A self-deformable gel system with asymmetric shape change based on a gradient structure

Article abstract of DOI:10.1039/c8cc05893h

A self-deformable gel system is constructed by coupling a gradient structured gel with a chemical oscillating reaction. The system exhibits periodic and asymmetric shape change. The asymmetric shape change of the gel is based on the gradient structure.

Full text of DOI:10.1039/c8cc05893h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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Self-deformable gel system with asymmetric shape change based
on gradient structure
Received 00th January 20xx,
Accepted 00th January 20xx
ab
ab
c
a
a
a
Jie Li, Xiuchen Li, Guohe Xu, Zhaohui Zheng, Jinni Deng, * and Xiaobin Ding *
DOI: 10.1039/x0xx00000x
www.rsc.org/
4
A self-deformable gel system is constructed by coupling gradient ciliary motion, cellular beating, etc.. However, limited by the
structured gel with chemical oscillating reaction. The system isotropic gel structure, the shape change of these gels is
exhibits periodic and asymmetric shape change. The asymmetric symmetric. Thus, realizing the diversified deformation pattern
shape change of the gel is based on the gradient structure.
of the SPMs remains a great challenge.
Shape-changing polymer materials (SPMs), an important smart
polymer material, has been extensively investigated during last
two decades due to their stimuli responsiveness and
1
reversibility. A shape-changing process occurs when SPMs are
exposed to certain external stimuli (light, heat, pH, electric
field, magnetic field, etc.), and the shape change can recover
after the elimination of external stimuli. Based on the
corresponding relationship between external stimuli and the
geometrical shape change of material, SPMs have been utilized
to develop intelligent polymer devices, such as micro-sensor,
micro-actuator, mass transportation, drug delivery device,
2
etc..
In spite that SPMs have been widely studied, the realization of
stimuli responsive shape change essentially depends on the
ON-OFF switching of external stimuli in most cases. Developing
highly intelligent SPMs without external stimuli switching
remains a great challenge. There are many natural rhythmic
deformations that do not rely on external stimuli, such as
pbummion flower and heartbeat. The SPMs mimicking natural
rhythmic deformation shows a lot of advantages, including
autonomous and reversible shape change as well as self-
sustained feature. By coupling Ru-contained polymer gel and
self-oscillating reaction, Yoshida and co-workers developed a
smart gel system which presented autonomous and reversible Scheme 1 Concept of the self-deformable gel system based on chemical
3
volume change without ON-OFF switching of external stimuli.
Furthermore, by altering shape and size of the gel, a series
oscillating reaction.
interesting biomimetic function have accomplished, including To our knowledge, most attempts to realize advanced shape
change of SPMs point to building an asymmetric structure. For
example, Xiong et al. fabricated an ion-responsive hydrogel
microcantilever with gradient distributed cross-linking density
using multiphoton polymerization, which presented
5
controllable bending behavior. By utilizing patterning method,
3
Smith et al. developed heterogeneous Ru(II)(bpy) -gelatin
6
composites showed anisotropic deformation. Hu et al.
prepared a nepenthes mimetic gel device by constructing
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multi-layer thermo-responsive gel. In these cases, the (Ru(II)(bpy)
Journal Name
7
3
), the swelling degree of the gel is uneven,
deformation of the material is provided by the asymmetric resulting in asymmetric deformation of the gel. In this way, the
structure of SPMs, such as gradually varied cross-linking self-deformation of the gel is periodic, asymmetric and
density or multi-layer structure.
completely regulated by the system itself without ON-OFF
switching of external stimuli.
In this study, we prepared a COR responsive double network
3
gel with gradient Ru(II)(bpy) distribution. At first, a non-COR-
responsive poly(N-isopropylacrylamide) (PNIPAAm) network
st
was synthesized as framework (named 1 network) via free
radical polymerization, the multi-vinylated PNIPAAm micro-gel
was used as crosslinker. Then, the swelling degree regulating
3
group Ru(II)(bpy) (acted as the catalyst of COR) was
introduced to construct gradient structured poly(Ru(bpy) -co-
3
nd
NIPAAm) network (named 2 network) interpenetrating the
st
1
st
network. Specifically, the 1 network gel was partially
nd
immersed in the 2 network gelating solution (see ESI). A
spontaneous concentration graidient of Ru(II)(bpy)3 moiety
was formed inside the gel. After that, the gel underwent UV-
nd
irradiation to crosslink the 2 network. Due to the rapidity of
8
photo-polymerization, the Ru(II)(bpy) concentration gradient
nd
3
was fixed during the UV-irradiation. As a result, the 2
network possessed of gradient distribution of Ru(II)(bpy)3
moiety (Figure 1b). In this way, an interpenetrating
PNIPAAm/Poly(Ru(bpy) -co-NIPAAm) double network gel with
3
gradient distribution of Ru(II)(bpy)3 was achieved. To
intuitively investigate thermo and redox responsiveness of
Poly(Ru(bpy) -co-NIPAAm) gel, we prepared isotropic
3
poly(Ru(bpy) -co-NIPAAm) gel in a similar way. Detailed
3
preparation method and characterization can be seen in ESI.
By utilizing Belousov-Zhabotinsky (BZ) reaction (a typical COR
presents periodic redox variation) as in situ driving source, a
self-deformable gel system exhibiting autonomous and
periodic bending-stretching motion was established based on
3
-co-NIPAAm) gel. asymmetric swelling-deswelling behaviour of gradient
Figure 1 Preparation method of the gradient Poly(Ru(bpy)
(a) Synthesis of the Poly(Ru(bpy)
3
3
-co-NIPAAm) gel. (b) Illustration of the Poly(Ru(bpy) -co-NIPAAm) gel.
preparation process of gradient double network.
There were two aspects of concern when we tried to establish
the SGS. First, the polymer network should be flexible enough
Inspired by these works, we attempt to develop a self- to accomplish the reversible and periodic shape change.
nd
st
deformable gel system (SGS) based on gradient structured Therefore, the monomer concentration of both 1 and 2
polymer gel and chemical oscillating reaction (COR). In this network was set to be slightly above the critical solid content
system, the gradient structured polymer network is designed value to form comparatively sparse double network. It can be
to be responsive to the chemical environment variation (e.g. seen in Table 1, the solid content of prepared gel was 15.2 %,
pH or redox state) that induced by COR, and to be flexible and the gel was quite tender and easily deformable.
nd
enough to undertake reciprocating shape change. COR is the in Furthermore, the response of 2 network to COR should be
situ driving source that gives the gel autonomous, reversible rapid. It could help to reduce the hysteresis and mechanical
and periodic deforming character. As shown in Scheme 1, loss during reciprocating deformation. Thus the micro-gel with
st
when COR occurs, a periodic chemical variation spontaneously multi-vinyl group was selected as the crosslinker for both 1
nd
evolves and propagates in the gel, resulting in responsive and and 2 network, since the multi-vinylated micro-gel or nano-
9
reversible swelling degree change of the gel. On account of gel crosslinked networks were proved to be rapid responsive.
3
gradient distribution of the swelling degree regulating group The swelling degree responsiveness of the Poly(Ru(bpy) -co-
3
Table 1 Structural profiles of the gradient Poly(Ru(bpy) -co-NIPAAm) gel.
a
a
a
st
b
nd
b
c
NIPAAm
Microgel
(m %)
21.0
Ru(II)(bpy)
(m %)
-
3
1
Network
(m %)
2
Network
(m %)
Solid content
(
m %)
(m %)
14.3
-
st
1
2
Network
Network
79.0
-
-
-
-
nd
44.4
74.0
33.3
22.6
22.3
3.4
Gel
85.6
14.4
15.2
a
b
nd
c
Feed ratio. Determined by weighing the gel in thoroughly dry state before and after the construction of 2 network. Determined by weighing the gel in
thoroughly dry state and equilibrium swelling state
2
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NIPAAm) gel to the redox variation is crucial in SGS. It region (turned to light green) emerged in central part of the
determines the deformation degree of the gel during repeated gel and expanded along the length to both ends (Figure 3a).
shape change. Besides, temperature plays an important role in Then, almost the entire gel was oxidized as the chemical wave
swelling-deswelling process of the gel due to the thermo- propagated in the gel (Figure 3b). After that, oxidizing region
responsive nature of main component PNIPAAm. Therefore, was gradually reduced (restored to orange) (Figure 3c). The
equilibrium swelling ratio of isotropic Poly(Ru(bpy)
NIPAAm) gel as a function of temperature in oxidized/reduced both ends until the whole gel was back to original reduced
state was investigated firstly. Ce(SO and Ce (SO were state (Figure 3d). The concentration of Ru(II)(bpy) at left side
3
-co- chemical wave of reducing state propagated from centre to
4
)
2
2
4
)
3
3
used as oxidizing/reducing agent to keep the gels in Ru(II) or was higher than that of right side of the gel. The swelling
Ru(III) state. The gels were totally immersed in the redox degree at left side of the gel was higher when the gel was
solution for 20 min to reach equilibrium. The volume of the gel oxidized, so the gel was forced to bend to the right side.
o
in Ru(III) state at 10 C was set as 100 %. Equilibrium swelling Accordingly, the deswelling degree at left side of the gel was
ratio of the gels continuously decreased with increasing higher when the gel was reduced, which made the gel
o
nd
temperature, and tended to be stable at 35 C in both Ru(II) stretched. It is notable that the 2 network was COR
and Ru(III) states (Figure 2). At constant temperature, the responsive and in minority (Table 1) in the Poly(Ru(bpy) -co-
3
nd
volume of gels in Ru(III) state was larger than that in Ru(II) NIPAAm) gel. The deformation of the gel was caused by the 2
st
state because the molecule chain in Ru(III) state was more network, and the 1 network was forced to deform. Due to the
hydrophilic. It is easy to imagine that if we combine the robustness and high deformability that multi-vinylated micro-
nd
gradient structured gel with BZ oscillating reaction, a periodic gel cross-linker conferred to the 2 network, the deformation
9
and reversible self-deformable gel system can be established. of the gel could accomplished.
The Self-deforming behaviour of the gradient gel was
o
investigated at 20 C since the gel volume deviation between
o
Ru(III) and Ru(II) is largest at 20 C.
Figure 2 Equilibrium swelling ratio of the Poly(Ru(bpy) -co-NIPAAm) gel as a Figure 3 Image of the autonomous bending-stretching motion of the
3
o
function of temperature. The swelling ratio of oxidized gel at 10 C was set gradient Poly(Ru(bpy)
3
-co-NIPAAm) gel in one redox cycle. Green (oxidized)
as 100 %. Green square: [Ce
2
(SO
4
)
3
]=5 mM and [HNO
]=0.89 M.
3
]=0.89 M; Orange and orange (reduced) arrows represent the direction and range of redox
chemical wave, black arrows represent the motor direction of the gel. The
time interval is 100 s.
rhombus: [Ce(SO ]=5 mM and [HNO
4
)
2
3
The SGS was successfully constructed by immersing the
3
gradient Poly(Ru(bpy)
reaction substrate (aqueous solution of 0.89 M HNO
NaBrO , 62.5mM malonic acid (MA)). The gel was thoroughly oscillating amplitude gradually weaken over time.
3
-co-NIPAAm) gel (5×1×1 mm ) in BZ In a closed system, BZ reaction can self-sustain for several
3
, 84 mM cycles until all organic acid is consumed, and the redox
1
0
3
oxidized after approximate 270 s of induction period, and then Correspondingly, the self-deforming motion of gradient
the BZ reaction occurred inside the gel. Thereafter, the gel Poly(Ru(bpy) -co-NIPAAm) gel driven by BZ reaction showed
3
showed autonomous and periodic bending-stretching similar character. Figure 4 shows the self-deforming process
deformation without any external stimuli (ESI video S1). Figure included
5 complete bending-stretching cycles which
showed the colour and shape change of the gradient consumed 2.5×10 mol MA, and the deforming process
-4
3
o
3
Poly(Ru(bpy) -co-NIPAAm) gel in one redox cycle at 20 C. The continued for 2500 s. The period of one self-deforming cycle
dimension of the gel was larger than the wavelength of BZ was approximate 500 s. The stretching degree of the gradient
reaction, thus the gel cannot be synchronously oxidized or Poly(Ru(bpy) -co-NIPAAm) gel in oxidized state was relatively
3
reduced. A chemical wave of redox oscillation was generated large at early stage of the self-deforming process because
and propagated inside the gel periodically. At first, an oxidizing there was sufficient MA in the system. The direct distance
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between two ends of the gel was over 2700 μm. The stretching unimplemented deformation pattern. More broadly, the SGS
degree of the gel decreased as the MA concentration could be regarded as a template for constructing other
decreasing. BZ reaction in the gel stopped, and no more autonomous shape changing systems, by utilizing other
bending-stretching deformation occurred when all MA was asymmetric structured polymer materials and chemical
consumed. The direct distance between two ends reached the oscillating reaction (e.g. pH oscillator).
minimum value of 1814 μm (defined as L). It can be seen in
Figure 4a, the stretching degree and deforming amplitude This work was supported by the National Natural Science
decreased over time, as the concentration of MA reduced. It is Foundation of China (No. 51373175).
notable that the self-deforming motion can be restarted if
additional MA is added into the dormant system or mechanical
Conflicts of interest
stirring is carried out. Figure 4b shows the trajectory of
3
gradient Poly(Ru(bpy) -co-NIPAAm) gel’s one vertex over time. There are no conflicts to declare.
The trajectory reveals the reciprocating feature of the self-
deforming process. It can be expected that the attenuation
phenomenon will be suppressed by improving the diffusion References
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Figure 4 Autonomous and periodic bending-stretching deformation profiles
9
of the gradient Poly(Ru(bpy)
substrates ([HNO
3
-co-NIPAAm) gel in the solution of BZ
o
]=84 mM, [MA]=62.5 mM, 20 C). (a) L
,
3
]=0.89 M, [NaBrO
3
8
1
649-8664; E. J. Reusser, R. J. Field, J. Am. Chem. Soc. 1979,
01, 1063-1071.
is the direct distance between two ends of the gel at 2500 s (when MA is
completely consumed); ΔL is the displacement of two ends distance of the
gel during bending-stretching process. (b) (x,y) is the two dimension
coordinate of marked position on the gel. The time interval of data
1
1 P. Dayal, O. Kuksenok, A. C. Balazs, Langmuir, 2009, 25
4298-4301; P. Dayal, O. Kuksenok, A. C. Balazs, PNAS, 2013,
10, 431-436.
,
1
collection is 20 s
.
In conclusion,
a self-deformable gel system exhibiting
autonomous, periodic and asymmetric deformation was
established by coupling gradient structured gel with chemical
oscillating reaction. The asymmetric deformation of the gel
relies on the gradient structure of the gel. This work could
enlighten the development of diversified and self-sustained
SPMs and motivate further work on advanced smart polymer
materials. The two-step strategy of gradient structure provides
an opportunity to realize those theoretically feasible but
4
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