Mass fabrication of high-resolution hydrogels by a high-speed process using a thermal 3D screen printing method. (2019)
Type of ContentTheses / Dissertations
Thesis DisciplineMechanical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
Hydrogel is a class of material described as ‘solid water’: it maintains properties of liquid water while being physically constrained like a solid. The hydrophilic polymer networks of hydrogels have the strength to absorb and retain a significant amount of water, as much as 99% by weight. This makes it suitable for a diverse range of applications such as tissue engineering, chromatography purification and viral vector separation. Natural hydrogels, made of natural polymers such as starch, alginate, and cellulose, demonstrate common characteristics including permeability, nontoxicity, biocompatibility, and biodegradability.
Cellulose is the most abundant organic compound on Earth with fascinating structures and properties. Hydrogel developed from cellulose is of particular interest due to its characteristics of hydrophilicity, solute permeability, and nontoxicity.
Aqueous solutions of NaOH/urea are a common solvent for cellulose. Heating or cooling the solution causes an irreversible gel to form. When a non-solvent such as water or acetone is introduced to the gel, the cellulose precipitates, and if water is present, forms a stable hydrogel.
An advanced fabrication technique is necessary in order to create the desired geometry, such as a highly complex Schoen Gyroid, made of cellulose hydrogel. The fabricated cellulose hydrogel can be used in various applications including chromatography purification and viral vector separation. Additive manufacturing has the capability to fabricate complex geometries of cellulose hydrogel. It has rapidly emerged as a disruptive technology to build parts, enabling increased design complexity and an ever-increasing range of materials. However, the size and print resolution required for a complex cellulose hydrogel model have a dramatic impact on printing speed. As a result, large batch manufacturing of cellulose hydrogels using additive manufacturing has not been achieved to date. Current methods are also limited by their inability to manufacture complicated shapes. This is due to a lack of suitable support material, which plays an important role in holding up overhanging regions during the printing process. This thesis aims to overcome the current limitations of 3D printing hydrogels, proposing solutions that increase the speed of the process, enable the fabrication of large and complex parts, and ultimately make batch manufacturing hydrogels more economical without sacrificing high resolution.
This project has succeeded in developing a method of batch manufacturing large, high resolution cellulose hydrogel parts at a high-speed fabrication rate. A machine, referred to as a thermal 3D screen printer, was prototyped and successfully tested. For the 3D screen printing process, a three-dimensional CAD model is sliced into individual layers of a predefined thickness. A stencil made of thin metal shim is produced for each layer. Successive stencils are used to print wax, which forms a mould in which cellulose hydrogel is cast. After casting, the wax mould is removed in hot water and recycled for further use and the cellulose hydrogel object is fabricated.
Thermal 3D screen printed wax moulds can be used to print an extremely large range of material, such as polymers, metals and ceramics; however, the focus of this research was on printing cellulose solutions for the manufacture of structured cellulose hydrogels. The material selection process for the mould material showed that sacrificial wax is suitable for casting complex geometries. The properties investigated are rheological and thermophysical properties, wettability, surface roughness, material interaction, and the effects of different mould removal methods.
The thermal 3D screen printed wax mould was optimised using the Taguchi statistical method. Process parameter optimisation showed that the selected printing parameters have a significant effect on 3D screen print quality: listed in declining order of significance, these parameters are squeegee speed, squeegee pressure and printing temperature.
The main objective of this project was to explore a manufacturing method for mass fabricating monolithic cellulose hydrogel columns with a TPMS geometry. As a result, a thermal 3D screen printing method with an ultra-fast fabrication process capable of mass fabricating wax moulds was proposed, designed, prototyped, tested, and developed to successfully fabricate cellulose hydrogel.
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