Double Network Gel: Tough Hydrogel

~ Living organs work with fantastic functions in soft and wet gel-like states, except for hard compositions such as bone and tooth. Thus, hydrogels attract considerable attention as excellent wet materials, suitable for fabricating artificial organs for medical treatments. However, conventional hydrogels are too brittle to be employed in such applications. To address this problem, our laboratory succeeded in synthesizing tough double network hydrogels (DN gels) by combining rigid strong polyelectrolyte gels (the first network) with flexible neutral polymer gels (the second network).
除了骨骼和牙齿等硬组合物外，活体器官在柔软和湿润的凝胶状态下具有出色的功能。因此，水凝胶作为优异的湿材料引起相当大的注意，适合于制造用于医学治疗的人造器官。然而，常规水凝胶太脆而不能用于这种应用中。为了解决这个问题，我们的实验室通过将刚性强聚电解质凝胶（第一网络）与柔性中性聚合物凝胶（第二网络）相结合，成功合成了坚韧的双网络水凝胶（DN凝胶）。

DN gels are as strong as rubber, and their compressive fracture stress is about 10–60 MPa (100–600 kgf/cm2), which is approximately hundred times stronger than single network gels (0.3–0.6 MPa). In addition, some types of DN gels also show flexibility, with tensile fracture strains reaching 30.
DN凝胶与橡胶一样坚固，其压缩断裂应力约为10-60 MPa（100-600 kgf / cm2），比单网凝胶（0.3-0.6 MPa）强约100倍。此外，某些类型的DN凝胶也显示出柔韧性，拉伸断裂应变达到30。

These high mechanical properties of DN gels arise from the reciprocity of two networks, which suppress the progression of cracks. When force is applied to DN gels, the rigid and brittle first network breaks down. For single network gels, microcracks can spread and cause a breakdown of the network , while for DN gels, the existence of the soft second network can connect the fragments of the first network, thereby suppressing the scale-up of microcracks. Therefore, large amounts of microcracks are in co-existence because of the distribution of stress, and large amounts of energy are dissipated in this process.
DN凝胶的这些高机械性能源于两个网络的互易性，这抑制了裂缝的进展。当对DN凝胶施加力时，刚性和脆性的第一网络发生故障。对于单网络凝胶，微裂纹可以扩散并导致网络崩溃，而对于DN凝胶，软第二网络的存在可以连接第一网络的碎片，从而抑制微裂纹的扩大。因此，由于应力的分布，大量的微裂纹共存，并且在该过程中消耗了大量的能量。

We have also developed tough hydrogels that were constructed completely from natural materials. Bacterial cellulose gels (BC gels), produced by Acetobacter xylinum , show high tensile fracture stress, but cannot hold enough water within them. In contrast, gelatin gels derived from animals can hold a sufficient amount of water, but they possess poor mechanical properties. We have succeeded in synthesizing BC/gelatin DN hydrogels having excellent mechanical properties by combining these two kinds of gel in order to compensate for each of their individual weaknesses.
我们还开发了完全由天然材料制成的坚韧水凝胶。由木醋杆菌（Acetobacter xylinum）产生的细菌纤维素凝胶（BC凝胶）显示出高拉伸断裂应力，但不能在其中保持足够的水。相反，来自动物的明胶凝胶可以保持足够量的水，但它们具有差的机械性能。通过组合这两种凝胶，我们成功地合成了具有优异机械性能的BC /明胶DN水凝胶，以补偿它们各自的弱点。

We are trying to apply DN gels to regeneration medicine in co-operation with Prof. Yasuda's group (Graduate School of Medicine, Hokkaido University). Articular cartilages play very important roles such as cushioning when we perform exercises. Until now, it was believed that once an articular cartilage is broken, it would never fully regenerate. Hence, it has been difficult to repair a broken cartilage adequately. However, we succeeded in regenerating a cartilage by embedding a biocompatible DN gel in the defect of rabbit cartilage for four weeks. This is the world's first case of successful cartilage regeneration! For details, visit Prof. Yasuda's homepage
我们正在尝试将DN凝胶应用于Yasuda教授团队（北海道大学医学研究生院）的再生医学。当我们进行锻炼时，关节软骨起着非常重要的作用，例如缓冲。到目前为止，人们认为，一旦关节软骨破裂，它就永远不会完全再生。因此，难以充分修复破损的软骨。然而，我们通过在兔软骨缺损中嵌入生物相容性DN凝胶四周来成功地再生软骨。这是世界上第一例成功进行软骨再生的案例！有关详细信息，请访问 Yasuda教授的主页

Compared with common hydrogels, DN gels possess higher mechanical properties. However, there are some problems with regard to their application. The largest problem is that it is difficult to synthesize DN gels in free shape because of the poor mechanical strength of the first network gel. Apart from this issue, it is important to be able to shape gels freely for application as artificial cartilage. Recently, we succeeded in synthesizing freely shapeable DN gels by two methods: by synthesizing DN gels from a particle first network gel or by using a poly(vinyl alcohol) hydrogel mold.
与普通水凝胶相比，DN凝胶具有更高的机械性能。但是，它们的应用存在一些问题。最大的问题是由于第一网络凝胶的机械强度差，难以合成自由形状的DN凝胶。除了这个问题之外，重要的是能够自由地形成凝胶以用作人造软骨。最近，我们成功地通过两种方法合成可自由成形的DN凝胶：通过从颗粒第一网络凝胶合成DN凝胶或通过使用聚（乙烯醇）水凝胶模具。

References
Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Adv. Mater. 2003, 15, 1155.
Na, Y.-H.; Tanaka, Y.; Kurokawa, T.; Furukawa, H.; Sumiyoshi, T.; Gong, J. P.; Osada, Y. Macromolecules 2006, 39, 641.
Tanaka, Y.; Kawauchi, Y.; Kurokawa, T.; Furukawa, H; Okajima, T.; Gong, J. P. Macromol. Rapid Commun. 2008, 29, 1514.
Yasuda, K.; Kitamura, N.; Gong, J. P.; Arakaki, K.; Kwon, H. J.; Onodera, S.; Chen, Y. M.; Kurokawa, T.; Kanaya, F.; Ohmiya, Y.; Osada, Y. Macromol. Biosci., 2009, 9, 307.

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