Scalable Fabrication of Micro-Architected Water Filtering Membranes

Citation

Friedman, Andrew Collin (2023) Scalable Fabrication of Micro-Architected Water Filtering Membranes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rktj-2v55. https://resolver.caltech.edu/CaltechTHESIS:06022023-223752519

Abstract

Polymer-based filtration devices are predominantly mass manufactured via mechanical spinning or electrospinning of heated polymer materials or fiberglass to create a randomly oriented fibrous network. This technique, while effective at producing materials necessary for traditional filtering applications, fails to afford control over morphology, both macro- and microscopically. The filtering material produced often relies exclusively on its randomly assembled porosity (and occasionally on its surface charge) to capture materials from filtered fluids but provides little means for targeted analyte capture without bulk surface coating or functionalization. This thesis seeks to demonstrate a unique approach to filtration membrane manufacture via a novel high-throughput holographic lithography and contact lithography process in the visible spectrum that utilizes a customized negative-tone photoresist inherently capable of localized surface modification.

This thesis first describes the development of a large-scale holographic lithography process, from conceptualization to implementation, and demonstrates its efficacy by examining produced materials. A phase metasurface mask is utilized to produce a periodic intensity distribution of incident photons. This mask is irradiated at 0.23-0.25 W via linear raster scanning of a 2.2 mm diameter 532 nm laser at 1.5 mm/s and a scan offset of 0.4 mm to produce a homogeneous exposure profile in visible-light sensitized SU-8 negative-tone photoresist. Subsequent photoresist development results in 30–40 µm-thick nano-architected sheets with 2.1 × 2.4 cm² lateral dimensions and ~500 nm-wide struts organized in layered 3D brick-and-mortar-like patterns to result in ~50–70% porosity. Scanning electron micrographs of cross-sectioned materials reveal how pattern morphology varies with cure depth, and furthermore how the lack of complete porosity disqualifies this material for application as a membrane filter.

This thesis subsequently focuses on the development of a novel glycidyl methacrylate (GMA)-based negative-tone photoresist for implementation in the previously described lithography system to produce materials more amenable to functional membrane filter production. GMA is polymerized with a photo-caged aminated monomer, 2-((((2-nitrobenzyl)oxy)carbonyl)amino)ethyl 2-methyloxirane-2-carboxylate (ONBAMA) via free radical polymerization (FRP) and atom-transfer radical polymerization (ATRP) to produce ~30 kDa statistical co-polymers at an 85:15 monomer ratio, respectively. These linear co-polymers are then mixed with a photoacid generator (PAG) to produce a 532 nm sensitized negative-tone photoresist. Pre- and post-exposure bake temperatures are selected via glass-transition temperature identification (~62 °C) with differential scanning calorimetry (DSC) experiments, and cure depth varying with optical exposure dose is examined via establishment of contrast curves. The photoresist is then utilized in the previously described lithography system to produce square arrays of ~25 um circular holes, and the resulting films are characterized via optical and scanning electron microscopy.

This thesis concludes with an examination of the poly(GMA-rand-ONBAMA) films implemented as water-permeable filtration membranes. Efficacy of surface functionalization and solution capture explored via amine deprotection and subsequent tagging with fluorescein isothiocyanate (FITC) dye. The presence and intensity uniformity of tagged samples are examined via confocal microscopy. Transmission of water is justified analytical examination and phenomenologically demonstrated via droplet loading of supported membranes with methylene blue-dyed water. Results are preliminary but indicate potential application of manufactured films as water filters.

In summary, this thesis provides a foundation for the development of nano- and micro-architected materials at large scale and details its implementation for the design and preliminary testing of a GMA-based photoresist for water filtering membrane manufacture. Future research on optimizing photoresist design for mechanical stability could enable utilization of similar membranes for protein capture from biological fluids for use in diagnostic tools and assay automation.

Thesis Files