Cellulose nanomaterial hydrogels for freeform gel-in-gel printing

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Three-dimensional (3D) printing of polymers is used to build structures layer-by-layer by micro-extrusion of molten filaments in a mold-free environments, while computer software drives the movement of extrusion head. This process has limitations: long printing times, low-quality printed object and high costs. The primary goal of my Discovery Grant program is to address those shortcomings by printing hydrogels in another hydrogel environment. Hydrogels are water-holding polymer solutions or colloidal suspensions which form three-dimensional network structures through covalent bonds or physical associations. Through various formulation strategies, flow and curing characteristics of hydrogels can be designed to offer the required processing and finishing by 3D-printing. My proposal is the extrusion of relatively low-viscosity but curable gel (Gel #1) as printing material within another gel (Gel #2) environment which supports the 3D-printed structure. The challenge of using soft hydrogels (Gel #1) is to build complex structures with spanning beams or overhanging features. Those requirements can be met while building complex structures by extruding "weak" filaments directly into the medium of a supporting gel (Gel #2). The support gel prevents the collapse or deformation the printed object under its own weight. Once the printing is completed, the object is cured, removed from the support gel bath and rinsed off. Flow characteristics of hydrogels are crucial both for the development printing and support gels. The goal of this pioneering program is to develop cellulose-nanofiber(CNF)-based post-curable hydrogel inks (Gel #1) and cellulose-nanocrystal (CNC)-based support hydrogels (Gel #2) for free-form fabrication. This program will have two parts focusing on (1) micro-extrusion, bonding crosslinking of hydrogel filaments and (2) support gels that are compatible hydrogel printed filaments. During the course of this program, scientific knowledge will be generated in the fields of colloid science, thermodynamics, rheology, material science and biomaterials. Aggregation and association of CNFs and CNCs and their gel structures are the topics of colloid science. Phase separation, anisotropic organization, responses to pH, ionic and temperature of CNFs and CNCs hydrogels are the topics of thermodynamics. The role of viscoelasticity in the capillary flow, flow in gel bath, adhesion between stacked layers are the topics of rheology and material science. Over the five years of my Discovery Grant program, two PhD students, three MSc students and two undergraduate co-op students will be trained on additive manufacturing with 3D-printing of gels in gel support environment.