bilayer hydrogel actuator was prepared by a one-step sticking (Figure 1), other than the traditional two-step method,14 in which the second layer of hydrogel was synthesized on the previously synthesized layer.

Due to the adhesive capability of the PAA gel, the F-PAA

gel layer was adhered to the PAA gel layer tightly to produce the bilayer hydrogel actuator. This adhered state can be kept even immersed in pure water for more than 12 h.

The

actuators with different shapes were prepared by cutting the bilayer hydrogel according to design, such as a strip or a hand-shape.

Multilayer actuators were also fabricated in the

similar way by stacking required layers together with the PAA gel layer in between.

Characterization.

Tensile strength of the hydrogel was measured with a Shimadzu AG-X

plus testing system at ambient temperature on cylindrical samples with diameter of 3 mm after specified treatments.

The sample length between the jaws was about 30 mm, and the

crosshead speed was 100 mm/min.

The tensile strain was taken as the length change

related to the original length, and the tensile stress was evaluated with the cross section of the 6


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original sample. The T-peel strength test was conducted at room temperature according to the Chinese Standard GB/T 2791-1995 with a change in the sample length to examine the adhesive strength between the PAA and F-PAA gels.

A Shimadzu AG-X plus testing system was

used with a crosshead speed of 100 mm/min by clamping the bent and unbounded ends of the specimen.

The specimen consisted of two pieces of F-PAA gel adhered by one piece of

PAA gel in between with size of 100 mm × 25 mm × 1 mm. The actuator was constituted of the A4C2B2/F-A4C2B2 bilayer hydrogel, which was cut into a strip of 70 mm × 2 mm × 2 mm or designed shapes.

The actuation was detected

in aqueous solutions of 0.05 M HCl, 0.05 M NaOH, saturated NaCl, and buffers with desired pH and ionic strength (pH = 7 by Na2CO3/NaHCO3, pH = 9 and 11 by Tris/HCl, the ionic strength was adjusted by NaCl). Swelling ratio SR of the hydrogel was determined by the weight change as SR = (Wt -

Wd)/Wd, where Wt and Wd were the weight of the hydrogel swollen to time t and the weight of the dried hydrogel, respectively.

The gel samples of 70 mm × 2 mm × 1 mm were

immersed in large excess of solutions at 20 °C for 3 h, and weighed every 15 min.

RESULTS AND DISCUSSION Mechanical and Adhesive Properties.

Homogenous and transparent clay and BIS

cross-linked PAA-clay gels (Figure S1) are obtained with satisfactory mechanical property, which means that the clay suspension containing the ionic AA monomer is stable during the reaction.

FT-IR and EDS spectra of the PAA-clay gel, clay and AA monomer in Figure S2

indicate successful synthesis of the PAA-clay hydrogels (detailed description in Figure S2). Figure 2 shows the effect of clay and BIS contents on the tensile properties of the PAA gels.

The gels cross-linked with both BIS and clay exhibit significant high strength, but the

7



gels cross-linked merely by clay (A4C2) or BIS (A4B3) are quite weak.

For example, the

tensile strength of the A4C2B3 gel is ~0.34 MPa, about 6 times higher than that of the A4C2 or A4B3 gel, and the breaking strain is nearly 2000% for A4C2B3, similar to that of A4C2 but much higher than that of A4B3.

This should be attributed to the synergistic cross-linking

effect of the physical cross-linker clay and the chemical cross-linker BIS.18

Dynamic

mechanical data of the PAA gels in Figure S3 show that both the storage modulus G' and loss modulus G" of the A4B3 and A4C2B2 gels are almost unchanged over the testing frequency range with G’ much higher than G”, indicating the predominant elastic response of the The effective network chain density N of the A4C2B2 gel is higher than that of

hydrogels.

the A4B3 gel reflecting the higher cross-linking density in the former contributed by the clay.19

When increasing BIS content from 0.01 wt% to 0.03 wt% of AA, the strength

increases obviously and the strain at break decreases due to the increase of cross-linking density (Figure 2A).

As for the hydrogels with different clay contents, the maximum

strength and strain at break are achieved at clay content of 2 wt% for probably appropriate cross-linking density (Figure 2B); because high clay content leads to high modulus of the hydrogel, resulting in low elongation.19

0.4

0.4

A 0.3

0.2

B A4C2B2

Stress (MPa)

A4C2B3

Stress (MPa)

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A4C2B1 A4B3

0.1

A4C2B2

0.3 A4C3B2 0.2

A4C1B2 0.1

A4C2 0.0

0.0 0

500

1000

1500

2000

0

2500

500

Strain (%)

Figure 2.

1000

1500

2000

2500

Strain (%)

Tensile stress-strain curves of the PAA hydrogels with different BIS (A) and clay

(B) contents, A4C2 and A4B3 in A are the hydrogels cross-linked only with either clay or BIS. 8


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The PAA hydrogel can be reinforced by introducing physical cross-linking of PAA-Fe3+ through the ionic interaction between carboxyl groups and Fe3+ ions.17, 20-22

But a hard

shell would appear when the PAA hydrogels are directly immersed in FeCl3 solution, which prevents further cross-linking of the interior part of the hydrogel. binding of the AA groups with Fe3+ ions at the gel surface.

This is caused by the fast

Therefore, HCl is added into

the FeCl3 solution, for the binding of Fe3+ to the PAA is decelerated at low pH.23-26

The

competitive binding of Fe3+ and H+ to the -COO- groups of PAA shifts to form -COOH at high H+ concentration.

From tensile results (Figure S4), the hard shell still exists at low HCl

concentration (0.1 M), while high HCl concentration (≥ 0.3 M) causes a fast decrease in the tensile strength.

The optimal HCl concentration of 0.2 M was chosen for the treating

solution of 0.06 M FeCl3-0.2 M HCl solution (FeCl3/HCl for short) to reinforce the PAA hydrogels according to our previous study.17

Figure 3 depicts that the strength of the

A4C2B2 gel increases with the immersing time in FeCl3/HCl.

The strength of F-A4C2B2

is 1.7 MPa after immersed for 1 h, and increases to about 4 MPa when immersed for 5~6 h, At the same time, G' and effective network

while the strain at break is around 2000%.

chain density N of the F-A4C2B2 gel are greatly increased after the immersing treatment, while the molecular weight between cross-linking points Mc decreases correspondingly (Figure S3), confirming the formation of additional cross-linking points.

The increase of

G" with lowering shear frequency reflects the relaxation of the physical cross-linking at long time.

Based on the above results, all the F-PAA hydrogels used in the following work were

all prepared by immersing in FeCl3/HCl for 5 h.

9



5 6h

Stress (MPa)

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5h

4 4h 3 3h 2h

2 1h 1 original 0 0

500

1000

1500

2000

2500

Strain (%)

Figure 3.

Tensile stress-strain curves of the A4C2B2 before and after being immersed in

FeCl3/HCl solution for different times.

Additional to the good mechanical properties, the PAA hydrogel also exhibits good adhesive capability, which provides a facile way to construct bilayer or multilayer hydrogel actuators.

T-peel test was carried out to explore the adhesive strength between the PAA and

F-PAA gels as schematically illustrated in Figure 4A.

Figure 4B presents curves of the

peeling force for unit length versus the displacement during the T-peel test. gel behaves with adhesive strength of ~200 N/m.

The A4C2B2

For comparison, the A4B3 gel merely

cross-linked by BIS shows low adhesive strength of ~75 N/m, while the A4C2 gel cross-linked by clay expresses high adhesive strength of ~400 N/m.

Insert photos in Figure

4B present that the failure style changes from adhesive failure (bottom) to cohesive rupture (upper) as increase in the adhesive strength.

This implies that the adhesive capability of the

PAA gel mainly originates from the physically cross-linked PAA-clay structure, which can provide large amount of hydrogen bonding and electrostatic interaction at the interface of the gels to improve the adhesive strength.27

While the low adhesive strength of the chemically

cross-linked PAA gel may be due to the low mobility of the polymer chains in the network. The decrease in adhesive strength of the both clay and BIS cross-linked A4C2B2 gel is ascribed to the increase in the cross-linking density by adding BIS as mentioned above. 10


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The adhesive capability almost disappears for the F-PAA gel after the second cross-linking by Fe3+ due to the excessive cross-linking density and super high strength.

Nevertheless, the

F-PAA can be stuck by the PAA gel because of the enough adhesive capability of the latter (inserts in Figure 4B).

These results indicate that the adhesion of PAA gels can be adjusted

by regulating the content of clay and BIS.

For the present work, the A4C2B2 gel was

chosen to satisfy both mechanical and adhesive requirements.

500

Adhesive strength (N/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


B

A4C2

400 300 A4C2B2 200 A4B3

100 0 0

100

200

300

Displacement (mm)

Figure 4.

(A) Schema of the T-peel test sample; (B) curves of the adhesive strength (peeling

force per unit length) versus displacement for the PAA gels during the T-peel test.

Inserts

are photos of the corresponding samples during peeling.

Actuating Behavior of the Bilayer Actuator.

Strong F-PAA gels were fabricated from the

PAA gels by further cross-linking with Fe3+ ions.

These two kinds of hydrogels with

different cross-linking density were used to prepare Janus bilayer actuators, taking advantage of the adhesive capability of the PAA gel.

The difference in the swelling rate and swelling

ratio caused by different cross-linking densities of the hydrogels results in asymmetrical deformation of the Janus bilayer actuator.7, 9-10, 28

In the present work, the A4C2B2 and

F-A4C2B2 gels were applied in the bilayer and multilayer hydrogel actuators.

We define

the actuating degree as (L0 - L)/L0 to measure its bending extent, where L0 is the length of the sample, and L is the end-to-end distance of the sample during actuating as shown in Figure

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5.14

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The present actuator has two action directions when soaked in different solutions, so

plus (+) and minus (-) symbols are assigned to the bending direction to the PAA side and F-PAA sides, respectively.

Figure 5.

Schema and the corresponding photos for reversible actuation of the

A4C2B2/F-A4C2B2 Janus bilayer hydrogel of 70 mm× 2 mm× 2 mm in different solutions fixed by a clip for easy observation.

Symbols “+” and “-” stand for bending direction to the

PAA side and F-PAA side, respectively.

Figure 6A shows the actuation degree of the bilayer hydrogel A4C2B2/F-A4C2B2 in buffer solutions at pH = 7, 9, and 11 (ionic strength I = 0.01 M).

It is unexpected that the

bilayer hydrogel bends towards PAA side at pH = 9 and 11 with the actuation rate faster at pH =11 than that at pH = 9, while it bends towards F-PAA side at pH = 7.

This action is driven

by the unbalanced swelling of the two layers in the hydrogel swollen in the specified medium. The swelling ratio and swelling rate of ionic hydrogels depend on their cross-linking density,19 and on pH and ionic strength of the medium.29-30

As displayed in Figure 6B, the

PAA gel swells faster with higher swelling ratio than that of the F-PAA gel compared at the same pH and ionic strength due to the higher cross-linking density in the latter contributed by 12


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the Fe3+-COO- binding.

The F-PAA gel is hardly swollen at pH = 7, but the PAA gel swells,

driving the bilayer actuator bending towards the F-PAA side.

At pH = 9 and 11, on the

other hand, both the PAA and F-PAA gels swell, but the modulus of the F-PAA gel is much higher than that of the PAA gel.

Ultimately, the volume expansion of the swollen F-PAA

gel drives the bilayer actuator bending towards the PAA side, because the soft swollen PAA gel cannot stretch and bend the F-PAA gel back.

This fact implies that the bending

direction is determined by the volume change of the component gel with higher modulus in the two layers.

100

pH = 11

A

80

pH = 9

I = 0.01 M

60 40

PAA

20

F-PAA

+

pH=9

B

200

PAA F-PAA

100 80

SR

Actuation Degree (%)

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


pH=11

I = 0.01 M pH=7

60

0

-40

pH = 7

50

pH=9

20

F-PAA

-60 0

pH=11

40

_

-20

PAA 100 150 200

pH=7 0

400

600

0

800 1000

Figure 6.

1

2

3

Time (h)

Time (s)

(A) Actuating degree of the bilayer hydrogel A4C2B2/F-A4C2B2 in the buffer

solutions of indicated pH at ionic strength of I = 0.01 M; (B) the swelling ratio SR of the component hydrogels A4C2B2 and F-A4C2B2 in the corresponding buffer solutions.

According to the above finding, it suggests that the bending direction of the bilayer actuator would be controlled by regulating the swelling ratio and modulus of the component hydrogels.

To test this idea, we changed the ionic strength to adjust the swelling ratio of

the gels.

Figure 7A manifests the actuation degree of the bilayer hydrogel

A4C2B2/F-A4C2B2 actuator at pH = 9.

When the ionic strength is increased from 0.01 M

to 0.6 M, the bending direction turns from the PAA gel side to the F-PAA gel side. 13


Most


interesting behavior is that at I = 0.2 M the bending is towards the F-PAA gel side (–) at beginning and then reverses to the PAA gel side (+) when swelling beyond 550 s. The swelling behavior in Figure 7B confirms our expectation that the swelling ratio of the PAA gel decreases obviously with the increase in ionic strength, reducing the modulus decline induced by swelling.

Furthermore, the F-PAA gel slightly shrinks at I = 0.6 M,

causing bending towards the F-PAA gel side.

At I = 0.2 M, the F-PAA gel shrinks at

beginning and swells at ~800 s as known from its length.

This shrinking and swelling

result in the reverse actuation, indicating again that the bending is induced by the volume change of the component gel with higher modulus.

Consequently, the actuation direction

and rate can be regulated by pH and ionic strength of the medium.

300

100

A

80

I = 0.01 M

60

B

200

pH = 9

pH = 9

100

40

PAA

20

I = 0.01 M

+

40

I = 0.2 M

SR


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

Page 14 of 24

F-PAA

0 -20

20

_

-40 F-PAA

-60 -80

I = 0.6 M 0

50

I = 0.6 M

I = 0.2 M

PAA

0

100 150 200

400

600

0

800

1

Figure 7.

2

3

Time (h)

Time (s)

(A) Actuating degree of the A4C2B2/F-A4C2B2 bilayer hydrogel in the buffer

solutions of indicated ionic strength at pH = 9; (B) the swelling ratio SR of the gels A4C2B2 (solid symbols) and F-A4C2B2 (open symbols) in the corresponding buffer solutions.

According to the above principle, a facile way in regulating the actuation of the bilayer hydrogel was developed by alternately swelling in 0.05 M NaOH solution and recovering in saturate NaCl solution.

As demonstrated in Figure 8, the bilayer hydrogel actuator bends in

0.05 M NaOH solution to actuation degree of 80% in 70 s and completely recovers in saturate

14


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NaCl solution in only 10 s, achieving a fast actuation (Movie M1 in Supporting Information). Moreover, the reversible actuation has a perfect repeatability.

The swelling ratio in the

lower panel of Figure 8 indicates that the actuation is driven by the swelling and shrinking of the F-PAA gel.


100 NaCl

NaCl NaOH

NaOH

NaOH

80 60 0.05 M NaOH

40

Saturated NaCl

PAA F-PAA

20 0 6 A4C2B2

SR

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


4 F-A4C2B2

2

0

70 80

150160

230

Time (s)

Figure 8.

Reversible actuation of the A4C2B2/F-A4C2B2 bilayer hydrogel (upper panel)

and the swelling ratio of the component A4C2B2 and F-A4C2B2 gels in 0.05 M NaOH and saturated NaCl solutions.

The recovery of the bent bilayer hydrogel actuator is also realized by immersing in a HCl solution.

For example, it takes about 30 s and 450 s for a bent actuator to return its

original shape in 0.05 M HCl solution and in pure water, respectively (Figure S5 and Movies M2 and M3).

But the swelling ratio continuously becomes high during this process, thus

the repeat number is limited.

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Programming and Assembling of Actuation.

Controllable and programmable action is

necessary for practical soft actuators and robots.

Inspired by the above rapid and reversible

bidirectional actuations of the bilayer hydrogels, we designed a series of soft actuators capable to finish more complex actions under program control.

A bioinspired bilayer

hydrogel flytrap is demonstrated in Figure 9 with reversible bidirectional actions triggered by the medium.

The bilayer PAA/F-PAA structure allows the flytrap to bend to the PAA side

(+) from flat state in 0.05 M NaOH in 2 min (A→B), and the bent flytrap recovers quickly when immersed in 0.05 M HCl for 1 min (B→C), while the recovered flytrap bends to the F-PAA side (-) in saturate NaCl solution in 1 min (C→D), and recovers again upon immersing in 0.05 M NaOH (D→A).

Therefore, through adjusting the immersing media, the hydrogel

flytrap demonstrates a complete cycle of trap action rapidly and reversibly.

Figure 9.

A bioinspired PAA/F-PAA bilayer hydrogel flytrap with reversible bidirectional

action when immersed in the indicated media; A: original shape; B: actuation towards PAA gel side (“+” direction); C: recovered shape; D: actuation towards F-PAA gel side (“-” direction).

The inserts in B and D are the lateral view of the actuator.

16


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Figure 10.

Programmed actions of the hydrogel actuator: (A) assembled gel strips for

presenting the word “hydrogel”; (B) grasp and open actions of the hand-shaped actuator; (C) gestural hands for signals one, two, three, and four with the finger movements, and the inserts are the schema with red fingers from the F-PAA/PAA bilayer and black part from the F-PAA/PAA/F-PAA trilayer gels.

Furthermore, Figure 10 presents some programmed actions of the hydrogel actuator based on the specific swelling capability.

A designed multilayer hydrogel assembly forms

the word “hydrogel” when immersed in 0.05 M NaOH solution for 2 min (Figure 10A, detailed assembly in Figure S6) depending on the programmed gel layer stacking and position. A hydrogel hand is also designed using the F-PAA/PAA bilayer hydrogel for the fingers and the F-PAA/PAA/F-PAA trilayer hydrogel for the palm (Figure 10B). 17


The bilayer structure


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provides the bending action, and the trilayer structure keeps the flat gel unchanged.

Thus,

the hydrogel hand grasps and opens reversibly like a human hand by alternately immersing in 0.05 M NaOH and saturate NaCl solutions (Figure 10B and Movie M4).

Similarly, the

gestural hands for signals one, two, three, and four are achieved successfully by ingenious combination of the gel layers on necessary (Figure 10C). In order to generalize the concept mentioned above, other hydrogels were tested to assemble bilayer or multilayer soft actuators with adhesion of the PAA gel.

The

thermosensitive graphene oxide-clay-PNIPAm hydrogel used in our previous work31 was adopted, instead of the F-PAA gel, to build a PAA/PNIPAm bilayer hydrogel actuator.

As

illustrated in Figure 11, bending action of this actuator is induced by either pH increase (0.05 M NaOH for 20 s) or heating (80 °C water for 5 s).

Figure 11.

Actuation of the PAA/PNIPAm bilayer hydrogel actuator in 0.05 M NaOH

solution and hot water (~ 80 °C).

CONCLUSIONS In conclusion, we presented a facile sticking method to prepare Janus bilayer and multilayer hydrogel actuators by taking advantage of the adhesive PAA gel.

Combining two

cross-linking of clay and BIS, we endowed the PAA gel with both toughness and adhesion. This PAA gel was reinforced by further cross-linking with Fe3+ ions to form the F-PAA gel These hydrogels with different cross-linking densities exhibited different swelling capabilities 18


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and moduli in the media of different pH and ionic strengths, thus acted as ideal candidates for soft actuators to achieve specified actions.

Based on this idea, we prepared several

hydrogel actuators with rapid response and precise control of the acting direction, such as bidirectional bending flytrap, gel hand with grasp, open, and gesturing actions, as well as word-writing actuator.

In addition to the F-PAA gel, thermo-responsive hydrogel was also

adhered by the PAA gel to assemble the hydrogel actuator driven by heating.

This method

will open a new way in designing soft actuators with programmable and versatile properties.

ASSOCIATED CONTENT Supporting Information Photo of the PAA gel, FT-IR and EDS spectra, viscoelastic measurement, effective network chain density and molecular weight between cross-linkers of the PAA-clay gels, stress-strain curves of the PAA gel in different Fe3+ solutions, the bending and recovery actuation of the bilayer actuator, schema of designed assembly form of word writing actuators and movies of different actuating process.

This material is available free of charge via the Internet at

http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: E-mail: [email protected] (T. Wang) E-mail: [email protected] (Z. Tong) Tel: (86)-20-87112886; Fax: (86)-20-87110273 Notes The authors declare no competing ﬁnancial interest. 19



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ACKNOWLEDGEMENTS The financial support from the NSFC (51573060 and 21427805) and the Pearl River S&T Nova Program of Guangzhou (201710010146) is gratefully acknowledged.

The authors

thank Prof. Yuxi Jia of Shandong University for the valuable discussion about the actuation mechanism of the hydrogel actuators.

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Programmable and Bidirectional Bending of Soft Actuators Based on

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