Investigation of Hierarchical Fabrication Methods for High-Resolution Additive Manufacturing

This study investigates versatile multi-material and multi-scale fabrication approaches for high-resolution additive manufacturing, with a special focus on multi-material and multiscale 3D scaffolds for tissue engineering and high-resolution conductive traces for low-cost printed flexible electronics.;Firstly, a cost-effective layer-molding method is developed for macroscale 3D tissue engineering applications using photocrosslinkable biopolymers: thermoplastic material is used as supporting mold and functional photocrosslinkable material can be deposited on-demand into the mold in an arbitrary way. The building process is repeated following a layer-by-layer sequence until the desired thickness is reached. The final 3D structure can be obtained by removing the supporting material. In comparison with conventional stereolithography (SLA), the layer-molding method in this study enables highly-viscous functional photocrosslinkable materials to be fabricated into 3D structures and have multi-material integration capability, which are not possible for conventional SLA approach.;Second, Electrohydrodynamic (EHD) hot jet plotting is developed for microscale additive manufacturing. By applying strong spatial electrostatic field, Maxwell stress at the material/air interface results in pointed meniscus (Taylor Cone), from which microscale droplet or fine jet is issued. A 3D microfabrication method is obtained by taking advantage of this microscale material deposition mechanism, coupled with sub-micron position accuracy, augmented by appropriate processing conditions which will suppress inherent EHD instabilities. Moreover, a fully 3D multiscale fabrication method for 3D tissue engineering scaffold is demonstrated via combination of Solid Freeform Fabrication technology and EHD printing methods.;Third, a long-existing issue for EHD-based printing lies on residual charge on substrate, especially for highly insulated substrate with long characteristic charge decay time. In order to overcome this fundamental challenge, an AC EHD-jet printing method was developed with objective on both charge neutralization and high-resolution. A special AC plus based high voltage is used to drive the material flow. Consequently, a positively-charged droplet and a negatively-charged droplet with equal amount of charge will be ejected from meniscus - overlaying them will eliminated residual charge, making printing continuous features possible. Another advantage for this AC EHD-jet method over EHD printing using constant DC voltage is that printing frequency and printing resolution can be controlled separately.;Lastly, this study investigated the detailed mechanism for EHD-based fabrication. A complete printing process is separated into multiple stages with distinct characteristics, which includes droplet formation, droplet flight, and droplet impact/spreading/solidification. Droplet formation and droplet fight has been successfully addressed in this study. Droplet formation process is modeled by Finite Element Analysis (FEA) with respect to the electrostatic force and surface tension of the resulting pending droplets about to detach from pointed meniscus. The numerical results have reasonable agreement with experimental results obtained from Atomic Force Microscope (AFM), indicating this model can provide guidelines to predict the droplet dimensions at different process conditions. Droplets in-flight velocity and fluidic characteristics (e.g. Reynolds number and Weber number) are modeled using the results from FEA analysis. Droplet impact/spreading/solidification on non-porous media will be in future study.