A new approach for the development of DLP-3D printable functional materials

Within the advances in computer and processing technologies, new manufacturing processes known under the name of Additive manufacturing (AM), have broken into public consciousness over the past last years. Additive Manufacturing Technologies (AMT), popularly known as 3D printing, have become appealing methods for the fabrications of solid forms with controlled geometry. AM is an umbrella term for a group of technologies that can produce highly complex three dimensional objects using data generated by Computer Aided Design (CAD) systems without tools or molds. 3D techniques work using the concept of layered manufacturing: objects are fabricated layer by layer and produced through the addition of materials. The additive approach of manufacturing provides a cost-effective and time-efficient way to produce low-volume, customized products with complicated geometries and advanced material properties and functionality. Currently there are over thirty different types of additive manufacturing technologies. Their operation can be based on different physical principles and requires the use of different materials, however all have as their common denominator, the distinction of building the object layer by layer. The type of 3D printer chosen for an application often depends on the materials to be used and on the resolution needed. 3D printing technologies can be classified under seven main groups based on their mechanism: materials extrusion based methods, powder bed fusion methods, directed energy deposition methods, binder and material jetting methods, sheet lamination methods and photopolymerization (light induced) methods. Among the various 3D printing approaches, in my PhD l focused on photopolymerization based 3D printing methods. The main 3D technologies of this groups are: stereolithography (SLA) and digital light processing (DLP). In these techniques, the chemical process underlying the construction of the layers, and the creation of 3D objects, is the photopolymerization. Recently, 3D printing has rapidly grown and has shown great potential in various application fields, spanning from bioengineering, to microfluidics and electronics. There is a growing interest for 3D printing focused to the production of functional structures. The possibility to obtain functional elements by means of a 3D printer, such as batteries, antennas, membranes, sensors etc. is one of the key points of the evolution of this technology. Such a breakthrough requires a simultaneous development in new printing technologies and new materials. A constant effort to enhance and to extend the functionality of printing to meet the specific requirements of various applications is needed. The materials used in printing must undergo several developments: resolutions must continue to improve, building time must continue shortening and, in particular, more types of materials must become available. Great strides have been made in all these areas in the past years but the innovation is still on going. A robust choice of materials and the ability to control and predict their performance are essential to achieve broader use of 3D printing. Engineered materials specifically studied for being 3D printable, exhibiting optimized properties and multifunctionality, will offer huge potential and opportunities in myriad applications, resulting in better functionality of the manufactured device (e.g, biocompatibility, electrical conductivity, optical response, chemical sensitivity, mechanical behavior...) coupled with improved printability. The main approaches in the evolution of 3D printable materials consist in working with multiple materials or nanocomposite to create new combinations that have unique properties expanding the range of 3D printable materials. The research conducted during my PhD is centered in this frame, focusing on the development of new polymer nanocomposites for 3D printing technologies based on photopolymerization, in particular for digital light processing (DLP). With such technique, it is possible to tailor the final properties of the printed object by simply changing the reactive light-sensitive liquid formulations: a large variety of systems can be conceived to produce functional structures. The addition of nanofillers to the formulations could help in reaching the desired functionalities but, at the same time, it could modify the printing process introducing new issues: increased viscosity, limited light penetration depth, nanoparticles dispersion and stability. For these reasons, the strategy developed during my PhD consisted in realizing multifunctional materials by simply operating on the chemistry of the systems without affecting the printability. Using a “bottom-up approach”, liquid or soluble precursors of the desired nanoparticles can be added to the formulation in order to obtain the required functionality directly into the printed piece through a post processing step. Using this bottom-up approach two different works have been carried on. In the first work silica nanodomains were directly generated in a photocured 3D structured matrix dispersing metal alkoxide liquid precursors in the initial formulation and submitting the printed part to a sol-gel post-process in acidic vapors. The post sol-gel treatment in acidic vapors allowed the in-situ generation of the inorganic phase in a dedicated step. This method allows to build hybrid structures operating with a full liquid formulation without meet with the drawbacks of incorporating inorganic powders into 3D printable formulations. Following the same strategy, the second work deals with the incorporation of silver nitrate into a reactive mixture in order induce the in-situ generation of silver nanoparticles during a thermal or UV post processing of the printed object. The developed process allows a good distribution of the metal nanoparticles in the polymer matrix, producing very complex geometries with improved electrical properties. In the last chapter of the this thesis, Chapter 4, I summarize the activity conducted during my exchange period in the laboratory of Micro-Nano-Bio Systems Laboratory (MNBS Lab) of Prof. Jun Yang, University of Western Ontario Canada. During this period, I have familiarized with the use of 3D technologies for the fabrication of conductive hydrogels. Conductive polymer hydrogels are promising materials, especially in bio-medical and bio-electronics fields. They showed great potential in drug release, bioactive electrode, actuator and as scaffolds for tissue engineering. However, it is extremely challenging to build this hydrogel in complex 3D structures with conventional fabrication methods. The idea of the work was to couple 3D printing with interfacial polymerization in order to obtain electro-active hydrogels with complex and defined geometry. Polypyrrole/PEGDA conductive hydrogels were fabricated through a two-step procedure. First 3D printing technology was used to fabricate PEGDA microstructures, functioning as the supportive structure. Second, by means of interfacial polymerization (IP), pyrrole (PY) was polymerized at the interface, leading to the formation of polypyrrole (PPY) into the hydrogel matrix, thus synthesizing a conductive hydrogel. All the approaches presented in the thesis allowed to build nanocomposites structures operating with a full liquid formulation without meeting the drawbacks of incorporating inorganic fillers into 3D printable formulations. In all the works, the composites prepared present excellent printability and mechanical stability saving the high accuracy of the 3D process. These works open the possibility of developing functional objects with complex geometries through simple but very efficient processes.