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Smart Hydrogels with Inhomogeneous Structures Assembled Using Nanoclay-Crosslinked Hydrogel Subunits as Building Blocks Chen Yao, Zhuang Liu, Chao Yang, Wei Wang, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07713 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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ACS Applied Materials & Interfaces

Figure 1 177x140mm (300 x 300 DPI)

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Smart Hydrogels with Inhomogeneous Structures Assembled Using Nanoclay-Crosslinked Hydrogel Subunits as Building Blocks Chen Yao,† Zhuang Liu,*,† Chao Yang,† Wei Wang,†,‡ Xiao-Jie Ju,†,‡ Rui Xie,†,‡ and Liang-Yin Chu*,†,‡,§

†

School of Chemical Engineering, Sichuan University, No. 24, Southern 1 Section, Yihuan

Road, Chengdu, Sichuan 610065, P. R. China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu,

Sichuan 610065, P. R. China §

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing,

Jiangsu 211816, P. R. China

1 ACS Paragon Plus Environment

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ABSTRACT A novel and facile assembly strategy has been successfully developed to construct smart nanocomposite (NC) hydrogels with inhomogeneous structures using nanoclay-crosslinked stimuli-responsive hydrogel subunits as building blocks via rearranged hydrogen bonding between

polymers

and

clay

nanosheets.

The

assembled

thermo-responsive

poly(N-isopropylacrylamide-co-acrylamide) (poly(NIPAM-co-AM)) hydrogels with various inhomogeneous structures exhibit excellent mechanical properties due to plenty of new hydrogen bonding interactions created at the interface for locking the NC hydrogel subunits, which are strong enough to tolerate external forces such as high levels of elongations and multi-cycles of swelling/deswellling operations.

The proposed approach is featured with

flexibility and designability to build assembled hydrogels with diverse architectures for achieving

various

responsive

deformations,

which

are

highly

promising

for

stimuli-responsive manipulation such as actuation, encapsulation and cargo transportation. Our assembly strategy creates new opportunities for further developing mechanically strong hydrogel systems with complex architectures that composed of diverse internal structures, multi-stimuli-responsive properties, and controllable shape deformation behaviors in the soft robots and actuators fields. KEYWORDS Stimuli-responsive materials; Hydrogels; Actuators; Nanocomposite materials; Hydrogen bonding



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INTRODUCTION Smart hydrogels with controllable volume/shape changes in response to external stimuli such as temperature,1-4 pH,5-7 light,8-10 special ions or molecules,11-13 ionic strength or electric field,14-18 have been considered to be versatile in various applications in biological and chemical fields.19-23

Smart hydrogels with inhomogeneous structures enable their shapes to

achieve designable and controllable deformations due to the asymmetrical responsive properties of different parts of hydrogels.24-30

Because of the asymmetric deformations,

such smart hydrogels have been successfully developed as biomimetic soft robotics for vehicles, manipulators and crawlers.3,8,31-35

For controllable stimuli-triggered manipulation

such as actuation, locomotion and cargo transportation, excellent mechanical properties such as high strength, elasticity and toughness are indispensable to tolerate external forces.36 Therefore, development of smart hydrogels with inhomogeneous structures and excellent mechanical properties is of significant scientific and technological importance. Up to now, several strategies have been developed for fabricating smart hydrogels with inhomogeneous structures.

One strategy is controlling the hydrogel preparation conditions

to generate discrepant distribution in the structure of responsive hydrogels.24,26,37-40

For

example, Hayward and co-workers24 have developed a method of halftone gel lithography using photomasks to fabricate temperature-responsive gels.

With highly cross-linked dots

formed in a lightly cross-linked matrix by adjusting irradiation dose, the gel can transform between a flat state and a prescribed three-dimensional shape.

Akashi and co-workers39

have reported a thermo-responsive hydrogel with an asymmetrical pore structure by applicable subsequent silica extraction, which is fabricated via electrophoresis and subsequent photo-polymerization.

The gradient in the structure of the responsive hydrogel

is the driving force behind the bending of the gel.

The inhomogeneous structure designed

via controlling preparation conditions of stimuli-responsive hydrogels advances the


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development of functional gels for intelligent soft-actuator systems.

However, such a

strategy may be laboursome to realize diversified architectures of hydrogels as sophisticated responsive actuator systems.

Alternatively, assembly of different hydrogels via

physical/chemical interaction such as molecular recognition,41-43 could establish smart hydrogels with inhomogeneous structures for diversified actuator systems.

Smart hydrogels

can be facilely designed for creating complex architectures with such an assembly strategy. For example, Xie and co-workers43 have constructed a variety of three-dimensional (3D) responsive material architectures by Lego assembly approach based on the host-guest supramolecular interaction between β-cyclodextrin (β-CD) rings and ferrocene.

The

assembled hydrogels exhibit complex deformation shapes in response to pH and ionic strength.

The

complexity

of

the

shape

stimuli-responsiveness of the building blocks.

changing

behavior

depends

on

the

However, these assembled smart hydrogels

based on host-guest interaction may suffer from poor mechanical properties due to the weak super-molecular force.43-44

Recently, several strategies have been developed to improve the

mechanical properties of smart hydrogels, including generating double-network hydrogels consist of two interpenetrating networks,45-46 synthetizing slide-ring polyrotaxane cross-linkers for fabricating topological hydrogels,47-48 or using exfoliated clay nanosheets as crosslinkers for synthesizing nanocomposite hydrogels.49-53

However, the diversified

assembly of the smart hydrogels fabricated with these strategies is still difficult to be achieved.

Therefore, easy fabrication of smart hydrogels with inhomogeneous structures

and excellent mechanical properties for diversified actuator systems still remains a challenge. In this study, we report on a flexible assembly strategy for fabricating smart hydrogels with inhomogeneous structures and excellent mechanical properties, which combines diverse hydrogel subunits to create complex actuator systems via rearranged strong hydrogen bonding between polymers and clay nanosheets.

Nanocomposite (NC) hydrogels using clay



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nanosheets as crosslinkers have been developed with excellent mechanical property and stimuli-responsive property.52-55

The polymer chains in nanocomposite hydrogels are

connected with clay nanosheets through hydrogen bonding interactions.52-55

The

noncovalent bonds are formed between the amide side groups (-CONH(R)) on the polymers and the SiOH, Si-O-Si units on the surfaces of the clay sheets.52-55

Via rearrangement of

hydrogen bonding interactions between polymer chains and clay nanosheets at the interface of different nanocomposite hydrogel subunits, the hydrogel subunits with different responsive properties can be assembled easily to form diversified inhomogeneous architectures.

Due to

the different responsive properties of the hydrogel subunits, the assembled hydrogel complexes can realize a variety of diversified deformations in response to environmental stimuli, which are highly attractive for developing smart hydrogels with diversified architectures for soft robots, actuators and artificial muscles.

EXPERIMENTAL SECTION Materials. recrystallization.

N-isopropylacrylamide (NIPAM, Sigma-Aldrich) was purified by The Irg.500, which was a mixture of 1-hydroxy-cyclohexyl-phenylketone

(Irg.184, TCI Shanghai) and benzophenone (Ph2CO, Tianjin Bodi Chemicals) with mass ratio

of

1:1,

was

used

as

a

photo-initiator.

The

Laponite

XLG

([Mg5.34Li0.66Si8O20(OH)4]Na0.66, Rockwood), which was used as the clay crosslinker, was dried at 130 °C for 4 h before use. Chengdu Kelong Chemicals.

Acrylamide (AM) and ethanol were purchased from

Methylene blue was purchased from Tianjin Bodi Chemicals,

crystal violet was purchased from Beijing J&K Scientific Ltd., and brilliant green was purchased from Aladdin. further purification.

All other reagents were of analytical grade and used without

Deionized water (18.2 MΩ at 25 °C) from a Milli-Q Plus water

purification system (Millipore) was used throughout the experiments.


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Preparation of NC Hydrogels.

Uniform aqueous solution containing monomers

(NIPAM and AM), crosslinker (Laponite XLG), dyestuff (crystal violet, brilliant green or methylene blue) and photo-initiator (Irg.500) was prepared.

The dyestuff was first dispersed

in water and stirred for about 10 min, and then the clay was added into the dyestuff solution and stirred for 4 h.

Subsequently, monomers NIPAM and AM were added into the dyestuff

and clay suspension, and then the suspension was stirred in an ice-water bath for another 2 h. The concentration of clay was fixed at 5×10-2 mol L-1, and the total monomer concentration was fixed at 1.0 mol L-1.

In our previous study, the effects of clay concentration on the

mechanical and responsive behaviors of the nanocomposite hydrogels have been investigated,38 and the results show that the tensile strength of the nanocomposite hydrogel increases with increasing the clay concentration.

Therefore, in this study we fixed the clay

concentration as 5 mmol/L, because the nanocomposite hydrogels could possess high tensile strength with the clay concentration of 5 mmol/L.38

The concentration of dyestuff (crystal

violet, brilliant green or methylene blue) was fixed at 0.2 g L-1.

The concentration ratio of

photo-initiator (Irg.500) to monomer was fixed at 0.8 wt %. The prepared homogeneous poly(N-isopropylacrylamide-co-acrylamide) (poly(NIPAM-co-AM)) nanocomposite (NC) hydrogel is coded as “NC5[x]”, in which “NC” represents the “nanocomposite”, “5” represents the clay concentration in the hydrogel is fixed at 5×10-2 mol L-1, and “x” stands for the molar percentage of acrylamide (AM) content in the total monomer.

As a typical

example, to prepare a NC5[10%] hydrogel, a transparent aqueous solution was prepared with deionized water (10 mL), brilliant green (2 mg), clay (0.3808 g), NIPAM (1.017 g) and AM (0.071 g).

Then, Irg.500 (8.704 mg) dissolved in 50 µL ethanol solution was added under

the condition of ice-water bath with stirring.

The aqueous reaction mixture was injected

into a mold with space of 50 mm × 50 mm × 1 mm, which was formed by a polytetrafluoroethylene (PTFE) plate, a transparent quartz plate and a PTFE spacer with



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thickness of 1 mm.

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Next, the mixture in the mold was irradiated from the transparent quartz

side under UV light (365 nm) for 5 min in an ice-water bath.

The prepared NC5[10%]

hydrogel was purified with excessive water thoroughly. Preparation of Assembled NC Hydrogels.

The assembly of smart hydrogel subunits

for inhomogeneous structures via rearranged strong hydrogen bonding between polymers and clay nanosheets at the interface of different nanocomposite hydrogel subunits is schematically

illustrated

in

Figure

1.

Typically,

the

hydrogel

subunits

are

poly(N-isopropylacrylamide-co-acrylamide) (Poly(NIPAM-co-AM)) hydrogels crosslinked by clay nanosheets with homogeneous structure, excellent mechanical property and thermo-responsiveness

(Figures

S1-S4,

see

Supporting

Information).

The

nanoclay-crosslinked subunits of NC5[x1] and NC5[x2] hydrogels are stacked together (Figure 1a,b), which are slowly dried under atmospheric condition with relative humidity of 70% at room temperature (25 ºC) for 24 hours.

During the drying treatment, the significant

decrease in thicknesses of subunits causes close contact between neighboring polymer chains and clay nanosheets at the interface of the two nanocomposite hydrogel subunits (Figure 1c). As a result, plenty of new hydrogen bonding interactions between polymer chains and clay nanosheets at the interface of the two nanocomposite hydrogel subunits are formed, which enable the two subunits to firmly connect together even after re-swelling in deionized water at 20 ºC (Figure 1d).

Thus, a hydrogel with inhomogeneous structure, NC5[x1]-[x2], is

assembled via rearranged strong hydrogen bonding between polymers and clay nanosheets at the interface of nanocomposite hydrogel subunits NC5[x1] and NC5[x2].

Assembled NC

hydrogels with diversified architectures could be prepared by the same method.

Such

assembly strategy is flexible to build the hydrogels with diversified architectures using nanoclay-crosslinked subunits with thermo-responsive,52-55 light-responsive,50 and other stimuli-responsive properties,56 which enables to enrich the development of functional


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hydrogels and expand the scope of applications of soft sensors, manipulators, crawlers, artificial muscles, and so on. SEM Characterization of Inhomogeneous NC Hydrogels.

The microstructures of

freeze-dried assembled NC hydrogels were observed by SEM (G2 Pro, Phenom).

To

prepare freeze-dried samples, the assembled NC hydrogel samples in deionized water were frozen in liquid nitrogen for 10 min, which might result in the fissures of the ice due to the rapid freezing.

After lyophilized by a freeze drier (FD-1C-50, Beijing BoYiKang) at -35 °C

for about 48 h, the natural fracture of the sample was formed. Mechanical Property Tests of NC Hydrogels.

The adhesion property of assembled

NC hydrogels was measured using a tensile machine (EZ-LX, Shimadzu) at room temperature.

After deswelling in hot water bath (50 °C) for 5 min, the hydrogel samples in

collapsed state with water content of 80%±6% were cut into dumbbell shapes of standardized JIS-K6251-7 size (length 35 mm, width 2 mm, and gauge length 20 mm) with a sample-cutting machine.

Both ends of the dumbbell-shaped samples were clamped, and

stretched at a constant velocity of 200 mm min-1.

The tensile stress was defined as the force

applied on the deformed hydrogel divided by the real-time characteristic cross-sectional area of the deformed hydrogel.

In general case, the asymmetric deformation of the assembled

hydrogels is caused by the shrinking of the hydrogels according to the stimuli (e.g., temperature change), which results in the application of the hydrogel for biomimetic soft robotics such as vehicles and manipulators.

Due to the shrinking and/or shrinking-induced

bending, the soft robotics can embed or grapple desired objects.

The mechanical properties

of the responsive hydrogels in collapsed state are crucial for holding the objects, so the tensile tests were performed in completely collapsed state. Dynamic Thermo-Responsive Bending Behaviors of Assembled NC Hydrogels. The NC5[0%]-[y] (y=10% or 20%) hydrogels were all cut into strips of 1.0 cm in length and



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0.1 cm in width.

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Then, the NC5[0%]-[y] hydrogel was put in transparent quartz holder that

full of deionized water, which was placed on a heating stage (TS62, Instec).

The

NC5[0%]-[y] hydrogel strip had been equilibrated at 25 °C for more than 30 min to reach the fully swollen state before each dynamic shrinking experiment.

The heating operations were

simply achieved by rapidly changing the temperature from 25 to 50 °C with the thermostatic stage system.

The ambient temperature of NC5[0%]-[y] hydrogels at different time

intervals was measured by a thermocouple.

The whole dynamic shrinking process was

recorded using a digital camera from the top of the quartz holder.

Several NC5[0%]-[y]

hydrogels with the same y value were measured to obtain statistic data for the thermo-responsive volume change characteristics. Reversible and Reproducible Thermo-Responsive Behaviors of NC5[0%]-[20%] Hydrogels. The NC5[0%]-[20%] hydrogel strip was put in a transparent quartz holder that full of deionized water, which was placed on a heating stage (TS62, Instec).

The hydrogel

strip was kept in water at 25 °C for more than 30 min to reach equilibrated swollen state before each dynamic bending experiment.

Subsequently, the temperature was alternated

between 25 and 50 °C with thermostatic stage system.

The heating-cooling cycle was

repeated for five times to test the reversibility and reproducibility of the hydrogel strip.

The

ambient temperature of NC5[0%]-[20%] hydrogel was measured by a thermocouple.

The

whole dynamic bending process was recorded using a digital camera equipped on the top of the quartz holder.

Several NC5[0%]-[20%] hydrogel strips were measured to obtain statistic

data for the thermo-responsive volume change characteristics. Demonstrations

of

NC5[0%]-[20%]

Hydrogels

as

Temperature-Controlled

Actuators. The rectangle and cross-shaped NC5[0%]-[20%] hydrogels were respectively put into glass containers that full of deionized water at 20 °C for more than 30 min to reach the fully swollen state before heating.

Plastic beads were used as model target objects for


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encapsulation.

The ambient temperature of NC5[0%]-[20%] hydrogels was measured by a

thermocouple.

Furthermore, the cross-shaped NC5[0%]-[20%] hydrogel was connected to a

string with two pieces of thin magnetic sheet.

After being kept at 19 °C for more than 30

min to reach the equilibrated swollen state, the hydrogel was transferred into a hot water bath (48 °C) to grasp a PTFE block.

The hydrogel was then transferred back into the cold water

bath (19 °C) to release the PTFE block.

All the triggered manipulation processes of

hydrogels were recorded using a digital camera.

RESULTS AND DISCUSSION Preparation and Characterization of Inhomogeneous NC Hydrogels.

In this study,

the NC5[0%] (purple, Figure 2a), NC5[10%] (green, Figure 2b) and NC5[20%] (blue, Figure 2c) hydrogels are selected as subunits, which can be assembled to build complex hydrogels with diversified architectures, as shown in Figure 2d-r.

After drying treatment for the

rearrangement of hydrogen bonding between polymer chains and clay nanosheets at the interface of hydrogel subunits, NC hydrogels with inhomogeneous structures such as NC5[0%]-[10%] and NC5[0%]-[20%] can be prepared with various shapes or architectures (Figure 2d-r).

The results show that our assembly strategy is flexible and applicable to

design and fabricate NC hydrogels in terms of stimuli-sensitivity and complex architecture. The interfacial microstructures of three assembled hydrogels, NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%], are observed by scanning electron microscope (SEM) to show the combination of the subunits by the hydrogen bonding rearrangement strategy. As shown in Figure 3, the assembled NC hydrogels show uniform honeycomb-like structure with dense cell walls, which are resulted from the ice crystals that presented in the swollen hydrogels acting as templates to generate pores in the freeze-drying process for preparing SEM samples.57

At the interface of the subunits, plenty of crosslinks are newly formed



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across the interface due to the hydrogen bonding re-interactions of polymer chains and neighboring clay nanosheets at the interface during the drying treatment (Figure 3a-c).54

As

a result, the re-associated hydrogen bonding interactions tightly lock the two hydrogel subunits together, which are sufficient to tolerate several swelling-deswelling cycles in response to temperature variation.

The SEM images show that the subunits in assembled

NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels are still combined very strongly at the interface after ten cycles of swelling-deswelling (Figure 3d-f). Furthermore, the tensile tests of assembled NC hydrogels are systematically investigated at room temperature.

As shown in Figure 4a, the NC5[0%]-[10%] hydrogel sample (Figure

2m) is combined very well by the rearranged hydrogen bonding, and is strong enough to withstand stretching even twisting (Figure 4a, see Supporting Information for Movie S1). The assembled NC5[0%]-[10%] hydrogel does not detach or break due to the achievement of strong hydrogen bonding interactions at the interface of the assembled NC hydrogel subunits. The quantitative investigations on the mechanical properties of the assembled NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels have been carried out with a tensile machine.

The NC5[0%] hydrogel is dried once and reswelled, and is used as a control

sample coded as NC5[0%]-D. The assembled NC hydrogels have equal overlap lengths, which are about 8 mm and circled with red frames (Figure 4b).

All the hydrogel samples

are cut into dumbbell shapes with a sample-cutting machine.

Both ends of the

dumbbell-shaped samples are clamped, and stretched at a constant velocity of 200 mm min-1. During the elongation tests, the overlap regions of assembled NC hydrogels do not show interface slippage, while the breaking points are outside the joints and indicated by blue arrows (Figure 4b, see Supporting Information for Movie S2-S5).

Typical stress-strain

curves of NC5[0%]-D, NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels are quite close to each other (Figure 4c).

The results of the fracture stress and fracture strain are


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summarized in Figure 4d.

The fracture stress of NC5[0%]-D hydrogel in the contraction

state at room temperature is 4.5 MPa, which is much larger than that in the swollen state with 90% water content just after as-prepared (Figure S2). affects the fracture stress of NC hydrogel.

That is, the water content greatly

For the assembled NC5[0%]-[0%],

NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels, the fracture stress slightly decreases with increasing the AM content, because more AM content results in more water in the hydrogel even in contraction state.

The water contents of assembled NC5[0%]-[0%],

NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels are 76%, 79% and 86%, respectively.

The

fracture strains of tested hydrogels are all about 630%, which implies the assembled NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels are all featured with good elastic properties.

In general, the assembled NC hydrogels perform excellent mechanical

properties with large elongation ratio (>630%) and high tensile stress (>3.3 MPa) at room temperature without adhesive failure, which reveals that the hydrogen bonding interactions between NC hydrogel subunits are quite strong. Responsive Properties of Assembled Inhomogeneous NC Hydrogels.

Due to the

various responsive properties of diverse hydrogel subunits, the assembled NC hydrogels with inhomogeneous structures could exhibit desired responsive behaviors.

For example, the

subunits of NC5[0%], NC5[10%] and NC5[20%] hydrogels after drying treatment are featured with different volume phase transition temperatures (VPTTs) (Figures S5, see Supporting

Information).

So,

the

assembled

strip-shaped

NC5[0%]-[10%]

and

NC5[0%]-[20%] hydrogels exhibit different thermo-responsive bending behaviors (Figure 5). As shown in Figure 5a, the NC5[0%]-[10%] hydrogel strip with length of 1 cm is straight in deionized water at 24 °C. With increasing the temperature suddenly from 24 to 42 °C, the NC5[0%]-[10%] hydrogel strip gradually bends towards the NC5[0%] side, because the NC5[0%] hydrogel shrinks earlier than the NC5[10%] hydrogel due to their different VPTTs.



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When the shrinking degree of NC5[0%] hydrogel layer is much larger than that of the NC5[10%] hydrogel layer, the NC5[0%]-[10%] hydrogel strip forms a circle at 4 min. However, after the complete deswelling of the NC5[0%] and NC5[10%] hydrogel layers, the NC5[0%]-[10%] hydrogel strip unfolds slowly to be straight again in the end.

While, for

the NC5[0%]-[20%] hydrogel strip, the bending behavior is much more significant due to the remarkable difference between the thermo-responsive properties of the NC5[0%] and NC5[20%] hydrogel layers (Figure 5a and Figures S6, see Supporting Information).

Under

this temperature condition, the bending shape of the NC5[0%]-[20%] hydrogel strip remains unchanged even when it is allowed to stay longer.

When the temperature is increased to 54

ºC, the NC5[20%] layer would further shrink, which results in slightly unfolding of the NC5[0%]-[20%] hydrogel strip (Figures S7, see Supporting Information). bending degree is reflected by curvature value.

The responsive

Accordingly, the relative length is defined

as the ratio of the length of hydrogel strip at certain time to its initial length.

Both dynamic

thermo-responsive curvature value and relative length of the NC5[0%]-[10%] and NC5[0%]-[20%] hydrogel strips upon increasing the temperature from 24 to 42 °C within 10 min in water are shown in Figure 5b. the larger the curvature value.

The higher the bending degree of the hydrogel strip is,

The bending degree of NC5[0%]-[20%] hydrogel strip is

higher than that of NC5[0%]-[10%] hydrogel strip (Figure 5b).

Both of the NC5[0%]-[20%]

and NC5[0%]-[10%] hydrogel strips reach their maximum curvature values within 4 min, due to the same responsive property of NC5[0%] hydrogel layer.

However, their maximum

curvature values are much different, ca. 0.9 mm-1 for NC5[0%]-[10%] hydrogel strip but ca. 2.3 mm-1 for NC5[0%]-[20%] hydrogel strip, because of different responsive properties of NC5[10%] and NC5[20%] hydrogel layers.

The curvature value of NC5[0%]-[10%]

hydrogel strip decreases gradually after 4 min, while that of NC5[0%]-[20%] hydrogel strip remains nearly unchanged after 4 min, because the NC5[10%] hydrogel layer deswells more


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significantly than the NC5[20%] hydrogel layer at 42 °C (Figure S6).

The relative lengths

of hydrogel strips decrease quickly with increasing temperature from 24 to 42 °C.

The final

relative length of NC5[0%]-[20%] hydrogel strip is slightly longer than that of NC5[0%]-[10%] hydrogel strip.

These results of bending degree and relative length are

crucial and valuable for rationally designing the sensors and biomimetic soft robotics. Figure 6 shows the rapid, reversible and repeatable thermo-responsive behavior of a typical NC5[0%]-[20%] strip with different thicknesses of the subunits.

The NC5[0%]-[20%]

hydrogel strip bends with increasing temperature from 24 to 42 °C and unfolds with decreasing temperature from 42 to 24 °C reversibly (Figure 6a).

The changes of curvature

and relative length represent that the NC5[0%]-[20%] hydrogel strip exhibits rapid bending but gradual unbending actions (Figure 6b).

In short, the responsive property of the

assembled NC hydrogel is depended on the responsive properties of the hydrogel subunits. So, the assembled NC hydrogels can be designed with diversified architectures as soft actuators for shape-actuation, encapsulation and manipulation. Demonstration of Assembled NC Hydrogels with Inhomogeneous Structures as Temperature-Controlled Actuators.

Because the bending degree of NC5[0%]-[20%] is

optimal due to the significant difference of shrinking volume ratio between NC5[0%] and NC5[20%] layers, the NC5[0%] and NC5[20%] hydrogel subunits are selected to assembly temperature-responsive actuators.

Figure 7 shows three assembled hydrogel strips with

different inhomogeneous architectures that designed to form the shapes of capital letters “SCU” (the abbreviation of Sichuan University) with increasing temperature from 24 to 40 °C (see Supporting Information for Movie S6).

The hydrogel strips are assembled by

combining NC5[0%] subunit with NC5[20%] subunit in different manners.

The

deformation shapes of the assembled inhomogeneous NC5[0%]-[20%] hydrogels can be easily designed by varying the number, size and shape of the hydrogel subunits.



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To demonstrate the flexibility of our strategy, various hydrogels with different inhomogeneous architectures are assembled from NC5[0%] and NC5[20%] hydrogel subunits, and are designed as temperature-controlled actuators for encapsulation and manipulation (Figure 8). For example, the NC5[0%] and NC5[20%] hydrogel subunits are assembled to form two-layered rectangular films, which could roll into hydrogel tubes in deionized water for encapsulation in response to temperature increase (Figure 8a, see Supporting Information for Movie S7).

A cross-shaped NC5[0%]-[20%] hydrogel, which

bonds four pieces of NC5[0%] subunits to the cross-shaped NC5[20%] subunit (see Figure 2r), could fold towards the center to form a holder like basketry with increasing temperature from 22 to 66 °C (Figure 8b, see Supporting Information for Movie S8).

Moreover, another

type of cross-shaped NC5[0%]-[20%] hydrogel sheet (see Figure 2l) is designed to manipulate and transfer a target in deionized water (Figure 8c, see Supporting Information for Movie S9).

The cross-shaped NC5[0%]-[20%] hydrogel is connected to a string with two

pieces of thin magnetic sheet, and is used to pick up and transfer the target object like the grasp and release actions of human hand.

The cross-shaped NC5[0%]-[20%] hydrogel

actuator is fully expanded in water at 19 °C.

After transferred onto the

polytetrafluoroethylene (PTFE) block in water bath maintained at 48 °C, it bends rapidly towards the NC5[0%] side and clamps the PTFE block.

Then, it unfolds and releases the

PTFE block after transferred back into the water bath at 19 °C.

Overall, the demonstrations

have confirmed the flexibility and designability of our assembly method via rearranged strong hydrogen bonding between polymers and clay nanosheets.

Our assembly method

enables to build complex hydrogel actuators by designing different subunits with various responsive properties, which remains difficult to do with conventional fabrication method alone.


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CONCLUSIONS In summary, we have developed an easy and flexible method for fabricating smart hydrogels with inhomogeneous structures by assembling nanoclay-crosslinked stimuli-responsive hydrogel subunits via rearranged strong hydrogen bonding between polymers and clay nanosheets.

The assembled hydrogels exhibit excellent mechanical properties and quite

strong hydrogen bonding interactions between the hydrogel subunits.

The proposed

approach is featured with flexibility and designability to build assembled hydrogels with diversified architectures by using various stimuli-responsive subunits as building blocks for constructing inhomogeneous structures, resulting in desirably responsive actuating behaviors. Our assembly method enables the rearrangement of hydrogen bonding interactions between polymers and clay-nanosheets at the interface of subunits, makes it possible to design and fabricate mechanically strong hydrogel systems with complex architectures that composed of diverse internal structures, multi-stimuli-responsive properties, and controllable shape deformation behaviors.

Such a strategy is highly promising for advancing the development

of stimuli-responsive hydrogel actuators in the soft robots and actuators fields.

ASSOCIATED CONTENT: The Supporting Information is available free of charge via the Internet at http://pubs.acs.org or from the author.

The SEM micrographs, typical stress-strain curves, equilibrium and

dynamic thermo-responsive volume phase transition characteristics of NC hydrogel subunits with different AM contents.

The optical photographs of the dynamic thermo-responsive

behavior of NC5[0%]-[20%] hydrogel strip with increasing temperature rapidly from 24 to 54 °C.



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Page 26 of 42

Movies of a simple tensile test performed on assembled NC5[0%]-[10%] hydrogel; mechanical tensile stress experiment performed on NC5[0%]-D hydrogel; mechanical tensile stress experiment performed on assembled NC5[0%]-[0%] hydrogel; mechanical tensile stress experiment performed on assembled NC5[0%]-[10%] hydrogel; mechanical tensile stress experiment performed on assembled NC5[0%]-[20%] hydrogel; three different assembled NC5[0%]-[20%] hydrogel strips bending to form a shape of the abbreviation “SCU”; rectangular assembled NC5[0%]-[20%] hydrogels encapsulating plastic beads; a cross-shaped assembled NC5[0%]-[20%] hydrogel encapsulating plastic beads (Upper: Top view;

Lower: Side view); a cross-shaped assembled NC5[0%]-[20%] hydrogel gripper

moving a PTFE block

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (L.-Y.C.); * E-mail: [email protected] (Z. L.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202, 21322605, 81321002), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).


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Page 34 of 42

FIGURES

Figure 1.

Schematic illustration of the assembly process of smart hydrogel subunits for

constructing NC5[x1]-[x2] hydrogel with inhomogeneous structure via rearranged hydrogen bonding between polymers and clay nanosheets.

(a,b) The typical subunits, i.e., NC5[x1]

and NC5[x2] hydrogels (a) with the same clay content fixed at 5×10-2 mol L-1 but with different acrylamide (AM) contents are stacked together on a glass plate (b).

(c) During the

drying process under atmospheric condition of 70% relative humidity at room temperature, the hydrogen bonding between polymers and clay nanosheets of NC5[x1] and NC5[x2] hydrogel subunits at the interface is rearranged to firmly “lock” the two subunits.

(d) After

re-swelling in deionized water, the NC5[x1]-[x2] hydrogel with inhomogeneous structure is formed due to the strong hydrogen bonding between polymers and clay nanosheets of NC5[x1] and NC5[x2] hydrogel subunits at the interface.


Page 35 of 42

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Figure 2.

Optical photographs of NC5[0%] (a), NC5[10%] (b) and NC5[20%] (c) hydrogel

subunits as well as the assembled NC5[0%]-[20%] (d-l, n-r) and NC5[0%]-[10%] (m) hydrogels with various architectures.

Scale bars are 1 cm.



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Figure 3.

Page 36 of 42

SEM micrographs of typical assembled NC5[0%]-[0%] (a, d), NC5[0%]-[10%]

(b, e) and NC5[0%]-[20%] (c, f) hydrogels before (a-c) or after (d-e) ten cycles of swelling/deswelling operation.

Scale bar is 50 µm.


Page 37 of 42

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Figure 4.

Tensile tests of NC5[0%]-D, NC5[0%]-[0%], NC5[0%]-[10%] and

NC5[0%]-[20%] hydrogels.

(a) Optical images showing the process of an assembled

NC5[0%]-[10%] hydrogel being stretched by hand at room temperature..

(b) Optical images

showing the NC5[0%]-D, NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels with standard dumbbell-shapes stretched by a tensile machine at room temperature..

The

overlapping portions of NC5[0%]-[0%], NC5[0%]-[10%] and NC5[0%]-[20%] hydrogels are circled with red frames, while the rupture positions are pointed out by blue arrows.

(c,d)

The typical stress-strain curves (c) and fracture stress and strain values (d) of NC hydrogels obtained in mechanical tensile stress tests at room temperature.



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Figure 5.

Page 38 of 42

Dynamic thermo-responsive behaviors of NC5[0%]-[10%] and NC5[0%]-[20%]

hydrogel strips in water with increasing temperature rapidly from 24 to 42 °C. (a) Optical photographs of the dynamic processes.

(b) Dynamic changes of the curvature values and

the relative lengths of NC5[0%]-[10%] and NC5[0%]-[20%] hydrogel strips during the heating process.


Page 39 of 42

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Figure 6.

Repeatable thermo-responsive behavior of a NC5[0%]-[20%] hydrogel strip in

water by switching temperature rapidly between 24 to 42 °C. thermo-responsive

behavior

of

typical

(a) Optical photographs of the

NC5[0%]-[20%]

hydrogel

strip.

(b)

Thermo-responsive change of the curvature and the relative length of NC5[0%]-[20%] hydrogel strip during the heating-cooling cycles.



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Figure 7. Optical photographs of three NC5[0%]-[20%] hydrogel strips assembled with different architectures actuating to form the shapes of capital letters “S” (a), “C” (b) and “U” (c) (“SCU” is the abbreviation of Sichuan University) in response to an increase of temperature from 24 to 40 °C.


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Figure 8. actuators.

Demonstrations of NC5[0%]-[20%] hydrogels as temperature-controlled (a,b) Encapsulation of objects (such as plastic beads) using the assembled

hydrogels to actuate as tubes (a) or holder like basketry (b).

(c) Capture and transportation

of an object (such as PTFE block) using the assembled hydrogel to actuate as manipulator.



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TOC FIGURE


Smart Hydrogels with Inhomogeneous Structures ... - ACS Publications

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