3D printing of self-healing materials

Over the past few yeas there has been a great deal of interest in the development of self-healing materials with tunable structural, mechanical and rheological properties. Self-healing materials are capable to heal in response to damage whenever and wherever it occurs in the material. This unique feature makes them really attractive for applications in biomedicine, sensoring, soft robotics, and energy harvesting systems. The other great emerging and promising research field is the additive manufacturing (AM), also known as three dimensional (3D) printing, which enables to produce structures with hierarchical architectures, desired spatial arrangement, and with high degree of complexity such as hollow objects and graded or multimaterial structures. The aim of this work is to join these two advanced research fields together and, by understanding the mechanisms involved in self-healing, as well as the peculiarities of different 3D-printing techniques, to describe the main results obtained so far in the literature in order to open the possibility for future researches in the field of 3D-printing of self-healing materials. Therefore, in the first chapter the different self-healing mechanisms and chemistries, extrinsic (capsule or vascular) and intrinsic (based on either supramolecular interactions or dynamic covalent bonds), will be described with some examples for each type of strategy. The main results will be used to assess the ability of materials to self-heal, specifying whether the material need an external stimulus or not to trigger the repair process. During the description, critical issues are outlined in order to better understand how is possible to improve a determinate system. At the end of the chapter, the final remarks help the reader to resume in few lines the content of the entire chapter. In the subsequent chapter, the second one, there will be a description of the basis of the main 3D-printing techniques adopted in printing of self-healing materials. The second part of the chapter will focus on all the 3D-printed self-healing materials with the aim to give a state of the art in this field. The different results will be described dividing them according to the 3D-printing method utilized and the mechanism of self-healing involved. Unfortunately, due to the restriction in the number of people in laboratories (DISAT of Politecnico di Torino) caused by the continuous spreading of the virus Covid-19, it was not possible to develop a deep experimental analysis on 3D-printable self-healing systems. Thus, as final chapter, a little experimental part, performed in collaboration of the PhD student Matteo Caprioli, will be described and the main results obtained will be listed and discussed. Finally, the conclusion will resume all the topic cover in this work with the aim of enticing the reader to further explore the world of 3D-printable self-healing materials.