Organic solvent-assisted control of hydrogel structures and properties

• Main Author: Zhou, Quan
• Other Authors: Huang Changjin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/177117
• Institution: Nanyang Technological University

Hydrogels are three-dimensional (3D) cross-linked polymeric networks containing a significant amount of water and have found their applications in various fields, including tissue engineering, drug delivery, etc. However, major limitations still exist for hydrogels, especially their mechanical weakness, which limits their use in load-bearing applications. In addition, due to their soft and wet nature, it remains technically challenging to generate hydrogel structures with complex geometries as the well-established techniques developed for shaping traditional engineering materials, such as machining and lithography, are no longer useful. Continuous advancement in hydrogel-based applications requires both novel hydrogel materials with superior properties and effective strategies that can facilitate the easy generation of complex hydrogel structures. Extrusion-based 3D printing has recently emerged as a powerful approach to generating customized hydrogel structures. However, the low viscosity of hydrogel precursor solutions makes it hard to directly write the hydrogel ink to form desired 3D structures without additional special treatments, such as incorporating solid thickeners, printing into a highly viscous bath to support the printed pattern or adopting more complicated printing system setup. In this dissertation, we explored the possibility of controlling the micro/macro structures and mechanical properties of hydrogels by introducing ethanol into the hydrogel precursor solution. First of all, we demonstrated that incorporating ethanol into sodium alginate (SA) solution could allow direct printing of SA hydrogel solution to generate desired 3D structures. This became possible for the following two reasons: 1) the addition of ethanol makes SA solution exhibit the upper critical solution temperature (UCST) behavior in which gelation occurred when the temperature was reduced below the sol-to-gel transition temperature, and 2) the presence of ethanol significantly increased the viscosity of SA solution and enhanced its shear-thinning feature, thus making it ideal for 3D printing. Secondly, we demonstrated that hydrogel structures could be manipulated by systematically tuning the rate of ethanol gasification and hydrogel gelation kinetics. Highly porous and hollow hydrogel structures were successfully obtained by controlling the temperature under which gelation proceeded. At last, we found that introducing ethanol into hydrogel precursor solution to a certain concentration range could significantly increase the toughness of hydrogels by several folds. In summary, this dissertation provides a novel strategy to enhance the printability and mechanical toughness of hydrogels and a new approach to generating complex hollow structures.