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Programmable responsive shaping behavior induced by visiblemulti-dimensional gradients of magnetic nanoparticles†

Yang Liu,*ab Makoto Takafuji,b Hirotaka Ihara,*b Meifang Zhu,c Mingshan Yang,a Kai Gua and Wenli Guoa

Received 18th November 2011, Accepted 30th January 2012

DOI: 10.1039/c2sm07206h

Herein, we report a new ‘programmable’ responsive shaping

behavior induced by the visible multi-dimensional gradient of

magnetic nanoparticles (MNP): the materials exhibit different local

curvatures and a sequence of responsive shapes during the responsive

process; the sequence and the local curvature are accurately defined

by ‘programmed instructions’—MNP gradients in the materials.

Responsive shaping behavior is defined as materials that change their

geometric shapes under external stimuli, e.g. the shape change of

smart materials and the shape recovery of shape memory materials.

Many different shaping behaviors have been developed, like bending

of a one-dimensional (1D) strip,1 folding or buckling of two-dimen-

sional (2D) sheet,2 multi-shape recovery of shape memory materials.3

These behaviors are triggered by different external stimuli, including

heat,2a,3,4 light,2b,5 pH,1d,6 humidity,7 magnetic field8 and electric field.9

This shaping behavior is the essential property for many applications

of smart materials and shape memory materials, such as sensors,10

actuators,11 logic components,1d valves,12 sensitive patterns,13 and

artificial muscles. Recently, some researchers reported that the

shaping behaviour of hydrogels can be controlled by introducing

gradients, such as gradients in monomer concentration,2a and com-

position.1d,4d,6 The internal gradients result in a gradient of the

de-swelling ratio in hydrogels under an external stimulus, e.g. tem-

perature2a or pH,6 which gives rise to responsive shaping behavior.

However, the gradients are in a one dimensional gradient along the

diameter 2a or vertical directions,1d,4d,6 and they are invisible to the

naked eye, which makes it more difficult to modulate the gradient

structure and directly investigate the relationship between the

gradient and the responsive shaping behavior. Here, we insert

a visible multi-dimensional gradient structure (i.e. gradient distribu-

tions of magnetic nanoparticles (MNP)) into clay/poly(N-iso-

propylacryl-amide) (PNIPAAm) nanocomposite hydrogels (NC gel).

aBeijing Key Lab of Special Elastomer Composite Materials, Departmentof Material Science and Engineering, Beijing Institute of PetroleumTechnology, 19 North Qingyuan Road, Beijing 102617, China. E-mail:yang.liu@bipt.edu.cn; Fax: +86 10 81292129; Tel: +86 10 81292129bDepartment of Applied Chemistry and Biochemistry, KumamotoUniversity, Kumamoto 860-8555, Japan. E-mail: ihara@kumamoto-u.ac.jpcState Key Lab for Modification of Chemical Fibers & Polymer Materials,College of Material Science and Engineering, Donghua University, 2999Ren-min Road, Shanghai, 201620, China

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2sm07206h

This journal is ª The Royal Society of Chemistry 2012

Different MNP gradients can be ‘programmed’ and frozen in gels by

external magnetic field and in situ polymerization, illustrated in

Scheme 1a. The details of ‘the programming process’ are illustrated in

the ESI† (Scheme S3, S4, S5, S6, S7). Based on different internal

gradients, the responsive shaping process of these gels runs like an

accurate ‘program’ exhibiting a desired sequence of multiple

responsive shapes in three-dimensional (3D) spatial dimensions under

an external uniform stimulus with time elapsing, illustrated in Scheme

1b. Namely, the responsive shaping behavior of the gels is

programmable according to the inserted ‘programmed instruc-

tions’—the MNP gradients. Moreover, because MNPs are visible to

the naked eye, it is easy to directly observe the relationship between

the responsive shaping behavior and different MNP gradients. The

relationship and the hydrogel materials with programmable respon-

sive shaping behavior will enlighten the design of responsive shaping

behavior and widen the applications of smart materials and shape-

memory materials.

Fig. 1a shows a responsive shaping process of a ‘programmed’ gel

strip, which is the same as that of the strip I illustrated in Scheme 1b.

This gel is called ‘BL gel’ because it has a visible bi-layer structure: the

black lower layer containing concentrated MNP and the transparent

Scheme 1 a) The preparation of ‘programmed’ gels inserted by ‘pro-

grammed instructions’—MNP gradients. MNP content in as-prepared

hydrogels: 2.2wt%; b) Programmable responsive shaping behavior of

strips with a sequence of responsive shapes and with controllable local

curvature.

Soft Matter, 2012, 8, 3295–3299 | 3295

Fig. 1 Programmable responsive shaping behavior of a BL gel strip a)

Photos of BL gel strip at 40 �C and 20 �C (white bar is 5 mm); b) the

relationship between the length of two layers and the time.

Fig. 2 a) Microscopy images of BL gel strip. b) Mechanism for the

programmable responsive shaping behavior of BL gel strip.

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upper layer without MNP, shown in Fig. 1a–1. It is well known that

PNIPAAm NC gels de-swell as the temperature increases past the

LCSTof PNIPAAm (32 �C).14BLgel strips exhibit a sequence of two

shapes under an external uniform stimulus (i.e. 40 �C): the gel firstbends in the lower direction, then bends in the upper direction. The

relationship between the length of the two layers and the time indi-

cates that the lower layer (containing MNP) shows a higher de-

swelling rate and a lower equilibriumde-swelling ratio comparedwith

the upper layer (without MNP), shown in Fig. 1b. During the

responsive process, the length of the lower layer initially decreases

faster than the upper layer: the gel strip bending in the lower direc-

tion; then the length of the lower layer stops decreasing and the length

of the upper layer continues to decrease: the gel strip starts to bend in

the upper direction. This means that the existence of MNP simulta-

neously affects both the equilibrium de-swelling ratio and the de-

swelling rate. After the hydrogel strip was immersed in water at 40 �Cfor 120 min, it was taken out and put into cold water at 20 �C. Thestrip immediately bends from a positive bending angle to a minus

bending angle within 1.5 min, shown in Fig. 1a–9. Then, the bending

angle increases to the maximum value of 125� at 15 min (Fig. 1a–11).

3296 | Soft Matter, 2012, 8, 3295–3299

After that, the bending angle decreases slowly and the hydrogel strip

gradually recovers its original straight shape of 0� at 2430 min.

Fig. 2a shows the microscopy images of a BL gel strip. There are

many micrometer MNP aggregates in the lower layer. The large size

of the aggregates maymake the hydrogel loose, which is good for the

discharge of water. Moreover, the aggregates remain hydrophilic in

hydrogel and work as water channels to help water discharge when

PNIPAAm becomes hydrophobic at a temperature above the LCST

(32 �C). So the lower layer shows a higher de-swelling rate compared

with the upper layer, which makes the BL gel strip bend to the lower

direction at first. As shown in Fig. 2a–2, 2a–3, 2a–4, the MNP

content and the amount of aggregates increase from the central area

to the bottom area. This means there are two vertical gradient

distributions in the lower layer: the gradient of MNP aggregates and

the gradient ofMNP content. The first gradient leads to a gradient of

the initial de-swelling rate. The second gradient results in a gradient of

the equilibrium de-swelling ratio at the end of the de-swelling process

(i.e. higher solid content means lower equilibrium de-swelling ratio).

The de-swelling behavior results from the conformation transition of

PNIPAAm molecules from the hydrophilic coil to the hydrophobic

globule. In the lower layer, the MNP occupy some space in the

hydrogels. If the MNPs were replaced by PNIPAAm molecules, the

volume of these molecules would shrink above the LCST due to

the transition.While the volume of theseMNPwill not change above

the LCST, this means that the de-swelling ratio of the MNP-loaded

This journal is ª The Royal Society of Chemistry 2012

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layer at the equilibrium shrinking state is smaller than that of the

corresponding pure PNIPAAm layer (i.e. the upper layer). This is

also confirmed by the de-swelling behavior of a uniform MNP-

loaded PNIPAAm NC hydrogel and a pure PNIPAAm NC

hydrogel (Fig. S8, ESI†). The synergy of the two gradients gives rise

to the whole responsive shaping behavior of the BL gel strip. In

addition, the re-swelling behavior of the BL gel strip is interesting: the

re-swelling rate of the upper layer is faster than that of the lower layer.

The probable reason is that the lower layer (containing MNP)

formed a dense structure after the shrinking process. The big

micrometer MNP aggregates are packed tightly together by the

shrinking force during the de-swelling process, which forms a denser

structure compared to the upper layer. As the re-swelling process

begins, the conformation of the PNIPAAmmolecules transfers from

globule to coil: the volume of the molecules increase, then they have

to push away the big MNP aggregates, which slow down the

re-swelling rate of the lower layer. Thus, the re-swelling rate of the

upper layer without MNP is faster than that of the lower layer.

As we see, the BL gel strip realizes shape control in the temporal

dimension by a sequence of two shapes during the responsive process.

Fig. 3 Programmable responsive shaping behavior of HBL gel strip: a)

Photos of HBL gel strip at 40 �C and 20 �C; b, c) Mechanism for the

shaping behavior of HBL gel strip.

This journal is ª The Royal Society of Chemistry 2012

But the curvature of every part of the equilibrium de-swollen BL gel

strip is almost the same. Based on the BL gel strip, it has been

concluded that the existence of theMNP gradient affects both the de-

swelling rate and the equilibrium de-swelling ratio. If there is another

MNP gradient along the gel strip, the responsive process of different

local areas along the gel strip will be different. Thus, if we want to

control the local curvature of the gel strip, we need to insert another

horizontal MNP gradient (i.e. along the strip) into the gel. Such a gel

strip has been prepared on the basis of BL gel, shown in Fig. 3a. This

gel is called HBL gel, which means BL gel inserted with a horizontal

gradient. As shown in Fig. 3a–1, the thickness of the lower black layer

changes along the strip, indicating there is a horizontal gradient of

MNP content. The local curvature of the equilibrium bending state is

different along the strip: the thicker the lower layer is, the higher the

local curvature will be, as shown in Fig. 3a (30–120min). In addition,

the left part of the HBL gel strip, with lowMNP content, lags behind

the right part, with highMNP content, during the responsive shaping

process: the right part begins the second stage (i.e. bending in the

upper direction), while the left part still stays in the first stage (i.e.

bending in the lower direction). So, every local part of the strip

asynchronously changes from convex to concave along the strip from

Fig. 4 Programmable responsive shaping behavior of Gel Plate I with

a 3D symmetric gradient of MNP at 40 �C (white bar is 5 mm).

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Fig. 5 Programmable responsive shaping behavior of Gel Plate II with

a 3D asymmetric gradient of MNP at 40 �C (white bar is 5 mm).Publ

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right to left during the responsive process. This results from the two

horizontal gradients of the MNP content and MNP aggregates,

illustrated in Fig. 3b,3c.

For the BL gel strip, the gradient is one-dimensional, vertical; for

the HBL gel strip, the gradient structure is two-dimensional: vertical

and horizontal. Their shaping behavior happens in the 2D plane. If

a 3D gradient is inserted in a gel plate, the responsive process will be

more interesting and complicated: the local curvature on the plate can

be controlled in both 3D space and temporal dimension.

Table 1 Programmable responsive shaping behavior defined by ‘programme

Materials ‘Programmed instructions’

BL gel strip 1D gradient of MNP

HBL gel strip 2D gradient of MNP

Gel plate I 3D symmetric gradient of MNP

Gel plate II 3D asymmetric gradient of MNP

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Fig. 4 is the responsive process of a gel plate inserted with a 3D

gradient of MNP. As shown in Fig. 4, the side view of this gel indi-

cates that theMNP gradient along the AD side is similar to the HBL

strip, and there is a basin of MNP content in the area of ODOC.

This means that the MNP gradient in the whole gel plate is 3D, and

the MNP content in the basin ofODOC is lower than that in other

parts. As we know from the HBL strip, a lowerMNP content means

a slower de-swelling process. Thus, the responsive shaping process of

the ODOC area lags behind other parts, as shown in Fig. 4. First,

including ODOC area, the whole plate exhibits a convex shape (0–

7 min), and the ODOC still remains convex while other parts

become concave (7–14 min). Finally, ODOC gradually changes to

concave (14–30 min).

Here, the basin of ODOC is approximately symmetrical to line

EF, so the whole responsive shaping process is nearly symmetrical to

line EF. E and F are the middle points of the AB and DC sides,

respectively. Furthermore, if the 3D gradient of MNP is asymmetric,

we can get an asymmetric response behavior, shown in Fig. 5. There

is an asymmetric basin of MNP content in ODOC. The basin lags

behind other parts during the shaping process: the basin remains

convexwhile other parts become concave (see the right view photos in

Fig. 5). In addition, because of the asymmetry of the basin, the

responsive shaping behavior is also asymmetric to line EF: the right

part (rectangle EBCF) of the gel plate lags behind the left part

(rectangle AEFD) during the shaping process, shown in the front-

view photos.

In conclusion, it has been proven that the insertion of MNP

gradients into PNIPAAm NC gels affects both the equilibrium de-

swelling ratio and the de-swelling rate and the gels exhibit a sequence

of responsive shapes during their responsive processes. The MNP

gradient can form 1D, 2D, 3D, symmetric or asymmetric structure if

the pre-polymerization solution is ‘programmed’ by different external

magnetic fields during in situ polymerization. These ‘programmed’

gradients accurately define the responsive process: the sequence of

responsive shapes, the local curvature of the gels, synchrony/asyn-

chrony, symmetry/asymmetry, and so on. Therefore, the responsive

shaping behavior can be predicted based on the ‘programmed

instructions’ of theMNPgradients, which are summarized inTable 1.

In addition, the introduction of MNP does not harm the original

excellent mechanical strength of NC gel (Fig. S9, ESI†), whichmakes

the material more practical. Thus, we believe that a gel with such

programmable responsive shaping behavior will find practical use in

sensors, actuators, sensitive patterns, and artificial muscles; the

concept of programmable responsive behavior in both 3D space and

the temporal dimension could also be applied in their fields to develop

new materials with such behavior, like inorganic, ceramic materials

and so on.

d instructions’—MNP gradients

Programmable responsive shaping behavior

A sequence of two shapes; curvature changingsynchronously along the strip; even local curvatureA sequence of three shapes; curvature changingasynchronously along the strip; different local curvature along the stripA sequence of several shapes; curvature changingasynchronously and symmetrically based on the 3D gradientA sequence of several shapes; curvature changingasynchronously and asymmetrically based on the 3D gradient

This journal is ª The Royal Society of Chemistry 2012

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Acknowledgements

This research is financially supported by the Japan Society for the

Promotion of Science for Foreign Researchers (P08043), Beijing

Municipal Natural Science Foundation (No.2122015), and National

Natural Science Funds for Distinguished Young Scholars

(No. 50925312).

Notes and references

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