SCIENCE CHINA Chemistry. Self-assembled structures of a semi-rigid polyanion in aqueous solutions and hydrogels

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1 SCIENCE CHINA Chemistry REVIEWS May 2012 Vol.55 No.5: SPECIAL ISSUE In Honor of the 80th Birthday of Professor WANG Fosong doi: /s x Self-assembled structures of a semi-rigid polyanion in aqueous solutions and hydrogels SUN TaoLin 1, WU ZiLiang 1 & GONG JianPing 2* 1 Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo , Japan 2 Faculty of Advanced Life Science, Graduate School of Science, Hokkaido University, Sapporo , Japan Received October 5, 2011; accepted November 22, 2011; published online February 21, 2012 Poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT), a kind of liquid-crystalline (LC) molecule, has high molecular weight, negative charge and a semi-rigid structure. The aqueous solution of PBDT shows nematic liquid crystalline state above a critical PBDT concentration, C LC *, of 2 wt% 3wt%. Different from the flexible polyelectrolyte, PBDT shows a variety of self-assembling structures in aqueous solution with and without salt due to the semi-rigid nature and highly charged property. In addition, the hydrogels with ordered structure are developed by polymerizing a cationic monomer N-[3-(N,N-dimethylamino) propyl] acrylamide methyl chloride quarternary (DMAPAA-Q) in the presence of a small amount of PBDT below the C LC *. During the polymerization of cationic monomer, the polycations form a complex with semi-rigid PBDT through electrostatic interaction; these complexes self-assemble into ordered structures that are frozen in the hydrogel. Several different structures, including the anisotropic, dual network-like structure, and cylindrically symmetric structure, with various length scales from micrometer to millimeter, are observed. The hydrogels with ordered liquid crystalline assemblies and particular optical properties should promise applications in many fields, such as in bionics, tissue engineering, and mechano-optical sensors. hydrogels, self-assembl, polyelectrolyte, semi-rigid, polyion complex 1 Introduction Most biotissues, ranging from the hard to the soft, have mesoscopically well-ordered composite structures, which play a crucial role in executing the functions of living organisms [1, 2]. For example, myosin shows a liquidcrystalline (LC) structure in sarcomere, contributing to the formation and smooth motion of muscle fibers. In the reticular connective tissue, collagen fibrils (type III) crosslink to form a fine reticulum in the extracellular spaces of tissues, and act as a supporting skeleton to maintain the space and position for ground substance and cells (for lymph nodes, the size of the reticular mesh is several tens of micrometers) [2 5]. The ground substance, primarily composed of glycosaminoglycans (GAGs), is filled in the reticulum of col- *Corresponding author ( gong@mail.sci.hokudai.ac.jp) lagen fibrils and has a soft gel-like state, providing a micro-environment for cells and predominating in the balance and translocation of tissue fluid [5]. The biomacromolecules, such as deoxyribonucleic acid (DNA), microtubule (MT), and actin filaments (F-actin), should be crucial for the formation of these ordered structures in living organisms. These biomacromolecules usually possess negative charges and have rigid or semi-rigid structures, endowing them with great abilities to form advanced architectures by themselves or via electrostatic interaction with cations and cationic lipids. Inspired by nature, efforts should be made to introduce ordered and hierarchical structure into hydrogels. Among the strategies developed to date, self-assembly is a convenient and powerful method for producing ordered structures [6]. Molecular self-assembly that is directed through weak, noncovalent interactions has received tremendous attention for the development of systems with ordered structures and Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com

2 736 Sun TL, et al. Sci China Chem May (2012) Vol.55 No.5 resultant functions [7 10]. Biomacromolecules and block copolymers have been used to develop self-assembled architectures in synthetic physical or chemical hydrogels via intermolecular noncovalent interactions, such as ionic bonding, hydrophobic interactions and hydrogen bonding [11 14]. Liquid-crystalline (LC) molecules and their assemblies have excellent optical, electric, and magnetic properties and can be incorporated in physical and chemical hydrogels as a monomer or dopant during the polymerization. These gels have relatively good mechanical properties and are widely used in developing chemomechanical actuators, such as mechano-optical sensors. Osada developed hydrogels with shape memories by the copolymerization of the water-soluble and hydrophobic monomers, acylic acid (AA) and crystalline stearyl acylate (SA) [15, 16]. Furthermore, by the copolymerization of LC 11-(4thcyanobiphenyloxy) undecyl acrylate (11CBA) and AA monomers, poly (11CBA-co-AA) LC hydrogels were synthesized [17 21]. This structured gel showing anisotropic shrinkage in one direction with increasing temperature is promising for use as soft actuators. Poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT), another kind of liquid-crystalline (LC) molecule, has a high molecular weight, negative charge, and a semi-rigid structure, similar to natural biopolymers. The aqueous solution of PBDT shows a nematic liquid crystalline state above a critical PBDT concentration, C LC *, of 2 wt% 3wt%. In aqueous solutions with different concentrations of PBDT and NaCl, PBDT shows various self-assembling structures. On the other hand, if we polymerize a cationic monomer N-[3-(N,N-dimethylamino) propyl] acrylamide methyl chloride quarternary (DMAPAA-Q) and chemical crosslinker in the presence of PBDT as a dopant, the polymerized polycations interact with the oppositely charged PBDT to form a semi-rigid polyion complex and self-assemble into ordered structures that are frozen in the hydrogels by the subsequent gelation process. At different conditions of gel preparation, the synthesized gels possess different ordered structures and functions. The chemical structure of PBDT and DMAPAA-Q are shown in Scheme 1. In this article, we generally introduce the typical selfassembling behaviors of PBDT and its polyion complex in aqueous solution and synthetic hydrogels. 2 Specific behavior of PBDT in aqueous solution PBDT with different molecular weight was prepared by an interfacial polycondensation reaction, as described elsewhere. In the study of self-assembling behavior of PBDT in aqueous solution, the number-average molecular weight, M n, weight-average molecular weight, M w, and polydispersity (M w /M n ) of PBDT used were , 88000, and 1.2, respectively [22]. The aqueous solutions of PBDT were prepared by the dilution method. A mother solution was prepared at 5 wt% and subsequently diluted with pure solvent (water) in order to achieve a range of lower concentrations. On the other hand, PBDT + NaCl aqueous solutions were prepared according to a direct dissolution method consisting of preparing PBDT aqueous solutions with various PBDT concentrations by directly dissolving proper amounts of NaCl powder to achieve predetermined concentrations. PBDT behaves as a semi-rigid molecular in an aqueous solution and shows a variety of the self-assembling structures. Different from the flexible polyelectrolyte, the PBDT solution exhibits two characteristic polymer concentrations, crossover concentration C*~0.3 wt% and lower critical concentration of lyotropic liquid crystalline state C LC *~3 wt%. Accordingly, there are three different regions divided by the two characteristic concentrations. The addition of NaCl to the aqueous solution induces a different behavior in each of the three different regions. The specific behaviors of PBDT in aqueous solution with salt or without salt in these regions, including the reduced viscosity and the aggregate structure, are discussed [14, 23]. 2.1 Salt-free PBDT aqueous solution Scheme 1 Chemical structures of the N-[3-(N,N-dimethylamino)propyl] acrylamide methyl chloride quarternary (DMAPAA-Q) and poly(2,2'- disulfonyl-4,4'-benzidine terephthalamide) (PBDT). (1) In dilute aqueous solution of PBDT, below the crossover concentration C*, the reduced viscosity in the salt-free condition shows a simple increase with the decrease of the PBDT concentration C P. This behavior is similar to that of the diluted aqueous solution of the flexible polyelectrolyte, poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS). The reduced viscosity of the flexible polyelectrolyte also shows an anomalous increase as the polymer concentration decreases, as shown in Figure 1. PBDT molecules are semi-rigid and exist in the state of a single molecule state without molecular association in this region, as described in Figure 2(a).

3 Sun TL, et al. Sci China Chem May (2012) Vol.55 No Figure 1 Changes in the reduced viscosity of aqueous solutions of polyelectrolytes as a function of polymer concentrations. ( ) PBDT; ( ) PAMPS. Inset photographs show the microscopic texture of an aqueous solution of PBDT under the crossed polarizer at 25 C. (a) Concentration of 5.0 wt%; (b) concentration of 2.0 wt%; (c) concentration of 2.0 wt% when a shear force was applied by sliding the glass plate covering the sample. The PBDT used with the number-average molecular weight, M n, weight-average molecular weight, M w, and polydispersity (M w /M n ) are , 88000, and 1.2, respectively. Reproduced with permission from ref. [14]. (2) In a semi-dilute aqueous solution of PBDT in the range of C* < C P < C LC *, a steep increase in the reduced viscosity is observed as the polymer concentration increases (Figure 1). This is different from a flexible polyelectrolyte, as shown by the behavior of PAMPS. In a semi-dilute aqueous solution of flexible polyelectrolyte, the reduced viscosity decreases as the polymer concentration increases, due to the screening effect. In this range, the homogenous dark-field view is observed, indicating the solution is optically isotropic as shown in the inset of Figure 1(b). Interestingly, the shear stress applied to the sample induces the birefringence resulting in the bright vision (inset of Figure 1(c)). PBDT molecules form associations and the size of these association increases with C P (Figure 2(b)). We consider that the molecular association below C LC * originates from the hydrophobic nature of the PBDT main chain backbone or from the hydrogen bonds between PBDT main chains. (3) An aqueous solution of PBDT shows a nematic liquid crystalline state above the lower critical concentration, C LC *, of 3 wt%. This value of C LC * is much lower than those of common LC macromolecules. For instance, cellulose derivatives [24] generally show the LC state with a C LC * of about 20 wt%. The quite low C LC * of PBDT should be attributed to its intrinsic structural rigidity and its polyelectrolyte nature. The reduced viscosity shows a continuous increase with an increase in C PBDT (Figure 1). The strong birefringence of the nematic phase and snow like structures were observed in this range under the crossed polarizing microscope and transmission electron microscope (TEM), as showed in the Figure 1(a) and Figure 2(c, d). 2.2 PBDT aqueous solution with addition of salt Figure 2 TEM images of PBDT association of various PBDT concentrations, C P, around the crossover concentration of PBDT, C*, and the lower critical concentration of lyotropic liquid crystal, C LC *. (a) C P = 0.1 wt% (C P < C*), (b) C P = 0.5 wt% (C* < C P < C LC *), (c) C P = 3 wt% (C P C LC *), and (d) C P = 5 wt% (C P > C LC *). Reproduced with permission from ref. [23]. (1) In the dilute PBDT solution of C P < C*~0.3 wt%, the addition of salt alters the rigidity of PBDT so that it becomes slightly flexible; consequently, the reduced viscosity slightly decreases. (2) In the semi-dilute PBDT solution of C* < C P < C LC *~3 wt%, the cluster-like isotropic aggregates transits to a fiber-like anisotropic structure as the NaCl concentration C s increases, where the apparent size does not change. However, as C s increases further over a critical value of [Na + Cl ]/[Na + SO 3 ] = 1, PBDT forms a network-like structure, so that the reduced viscosity steeply increases. As C s increases still further, PBDT begins to precipitate. (3) In a semi-concentrated PBDT solution of C P > C LC *, the addition of salt destroys the LC phase, and the LC phase and the amorphous phase coexist. The LC phase vanishes as C s increases. Subsequently, the precipitates also form when the charge ratio exceeds the critical value of [Na + Cl ]/ [Na + SO 3 ] = 1 for C P > C LC * due to the salting-out effect. The self-assembling behaviors of PBDT in aqueous solutions are summarized into a phase diagram of C P vs. C s, as shown in Figure 3. 3 Hydrogel with ordered structure by selfassembly of semi-rigid polyion complex The positively charged poly(dmapaa-q) has a flexible main-chain structure and does not show any LC phase by itself. However, structured hydrogels with high water content can be obtained by polymerization of cationic monomer

4 738 Sun TL, et al. Sci China Chem May (2012) Vol.55 No.5 This birefringence was related to microscopic domains, which are randomly oriented in the bulk. We assume that the cationic polymer emerges and forms an ion complex with anionic PBDT when polymerization starts. The polyion complexes and small aggregates self-assemble to form large size anisotropic domains, which are immediately frozen by the gelation to result in anisotropic gels. These anisotropic hydrogels can also be synthesized by photo polymerization. 3.2 Network-like structure [27 29] Figure 3 Phase diagram of PBDT concentration, C P, vs. NaCl concentration, C s, where C* is the crossover concentration of PBDT and C LC * is its lower critical concentration of lyotropic liquid crystal. Reproduced with permission from ref. [23]. DMAPAA-Q in the presence of a small amount of semirigid, anionic PBDT as dopant, even below C LC * of PBDT. 3.1 Anisotropic gels [25, 26] The PBDT used here showed a nematic state above the critical concentration C LC * of 2.8 wt%. The hydrogels were synthesized by thermal radical polymerization at 60 C for 6 h from a precursor solution containing a prescribed amount of PBDT as dopant, 2 M DMAPAA-Q as monomer, 2 mol% N,N -methylenebisacrylamide (MBAA) as the chemical crosslinker, and 0.1 mol% potassium persulfate (KPS) as thermal initiator. Before polymerization, the solution of the cationic monomer with a small amount of PBDT(C P < C LC *) was optically isotropic and without specific structure, except for some preliminary aggregates due to the spontaneous aggregation of PBDT. However, the synthesized hydrogel was transparent and showed the strong birefringence under crossed polarizing microscopy, as shown in Figure 4. Here, the M w and M w /M n of PBDT used were and 1.2, respectively. The aqueous solutions show the significant low critical concentration of nematic liquid crystal, C LC * of 2.2 wt%. The PBDT-containing hydrogels were synthesized by a photo-polymerization. Photo-polymerization was carried out at room temperature under an argon atmosphere with an ultraviolet (UV) lamp for 6 h, from a precursor solution containing prescribed amount of DMAPAA-Q as monomer, PBDT as dopant, 2 mol% MBAA as crosslinker, and 0.15 mol% 2-ketoglutaric acid as initiator. Samples of hydrogels were denoted as Q P -C Q -C P, where C Q is the monomer concentration of DMAPAA-Q in M, C P is the PBDT concentration in wt%. The precursor aqueous solutions were transparent and optically isotropic at C P < C LC *. The hydrogels synthesized with relatively low C Q (C Q < 1.5 M) were turbid, becoming transparent at relatively high C Q (C Q < 1.5 M) (Figure 5(a)). The transparent gels exhibited strong birefringence ( n > 10 5 ) with anisotropic domains of 1 3 mm in size at C Q > 1.75 M and C P > 0.5 wt% (Figure 5(c)), similar to that synthesized by thermal polymerization, as shown in Figure 4. The turbid gels, indicating the occurrence of phase separation, possessed a unique well-ordered network-like structure of micrometer size, despite exhibiting weak birefringence (Figure 5(b)).When the polymerization starts, the generated polycation interacts with the semi-rigid anionic PBDT to form a polyion complex that self-assembles into small anisotropic aggregates with high viscosity. When the polymerized solution has a low ionic strength (C Q 1.5 M), the polyion complexes are unstable and phase separation occurs. The phase separation rapidly develops into viscoelastic phase separation owing to the dynamical asymmetry, form- Figure 4 Template polymerization of polyion-complex gels. The critical concentration of lyotropic liquid crystal C LC * is 2.8 wt%. (a) Crossed polarizing microscope image of a 2 wt% PBDT aqueous solution before polymerization; (b) crossed polarizing microscope image of an as-prepared gel; C LC is 2 wt%; (c) crossed polarizing microscope image of the equilibrium swollen gel in water; C LC is 0.14 wt %. Reproduced with permission from ref. [25].

5 Sun TL, et al. Sci China Chem May (2012) Vol.55 No Figure 5 (a) Appearances of as-prepared gels QP-C Q -C P synthesized with different C Q and C P ; (full triangles): turbid gel, (diamonds): transparent gel. The insets show the representative appearances of the synthesized gels; (A) QP , (B) QP (b, c) Optical micrographs of as-prepared gels QP-C Q under parallel and crossed (corresponding insets) polarizers; (b) C Q = 1.5 M, (c) C Q = 2 M. Reproduced and modified with permission from ref. [28]. ing a network-like structure of ~40 μm in size, which is permanently frozen by the subsequent chemical crosslinking reaction. The necessary conditions for developing the dual network gel have been elucidated as follows; (1) a relatively high PBDT concentration (C P > C*), (2) a low cationic monomer concentration C Q (low ionic strength), and (3) a certain amount of chemical cross-linker. not change while rotating the sample, indicating that PBDT molecules had formed an oriented structure with cylindrical symmetry during polymerization (Figure 6(b, c)). We further characterized the microstructure of PBDT and its polyion complex in the cylindrical hydrogels by polarizing microscopy with a 530 nm tint plate, and by small-angle X-ray 3.3 Ordered structure of cylindrical symmetry [30, 31] As suggested by the above findings on the PBDT-containing hydrogels with millimeter-scale anisotropic domains, if we could control the self-assembly direction of the polyion complex, it would be possible to develop a hydrogel with a macroscopically ordered structure. A strategy we have taken is to polymerize the precursor solution in a glass tube by ultraviolet irradiation from all sides, as shown in Figure 6(a). The reaction should proceed from the outer region to the central region that controls the self-assembly direction, and could result in macroscopically structured hydrogels. In the experiments, the precursor solutions were poured into a glass tube placed vertically in the center of a square arrangement of four ultraviolet lamps. After the polymerization, a thin disc of the gel cut from the bulk cylinder showed a huge Maltese cross under the crossed polarizers that did Figure 6 (a) Scheme of experimental setup for synthesis of cylindrical gel with ordered structures. Glass tube with precursor solution is placed vertically in the center of four UV lamps arranged in a square; (b) appearance of gel swelled in 1 M sodium chloride aqueous solution. The disk piece is cut from the cylinder gel; (c) polarizing micrographs of precursor solution in glass tube, as-prepared cylindrical gel, and swollen gel (QP-2.5-1). Reproduced with permission from ref. [30].

6 740 Sun TL, et al. Sci China Chem May (2012) Vol.55 No.5 scattering. The polyion complex self-assembled into a radial structure in the outer region of the cylinder gels, whereas in the inner region, they formed a concentric structure and aligned parallel with the axial direction, as shown in Figure 7. Recent study suggests that the radial (homeotropic) orientation of the semi-rigid polyion complex in the outer region of the gel was probably due to swelling-induced orientation. In the inner region, the complex was oriented in concentric and axial directions due to heterogeneous polymerization (through light absorbance by the PBDT), which led to monomer diffusion and orientation of the polyion complexes perpendicular to the diffusion direction. These gels show a sensitive response to external force by changing birefringence colors and are expected to find applications in mechano-optical sensing. Based on the above mechanism of structure formation, the concentric cylinder domains at millimeter-scale in cubic packing are observed in the hydrogels synthesized by the photo-initiated polymerization from the two sides of the reaction cell [30], as shown in Figure 8. In the outer layer, the polyion complexes orient perpendicular to the gel surface. In the inner region, the polyion complexes selfassemble to form well-oriented concentric cylinders with their axes perpendicular to the plate gel surface. The diameter of the concentric cylinders universally depends on the thickness of the swollen gel, and the latter is tunable by Figure 8 (a, b) Optical polarizing micrographs of the swollen gel QP observed from the top (a) and the side (b); the upper and lower images are observed without and with 530 nm tint plate, respectively. The diameter D of the concentric domains is shown in the lower image of (a); three regions (1, 2, 3) of which the molecules are vertically orientated with each other are shown in the lower image of (b). (c) A simple illustration of molecular orientation of PBDT and its semi-rigid polyion complex in gel. X : Fast axis of the tint plate; Z : Slow axis of the tint plate. Reproduced with permission from ref. [31]. changing the silicone spacer thickness for polymerization of the plate gel. 4 Macroscopically anisotropic hydrogels Figure 7 Polarizing micrographs of gel QP-2-1 swelled in 1 M NaCl aqueous solution and schematic structures of PBDT in the synthetic gel. Micrographs of disk gel observed from the top (a) and its central slice observed from the side (b) under a microscope with crossed polarizers and insertion of 530 nm tint plate. The insets show the specimen position for the observation and observation direction; the central slice cut from the disk gel as guided by red dotted line; (c) a proposed microstructure of hierarchically oriented PBDT in the cross section and axial plane of polymeric gels. Reproduced with permission from ref. [30]. The hydrogels with millimeter-scale anisotropic domains were successfully developed by polymerizing a cationic monomer in the presence of a small amount of semi-rigid polyanion as dopant [31]. Further increase in the size of anisotropic domains is difficult to achieve because of the occurrence of polyion condensation and resultant gel deformation. A significant route to develop hydrogels with macroscopically anisotropic structure is through the reaction-diffusion process; the reaction and diffusion compete with each other and result in intricate spatial or temporal structures. A strategy we have taken is to synthesize physically cross-linked LC hydrogel by dialysis of PBDT in CaCl 2 solution, as shown in Figure 9 [32]. By the uniaxial diffusion of Ca 2+ into two ends of a thin rectangular reaction cell containing semi-rigid polyanion PBDT aqueous solution, centimeter-scale anisotropic hydrogels with the PBDT molecules and their self-assembled fibrous bundles align in perpendicular to the Ca 2+ diffusion direction have been obtained.

7 Sun TL, et al. Sci China Chem May (2012) Vol.55 No Figure 9 (a) Scheme of the reaction cell used for synthesizing physically cross-linked LC hydrogel by dialysis of PBDT solution in CaCl 2 solution; dimension of cell: 30 mm (length) 20 mm (width) 1 mm (thickness); (b, c) images of synthesized 1 wt% gel observed under a polarizing microscope; crossed polarizers (b), insertion of 530 nm tint plate (c). Arrows show the Ca 2+ diffusion direction during the synthesis of the gel. The outer and central regions of the gel are denoted as region I and region II, respectively, as shown in part b; SAXS and tensile samples cut from the outer region of LC gel are shown in part c. Key: A, analyzer; P, polarizer; X, fast axis of the tint plate; Z, slow axis of the tint plate. Reproduced with permission from ref. [32]. The orientation and gelation of PBDT molecules are induced and controlled by the diffusion process of Ca 2+. The electrostatic complexation between the cationic Ca 2+ and anionic PBDT results in the self-assembly of PBDT molecules into mesoscopic fibrous bundles that align in parallel to the Ca 2+ flux front. At the local flux front, PBDT molecules diffuse from the solution phase due to the concentration gradient, resulting in the PBDT alignment parallel to the diffusion direction of Ca 2+ to form a thin anisotropic layer due to the favorable longitudinal diffusion during PBDT transportation. Finally, the different oriented structures of PBDT are frozen to form LC gels, of which PBDT aligns perpendicular to the diffusion direction of Ca 2+ in the outer regions and parallel to the diffusion in the narrow central region. The extraordinary molecular reorientation at the diffusion flux front and complex molecular alignments in the physical hydrogel were observed for the first time and expected to merit revealing the formation of oriented structures in living organisms and find applications in materials sciences, such as optical sensors. This ordered structure frozen by physically cross-linking was not stable and was weak when the gel was immersed in the simple salt solution with strong ionic strength. Combining with the double-network (DN) technology, a robust, macroscopically anisotropic hydrogel has been developed by insetting the first physically cross-linked network of PBDT into the second chemically cross-linked network of flexible polyacrylamid (PAAm) [33]. These hydrogels possessed robust mechanical properties, especially good extensibility, as shown in Figure Conclusions and prospective PBDT, a kind of liquid-crystalline (LC) molecule with a semi-rigid structure, exhibits special self-assembling structures, ranging from isotropic clusters to anisotropic fibers, in aqueous solution either with and or without salt. The hydrogels with ordered structure were developed by the polymerization of cationic DMAPAA-Q with a small amount of semi-rigid polyanion PBDT as dopant. The synthesized polycations interact with the oppositely charged PBDT to form a semi-rigid polyion complex and selfassemble into ordered structures that are frozen in the hydrogels. Ordered structures in these PBDT-containing hy- Figure 10 Stress-strain curves of A-DN gel in parallel and vertical direction of PBDT orientation, PBDT-containing PAAm gel, and PAAm gel. Reproduced with permission from ref. [33].

8 742 Sun TL, et al. Sci China Chem May (2012) Vol.55 No.5 drogels, including the network and cylindrically symmetric structure, with various length scales from micrometer to millimeter were observed. In addition, combining with the DN technology, robust, macroscopically anisotropic hydrogels have been developed, which endow the hydrogels with additional functions with promising applications in tissue engineering and stress-optical sensors. This research was financially supported by a Grant-in-Aid for the Specially Promoted Research ( ) from the Ministry of Education, Science, Sports and Culture of Japan. The authors sincerely thank Prof. Osada Y, Prof. Ueda M, Prof. Kaneko T, Prof. Furukawa H, Dr. Kurokawa T, and the graduate students in Laboratory of Soft and Wet Matter, Hokkaido University for their contributions to this work. 1 Gartner LP, Hiatt JL. Color Textbook of Histology. Philadelphia: Saunders, Langevin HM, Cornbrooks CJ, Taatjes DJ. 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