Deformation and Fracture of Tough Microstructured Hydrogels

Many natural materials are structured on different length scales. This structuring often leads to an intricate interplay between soft and stiff components, which significantly improves the fracture toughness of these materials. A class of soft material that is frequently employed by natural organisms as load-bearing elements, such as tendons, are hydrogels. These are polymeric materials that are swollen with large amounts of water. However, in contrast to their natural counterparts, synthetic hydrogels often suffer from intrinsic tradeoffs between mechanical properties, such as stiffness and toughness. These limitations hamper the use of hydrogels for more advanced applications, for example in soft robotics or biomedical engineering. The current weaknesses of synthetic hydrogels are, to a big share, related to their homogeneous structure. A promising, bio-inspired route to design tough and load-bearing synthetic hydrogels is hence to introduce microstructures into them. In this dissertation, I investigate how the microstructure of bulk hydrogels that are reinforced with hydrogel microparticles, microgels, influences their ability to resist deformation and fracture. I show that the fracture toughness of microgel-reinforced hydrogels (MRHs) is independent of the size of reinforcing microgels, and only depends on their effective volume fraction regardless of their degree of swelling. In contrast, the stress at break of MRHs is dependent on the microgel-size. I demonstrate that the microstructure in soft and tough materials is key to improve their stiffness and work of extension. To precisely control this important parameter, I present a microfluidic trapping device that allows to introduce abrupt local compositional changes into thin hydrogel sheets on the 100 ÎŒm length scale. I believe that the field of soft materials will strongly benefit from the insights gained in this thesis to develop stiffer and tougher soft materials by introducing microstructures into them, which will likely open up new applications in the fields of biomedical science and soft robotics. Further, the introduction of abrupt, local compositional variations into soft materials likely enables the development of advanced soft actuators, electrical switches or hydrogel batteries. In summary, I show that the microstructure of soft, tough materials is an important design parameter to improve their stiffness and strength, and introduce a microfluidic device to precisely control it.