Dissertation/Thesis Abstract

Digital-Lithographic Fabrication of Unconventional 3D Microstructures
by He, Ran, Ph.D., Lehigh University, 2017, 131; 10604386
Abstract (Summary)

Developing advanced microfabrication technologies have been regarded as an important direction since many opportunities in technological innovations depend on the ability to fabricate novel structures in microscale. Commercially, high-throughput devices fabricated by cost-effective and efficient techniques that are suitable for mass production are highly desired. Scientifically, the ability to generate unconventional microstructures for improving the design and control experiments has attracted great interest of scientists. This dissertation presents five innovative techniques based on digital-lithographic fabrication to generate unconventional structures as better solutions in various applications. The techniques were firstly developed to advance the microfluidic areas, and then extended to 3D printing fields.

Firstly, a micro-patterning technology was developed to generate customizable micro-wavy patterned microfluidic devices. This method utilized the grayscale Gaussian distribution effect to model inaccuracies inherent in the polymerization process. An accurate pattern can be generated with customizable parameters (wavelength, amplitude, wave shape, pattern profile, and overall dimension), demonstrating the ability of this method to generate wavy patterns with precisely controlled features. Wavy patterns can be generated with the wavelength ranging from 12μm to 2100μm, and an amplitude-to-wavelength ratio as large as 300%. Microfluidic devices with pure wavy and wavy-herringbone patterns suitable for capture of circulating tumor cells were made as a demonstrative application. Secondly, build upon the first development, we developed a dual-projection lithography technique to fabricate microfluidic channels with circular and elliptical cross sections. The method utilized two projecting systems to expose grayscale image face-to-face and simultaneously polymerize the photocurable material. The cross-sectional profiles of the fabricated microchannel were consistent with mathematical predictions and, therefore, demonstrate the capability of controlling the channel shapes precisely. Customized circular microchannel can be generated with complex features such as junctions, bifurcations, hierarchies, and gradually changed diameters. This method was capable of fabricating circular channels with a wide range of diameters (39 μm–2 mm) as well as elliptical channels with a major-to-minor axis ratio up to 600%. Microfluidic devices with circular cross sections suitable for particle analysis were made as a demonstrative application in nanoparticle binding and distribution within a mimetic blood vessel. Thirdly, we developed an in situ fabrication technology to generate controllable actuators through two-color inhibited lithography. The method utilized the light inhibition effect to selectively form an unpolymerized layer that prevents the adhesion between polymerized structures and microfluidic substrates. A novel material system and an optimized optical system were established to allow the simultaneous control of both lights. Magnetic nanoparticles were introduced in the photocurable materials and acoustically aligned. Thus, under magnetic field, remote and precise control of the rotation angle was achieved for a fabricated rotor structure.

Next, we further extended the digital-lithographic fabrication to additive manufacturing and developed two 3D printing technologies. The first technology was based on a scanning lithography that allowed UV projector to continuously cure resin while scanning over the build area. This approach can be regarded as a combination of both laser based 3D printing and DLP (Digital Light Processing) based 3D printing, adopting laser’s mobility and DLP’s ability to exposure an area at once. To 3D print large objects, 3D models were sliced in to layer-by-layer “maps”, which were ultrahigh-resolution images. Each “map” was further divided into sub-region images to fit the projector’s resolution. During the movement of the projector, the sub-region images were dynamically exposed to the photocurable materials and synchronized with the scanning of the projector, causing a still exposure pattern to appear on the target build surface. Therefore, 3D printing of customized build volumes on a large-scale (greater than 1 m3) can be achieved with micro-scale features. The second 3D printing technology utilized an oxygen inhibited lithography approach to achieve continuous fabrication of three-dimensional objects. The oxygen inhibited the polymerization and creates an isolated layer, enabling ultra-high-speed printing with layer-less finish. The tensioned vat system and pressure controlled chamber enabled a close-loop control to provide robust solutions for printing objects with solid inner structures and large cross-sections. Unconventional structures with microscale features that can hardly be fabricated by other methods were generated rapidly.

With these developed microfabrication techniques, we expect the work to provide solutions for effectively generating features that can be extensively applied in various applications.

Indexing (document details)
Advisor: Liu, Yaling Liu
Commitee: Krick, Brandon, Ou-Yang, H. Daniel, Zhou, Chao
School: Lehigh University
Department: Mechanical Engineering
School Location: United States -- Pennsylvania
Source: DAI-B 79/02(E), Dissertation Abstracts International
Subjects: Mechanical engineering
Keywords: 3d printing, Additive manufacturing, Lithography, Microfabrication, Microfluidics, Microstructure
Publication Number: 10604386
ISBN: 978-0-355-24590-5
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