Mechanistic Models of the Mechanical Response of Self-healing Hydrogels

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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.