3D/4D printing tough hydrogel composites: a pathway to functional devices

Hydrogels are a class of hydrated and compliant polymer materials that display a variety of desirable properties for engineering applications including actuation, conductivity, and biocompatibility. Despite their unique range of material properties, the implantation of hydrogels into real world applications has been restricted due to their poor mechanical performance. In recent years a number of toughening mechanisms have been designed to improve the mechanical characteristics of hydrogels. This thesis investigates the use 3D printing as a means to process these ‘tough hydrogels’ into complex structures and functional devices. Two different methods are designed for extrusion printing ionic-covalent entanglement hydrogels. The printing inks were formulated to match the rheological requirements of the printing machines available and the chemical composition optimised to permit rapid crosslinking of the hydrogel components. Multi-material printing techniques are then used to print these tough hydrogel inks alongside other inks of other structural polymers to create a variety of composite architectures including fibre and particulate reinforcement. The mechanical properties of a series of composite structures were measured and compared with established composites theory. The material properties of these hydrogel composites are dependent on the volume fraction of hydrogel present and can be programed into a printed object through digital modelling software. With these printing techniques mechanical gradients have been constructed and a preliminary prototype artificial cartilage demonstrated. These printing techniques have been further developed to incorporate an ink for printing a temperature-sensitive hydrogel that actuates. Combining this ink with other structural materials, co-called “4D printing” was demonstrated and a smart valve device was designed and fabricated to illustrate the potential usefulness of this new manufacturing approach.