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Cyclodextrin, a general term for a class of cyclic oligosaccharides obtained by starch decomposition, can be divided into α- cyclodextrin, β- cyclodextrin and γ- cyclodextrin according to the number of glucose molecules contained (six, seven and eight). Their most important feature is amphiphilicity: internal hydrophobic, external hydrophilic. Therefore, the cavity of cyclodextrin is often filled with hydrophobic small molecules to form a clathrate. In 1990, Harada and Kamachi first passed polyethylene glycol (PEG) polymer chains through α-cyclodextrin (α-CD)[1,2], successfully constructing a cyclic paste precision polycythane hydrogel, laying the foundation for the study of sliding cyclic hydrogel materials.
Polyethylene glycol passes through α-cyclodextrin. Image source: Nature [2]
In 2001, Kohzo Ito and Yasushi Okumura developed a supramolecular topological gel structure [3]. The polyethylene glycol is used as the guest to penetrate the α-ring dextrin, and then the end is terminated by a large estost group, so that the ring dextrin unit can move freely like a pulley. The cyclodextrin is then cross-linked to form a "figure 8" sliding ring hydrogel. Since then, the researchers have expanded the subject and object, such as α, β, γ-cyclodextrin and various derivatives thereof, polyvinyl alcohol instead of polyethylene glycol, and self-healing properties of materials [4,5].
Schematic diagram of sliding ring hydrogel. Image credit: J. Appl. Polym. Sci. [4]
Recently, researchers such as Kohzo Ito and Koichi Mayumi of the University of Tokyo in Japan published a paper in the journal Science that novelized the polyethylene glycol/cyclodextrin sliding ring hydrogel. Or the use of polyethylene glycol chain as a "line", through the hydroxypropyl-α-cyclodextrin sliding "ring", hydrogel hydroxypropyl-α-cyclodextrin "ring" is still crosslinked. The difference is that the authors carefully controlled the amount of cyclodextrin so that there are only about 8 cyclodextrin molecules on a polyethylene glycol chain. Don't look at this small parameter control, but the impact is huge - the resulting hydrogel material has obtained excellent toughness like natural rubber, which is an order of magnitude higher than ordinary covalent cross-linked polyethylene glycol hydrogels, and can be quickly and completely recovered after the disappearance of external forces, and can still maintain good performance after hundreds of cycles of stretching. The authors attribute the excellent toughness of this hydrogel to the non-damage-free self-reinforcement of strain-induced crystallization similar to that of natural rubber.
Stretching comparison of conventional hydrogels (left) with sliding ring hydrogels (right). Photo credit: University of Tokyo [6]
The reason why sliding ring hydrogels have good toughness is first derived from the "pulley effect", that is, the crosslinking point can move freely, balancing the tension of the threaded polymer chain in a manner similar to a pulley, improving the toughness of the hydrogel. In conventional hydrogels, polymers form a network through fixed crosslinking, and the toughness of the material under stretching or impact is achieved through an energy dissipation mechanism. However, crosslinking occurs on polymer chains, and long polymer chains are divided into short chains of different lengths, and stress is mainly concentrated on shorter chains, and fractures are most likely to occur. And this loss is difficult to recover quickly, and it usually takes minutes or even hours after the external force disappears. This results in a significant decrease in the mechanical properties of the hydrogel during recycling.
The pulley effect changes the length of the chain between the crosslinking points. Image credit: J. Appl. Polym. Sci. [4]
In this paper, the researchers used polyethylene glycol with an average molecular weight of 35,000 g/mol as the guest and passed through the hydroxypropyl-α-cyclodextrin body. Their previous research found that reducing the number of α-cyclodextrin molecules on the polyethylene glycol chain increased the sliding distance of the crosslinking point, thereby increasing the elongation of the hydrogel. Therefore, they controlled the α-cyclodextrin coverage (the lower the coverage, the smaller the number of α-cyclodextrin molecules on the polyethylene glycol chain) at about 2%, with an average of 8 α-cyclodextrin rings on each polyethylene glycol molecular chain, and the average molecular weight between adjacent rings was ~4400 g/mol. They wanted to reduce coverage further, but geling could not occur at 1% coverage and gave up. Another optimized parameter is the polymer concentration, which tested hydrogels with different polyethylene glycol volume fractions (0.18, 0.30, 0.38).
Experiments have shown that this sliding ring hydrogel is flexible when there is no strain, which is no different from ordinary hydrogels, and will quickly harden and rigid under strain, and then quickly return to the flexible hydrogel state after the strain disappears. The toughness under uniaxial stretching can reach 6.6-22 MJ m-2, which is an order of magnitude higher than traditional covalent cross-linked polyethylene glycol hydrogels. What's more, this sliding ring hydrogel has excellent reversible stretchability. In mechanical performance testing, reversible performance is often expressed as the ratio of the coverage area of the second cycle curve to the coverage area of the first cycle. In 100 cycle experiments, the sliding ring hydrogel exhibited 98-100% reversible stretchability.
Sliding ring hydrogel compared to conventional polyethylene glycol hydrogel tensile properties. Image source: Science
Where does this sliding ring hydrogel come from for its excellent performance? Researchers believe that in addition to the "pulley effect" of α-cyclodextrin, there is also a strain-induced crystallization mechanism similar to that found in natural rubber. Under uniaxial stretching, the crosslinked α-cyclodextrin slides along the chain, close to each other, and the polymer chains between the crosslinking points are stretched in a directional manner and arranged in parallel (Figure b below). Under extreme strain, strain-induced crystallization occurs in polyethylene glycol chains that are exposed to each other and are highly oriented (Figure c below). The strain disappears, the tightly packed crystal structure formed is destroyed, and the polyethylene glycol chain and α-cyclodextrin can be quickly restored to their original structure (Figure a below).
Schematic diagram of the sliding ring hydrogel stretching process. Image source: Science
To prove these guesses, the researchers performed small-angle X-ray scattering (SAXS) analysis of the material. During stretching, due to the sliding ring action, the chaotic polyethylene glycol is arranged in parallel orientation and crystallized under strain induction, making the hydrogel hardened, and sharp streaks are observed in the small-angle X-ray scattering pattern. But when the strain is released, they quickly return to the flexible state of the gel, and the strain induces the crystalline phase to disappear. This is similar to the tensile recovery process of natural rubber, and therefore has the same good cyclic stretching properties as natural rubber.
Structural transformation under cyclic stress. Image source: Science
"The problem with existing hydrogels is poor mechanical properties, and current enhancement methods can only guarantee limited tensile cycles and cannot recover quickly," says Koichi Mayumi. "Because hydrogels have a water content of more than 50 percent and are highly biocompatible, they are critical for medical applications," Mayumi said, "The next phase of the study is to try different arrangements of molecules, simplify the structure, and reduce the cost of materials, which will help accelerate the application of materials in the medical industry." ”[6]
Tough hydrogels with rapid self-reinforcement
Chang Liu, Naoya Morimoto, Lan Jiang, Sohei Kawahara, Takako Noritomi, Hideaki Yokoyama, Koichi Mayumi, Kohzo Ito
Science, 2021, 372, 1078-1081, DOI: 10.1126/science.aaz6694
bibliography:
[1] A. Harada, M. Kamachi, Complex formation between poly(ethylene glycol) and α-cyclodextrin. Macromolecules, 1990, 23, 2821-2823. DOI: 10.1021/ma00212a039
[2] A. Harada, J. Li, M. Kamachi, The molecular necklace: a rotaxane containing many threaded α-cyclodextrins. Nature, 1992, 356, 325-327. DOI: 10.1038/356325a0
[3] Y. Okumura, K. Ito, The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-links. Adv. Mater., 2001, 13, 485-487. DOI: 10.1002/1521-4095(200104)13:7<485::AID-ADMA485>3.0.CO;2-T
[4] Y. Noda, Y. Hayashi, K. Ito, From topological gels to slide-ring materials. J. Appl. Polym. Sci., 2014, 131, 40509. DOI: 10.1002/APP.40509
[5] M. Nakahata, et al. Self-Healing Materials Formed by Cross-Linked Polyrotaxanes with Reversible Bonds. Chem, 2016, 1, 766-775. DOI: 10.1016/j.chempr.2016.09.013
[6] Healing hydrogels
https://www.u-tokyo.ac.jp/focus/en/press/z0508_00179.html
(This article is contributed by Xiao Xi)