Cellulose nanomaterial hydrogels for freeform gel-in-gel printing

聚合物的三维（3D）打印是通过在无模具的环境中对熔融细丝进行微挤压，逐层构建结构，而计算机软件驱动挤压头的运动。这种工艺有其局限性：打印时间长，打印对象质量低，成本高。我的发现基金项目的主要目标是通过在另一种水凝胶环境中打印水凝胶来解决这些缺点。水凝胶是持水聚合物溶液或胶体悬浮液，通过共价键或物理结合形成三维网络结构。通过各种配方策略，可以设计水凝胶的流动和固化特性，从而通过3d打印提供所需的加工和整理。我的建议是在另一个支持3d打印结构的凝胶（凝胶#2）环境中挤出相对低粘度但可固化的凝胶（凝胶#1）作为打印材料。使用软水凝胶（凝胶#1）的挑战是建造具有跨梁或悬垂特征的复杂结构。通过将“弱”细丝直接挤压到支撑凝胶（凝胶#2）的介质中，可以在构建复杂结构时满足这些要求。支撑凝胶可以防止打印物体在自身重量下坍塌或变形。一旦打印完成，对象被固化，从支撑凝胶浴中取出并冲洗掉。水凝胶的流动特性对于开发打印和支撑凝胶都是至关重要的。这个开创性项目的目标是开发基于纤维素-纳米纤维（CNF）的后固化水凝胶油墨（凝胶#1）和基于纤维素-纳米晶体（CNC）的支持水凝胶（凝胶#2），用于自由形状制造。该计划将包括两个部分，重点是(1)微挤压，水凝胶长丝的键合交联和(2)与水凝胶打印长丝兼容的支撑凝胶。在这个项目的过程中，科学知识将在胶体科学，热力学，流变学，材料科学和生物材料等领域产生。CNFs和CNCs的聚集和关联及其凝胶结构是胶体科学的主题。CNFs和CNCs水凝胶的相分离、各向异性组织、对pH、离子和温度的响应是热力学的主题。粘弹性在毛细管流动中的作用、凝胶浴中的流动、堆积层之间的粘附是流变学和材料科学的主题。在我的五年Discovery Grant项目中，将有两名博士研究生，三名硕士研究生和两名本科co-op学生接受在凝胶支持环境下3d打印凝胶的增材制造培训。

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.