Electrohydrodynamic 3D Printing of Aqueous Solutions

Three-dimensional (3D) printing has proven to be a suitable technique to face demanding social and technological challenges. In fact, several important fields such as electronics, energy harvesting, environment, and biomedicine, have been benefited from progress in 3D printing. The advantages of 3D printing are numerous, including increased design opportunities, accessibility, process simplification, optimization of resource use, and customization of the final product. However, due to the recency of the 3D printing concept, there are still several areas of opportunity for development, bringing new research paths and market opportunities.<br/>Many technologies can be considered as 3D printing, each with their advantages and disadvantages. Among them, three main groups can be distinguished: photocurable resins, powder sintering, and material extrusion. Due to technological advances many different materials can be processed, including hydrogels, plastics, ceramics, metals, and several intermediate combinations. However, there is still a relatively unexplored area; 3D printing with aqueous media ¹. In this regard, the largest focus has been with 3D bioprinting, however this has a narrow focus to include cells for living biomedical products. In this research, complex microstructures made from aqueous polymers solutions is of technological interest.<br/>To 3D print complex structures from water-based systems, we followed some of the principles established in Melt Electrowriting (MEW). In this way, we use an electrohydrodynamic (EHD) effect to form microfibers, and 3 axis stage for driving fibers accurate deposition². However, in opposition to MEW, we switch from heating thermoplastic materials, to freezing water-based systems. To explore beyond state-of-the-art, we use a cryogenic stage to collect EHD jets, allowing the conservation of fiber shape, allow the 3D growth of structures, and avoid the use of additional components. This new technology has been named cryogenic aqueous electrowriting (Cryo-AEW).<br/>To overview of Cryo-AEW, this study focusses on variables that impact the fabrication of microstructures. Three main areas have been studied: i) solution behavior (viscosity, surface tension and conductivity) and how to control through different materials combination; ii) printer set-up (applied voltage, nozzle dimension, flow rate and collector speed) and how the switch between different parameters allow obtaining different fibers arrangements; and iii) environmental conditions (temperature and humidity), and how them affect the final structure growth.<br/>With the sustainability as a goal, water is the main solvent for all the experiments. With an emphasis on the use of non-toxic, biobased, and renewable materials, silk fibroin obtained from silkworm cocoon is used. Silk fibroin has a unique property to be dissolved in water, and to later become insoluble as a highly water-stable material³. This opens the use for different applications, especially for biomedicine, where silk has been widely used as support material and cell-growth media. Natural thickeners and surfactants (such us polysaccharides and fatty acids) have allowed the control of the solution behavior, for accurate scaffolds development.<br/>With this approach, the suitability of a new type of microstructured and 3D-printed material is achieved, as the time that the complexity and requirements of a new technology are tested. Due to its technological novelty, the work has been approached in a broad way, to first give an overview of the possible effect of all the variables while identifying the guideline for future research and new technology development.<br/><br/>1. Shahrubudin, et al. <i>Procedia Manufacturing</i> <b>35</b>, 1286-1296, (2019).<br/>2 Kade et al. Polymers for Melt Electrowriting. <i>Adv Healthc Mater</i> <b>10, 1, 2001232 (2021)</b><br/>3 Reizabal,<i> et al.</i> <i>Journal of Hazardous Materials</i> <b>403</b>, 123675,