Design principles for strong and tough hydrogels 强韧水凝胶的设计原则
Xueyu Li Jian Ping Gong 李雪雨 建平宫
Abstract 摘要
Hydrogels are crosslinked polymer networks swollen with water. Owing to their soft and water-containing nature, hydrogels are promising materials for applications in many fields, such as biomedical engineering, soft robotics and environmental studies. One of the main obstacles to the practical application of hydrogels is their low mechanical strength and toughness. Since the 2000s, many breakthroughs in the development of mechanically strong and tough hydrogels have led to enormous advances in the study of soft materials and our understanding of their failure mechanisms. Research has also been conducted on long-term mechanical stability - that is, the cyclic fatigue resistance and self-strengthening properties of hydrogels - to enable their application as load-bearing materials. This Review provides a comprehensive overview of the design principles for tough hydrogels. Strategies to obtain self-growing and reinforced hydrogels that can adapt to their surrounding mechanical environment are also presented. 水凝胶是交联聚合物网络，含有水。由于其柔软和含水性质，水凝胶是许多领域的应用中有前景的材料，如生物医学工程、软机器人和环境研究。水凝胶在实际应用中的一个主要障碍是其低机械强度和韧性。自 2000 年以来，对机械强度和韧性较高的水凝胶的开发取得了许多突破，从而在软材料研究和我们对其失效机制的理解方面取得了巨大进展。还进行了关于长期机械稳定性的研究 - 即水凝胶的循环疲劳抗性和自强化性能，以使其能够作为承载材料应用。本综述提供了对坚韧水凝胶设计原则的全面概述。还介绍了获得自生长和加固水凝胶的策略，使其能够适应其周围的机械环境。
Sections 部分
Introduction 介绍
Model of basic hydrogel mechanics 基础水凝胶力学模型
Design strategies 设计策略
Conclusions and outlook 结论和展望
Review article 评论文章
Introduction 介绍
As a combination of crosslinked solid and liquid components, hydrogels are similar to soft tissues in the human body and are promising materials for uses such as scaffolds in tissue engineering , medical implants or wound dressings . Many applications require hydrogels to bear mechanical loads and to resist failure under static or cyclic loading conditions. The macroscopic failure of materials originates from the growth of small defects. The ability to resist defect growth - the toughness - is correlated to the fracture energy of materials, which is the energy required for crack growth per unit area . Typically, loadbearing biological tissues have a high toughness. For instance, typical fracture energies are for skeletal muscle, for cartilage and for tendons . However, conventional synthetic hydrogels, which are composed of a single non-uniform network of hydrophilic polymers, are in general very weak, with low fracture energy . This poor mechanical performance greatly limits the application of such hydrogels. In addition to the toughness, elastic modulus (stiffness), fracture strength (strength), fracture stretch ratio (deformability) and fatigue threshold are important mechanical parameters for applications. For a simple polymer network material, these mechanical properties are correlated and conflicting. Increases in elastic modulus and strength result in decreases in deformability, fracture energy and fatigue threshold. Moreover, as hydrogels are polymer networks swollen in water (solvent), these mechanical parameters are strongly related to the swelling ratio of the hydrogels. At the equilibrium swelling, the swelling ratio is determined by the balance of osmotic pressure and network elasticity . Therefore, designing and developing hydrogels with high toughness without sacrificing the modulus and strength is a challenge. 作为交联固体和液体组分的组合物，水凝胶类似于人体软组织，是组织工程支架、医用植入物或伤口敷料等用途的有前途的材料。许多应用需要水凝胶承受机械载荷并在静态或循环加载条件下抵抗破坏。材料的宏观破坏源于小缺陷的增长。抵抗缺陷增长的能力 - 韧性 - 与材料的断裂能量相关，即单位面积裂纹增长所需的能量。通常，承载载荷的生物组织具有较高的韧性。例如，骨骼肌的典型断裂能量为，软骨为，肌腱为。然而，由一种非均匀网络组成的传统合成水凝胶通常非常脆弱，具有较低的断裂能量。这种劣质的机械性能极大地限制了这类水凝胶的应用。 除了韧性外，弹性模量 （刚度）、断裂强度 （强度）、断裂延展率 （可变形性）和疲劳阈值 是应用中重要的机械参数。对于简单的聚合物网络材料，这些机械性能是相关且矛盾的。弹性模量和强度的增加导致可变形性、断裂能量和疲劳阈值的降低。此外，由于水凝胶是在水（溶剂）中膨胀的聚合物网络，这些机械参数与水凝胶的膨胀比 强相关。在平衡膨胀时，膨胀比由渗透压和网络弹性平衡决定 。因此，设计和开发具有高韧性的水凝胶而不牺牲模量和强度是一个挑战。
The poor mechanical performance of conventional hydrogels is attributed to several intrinsic features. One is the network inhomogeneity in polymer density distribution and polymer strand length between the crosslinking points. Thus, hydrogels are susceptible to stress concentration at loading, initiating cracks. Another is the rubber-like elasticity caused by a lack of energy dissipation mechanisms, resulting in low resistance against crack propagation. In addition, conventional hydrogels usually contain abundant water. The low amount of load-bearing solid phase further results in weak and fragile mechanical properties. Since the 2000s, the development of mechanically strong and tough hydrogels has led to important advances in the study of soft materials and our understanding of their failure mechanisms. The main strategies to improve the mechanical properties of hydrogels through their structural components can be classified into three categories: the design of topological structures, such as slide-ring gels , homogeneous four-arm gels and highly entangled gels , to distribute stress in single-network systems; the introduction of energy dissipation mechanisms by sacrificial bonds, such as in double-network (DN) hydrogels and dual-crosslinked hydrogels ; and the introduction of high-order structures, such as microphase separations , microcrystals, and fibrils or fabrics . From the perspective of mechanical dynamics, they can be classified into elastic and viscoelastic hydrogels that are strain-rate independent and dependent, respectively, in the common observation window. 传统水凝胶的机械性能差主要归因于几个固有特征。 其中一个是聚合物密度分布和交联点之间聚合物链长度的网络不均匀性。 因此，水凝胶容易在加载时出现应力集中，从而引发裂纹。 另一个是由于缺乏能量耗散机制而导致的橡胶状弹性，从而使其抗裂纹传播能力较低。 此外，传统水凝胶通常含有大量水。 负载承载固相的少量进一步导致机械性能薄弱和脆弱。 自 2000 年代以来，机械强度和韧性水凝胶的发展已经在软材料研究和我们对其失效机制的理解方面取得了重要进展。 通过其结构组分改善水凝胶的力学性能的主要策略可分为三类：设计拓扑结构，如滑环凝胶 、均匀四臂凝胶 和高度纠缠凝胶 ，以在单网络系统中分布应力；通过牺牲键引入能量耗散机制，如双网络（DN）水凝胶 和双交联水凝胶 ；引入高阶结构，如微相分离 、微晶体和纤维或织物 。从力学动力学的角度来看，它们可以分为弹性和粘弹性水凝胶，在常见观察窗口中分别是应变速率独立和依赖的。
Fatigue resistance - that is, long-term stability under cyclic loadsis vital for some applications such as artificial cartilage. Since the initial work in 2017 (ref. 6), the study of fatigue-resistant mechanisms and the development of fatigue-resistant hydrogels have attracted considerable interest. Numerous antifatigue design strategies have been proposed. For elastic gels, these strategies involve modulations at the molecular level, such as lengthening the polymer chain , increasing entanglement , and unfolding or degrading the crosslinker . For viscoelastic gels, mesoscale modifications have been proposed, such as microphase separation , microcrystallization and fibrils , and nanocomposites . The fatigue threshold is greatly enhanced for gels with hierarchical structures , resolving the conflict between the modulus and the fatigue threshold. 疲劳抗性 - 即在循环载荷下的长期稳定性对于某些应用，如人工软骨至关重要。自 2017 年的最初研究以来（参考文献 6），对抗疲劳机制的研究和疲劳抗性水凝胶的开发引起了广泛关注。已提出了许多抗疲劳设计策略。对于弹性凝胶，这些策略涉及分子水平的调节，如延长聚合物链 ，增加纠缠 ，展开或降解交联剂 。对于粘弹性凝胶，已提出了介观尺度的修改，如微相分离 ，微晶化和纤维 ，以及纳米复合材料 。对于具有分层结构的凝胶，疲劳阈值得到了极大增强 ，解决了模量和疲劳阈值之间的冲突。
After about two decades of effort, nowadays the fracture energy and fatigue threshold of hydrogels can reach (refs. 16,44) and (ref. 18), respectively. Elastic modulus from submegapascal to hundreds of megapascals, strength from submegapascal to tens of megapascals, and stretching ratio from several to hundreds can be achieved. Moreover, taking inspiration frombiological systems, efforts have been made to develop self-healing hydrogels and anisotropichydrogels. Unlike other solid materials, hydrogels are permeable to small molecules. Thus, hydrogels could be used as an open system to develop mechanically triggered new networkgrowth. Self-growing and strengthening hydrogels based on mechanochemistry mechanisms have also attracted attention for the purpose of developing materials that adapt to their surrounding environment, resembling biological systems . 经过大约二十年的努力，如今水凝胶的断裂能量和疲劳阈值分别可以达到 （参考文献 16,44）和 （参考文献 18）。弹性模量从亚兆帕斯卡到数百兆帕斯卡，强度从亚兆帕斯卡到数十兆帕斯卡，拉伸比从几到数百 都可以实现。此外，受生物系统启发，人们努力开发自修复水凝胶和各向异性水凝胶。与其他固体材料不同，水凝胶对小分子具有渗透性。因此，水凝胶可以用作开放系统，以发展机械触发的新网络生长。基于机械化学机制的自生长和强化水凝胶也引起了人们的关注，目的是开发能够适应周围环境的材料，类似于生物系统 。
The advancements in hydrogel design strategies, focusing on achieving comprehensive mechanical performance, have been summarized in several outstanding review papers, particularly those emphasizing the structure of polymer networks and the nonlinear elastic fracture mechanics . Given the importance of swelling and deswelling characteristics in hydrogels, and the practical need for hydrogels to be in an equilibrium state of swelling in a liquid medium, this Review explores design strategies to attain high mechanical performance with a focus on the perspective of swelling and deswelling. We initially use a basic hydrogel model to clarify the influences of the molecular structure and swelling or deswelling on the mechanical properties, encompassing the elastic modulus, extensibility, strength, toughness and fatigue resistance. We then discuss the main design strategies to obtain tough hydrogels, followed by advances in developing fatigue-resistant hydrogels and in understanding their underlying mechanisms. We highlight recently developed strategies for developing self-reinforcement hydrogels with tissue-like self-growing properties. To close, we emphasize challenges and trends in developing inelastic fracture mechanics theory to capture the large deformation behaviour and the next generation of tough hydrogels for practical biological applications. 水凝胶设计策略的进展主要集中在实现全面的力学性能上，已经在几篇杰出的综述论文中进行了总结，特别强调了聚合物网络结构和非线性弹性断裂力学。鉴于水凝胶中膨胀和脱胀特性的重要性，以及水凝胶在液体介质中处于膨胀平衡状态的实际需求，本综述探讨了通过关注膨胀和脱胀的角度来实现高力学性能的设计策略。我们首先使用基本水凝胶模型来阐明分子结构和膨胀或脱胀对力学性能的影响，包括弹性模量、延展性、强度、韧性和疲劳抗性。然后我们讨论了获得韧性水凝胶的主要设计策略，接着是发展抗疲劳水凝胶和理解其基本机制的进展。我们重点介绍了最近开发的具有组织样自生长特性的自强化水凝胶的策略。 总的来说，我们强调发展不可塑性断裂力学理论以捕捉大变形行为和下一代用于实际生物应用的坚韧水凝胶的挑战和趋势。
Model of basic hydrogel mechanics 基础水凝胶力学模型
Various theoretical models, including the classic Flory-Rehner statistical model , Arruda-Boyce eight-chain constitutive model and Gent continuum mechanics model , along with their combination , have been explored to explain the swelling and large deformation of polymer networks. These models are discussed in reviews elsewhere . Here, we analyse a simple affine network to represent the structure of a hydrogel, and we discuss its rubber elasticity and fracture at different swelling states. The hydrogel features a uniform polymer network comprising strands characterized by the number of Kuhn monomers in each strand and the length of each monomer (b) (Fig. 1a). In the reference state, the end-to-end distance of each strand follows an ideal Gaussian chain, approximately given by , and the network has a strand density denoted as (refs. 76,83). On contact with water, the hydrogel undergoes a size change by a factor of in length ( 1indicates swelling, and indicates deswelling). Operating as an affine network, where the deformations of the bulk hydrogel and 各种理论模型，包括经典的 Flory-Rehner 统计模型 ，Arruda-Boyce 八链本构模型 和 Gent 连续力学模型 ，以及它们的组合 ，已被探讨用于解释聚合物网络的膨胀和大变形。这些模型在其他地方已经讨论过 。在这里，我们分析一个简单的仿生网络来代表水凝胶的结构，并讨论其橡胶弹性和在不同膨胀状态下的断裂。水凝胶具有由每个链中 Kuhn 单体的数量 和每个单体的长度（b）（图 1a）表征的均匀聚合物网络。在参考状态下，每个链的端到端距离 遵循理想的高斯链，近似为 ，并且网络具有一个链密度表示为 （参考文献 76,83）。与水接触时，水凝胶的尺寸会按长度因子 发生变化（ 表示膨胀， 表示收缩）。作为一个仿生网络，其中水凝胶的整体变形和
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individual network strands are identical , the end-to-end distance of a strand becomes . Consequently, the strand density transforms to (Fig. 1a). Subsequently, we investigate the influence of on the elastic modulus, stretchability and fracture characteristics of the hydrogel in the absence of viscoelastic effect, unless specified. 个体网络链条是相同的 ，链条的端到端距离变为 。因此，链条密度转变为 （图 1a）。随后，我们研究了 对水凝胶的弹性模量、延展性和断裂特性的影响，在没有粘弹性效应的情况下，除非另有规定。
Elastic modulus 弹性模量
The Young's modulus ( ) of a hydrogel is determined by the product of the elasticity or stiffness per strand and the density of strands , expressed as . As the elasticity of a strand increases with swelling and strand number per volume decreases with swelling, exhibits a non-monotonic dependence on . In this context, we use the freely jointed chain model to represent the elastic energy of a strand, considering the finite extensibility effect (see Supplementary Information . By taking the second derivative of the elastic energy of a strand with respect to the stretching ratio, we derive the elasticity of the strand, expressed as: 年轻模量（ ）的水凝胶由弹性 或每股刚度与股密度 的乘积确定，表示为 。随着股的膨胀 和每单位体积的股数随膨胀减少， 对 表现出非单调依赖性。在这种情况下，我们使用自由联接链模型来表示股的弹性能量，考虑有限可伸展性效应（见补充信息 。通过对股的弹性能量关于拉伸比的二阶导数，我们推导出股的弹性，表示为：
where . Consequently, the Young's modulus of the hydrogel is given by: 在 。因此，水凝胶的杨氏模量由以下公式给出：
In Fig. 1a, we depict as a function of for 20 as a representative example. Here is the Young's modulus of the hydrogel at the reference state . The curve in Fig. 1a illustrates the non-monotonic evolution of the elastic modulus with swelling ratio. For small or modest , where is much smaller than the contour length of the strand , polymer strands can be approximated as Gaussian chains, and the elasticity is proportional to . Therefore, decreases as increases because the strand density is proportional to . For large where approaches , the stiffness of strands increases rapidly owing to the limited extensibility of the strand . The increase of strand stiffness with is much stronger than , surpassing the effect from the decrease of strand density and resulting in an increase of with . 在图 1a 中，我们以 20 为代表性例子，将 描绘为 的函数。这里 是水凝胶在参考状态 的杨氏模量。图 1a 中的曲线说明了弹性模量随着膨胀比的非单调演变。对于小或适度 ，其中 远小于链 的轮廓长度，聚合物链可以近似为高斯链，弹性与 成正比。因此， 随着 的增加而减少，因为链密度与 成正比。对于大 ，其中 接近 ，由于链 的有限延展性，链的刚度迅速增加。链刚度随 的增加要比 强得多，超过了链密度减少的影响，导致 随 的增加而增加。
For a deswelling hydrogel , when there are physical interactions between polymer strands, the elastic modulus comprises two terms, , where is from the primary network and increases with deswelling (Fig. 1a, regime C), and is from dynamic crosslinking of interstrand physical bonds and changes with the observation time (or strain rate ) relative to the characteristic relaxation time ( ) of the viscoelastic hydrogel. At low strain rate ( ), the dynamic bonds have little contribution to the modulus, and the hydrogel behaves as a soft solid with elastic modulus , like the elastic hydrogel. At high strain rate ( ), the dynamic bonds play a similar role to the permanent crosslinking, and the hydrogel behaves as a hard elastic solid with a large . In the intermediate strain rate, the modulus increases as the strain rate increases (viscoelastic regime). The modulus of viscoelastic hydrogels typically follows the time-temperature superposition principle, as the bond association time is strongly temperature dependent and influences (refs. ). 对于脱肿水凝胶 ，当聚合物链之间存在物理相互作用时，弹性模量包括两个项 ，其中 来自主要网络并随脱肿增加（图 1a，区域 C）， 来自链间物理键的动态交联，并随观察时间（或应变速率 ）相对于粘弹性水凝胶的特征弛豫时间（ ）而变化。在低应变速率（ ）下，动态键对模量的贡献很小 ，水凝胶表现为弹性模量 的软固体，类似于弹性水凝胶。在高应变速率（ ）下，动态键起到类似于永久交联的作用，水凝胶表现为具有大 的硬弹性固体。在中等应变速率下，模量随着应变速率的增加而增加（粘弹性区域）。粘弹性水凝胶的模量通常遵循时间-温度超定位原则，因为键的结合时间强烈依赖于温度并影响 （参考文献 ）。
Maximum stress and stretch ratio 最大应力和拉伸比例
The theoretical maximum stretching ratio ( ) and engineering stress strongly depend on the swelling behaviour. Here we use a simple model to see the effect of on and . Under deformation, the maximum stretch ratio of a single strand, which goes from to its physical limit or contour length ( ), is 理论最大拉伸比（ ）和工程应力 高度依赖于膨胀行为。在这里，我们使用一个简单的 模型来观察 对 和 的影响。在变形下，单根纤维的最大拉伸比，从 到其物理极限或轮廓长度（ ），是
The maximum engineering stress, which is the product of a single strand's rupture force and the areal density of strands crossing the plane perpendicular to the loading direction in the undeformed state , is given by 最大工程应力，即单股断裂力 与垂直于加载方向的平面上穿过股的面密度 的乘积，在未变形状态下给出
Here, the monomer concentration in the as-synthesized state is . From equations (3) and (4), the true maximum stress ) of the simple hydrogel is proportional to the bond rupture force and bond density of the network , independent of . 在这里，合成状态下的单体浓度为 。根据方程（3）和（4），简单水凝胶的真正最大应力 ）与网络的键断裂力和键密度成正比，与 无关。
Thus, the stretchability and strength of a hydrogel decrease by swelling. 因此，水凝胶的延展性和强度会随着膨胀而减少。
Stress concentration effect 应力集中效应
For real hydrogels, as for other brittle materials, the theoretical has never been observed macroscopically because of stress concentration at defects or crack tips. For hydrogels for which nonlinear elastic fracture mechanics apply, two characteristic lengths, the nonlinear elastic length , denoted as the distance from the crack tip below which the deformation is dominated by elastic nonlinearity at the onset of crack initiation and defect-insensitive length ( , denoted as the critical crack length for the transition from defect-insensitive to defectsensitive rupture or dissipative length, are important . The true tensile stress around the crack tip inversely scales with the distance from the crack tip ( ), provided that the length of a defect or crack (c) exceeds a certain nonlinear elastic length , and is in the range of (refs. 75,84) (Fig. 1b). 对于真实的水凝胶，就像其他脆性材料一样，由于缺陷或裂纹尖端的应力集中，理论 从未在宏观上观察到。对于适用于非线性弹性断裂力学的水凝胶，两个特征长度，非线性弹性长度 ，表示裂纹尖端以下的距离，在裂纹启动 时，变形由弹性非线性主导，以及不敏感缺陷长度（ ，表示从不敏感缺陷到敏感缺陷破裂 或耗散长度的临界裂纹长度，是重要的 。真正的拉伸应力 在裂纹尖端周围与距离裂纹尖端的距离成反比（ ），前提是缺陷或裂纹的长度（c）超过一定的非线性弹性长度 ，且 在 范围内（参见文献 75,84）（图 1b）。
Here, the fracture energy represents the energy required to create a unit surface by propagating a pre-existing crack. The true stress converges to the external stress applied to the material when the distance from the crack is much larger than the crack size . Therefore, even if the applied stress is still far below the theoretical maximum stress , the near a defect could already reach , resulting in material failure. Owing to this stress concentration effect at the crack tip, the elastic energy per unit volume that a soft material can store before undergoing rupture decreases inversely proportional to , provided that exceeds (ref. 75) (Fig. 1c). 这里，断裂能量 代表通过传播预先存在的裂纹来创建单位表面所需的能量。当距离裂纹远大于裂纹尺寸 时，真应力会收敛到施加在材料上的外部应力。因此，即使施加的应力仍远低于理论最大应力 ，存在缺陷附近的 可能已经达到 ，导致材料失效。由于裂纹尖端的应力集中效应，软材料在发生破裂之前可以存储的弹性能量每单位体积 与 成反比，前提是 超过 （参见 75 页）（图 1c）。
For an elastic hydrogel, corresponds to the area under the stress-strain curve of the material. Thus, is not an intrinsic material parameter and depends on the pre-existing crack length . Equation (7) explains why in real hydrogels, in which defects inevitably exist, the 对于弹性水凝胶， 对应于材料应力-应变曲线下的面积。因此， 不是固有的材料参数，而是取决于预先存在的裂纹长度 。方程（7）解释了为什么在现实水凝胶中，缺陷不可避免地存在。
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a Hydrogel swelling behaviour and its effect on elastic modulus 一种水凝胶膨胀行为及其对弹性模量的影响
d Fracture energy of hydrogels 水凝胶的断裂能量
: Lake-Thomas model ：湖-托马斯模型
b True stress as a function of distance to the crack tip b 真应力作为距离 到裂纹尖端的函数
C Critical strain-energy density at crack initiation as a function of crack length C 临界应变能密度在裂纹长度 作为函数的裂纹初始化
Polymer chain lying across the crack propagation plane 聚合物链横跨裂纹传播平面
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Abstract 摘要
Fig. 1|Hydrogel swelling, deswelling and fracture behaviours. a, Schematic depiction of hydrogel swelling behaviour and its influence on the elastic modulus. The top panel shows an affine network hydrogel in its reference state. In the middle panels, the hydrogel undergoes swelling or deswelling by a length factor of . The bottom panel shows the elastic modulus of the hydrogel as a function of , where is normalized by its value at the reference state , and is normalized by the maximum allowable value ( ). In practice, hydrogels exhibit distinct behaviours: typically, a neutral hydrogel swells modestly, and is in the regime where decreases (regime ); a strong polyelectrolyte hydrogel swells substantially, and can access the regime where increases (regime ); and a hydrogel containing dynamic bonds deswells in water, and is greater than 1 (regime C).b, True stress distribution along the principal axis as a function of distance to the crack tip for soft materials, based on nonlinear elastic fracture mechanics theory. and are the dissipative length and nonlinear elastic length, respectively.c, Critical strain-energy density at crack initiation as a function 图 1 | 水凝胶的膨胀、脱水和断裂行为。a，水凝胶膨胀行为的示意图及其对弹性模量的影响。顶部面板显示了亲和网络水凝胶处于其参考状态。在中间面板中，水凝胶通过长度因子 膨胀或脱水。底部面板显示了水凝胶的弹性模量 作为 的函数，其中 由其在参考状态 的值归一化， 由最大允许值（ ）归一化。在实践中，水凝胶表现出不同的行为：通常，中性水凝胶轻微膨胀， 处于 减小的区域（区域 ）；强聚电解质水凝胶大幅膨胀， 可进入 增加的区域（区域 ）；含有动态键的水凝胶在水中脱水， 大于 1（区域 C）。b，基于非线性弹性断裂力学理论，软材料中主轴上真应力沿裂纹尖端距离 的分布。 和 分别是耗散长度和非线性弹性长度。c，临界应变能密度 在裂纹启动时作为一个函数
of crack (defect) length is the critical strain-energy density for rupturing a material in the absence of large cracks of , Fracture energy of hydrogels. is the initial height. is the critical stretch ratio where the pre-existing crack starts propagation. In general, , where is correlated to the energy dissipation in the processing zone and increases with dissipation length , and is intrinsic fracture energy, correlated to the energy required to break polymer chains lying across the crack plane by a unit area. For a simple network, and can be described by the Lake-Thomas model. The typical processing zone sizes (or dissipation length) are and for simple network hydrogels, double-network hydrogels, dual-crosslinked viscoelastic hydrogels, strain-induced crystallization hydrogels and fibrereinforced composites, respectively . Panels and adapted with permission from ref. 75, Annual Reviews. Panel d adapted with permission from ref. 69, Royal Society of London. fracture stretch ratio and engineering stress vary from sample to sample and are much smaller than and predicted from their average structure. 裂纹（缺陷）长度 是在没有大裂纹的情况下材料破裂的临界应变能密度 ，水凝胶的断裂能 。 是初始高度。 是预先存在的裂纹开始传播的临界拉伸比。一般来说， ，其中 与加工区域中的能量耗散相关，并随着耗散长度 增加， 是固有断裂能，与在裂纹平面上横跨聚合物链所需的能量相关。对于简单网络， 和 可以用 Lake-Thomas 模型描述。简单网络水凝胶、双网络水凝胶、双交联粘弹性水凝胶、应变诱导结晶水凝胶和纤维增强复合材料的典型加工区域尺寸（或耗散长度）分别为 和 。面板 和 经授权改编自参考文献 75，年度评论。面板 d 经授权改编自参考文献 69，伦敦皇家学会。 骨折拉伸比 和工程应力 因样本而异，远小于从其平均结构预测的 和 。
Fracture energy 断裂能量
The fracture energy is an intrinsic material parameter for characterizing a material's toughness. For the swelling elastic hydrogel, the Lake-Thomas model provides a description for as the energy needed to break a single layer of strands intersecting the crack plane: 断裂能量 是表征材料韧性的固有材料参数。对于膨胀弹性水凝胶，Lake-Thomas 模型 提供了一个描述， 作为破裂平面相交的单层纤维所需的能量：
Here, represents the activation energy required to break a chemical bond in the backbone of strands. The Lake-Thomas model postulates that the energy needed to break an individual strand is directly proportional to the monomer number of that particular strand . This proportionality arises because all the bonds in the strand backbone are arranged in series, necessitating the stretching of each bond to the same energy state to induce bond rupture (Fig.1d). By inserting equations (5) and (8) into equation (6), the defect-insensitivity length or energy dissipative length for the model elastic hydrogel is found to be of the order of the end-to-end distance of the polymer strands (the network mesh size), . 这里， 代表在链条主干中断裂化学键所需的活化能。Lake-Thomas 模型假设，断裂单个链条所需的能量与该特定链条的单体数成正比 。这种比例关系是因为链条主干中的所有键都是串联排列的，需要将每个键拉伸到相同的能量状态 以诱导键的破裂（图 1d）。通过将方程（5）和（8）插入方程（6），模型弹性水凝胶的缺陷不敏感长度或能量耗散长度 被发现与聚合物链条的端到端距离（网络网格尺寸）的量级相同 。
To bolster a material's strength, reducing the stress concentration is crucial, as underscored by equations (6) and (7). Increasing , reducing the size of defects or cracks (c), and increasing the energy required for crack propagation are potential strategies. For improving , the central idea is to incorporate additional energy-dissipating mechanism from a processing zone at the crack tip that extends beyond the network mesh size. The total fracture energy can be expressed as the sum of two components: , where represents the dissipation directly related to breaking the polymer strands bridging the crack (the intrinsic fracture energy) and can be described by the Lake-Thomas model (Fig. 1d), and accounts for the contribution of mechanical energy dissipation from the process zone. The process zone also increases the defect-insensitivity length . Different molecular designs can substantially increase the crack-tip processing zone (Fig. 1d). For a deswelling hydrogel that is dual crosslinked, the viscoelasticity resulting from the dynamic bonds brings intrinsic energy dissipation mechanisms. 为了增强材料的强度，减少应力集中是至关重要的，正如方程（6）和（7）所强调的。增加 ，减小缺陷或裂纹（c）的尺寸，以及增加裂纹传播所需的能量 都是潜在的策略。为了改善 ，中心思想是从裂纹尖端的加工区域中引入额外的能量耗散机制，该机制延伸至网络网格尺寸之外。总断裂能量可以表示为两个组成部分的总和： ，其中 代表与断裂聚合物链桥直接相关的耗散（固有断裂能量），可以用 Lake-Thomas 模型（图 1d）描述， 表示来自过程区的机械能耗散的贡献。过程区还增加了缺陷不敏感长度 。不同的分子设计可以显著增加裂纹尖端的加工区域（图 1d）。对于双重交联的脱肿水凝胶，由于动态键带来的粘弹性带来了固有能量耗散机制。
The of elastic gels can be experimentally measured based on Griffith's theory for a brittle fracture . The theory postulates that a pre-existing crack will extend when the rate of strain-energy release from the stress field around the crack is at least equal to the rate at which it is absorbed by crack extension. Commonly used experimental setups for measuring the fracture energy for the crack initiation (the onset of crack propagation) can be found in the literature . 弹性凝胶的 可以根据格里菲斯理论进行实验测量，该理论适用于脆性断裂 。该理论假设，当裂纹周围的应力场释放应变能的速率至少等于被裂纹延伸吸收的速率时，预先存在的裂纹将延伸。用于测量裂纹起始处的断裂能量（裂纹传播开始）的常用实验设置可以在文献中找到 。
Design strategies 设计策略
In practice, hydrogels typically undergo swelling when immersed in water after synthesis. The reference state of the model hydrogel described in the theoretical section may correspond to the as-synthesized hydrogel. The swelling ratio of a hydrogel in the equilibrium state is governed by the equilibrium between the osmotic pressure arising from the polymer-solvent (water) mixing free energy and the network elasticity of the hydrogels, which can be described by the Flory-Rehner theory . 在实践中，水凝胶通常在合成后浸泡在水中时会发生膨胀。理论部分描述的模型水凝胶的参考状态可能对应于合成后的水凝胶。水凝胶在平衡状态下的膨胀比 由聚合物-溶剂（水）混合自由能引起的渗透压和水凝胶的网络弹性之间的平衡所控制，这可以用 Flory-Rehner 理论 来描述。
Generally, hydrogels made from neutral polymers display modest swelling, leading to a decrease in the elastic modulus on swelling (Fig. 1a, regime A). By contrast, polyelectrolyte hydrogels, where ionic osmotic pressure from Donnan equilibrium contributes to excessive swelling in pure water, result in an increase in with swelling (Fig. 1a, regime ). When physical bonds can be formed between polymer strands, hydrogels undergo deswelling after synthesis . 通常，由中性聚合物制成的水凝胶显示出适度的膨胀，导致在膨胀时弹性模量降低（图 1a，区域 A）。相比之下，聚电解质水凝胶，其中来自唐南平衡的离子渗透压导致在纯水中过度膨胀，导致弹性模量随膨胀而增加（图 1a，区域 2）。当聚合物链之间可以形成物理键时，水凝胶在合成后会发生脱水。
For a swelling gel , the equilibrium increases as increases , whereas for a deswelling gel , the equilibrium is barely dependent on (refs. 41,42). decreases as chemical crosslinker density increases. Swelling hydrogels typically exhibit elastic behaviour with negligible interstrand interaction. On the other hand, deswelling hydrogels usually exhibit viscoelastic behaviour due to physical bonds among polymer strands (Fig. 1a). It is important to note that there are exceptions to these patterns . This Review focuses on discussing these two typical patterns. 对于膨胀凝胶 ，平衡 随 增加而增加 ，而对于收缩凝胶 ，平衡 几乎不依赖于 （参考文献 41,42）。 随着化学交联剂密度的增加而减少。膨胀水凝胶通常表现出弹性行为，几乎没有链间相互作用。另一方面，收缩水凝胶通常由于聚合物链之间的物理键而表现出粘弹性行为（图 1a）。重要的是要注意这些模式存在例外 。本综述重点讨论这两种典型模式。
Elastic hydrogels (typically ) 弹性水凝胶（通常 ）
In real hydrogels, there are inherent variations in both the distribution of polymer density and the length of a polymer strand (Fig. 2a). Therefore, hydrogels naturally contain numerous defects. A single, relatively large defect produces remarkable stress concentration and triggers crack propagation, resulting in the failure of hydrogels at macroscopic stress levels that are much lower than theoretical averages would suggest (equations (6) and (7)). Because elastic hydrogels have no energy dissipation mechanism during deformation, real swelling hydrogels are typically very weak, and the failure behaviour is hard to predict, owing 在真实的水凝胶中，聚合物密度的分布和聚合物链的长度都存在固有的变化（图 2a）。因此，水凝胶自然包含许多缺陷。一个单一的、相对较大的缺陷会产生显著的应力集中，并触发裂纹传播，导致水凝胶在宏观应力水平下的失效远低于理论平均值所建议的（方程（6）和（7））。由于弹性水凝胶在变形过程中没有能量耗散机制，真实膨胀的水凝胶通常非常脆弱，失效行为难以预测，这是因为。
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C Double-network hydrogel C 双网络水凝胶
First network: rigid and brittle 第一个网络：刚性和脆弱
Second network: flexible and stretchable 第二网络：灵活和可伸缩
No interaction between first network and second network 第一个网络和第二个网络之间没有交互
Interaction between first network and second network 第一个网络和第二个网络之间的互动
Deswelling in water (viscoelastic) 在水中脱肿（粘弹性）
Fig. 2 | Swelling (elastic) hydrogels. a, Structure of conventional hydrogel. b, Topological structure hydrogels. c, Double-network hydrogels. Panel b adapted with permission from ref. 26, AAAS, and with permission from ref. 108, AAAS. Panel c adapted from ref. 200, Springer Nature Limited. 图 2 | 膨胀（弹性）水凝胶。a，传统水凝胶的结构。b，拓扑结构水凝胶。c，双网络水凝胶。面板 b 经授权改编自参考文献 26，AAAS，并经授权改编自参考文献 108，AAAS。面板 c 经授权改编自参考文献 200，Springer Nature Limited。
to their heightened vulnerability to defects. Enhancing the toughness of elastic hydrogels is a definite challenge. 由于其更容易受到缺陷的影响，增强弹性水凝胶的韧性是一个明显的挑战。
Topological structure. Reducing spatial heterogeneities through the realization of an ideally homogeneous network, movable crosslinkers or scission mechanisms can improve stress homogenization (Fig. 2b). Hydrogels made from well-defined symmetrical tetrahedron-like polyethylene glycol (PEG) macromonomers have fewer spatial heterogeneities than hydrogels obtained from free radical polymerization and thus show improved failure strength and extensibility due to a lower amount of defects . However, the fracture energy of such gels lies in the range estimated from the Lake-Thomas model , because the network homogeneity improves the strength but not the fracture energy, consistent with the theorem for simple elastic network fracture. 拓扑结构。通过实现理想均匀网络，可通过可移动的交联剂或切割机制减少空间异质性，从而改善应力均匀化（图 2b）。由明确定义的对称四面体状聚乙二醇（PEG）大单体制成的水凝胶 比由自由基聚合得到的水凝胶具有更少的空间异质性 ，因此由于缺陷量较低，显示出改善的破坏强度和延展性 。然而，这类凝胶的断裂能量处于从 Lake-Thomas 模型估计的范围内 ，因为网络的均匀性提高了强度但并未提高断裂能量，与简单弹性网络断裂定理一致。
For a purely elastic hydrogel, a large (which can be achieved by reducing the chemical crosslinking density) results in increases in stretchability and fracture energy , but decreases in elastic modulus and strength . Therefore, there exists a tradeoff between these quantities. Hydrogel swelling reduces and . Hence, simultaneously optimizing these properties in purely elastic hydrogels necessitates molecular designs that go beyond the constraints of a basic elastic network. For example, slide-ring hydrogels can equalize the tension of polymer chains. In these hydrogels, the polymer chains are topologically interlocked by figure-of-eight polyrotaxane crosslinks, which can pass along the polymer chains freely in a manner similar to pulleys. As a result, the strand length between two crosslinked junctions ( ) increases as the figure-ofeight crosslinks slide away from each other and redistribute loads , resulting in a higher extensibility and stronger crack growth resistance compared with fixed crosslink gels . Another strategy to transmit tension around the notch consists in applying dense entanglements as slip links to fabricate single-network polyacrylamide (PAAm) hydrogels - with polymer volume fraction around 10 vol - that are stiff ( ) and stretchable (fracture stretch ratio ) and have superior fracture energy compared with conventional PAAm hydrogels of similar modulus ( up to compared with to . The large number of trapped entanglements 对于纯弹性水凝胶，较大的 （可以通过降低化学交联密度实现）导致了拉伸性 和断裂能量 的增加，但弹性模量 和强度 的降低。因此，这些量之间存在权衡。水凝胶膨胀 减少 和 。因此，在纯弹性水凝胶中同时优化这些性质需要超越基本弹性网络的分子设计。例如，滑环水凝胶 可以使聚合物链的张力相等。在这些水凝胶中，聚合物链通过八字形聚醚环交联点进行拓扑锁定，这些交联点可以自由地沿着聚合物链传递，类似于滑轮。因此，两个交联结点之间的链长（ ）随着八字形交联点相互滑动并重新分配载荷 而增加，与固定交联凝胶 相比，导致更高的延展性和更强的裂纹生长抗力。 另一种传递缺口周围张力的策略是应用密集的纠缠作为滑移链，以制备单网络聚丙烯酰胺（PAAm）水凝胶 - 其聚合物体积分数约为 10 vol - 这些水凝胶坚硬（ ）且具有可拉伸性（断裂拉伸比 ），与类似模量的传统 PAAm 水凝胶相比具有更优越的断裂能量（ 相比 ，与 到 相比。被困在其中的大量纠缠
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contribute to the modulus and strength, whereas the slip of entanglements at the crack tip lengthens the strands, resulting in large fracture energy simultaneously. Weak side-crosslinker scission is another strategy, in which the side crosslinkers are preferentially cleaved under force, resulting in lengthening of the strands. This strategy can enhance the fracture energy ninefold without sacrificing the modulus . To demonstrate the toughening effect of hydrogels fabricated through different strategies, we summarize the mechanical performances and corresponding water content of various types of hydrogels (Table 1). We can see that the tradeoff between modulus and fracture energy can be resolved by movable or weak crosslinker mechanisms (Table 1). Hydrogels with topological designs have the advantage of negligible hysteresis, which can avoid stress softening and relaxation during practical applications. Because they have no built-in energy dissipation mechanisms, the fracture energy of such systems, in principle, can be described by a modified LakeThomas model in which multiple network layers near the crack plane participate in dissipating energy . 有助于模量和强度，而纠缠在裂纹尖端的滑移会延长纤维，同时导致大的断裂能量。弱侧交联剂裂解是另一种策略，其中侧交联剂在受力下被优先切断，导致纤维延长。这种策略可以将断裂能量提高九倍，而不牺牲模量。为了展示通过不同策略制备的水凝胶的增韧效果，我们总结了各种类型水凝胶的力学性能和相应的含水量（表 1）。我们可以看到模量和断裂能量之间的权衡可以通过可移动或弱交联剂机制来解决（表 1）。具有拓扑设计的水凝胶具有可忽略的滞后，可以避免在实际应用中的应力软化和松弛。因为它们没有内置的能量耗散机制，所以这类系统的断裂能量原则上可以通过修改后的 LakeThomas 模型来描述，在该模型中，靠近裂纹平面的多个网络层参与能量耗散。
Double-network structure. Swelling hydrogels are elastic and thus do not dissipate energy during deformation. The invention of the DN structure with built-in sacrificial bonds and energy dissipation mechanisms was a breakthrough for that class of hydrogels . DN hydrogels consist of two interpenetrating elastic networks with contrasting properties (Fig. 2c) obtained by a two-step sequential polymerization process. The first network, which is rigid and brittle, acts as a sacrificial bond network that effectively dissipates energy. Meanwhile, the second network is soft and ductile, ensuring the hydrogel's integrity during deformation . Despite having a high water content ( ), DN hydrogels are stiff, strong and tough; the fracture energy is even comparable to industrial rubbers and natural cartilage . Achieving exceptionally high elastic modulus, strength and fracture energy in DN hydrogels revolves around two key aspects. The first network should be more brittle and weaker than the second network, that is, it should have a smaller fracture stretch ratio ) and fracture stress ( ). Conventional DN gels usually use highly crosslinked polyelectrolyte networks (such as PAMPS (poly(2-acylamido-2-methylpropanesulfonic acid)) and PAAc (poly(acrylic acid))) as the first network, and slightly crosslinked neutral polymers (such as PAAm, PDMAAm (poly(N,N-dimethylacrylamide)) and PHEMA (poly(2-hydroxyethyle methacrylate))) as the second network. The highly crosslinked polyelectrolyte network, which over-swells in water owing to high osmotic pressure from counterions (Fig. 1a, regime B), is both brittle (small ) and weak (small ) while exhibiting a high Young's modulus . In addition, the monomeric molar ratio between the sparsely crosslinked second neutral network and the first network should range from several to a few tens, resulting in larger and . The mechanical contrast between the two elastic networks results in inelastic behaviour, the DN hydrogel exhibiting yielding and necking during tensile loading and substantial hysteresis during cyclic loading . Because the second network suppresses the stress concentration in the defects of the first network, the yielding stress is much higher in comparison to the fracture stress of the first network alone. Above the yielding point, the short-strand first network is considered to rupture into fragments, dissipating energy, whereas the stretchable second network maintains structural integrity and sustains further elongation . In fracture tests, covalent bond rupture in the first network ahead of the propagating crack forms a damage zone with a thickness of several hundred micrometres, and the ruptured first network causes large softening and stress-strain hysteresis, contributing to a substantial (refs. 115-118) (Fig. 1d). This internal fracture phenomenon has been confirmed by luminescent probes and real-time birefringence . Furthermore, the broken fragments of the first network could serve as sliding crosslinks to further delocalize the stress concentration around the crack tip and prevent chain scission . Therefore, the combination of the two networks enhances resistance to crack propagation, resulting in high fracture energy comparable to cartilage and tendon . 双网络结构。膨胀水凝胶具有弹性，因此在变形过程中不会消耗能量。具有内置牺牲键和能量耗散机制的 DN 结构的发明对该类水凝胶来说是一项突破。DN 水凝胶由两个相互渗透的具有对比性质的弹性网络组成（图 2c），通过两步顺序聚合过程获得。第一个网络是刚性且脆弱的，作为一个牺牲键网络，有效地耗散能量。同时，第二个网络是柔软且韧性的，确保水凝胶在变形过程中完整性。尽管含有大量水分，DN 水凝胶却具有硬度、强度和韧性；其断裂能量甚至可与工业橡胶和天然软骨相媲美。在 DN 水凝胶中实现极高的弹性模量、强度和断裂能量围绕着两个关键方面。第一个网络应比第二个网络更脆弱和更弱，即应具有较小的断裂拉伸比和断裂应力。 传统的 DN 凝胶通常使用高度交联的聚电解质网络（如 PAMPS（聚（2-丙烯磺酸甲酰胺-2-甲基丙烯酸））和 PAAc（聚丙烯酸））作为第一个网络，以及轻度交联的中性聚合物（如 PAAm，PDMAAm（聚（N，N-二甲基丙烯酰胺））和 PHEMA（聚（2-羟基乙基甲基丙烯酸酯））作为第二个网络。高度交联的聚电解质网络由于对离子的高渗透压而在水中过度膨胀（图 1a，区域 B），既脆弱（小 ）又弱（小 ），同时具有较高的杨氏模量 。此外，稀疏交联的第二中性网络与第一个网络之间的单体摩尔比应在几个到几十之间，导致更大的 和 。两个弹性网络之间的机械对比导致非弹性行为，DN 凝胶在拉伸加载过程中表现出屈服和颈缩 ，在循环加载过程中表现出明显的滞后 。 由于第二网络抑制了第一个网络缺陷处的应力集中，相比于仅有第一个网络的断裂应力，屈服应力要高得多。在屈服点以上，短链第一个网络被认为会破裂成碎片，释放能量，而可伸缩的第二网络保持结构完整并继续延展。在断裂测试中，第一个网络中的共价键破裂在传播裂纹前形成了一个厚度为几百微米的损伤区，破裂的第一个网络导致了大量软化和应力-应变滞后，对实质性贡献（参考文献 115-118）（图 1d）。这种内部断裂现象已被发光探针和实时双折射证实。此外，第一个网络的破碎碎片可以作为滑动交联点，进一步使裂纹尖端周围的应力集中分散，并防止链断裂。 因此，这两个网络的结合增强了抗裂纹传播的能力，导致高断裂能量，与软骨和肌腱相当。
Viscoelastic hydrogels (typically ) 粘弹性水凝胶（通常 ）
Viscoelastic hydrogels have an intrinsic energy dissipation mechanism, and they typically exhibit a large, self-recoverable mechanical hysteresis because the physical bonds dynamically break and reform. The mechanical properties strongly depend on the structure (dynamic bond strength and density) and the observation condition (strain rate and temperature). The fracture energy strongly depends on the crack propagation velocity and temperature , and can be expressed as ref. 87). When approaches zero, the fracture energy reduces to , which is determined by the primary network structure (equation (8)). 粘弹性水凝胶具有固有的能量耗散机制，通常表现出较大的、可自我恢复的机械滞后，因为物理键动态断裂和重组。机械性能强烈依赖于结构（动态键强度和密度）和观察条件（应变速率和温度）。断裂能量强烈依赖于裂纹传播速度 和温度 ，可以表示为 ref. 87)。当 接近零时，断裂能量减少到 ，由主要网络结构（方程（8））确定。
Dual-crosslinked hydrogels. Because deswelling hydrogels typically have two crosslinking mechanisms (covalent and non-covalent), they are also known as dual-crosslinked hydrogels. The non-covalent bonds can dissociate and reassociate dynamically, dissipating energy and protecting the primary network crosslinked by chemical bonds from stress overshoot. Thus, the dynamic bonds function as reversible sacrificial bonds to dissipate energy. Various kinds of non-covalent bonds, including borate/di-diol complexation, electrostatic interaction, hydrogen bonds, metal-ligand coordinate bonds, hydrophobic interaction, interaction, cation- interaction and host-guest interaction, have been used (Fig. 3a). Synthesis of this type of hydrogel is simple and usually involves a one-step reaction to form the chemically crosslinked primary network. Non-covalent interactions are induced by posttreatments, such as dialysis to remove counterions for polyampholytes, or immersion in a multivalent ion solution to form coordination complexes. During post-treatment, intrachain and interchain dynamic bonds are formed and reorganized to form aggregations, causing the polymer strands to collapse into globule conformation, analogous to the folded structure of proteins. Therefore, these hydrogels deswell in water after post-treatment, resulting in a high polymer volume fraction (around 50 vol%). 双重交联水凝胶。由于脱水水凝胶通常具有两种交联机制（共价和非共价），它们也被称为双重交联水凝胶。非共价键可以动态解离和重新结合，耗散能量并保护由化学键交联的主要网络免受应力过载。因此，动态键作为可逆牺牲键来耗散能量。各种非共价键，包括硼酸/二醇络合物、静电相互作用、氢键、金属配体配位键、疏水相互作用、π-π相互作用、阳离子-阴离子相互作用和宿主-客体相互作用，已被使用（图 3a）。这种类型的水凝胶的合成简单，通常涉及一步反应形成化学交联的主要网络。非共价相互作用是通过后处理诱导的，例如通过透析去除多电离物质以形成多电离物质络合物。 在后处理过程中，形成和重新组织了链内和链间动态键，形成聚集体，导致聚合物链坍缩成球状构象，类似于蛋白质的折叠结构。因此，这些水凝胶在后处理后在水中脱水 ，导致高聚合物体积分数（约 50 体积%）。
Here we take the hydrogen bond as an example, but examples of hydrogels containing other non-covalent bonds can be found in comprehensive reviews . Hydrogen bonds are ubiquitous in living systems, contributing to the formation of secondary structures in proteins. When designing tough hydrogels based on hydrogen bonds, the presence of water molecules in hydrogels can disrupt the bonds, potentially limiting the stability of dynamic bonds . Therefore, to stabilize the hydrogel, formation of multiple hydrogen bonds between monomer pairs is required , or the hydrogen bonds can be further stabilized by hydrophobic interactions . For instance, the chemically crosslinked PAAm hydrogel exhibits swelling behaviour and no selfhealing property because its hydrogen bonding is attacked by water. By contrast, after the PAAm hydrogel is synthesized in the presence of tannic acid, which can form multiple hydrogen bonds between the PAAm polymer and tannic acid, the gel deswells in water with self-healing ability . The deswelling behaviour of PAAm-tannic-acid hydrogel 在这里，我们以氢键为例，但包含其他非共价键的水凝胶的例子可以在综合评论中找到。氢键在生物系统中无处不在，有助于蛋白质中次级结构的形成。在基于氢键设计坚韧水凝胶时，水凝胶中的水分子可能会破坏键，潜在地限制动态键的稳定性。因此，为了稳定水凝胶，需要在单体对之间形成多个氢键，或者氢键可以通过疏水相互作用进一步稳定。例如，化学交联的 PAAm 水凝胶表现出膨胀行为和无自愈性质，因为其氢键受到水的影响。相比之下，当 PAAm 水凝胶在鞣酸存在下合成时，鞣酸可以在 PAAm 聚合物和鞣酸之间形成多个氢键，水中的凝胶具有自愈能力。PAAm-鞣酸水凝胶的脱水行为
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Table 1| Mechanical properties of representative swelling and deswelling hydrogels 表 1 | 代表性膨胀和脱胀水凝胶的力学性能
is conducive to high extensibility ( up to 16 ) and tensile strength ( up to ). In addition, the multiple hydrogen bonds between the polymer and tannic acid effectively dissipate energy and suppress crack propagation, imparting a high fracture energy to the hydrogel ( up to ), nearly 30 times higher than that of PAAm gels solely crosslinked through chemical crosslinking. 有利于高可延展性（ 高达 16）和拉伸强度（ 高达 ）。此外，聚合物与丹宁酸之间的多个氢键有效地耗散能量并抑制裂纹扩展，赋予水凝胶高断裂能量（ 高达 ），几乎比仅通过化学交联交联的 PAAm 凝胶高出 30 倍。
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Double-network gels with physical interactions. Traditional DNgels are composed of two chemically crosslinked elastic networks, but DN gels can also be formed by combining a physically crosslinked and a chemically crosslinked network. For example, DN hydrogels from crosslinked alginate network and chemically crosslinked PAAm network exhibit partial self-recovery and a high fracture energy of in the as-prepared state, highlighting the beneficial effect of the dynamic bonds . However, owing to osmotic pressure, this DN hydrogel swells in water and becomes weaker than in its as-prepared state . DN gels containing no chemically crosslinked network have also been developed. For example, combining an amphiphilic triblock copolymer as the first network and a linear PAAm as the second network, a deswollen water-equilibrated block copolymer PAAm DN hydrogel was synthesized (water content . The amphiphilic triblock copolymer PBMA-b-PMAA-b-PBMA (poly(butyl methacrylate)-b-poly(methacrylic acid)-b-poly(butyl methacrylate)) forms a hyperconnective physical network through strong hydrophobic associations of the end-block PBMA . Simultaneously, the midblock PMAA forms hydrogen bonds with the linear PAAm chains to allow energy dissipation (Fig. 2c). The hydrogels show abnormally large non-softening, and quasilinear but inelastic deformation, and they have high elastic modulus ( ), fracture energy , strength ( , extensibility and quick self-recovery ( recovery within ). 双网络凝胶具有物理相互作用。传统的 DN 凝胶由两个化学交联的弹性网络组成，但 DN 凝胶也可以通过结合物理交联和化学交联网络来形成。例如，由交联的海藻酸盐网络和化学交联的 PAAm 网络组成的 DN 水凝胶在原始状态下表现出部分自我恢复和高 的断裂能量，突显了动态键的有益效果 。然而，由于渗透压，这种 DN 水凝胶在水中膨胀，变得比原始状态下更脆弱 。也已经开发了不含化学交联网络的 DN 凝胶。例如，将一种两性三嵌段共聚物作为第一个网络，线性 PAAm 作为第二个网络，合成了一种脱水水平衡块共聚物 PAAm DN 水凝胶（含水量 。两性三嵌段共聚物 PBMA-b-PMAA-b-PBMA（聚丙烯酸丁酯-b-聚甲基丙烯酸-b-聚丙烯酸丁酯）通过末端块 PBMA 的强热亲和力形成超连接的物理网络 。 同时，中间 PMAA 与线性 PAAm 链形成氢键以实现能量耗散（图 2c）。这种水凝胶表现出异常大的非软化、准线性但非弹性变形，具有高弹性模量（ ）、断裂能量 、强度（ ）、延展性 和快速自恢复（ 恢复在 内）。
Hydrogels with high-order structure 具有高阶结构的水凝胶
Introducing high-order structures, such as microphase separation structures, microcrystals and fibrils, fibre-woven fabrics and nanocomposites, are effective approaches for toughening hydrogels (Fig. 3b). Interplays between hierarchical structures from molecular to mesoscopic scales endow strong crack resistance. Particularly, the processing zone is enlarged because of the increase in the load-transfer length by submicrometre-scale to centimetre-scale stiff constituents, resulting in a large . Meanwhile, the can be enhanced by two to four orders of magnitude. Consequently, incorporating reinforcement components is one of the most effective strategies to substantially enhance both fracture energy and fatigue threshold simultaneously, to a level even better than bio-tissues (Fig. 4). 引入高阶结构，如微相分离结构、微晶体和纤维，纤维编织织物和纳米复合材料，是增强水凝胶韧性的有效方法（图 3b）。从分子到中等尺度的分层结构之间的相互作用赋予了强大的抗裂性。特别是，由于亚微米尺度到厘米尺度的硬质成分增加了载荷传递长度，加工区域扩大，导致大 。同时， 可以提高两到四个数量级。因此，将增强组分纳入是显著增强断裂能量和疲劳阈值的最有效策略之一，甚至可以达到比生物组织更好的水平（图 4）。
Microphase separation. Deswelling hydrogels are prone to microphase separations, forming polymer-dense phases and polymer-sparse phases at scales much larger than the dual-crosslinked structure (Fig. 3b). The size of the microphases is governed by the competition between enthalpic gain of the physical interactions and entropic penalty of the primary network. The interplay of the molecular and mesoscale hierarchical structures dissipates a substantial amount of energy through a large load-transfer length. For example, polyampholyte hydrogels (PA gels) copolymerized from cationic and anionic monomers at high monomer concentrations around the charge balance point form a bicontinuous microphase separation, in which the hard microphase network and soft microphase network are interpenetrated . The PA gels shrink to approximately in volume when immersed in water to dialyse counterions. During dialysis, the conformation of the polyampholyte strands changes from coil to aggregated globule. Water-equilibrated PAgels exhibit a polymer volume fraction -50 vol , 微相分离。脱水凝胶容易发生微相分离，形成聚合物致密相和聚合物稀疏相，其尺度远大于双交联结构（图 3b）。微相的大小由物理相互作用的焓增益和主要网络的熵惩罚之间的竞争所决定。分子和介观尺度层次结构的相互作用通过较大的载荷传递长度耗散了大量能量。例如，高单体浓度下由阳离子和阴离子单体共聚合而成的聚离子凝胶（PA 凝胶）在电荷平衡点附近形成了一个双连续微相分离，其中硬微相网络和软微相网络相互渗透。PA 凝胶在水中浸泡以透析对离子时，体积大约缩小 。在透析过程中，聚离子链的构象从卷曲变为聚集的球状。水平衡的 PA 凝胶表现出聚合物体积分数约为-50 vol 。
b Hydrogel containing load-bearing high-order structure 含有承载高阶结构的水凝胶
Deswelling in water (viscoelastic) 在水中脱肿（粘弹性）
Partly deswelling in water (viscoelastic) 在水中部分脱肿（粘弹性）
Fig. 3 | Deswelling (viscoelastic) hydrogels and hydrogels with high-order structure. a, Dual-crosslinked hydrogel composed of a primary network from chemical crosslinking and/or trapped entanglement and dynamic networks 图 3 | 脱肿（粘弹性）水凝胶和具有高阶结构的水凝胶。a，由化学交联和/或被困纠缠以及动态网络组成的双重交联水凝胶
Bilayer lamellar structure 双层片状结构
Usually used in the as-prepared state Nanocomposite 通常用于原始状态的纳米复合材料
from physical crosslinking. b, Hydrogel containing high-order structure. PVA, polyvinyl alcohol. Panel b adapted with permission from ref. 40, APS, ref.177, Wiley, and ref.156, ACS. 从物理交联。 b，含有高阶结构的水凝胶。 PVA，聚乙烯醇。 面板 b 经许可改编自参考文献 40，APS，参考文献 177，Wiley，和参考文献 156，ACS。
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a Fracture energy and fatigue threshold versus stiffness 断裂能量和疲劳阈值与刚度之间的关系
b Fracture energy and fatigue threshold versus strength 断裂能量和疲劳阈值与强度
C Fatigue threshold versus fracture energy C 疲劳阈值与断裂能量
Fig. Fracture energy and fatigue threshold versus elastic modulus and strength of hydrogels fabricated by various strategies. a, Fracture energy and fatigue threshold versus elastic modulus. , Fracture energy and fatigue threshold versus strength (engineering fracture stress). c, Fatigue threshold versus fracture energy. Elastic hydrogels including simple network , 图 断裂能量和疲劳阈值与各种策略制备的水凝胶的弹性模量和强度之间的关系。a，断裂能量和疲劳阈值与弹性模量的关系。 ，断裂能量和疲劳阈值与强度（工程断裂应力）的关系。c，疲劳阈值与断裂能量的关系。包括简单网络的弹性水凝胶 。
topological network and double network (DN) (I). Viscoelastic hydrogels with dynamic bonds (II). Hydrogels containing high-order structure (III). Load-bearing biotissues (IV). GO, graphene oxide; PA, polyampholyte; PAAm, polyacrylamide; PEG, polyethylene glycol;PVA, polyvinyl alcohol. independent of chemical crosslinking density but slightly dependent on the chemical structure of the monomeric units . The hierarchical structures, including the transient network due to dynamic bonds, the primary network, and the bicontinuous phase networks, which exist at scales of nanometres, tens of nanometres and hundreds of nanometres, respectively, play a crucial role in achieving exceptional mechanical properties ( up to -35 , modulus up to , strength up to , self-healing capability up to , and fracture energy up to ) 拓扑网络 和双网络（DN） （I）。具有动态键的粘弹性水凝胶 （II）。含有高阶结构的水凝胶 （III）。承载生物组织 （IV）。GO，氧化石墨烯；PA，多元离子聚合物；PAAm，聚丙烯酰胺；PEG，聚乙二醇；PVA，聚乙烯醇。独立于化学交联密度，但略微依赖于单体单位的化学结构 。包括由于动态键而存在的瞬时网络、主要网络和双连续相网络的分层结构，分别存在于纳米、数十纳米和数百纳米的尺度上，对实现出色的机械性能起着至关重要的作用（ 高达-35，模量高达 ，强度高达 ，自愈能力高达 ，断裂能高达 ）
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through multistep energy dissipation . The hierarchical structures in PAgelsalso contribute to excellent mechanical adaptation to cyclic training . The value of is very close to the theoretical maximum stretch ratio predicted from the average network structure, suggesting that such PA hydrogels have a defect-insensitive length much larger than the mesh size of the primary network. The small-strain moduli, large deformation energy dissipation, and fracture energy of PAgels not only obey the time-temperature superposition principle , but also obey the time-salt superposition principle, where salt ions screen the ionic bonding between polyampholytes, accelerating the dynamic processes like the temperature, and the dynamic mechanical behaviour for different salt concentrations can be shifted onto a single master curve using saltconcentration-dependentshift factors for the frequency . Through time-salt superposition, one can access a wide range of timescales that are difficult to access at room temperature. 通过多步能量耗散 。PAgels 中的分层结构也有助于对循环训练的优秀机械适应性 。 的值非常接近从平均网络结构预测的理论最大拉伸比 ，表明这种 PA 水凝胶具有一个缺陷不敏感的长度 ，远大于主要网络的网格尺寸。PAgels 的小应变模量、大变形能耗散和断裂能量不仅遵循时间-温度超定原则 ，而且遵循时间-盐超定原则，其中盐离子屏蔽了聚离子之间的离子键合，加速了像温度这样的动态过程，不同盐浓度下的动态机械行为可以使用盐浓度依赖的频率 的移位因子转移到单一主曲线上。通过时间-盐超定，可以访问在室温下难以访问的广泛时间尺度范围。
Microcrystals and microfibres. Hydrogels containing microcrystals and microfibres typically have coexisting swelling (amorphous) and deswelling (crystalline or fibre) domains (Fig. 3b). Because water weakens polymer-polymer interactions, special strategies are required to introduce microcrystalline domains or fibrils into hydrogels. Common strategies include freeze-thaw , mechanical stretching , solvent exchange , thermal drawing and rapid quenching , and salting out . The formation of microcrystals can suppress crack propagation because the crystalline domains have high chain density, and the energy for rupturing a crystalline domain is higher than that for rupturing the amorphous polymers . Meanwhile, under loading, microcrystals convert to fibrils , which are highly anisotropic and prone to crack deflection. For example, freeze-thawing and air-drying resulted in polyvinyl alcohol (PVA) hydrogels with high crystallization, which showed superior crack resistance and therefore superior mechanical properties . Combining freeze-casting and salting out led to the PVA hydrogels with a highly anisotropic hierarchical structure, consisting of micrometre-scale honeycomb-like pore walls and interconnected nanofibril meshes . This anisotropic hierarchical structure contributes to the suppression of crack propagation, leading to superior mechanical performances and . 微晶体和微纤维。含有微晶体和微纤维的水凝胶通常具有共存的膨胀（无定形）和脱水（结晶或纤维）领域（图 3b）。由于水使聚合物-聚合物相互作用减弱，因此需要特殊策略将微晶领域或纤维引入水凝胶中。常见策略包括冻融 ，机械拉伸 ，溶剂交换 ，热拉伸和快速淬火 ，以及盐析 。微晶体的形成可以抑制裂纹传播，因为结晶领域具有较高的链密度，破裂结晶领域所需能量高于破裂无定形聚合物 。同时，在加载下，微晶体转变为纤维 ，这些纤维具有高各向异性，易于裂纹偏转。例如，冻融和空气干燥导致聚乙烯醇（PVA）水凝胶具有高结晶度，表现出优异的抗裂性和因此优越的机械性能 。 将冷冻铸造和盐析结合起来，可以制备出具有高度各向异性分级结构的 PVA 水凝胶，由微米级蜂窝状孔壁和相互连接的纳米纤维网构成。这种各向异性分级结构有助于抑制裂纹传播，从而导致卓越的机械性能。
Strain-induced crystallization. Similar to natural rubber , watercontaining hydrogels reinforced by strain-induced crystallization have also been developed. Strain-induced crystallization can serve as a damage-free reinforcement strategy for slide-ring PEG hydrogels with reduced slidable crosslinker hydroxypropyl- -cyclodextrin rings . Because the crosslinks can slide along the PEG chains, the initially amorphous polymer strands between the crosslinks can become long and uniformly stretched under large deformation, resulting in highly oriented PEG chains. Microcrystals form and melt with elongation and retraction, respectively. Consequently, the crack resistance during loading is substantially enhanced, resulting in a fracture energy of up to , much higher than gels with fixed crosslinker or non-strain-induced crystallization systems ( from about 0.05 to ). 应变诱导结晶。类似于天然橡胶，含水水凝胶通过应变诱导结晶进行了增强。应变诱导结晶可以作为一种无损增强策略，用于具有减少可滑动交联剂羟丙基-β-环糊精环的滑环 PEG 水凝胶。由于交联点可以沿着 PEG 链滑动，因此在大变形下，最初无定形的聚合物链在交联点之间可以变得又长又均匀地拉伸，从而形成高度定向的 PEG 链。微晶体随着拉伸和回缩而形成和融化。因此，在加载过程中，抗裂性大大增强，导致断裂能量高达，远高于具有固定交联剂或非应变诱导结晶系统的凝胶（约从 0.05 到）。
Bilayer lamellar structure. The unidirectionally aligned hydrophobic lamellar bilayers embedded in simple hydrophilic networks lead to hydrogels with coexisting swelling and deswelling layers. A unique example is anisotropic photonic poly(dodecyl glyceryl itaconate)/ polyacrylamide (PDGI/PAAm) hydrogels, which consist of macroscopic, single-domain, periodical stacking of integrated microscopic lamellar bilayers (PDGI) inside the polymer matrix (PAAm) (Fig. 3b). Owing to the water-impermeable nature of the hydrophobic bilayers, these hydrogels show 1D swelling, anisotropic molecular permeation and diffusion, and substantial mechanical anisotropy along the thickness and in-plane directions. The rigid PDGI lamellar layers, having a lipidlike mobile nature, act as reversible sacrificial bonds that disassociate during deformation, thereby contributing to substantial energy dissipation. Furthermore, at large deformation, the single-domain PDGI lamellar bilayers transform into a hierarchical fibrous structure consisting of micrometre-thick fibre bundles made from nanometre-thick fibrils, which impedes crack growth and results in crack blunting . Therefore, the hydrogels exhibit large hysteresis, high strength and extensibility, and extraordinary toughness. Lamellar hydrogels are often called photonic hydrogels because they usually exhibit structural colours due to Bragg's reflection on the multilayer planes. The colours can be dynamically tuned over a broad spectral range, spanning from the ultraviolet-visible region to the near-infrared region, by tuning the hydrogel layer thickness through the application and release of stress or strain. Therefore, these tough gels could be used as stress or strain sensors and deformation-based colour displays. Various chemical structures have been incorporated into the soft layers to achieve different functionalities. For instance, PDGI- -PAAm (partially hydrolysed PAAm) hydrogels have ultrafast colour response over the whole visible wavelength , and PDGI-PAAcNa (sodium polyacrylate) hydrogels show colour tunability under perpendicular or parallel electric fields in the directions of the hydrogel layers . 双层片状结构。单向排列的疏水性片状双层嵌入简单的亲水性网络，导致具有共存膨胀和收缩层的水凝胶。一个独特的例子是各向异性光子聚(十二烷基丙二酸甘油酯)/聚丙烯酰胺(PDGI/PAAm)水凝胶，它由宏观、单域、周期性堆叠的集成微观片状双层(PDGI)构成，位于聚合物基质(PAAm)内部 （图 3b）。由于疏水性双层的不透水性质，这些水凝胶表现出一维膨胀、各向异性分子渗透和扩散，以及在厚度和平面方向上的显著机械各向异性。刚性的 PDGI 片状层具有类似脂质的移动性质，作为可逆牺牲键，在变形过程中解离，从而有助于大量能量耗散。此外，在大变形下，单域 PDGI 片状双层转变为由纳米厚纤维束制成的微米厚纤维结构，阻碍裂纹扩展并导致裂纹钝化 。 因此，水凝胶表现出很大的滞后性、高强度和延展性，以及非凡的韧性。层状水凝胶通常被称为光子水凝胶，因为它们通常由于多层平面上的布拉格反射而呈现出结构颜色。通过调节水凝胶层厚度，可以在广泛的光谱范围内动态调节颜色，从紫外可见区域到近红外区域，通过施加和释放应力或应变。因此，这些坚韧的凝胶可以用作应力或应变传感器和基于变形的颜色显示器。已将各种化学结构纳入软层中以实现不同的功能。例如，PDGI- -PAAm（部分水解的 PAAm）水凝胶在整个可见波长范围内具有超快的颜色响应 ，而 PDGI-PAAcNa（聚丙烯酸钠）水凝胶在垂直或平行电场方向上显示颜色可调性，沿着水凝胶层的方向 。
Organic-inorganic nanocomposite. Polymer networks that are physically adsorbed on hard inorganic nanoparticles , nanotubes or nanosheets form one type of organic-inorganic nanocomposite. The inorganic components act as a high-energy phase to toughen the hydroge (Fig. 3b). To achieve superior mechanical performances in the water-equilibrated state, one should choose a hydrogel showing deswelling behaviour or containing crystalline structure as polymer matrix to enhance the interfacial affinity , or the polymer matrix should be post-crosslinked after the formation of a strong interface between the polymer and the inorganic components . Otherwise, the swelling mismatch between the polymer network and inorganic components weakens the load-transfer interface. Such nanocomposite gels with a swelling mismatch in organic-inorganic components have superior performance in the as-synthesized state (usually , and for an appropriate amount of polymer and inorganic components) , whereas their mechanical properties become weak at the swollen equilibrium state . 有机-无机纳米复合材料。物理吸附在硬无机纳米颗粒 、纳米管 或纳米片 上的聚合物网络形成一种有机-无机纳米复合材料。 无机组分作为高能量相来增强水凝胶的韧性 （图 3b）。为了在水平衡状态下实现卓越的机械性能，应选择表现出脱水行为或含有结晶结构作为聚合物基体以增强界面亲和性的水凝胶 ，或者在聚合物与无机组分之间形成强界面后应该进行后交联 。否则，聚合物网络与无机组分之间的膨胀不匹配会削弱载荷传递界面。这种有机-无机组分中存在膨胀不匹配的纳米复合凝胶在合成状态下表现出卓越性能（通常 、 和 适量的聚合物和无机组分） ，而在膨胀平衡状态下其机械性能变弱 。
Fibres and fibre-woven fabric composite. Incorporating macroscale stiff fibres or fibre-woven fabrics into hydrogels is a powerful strategy for fabricating strong and tough composites (Fig. 3b). This strategy relies on two key principles :strong interface adhesion between the fibres or fabrics and the soft matrix to effectively disperse stress at the crack tip; and a large modulus contrast between the stiff fibres or fabrics and the soft hydrogel. The load-transfer length of unidirectional fibre-reinforced soft composites correlates with the modulus contrast between the fibre and matrix, , where is the Young's modulus of the fibre, is the shear modulus of the matrix, and is the cross-sectional area of a fibre . Along this line, polymer fibres have been introduced into swelling hydrogels . To achieve good adhesion and load transfer between the matrix and polymer fibres, covalent bonds were used for interlinking . Such composite 纤维和纤维编织织物复合材料。将宏观刚性纤维或纤维编织织物纳入水凝胶中是制备强韧复合材料的有效策略（图 3b）。该策略依赖于两个关键原则 ：纤维或织物与软基体之间的强界面粘附，以有效分散裂纹尖端的应力；以及刚性纤维或织物与软水凝胶之间的大模量对比。单向纤维增强软复合材料的载荷传递长度 与纤维和基体之间的模量对比相关 ，其中 是纤维的杨氏模量， 是基体的剪切模量， 是纤维的横截面积 。沿着这条线，聚合物纤维已被引入膨胀水凝胶 。为了实现基体和聚合物纤维之间的良好粘附和载荷传递，共价键被用于交联 。这种复合
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Fig. Fatigue fracture measurement and the mechanisms proposed to enhance the fatigue resistance of hydrogels. a, Pure shear test for cyclic fatigue test. From the fatigue resistance curve versus ), one can obtain the fatigue threshold below which the crack does not grow. b, Chain scission mechanism for elastic hydrogels. c, Fatigue resistance of viscoelastic hydrogels. d, Fatigue resistance enhanced by high-order structures. hydrogels showed a certain reinforcement in the as-synthesized state, but were rarely used in water equilibrium state, since swelling resulted in weak mechanical performance owing to stress mismatch between the hydrogel matrix and the fibres. Using deswelling hydrogels to form the fibre-reinforced hydrogels is a more successful strategy. For example, composites obtained from glass fibre-woven fabric and polyampholyte hydrogels show exceptional mechanical performance with a superior tearing strength of , a tensile modulus of and a fracture energy of , which are several orders of magnitudegreater than individual neat materials . Such excellent mechanical properties stem from the strong physicaladhesion of the deswelling polyampholyte matrix to the negatively charged glass fibre surface in water as well as from the high viscoelastic energy dissipation density of the polyampholyte matrix. 图 疲劳断裂测量和提出的增强水凝胶疲劳抗性的机制。a，纯剪切试验用于循环疲劳试验。从疲劳阻力曲线 与 ），可以获得疲劳阈值 ，在此阈值以下裂纹不会增长。b，弹性水凝胶的链切断机制。c，粘弹性水凝胶的疲劳抗性。d，高阶结构增强的疲劳抗性。水凝胶在合成状态下表现出一定的增强，但很少在水平衡状态下使用，因为膨胀导致了水凝胶基质与纤维之间的应力不匹配，从而导致机械性能较弱。使用脱水水凝胶形成纤维增强水凝胶是一种更成功的策略。例如，由玻璃纤维编织织物和聚离子水凝胶制成的复合材料表现出卓越的机械性能，具有优越的撕裂强度 ，拉伸模量 和断裂能 ，比单个纯材料 高几个数量级。 这种优异的机械性能源于脱肿聚离子复合物基质在水中与带负电玻璃纤维表面的强物理粘附，以及聚离子复合物基质的高粘弹性能量耗散密度。
Fatigue resistance of hydrogels 水凝胶的疲劳抗性
Long-term mechanical stability under prolonged cyclic loads is extremely important for tough hydrogels to emulate load-bearing biological tissues, such as muscles and cartilage, that are constantly undergoing reciprocal cycles in daily life. An early study on the fatigue of hydrogels was reported in 2017 (ref. 6). Fatigue damage occurs when a sample is subjected to irreversible changes in mechanical properties under cyclic loading . Fatigue is usually characterized by two aspects: the critical energy release rate for a pre-existing crack to grow, and the crack growth rate per cycle ( ) versus the energy release rate of cyclic loading (Fig. 5a). Typically, a sample with a pure shear or single-edge notch geometry is used for cyclic fatigue tests. In an example of pure shear geometry (Fig. 5a), a pre-notch perpendicular to the loading direction is made at the middle edge of the sample. At a certain loading 长期承受长时间循环载荷的机械稳定性对于模拟负载承受生物组织（如肌肉和软骨）的坚韧水凝胶非常重要，这些生物组织在日常生活中不断进行相互循环。关于水凝胶疲劳的早期研究报告于 2017 年发布（参考文献 6）。当样品在循环加载下经历机械性能的不可逆变化时，疲劳损伤就会发生。疲劳通常由两个方面来表征：预先存在的裂纹生长的临界能释放速率以及每个循环的裂纹生长速率与循环加载的能量释放速率之间的关系（图 5a）。通常，纯剪切或单边缺口几何形状的样品用于循环疲劳测试。在纯剪切几何形状的示例中（图 5a），在样品的中间边缘处垂直于加载方向制作一个预先缺口。在某个加载时刻
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amplitude ( ) and cycle period (or speed), the crack length grows above a certain energy release rate , and increases gradually with number of cycles . The energy release rate can be obtained from the stress-stretch curve of the corresponding unnotched sample at the same loading condition, by , where is the strain energy when the cyclic loading curve reaches steady state, and is the initial sample height along loading direction. From the crack growth rate per cycle versus the energy release rate, one obtains the fatigue-resistant curve and the fatigue threshold below which the crack does not grow (infinite lifetime). A comparison of between synthetic hydrogels (including elastic, viscoelastic hydrogels and hydrogel containing high-order structures) and bio-tissues is presented in Fig. 4. When comparing among different materials, attention should be paid to the observation resolution of the fatigue threshold. is denoted as the threshold below which crack growth is undetectable at the camera resolution under certain fatigue cycles. 振幅（ ）和周期（或速度），裂纹长度 在某一能量释放率 以上增长，并且 随着循环次数 逐渐增加。能量释放率可以从相应未切口样品在相同加载条件下的应力-拉伸曲线中获得，通过 ，其中 是循环加载曲线达到稳定状态时的应变能量， 是沿加载方向的初始样品高度。通过每个循环的裂纹增长速率与能量释放率的关系，可以得到疲劳曲线和疲劳阈值 ，低于该阈值裂纹不会增长（寿命无限）。图 4 中呈现了合成水凝胶（包括弹性、粘弹性水凝胶和含有高阶结构的水凝胶）与生物组织之间的比较。在不同材料之间进行比较时，应注意观察疲劳阈值的分辨率。 被定义为在某些疲劳循环下，裂纹增长在相机分辨率下是不可检测的阈值。
The observation resolution of several representative hydrogels is listed in Table 1 . 几种代表性水凝胶的观察分辨率列在表 1 中。
Fatigue-resistant behaviour of elastic hydrogels. Currently, the fatigue mechanisms in hydrogels are mainly explained by rubber elastic theory . The fatigue threshold of a single-network hydrogel follows the chain scission mechanism (Fig. 5b), that is, the Lake-Thomas model (equation (8)), which was also confirmed in tetra-PEG gels with a relatively homogeneous network structure . Therefore, increasing in simple network hydrogels is an effective strategy to improve . However, a large results in small elastic modulus (ref. 92). To solve the issue, fatigue-resistant hydrogels that use tandem-repeat proteins (polyproteins) as crosslinkers and coiled PAAm for percolating networks were developed . During cyclic fatigue tests, the foldable polyprotein crosslinkers around the crack tip unfold, yielding a fatigue threshold that is substantially superior to that of 弹性水凝胶的抗疲劳行为。目前，水凝胶中的疲劳机制主要由橡胶弹性理论解释。单网络水凝胶的疲劳阈值遵循链切断机制，即 Lake-Thomas 模型，这也在具有相对均匀网络结构的四 PEG 凝胶中得到确认。因此，增加简单网络水凝胶中的交联密度是改善的有效策略。然而，较大的交联密度会导致较小的弹性模量。为解决这个问题，开发了使用串联重复蛋白（多蛋白）作为交联剂和螺旋 PAAm 作为渗透网络的抗疲劳水凝胶。在循环疲劳测试期间，围绕裂纹尖端的可折叠多蛋白交联剂展开，产生一个明显优于的疲劳阈值。
a Force-induced mechanical activation of dibromocyclopropane mechanophores 二溴环丙烷机械感应激活机械感受器
b Force-induced radicals in DN gel b 力诱导的 DN 凝胶中的自由基
Fig. 6| Strategies proposed for mechanochemical strengthening and self-growing hydrogels. a, Self-strengthening through ring-opening reaction of gem-dibromocyclopropanes and subsequent crosslinking by nucleophilic displacement reactions with TBASA (ditetrabutylammonium salt of sebacic acid). b, Force-induced mechanoradicals in double-network (DN) gel, which can 图 6|机械化学强化和自生长水凝胶的策略。a，通过环氧化反应自强化的 gem-二溴环丙烷，然后通过与 TBASA（癸二酸四丁基铵盐）的亲核置换反应进行交联。b，双网络（DN）凝胶中的力诱导机械自由基。
C Force-induced cyanofluorene radicals to initiate the polymerization of side chains C 力诱导的氰基芴自由基来启动侧链的聚合
induce polymerizing monomers to reconstruct and strengthen the network. Compared with the first network crosslinked by strong bonds, the weak azoalkane crosslinked first network can produce more mechanoradicals in DN gels. c, Force-induced cyanofluorene radicals to initiate the polymerization of side chains. Panel a adapted from ref. 185, Springer Nature Limited. 诱导聚合单体重新构建和加强网络。与由强 键交联的第一个网络相比，由弱偶氮烷交联的第一个网络可以在 DN 凝胶中产生更多的机械自由基。c，力诱导氰基芴自由基启动侧链的聚合。面板 a 改编自参考文献 185，Springer Nature Limited。
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a Force-induced self-growth and reinforcement 一种力量诱导的自我增长和强化
b Sustainable mechanochemical growth supported by vascular-like perfusion through the inner channel 通过内部通道支持的类血管灌注的可持续机械化学生长
C Force-triggered rapid microstructure growth on surface C 力触发表面快速微观结构生长
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Fig. Force-induced self-growing and strengthening in DN hydrogels. a, Self-growing and strengthening of double network (DN) hydrogel through rupturing the first network to produce radicals and subsequent polymerization to reconstruct network. is force and is sample length. b, Sustainable mechanochemical growth of channel-containing DN gel under cyclic training. The channel can supply monomers for polymerization and water for avoiding sample dry. c, Microstructure growth on DN hydrogel surface via force-induced self-growing strategy. , advancing angle; , receding angle; , displacement; PAAm, polyacrylamide; PNaAMPS, poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt). Panel a reprinted with permission from ref. 72, AAAS. Panel reprinted with permission from ref.187, RSC. Panel c reprinted from ref.188, CC BY 4.0. 图 力诱导的 DN 凝胶自生长和强化。a，通过破坏第一个网络产生自由基，随后聚合重建网络，实现双网络（DN）凝胶的自生长和强化。 为力， 为样品长度。b，循环训练下通道含 DN 凝胶的可持续力化学生长。通道可以提供单体进行聚合和水以避免样品干燥。c，通过力诱导的自生长策略在 DN 凝胶表面的微结构生长。 ，前进角； ，后退角； ，位移；PAAm，聚丙烯酰胺；PNaAMPS，聚（2-丙烯酰胺-2-甲基-1-丙烷磺酸钠盐）。面板 a 经授权转载自参考文献 72，AAAS。面板 经授权转载自参考文献 187，RSC。面板 c 经授权转载自参考文献 188，CC BY 4.0。
PAAm hydrogel crosslinked by bisacrylamide , which has about half the modulus of the polyprotein crosslinked PAAm. Inducing highly entangled polymers into single-network elastic hydrogels is another strategy to improve . For instance, PAAm hydrogel with dense entanglements fabricated through a high concentration of PAAm and sparse crosslinks has high modulus ( ) and . The sparse chemical crosslinks impeding the disentanglement of polymer strands contribute to the high modulus, and the dense entanglements allowing the transmission of tension in the polymer strands at the crack tip to other strands contribute to the fatigue threshold. PAAm 水凝胶由双丙烯酰胺交联 ，其模量约为聚蛋白交联 PAAm 的一半。将高度纠缠的聚合物引入单网络弹性水凝胶是另一种改善 的策略。例如，通过高浓度的 PAAm 和稀疏交联制备的密集纠缠的 PAAm 水凝胶具有高模量（ ）和 。稀疏的化学交联阻碍聚合物链的解缠，有助于高模量，而密集的纠缠允许张力在裂纹尖端的聚合物链之间传递，有助于疲劳阈值。
As a special type of elastic material, chemically crosslinked DN hydrogels can resolve the conflict between and (refs. 49,50). The high elastic modulus and robust fatigue resistance are ascribed to the densely crosslinked first network and sparse second network, respectively. During cyclic fatigue tests, the first network fractures into small fragments, effectively serving as mobile crosslinks for the second network, which features long strands (large ). Consequently, DN hydrogels with loosely crosslinked second networks can achieve a fatigue threshold of up to with a high Young's modulus of 0.3 MPa while still containing around water 作为一种特殊类型的弹性材料，化学交联的 DN 水凝胶可以解决 和 之间的冲突（参考文献 49,50）。高弹性模量和强韧疲劳抗性归因于密集交联的第一网络和稀疏的第二网络，分别。在循环疲劳测试中，第一网络会断裂成小碎片，有效地作为第二网络的移动交联，第二网络具有长链（大 ）。因此，具有松散交联第二网络的 DN 水凝胶可以实现高达 的疲劳阈值，同时仍含有约 的水。
Fatigue resistance of viscoelastic hydrogels containing dynamic bonds or weak crosslinking. Although dynamic bonds substantially enhance the toughness of viscoelastic hydrogels, they contribute little to the fatigue threshold. The dynamic bonds are gradually destroyed under cyclic loading, and the fatigue threshold of such tough hydrogels also follows the chain scission mechanism described by the LakeThomas model (Fig. 5c). For example, -alginate/PAAm hydrogels with interchain ionic bonds have a high fracture energy of several kilojoules per square metre but a low of , which is close to the corresponding -alginate/PAAm without interchain ionic bonds . Nevertheless, the presence of dynamic bonds does reduce the crack growth rate above . Notably, whether dynamic bonds contribute to gel depends on the observation timescale relative to the relaxation time of the hydrogel. By extending the observation timescale of PA gels by the time-salt superposition principle was observed to be rate independent and matched the predicted value from the Lake-Thomas model only when the fatigue test was performed at a strain rate in the elastic regime. By contrast, increased with increasing the strain rate with scaling parameter when the fatigue test was performed in the viscoelastic regime . Thus, the fatigue threshold can be improved by tuning the sacrificial bond dynamics of the viscoelastic hydrogels. Similar to dynamic bonds, weak side-chain crosslinks are also gradually destroyed under cyclic loading. Triggering the rupture of weak side-chain crosslinks using force can lengthen the bridging strands around the crack tip, improving the fatigue threshold . 含有动态键或弱交联的粘弹性水凝胶的疲劳抗性。尽管动态键显著增强了粘弹性水凝胶的韧性，但对疲劳阈值的贡献很小。在循环加载下，动态键逐渐破坏，这种坚韧水凝胶的疲劳阈值也遵循 LakeThomas 模型描述的链切断机制（图 5c）。例如，具有链间离子键的-alginate/PAAm 水凝胶具有每平方米几千焦耳的高断裂能量，但是 为 ，接近不具有链间离子键的相应 -alginate/PAAm 。然而，动态键的存在确实降低了 以上的裂纹增长速率。值得注意的是，动态键是否有助于凝胶 取决于观察时间尺度相对于水凝胶的弛豫时间。通过时间-盐叠加原理延长 PA 凝胶的观察时间尺度 被观察到是速率独立的，并且仅当疲劳测试在弹性区域的应变速率下进行时，才与 Lake-Thomas 模型的预测值相匹配。 相比之下，当在粘弹性区域进行疲劳测试时， 应变速率增加， 比例参数增加， 也随之增加。因此，通过调节粘弹性水凝胶的牺牲键动力学，可以提高疲劳阈值。类似于动态键，弱侧链交联也会在循环加载下逐渐破坏。利用力量触发破裂弱侧链交联可以延长裂纹尖端周围的桥接链条，提高疲劳阈值 。
Fatigue resistance improved by high-order structure. Introducing high-order structures, such as microphase separations , microcrystals , fibrils and nanocomposites into polymer networks can substantially improve the fatigue resistance of hydrogels. 高阶结构提高了疲劳抗性。将高阶结构引入聚合物网络，如微相分离、微晶体、纤维和纳米复合材料，可以显著提高水凝胶的疲劳抗性。
Rather than the nanometre-scale polymer chains, the large-scale stiff constituents are broken to advance the crack during cyclic loading. Therefore, the intrinsic energy required to advance the crack per unit area equals the covalent energy of a layer of the reinforcement constituents per unit area. The Lake-Thomas model can be generalized as (refs. 183,184), where is the size of reinforcement constituents, is the number of the reinforcement constituents per unit volume, and is the energy required to break the reinforcement constituents. Compared with typical hydrogels without reinforcement components, can be increased by two to eight orders of magnitude, and can be increased by fourteen orders of magnitude . Meanwhile, can be adjusted within a broad range. As a consequence, can be enhanced by two to four orders of magnitude. These hydrogels show fracture energy and fatigue thresholds comparable to or even better than bio-tissues (Fig. 4). 与纳米级聚合物链不同，大尺度的硬质成分在循环加载过程中被破坏以推进裂纹。因此，每单位面积推进裂纹所需的固有能量等于每单位面积的增强成分层的共价能量。Lake-Thomas 模型可以概括为 （参考文献 183,184），其中 是增强成分的尺寸， 是每单位体积的增强成分数量， 是破坏增强成分所需的能量。与没有增强成分的典型水凝胶相比， 可以增加两到八个数量级， 可以增加十四个数量级 。同时， 可以在广泛范围内调整。因此， 可以增加两到四个数量级。这些水凝胶显示出与生物组织相当甚至更好的断裂能量和疲劳阈值（图 4）。
Hydrogels with hierarchical structures may exhibit multilevel fatigue resistance compared with elastic hydrogels, which is derived from the minor slope of the curve of versus (Fig. 5a). For example, hierarchical PA gels that include ionic bonds, primary polymer network, and -nm bicontinuous phase network structures (Fig.5d) exhibit multilevel fatigue-resistant behaviour . The fatigue threshold is mainly correlated with the mesh size of the primary polymer network bridged by both chemical crosslinking and trapped entanglement. Above , a transition point marking the jump from slow to fast crack growth occurs in PA gels with strong phase separation. is correlated to whether the damage occurs in the hard phase network. Fatigue fracture is delayed to a larger when there is abigger phase contrast, whereas gels without phase separation exhibit only a fast crack growth mode (without ). 具有分层结构的水凝胶可能表现出比弹性水凝胶更多级别的疲劳抗性，这是由于 与 曲线的较小斜率导致的（图 5a）。例如，包括 离子键， 主要聚合物网络和 纳米双连续相网络结构（图 5d）的分层 PA 凝胶表现出多级别的疲劳抗性行为 。疲劳阈值 主要与由化学交联和被困纠缠桥接的主要聚合物网络的网格尺寸相关。在 以上，具有强相分离的 PA 凝胶中发生从缓慢到快速裂纹生长的转变点 。 与损伤是否发生在硬相网络中有关。当存在更大的相对比时，疲劳断裂会延迟到更大的 ，而没有相分离的凝胶只表现出快速裂纹生长模式（没有 ）。
Microcrystalline domains are comprised of densely folded chains. Pulling out the polymer chain from a microcrystalline domain needs energy multiple times what is needed to fracture a single polymer chain. In addition, mechanically rupturing the microcrystalline domain demands energy several times that required to rupture the corresponding amorphous polymers . Therefore, microcrystalline domains act as intrinsic high-energy phases. For example, the introduction of microcrystalline domains into amorphous PVA hydrogels using a freezethawing and air-drying approach enhanced the fatigue threshold from 10 to (ref. 150). Meanwhile, the elastic modulus of such PVA hydrogels can reach , effectively resolving the tradeoff between modulus and fatigue threshold. Similar to microcrystals, fibres can act as strong barriers to fatigue crack extension, resulting in a high fatigue threshold , especially when the fibres are aligned perpendicular to the crack extension direction (Fig. 5d). 微晶颗粒区域由密集折叠的链组成。从微晶颗粒区域中拉出聚合物链需要的能量是破裂单个聚合物链所需能量的多倍。此外，机械破裂微晶颗粒区域需要的能量是破裂相应无定形聚合物所需能量的数倍。因此，微晶颗粒区域起着内在高能量相的作用。例如，通过冻融和空气干燥方法将微晶颗粒区域引入无定形 PVA 水凝胶，可以将疲劳阈值从 10 提高到（参考文献 150）。同时，这种 PVA 水凝胶的弹性模量可以达到，有效解决了模量和疲劳阈值之间的权衡。与微晶类似，纤维可以作为疲劳裂纹扩展的强障碍物，导致高疲劳阈值，尤其是当纤维与裂纹扩展方向垂直排列时。
Mechanochemical strengthening and self-growing hydrogels Living soft tissues possess the remarkable ability to autonomously grow, remodel and strengthen themselves in direct response to mechanical forces. For instance, skeletal muscles can increase in mass and strength when subjected to cyclic training. There has thus been increasing interest in developing self-growing and self-strengthening polymer networks based on force-triggered mechanoradical generation and subsequently 机械化学强化和自生长水凝胶。活体软组织具有自主生长、重塑和强化的显著能力，以直接响应机械力。例如，骨骼肌在经受循环训练时可以增加质量和力量。因此，越来越多的人对基于力触发的机械自由基生成和随后发展自生长和自强化聚合物网络感兴趣。
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induced chemical reactions. Mechanical forces typically cause bond scission in hydrogels, resulting in material failure. These mechanoradicals, in turn, initiate processes such as crosslinking or polymerization propagation, effectively converting mechanical energy into an increase in material strength (Fig. 6). Force-induced crosslinking was pioneered by the application of shear forces to trigger the ring opening of gem-dibromocyclopropanes to generate allylic bromides, which were subsequently crosslinked through nucleophilic substitution reactions with a bifunctional carboxylate (Fig. 6a). The mechanically triggered crosslinking outcompetes the destructive shearing forces, resulting in an increase in the material's modulus by several orders of magnitude. In 2019, a 'self-growing' strategy was introduced that relies on forceinduced mechanoradical generation and subsequent polymerization within DN hydrogels . The densely crosslinked initial brittle network undergoes ruptures when subjected to loads, concurrently generating mechanoradicals (Fig. 6b). These mechanoradicals then initiate the polymerization of monomers, forming a new polymer network. This process, similar to open and dynamic biological systems , leads to an increase in the mass of DN gels under repeated mechanical loading through structural destruction and reconstruction with a sustained monomer supply, substantially strengthening the DN gels (Fig. 7a). By constructing a vascular-like circulatory system to supply monomers to channel-containing DN gels, sustainable self-growth of hydrogels in air was achieved (Fig. 7b). The force-induced self-growing strategy within the DN gel system allows for local remodelling and for the creation of microstructures on the hydrogel surface on demand , which provides a facile way to imbue the hydrogel surface with specific properties, such as wettability and celladhesion (Fig. 7c). In such systems, the active mechanoradical concentration plays a crucial role on the reconstruction of the ruptured network. DN hydrogels are unusual mechanochemical materials, as numerous mechanoradicals are generated by the internal fracture of the brittle first network. The efficiency of the mechanoradical generation can be further increased by using weak azoalkane crosslinkers in the first network, as the bond (azogroup connected to saturated carbon atoms) in azoalkane has a lower bond dissociation energy than the saturated C-C and C-N bonds in traditional crosslinker (Fig. 6b). 诱导化学反应。机械力通常会导致水凝胶中的键裂解，导致材料失效。这些机械自由基反过来会启动诸如交联或聚合传播等过程，有效地将机械能转化为材料强度的增加（图 6）。力诱导的交联是通过施加剪切力来触发双溴代环丙烷的环开启，生成烯丙基溴化物，随后通过与双功能羧酸酯的亲核取代反应交联（图 6a）。机械触发的交联胜过破坏性的剪切力，导致材料模量增加数个数量级。2019 年，引入了一种“自生长”策略，依赖于力诱导的机械自由基生成和 DN 水凝胶内的后续聚合。密集交联的初始脆性网络在受到载荷时发生破裂，同时产生机械自由基（图 6b）。这些机械自由基然后启动单体的聚合，形成新的聚合物网络。 这个过程类似于开放和动态的生物系统，通过结构破坏和重建以及持续的单体供应，在重复的机械加载下导致 DN 凝胶质量增加，大大加强了 DN 凝胶。通过构建类似血管的循环系统，向含通道的 DN 凝胶供应单体，实现了空气中水凝胶的可持续自我生长。在 DN 凝胶系统内引入力诱导的自我生长策略，可以进行局部重塑，并根据需要在水凝胶表面创建微结构，从而为水凝胶表面赋予特定性能，如润湿性和细胞粘附。在这种系统中，活性机械自由基浓度对破裂网络的重建起着至关重要的作用。DN 水凝胶是不寻常的机械化学材料，因为内部脆性第一网络的破裂会产生大量机械自由基。 机械自由基生成的效率可以通过在第一个网络中使用弱偶氮烷交联剂进一步提高，因为偶氮烷中的 键（连接到饱和碳原子的偶氮基）的键解离能低于传统交联剂 中的饱和 C-C 和 C-N 键（图 6b）。
In common soft materials comprising a single network, the breaking of covalent bonds often leads to the failure of the entire material. Therefore, weak moieties (for which bond dissociation energy is lower than that of the bond), such as difluorenylsuccinonitrile , diselenide and disulfide , have been introduced into soft materials to achieve mechanoradical-induced self-growing and strengthening (Fig. 6). In polyurethanes, for example, the dissociation of weak difluorenylsuccinonitrile moieties in the main chains (Fig. 6c) resulted in the production of pink cyanofluorene (CF) radicals. These CF radicals initiated the polymerization of side chains with methacrylate groups, leading to new chemical crosslinking in the network. Therefore, the modulus increased markedly with increasing mechanical stimuli cycles. Additionally, the generated CF radicals displayed notable resilience to oxygen exposure, making this approach well suited for applications in ambient air conditions. This strategy simultaneously incorporates damage reporting (a pink colour appears, owing to the generation of radicals) and strengthening capabilities. 在由单一网络组成的常见软材料中，共价键的断裂通常会导致整个材料的破坏。因此，弱基团（其键解离能低于 键的基团），如二氟苯丙二腈 、二硒化物 和二硫化物 ，已被引入软材料中，以实现机械自由基诱导的自生长和强化（图 6）。例如，在聚氨酯中，主链中弱二氟苯丙二腈基团的解离 （图 6c）导致产生粉红色氰基二氟苯（CF）自由基。这些 CF 自由基启动了具有丙烯酸甲酯基团的侧链的聚合，导致网络中新的化学交联。因此，模量随着机械刺激循环次数的增加而显著增加。此外，生成的 CF 自由基对氧气暴露表现出显着的弹性，使得这种方法非常适用于在常温下的应用。这种策略同时结合了损伤报告（出现粉红色，由于 自由基的生成）和强化能力。
Conclusions and outlook 结论和展望
Since the 2000 s, various successful designs for tough hydrogels have been proposed, strengthening the position of hydrogels as promising soft and wet materials for diverse applications, especially in biomedical fields. However, challenges in fundamental science and engineering still limit their real-life applications. From the fundamental point of view, cutting-edge fracture mechanics theories were developed for nonlinear elastic materials, which assume no strain-rate dependency. However, hydrogels with dynamic bonds are strongly strain-rate dependent, like most biological tissues. Moreover, hydrogels with relatively strong dynamic bonds show loading-history dependence, resulting in richer and more complex mechanical behaviours. New theories and experimental approaches are required to understand the toughening and antifatigue mechanisms related to such nonlinear viscoelastic effects. Resulting progress in understanding the dynamic aspect of the materials and the molecular mechanisms should assist in the design of new structures in material development. 自 2000 年以来，各种成功的坚韧水凝胶设计被提出，加强了水凝胶作为有前途的软性和湿性材料在各种应用中的地位，特别是在生物医学领域。然而，基础科学和工程方面的挑战仍然限制了它们在现实生活中的应用。从基础的角度来看，为非线性弹性材料开发了尖端断裂力学理论，假设没有应变速率依赖性。然而，具有动态键的水凝胶具有强烈的应变速率依赖性，就像大多数生物组织一样。此外，具有相对强动态键的水凝胶表现出加载历史依赖性，导致更丰富和更复杂的机械行为。需要新的理论和实验方法来理解与这种非线性粘弹性效应相关的增韧和抗疲劳机制。对材料动态方面和分子机制的理解进展应有助于设计材料开发中的新结构。
From an engineering point of view, developing hydrogels with comprehensive mechanical properties that match those of specific biological tissues is indispensable. Currently, loading-induced softening, substantial stress relaxation, and large hysteresis loops are the main mechanical characteristics of tough hydrogels. Introducing protein-like intrachain folded structures into hydrogels may be a future strategy to create hydrogels with fast hysteresis recovery. Moreover, one needs to address issues related to biocompatibility and biochemical stability in vivo, ensuring that implanted hydrogel artificial tissues can maintain their structure and functionality. Because hydrogels are an open system, when hydrogels are implanted into the body they may interact with various minerals and biomolecules during prolonged use. Using biopolymers as constituents or designing copolymers possessing monomer sequences that feature amino acids from functional proteins could be possible strategies to solve this issue. For the latter, molecular design, notably through machine learning from protein structures, and precise control of the designed monomer sequences are key challenges. 从工程角度来看，开发具有与特定生物组织相匹配的全面机械性能的水凝胶是不可或缺的。目前，受载软化、显著的应力松弛和大的滞后回路是坚韧水凝胶的主要机械特性。将类蛋白质的链内折叠结构引入水凝胶可能是创造具有快速滞后恢复的水凝胶的未来策略。此外，需要解决与体内生物相容性和生化稳定性相关的问题，确保植入的水凝胶人工组织能够保持其结构和功能。由于水凝胶是一个开放系统，当水凝胶植入体内时，它们可能在长时间使用过程中与各种矿物质和生物分子发生相互作用。使用生物聚合物作为组分或设计具有来自功能蛋白质的氨基酸序列的共聚物可能是解决这个问题的可能策略。对于后者，分子设计，特别是通过从蛋白质结构中学习的机器学习，以及对设计的单体序列的精确控制是关键挑战。
Tough hydrogels bring new opportunities beyond their loadbearing use. Because they are open systems and can be permeated by small molecules, tough hydrogels are promising as force-catalysing materials that use mechanical energy to drive chemical reactions. As a proof of concept, tough DN hydrogels could enable the development of mechanochemical-based metabolic-like active materials. These new materials could achieve functions such as self-strengthening, self-growth or morphogenesis by supplying specific monomers during mechanical loading. To develop such adaptive materials - whose structure undergoes repeated destruction and reconstruction under load - challenges include the removal of the destroyed residuals and the maintaining of the internal fracture characteristics of the DN that are essential for material regeneration after many loading cycles. For the former, using a reversible chemical reaction pathway could be a solution, whereas for the latter, a fundamental understanding of the mechanisms of internal destruction and reformation is key. Exploring this new field of soft materials will require interdisciplinary research in fracture mechanics, polymer physics and polymer chemistry. 坚韧的水凝胶带来了超越承载用途的新机遇。由于它们是开放系统，可以被小分子渗透，坚韧的水凝胶有望作为力催化材料，利用机械能驱动化学反应。作为概念验证，坚韧的 DN 水凝胶可以促进基于机械化学的代谢活性材料的发展。这些新材料可以通过在机械加载过程中提供特定单体来实现自强化、自生长或形态发生等功能。为了开发这种适应性材料 - 其结构在负载下经历重复破坏和重建 - 挑战包括清除破坏残留物和保持 DN 的内部断裂特性，这对于材料在多次加载循环后再生至关重要。对于前者，使用可逆化学反应途径可能是一个解决方案，而对于后者，对内部破坏和重组机制的基本理解是关键。 探索这个新的软材料领域将需要在断裂力学、聚合物物理学和聚合物化学方面进行跨学科研究。
Published online: 7 May 2024 在线发布日期：2024 年 5 月 7 日
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Acknowledgements 致谢
This work was supported by JSPS KAKENHI (grant nos. JP22H04968, JP22K21342, JP22K2O521 and JP23K13796). 本工作得到 JSPS KAKENHI（授予编号 JP22H04968、JP22K21342、JP22K2O521 和 JP23K13796）的支持。
Author contribution 作者贡献
Both authors contributed to the discussion, writing, reviewing and editing of the manuscript. 两位作者都参与了讨论、撰写、审阅和编辑手稿。
Competing interests 竞争利益
The authors declare no competing interests. 作者声明没有竞争利益。
Additional information 额外信息
Supplementary information The online version contains supplementary material available a https://doi.org/10.1038/s41578-024-00672-3 补充信息 在线版本包含附加材料，可在 https://doi.org/10.1038/s41578-024-00672-3 获取。
Peer review information Nature Reviews Materials thanks Kozho Ito; Shaoting Lin, who co-reviewed with Zhaohan Yu; and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. 《自然评论材料》感谢 Kozho Ito；与余兆翰共同审阅的林少婷；以及其他匿名审稿人对本文的审稿贡献。
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Laboratory of Soft and Wet Matter, Faculty of Advanced Life Science, Hokkaido University, Sapporo, Japan 北海道大学高级生命科学学院软湿物质实验室，日本札幌
Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Sapporo, Japan. Je-mail: lixueyu@sci.hokudai.ac.jp; gong@sci.hokudai.ac.jp 北海道大学化学反应设计与发现研究所（WPI-ICReDD），日本札幌。电子邮件：lixueyu@sci.hokudai.ac.jp；gong@sci.hokudai.ac.jp
