Circular plastics economies for Mars and beyond : 3D printing a sustainable future

My graduate research explored the synthesis, characterization, and applications of synthetically- and microbially-derived biodegradable polymers. The broad scope of my work was motivated through the NASA institute, CUBES (Center for the Utilization of Biological Engineering in Space). The goal of this group was to use the tools of biological engineering to create a proof-of-concept architecture for a manned mission to Mars. Out of the four interconnected branches that comprised CUBES, my research was part of Biofuel and Biomanufacturing Division (BBMD). This group focused on tool manufacturing, and as such, our goal was to derive polymers through microbial engineering and use them to manufacture useful mission parts via 3D printing. I showed that Poly-3-hydroxybutyrate (P3HB) could be used for Fused Filament Fabrication (FFF) by engineering solutions to control part warping and prevent delamination. Though I successfully produced mission-relevant parts using FFF, I was not able to overcome low tolerances on filament production which ultimately constrained the largest printable parts to a few grams. These limitations helped motivate much of the microbial work of the BBMD in CUBES. Methanotrophs and other microbes rely on sources of CO2, CH4, and O2 for both cell growth and energy storage as P3HB. By feeding microbes with 13C-labeled CO2 and CH4, I could trace their uptake into P3HB and thereby elucidate metabolic features. Ultimately, I showed that CO2 contributed to the more oxidized C1 and C3 carbons of the P3HB repeat while the CH4 contributed to the more reduced C2 and C4 carbons. This provided strong support for CH4 incorporation as formate in the serine cycle and CO2 utilization in the biosynthesis of oxaloacetate to ultimately form P3HB. With knowledge of the metabolic pathways for P3HB biosynthesis, microbes could be engineered to take up novel substrates. Cupriavidus necator H16 was engineered to uptake novel aryl substrates into the PHA backbone by means of heterologous hydroxyacyl-CoA transferase and mutant PHA synthase. Through NMR studies I showed the first formation of biological polyesters incorporating aromatic rings in the backbone (hydroxyphenylic and a hydroxyfuranoic acid). In parallel to these biological efforts, I worked to develop novel organic ring-opening polymerization (ROP) catalysts for synthesis of degradable polyesters. Synthesis affords quicker small-batch synthesis and tighter control over polymer architecture, composition, and molecular weight distribution, allowing more rapid screening of materials properties. Adapting ROP systems to flow reactor settings has allowed for easy production control at these timescales and for system properties to be quickly screened. I developed new catalysts systems for use in these flow reactors, ultimately showing reaction times could be reduced to milliseconds while offering good polymer composition control. With bases as simple and widely available as potassium tert-butoxide, these systems offer a green alternative to existing metal-centered catalysts. By initiating ROP from branched alcohols using these catalysts, I created low molecular weight liquid star polymers. By acrylate functionalizing the polymer end groups I created photo-initiated radically polymerizable liquid resins. These resins were then adapted for use in Digital Light Processing (DLP) 3D printers and resultant part properties were analyzed. This technology could be used to one day replace manufactured parts with 3D printable degradable alternatives. Ultimately, both degradable synthetic and bioderived plastics will play important roles in building a sustainable future on Earth, Mars, and beyond

Keywords: Humanities and Sciences.
Chemistry.
Advisor: Waymouth Robert M.
Criddle Craig
Xia Yan