Photoswitchable Supramolecular Hydrogels Formed by Cyclodextrins and Azobenzene Polymers
Photoswitchable Supramolecular Hydrogels Formed by Cyclodextrins and Azobenzene Polymers
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DOI：
10.1002/anie.201003567
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Harada, Akira
中科院分区：
文献类型：
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作者：
Tamesue, Shingo;Takashima, Yoshinori;Harada, Akira
水凝胶在自然界中普遍存在，特别是在生物体中。[1]合成水凝胶用于例如组织工程、药物递送系统（DDS）和医学治疗。[2]凝胶主要有两种：物理凝胶[3]和化学凝胶。[4]具有非共价交联点的水凝胶被认为比具有共价键的水凝胶更通用且更容易调节。虽然低分子量物理凝胶表现出光响应性弹性和光开关特性，[5-9]通过主客体相互作用形成的水凝胶应该比低分子量物理凝胶对外部刺激更敏感。[10-17]虽然有一些关于经历相变的刺激响应性水凝胶的报道，[8-9]但是关于通过主客体相互作用形成的光响应性水凝胶的报道很少。我们先前发现，含有环糊精（CD）的主体聚合物和含有偶氮苯（Azo）的客体聚合物的混合物在光照射后显示出粘度变化。[18]然而，我们不能从两种聚合物的混合物获得水凝胶。为了获得水凝胶，我们决定使用凝胶多糖（β-1，3葡聚糖，CUR）作为骨架，因为CUR具有相当刚性的结构，并且CD可以连接到聚合物链中的每个单体单元。我们通过混合用CD官能化的CUR（CD-CUR）和具有连接到其上的偶氮部分的客体聚合物成功地获得了超分子水凝胶（方案1）。这是从CD聚合物和客体聚合物获得的光响应性水凝胶的第一个实例。通过炔丙基酰胺修饰的α-CD和6-叠氮基-凝胶多糖之间的环加成反应制备CD-CUR（参见支持信息）。用修饰后的α-CD对CUR的C6位进行取代，得到取代度为98%的CD-CUR。当偶氮改性的聚（丙烯酸）（pAC 12 Azo）（8.6重量%）和CD-CUR（4.0重量%）以1：1的单体单元比例在水中混合时，溶液的粘度首先增加，然后溶液转化为凝胶（支持信息中的图S7）。另一方面，CUR（不含α-CD单元）/pAC 12 Azo混合物、CD-CUR/聚（丙烯酸）（pAA，不含偶氮单元）混合物和CD-CUR/偶氮客体二聚体（Azo 2Dimer）混合物未形成超分子水凝胶（支持性信息中的图S7）。这些结果表明，在超分子水凝胶的形成过程中，不仅α-CD单元与偶氮单元的结合，而且CD-CUR与pAC 12 Azo之间的多重交联也起着重要作用。图1a-d分别显示了CDCUR/pAC 12 Azo（a）、CUR/pAC 12 Azo（B）、CD-CUR/pAA（c）和CD-CUR/Azo 2二聚体（d）的零剪切粘度（η0）。尽管CUR/pAC 12 Azo、CUR/Azo 2Dimer和CD-CUR/pAA显示出低粘度（小于3 Pa s），但CD-CUR/pAC 12 Azo显示出54 Pa s的粘度，其比可比较系统的粘度高18倍。研究了主客体比例对体系粘度变化的影响。当以不同的单体单元比（CD/Azo= 4：1、1：1、1：4）混合CD-CUR和pAC 12 Azo时，对于1：1的CD-CUR/pAC 12 Azo单体比获得最大η0值，从而表明有效形成互补包合复合物（图1 e-g）。
Hydrogels are ubiquitous in nature, especially in living organisms.[1] Synthetic hydrogels are used, for example, for tissue engineering, drug delivery systems (DDS), and medical treatment.[2] There are mainly two kinds of gels: physical gels [3] and chemical gels.[4] Hydrogels with noncovalent crosslinking points are supposed to be more versatile and easily tunable than those with covalent bonds. Although lowmolecular-weight physical gels exhibit photoresponsible elastic and photoswitchable properties,[5–9] hydrogels formed by host–guest interactions should be more sensitive to external stimuli than low-weight-molecular low-molecular-weight physical gels.[10–17] Although there are some reports on stimuli-responsive hydrogels that undergo a phase transition,[8–9] there have been few reports on light-responsive hydrogels formed by host–guest interactions. We found previously that the mixture of a host polymer that contains cyclodextrins (CDs) and a guest polymer that contains azobenzene (Azo) showed a viscosity change after photoirradiation.[18] However, we could not obtain a hydrogel from the mixture of both polymers. To obtain a hydrogel, we decided to use curdlan (β-1, 3 glucan, CUR) as a backbone because CUR has a rather rigid structure and CDs can be attached to each monomer unit in the polymer chain. We succeeded in obtaining a supramolecular hydrogel by mixing a CUR that is functionalized with CDs (CD-CUR) and a guest polymer that has Azo moieties attached to it (Scheme 1). This is the first example of a light-responsive hydrogel obtained from a CD polymer and a guest polymer. CD-CUR was prepared by a cycloaddition reaction between propargylamide-modified α-CD and 6-azido-curdlan (see the Supporting Information). CUR was substituted at theC6 positions by the modified α-CD to obtain CD-CUR with a substitution degree of 98%. When the azo-modified poly-(acrylic acid)(pAC12Azo), 8.6 wt%) and CD-CUR (4.0 wt%) were mixed in a 1: 1 ratio of the monomer units in water, the viscosity of the solution was first increased and then the solution transformed into a gel (Figure S7 in the Supporting Information). On the other hand, the CUR (without the α-CD unit)/pAC12Azo mixture, the CD-CUR/poly (acrylic acid)(pAA, without the azo unit) mixture, and the CD-CUR/azo-guest-dimer (Azo2Dimer) mixture did not form supramolecular hydrogels (Figure S7 in the Supporting Information). These results indicate that not only the combination between the α-CD unit and the azo unit but also multiple cross-links between CD-CUR and pAC12Azo play important roles in the formation of the supramolecular hydrogel. Figure1a–d shows the zero-shear viscosities (η0) for CDCUR/pAC12Azo (a), CUR/pAC12Azo (b), CD-CUR/pAA (c), and CD-CUR/Azo2Dimer (d), respectively. Although CUR/pAC12Azo, CUR/Azo2Dimer, and CD-CUR/pAA showed low viscosities (less than 3 Pa s), CD-CUR/pAC12Azo showed a viscosity of 54 Pa s, which is eighteen times higher than the viscosity of comparable systems. We have studied the effects of the host–guest ratio on the viscosity change of the system. When CD-CUR and pAC12Azo were mixed in different monomer unit ratios (CD/Azo= 4: 1, 1: 1, 1: 4), the largest η0 value was obtained for the 1: 1 monomer ratio of CD-CUR/pAC12Azo, thus indicating the effective formation of complementary inclusion complexes (Figure 1e–g).