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Highly Extensible Double-Network Gelswith Self-Assembling Anisotropic Structure**

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By Wei Yang, Hidemitsu Furukawa, and Jian Ping Gong*

Novel hydrogels that possess highly ordered anisotropic

structures have been synthesized by controlling the diffusion

direction of multivalent ions that act as cross-linkers into a

semi-rigid polyelectrolyte solution. Furthermore, by using the

double network (DN) technique, highly ordered DN gels have

been synthesized with excellent mechanical properties, in

particular high extensibility, which exceeds 2200% of the

original length under uniaxial elongation, although the gels

contain plentiful water, about 97%.

Tissues of living organisms, which contain a large amount of

water and are in a soft and wet gel-like state, usually have

well-ordered structures. For example, myosin shows a

liquid-crystalline (LC) structure in sarcomere and contributes

to the formation and smooth motion of muscle fibers.[1–3]

However, most commonly synthesized soft and wet gels lack an

ordered structure and have weak mechanical strength. We

have recently succeeded in synthesizing double network (DN)

hydrogels with anomalously high mechanical strength, which

makes this an epoch-making breakthrough.[4,5] DN gels are

composed of a polyelectrolyte as the first network and a

hydrophilic neutral polymer as the second network. Although

DN gels contain 90% water, they have significant hardness

(elastic modulus of 0.3MPa) and toughness (fracture stress of

20MPa) values, which are comparable to those of cartilage and

tendons. However, DN gels are amorphous and isotropic and

do not have a well-ordered and anisotropic structure like the

sarcomere in a muscle. Hence, our aim is to obtain tough DN

gels with a well-ordered and anisotropic structure.

Here, we introduce a well-ordered structure into a DN gel

based on a semi-rigid polyelectrolyte, poly(2,20-disulfonyl-

4,40-benzidine terephthalamide) (PBDT),[6–8] whose chemical

structure is presented in Figure 1a. PBDT has a semi-rigid

main chain and its aqueous solution shows a nematic liquid

crystalline state above a low critical concentration (CLC*).[9–11]

Because of its semi-rigid structure, PBDT shows various types

of self-assembled structures. For example, by addition of NaCl,

the structure of PBDT alternates from a cluster-like isotropic

self-assembly to a fiber-like anisotropic one. This alternation in

[*] Prof. J. P. Gong, Dr. W. Yang, Prof. H. FurukawaDepartment of Biological SciencesGraduate School of ScienceHokkaido UniversitySapporo, 060-0810 (Japan)E-mail: gong@sci.hokudai.ac.jp

[**] This research was financially supported by a Grant-in-Aid for SpeciallyPromoted Research (No. 18002002) from the Ministry of Education,Science, Sports and Culture of Japan.

Adv. Mater. 2008, 20, 4499–4503 � 2008 WILEY-VCH Verlag G

structure depends on the amount of salt added. At the

stoichiometric charge ratio of the added salt to the sulfonic

moiety of PBDT, a network-like structure was observed.[12]

However, these ordered structures can only be obtained with a

dimension of several micrometers, with spatial correlation

being lost for a longer scale. A suitable method is needed to

achieve global orientation in order to obtain an anisotropic gel

on a macroscopic scale.

Dobashi and co-workers[13] have successfully developed a

method to self-assemble rigid polyelectrolytes into an ordered

structure based on ion-diffusion-inducedmolecular orientation

and complex formation. For example, a radial structure like a

spherocrystal has been obtained by the simple diffusion of a

multivalent salt such as CaCl2 into a semi-rigid polysaccharide

solution, in which the Ca2þ ion behaves as a physical

cross-linker.

Thus, in this research, we attempted to synthesize a DN gel

with an anisotropic structure, termed ‘A-DN gel’, by using

Dobashi’s method to obtain uniaxially oriented PBDT gels as

the first network and then polymerizing acrylamide (AAm) in

the PBDT gel to obtain PAAm as the second network. In the

first step, an anisotropic PBDT gel was physically formed by

the uniaxial diffusion of CaCl2 into the PBDT solution in a

reaction mold. As illustrated in Figure 1b, the reaction mold

consisted of a pair of glass plates, separated by a silicone spacer

with a thickness of 1mm. Three sides of the silicone spacer

were closed, and the remaining side was left open, in order to

allow the Ca2þ ions to only diffuse along the axial direction

perpendicular to the open side. The obtained PBDT gel, which

was physically cross-linked by Ca2þ, was semi-transparent and

showed dark and bright images in 08 and 458 directions undercrossed Nicols. This result implies that the PBDT gel had a

uniaxially oriented structure formed by cross-linking of the

multivalent ion.

In the second step, we immersed the uniaxially oriented

PBDT gel in an acrylamide (AAm) solution that contained the

chemical cross-linker and chemically synthesized the PAAm

gel in the PBDT gel to obtain the A-DN gel. The A-DN gel,

which finally contained 97% of water in an equilibrium swollen

state, became semi-transparent and the dark and bright images

in 08 and 458 directions were identified under crossed Nicols, as

shown in Figure 1c. This implies that the uniaxially oriented

structure of the PBDT gel was preserved even in theA-DN gel.

In order to elucidate how the orientation changes with the

concentration of PBDT (CP), wemeasured the birefringence of

the first PBDT and the A-DN gels prepared at different CP

values using polarized optical microscopy (POM).[14] After

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Figure 1. a) Chemical structure of PBDT. b) Schematic of the structure formation of ananisotropic gel (first network) between a pair of parallel glass plates by uniaxial diffusion.c) Polarized photographs of the prepared A-DN gel synthesized at 1 wt % PBDT, under crossedNicols, where the gel shows dark and bright images for every 458 rotation, indicating ananisotropic orientation.

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attaining equilibrium swelling in water, both the PBDT and

A-DN gels exhibited high birefringence (Dn), which increased

with the CP, over a range of 10�5–10�3 for CP¼ 0.1–3wt %.

Compared with the PBDT gels, the corresponding A-DN gels

showed only a small decrease in Dn.

The uniaxially oriented structures were confirmed by

scanning electron microscopy (SEM) micrographs. Figure 2a

shows a network-like structure with uniform orientation in the

first PBDT gel prepared from 1wt % PBDT. The rod-like

aggregates formed a well-oriented grid-like network along the

axial direction, with an average grid distance of approximately

20–50mm. It is noteworthy that at higher magnification, CaCl2crystals can be observed on the surfaces. It is probable that the

freeze-drying process of the sample preparation promoted the

formation of the CaCl2 crystals. Figure 2b shows a well-ordered

hierarchical network-like structure of the A-DN gel prepared

from 1wt % PBDT. In contrast to the first PBDT gel, the

presence of CaCl2 crystals could not be confirmed on the

surfaces. In addition to bundle-like structures, whose orienta-

tion was similar to that of the PBDT gel, the A-DN gel showed

polymer network entanglement with thick bundle-like struc-

tures, which constituted hierarchical structures. Elementary

analysis revealed that themolar ratio of PBDT to PAAm in the

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A-DN gel was 1: 99 (which corresponds to a

weight ratio of 1: 27), which indicates that

the gel contained a significant excess of

PAAm.

It is believed that the A-DN gel is formed

first by the well-oriented anisotropically

structured PBDT gel acting as a template,

and the PAAm polymers as the second

network, whose monomers diffuse into the

anisotropic template of the PBDT aggre-

gates beforehand and thereby possesses an

anisotropic entangling structure as well.

Considering this, Figure 2c presents a

possible well-ordered structure in the

A-DN gel.

For the uniaxially oriented anisotropic

A-DN gels, tensile experiments were per-

formed in two different directions. One was

the axial direction, which was parallel to the

diffusion direction of the Ca2þ ions, and the

other was the vertical direction, which

was perpendicular to the axial direction.

Figure 3a shows the stress–strain curves of

two samples sliced from these two directions

of a A-DN gel (CP¼ 1wt%<CLC*,CLC*¼1.5wt %). Figure 3b shows enlarged stress–

strain curves of each gel at the initial

elongation. As expected, the A-DN gel

showed anisotropic mechanical properties,

depending on the tensile directions. The

elongation in the axial direction showed a

nice J-curve and the elongation stress at

fracture was much higher than that in the

vertical direction. On the other hand, the elastic modulus

(initial slope of the stress–strain curves) in the vertical

direction (0.036MPa) was higher than that in the axial

direction (0.009MPa). Regardless of the direction in which

the gel was elongated, the gel showed extraordinary extensi-

bility that could reach 22 times the original length, as

demonstrated in Figure 3c. The elongations in both directions

were reversible before fracture, as shown in Figure 3d. Both the

A-DN gels before and after the extension experiment

exhibited almost the same birefringence, Dn. This implies that

the degree of order of the anisotroic structure in the A-DN gel

was preserved even during the stretching. In Figure 3d, the

colors of the A-DN gels before and after the extension

experiment look slightly different. This is only a result of the

photo quality, as in fact the colors were the same. It is worth

noting that all the sample gels elongated along the axial

direction split at the centre of the samples. In contrast, all the

samples that elongated in the vertical direction did not break

down but only escaped from one of the two chucks, which

indicates that the actual strain exceeded 2200%.

In order to elucidate the effect of the individual single

network (SN) of the first gel (PBDT SN gel) and the second gel

(PAAm SN gel), we compared the two types of individual SN

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Figure 2. SEM micrographs (top view) of gels containing 1wt % PBDT:a) The first gel consists of PBDT and CaCl2. b) The A-DN gel consists ofPBDT and PAAm. The pictures were taken at room temperature afterfreeze-drying and sputter-coating. c) Schematic representation of apossible well-ordered structure in the A-DN gel.

gels and A-DN gels in different directions. The results are

shown in Figure 3b. The SN PBDT gel that was stretched in the

vertical direction had the highest elastic modulus of around

0.081MPa. However, it fractured at a small tensile strain

(70%), which indicates that the physically cross-linked PBDT

gel was brittle and lacked elasticity. This should be attributed

to the packing structure of the anisotropically self-assembled

PBDTmolecules even at CP<CLC*.[12] On the other hand, the

PAAm SN gel was softer than the A-DN gel, and large

extensions were not reversible, probably because of the loose

cross-linking. These results indicate that although the amount

of PBDT is significantly lower than that of PAAm (1:99), it

enhances the mechanical strength of the A-DN gel dramati-

cally because of its well-oriented anisotropic structure.

The high initial elastic modulus and yielding strength of the

A-DN gel in the vertical directionmight be a result of the break

in the PBDTpacking; subsequently, the untwisting effect of the

second flexible PAAm chains contribute to a large strain. In

Adv. Mater. 2008, 20, 4499–4503 � 2008 WILEY-VCH Verl

contrast, the A-DN gel in the axial direction can release the

elastic modulus at the initial elongation by anisotropically

entangling the PAAm chains, whose monomer diffused into

the anisotropic template of the first PBDT structure. Follow-

ing this, the slipping effect of the PBDT molecules against

themselves in conjunction with the unentangling effect of

the PAAm chains around the PBDTmolecules are responsible

for a typical J-curve and a larger increase in the effective

strength of the A-DN gel than that possessed in a single

PAAm gel.

We performed tensile tests on A-DN gels synthesized at

different CP values (0.5–3wt %). It was found that the

mechanical properties of the A-DN gels strongly depended on

the PBDT concentration CP. A-DN gels synthesized at

CP�CLC* were hard but brittle, and the elastic modulus

could reach as high as 0.35MPa. In contrast, when CP<CLC*,

the gels became comparatively flexible and exhibited excellent

elasticity, as shown in Figure 3. This implies that the difference

in mechanical properties is caused by the different structures

formed at different CP values, which were observed in the

previous study.[12] It is worth noting that all the samples

(CP<CLC*) finally return to their original size after the first

tensile test. Thus, cyclic tensile tests on the same gel samples

were simultaneously carried out to verify their modulus

changes during the tensile test and to determine whether any

obvious change was observed in the stress–strain curves in the

axial or vertical direction, although the gels endured stretching

that was three times their original length. This implies that the

A-DN gels (CP<CLC*) possess intriguingly high extensibility,

which results in shape retention after sustaining an elongation

from a strong stretch, without any other external effects. This

effect is probably observed because of the self-assembling

network structure built by non-covalent interactions.

In summary, novel highly extensible anisotropic hydrogels

have been synthesized by the self-assembly of a semi-rigid

polyelectrolyte PBDT and the DN technique. Even for 1wt %

PBDT, which was below the critical concentration of the

nematic phase in PBDT aqueous solution, the uniaxially

oriented anisotropic gels were spontaneously induced by the

unidirectional diffusion of multivalent ions into the PBDT

solution, and the LC-like orientation could be retained

permanently in A-DN gels even after reaching equilibrium

swelling in water. The A-DN gel synthesized at CP<CLC*

exhibited excellent mechanical properties, particularly an

outstanding extensibility of sustaining an elastic elongation

that exceeded 22 times its original length, and a shape retention

feature that allowed restoration to its original length, although

it contains a tremendous amount of water (about 97%).

Elementary analysis and SEM observation clarified that the

first gels (CP<CLC*) possessed a well-oriented network-like

structure of PBDT aggregates with uniform orientation by

the diffusion and cross-link effect of themultivalent ions. In the

corresponding A-DN gels, the highly ordered structure was

maintained by the entanglement of the second flexible PAAm

chains around the first PBDT well-ordered aggregates, which

constituted the hierarchical structures.

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Figure 3. High extensibility of the A-DN gel containing 1wt % PBDT: a) Tensile stress–straincurves of a A-DN gel in axial and vertical directions. b) Enlarged curves at the initial elongation ofthe A-DN gel and corresponding two individual single network gels (PBDT SN gel and PAAm SNgel). c) Photograph demonstrating the tensile capability of the A-DN gel that can elongate to over22 times of its original length. d) Photograph representing the shape retention feature of theA-DN gel after large extension.

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Experimental

Preparation of A-DN Gels that Contain PBDT: A-DN gels weresynthesized by a two-step network formation technique. First, anappropriate amount of PBDT (Mw¼ 2 600 000, CLC*¼ 1.5wt %) wasdissolved in Milli-Q water at 1 or 3wt %. After being bubbled withnitrogen gas for 30min, the PBDT solution was poured into a mold,which consisted of a pair of parallel glass plates separated by a siliconespacer with a 1mm thickness. Three sides of the spacer were closed andthe remaining side was left open to permit uniaxial diffusion of Ca2þ

ions into the mold. The mold was wrapped in dialysis membranes(BX-100, Bel-Art Products, USA). Themold that contained the PBDTsolution was dialyzed with 200mL of 0.5 M calcium chloride (CaCl2)solution at 25 8C, and the formation of the gel progressed from the openrim to the interior of the mold. The PBDT solution gelled within 24 h,and a plate-like PBDT gel was obtained after another 2 days in order toreach the diffusion equilibrium. This first physical gel was thenimmersed in an aqueous solution of 2 M acrylamide (AAm) (JunseiChemical Co. Ltd) recrystallized from chloroform, which contained0.01mol%N,N0-methylenebis(acrylamide) (MBAA; Tokyo Kasei Co.Ltd) as a cross-linker and 0.01mol % 2-oxoglutaric acid (Wako PureChemical Industries, Ltd) as a UV initiator, for at least two days untilthe diffusion equilibrium was reached. The polymerization of thesecond network was subsequently carried out under a nitrogen

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atmosphere with a UV lamp for 7 h. The preparedgel thus consisted of two independent polymernetworks entangled with each other.

Observation under Crossed Nicols and PolarizedOptical Microscopy (POM): The ordered struc-tures of the first PBDT gels and the A-DN gelswere identified using crossed Nicols and POM. Toobserve the overall structure of entire gels, theplate-like gel samples were sandwiched between apair of glass plates and the upper surfaces wereobserved under crossed Nicols. On the other hand,optical microscopy observation was performedusing crossed polarizing microscopy (Olympus,BH-2). Sample gels were placed on glass plates andthe upper free surfaces were observed. Thebirefringence, Dn, was then measured using acrossed polarizing microscope with a Berekcompensator for retardation [14], and an averageof five measurements was recorded for eachsample.

Tensile Properties of A-DN gels: Tensilemecha-nical properties were measured with a commercialtest machine (Tensilon RT-1150A, Orientec Co.).After reaching equilibrium swelling in water, thegels were cut into a dumbbell shape standardizedas JIS-K6251-7 sizes (length 35mm, width 6mm,thickness 3–4mm, gauge length 12mm, innerwidth 2mm) with a gel cutting machine (DumbBell Co., Ltd). The sample length between twochucks was 15–20mm. The stress was recordedwhile the sample gel was stretched at a constantrate of 100mmmin�1. In the case of uniaxialdiffusion, two kinds of samples were cut out indirections parallel and perpendicular to the axialdirection (diffusion direction).

Scanning Electron Microscopy (SEM): Themorphology of the first gels and the A-DN gelswas analyzed by SEM (HITACHI, S-3500N). Priorto observation by SEM, each sample was free-ze-dried and sputter-coated using a Pt-Pd target(E-1030, HITACHI). For each sample, more thanfive different areas were selected and observed.

Received: May 21, 2008Published online: October 6, 2008

[1] M. Spencer, W. Fuller, M. H. F. Wilkins, G. L. Brown, Nature 1962,

194, 1014.

[2] C. M. Coppin, P. C. Leavis, Biophys. J. 1992, 63, 794.

[3] H. I. Petrache, N. Gouliaev, S. Tristram-Nagle, R. Zhang, R. M. Suter,

J. F. Nagle, Phys. Rev. E 1998, 57, 6.

[4] J. P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada, Adv. Mater. 2003,

15, 1155.

[5] Y.-H. Na, T. Kurokawa, Y. Katsuyama, J. P. Gong, Y. Osada, S.

Okabe, T. Karino, M. Shibayama, Macromolecules 2004, 37, 5370.

[6] E. J. Vandenberg,W. R. Diveley, L. J. Filar, S. R. Pater, H. G. Barth, J.

Polym. Sci, Part A: Polym. Chem. 1989, 27, 3745.

[7] N. Sarkar, L. D. Kershner, J. Appl. Polym. Sci. 1996, 62,

393.

[8] S. Viale, A. S. Best, E. Mendes, W. F. Jager, S. J. Picken, Chem.

Commun. 2004, 1596.

[9] T. Funaki, T. Kaneko, K. Yamaoka, Y. Ohsedo, J. P. Gong, Y. Osada,

Y. Shibasaki, M. Ueda, Langmuir 2004, 20, 6518.

[10] Y. Shigekura, Y. M. Chen, H. Furukawa, T. Kaneko, D. Kaneko, Y.

Osada, J. P. Gong, Adv. Mater. 2005, 17, 2695.

Adv. Mater. 2008, 20, 4499–4503

COM

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[11] Y. Shigekura, H. Furukawa, W. Yang, Y. M. Chen, D. Kaneko, Y.

Osada, J. P. Gong, Macromolecules 2007, 40, 2477.

[12] W. Yang, H. Furukawa, Y. Shigekura, K. Shikinaka, Y. Osada, J. P.

Gong, Macromolecules 2008, 41, 1791.

Adv. Mater. 2008, 20, 4499–4503 � 2008 WILEY-VCH Verl

[13] T. Dobashi, K. Furusawa, E. Kita, Y. Minamisawa, T. Yamamoto,

Langmuir 2007, 23, 1303.

[14] M. Born, E. Wolf, Principles of Optics, Cambridge University Press,

London, Cambridge 1959.

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