Tissue-like hydrogels by design

Abstract

Human bodies are mostly made of soft, wet yet robust biological hydrogels such as cartilages, ligaments, and muscles. The biological hydrogels commonly possess mechanical properties such as high toughness, resilience and fatigue resistance to guarantee the bodies' reliable functions and activities. While hydrogels with tissue-like mechanical properties are highly desirable in applications as diverse as tissue engineering, drug delivery and soft machines, these properties are rarely achieved in synthetic hydrogels. The first part of this dissertation is aimed to design synthetic hydrogels that possess tissue-like mechanical properties, including high toughness, resilience and fatigue threshold, through combined theory, modeling and experiments. First, we develop a coupled cohesive-zone and Mullins effect model to predict the fracture energies of tough hydrogels. Based on the model, we further provide a toughening diagram that can guide the design of new tough hydrogels.

Second, we propose that delaying mechanical dissipations in tough hydrogels can make the hydrogels resilient under moderate deformation while still high toughness and high resilience for hydrogels. Third, we study fatigue fracture in hydrogels and show that the introduction of nanocrystalline domains and aligned nanofibrils can substantially increase hydrogels' fatigue-resistant properties. In the second part of this dissertation, we study mechanical instabilities in hydrogels. Under tension, a layer of confined elastic material such as hydrogel can exhibit various modes of mechanical instabilities, including cavitation, fingering and fringe instabilities. While the cavitation has been extensively studied, the fingering and fringe instabilities have not been well understood, and the relations and interactions of these instabilities have not been explored yet.

We systematically study the formation, transition, interaction and co-existence of mechanical instabilities in confined elastic layers under tension. Through combined experimental, numerical and theoretical analysis, we find that the mode of instability is determined by both geometry and mechanical properties of the elastic layer through two non-dimensional parameters: layer's lateral dimension over its thickness and elastocapillary length over the defect size. A phase diagram is calculated to quantitatively predict the occurrence of any mode of instability, which can help the design of robust adhesives by rationally harnessing the desired mode of instabilities while suppressing the other modes.