Mechanistic Models of the Mechanical Response of Self-healing Hydrogels

水凝胶是一种分子链网络，水被吸收和捕获，形成一种通常含有90%或更多水的材料。水凝胶具有广泛的当前和潜在的应用，如实验室培养组织的支架，药物输送系统，生物传感器和消费产品。水凝胶通常在低水平拉伸时破裂，限制了它们在承载应用中的使用。新开发的水凝胶由刚性和柔软的互穿网络组成，可以像橡胶一样拉伸。然而，一旦它们开始撕裂，损害是不可逆转的。通过用可以重组的键取代刚性网络的共价键，材料可以自我修复。该项目旨在了解这种凝胶的物理和力学，并开发数学模型来解释自愈行为。这些模型将建立分子尺度特征和机械响应之间的联系，使研究人员能够建立工程水凝胶系统的模拟模型。该模型将嵌入在力学中广泛使用的分析代码的上下文中。我们设想这样的模型将有助于自愈凝胶的潜在承载应用工程，如人造软骨和软机器的致动器。该项目还将为我们提供机会，通过亲身实践发现自我修复水凝胶的意想不到的特性，从而接触到未来的学生。近年来，聚合物化学家在合成生物相容性、韧性、低摩擦、自愈的水凝胶方面取得了巨大进展。目前，缺乏将抗疲劳、断裂和时间依赖的自愈行为与潜在的、速率依赖的键断裂和重组过程联系起来的模型。本研究通过：(a)设计和执行实验，使用模型材料系统来测量这些力学行为，(b)定义将这些行为与键断裂和重组动力学联系起来的定量模型，弥补了这一空白。实验将包括张力测试、测量裂纹扩展速率测试和单调和循环加载下的裂纹愈合。该研究将提供对观察到的宏观特性（强度、时间依赖性、断裂、疲劳和自愈）如何与分子水平上潜在的变形和分离机制相关的理解。本构模型和失效模型还将提供这些可观察到的力学性能与相关微尺度参数（如不同类型的物理键、它们的强度和断裂/愈合动力学）之间的定量联系。该项目将通过建立与微观结构直接相关的本构和断裂模型框架，推动力学领域的发展，并可用于结构有限元模拟。

A hydrogel is a network of molecular chains into which water is absorbed and trapped, forming a material that is typically 90 percent or more water. Hydrogels have a wide range of current and potential applications such as scaffolds for laboratory grown tissue, drug delivery systems, biosensors, and consumer products. Hydrogels typically break at low levels of stretching, limiting their use in load bearing applications. Newly developed hydrogels consisting of stiff and soft interpenetrating networks can stretch like rubber. However, once they start to tear the damage is irreversible. By replacing the stiff network's covalent bonds with bonds that can reform, the material can become self-healing. This project seeks to understand the physics and mechanics of such gels and to develop mathematical models that will explain the self-healing behavior. Such models will establish connections between molecular scale features and mechanical response and enable researchers to build simulation models of engineered hydrogel systems. The model will be embedded in the context of an analysis code widely used in mechanics. We envision that such models would aid in the engineering of potential load-bearing applications of self-healing gels such as artificial cartilage and actuators for soft machines. The project will also provide opportunities for us to reach out to prospective students through hands-on discovery of the unexpected properties of self-healing hydrogels. In recent years polymer chemists have made tremendous strides in the synthesis of biocompatible, tough, low friction, self-healing hydrogels. Currently, models are lacking that link fatigue resistance, fracture, and time dependent, self-healing behavior to the underlying, rate dependent bond breaking and reformation processes. This research bridges this gap by: (a) designing and executing experiments to measure these mechanical behaviors using a model material system, (b) defining quantitative models relating these behaviors to bond breaking and reformation kinetics. Experiments will include tension testing, measurements crack growth rate tests and crack healing under monotonic and cyclic loading. The research will provide understanding of how the observed macroscale properties (strength, time dependence, fracture, fatigue and self-healing) are related to underlying deformation and separation mechanisms at the molecular level. The constitutive and failure models will also provide a quantitative link between these observable mechanical properties and relevant microscale parameters such as the different types of physical bonds, their strengths, and breaking/healing kinetics. The project will advance the field of mechanics by building a framework for constitutive and fracture models that are linked directly to the microstructure and that can be used in structural finite element simulations.